Technical Documentation for use of HEC-HMS with …sscafca.org/development/documents/DPM/Technical...Technical Documentation for use of HEC-HMS with the Development Process Manual

Technical Documentation for use of HEC-HMS with the Development Process Manual

Technical Documentation for use of HEC-HMS with the Development Process Manual

Stantec Consulting Inc.8211 S. 48th StreetPhoenix, AZ 85044Tel. 602.438.2200Fax. 602.431.9562www.stantec.com

SSCAFCA1041 Commercial Dr. S.E.Rio Rancho, NM 87124Tel. 505.892.RAIN (7246)Fax. 505.892.7241www.sscafca.com

TECHNICAL DOCUMENTATION FOR USE OF HEC-HMS WITH THE DEVELOPMENT

PROCESS MANUAL

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Executive Summary

Hydrologic modeling within the Southern Sandoval County Arroyo Flood Control Authority

(SSCAFCA) jurisdictional area is generally accomplished using the AHYMO computer program.

AHYMO is a MS-DOS based, batch program that reads an ascii text file (formatted similarly to

HEC-1, TR-20, etc.) of data and / or input parameters. Many of the methods and parameters

used for hydrologic modeling and encoded in AHYMO are based on local data and are therefore

unique in many ways.

SSCAFCA, through the issuance of a Development Process Manual (DPM), will offer engineers

and hydrologists the opportunity to use an equivalent model to AHYMO for estimating runoff

magnitudes. It is desired that the equivalent model be a “main-stream” public domain model. In

addition, the model should be easy to use and yield equivalent results as AHYMO.

The equivalent model selected is HEC-HMS. Selection of HEC-HMS was based on a

qualitative evaluation of the hydrologic models on FEMA’s approved model list. The major

factors in the selection of HEC-HMS are that HEC-HMS:

• is a public domain model maintained by the U.S. Army Corps of Engineers

• has a graphical user interface

• has a range of methodologies that can be selected and/or tailored to yield equivalent

results to AHYMO

The benchmark used for evaluation and selection of equivalent methods is that HEC-HMS

runoff magnitudes should be within ± 5 to 10 percent of AHYMO runoff magnitudes. Through

various trials and testing, the following methods and parameters are recommended for

implementing the DPM in HEC-HMS:

• Rainfall: User Specified Hyetograph with input determined using the existing suite of

equations.

• Rainfall Loss: Initial and Constant Loss with input based on the existing base

parameters for the specified land treatment types.

• Unit Hydrograph: Clark Unit Hydrograph with input for time of concentration determined

using the existing suite of equations. Input for the storage coefficient determined using a

new equation derived through a regression analysis.

• Channel Routing: Muskingum-Cunge with input data derived from the physical

conditions of the watercourse.

• Storage Routing: Modified Puls, Level Pool with input data derived from the physical

conditions of the storage basin.

• Sediment Bulking: Hydrograph ratio using the factors specified in the DPM.

TECHNICAL DOCUMENTATION FOR USE OF HEC-HMS WITH THE DEVELOPMENT PROCESS

MANUAL EXECUTIVE SUMMARY

October 24, 2008


As a practical test of the recommended methodologies and parameters two existing watershed

models were converted from AHYMO to HEC-HMS. Those watersheds are the Montoyas

Arroyo and Black Arroyo. Through the testing process, important limitations of the AHYMO

model were identified. Those limitations involved the implementation of the Muskingum-Cunge

channel routing in AHYMO. Specifically, the Fread correction factors for the routing coefficients

resulted in artificially long floodwave travel times. The Fread correction factors are only used

when numerical instability problems using the preferred correction factors are detected. The

AHYMO code was subsequently revised by the model developer. The two test watersheds

were rerun with the revised AHYMO model and compared with the results from HEC-HMS. A

comparison of the peak discharge results for the two models is shown graphically in Figure A. A

comparison of runoff volume results for the two models is shown in Figure B.

At the subbasin level, the recommended methodologies, parameters and procedures for

estimating runoff magnitudes matched closely with AHYMO runoff magnitudes. For the two test

watershed used for evaluation purposes, approximately 94 percent of the 201 subbasins were

within the ±10 percent target for peak discharge. Looking at runoff volumes, 99.5 percent of the

201 subbasins were within the ± 10 percent target. Although the revised AHYMO executable

improves the comparison of results, minor routing differences between the two models still

pushes the peak discharge outside of the target range. For the two test watersheds, 83 percent

of the peak discharges at the model junctions were within the target range.

Figure A – Comparison of peak discharge results for the test watersheds

0

2,000

4,000

6,000

8,000

10,000

12,000

0 2,000 4,000 6,000 8,000 10,000 12,000

HE

C-H

MS

Pe

ak

Dis

ch

arg

e,

in c

fs

AHYMO Peak Discharge, in cfs

Montoyas Watershed

Black Watershed

Line of Agreement10% Error Band


MANUAL EXECUTIVE SUMMARY

October 24, 2008


Figure B – Comparison of runoff volume results for the test watersheds

0

400

800

1,200

1,600

2,000

0 400 800 1,200 1,600 2,000

HE

C-H

MS

Ru

no

ff V

olu

me,

in a

cre

-ft

AHYMO Runoff Volume, in acre-ft

Montoyas Watershed

Black Watershed



PROCESS MANUAL

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Table of Contents

1.0 INTRODUCTION ..............................................................................................................1.1

2.0 EXISTING DPM ................................................................................................................2.1

2.1 RAINFALL.........................................................................................................................2.1

2.2 RAINFALL LOSS ..............................................................................................................2.4

2.3 UNIT HYDROGRAPH.......................................................................................................2.6

2.4 HYDROLOGIC ROUTING.................................................................................................2.8

2.4.1 Channel Routing.................................................................................................2.8

2.4.2 Reservoir Routing.............................................................................................2.14

2.5 FLOW DIVERSIONS.......................................................................................................2.14

2.6 SEDIMENT BULKING / SEDIMENT TRANSPORT.........................................................2.14

3.0 ALTERNATIVE WATERSHED MODEL ...........................................................................3.1

3.1 FEMA ACCEPTED MODELS............................................................................................3.1

3.2 MODEL EVALUATION......................................................................................................3.3

3.3 MODEL RECOMMENDATION..........................................................................................3.4

4.0 EQUIVALENT PARAMETERS .........................................................................................4.1

4.1 RAINFALL.........................................................................................................................4.1

4.2 RAINFALL LOSS ..............................................................................................................4.3

4.3 UNIT HYDROGRAPH.......................................................................................................4.6

4.3.1 Unit Hydrograph Selection..................................................................................4.7

4.3.2 Parameter Adjustments....................................................................................4.11

4.4 HYDROLOGIC ROUTING...............................................................................................4.14

4.4.1 Channel Routing...............................................................................................4.14

4.4.2 Storage Routing ...............................................................................................4.17

4.5 FLOW DIVERSIONS.......................................................................................................4.18

4.6 SEDIMENT BULKING.....................................................................................................4.18

5.0 SUMMARY .......................................................................................................................5.1

TECHNICAL DOCUMENTATION FOR USE OF HEC-HMS WITH THE DEVELOPMENT PROCESS MANUAL

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List of Tables

Table 1 – Rainfall Loss Parameters .........................................................................................2.5

Table 2 – Conveyance Factor ..................................................................................................2.7

Table 3 – Basin Factor .............................................................................................................2.7

Table 4 – Routing Example: Hydraulic Rating ........................................................................2.10

Table 5 – Routing Example: Ponce Routing Coefficients .......................................................2.12

Table 6 – Routing Example: Fread Routing Coefficients ........................................................2.13

Table 7 – Rainfall-Runoff Model Evaluation Matrix...................................................................3.5

Table 8 – Comparison of Rainfall Loss Estimates ....................................................................4.5

Table 9 – Peak Discharge Comparison Descriptive Statistics ................................................4.13


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List of Figures

Figure A – Comparison of peak discharge results for the test watersheds ................................. 2

Figure B – Comparison of runoff volume results for the test watersheds.................................... 3

Figure 1 – Depth-area Reduction .............................................................................................2.3

Figure 2 – Time Distribution of Rainfall ....................................................................................2.3

Figure 3 – Comparison of 6- and 24-Hour Mass Curves ..........................................................2.4

Figure 4 – Representation of Rainfall Loss Methodology .........................................................2.5

Figure 5 – AHYMO Dimensionless Unit Hydrograph ................................................................2.8

Figure 6 – Mass Rainfall Curve Comparison ............................................................................4.2

Figure 7 – AHYMO – Clark Unit Hydrograph Comparison........................................................4.8

Figure 8 – AHYMO – Snyder Unit Hydrograph Comparison.....................................................4.8

Figure 9 – AHYMO – SCS Dimensionless Unit Hydrograph Comparison.................................4.9

Figure 10 – AHYMO – Clark Unit Hydrograph with Adjusted Parameters.................................4.9

Figure 11 – AHYMO –Snyder Unit Hydrograph with Adjusted Parameters.............................4.10

Figure 12 – AHYMO – SCS Unit Hydrograph with Adjusted Parameters................................4.10

Figure 13 – Peak Discharge Comparison with the Clark Unit Hydrograph..............................4.14

Figure 14 – Routing Peak Discharge Comparison..................................................................4.16

Figure 15 – Revised Routing Peak Discharge Comparison....................................................4.17

Figure 16 – Histogram of subbasin peak discharge comparison ..............................................5.2

Figure 17 – Histogram of subbasin runoff volume comparison.................................................5.2

Figure 18 – Histogram of model junction peak discharge comparison......................................5.3

Figure 19 – Histogram of model junction peak discharge comparison......................................5.3


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List of Appendices

Appendix A – AHYMO Unit Hydrograph Documentation

Appendix B – Regression Analysis

Appenidix C – Watershed Model Subbasin Comparison

Appendix D – Watershed Model Channel Routing Comparison

Appendix E – Digital Files on CD


PROCESS MANUAL

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1.0 Introduction

Hydrologic modeling within the Southern Sandoval County Arroyo Flood Control Authority

(SSCAFCA) jurisdictional area is currently conducted in accordance with the methodologies,

techniques and procedures set forth in Section 22.2, Hydrology of the Development Process

Manual; January 1993. That document, herein referred to as DPM, was prepared for the

Albuquerque Metropolitan Arroyo Flood Control Authority in cooperation with the City of

Albuquerque and Bernalillo County. The DPM presents methodologies and parameters that are

intended to be used for estimating runoff magnitudes for both rural and urban watersheds

Many of the methods and parameters set forth in the DPM are based on local data and are

therefore unique in many ways. Implementation of the methodologies, techniques and

procedures set forth in the DPM is generally accomplished with the AHYMO computer program.

AHYMO is an arid lands hydrologic model based on the HYMO computer program. The HYMO

program was developed by Jimmy R. Williams and Roy W. Hann, Jr. in the early 1970’s for the

USDA Agricultural Research Service in cooperation with the Texas Agricultural Experiment

Station, Texas A&M University. During the 1980’s, HYMO was reformulated, enhanced and

renamed to AHYMO by Cliff Anderson to simulate rainfall-runoff processes characteristic of the

Albuquerque area. The current version of the program was issued in 1997.

AHYMO is a MS-DOS based, batch program that reads an ascii text file (formatted similarly to

HEC-1, TR-20, etc.) of data and / or input parameters. Basic input to the program is:

• Drainage Area

• Computational time increment

• Time distribution of rainfall

• Rainfall loss parameters

• Unit hydrograph parameters

• Hydrologic routing parameters

• Sediment bulking and transport parameters

Input parameters required by AHYMO are either calculated external to the program according to

the procedures in the DPM or calculated internally to the program given the proper data. Model

computational order / logic is controlled by the assignment of identification numbers. Proper

use and understanding of identification numbers is crucial to successful implementation of

AHYMO. Use of specific routines is invoked by a command, such as “COMPUTE NM HYD”.

There are 31 commands in AHYMO. Several commands have optional or alternative elements

that are invoked by the input of a numeric toggle. The toggles can be the negative of the actual

input value or a specific number like -999.


MANUAL Introduction

October 24, 2008


Program output is runoff hydrographs and summaries of runoff magnitudes. The amount of

output generated by the program is controlled by the user. Copious output can be generated

allowing the user to thoroughly check the program computations.

The array of optional forms of input and commands coupled with locally derived parameters

results in AHYMO being an extremely flexible and capable program for estimating runoff

magnitudes. However, this flexibility increases the complexity of model use and when

combined with the somewhat cryptic nature of the text based input format and importance of

properly assigned identification numbers results in opportunities for user error.

SSCAFCA, through the issuance of a DPM, will offer engineers and hydrologists the opportunity

to use an equivalent model to AHYMO for estimating runoff magnitudes. It is desired that the

equivalent model be a “main-stream” public domain model. In addition, the model should be

easy to use and yield equivalent results as AHYMO.

The purpose of this document is to present the process by which an equivalent model to

AHYMO was selected. More importantly, this document presents the approach, analyses and

test results used to evaluate and select methodologies and input parameters that, when applied

properly in the alternate model, will yield similar results to AHYMO


PROCESS MANUAL


2.0 Existing DPM

Successful transition from the use of AHYMO to another software package requires the

understanding of the methodologies presented in the DPM and the implementation of those

methodologies in AHYMO. In general, the methodologies set forth in the DPM are organized

into five elements: rainfall, rainfall losses, unit hydrograph, hydrologic routing and

sedimentation. A brief discussion of each element is presented in the following sections.

2.1 RAINFALL

Design rainfall criteria required for estimating runoff from the 100-year event consists of point

precipitation depth, depth-area reduction and temporal distribution. Also provided in the DPM

are criteria for estimating the Probable Maximum Precipitation (PMP). Criteria for estimating the

PMP are not considered at this time.

Point rainfall depth for the 100-year event is determined from the NOAA Atlas 2. The NOAA

Atlas 2 presents depth-duration-frequency statistics as a series of isopluvial maps. Isopluvial

maps specific to Bernalillo County were taken or created from the maps and data in the NOAA

Atlas 2. Isopluvial maps specific to Bernalillo County are provided in the Hydrology Manual as

Figures C-1 through C-3. Isopluvial maps specific to Sandoval County are not included in the

Hydrology Manual. In the future, rainfall depths will be derived from the NOAA Atlas 14. Work

on the new rainfall depth criteria for the DPM is being done by others.

The rainfall depths from the isopluvial maps in Figures C-1 through C-3 (and eventually

corresponding figures based on the NOAA Atlas 14) are 100-year point rainfall depths for

specified durations. This depth is not the areally-averaged rainfall over the basin that would

occur during a storm. A reduction factor is used to convert the point rainfall to an equivalent

uniform depth over the entire watershed. As the watershed area increases, the reduction factor

decreases. Reduction factors recommended in the DPM are taken from the NOAA Atlas 2 and

depicted graphically in Figure C-4. That figure is reproduced here as Figure 1. Reduction

factors are applied to point rainfall depths for watersheds greater than 5 square miles in size.

For large, complex watersheds that are controlled by several dams and partial diversions, the

method of successive subtraction (USBR, 1989) may be used.

Temporal distribution of the areally-averaged rainfall depth is accomplished using a suite of

equations (Equations C-1 through C-6 of the DPM) that are a function of the 1-, 6- and 24-hour

depths. Those equations are reproduced here as Equations 1 through 6. The distribution is

front loaded with the peak intensity set at 85.3 minutes (1.42 hours). Approximately 88 percent

of the total depth occurs in first two hours of the storm. Equations 1 through 5 apply to the 6-

hour storm. To illustrate the pattern, the 6-hour distribution for a specific location in Bernalillo

County is shown in Figure 2. For the 24-hour storm, Equation 6 is used in addition to Equations

1 through 5. A comparison of the 6- and 24-hour rainfall mass diagrams is shown in Figure 3.


MANUAL Existing DPM

October 24, 2008


There are two options for inputting rainfall data to AHYMO. The first option requires the

specification of the desired rainfall distribution (Rainfall “TYPE”), along with the appropriate

areally-averaged x-hour rainfall depths. For this option, incremental rainfall is calculated internal

to AHYMO. The second option is the direct input of a mass rainfall table. For this option,

cumulative rainfall for the desired storm event is input by the user.

For 0 ≤ t ≤ 60 Eqn-1

For 60 < t < 67 Eqn-2

For 67 ≤ t < 85.3 Eqn-3

For 85.3 ≤ t < 120 Eqn-4

For 120 ≤ t ≤ 360 Eqn-5

For 360 < t < 1440 Eqn-6

Where:

( )

−−−=

A

A

T

tPPP

605.15.1**334.2 60360

−−+= =

9.0

9.0

606060

5.15.0*4754.0*t

PPP TT

( ) ( )( )2.3

6060 60*000018338.060*0001818182.0* −+−+= = ttPPP TT

( ) ( )( )0985865.1

6060 85*0404768.01886.160*07.0* −−−−+= = ttPPP TT

( )( )

AA

AA

TT

t

PPPPP33

33

36060603604.04.4

6.160

4.4

−

−−−++= =

( )( )

BB

BB

T

t

PPPP1230

660

30*14403601440

−

+−−+=

( )0.6

60

360

Log

PP

Log

A

=

( )0.4

360

1440

Log

PP

Log

B

=


MANUAL Existing DPM

October 24, 2008


Figure 1 – Depth-area Reduction

88

90

92

94

96

98

100

0 20 40 60 80 100

Area, in sq. miles

Perc

en

t o

f P

oin

t P

rec

ipit

ati

on

6-Hour

24-Hour

Figure 2 – Time Distribution of Rainfall

0.00

0.05

0.10

0.15

0.20

0.25

0 1 2 3 4 5

Time, in hours

Incre

men

tal R

ain

fall,

in in

ch

es

P1-HR = 1.95 inches

P6-HR = 2.25 inches

6

Adapted from NOAA Atlas 2


MANUAL Existing DPM

October 24, 2008


Figure 3 – Comparison of 6- and 24-Hour Mass Curves

0

1

2

3

0 6 12 18 24

Time, in hours

Cu

mu

lati

ve R

ain

fall

, in

in

ch

es

6-hour

24-hour

P1-HR = 1.95 inches

P6-HR = 2.25 inches

P24-HR = 2.68

inches

2.2 RAINFALL LOSS

Rainfall losses are estimated separately for pervious and impervious portions of the watershed.

Rainfall loss magnitudes are estimated for both land cover conditions using the combination of

an initial abstraction and an infiltration rate. For pervious land cover conditions, the infiltration

rate is constant. For impervious land cover conditions, the infiltration rate varies. An illustration

of the application of the rainfall loss method is provided as Figure A-2 in the Hydrology Manual

and that figure is recreated in this document as Figure 4. For watersheds with impervious

areas, AHYMO automatically varies the Infiltration Rate (INF) by the following:

• INF = 0.04 inches per hour from t = 0 to t = 180 minutes.

• From t = 180 to t = 360 minutes INF is reduced from 0.04 to 0.0 inches per hour. The

decay rate is linear.

• For t > 360 minutes INF = 0.

Recommended values for the Initial Abstraction (IA) and Infiltration (INF) are related to land

treatment classifications. The DPM identifies four land treatment classifications. Three of those

classifications (A, B and C) are for pervious conditions and one classification (D) is for

impervious conditions. Values of IA and INF are provided for each treatment classification in

Tables A-6 and A-7 of the DPM and those tables are reproduced in this document as Table 1.


MANUAL Existing DPM

October 24, 2008


From a footnote in Table A-7, inference is made to infiltration losses being accounted for at the

start of the storm concurrent with initial abstractions. Inspection of Figure A-2 (Figure 4 herein)

suggests that infiltration does not start until initial abstractions are satisfied. That footnote to the

infiltration rate is in reference to Land Treatment Type D, only.

For watersheds with multiple pervious treatment classifications, areally-averaged values for IA

and INF are to be used. There are two options for this in AHYMO. First, values of IA and INF

can be hard coded by the user. Second, weighted values of IA and INF can be computed by

AHYMO based on the input of the area associated with each treatment classification.

Regardless of the selected input option, the weighted values are a simple arithmetic average.

Figure 4 – Representation of Rainfall Loss Methodology

Table 1 – Rainfall Loss Parameters

Treatment Initial Abstraction (IA) Infiltration (INF)

Type inches inches/hour

A 0.65 1.67

B 0.50 1.25

C 0.35 0.83

D 0.10 0.04*

* - Treatment D infiltration rate is applicable from 0 to 3 hours; use uniform reductions from 3 to 6 hours, with no infiltration after 6 hours


MANUAL Existing DPM

October 24, 2008


2.3 UNIT HYDROGRAPH

Rainfall excess is transformed into a runoff hydrograph through the application of a

dimensionless unit hydrograph. There are several unit hydrograph options available in AHYMO.

For SSCAFCA, the unit hydrograph to be used is invoked by the Compute NM HYD command.

The shape of the NM HYD unit hydrograph is defined in three segments as shown in Figure 5.

Documentation on the unit hydrograph is provided in Appendix A. The unit hydrograph

ordinates are calculated internal to AHYMO as a function of two parameters: recession constant

(k) and time to peak (tp).

Time to peak is calculated as 67 percent of the basin Time of Concentration tc and cannot be

less than 8 minutes. Three equations (B-1, B-6 and B-7) are provided in the DPM for the

estimation of tc. Those equations are reproduced here as Equations 7 through 9. Selection of

the appropriate equation is a function of the longest flow path length. A single tc is estimated for

a watershed and applied to both pervious and impervious portions.

For flow path lengths (L) less than 4,000 feet:

Eqn-7

For flow path lengths (L) between 4,000 and 12,000 feet:

Eqn-8

For flow path lengths (L) greater than 12,000 feet:

Eqn-9

Where: L = Flow path length, in feet

K = Conveyance factor from, Table 2

S = Slope of flow path, in feet per foot

Lca = Distance along L from point of concentration to a point opposite

centroid of drainage basin, in feet

Kn = Basin factor from Table 3

( )∑=

=n

i i

i

cSK

Lt

1 *10

( )

165.0

33.0

*2.552

*000,4

**000,72

000,12

S

LL

L

SK

Lt

ca

c

−

+−

=

( ) 33.02 *280,5*280,5

*26*

3

4

S

LLKt ca

nc

=


MANUAL Existing DPM

October 24, 2008


Table 2 – Conveyance Factor

K Conveyance Condition

0.7 Turf, landscaped areas and undisturbed natural areas (sheet flow only*)

1 Bare or disturbed soil areas and paved areas (sheet flow only*)

2 Shallow concentrated flow (paved or unpaved)

3 Street flow, storm sewers and natural channels, and that portion of subbasins (without constructed channels) below the upper 2,000 feet for subbasins longer than 2,000 feet.

4 Constructed channels (for example: riprap, soil cement or concrete lined)

* Sheet flow is flow over plane surfaces, with flow depths up to 0.1 feet. Sheet flow only applies to the upper 400 feet (maximum) of a subbasin.

Table 3 – Basin Factor

Kn Basin Condition

0.042 Mountain Brush and Juniper

0.033 Desert Terrain (Desert Brush)

0.025 Low Density Urban (minimum improvements to watershed channels)

0.021 Medium Density Urban (flow in streets, storm sewers and improved channels)

0.016 High Density Urban (concrete and riprap lined channels)

The recession constant, k, is a function of drainage area, rainfall depth and land cover

treatment. A value of k is calculated for each land cover treatment present in the watershed.

Three sets of equations (Equations C-13 through C-26) are provided for the estimation of k.

Selection of the appropriate set is based on basin area. If multiple pervious land cover

treatments are present, an arithmetically area-weighted value is calculated.

In AHYMO, both tp (0.67 tc) and k can be calculated internally by the program, but only tp can be

input by the user. To invoke the internal calculation of tp, the user must input the proper data on

the Compute LT TP command.

For subbasins with both pervious and impervious areas, a composite runoff hydrograph is

calculated. Output for each subbasin includes the parameters for estimating the unit

hydrograph ordinates, unit peak discharge, unit volume, peak runoff discharge and runoff

volume. If desired by the user, the runoff hydrograph ordinates can be listed and plotted.


MANUAL Existing DPM

October 24, 2008


Figure 5 – AHYMO Dimensionless Unit Hydrograph

2.4 HYDROLOGIC ROUTING

There are two types of hydrologic routing supported by AHYMO; channel routing and reservoir

routing. Channel routing describes the movement of a flood wave (hydrograph) down a

watercourse. For most natural rivers, as a flood wave passes through a given reach, the peak

of the outflow hydrograph is usually attenuated and delayed due to flow resistance in the

channel and the storage capacity of the river reach. Reservoir routing is used to simulate the

attenuation of peak discharge due to storage in a detention basin or behind a dam.

2.4.1 Channel Routing

Channel routing, though not specified in the DPM, is accomplished using the Muskingum-Cunge

method (though another method is available but seldom used). This method requires the

specification of two commands; COMPUTE RATING CURVE and ROUTE MCUNGE. The

COMPUTE RATING CURVE command is used to input the physical characteristics of the

channel reach (slope, roughness and geometry). From that data, a rating curve describing the

stage-discharge relation for the reach is computed. The ROUTE MCUNGE command is used to

set the numerical stability parameters require for solution of the Muskingum-Cunge routing

method. The numerical stability parameters are key to the success of the routing calculations.


MANUAL Existing DPM

October 24, 2008


The governing equation used in the routing calculations is the continuity equation and the

diffusive form of the momentum equation. That equation is provided as Eqn-10. The

Muskingum-Cunge approximation of that equation is given by Eqn-11.

Eqn-10

Where: Q = discharge, in cfs

t = time, in seconds

ck = kinematic wave velocity, in fps

µ = hydraulic diffusivity, in ft2/s

Ot = C1It-1 + C2It + C3Ot-1 Eqn-11

Where: Ot = routed discharge at time t, in cfs

It= inflow discharge at time t, in cfs

C1, 2 and 3= Routing coefficients, dimensionless

The routing coefficients (C1,2 and 3) are a function of ck and µ discretized over time (∆t) and space

(∆x) and these parameters control the numerical stability of the solution. Numerical stability

issues are a byproduct of approximations of the partial differential equations and are not a

deficiency in the AHYMO program. For the Muskingum-Cunge approximation, numerical

instability often results in oscillations in the routed hydrograph or in surging (higher peak or

runoff volume in the routed hydrograph than the inflow hydrograph).

In AHYMO different approaches for establishing the routing coefficients are used to address

numerical instability. Use of one solution scheme over the other is determined through a set of

error checks that test for oscillations and surging. The first solution scheme uses Ponce’s

recommended constraints on the routing coefficients. If the error checks fail on this solution

scheme, the second solution scheme is employed. The second solution scheme uses Fread’s

recommended constraints on the routing coefficients. In general, the second solution scheme

results in a numerically stable solution by artificially slowing the floodwave velocity. However,

this results in unrealistically long floodwave travel times. This is illustrated through a simple

example taken from an existing AHYMO model in the SSCAFCA jurisdictional area.

Physical Routing Data (COMPUTE RATING CURVE Command)

• Channel Slope: 0.0176 ft/ft

• Manning’s n-value: 0.035

• Bottom width: 50 feet

• Geometry: rectangular channel

• Depth: 5 feet

2

2

x

Q

x

Qc

t

Qk

δ

δµ

δ

δ

δ

δ=+


MANUAL Existing DPM

October 24, 2008


From the physical parameters, a hydraulic rating table is generated (see Table 4).

Table 4 – Routing Example: Hydraulic Rating

Water

Surface Flow Flow Top Average Elevation Area Rate Width Velocity

feet sq. feet cfs feet fps (1) (2) (3) (4) (5)

0.00 0.00 0.00 50.00 0.00

0.26 13.16 30.33 50.00 2.30

0.53 23.32 95.95 50.00 4.11

0.79 39.47 187.95 50.00 4.76

1.05 52.63 305.53 50.00 5.81

1.32 65.79 437.32 50.00 6.65

1.58 78.95 590.59 50.00 7.48

1.84 92.11 761.02 50.00 8.26

2.11 105.26 947.50 50.00 9.00

2.37 118.42 1,149.15 50.00 9.70

2.63 131.58 1,365.17 50.00 10.38

2.89 144.74 1,594.89 50.00 11.02

3.16 157.89 1,837.71 50.00 11.64

3.42 171.05 2,093.07 50.00 12.24

3.68 184.21 2,360.49 50.00 12.81

3.95 197.37 2,639.52 50.00 13.37

4.21 210.53 2,929.75 50.00 13.92

4.47 223.68 3,230.80 50.00 14.44

4.74 236.84 3,542.32 50.00 14.96

5.00 250.00 3,863.97 50.00 15.46

In Table 4, Columns 1 – 4 are taken directly from the AHYMO output. Column 5 is

calculated externally and shown only for comparison purposes with other data presented

later in this example.

Routing Data (ROUTE MCUNGE)

• Length: 3,300 feet

• ∆t: 3 minutes


MANUAL Existing DPM

October 24, 2008


From the routing data and the hydraulic rating, AHYMO then calculates the routing

coefficients at the same depth increments as the hydraulic rating and applies the Ponce

corrections. Those calculations are provided in Table 5.

Using the Ponce routing coefficients results in a failure of the numerical stability checks

AHYMO uses and therefore, the routing coefficients are recalculated using the Fread

corrections. Those results are provided in Table 6. The key difference in the two data

sets is the predicted floodwave velocity, ck. Using the Ponce routing coefficients, the

floodwave velocities are always between 1.33 and 1.67 times greater than the average

flow velocity. Using the Fread routing coefficients, the floodwave velocities are always

less than the average flow velocities. Floodwave velocities should always be greater

than average flow velocities and according to accepted literature, that factor should be

between 1.33 and 1.67 for prismatic channels.

For this example, the inflow peak discharge of 230 cfs at 1.6 hours is translated to the

bottom of the routing reach with a peak discharge of 220 cfs at 1.85 hours. This implies

that it takes the floodwave 15 minutes to travel 3,300 feet at 3.7 fps. Based on the

normal depth hydraulic calculations provided in Table 4, the average channel velocity for

a flow rate of 220 cfs is between 4.8 and 5.8 fps.

In the example, it is important to note that the numerical instabilities that result from use of the

Ponce routing coefficients is not an error in the program. It is simply the byproduct of user input

and / or decisions, in particular the selection of the routing time step ∆t. The ∆t used in this

example is 3 minutes which is the same as the model computation time interval. A routing time

step that is the same as the model computation time interval is the default condition. If a

smaller ∆t is used, say 45 seconds, only the Ponce routing coefficients are generated (i.e.

numerical stability is achieved) and the routed peak discharge is 223 cfs at 1.7 hours. The 6

minute travel time that results with the shorter routing time step corresponds to a velocity of 9

fps.

It is also important to note that the use of a ∆t equal to the computational time interval (3

minutes in the previous example) does not constitute an error in user judgment or even an error

in application. It does however illustrate an important limitation with AHYMO. AHYMO is hard

coded with a limited number (600) of hydrograph ordinates. This limitation was typical of similar

programs in the time period AHYMO was developed and is due to computer memory issues, not

programming inadequacies. The cost of this limitation is that running the routing calculation at a

smaller time step can result in an incomplete runoff hydrograph (total runoff volume would not

be accounted for). This is only an issue if determination of runoff volume is the primary

objective of the model. If determination of runoff peak discharge is the primary objective, using

smaller routing time steps may not present an issue.


MANUAL Existing DPM

October 24, 2008


Table 5 – Routing Example: Ponce Routing Coefficients

Ta

ble

5 -

Po

nce

Ro

uti

ng

Co

eff

icie

nts

Flo

wF

low

Flo

wT

rav

el

To

pA

vera

ge

Dep

thA

rea

Ra

teT

ime

Wid

thck

Ve

locit

yC

DC

1C

2C

3

fee

tsq

. fe

et

cfs

ho

urs

feet

fps

0.0

00.0

00

0.5

45

03.0

61

.07

1.0

00.0

01

.00

0.0

00

.00

0.2

613

.20

30

0.4

05

03.8

32

.30

1.2

50.0

20

.99

0.1

2-0

.10

0.5

326

.30

96

0.2

55

06.0

53

.65

1.9

80.0

30

.98

0.3

4-0

.32

0.7

939

.50

18

80.1

95

07.8

94

.76

2.5

80.0

50

.97

0.4

5-0

.42

1.0

552

.60

30

30.1

65

09.5

05

.75

3.1

10.0

70

.97

0.5

2-0

.49

1.3

265

.80

43

70.1

45

010

.97

6.6

53

.59

0.0

80

.97

0.5

7-0

.54

1.5

878

.90

59

10.1

25

012

.32

7.4

84

.03

0.1

00

.96

0.6

1-0

.57

1.8

492

.10

76

10.1

15

013

.58

8.2

64

.44

0.1

20

.96

0.6

4-0

.60

2.1

11

05

.30

94

80.1

05

014

.76

9.0

04

.83

0.1

30

.96

0.6

7-0

.62

2.3

711

8.4

01

,14

90.0

95

015

.88

9.7

05

.20

0.1

50

.95

0.6

9-0

.64

2.6

31

31

.60

1,3

65

0.0

95

016

.95

10

.38

5.5

50.1

70

.95

0.7

0-0

.65

2.8

91

44

.70

1,5

95

0.0

85

017

.96

11.0

25

.88

0.1

80

.95

0.7

2-0

.67

3.1

61

57

.90

1,8

38

0.0

85

018

.94

11.6

46

.20

0.2

00

.95

0.7

3-0

.68

3.4

21

71

.10

2,0

93

0.0

85

019

.87

12

.24

6.5

00.2

20

.94

0.7

4-0

.69

3.6

81

84

.20

2,3

61

0.0

75

020

.77

12

.81

6.8

00.2

40

.94

0.7

5-0

.69

3.9

51

97

.40

2,6

40

0.0

75

021

.64

13

.37

7.0

80.2

50

.94

0.7

6-0

.70

4.2

12

10

.50

2,9

30

0.0

75

022

.47

13

.92

7.3

60.2

70

.94

0.7

7-0

.71

4.4

72

23

.70

3,2

31

0.0

65

023

.28

14

.44

7.6

20.2

90

.94

0.7

8-0

.71

4.7

42

36

.80

3,5

42

0.0

65

024

.06

14

.96

7.8

80.3

00

.93

0.7

8-0

.72

5.0

02

50

.00

3,8

64

0.0

65

024

.83

15

.46

8.1

30.3

20

.93

0.7

9-0

.72


MANUAL Existing DPM

October 24, 2008


Table 6 – Routing Example: Fread Routing Coefficients

Ta

ble

6 -

Fre

ad

Ro

uti

ng

Co

eff

icie

nts

Flo

wF

low

Flo

wT

rav

el

To

pA

vera

ge

Dep

thA

rea

Ra

teT

ime

Wid

thck

Ve

locit

yC

DC

1C

2C

3

fee

tsq

. fe

et

cfs

ho

urs

feet

fps

0.0

00.0

00

0.5

45

03.0

61

.07

1.0

00.0

01

.00

0.0

00

.00

0.2

613

.20

30

0.4

05

03.1

22

.30

1.0

20.0

20

.98

0.0

20

.00

0.5

326

.30

96

0.2

55

03.2

43

.65

1.0

60.0

60

.94

0.0

60

.00

0.7

939

.50

18

80.1

95

03.4

04

.76

1.1

10.1

10

.90

0.1

00

.00

1.0

552

.60

30

30.1

65

03.5

95

.75

1.1

70.1

70

.85

0.1

50

.00

1.3

265

.80

43

70.1

45

03.7

86

.65

1.2

40.2

40

.81

0.1

90

.00

1.5

878

.90

59

10.1

25

03.9

97

.48

1.3

10.3

10

.77

0.2

30

.00

1.8

492

.10

76

10.1

15

04.2

08

.26

1.3

70.3

70

.73

0.2

70

.00

2.1

11

05

.30

94

80.1

05

04.4

19

.00

1.4

40.4

40

.69

0.3

10

.00

2.3

711

8.4

01

,14

90.0

95

04.6

29

.70

1.5

10.5

10

.66

0.3

40

.00

2.6

31

31

.60

1,3

65

0.0

95

04.8

410

.38

1.5

80.5

80

.63

0.3

70

.00

2.8

91

44

.70

1,5

95

0.0

85

05.0

511

.02

1.6

50.6

50

.61

0.4

00

.00

3.1

61

57

.90

1,8

38

0.0

85

05.2

611

.64

1.7

20.7

20

.58

0.4

20

.00

3.4

21

71

.10

2,0

93

0.0

85

05.4

712

.24

1.7

90.7

90

.56

0.4

40

.00

3.6

81

84

.20

2,3

61

0.0

75

05.6

812

.81

1.8

60.8

60

.54

0.4

60

.00

3.9

51

97

.40

2,6

40

0.0

75

05.8

913

.37

1.9

30.9

30

.52

0.4

80

.00

4.2

12

10

.50

2,9

30

0.0

75

06.0

913

.92

1.9

90.9

90

.50

0.5

00

.00

4.4

72

23

.70

3,2

31

0.0

65

06.3

014

.44

2.0

61.0

60

.49

0.5

20

.00

4.7

42

36

.80

3,5

42

0.0

65

06.5

014

.96

2.1

31.1

30

.47

0.5

30

.00

5.0

02

50

.00

3,8

64

0.0

65

06.7

015

.46

2.1

91.1

90

.46

0.5

40

.00


MANUAL Existing DPM

October 24, 2008


2.4.2 Reservoir Routing

Reservoir routing is accomplished using the storage-indication method (inflow – outflow =

change in storage). Reservoir routing is invoked with the ROUTE RESERVOIR command.

Input required for reservoir routing is the stage-storage-discharge relation for the detention

facility. This is a standard routing procedure employed my numerous similar programs.

2.5 FLOW DIVERSIONS

There are several different ways in which flow at a model location can be bifurcated. Flow

diversions are invoked in AHYMO using the DIVIDE HYD command. Runoff hydrographs can

be split on a percentage basis, by a maximum flow rate or based on a rating curve.

2.6 SEDIMENT BULKING / SEDIMENT TRANSPORT

Perhaps one of the most unique aspects of AHYMO is the capability to consider sediment

bulking and sediment transport. There are three commands in AHYMO that are used to

simulate sediment bulking and sediment transport: SEDIMENT BULK, SED WASH LOAD and

SEDIMENT TRANS.

Sediment bulking is used to “bulk” or increase the ordinates of a runoff hydrograph by a user

specified amount to simulate the effect of sediment. Bulking factors can be input at any location

in the model and can be a constant value or varied by discharge.

The SED WASH LOAD command is used to estimate the sediment yield from a basin that can

be transported as wash load in a river reach. Sediment yield is calculated using the Modified

Universal Soil Loss Equation (MUSLE).

The potential sediment bed load transport capacity of a river reach can be estimated using the

SEDIMENT TRANS command. Transport capacity is estimated using a power function relating

channel hydraulic properties with wash load concentrations. Wash load concentrations can be

constant, varied by discharge or calculated using the SED WASH LOAD command.


PROCESS MANUAL


3.0 Alternative Watershed Model

There are a number of different mathematical models that are available and capable of

simulating rainfall-runoff processes. In general, the choice of a hydrologic model should be

based on an understanding of the watershed conditions to be modeled, model limitations and

computational procedures, desired output, ease of use and agency acceptance.

In this case, the goal of the model selection is that the model be an alternative to AHYMO, not

necessarily a replacement. As such, the alternative model must also be capable of yielding

similar results as are currently estimated using AHYMO. As discussed in Section 2, AHYMO is

based on several unique methodologies and / or procedures. Thus potential companion models

must either allow substantial user control of the employed methodologies or employ a wide

variety of methods from which a compatible method can be selected.

Selection of an alternative watershed model involved the compilation of a candidate model list,

development of an evaluation matrix and evaluation of the recommended model. Each of these

steps is discussed in the following sections.

3.1 FEMA ACCEPTED MODELS

The easy starting point for a list of candidate models is from FEMA’s Nationally Accepted

Models. FEMA acceptance is an important criterion. AHYMO is currently on FEMA’s Locally

Accepted Model list. FEMA acceptance of AHYMO was sought during development. The

FEMA review process required several changes to methods employed in AHYMO. Selecting a

pre-approved model will avoid this potential complication and will reduce potential issues for

future CLOMR / LOMR submittals. FEMA’s Nationally Accepted Models currently includes the

following:

• HEC-1 version 4.0.1 and up: a public domain model developed by Corps of Engineers

Hydrologic Engineering Center. HEC-1 is a lumped parameter, single storm event

model that simulates surface runoff response of a watershed to precipitation by

representing the basin as an interconnected system of hydrologic and hydraulic

components (stream channels or reservoirs). Modeling results in hydrographs at points

of interest. A variety of methodologies are available to input and model rainfall, losses,

runoff transformation and translation and diversion. The Corps no longer supports

HEC-1. The last version, 4.1 was released in 1998.

• HEC-HMS version 1.1 and up: HEC-HMS is a lumped parameter, single event model

and the successor to HEC-1. Not all of the original HEC-1 functions are available in

HEC-HMS, however there is additional functionality than that available in HEC-1. Many

of the original HEC-1 algorithms are updated and combined with new algorithms.

HEC-HMS is a windows based program and in the public domain.


MANUAL Alternative Watershed Model

October 24, 2008


• TR-20 Win version 1.00: a lumped parameter, single event model that is the successor

to TR-20 and in the public domain. Several aspects of the computational procedure for

estimating rainfall excess are revised to address some of the procedural and theoretical

concerns associated with the SCS CN methodology. It should be noted that these

changes in the methodology are not incorporated into HEC-HMS and perhaps other

programs that include the NRCS Curve Number methodology.

• WinTR-55 version 1.0.08: a lumped parameter, single event model that is the successor

to TR-55 and in the public domain. The model is intended for use simulating the rainfall-

runoff process on small urban watersheds. WINTR-20 is the driving engine for

hydrograph and routing procedures.

• SWMM 5 version 5.0.005: the Storm Water Management Model (SWMM) was originally

developed for EPA as a single-event or long term (continuous) simulation model for the

analysis of combined sewer overflows. SWMM is a public domain model. The model is

primarily intended to be applied to urban watersheds. SWMM is a physically based,

discrete-time model, which can simulate stormwater quantity and quality. SWMM can

utilize a variety of loss and runoff translation methods, applicable to SSCAFCA. SWMM

can account for evaporation of standing surface water, snow accumulation and melting,

percolation and storage.

• MIKE 11 UHM: a proprietary windows-based software package developed by Danish

Hydraulic Institute (DHI) for the simulation of flow, water quality and sediment transport

in rivers, channels and reservoirs. MIKE 11 consists of a core module (HD) and

numerous add-on modules. The rainfall-runoff module (RR) contains a number of

methods, which can be utilized to estimate runoff. MIKE 11 is able to model a complex

watershed network, including unique parameters such as snow storage. The price of an

unlimited structure and point HD and RR is approximately $18,000, plus an annual

software maintenance agreement is required.

• PondPack v.8 and up: a program for analyzing watershed networks and aiding in sizing

detention or retention ponds. Only the NRCS Unit Hydrograph method and NRCS time

of time of concentration formulas approved by State agencies in charge of flood control

or floodplain management are acceptable for use within the subject State. Pond Pack

can handle an unlimited number of synthetic or real storm events of any duration or

distribution.

• XP-SWMM version 8.52 and up: a proprietary version of SWMM 5.

• Xpstorm version 10.0: provides the same functionality as XP-SWMM

• HSPF version 10.10 and up: a public domain model that simulates for extended periods

of time the hydrologic, and associated water quality, processes on pervious and

impervious land surfaces and in streams and well-mixed impoundments. HSPF uses

continuous rainfall and other meteorological records to compute streamflow hydrographs

and pollutographs. HSPF simulates interception soil moisture, surface runoff, interflow,

base flow, snowpack depth and water content, snowmelt, evapotranspiration, ground-



October 24, 2008


water recharge, dissolved oxygen, biochemical oxygen demand (BOD), temperature,

pesticides, conservatives, fecal coliforms, sediment detachment and transport, sediment

routing by particle size, channel routing, reservoir routing, constituent routing, pH,

ammonia, nitrite-nitrate, organic nitrogen, orthophosphate, organic phosphorus,

phytoplankton, and zooplankton. The program can simulate one or many pervious or

impervious unit areas discharging to one or many river reaches or reservoirs.

Frequency-duration analysis can be done for any time series. Any time step from 1

minute to 1 day that divides equally into 1 day can be used. Any period from a few

minutes to hundreds of years may be simulated. HSPF is generally used to assess the

effects of land-use change, reservoir operations, point or non-point source treatment

alternatives, flow diversions, etc. Programs, available separately, support data

preprocessing and post processing for statistical and graphical analysis of data saved to

the Watershed Data Management (WDM) file. This is a continuous event model.

Calibration to actual flood events is required.

• Mike 11 RR: a lumped-parameter hydrologic model capable of continuously accounting

for water storage in surface and sub-surface zones that is an add-on to Mike 11 UHM.

• PRMS version 2.1: a deterministic, distributed-parameter modeling system developed to

evaluate the impacts of various combinations of precipitation, climate, and land use on

streamflow, sediment yields, and general basin hydrology. Basin response to normal

and extreme rainfall and snowmelt can be simulated to evaluate changes in water-

balance relationships, flow regimes, flood peaks and volumes, soil-water relationships,

sediment yields, and ground-water recharge. Parameter-optimization and sensitivity

analysis capabilities are provided to fit selected model parameters and evaluate their

individual and joint effects on model output. The modular design provides a flexible

framework for continued model-system enhancement and hydrologic-modeling research

and development. PRMS is a public domain model.

3.2 MODEL EVALUATION

Candidate models were qualitatively evaluated and compared using two basin criteria;

community acceptance and results compatibility with AHYMO. Definitions for these criteria are:

Community Acceptance: this criterion reflects how well a model is documented and

whether the input structure is based on a DOS platform with a command driven structure

(similar to AHYMO) or a Window based platform with interactive menu driven user

interface. This ease of use element directly impacts how the community (engineers and

hydrologists) will accept the alternative model. The alternative model could be the most

physically representative model for simulating rainfall-runoff processes that exactly

matches AHYMO results, but if it is difficult to use, is not well documented and / or is

costly to purchase than the likelihood of acceptance is diminished.



October 24, 2008


Results Compatibility with AHYMO: it is anticipated that an alternative model capable of

yielding similar results as AHYMO will employ a number of different methodologies for

simulating rainfall, rainfall loss, runoff transformation and runoff translation. The more

methodologies encoded in the software, the greater the chances that the model will be

able to yield similar results as AHYMO.

Qualitative evaluations of each model against the criteria are organized into a matrix. That

matrix is provided as Table 7.

.

3.3 MODEL RECOMMENDATION

Given the specific nature of the SSCAFCA modeling methodologies and based on the review of

available models and their relative merits, it is recommended that HEC-HMS be used for

rainfall-runoff modeling by SSCAFCA. This model is recommended because it incorporates a

wide range of methodologies that may be equivalent to the SSCAFCA methodologies, or at a

minimum can accommodate the SSCAFCA methodologies through generic user input options

(such as a user defined rainfall hyetograph). In addition, HEC-HMS is a public domain product

(maintained and supported by the USACE), it is currently in wide use already in the southwest

and nationwide. Finally, this model is accepted for use in support of floodplain delineation

studies by the Federal Emergency Management Agency (FEMA).



October 24, 2008


Table 7 – Rainfall-Runoff Model Evaluation Matrix


PROCESS MANUAL


4.0 Equivalent Parameters

Recommendations presented herein reflect the position that future rainfall-runoff modeling must

reproduce, to a reasonable extent, runoff magnitudes currently obtained using the AHYMO

computer program. The benchmark for achieving this goal is that runoff magnitudes from HEC-

HMS should be within ± 5 to 10 percent of the AHYMO runoff magnitudes. Secondary to this

goal is the use of existing data, procedures and parameters where possible. Use of existing

data, procedures and parameters is desirable for two reasons. First, the difficulties of learning a

new tool (HEC-HMS) for rainfall-runoff modeling will not be added to by also learning new

parameter development procedures. Second, FEMA approval should be more readily

obtainable.

Identification of recommended methodologies available in HEC-HMS and determination of

required input parameters relied heavily on comparisons to existing AHYMO results from the

Montoyas Arroyo and Black Arroyo watershed models. AHYMO input and output files for the

Montoyas and Black Arroyo watershed models are provided digitally on CD as Appendix E.

Throughout the following sections, specific subbasins and channel routing reaches from the

Montoyas and Black Arroyo watershed models were selected for comparison with results from

proposed parameters to be used with HEC-HMS. The selection of specific subbasins and

channel routing reaches were intended to provide a reasonable sampling of the range of

hydrologic conditions present in those watersheds and ideally a reasonable representation of

the range of hydrologic conditions within the region.

4.1 RAINFALL

As stated in Section 2.1, rainfall criteria required for rainfall-runoff modeling consists of three

elements; point rainfall depth, depth-area reduction and temporal distribution. Of these,

temporal distribution of the rainfall depth is the key component when considering the use of a

companion model to AHYMO. HEC-HMS has several predefined rainfall distributions and also

allows for the input of a user-specified distribution. Predefined rainfall distributions available in

the current version of HEC-HMS (version 3.2) are:

Hypothetical: Centrally nested pattern for durations between 5 minutes and 10 days

constructed from depth-duration data that can be applied to multiple return periods. Use

of depth-duration data from the study watershed makes this method site specific.

SCS Type I, Ia, II and III: Developed from the National Weather Service depth-duration-

frequency statistics (NOAA Atlas 2 and TP-40) for durations up to 24 hours and

frequencies from 1 to 100 years. The Type II distribution is representative of areas in

which high rates of runoff from small areas are typically generated during summer

thunderstorms.


MANUAL Equivalent Parameters

October 24, 2008


Mass rainfall curves for the 24-hour Hypothetical and SCS Type II rainfall distributions are

shown in Figure 6 along with the AHYMO 24-hour distribution.

Figure 6 – Mass Rainfall Curve Comparison

0

20

40

60

80

100

0 4 8 12 16 20 24

Time, in hours

Cu

mu

lati

ve r

ain

fall

, in

perc

en

t o

f to

tal

Hypothetical

SCS Type II

AHYMO

Selection of the temporal distribution is an important element in regard to the generation of

runoff even if the different distributions have similar maximum intensities. Front loaded rainfall

distributions tend to yield lower runoff magnitudes than distributions with peak intensities that

occur latter in time. The closer the peak intensity is to the beginning of the storm the less

rainfall that is available for satisfying the rainfall loss capacity, particularly the initial abstraction,

without “eating into” the peak intensity. Therefore, more rainfall during the most intense portion

of the storm can be lost resulting in lower runoff magnitudes.

Although the AHYMO rainfall distribution or similar front loaded distribution is not encoded in

HEC-HMS, it can be input manually through the Specified Hyetograph option. This can either

be input directly into HEC-HMS or “cut and pasted” to the model from an existing file, such as a

spreadsheet. Continued use of the AHYMO rainfall distribution has several benefits:

• FEMA acceptance

• Community acceptance

• Avoids the potential problems integrating existing models into new models

• Provides a consistent basis of design for new structures that are either upstream or downstream of existing structures



October 24, 2008


For these reasons, and in addition to the relative ease of calculation and input of the AHYMO

rainfall distribution into HEC-HMS, it is recommended that the Specified Hyetograph option of

HEC-HMS be utilized to implement the current DPM rainfall distribution.

4.2 RAINFALL LOSS

AHYMO employs a combination of an initial abstraction plus an infiltration loss rate for

estimation of rainfall losses. For pervious land use / land cover conditions the infiltration rate is

constant. For impervious conditions, the infiltration rate is constant up to hour 3 of the storm.

After 3 hours, the infiltration rate is reduced linearly from 0.04 inches/hour to zero over a 3 hour

time period. Recommended values for the initial abstraction and infiltration rate are provided in

Table 1.

The current version of HEC-HMS includes six different single event simulation rainfall loss

methodologies, two of which can be applied in both a gridded or lumped parameter form. Those

methods are:

• Initial and Constant Loss – this method employs an initial loss that is applied from the

beginning of rainfall and a constant loss that is applied once the initial loss is satisfied.

This method accounts for impervious area which is expressed as a percent of the total

drainage area. Impervious area is assumed to be directly connected to the basin outlet

and is used to convert rainfall directly to rainfall excess.

• Deficit and Constant – this method is a variation of the initial and constant loss method.

The difference is that the initial loss can recover after a prolonged period of no rainfall.

The recovery of the initial loss is based on specification of an initial moisture deficit and a

maximum moisture deficit. This method can also be implemented in a gridded

approach.

• Exponential Loss – this is an empirical method that relates the loss rate to rainfall

intensity and accumulated losses. It requires the input of four parameters that establish

the starting conditions and control the loss rate decay over time. This method can be

used to simulate an initial and constant loss method or a simple decay method without

an initial loss. Treatment and simulation of impervious area is the same as described for

the initial and constant loss.

• Green and Ampt – a physically based method that combines an initial loss with a time

variable infiltration rate. Infiltration begins once the initial loss is satisfied. The rate of

change in infiltration is controlled by three parameters that represent the physical soil

characteristics. The minimum infiltration rate occurs at soil saturation. This method can

be used to simulate the initial loss and constant loss method by setting the soil moisture

deficit as saturated. Treatment and simulation of impervious area is the same as

described for the initial and constant loss.



October 24, 2008


• SCS Curve Number – an empirical equation relating soil retention capacity and

precipitation to direct runoff. This method was originally intended for evaluation of total

direct runoff for total rainfall and is therefore not explicitly related to time.

Implementation of this method is generally accomplished with a single parameter, the

Curve Number. Treatment and simulation of impervious area is the same as described

for the initial and constant loss. This method can also be implemented in a gridded

approach.

• Smith Parlange – a physically based method similar to Green and Ampt but does not

consider an initial loss. The method is implemented through specification of six

parameters and can also account for the effect temperature has on infiltration.

Treatment and simulation of impervious area is the same as described for the initial and

constant loss. This method can also be implemented in a gridded approach.

Of these six methodologies, the initial and constant loss rate method is the most similar to the

AHYMO loss methodology. However, there is a key difference and that is in the treatment of

impervious area. In AHYMO, it is assumed that even though a surface is “impervious” some

portion of rainfall is lost and the loss rate is not constant. Additionally, runoff from impervious

portions of a drainage area is calculated completely independently from pervious areas. The

independent calculations include the rainfall losses as well as the transformation of rainfall

excess to a runoff hydrograph. The total runoff from a drainage area is the sum of the runoff

from the pervious and impervious portions. In HEC-HMS, there are no losses estimated for the

impervious area. And, for each time step in the simulation, the percentage of impervious area

within the subbasin is used to convert that portion of rainfall directly to rainfall excess. The

remaining portion of rainfall is then available for the calculation of losses. The resulting

combined excess is then transformed to runoff.

To test the significance of this difference, selected subbasins from the Montoyas Arroyo and

Black Arroyo watersheds are recreated in HEC-HMS and the resulting runoff volumes are

compared. Input to HEC-HMS for the initial loss and constant loss parameters are the initial

abstraction (IA) and infiltration rate (INF) for the pervious portion of each AHYMO subbasin.

The percent impervious input to HEC-HMS is taken from the percent area for Land Treatment

Type D for each AHYMO subbasin. Input data for each subbasin is listed in Table 8. Also listed

in Table 8 are the corresponding runoff volumes for both AHYMO and HEC-HMS.

Review of the results listed in Table 8 shows that for subbasins without impervious area, the

HEC-HMS runoff volumes calculated using the initial loss constant loss method are identical to

AHYMO. For subbasins with impervious area, the HEC-HMS runoff volumes are always higher

than the runoff volume calculated by AHYMO. The result of higher runoff volumes from HEC-

HMS is consistent with the differences in methodology between the two programs. In general

the increase in runoff volume calculated by HEC-HMS for subbasins with impervious area is

less than 5 percent



October 24, 2008


The rainfall loss values used in this simple comparison are taken directly from the DPM with the

percent area from Land Treatment Type D being used as the percent impervious input to HEC-

HMS. To bring the HEC-HMS results more in-line with AHYMO it is possible that the input

parameters listed in the DPM could be adjusted slightly when HEC-HMS is used. One possible

adjustment would be to apply a reduction factor to the percentage of Land Treatment Type D

that would be estimated for an equivalent AHYMO model. However, the difference in results

between the two models being is on the order of 5 percent. By most hydrologic standards,

accuracy of hydrologic modeling within 5 percent of any other model / method or observed data

is typically considered extremely reasonable. Compared to the potential confusion that could

result from the use of different parameters between the two models, it is recommended that the

initial and constant loss method encoded in HEC-HMS be used in conjunction with the current

rainfall loss parameters prescribed in the existing DPM.

Table 8 – Comparison of Rainfall Loss Estimates

Subbasin Land Treatment Perct. Area Initial Const. Percent Runoff Volume

ID Area A B C D Loss Loss Imperv. AHYMO HEC-HMS Diff.

sq. mi. inches in/hr % acre-ft acre-ft %

L2.107 0.340 71.5 18.6 5.0 4.9 0.60 1.54 4.9 10.2 10.4 2.0

L2a.108 0.490 71.6 9.1 9.3 10.0 0.60 1.54 10.0 17.4 18.0 3.4

M10.100 0.500 90.0 5.0 5.0 0.0 0.63 1.61 0.0 11.6 11.6 0.0

M14.102-R1 0.230 15.3 14.5 42.8 27.4 0.44 1.09 27.4 14.9 15.7 5.4

M16.100 0.330 27.2 9.2 48.6 15.0 0.46 1.14 15.0 16.9 17.6 3.9

M18.108 0.850 63.2 10.9 11.2 14.7 0.59 1.51 14.7 35.3 36.8 4.2

M2.106 0.570 95.0 5.0 0.0 0.0 0.64 1.65 0.0 12.6 12.6 0.0

M20.102 0.990 85.3 5.9 6.0 2.8 0.62 1.59 2.8 26.3 26.6 1.1

M20.116 0.450 18.2 21.7 22.3 37.8 0.49 1.22 37.8 32.3 34.4 6.5

M20.120 0.680 41.5 16.3 14.6 27.6 0.56 1.41 27.6 39.1 41.3 5.9

M20a.100 1.420 62.6 10.5 10.8 16.1 0.59 1.51 16.1 61.0 63.7 4.4

M4.102 0.340 95.0 5.0 0.0 0.0 0.64 1.65 0.0 7.5 7.5 0.0

M8.104 0.820 90.9 5.0 4.1 0.0 0.63 1.61 0.0 18.8 18.8 0.0

P10.106 0.720 95.0 5.0 0.0 0.0 0.64 1.65 0.0 15.9 15.9 0.0

P4.145 0.064 95.0 5.0 0.0 0.0 0.64 1.65 0.0 1.4 1.4 0.0

103 0.267 80 11 4 5 0.62 1.59 5 7.9 8.1 1.9

105 0.154 5 53 17 25 0.48 1.18 25 9.3 9.8 5.7

106 0.466 50 28 9 13 0.57 1.45 13 19.3 20.1 3.9

114 0.071 2 55 18 25 0.47 1.16 25 4.3 4.5 4.3

153 0.165 100 0 0 0 0.65 1.67 0 3.7 3.7 0.0

211 0.184 2 43 14 41 0.47 1.16 41 13.9 14.9 6.9

213 0.108 2 43 14 41 0.47 1.16 41 8.2 8.7 6.4

215 0.112 4 36 12 48 0.48 1.19 48 9.2 9.8 6.8



October 24, 2008


4.3 UNIT HYDROGRAPH

In the current version of HEC-HMS there are five synthetic unit hydrographs and two user

defined options available for transforming rainfall excess to a runoff hydrograph. Of the 5

synthetic unit hydrograph options, four are lumped parameter approaches and one is

distributive. Only the lumped parameter approaches are considered and those are described

below. The user defined options are not considered viable options due to the labor intensive

effort needed for multiple subbasin models.

• Clark Unit Hydrograph – a three parameter hydrologic routing approach that is based on

the direct application of the continuity equation. Two of the three parameters are

numeric values representing the Time of Concentration (Tc) and Storage Coefficient (R).

The time of concentration represents the time from the end of effective rainfall to the

inflection point on the recession limb of the runoff hydrograph. The storage coefficient

accounts for the effect that temporary storage in the watershed has on the runoff

hydrograph. The third parameter is the time-area relation that defines the shape of the

unit hydrograph. In HEC-HMS the time-area relation is hard coded.

• Kinematic Wave – a physically based, hydraulic routing procedure that uses the

kinematic form of the momentum equation. It is primarily intended for application with

urban watersheds where the watershed is represented as a distributive system of

overland flow planes and open channels. Implementation requires the specification of

the average characteristics for the range of overland flow planes and open channels

within the drainage area.

• Snyder Unit Hydrograph – a two parameter graphical approach derived from a study of

watersheds in the Appalachian Mountains. The first parameter is the Lag Time (Tp) and

represents the time from the rainfall excess center of mass to the peak of the unit

hydrograph. The second parameter is the Peaking Coefficient (Cp) that accounts for

storage in the watershed. The time distribution for the Snyder Unit Hydrograph is

estimated by calculating equivalent Clark Unit Hydrograph parameters and the hard

coded time-area relation.

• SCS Dimensionless – a single parameter graphical approach derived from a study of

numerous small rural watersheds different geographic areas. The single parameter is

the basin Lag Time (Tp) and represents the time from the center of mass of rainfall

excess to the peak rate of runoff. The shape of the unit hydrograph is fixed such that

37.5 percent of the area under the hydrograph occurs from the origin to the peak.

Unlike the rainfall loss methodologies there is no one unit hydrograph method available in HEC-

HMS that stands out as a clear choice to emulate the AHYMO unit hydrograph. However, there

is one method that is the most dissimilar and that is the Kinematic Wave method. The

Kinematic Wave methodology is primarily intended to be used for relatively homogeneous urban



October 24, 2008


watersheds. Because of this limited range of applicability, this method is not recommended for

consideration.

Selection of a recommended unit hydrograph approach for HEC-HMS was conducted in two

phases. First, the unit hydrograph methodology (Clark, Snyder or SCS) was selected and

second modifications to input parameters were derived.

4.3.1 Unit Hydrograph Selection

Selection of the unit hydrograph method is based on simple testing and comparisons using the

example watershed C-2 provided in the DPM. Example C-2 was selected for the initial testing

because it has 28 percent impervious area associated with it. Subbasin data for that example

was input to HEC-HMS and applied to the Clark, Snyder and SCS Dimensionless unit

hydrographs. The corresponding parameters for each of the candidate HEC-HMS unit

hydrographs to the AHYMO hydrograph are:

• Clark Tc and R = AHMYO tp and k, respectively

• Snyder Tp and Cp = AHMYO tp and k, respectively

• SCS Dimensionless Tp = AHMYO tp

The AHYMO unit hydrograph for both the pervious and impervious portions of the basin are

calculated using the equations in Appendix A and shown graphically in Figures 7, 8 and 9. Also

shown in Figures 7, 8 and 9 are the corresponding unit hydrographs for the Clark, Snyder and

SCS Dimensionless methodologies, respectively. Based on the initial comparison, the SCS

Dimensionless unit hydrograph using the AHYMO tp as the input yields a very close match to

the AHYMO unit hydrograph for the impervious portion of the subbasin, but does not fit the

pervious portion. The Clark unit hydrograph fits the AHYMO unit hydrograph for the pervious

portion of the watershed slightly better than the SCS, but does not yield as good of fit for the

impervious portion of the watershed. The Snyder unit hydrograph yields a very poor fit for both

the pervious and impervious portions of the subbasin.

To improve the fit, adjustments to the Clark, Snyder and SCS Dimensionless unit hydrograph

input parameters are made in order to get a sense of the magnitude of the adjustment that

would be necessary or if it is even possible to get a match with the AHYMO unit hydrograph.

For the Clark and Snyder unit hydrographs adjustments to each of the two parameters were

made independently as well as adjustments to both parameters. Additionally, separate

adjustments to the input parameters are made for the pervious and impervious portions of the

subbasins. The adjustments that resulted in the best fit with AHYMO are shown in Figures 10,

11 and 12 for the Clark, Snyder and SCS Dimensionless, respectively. The adjusted

parameters for each unit hydrograph method are shown on the corresponding figure.



October 24, 2008


Figure 7 – AHYMO – Clark Unit Hydrograph Comparison

0

500

1,000

1,500

2,000

0.0 0.5 1.0 1.5 2.0

Time, in hours

Dis

ch

arg

e,

in c

fs

AHYMO - Pervious

AHYMO - Impervious

Clark - Pervious

Clark - Impervious

Figure 8 – AHYMO – Snyder Unit Hydrograph Comparison

0

500

1,000

1,500

2,000

0.0 0.5 1.0 1.5 2.0

Time, in hours

Dis

ch

arg

e, in

cfs

AHYMO - Pervious

AHYMO - Impervious

Snyder - Pervious

Snyder - Impervious



October 24, 2008


Figure 9 – AHYMO – SCS Dimensionless Unit Hydrograph Comparison

0

500

1,000

1,500

2,000

0.0 0.5 1.0 1.5 2.0

Time, in hours

Dis

ch

arg

e, in

cfs

AHYMO - Pervious

AHYMO - Impervious

SCS - Pervios

SCS - Impervious

Figure 10 – AHYMO – Clark Unit Hydrograph with Adjusted Parameters

0

500

1,000

1,500

2,000

0.0 0.5 1.0 1.5 2.0

Time, in hours

Dis

ch

arg

e,

in c

fs

AHYMO - Pervious

AHYMO - Impervious

Clark - Pervious

Clark - Impervious

Parameters Adjustments: Pervious Clark Tc = AHYMO tp Clark R = 1.4 * AHYMO k Impervious Clark Tc = 1.2 * AHYMO tp Clark R = AHYMO k



October 24, 2008


Figure 11 – AHYMO –Snyder Unit Hydrograph with Adjusted Parameters

0

500

1,000

1,500

2,000

0.0 0.5 1.0 1.5 2.0

Time, in hours

Dis

ch

arg

e, in

cfs

AHYMO - Pervious

AHYMO - Impervious

Snyder - Pervious

Snyder - Impervious

Parameters Adjustments: Pervious Snyder Tp = AHYMO tp Snyder Cp = 2 * AHYMO k Impervious Snyder Tp = AHYMO tp Snyder Cp = 4.5 * AHYMO k

Figure 12 – AHYMO – SCS Unit Hydrograph with Adjusted Parameters

0

500

1,000

1,500

2,000

0.0 0.5 1.0 1.5 2.0

Time, in hours

Dis

ch

arg

e, in

cfs

AHYMO - Pervious

AHYMO - Impervious

SCS - Pervious

SCS - Impervious

Parameters Adjustments: Pervious SCS Tp = 1.3 *AHYMO tp Impervious SCS Tp = 1.3* AHYMO tp



October 24, 2008


For both the Clark and Snyder unit hydrographs the adjusted parameters greatly improved the

fit with the AHYMO unit hydrograph. For the pervious portion of the watershed, the adjustment

that yielded the best fit was increasing the storage parameter. For the impervious portion,

adjusting the Clark unit hydrograph Tc parameter yielded the best fit with AHYMO while for the

Snyder the best fit was obtained by again adjusting the storage parameter. Since the SCS

Dimensionless unit hydrograph is a single parameter model, the effectiveness of parameter

adjustments was limited.

From this simple analysis, it is recommended that the Clark unit hydrograph be considered for

further analysis and refinement of input parameters. The primary reason the Clark is selected

over the Snyder unit hydrograph is that HEC-HMS converts the Snyder input parameters to

equivalent Clark parameters in order to calculate the ordinates of the unit hydrograph. Selection

of the Clark unit hydrograph saves this computational step.

4.3.2 Parameter Adjustments

The analysis conducted for the selection of the unit hydrograph method was based on the single

example watershed presented in the DPM. Though simplistic, this initial analysis was sufficient

to highlight important challenges in the development of equivalent Clark unit hydrograph

parameters.

• The analysis conducted for the selection of the unit hydrograph method treated the

pervious and impervious portions of the subbasin separately. Separate adjustment

factors were identified for the two areas. From a practical / application perspective, this

would not be the recommended approach in HEC-HMS. Rather, it is envisioned that the

pervious and impervious portions will be lumped together. Therefore, there must be an

adjustment that works for subbasins without any impervious area as well as subbasins

with impervious area.

• The analysis was based on a single set of physical and land use conditions. Given the

dependent nature of the AHYMO tp and k parameters to the physical and land use

conditions, it is unlikely that the same adjustment factor will work for the range of

watershed characteristics present in the SSCAFCA jurisdictional area.

• For the pervious portion of the subbasin used in the analysis, the adjustment factor with

the best fit was applied to the recession constant. Generally, because of the complexity

in calculation, the recession constant is calculated internally to AHYMO. Development

of a suite of adjustment factors that accounted for the range in hydrologic conditions is

not practical from an application perspective.

To address these challenges, further testing was conducted to establish if a single Clark unit

hydrograph parameter can be adjusted that would yield similar results as AHYMO for subbasins

with both pervious and impervious areas and to determine the range in the adjustment factor for

different hydrologic conditions. The result of that testing was the determination that the storage



October 24, 2008


coefficient is the key parameter to adjust regardless of the presence of impervious area. In

addition, the adjustment factor was found to range from 0.9 times to 1.5 times the AHYMO

recession constant (k). The general trend in the variation of the adjustment factor is that the

factor decreases with increasing rainfall excess. The relationship of the Clark unit hydrograph

storage coefficient (R) to rainfall losses and other parameters was derived through a regression

analysis. A discussion of the regression analysis is provided in Appendix B. The result of the

analysis is the derivation of a single equation (Eqn. 12) for calculating the Clark unit hydrograph

storage coefficient.

Eqn-12

Where: Tc = Time of concentration, in hours (from Eqn. 7, 8 or 9)

INF = Infiltration loss rate for the subbasin, in in/hr

IA = Initial abstraction for the basin, in inches

RTIMP = Impervious area expressed as a fraction

The adequacy of this equation when used in combination with the AHYMO tp equations for

estimating the Clark unit hydrograph parameters is tested using the subbasin data for the

Montoyas and Black Arroyo watersheds. For this test, the entire model for each watershed is

recreated in HEC-HMS. Rainfall loss parameters are input to HEC-HMS according to the

recommendations presented in Section 4.2. The Clark unit hydrograph parameter Tc is taken

directly from the corresponding AHYMO input for tp. The Clark unit hydrograph parameter R is

calculated according to Equation 12.

Summary tables listing the AHYMO and HEC-HMS peak discharge and time to peak for each

subbasin are provided as Tables C-1 and C-2 of Appendix C for the Montoyas and Black Arroyo

watersheds, respectively. A graphical comparison of peak discharge rates for each subbasin

within both watersheds is provided in Figure 13. The descriptive statistics of the peak discharge

comparison both in terms of the raw differential and the percent difference is provided in Table

9. A more general comparison of the peak discharge results as well as the time to peak is

summarized as:

• Montoyas Arroyo

o 84 percent of subbasin peak discharge estimated using HEC-HMS within 5

percent of AHYMO peak discharge



o Maximum difference in subbasin time to peak is one computation interval (3

minutes)

o 81 percent of subbasin time to peak from HEC-HMS are identical to AHYMO

subbasin time to peak

( )40.040.145.0***165.1 RTIMPIAINFTR c −=



October 24, 2008


• Black Arroyo





o Maximum difference in subbasin time to peak is one computation interval (3

minutes)

o 74 percent of subbasin time to peak from HEC-HMS are identical to AHYMO

subbasin time to peak

Table 9 – Peak Discharge Comparison Descriptive Statistics

Montoyas Arroyo Black Arroyo

Raw Percent Raw Percent

Statistic Differential Difference Differential Difference

Mean 4.4 2.9 2.9 4.6

Standard Error 0.5 0.3 0.3 0.5

Median 3.6 1.9 2.3 2.3

Mode 8.9 4.0 1.7 ---

Standard Deviation 5.5 3.4 2.3 4.4

Sample Variance 30.4 11.7 5.1 19.0

Kurtosis 75.7 33.3 0.9 -0.1

Skewness 7.7 4.6 0.8 0.9

Range 59.6 31.0 11.0 17.3

Minimum 0.0 0.0 0.0 0.0

Maximum 59.6 31.0 11.0 17.3

Count 135 135 70 70

Confidence Level (95.0%) 0.9 0.6 0.5 1.0

Based on these results, the recommendation for calculation of the Clark unit hydrograph time of

concentration is to use the AHYMO tp equations, presented in Section 2.3. The

recommendation for calculation of the Clark unit hydrograph storage coefficient is to use

Equation 12. Equation 12 is to be used for any subbasin regardless of the physical

characteristics or land use.



October 24, 2008


Figure 13 – Peak Discharge Comparison with the Clark Unit Hydrograph

0

200

400

600

800

1,000

1,200

0 200 400 600 800 1,000 1,200

HEC-HMS Peak Discharge, in cfs

AH

YM

O P

eak

Dis

ch

arg

e,

in c

fs

Black Watershed

Montoyas Watershed

4.4 HYDROLOGIC ROUTING

Similar to AHYMO, HEC-HMS supports both channel and storage routing methodologies.

Recommended methodologies for each are discussed in the following sections.

4.4.1 Channel Routing

There are several methods available in the current version of HEC-HMS for routing runoff

hydrographs, including Muskingum-Cunge. Implementation of the Muskingum-Cunge routing

method in HEC-HMS is identical to implementation in AHYMO with the following exceptions:

• In HEC-HMS, specification of roughness coefficients is limited to three values; left

overbank, channel and right overbank,

• Channel geometry in HEC-HMS is limited to 8 points,

• HEC-HMS calculates the numerical stability parameters ∆t and ∆x internally and does

not use correction factors for the routing coefficients,

• HEC-HMS uses a slightly different iteration scheme, and

Line of Agreement



October 24, 2008


• There is no practical limit to the number of hydrograph ordinates using HEC-HMS.

Of the differences in implementation, the last two are the most important. Without a practical

limit on the number of hydrograph ordinates, HEC-HMS can use numerical stability parameters

sufficiently small to minimize computational error. Consequently, adjustments to the routing

coefficients (C1,2 and 3) are not required and the resulting floodwave velocities are more

consistent with the values expected based on current literature. In AHYMO, if the user specifies

a routing time step (∆t) that is too long, numerical instabilities may result. In those instances,

AHYMO forces a stable solution through the application of the Fread correction for the routing

coefficients. The result is artificially long floodwave travel times. In the upper reaches of a

watershed, the artificially long travel time does not yield significantly different results than if a

much smaller routing time step were used. However, as the upstream hydrographs are routed

downstream and combined with local runoff, the time shift results in miss aligned peaks. At the

watershed outlet, this cumulative effect can result in significantly lower peak discharges. This

miss alignment of peaks is the case in the versions of the Montoyas and Black Arroyo

watershed models provided for comparison purposes.

The influence of the miss aligned peaks in peak discharge estimates can be seen through a

comparison of the HEC-HMS and AHYMO model results for the two watersheds. Listings of the

routed hydrograph peak discharge results are provided in Tables D-1 and D-2, of Appendix D

for the Montoyas Arroyo and Black Arroyo watersheds, respectively. Included in those tables is

the percent difference in peak discharge for each routing reach between the two models. A

comparison of the AHYMO and HEC-HMS routed hydrograph results are also compared

graphically in Figure 14. For lower discharges, the results compare well between AHYMO and

HEC-HMS. These lower discharges are representative of runoff from the upper portions of the

watersheds. However, as peak discharge increases, so does the separation in results between

the two models with HEC-HMS yielding higher peak discharges. Of the data shown in Figure

14, the higher peak discharge values will represent runoff from larger areas in the watersheds.

For these lower reaches of the watershed, the cumulative effects of the travel time calculations

are very clearly illustrated. For the Montoyas Arroyo watershed, the maximum increase in

routed peak discharge estimated by HEC-HMS is 104 percent. The maximum differential in the

Black Arroyo watershed is 48 percent. As can be seen from Figure 14, the Montoyas Arroyo

watershed is more sensitive to the travel time effect than the Black Arroyo watershed. This is

primarily due to the differences in channel geometry between the two watersheds. The Black

Arroyo watershed is more heavily urbanized with smaller channels and steeper slopes. The

greater flow velocities in the routing reaches of the Black Arroyo watershed result in more

reaches that yield numerically stable results using the Ponce correction for the routing

coefficients. Therefore, fewer reaches are forced into a stable solution using the Fread

correction.



October 24, 2008


Figure 14 – Routing Peak Discharge Comparison

0

2,000

4,000

6,000

8,000

10,000

12,000

0 2,000 4,000 6,000 8,000 10,000 12,000

HE

C-H

MS

Pe

ak

Dis

ch

arg

e,

in c

fs


Montoyas Watershed

Black Watershed


In order to verify that it is the Fread correction factors driving the differences between AHYMO

and HEC-HMS, SSCAFCA requested that Cliff Anderson review the modeling for Montoyas and

Black Arroyos and data provided by Stantec. Cliff was asked to first confirm the program

limitations of the AHYMO channel routing routine; and second, recommend potential changes to

AHYMO that would lead to both programs yielding more similar results. Cliff’s findings were the

confirmation that AHYMO, when defaulting to the Fread correction factors, does indeed

artificially reduce floodwave travel times to achieve numerical stability. Second, Cliff confirmed

that reducing the AHYMO routing ∆t will result in peak discharges nearly identical to that of

HEC-HMS. Cliff also confirmed that reducing the routing time step to values consistent with

HEC-HMS can result in an incomplete runoff hydrograph due to the limitation of hydrograph

ordinates. It was therefore requested by SSCAFCA that Cliff enhance the AHYMO code to

avoid this artificial limitation and recommend a routing time step that was equivalent to the ∆t

calculated by HEC-HMS.

The revisions to the AHYMO code made by Cliff include increasing the number of hydrograph

ordinates to 4,000 and bypassing the Fread formulation of correction factors. In addition,

through his analysis, Cliff recommended that channel routing ∆t be stipulated as 0.01 hours.

Using the revised AHYMO code, the Montoyas Arroyo and Black Arroyo watersheds were rerun.



October 24, 2008


The updated results were then compared back to the HEC-HMS model results. Those

comparisons are provided in Tables D-3 and D-4 of Appendix D. A graphical comparison is also

provided as Figure 15.

Figure 15 – Revised Routing Peak Discharge Comparison

0

2,000

4,000

6,000

8,000

10,000

12,000

0 2,000 4,000 6,000 8,000 10,000 12,000

HE

C-H

MS

Pe

ak

Dis

ch

arg

e,

in c

fs


Montoyas Watershed

Black Watershed


Based on the routing comparisons between AHYMO and HEC-HMS and the enhancements to

the AHYMO code, it is recommended that the Muskingum-Cunge channel routing method be

adopted in the DPM when the HEC-HMS model is used. Physical channel parameters will be

identical to what would be input to AHYMO with the possible exceptions of the three roughness

coefficient value and 8 point geometry limitations.

4.4.2 Storage Routing

Storage routing in HEC-HMS is accomplished using the same storage indication method as in

AHYMO. Input options can either be user defined stage-storage-discharge rating curves or the

specification of predefined structures (outlets, spillways and dam overtopping). Additional

options for input of seepage loss and dam break simulation are also available. Consequently,

no specific recommendation is required.



October 24, 2008


4.5 FLOW DIVERSIONS

HEC-HMS supports routines for diverting of flow. Similar to the data options in HEC-HMS for

storage routing, diversion data can be specified either as a user defined rating curve or a

defined structure. At this time, the defined structures are lateral weir and pump. Again, similar

to the storage routing, no specific recommendation is required.

4.6 SEDIMENT BULKING

At this time, HEC-HMS does not support the same sediment functionality as AHYMO. Future

releases of HEC-HMS will incorporate sediment yield methodologies. Until that time, sediment

bulking can be simulated using a ratio applied to the runoff hydrograph of all subbasins within

the watershed. This method is identical to the sediment bulking ratio option of AHYMO when

the actual sediment modeling options are not utilized. The ratio is specified by the user and can

be any number. The ratio is applied to each ordinate of the runoff hydrograph for a subbasin.


PROCESS MANUAL


5.0 Summary

There were two primary goals for the selection of an equivalent model to AHYMO that could be

used for rainfall-runoff modeling in the SSCAFCA jurisdiction. First, is that the equivalent model

must reproduce, to a reasonable extent, runoff magnitudes currently obtained using the AHYMO

computer program. The benchmark for achieving this goal is that runoff magnitudes from HEC-

HMS should be within ± 5 to 10 percent of the AHYMO runoff magnitudes. Second, the model

should be in the public domain, well documented and relatively easy to use.

HEC-HMS clearly satisfies the second goal. The first goal was also achieved, but not to the

extent desired. At the subbasin level, the recommended methodologies, parameters and

procedures for estimating runoff magnitudes matched closely with AHYMO runoff magnitudes.

For the two test watershed used for evaluation purposes, approximately 94 percent of the 201

subbasins were within the ±10 percent target for peak discharge as illustrated in the histogram

provided as Figure 16. Looking at runoff volumes, 99.5 percent of the 201 subbasins were

within the ± 10 percent target, as shown in Figure 17. Although the revised AHYMO executable

provided by Cliff Anderson improves the comparison of results, minor routing differences

between the two models still pushes the peak discharge outside of the target range. For the two

test watersheds, 83 percent of the peak discharges at the model junctions were within the target

range as shown in Figure 18. In general the majority of the junction results that fall outside of

the target range are for large watershed areas as illustrated in Figure 19.


MANUAL Summary

October 24, 2008


Figure 16 – Histogram of subbasin peak discharge comparison

0

10

20

30

40

50

60

70

80

90

-50 -40 -30 -20 -10 0 10 20 30 40 50

Fre

qu

en

cy

Percent Difference in Peak Discharge (AHYMO minus HEC-HMS)

35.8%

39.8%

0.5%

18.4%

4.5%

1.0%

Percent of sample within interval

Figure 17 – Histogram of subbasin runoff volume comparison

0

10

20

30

40

50

60

70

80

90

100

-50 -40 -30 -20 -10 0 10 20 30 40 50

Fre

qu

en

cy

Percent Difference in Runoff Volume (AHYMO minus HEC-HMS)

47.3%

37.8%

14.4%

0.5%



MANUAL Summary

October 24, 2008


Figure 18 – Histogram of model junction peak discharge comparison

0

10

20

30

40

50

60

70

80

90

-50 -40 -30 -20 -10 0 10 20 30 40 50

Fre

qu

en

cy

Percent Difference in Runoff Volume (AHYMO minus HEC-HMS)

38.2%

20.3%

24.6%

6.8%


9.2%

1.0%

Figure 19 – Peak discharge comparison by drainage area

-25%

-20%

-15%

-10%

-5%

0%

5%

10%

0 10 20 30 40 50 60

Perc

en

t D

iffe

ren

ce

in

Pea

k D

isc

ha

rge

Drainage Area, in sq. miles

Montoyas Watershed

Black WatershedH

EC

-HM

S>

AH

YM

O

Appendix A

AHYMO Unit Hydrograph Documentation

Appendix B

Regression Analysis

Memo

mcg w:\active\185120283\reports\technical documentation\appendix b.doc

To: File From: Mike Gerlach

Phoenix

File: 185120283 Date: November 14, 2008

Reference: Unit Hydrograph Parameter Regression Analysis

GENERAL

From the initial testing of the unit hydrograph methodologies available in HEC-HMS, the Clark unit hydrograph was selected as an equivalent methodology to the AHYMO unit hydrograph. The analysis conducted for the selection of the unit hydrograph method was based on the single example watershed presented in the DPM. Though simplistic, this initial analysis was sufficient to highlight important challenges in the development of equivalent Clark unit hydrograph parameters.

• The analysis conducted for the selection of the unit hydrograph method treated the pervious and impervious portions of the subbasin separately. Separate adjustment factors were identified for the two areas. From a practical / application perspective, this would not be the recommended approach in HEC-HMS. Rather, it is envisioned that the pervious and impervious portions will be lumped together. Therefore, there must be an adjustment that works for subbasins without any impervious area as well as subbasins with impervious area.

• The analysis was based on a single set of physical and land use conditions. Given the dependent nature of the AHYMO tp and k parameters to the physical and land use conditions, it is unlikely that the same adjustment factor will work for the range of watershed characteristics present in the SSCAFCA jurisdictional area.

• For the pervious portion of the subbasin used in the analysis, the adjustment factor with the best fit was applied to the recession constant. Generally, because of the complexity in calculation, the recession constant is calculated internally to AHYMO. Development of a suite of adjustment factors that accounted for the range in hydrologic conditions is not practical from an application perspective.

To address these challenges, further testing was conducted to establish if a single Clark unit hydrograph parameter can be adjusted that would yield similar results as AHYMO for subbasins with both pervious and impervious areas and to determine the range in the adjustment factor for different hydrologic conditions. The result of that testing was

February 13, 2009

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the determination that the storage coefficient is the key parameter to adjust regardless of the presence of impervious area. In addition, the adjustment factor was found to range from 0.9 times to 1.5 times the AHYMO recession constant (k). The general trend in the variation of the adjustment factor is that the factor decreases with increasing rainfall excess. To further define this relationship a regression analysis was performed to develop an equation for calculation of the storage coefficient.

APPROACH

Existing watershed data that represents a full range of hydrologic conditions present within the SSCAFCA jurisdictional area is not available. Therefore, data for the regression analysis is created from an idealized suite of conditions. The idealized data was created from the following combination of hydrologic conditions:

• Five basin shapes,

• 50 different basin areas, ranging from 0.2 to 10 square miles in increments of 0.2 square miles,

• 50 different mean basin slopes, ranging from 0.001 to 0.05 feet/foot in increments of 0.001 feet/foot,

• 34 basin roughness coefficients, and

• 45 different combinations of the four land treatment

These various conditions are combined to form a total of 19,125,000 unique basin configurations. The five basin shapes were selected to represent different relations between the drainage area and the Clark unit hydrograph Time of Concentration (Tc) flow path length as well as the relation between the Tc flow path length and the length to centroid, Lca. These relations are illustrated in Figure 1.

The 34 basin roughness coefficients were broken out from the five values of roughness categories recommended in Table B-1 and B-2 of the DPM. Conveyance factors (roughness) presented in Table B-1 were used in the estimation of tp for flow path lengths less than 4,000 feet. Basin factors (roughness) presented in Table B-2 were used in the estimation of tp for flow path lengths greater than 12,000 feet. For flow path lengths between 4,000 and 12,000 feet, conveyance factors from both Tables B-1 and B-2 are used in the estimation of tp. Therefore, a consistent increment between each category for each table must be used. Based on the range of values between the individual categories for the two tables a total of 34 unique values were broken out.

There is a nearly unlimited number of potential land treatment combinations that can occur. To provide a reasonable of this full range, 45 unique combinations of the 4 land treatment types were developed for the regression analysis. Those combinations are listed in Table 1.

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Figure 1 – Generic basin configurations

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Table 1 - Land treatment combinations

Land treatment area, in percent

Combination A B C D

1 100 0 0 0

2 80 20 0 0

3 60 40 0 0

4 40 60 0 0

5 20 80 0 0

6 0 100 0 0

7 80 0 20 0

8 60 0 40 0

9 40 0 60 0

10 20 0 80 0

11 0 0 100 0

12 80 0 0 20

13 60 0 0 40

14 40 0 0 60

15 20 0 0 80

16 0 0 0 100

17 0 80 20 0

18 0 60 40 0

19 0 40 60 0

20 0 20 80 0

21 0 80 0 20

22 0 60 0 40

23 0 40 0 60

24 0 20 0 80

25 0 0 80 20

26 0 0 60 40

27 0 0 40 60

28 0 0 20 80

29 25 25 25 25

30 40 20 20 20

31 20 40 20 20

32 20 20 40 20

33 20 20 20 40

34 60 20 20 0

35 60 20 0 20

36 60 0 20 20

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Table 1 - Land treatment combinations (cont.)

Land treatment area, in percent

Combination A B C D

37 20 60 20 0

38 20 60 0 20

39 0 60 20 20

40 20 20 60 0

41 20 0 60 20

42 0 20 60 20

43 20 20 0 60

44 20 0 20 60

45 0 20 20 60

The watershed parameters for the 19,125,000 unique combinations of raw data were developed as an integrated step in the regression analysis. The regression analysis was performed as a four step process.

1. For each unique basin configuration, use AHYMO to compute the hydrologic parameters (rainfall loss and unit hydrograph) and corresponding runoff results.

2. Extract the hydrologic parameters and input that data into HEC-1. The rainfall loss parameters from AHYMO (IA and INF) are used directly as input to the initial loss and constant loss rate method in HEC-1. The AHYMO unit hydrograph tp parameter is used directly as input to the Clark unit hydrograph parameter Tc. The AHYMO unit hydrograph parameter k is adjusted according to the range of adjustment factors (0.9 – 1.5 and input as the Clark unit hydrograph parameter. For each unique basin configuration, HEC-1 is run 7 times incrementing the adjustment factor by 0.1 for each run. HEC-1 is used in this instance over HEC-HMS because it can be automated by a call from a program which was developed to perform all 19,125,000 iterations. There is essentially no difference in the implementation of the initial loss plus constant loss and Clark unit hydrograph methodologies between HEC-1 and HEC-HMS.

3. For each unique basin and for each of the 7 different adjustment factor runs, compare the HEC-1 results to the AHYMO results. Select the adjustment factor that most closely reproduces the AHYMO results.

4. For the selection set of 19,125,000 unique configurations select a fitting equation for the storage coefficient parameter of the Clark unit hydrograph and derive the coefficients and exponents using multiple linear regression. Selection of the fitting equation is based on inspection of the individual data parameter trends and co-relations.

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RESULTS

Calculation of k, as presented in the DPM, is a function of basin area, land treatment type, tp and the 1-hour precipitation depth. In the development of the raw data for the regression analysis, precipitation was not considered because 1-hour depths within the SSCAFCA jurisdictional area are relatively uniform. Drainage area was also dropped from the regression analysis because it is indirectly related to calculation of tp. Therefore, the key parameters selected for the fitting equation are tp and land treatment (IA, INF and impervious area).

According to the equations in the DPM k is directly related to tp, IA, INF and impervious area (designated herein as RTIMP). Based on inspection of the regression analysis data, k decreases as tp decreases. From the data it can also be observed that k decreases as rainfall losses decrease. This would imply that an equation for the storage coefficient (R) would have the following form:

edcb

c RTIMPINFIATaR ****=

Where: a = coefficient

b, c, d, e = exponents

However, for basins without impervious area, RTIMP will be zero. Therefore, a different form of the equation is required. Through testing of small segments of the overall data set, the following equation was derived.

( )edcb

c RTIMPIAINFTaR ** −=

Using the entire data set, the coefficients and exponents were determined to be:

a = 1.165

b = 1.0

c = 0.45

d = 1.40

e = 0.40

Using the above equation, R was calculated and compared against the R that yielded the best match in runoff to the original AHYMO calculations for each of the 19,125,000 basins. That data is plotted in Figure 2. In Figure 2, the observed values are the R values that yielded the best match in runoff to the original AHYMO calculations. The predicted values are the R values calculated using the above equation. The R2 for the above equation is 0.993. However, the R2 value can be somewhat miss leading for this analysis due to the extremely large number of observations. A somewhat simple but descriptive inspection of the fitness quality is to look at the percent difference between the predicted value and the observed value.

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For the 19,125,000 observations:

• 12% of the predicted values (2,447,755) are within 1% of the corresponding observed value,

• 77% of the predicted values (14,898,620) are within 5% of the corresponding observed value, and

• 94% of the predicted values (18,168,447) are within 10% of the corresponding observed value.

The remaining 6% of the predicted values (956,553) fell between 10% and 42% of the corresponding observed values. Of these, 948,226 are within 20% of the predicted values. All of the predicted values that were more than 10% off the observed value are on the edge of reasonable physical watershed conditions. The majority of these have basin areas greater than 6 square miles with very short time of concentrations, typically less than 1 hour. Other conditions that caused the predicted values to skew away from the observed values were basins with extremely high impervious area and some combination of large area or short time of concentration.

To further test the adequacy of the derived equation and the Clark unit hydrograph as an equivalent, alternative approach to AHYMO several subbasins from the Black and Montoyas Arroyo watersheds were recreated in HEC-HMS using the recommended rainfall loss and unit hydrograph methodologies and parameters. From each watershed two basins without any impervious are and two with impervious area were selected. Input data for those 8 subbasins are listed in Table 2. A plot of the unit hydrograph for each of the 4 subbasins in the Montoyas Arroyo watershed is provided as Figure 3. The Black Arroyo unit hydrographs are shown in Figure 4. Runoff hydrographs for each of the 4 subbasins from the Montoyas and Black Arroyo watersheds are shown in Figures 5 and 6.

Figures 3 and 4 illustrate the typical difference between the AHYMO and the Clark unit hydrographs. The Clark unit hydrograph tends to have a much sharper peak but with a slightly more gentle recession limb. However the general shape and scale of unit hydrographs for the two methods is very similar. The same differences in the unit hydrographs are manifested in the runoff hydrographs as can be seen from Figures 5 and 6. Similarly with the unit hydrographs to overall volume of runoff is generally distributed in a very similar manner.

The general conclusion of the regression analysis is that the equation can predict the Clark unit hydrograph storage coefficient parameter that can result in unit hydrograph ordinates that are very similar to the AHYMO unit hydrograph ordinates. Use of the Clark unit hydrograph in conjunction with the other recommended equivalent hydrologic methodologies results in very comparable runoff magnitudes between AHYMO and HEC-HMS. The next level of testing that should be performed is the use of HEC-HMS and the recommended methodologies and parameters for the full Montoyas and Black Arroyo watersheds.

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Figure 2 – Observed vs. predicted Storage Coefficient values

Table 2 – Subbasin parameters

Subbasin Arroyo Area IA INF RTIMP Tc R

ID sq. mi in in/hr % hrs hrs

M8.106 Montoyas 1.090 0.628 1.607 0 0.521 0.751

P14.102 Montoyas 0.770 0.538 1.357 0 0.392 0.573

M18.108 Montoyas 0.850 0.591 1.510 14.7 0.492 0.562

M20.116 Montoyas 0.450 0.490 1.222 37.8 0.287 0.282

102 Black 0.258 0.65 1.670 0 0.239 0.351

255 Black 0.110 0.65 1.670 0 0.244 0.358

106 Black 0.466 0.571 1.448 13 0.244 0.278

109 Black 0.089 0.473 1.175 33 0.212 0.210

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Figure 3 – Montoyas Arroyo subbasin unit hydrographs

0

100

200

300

400

500

600

700

0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00

Time, in hours

Dis

ch

arg

e,

in c

fs

M8.106 - AHYMO

M8.106 - Clark

P14.102 - AHYMO

P14.102 - Clark

M18.108 - AHYMO

M18.108 - Clark

M20.116 - AHYMO

M20.116 - AHYMO

Figure 4 – Black Arroyo subbasin unit hydrographs

0

100

200

300

400

500

600

700

0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00

Time, in hours

Dis

ch

arg

e,

in c

fs

102 - AHYMO

102 - Clark

255 - AHYMO

255 - Clark

106 - AHYMO

106 - Clark

109 - AHYMO

109 - AHYMO

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Figure 5 – Montoyas Arroyo subbasin runoff hydrographs

0

100

200

300

400

500

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Time, in hours

Dis

ch

arg

e,

in c

fs

M8.106 - AHYMO

M8.106 - Clark

P14.102 - AHYMO

P14.102 - Clark

M18.108 - AHYMO

M18.108 - Clark

M20.116 - AHYMO

M20.116 - Clark

Figure 6 – Black Arroyo subbasin runoff hydrographs

0

100

200

300

400

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Time, in hours

Dis

ch

arg

e,

in c

fs

102 - AHYMO

102 - Clark

255 - AHYMO

255 - Clark

106 - AHYMO

106 - Clark

109 - AHYMO

109 - Clark

Appendix C

Watershed Model Subbasin Comparison

Table C-1

Summary comparison of subbasin results for the Montoyas Watershed

Drainage Peak Discharge Runoff Volume Time to Peak, in hours

Operation Area AHYMO HEC-HMS Difference AHYMO HEC-HMS Difference AHYMO HEC-HMS Difference

ID sq. miles cfs cfs % acre-ft acre-ft % hours hours %

M4.100 0.390 228 220 3.3 8.6 8.7 1.2 1.60 1.60 0.0

M4.102 0.340 183 175 4.0 7.5 7.6 1.2 1.60 1.65 3.1

M4.104 0.390 209 200 4.0 8.6 8.7 1.2 1.60 1.65 3.1

M4.106 0.410 220 212 4.0 9.1 9.2 1.2 1.60 1.65 3.1

M4.108 0.460 244 235 3.5 10.2 10.3 1.2 1.60 1.65 3.1

M4.110 0.490 256 250 2.1 11.0 11.0 0.3 1.60 1.65 3.1

M4.112 0.490 194 191 1.2 11.3 11.2 0.8 1.70 1.70 0.0

M2.100 0.730 292 289 1.0 16.2 16.4 1.2 1.70 1.70 0.0

M2A.100 0.270 142 140 1.6 6.0 6.0 1.2 1.60 1.60 0.0

M2.102 0.410 220 212 4.0 9.1 9.2 1.2 1.60 1.65 3.1

M2.104 0.510 298 288 3.3 11.3 11.4 1.2 1.60 1.60 0.0

M2.106 0.570 333 322 3.3 12.6 12.8 1.2 1.60 1.60 0.0

M2.108 0.350 186 179 3.5 7.7 7.8 1.2 1.60 1.65 3.1

M2.110 0.500 192 252 31.0 11.1 11.2 1.2 1.70 1.70 0.0

M2B.100 0.610 199 199 0.2 13.5 13.7 1.2 1.75 1.75 0.0

M2.112 0.230 126 127 1.1 5.2 5.2 0.1 1.60 1.60 0.0

M2C.100 0.530 196 194 1.0 11.7 11.9 1.2 1.75 1.75 0.0

M2.114 0.300 153 151 1.2 6.9 6.9 0.8 1.65 1.65 0.0

M6.100 0.470 248 244 1.8 10.9 10.8 0.8 1.65 1.65 0.0

M6.102 0.460 156 155 0.7 10.6 10.5 0.8 1.80 1.75 2.8

M8.100 0.410 164 162 1.0 9.1 9.2 1.2 1.70 1.70 0.0

M8.102 0.410 187 182 2.3 9.2 9.2 0.3 1.65 1.65 0.0

M8b.100 0.490 136 137 0.5 11.3 11.2 0.3 1.85 1.85 0.0

M8a.100 0.620 264 261 1.2 13.7 13.9 1.2 1.70 1.70 0.0

M8a.102 0.560 220 217 1.2 12.8 12.8 0.5 1.70 1.70 0.0

Page 1 of 6

Table C-1





M8.104 0.820 237 237 0.1 18.8 18.8 0.0 1.85 1.85 0.0

M8.106 1.090 288 288 0.3 25.2 25.0 0.8 1.90 1.90 0.0

M8C.100 0.550 263 257 2.4 12.7 12.6 0.3 1.65 1.65 0.0

M8C.102 0.760 317 313 1.2 17.6 17.4 0.8 1.70 1.70 0.0

M8C.104 0.400 179 176 1.5 9.2 9.2 0.8 1.70 1.70 0.0

M8.108 0.440 228 224 1.9 10.2 10.1 0.8 1.65 1.65 0.0

M10.100 0.500 138 139 0.5 11.6 11.5 0.8 1.85 1.85 0.0

M12.100 0.690 323 315 2.3 15.9 15.8 0.8 1.65 1.65 0.0

M12.102 0.550 198 197 0.6 12.7 12.6 0.8 1.75 1.75 0.0

M12.104 0.670 186 187 0.5 15.5 15.4 0.8 1.85 1.85 0.0

M12a.100 0.610 271 268 1.4 14.1 14.0 0.8 1.70 1.70 0.0

M12A.102 0.650 324 318 2.0 15.0 14.9 0.8 1.65 1.65 0.0

M12A.104 0.720 252 251 0.5 16.6 16.5 0.8 1.75 1.75 0.0

M12A.106 0.500 224 220 1.4 11.6 11.5 0.8 1.70 1.70 0.0

M14.100 0.720 324 321 0.7 16.6 16.9 1.5 1.70 1.70 0.0

M14.102-R1 0.230 197 201 1.9 14.9 15.7 5.4 1.75 1.75 0.0

POND3 0.125 261 255 2.3 11.6 12.5 7.6 1.50 1.50 0.0

POND2 0.212 447 438 2.1 20.1 21.7 7.9 1.50 1.50 0.0

P0.100 0.571 281 276 1.7 12.6 12.8 1.2 1.65 1.65 0.0

P0.105 0.404 209 205 1.9 9.1 9.0 1.0 1.65 1.65 0.0

P0.130 0.175 109 113 3.4 4.0 3.9 1.2 1.55 1.60 3.2

P0.110 0.128 72 78 8.0 2.9 2.9 0.5 1.60 1.60 0.0

P0.115 0.069 54 57 6.0 1.5 1.5 1.2 1.50 1.55 3.3

P10.100 0.420 166 164 1.1 9.3 9.4 1.2 1.70 1.70 0.0

P0.120 0.117 74 77 4.1 2.6 2.6 1.2 1.55 1.55 0.0

Page 2 of 6

Table C-1





P0.125 0.109 76 81 7.0 2.4 2.4 1.2 1.55 1.55 0.0

P10A.100 0.430 211 208 1.7 9.5 9.6 1.2 1.65 1.65 0.0

P2.100 0.152 79 84 6.1 3.4 3.4 1.2 1.60 1.60 0.0

P2.110 0.108 67 70 4.3 2.4 2.4 1.2 1.55 1.55 0.0

P2.105 0.078 62 65 5.5 1.7 1.7 1.2 1.50 1.55 3.3

P2.115 0.087 62 68 8.7 1.9 1.9 1.2 1.55 1.55 0.0

P2.120 0.152 85 91 7.0 3.4 3.4 1.2 1.60 1.60 0.0

P2.125 0.135 88 91 3.5 3.0 3.0 1.2 1.55 1.55 0.0

P10.102 0.770 253 252 0.3 17.0 17.2 1.2 1.75 1.75 0.0

P10C.100 0.510 199 197 1.0 11.3 11.4 1.2 1.70 1.70 0.0

P10.104 0.360 163 159 2.2 8.0 8.1 1.2 1.65 1.65 0.0

P4.100 0.184 70 75 7.0 4.1 4.1 1.2 1.70 1.70 0.0

P4.135 0.112 58 63 8.6 2.5 2.5 1.2 1.60 1.60 0.0

P4.155 0.113 68 72 5.9 2.5 2.5 1.2 1.55 1.60 3.2

P4.105 0.323 161 158 1.8 7.2 7.2 1.2 1.65 1.65 0.0

P4.130 0.118 67 73 8.4 2.6 2.64 1.2 1.55 1.60 3.2

P4.140 0.049 34 37 10.0 1.1 1.1 1.2 1.55 1.55 0.0

P4.110 0.217 102 106 4.1 4.8 4.9 1.2 1.65 1.65 0.0

P4.115 0.360 172 169 1.9 8.0 8.1 1.2 1.65 1.65 0.0

P4.120 0.143 97 101 4.0 3.2 3.2 1.2 1.55 1.55 0.0

P4.125 0.181 102 107 5.1 4.0 4.1 1.2 1.60 1.60 0.0

P4.145 0.064 41 44 7.8 1.4 1.4 1.2 1.55 1.55 0.0

P4.150 0.168 97 102 6.1 3.7 3.8 1.2 1.60 1.60 0.0

P4.160 0.272 162 162 0.1 6.0 6.1 1.2 1.60 1.60 0.0

P4.165 0.238 138 141 1.7 5.3 5.3 1.2 1.60 1.60 0.0

Page 3 of 6

Table C-1





P4.170 0.298 168 164 2.6 6.6 6.7 1.2 1.60 1.60 0.0

P4.175 0.183 102 107 4.9 4.1 4.1 1.2 1.60 1.60 0.0

P10B.100 0.370 140 138 1.2 8.2 8.3 1.2 1.70 1.70 0.0

P10.106 0.720 190 191 0.6 15.9 16.1 1.2 1.85 1.85 0.0

P6.140 0.099 69 75 7.8 2.2 2.2 1.2 1.55 1.55 0.0

P6.145 0.101 57 63 9.4 2.2 2.3 1.2 1.55 1.60 3.2

P6.170 0.143 96 99 3.8 3.2 3.2 1.2 1.55 1.55 0.0

P6.100 0.252 140 140 0.2 5.6 5.6 1.2 1.60 1.60 0.0

P6.105 0.201 140 140 0.5 4.5 4.5 1.2 1.55 1.55 0.0

P6.115 0.179 120 121 1.2 4.0 4.0 1.2 1.55 1.55 0.0

P6.125 0.123 80 84 4.3 2.7 2.8 1.2 1.55 1.55 0.0

P6.135 0.098 66 70 6.8 2.2 2.2 1.2 1.55 1.55 0.0

P6.110 0.110 87 91 3.9 2.4 2.5 1.2 1.50 1.55 3.3

P6.120 0.212 140 138 1.3 4.7 4.7 1.2 1.55 1.55 0.0

P6.130 0.226 131 135 2.6 5.0 5.1 1.2 1.60 1.60 0.0

P6.150 0.186 104 109 4.8 4.1 4.2 1.2 1.60 1.60 0.0

P12.100 0.830 385 377 2.1 18.4 18.6 1.2 1.65 1.65 0.0

P6.155 0.105 61 66 7.5 2.3 2.4 1.2 1.55 1.60 3.2

P6.160 0.057 41 45 10.1 1.3 1.3 1.2 1.55 1.55 0.0

P6.180 0.066 52 55 6.1 1.5 1.5 1.2 1.50 1.55 3.3

P12.102 0.910 290 289 0.4 20.1 20.4 1.2 1.80 1.80 0.0

P12.104 0.960 295 295 0.1 21.3 21.5 1.2 1.80 1.80 0.0

P12A.100 0.840 346 342 1.1 18.6 18.8 1.2 1.70 1.70 0.0

P14.100 0.550 220 219 0.6 16.1 16.1 0.1 1.85 1.80 2.7

P14.102 0.770 359 355 1.0 22.9 23.0 0.5 1.75 1.75 0.0

Page 4 of 6

Table C-1





M16.100 0.330 334 340 1.5 16.9 17.6 3.9 1.65 1.65 0.0

POND1 0.184 384 375 2.1 17.1 18.4 7.6 1.50 1.50 0.0

M16.102 0.440 283 288 1.7 18.8 19.7 4.7 1.70 1.70 0.0

M18.100 0.470 207 204 1.3 10.9 10.8 0.8 1.70 1.70 0.0

M18A.100 0.280 142 142 0.1 6.5 6.4 0.8 1.65 1.65 0.0

M18B.100 0.390 191 186 2.3 9.0 8.9 0.8 1.65 1.65 0.0

M18.102 0.310 204 199 2.4 7.2 7.1 0.8 1.60 1.60 0.0

M18.104 0.490 270 261 3.3 11.3 11.2 0.8 1.60 1.65 3.1

M18C.100 0.730 221 222 0.2 16.9 16.7 0.8 1.85 1.80 2.7

M18.106 0.770 315 311 1.3 17.8 17.7 0.8 1.70 1.70 0.0

M18.108 0.850 384 392 2.0 35.3 36.7 4.1 1.90 1.85 2.6

M20.100 0.590 335 345 2.8 16.5 16.7 0.9 1.65 1.65 0.0

M20.102 0.990 327 340 3.9 26.3 26.4 0.5 1.85 1.80 2.7

M20A.100 1.420 743 753 1.4 61.0 63.6 4.3 1.80 1.80 0.0

M20.104 1.190 443 440 0.7 27.5 27.3 0.8 1.75 1.75 0.0

M20C.100 0.380 164 162 1.6 8.4 8.5 1.2 1.65 1.70 3.0

M20.108 0.910 333 331 0.6 21.0 20.9 0.8 1.75 1.75 0.0

M20.110 0.920 466 480 3.2 29.6 30.4 2.9 1.70 1.70 0.0

M20.112 0.570 425 430 1.1 21.2 21.9 3.0 1.60 1.60 0.0

M20.114 1.280 998 1005 0.7 66.0 69.6 5.6 1.70 1.70 0.0

M20.116 0.450 497 497 0.0 32.3 34.3 6.3 1.65 1.65 0.0

M20.118 0.340 127 131 3.6 9.2 9.4 2.5 1.80 1.75 2.8

M20D.100 0.910 684 687 0.5 54.4 57.8 6.2 1.75 1.75 0.0

M20.120 0.680 423 422 0.3 39.1 41.3 5.9 1.85 1.85 0.0

L2.100 0.940 349 347 0.7 21.7 21.6 0.8 1.75 1.75 0.0

Page 5 of 6

Table C-1





L2.102 0.930 252 253 0.3 21.5 21.3 0.8 1.90 1.85 2.6

L2A.100 1.040 380 378 0.6 24.0 23.9 0.8 1.75 1.75 0.0

L2.104 0.240 123 126 2.1 5.5 5.5 0.8 1.65 1.65 0.0

L2C.100 0.640 244 242 0.9 14.8 14.7 0.8 1.75 1.75 0.0

L2.106 0.230 133 134 1.2 5.3 5.3 0.8 1.60 1.60 0.0

L2B.100 0.370 154 152 1.3 8.6 8.5 0.8 1.70 1.70 0.0

L2D.100 0.420 157 156 0.7 9.7 9.6 0.8 1.75 1.75 0.0

L2.107 0.340 265 269 1.8 10.2 10.3 1.5 1.55 1.60 3.2

L2A.108 0.490 321 328 2.3 17.4 18.0 3.4 1.65 1.65 0.0

L2A.110 0.590 192 192 0.3 13.6 13.5 0.8 1.80 1.80 0.0

Page 6 of 6

Table C-2

Summary comparison of subbasin results for the Black Watershed




101 0.331 163 160 2.3 7.44 7.40 0.5 1.65 1.65 0.0

102 0.258 134 134 0.6 5.80 5.80 0.0 1.60 1.65 3.1

103 0.267 166 173 4.5 7.95 8.10 1.9 1.60 1.65 3.1

104 0.227 124 131 6.1 6.68 6.80 1.9 1.65 1.65 0.0

105 0.154 180 183 1.4 9.27 9.80 5.7 1.60 1.60 0.0

106 0.466 389 392 0.8 19.34 20.10 3.9 1.60 1.60 0.0

107 0.037 55 56 0.4 1.99 2.10 5.4 1.50 1.55 3.3

108 0.067 97 86 11.3 4.57 4.60 0.6 1.55 1.55 0.0

109 0.089 119 121 1.4 6.05 6.40 5.8 1.60 1.60 0.0

110 0.051 79 80 1.5 3.48 3.70 6.3 1.55 1.55 0.0

111 0.115 167 167 0.1 7.85 8.30 5.7 1.55 1.55 0.0

112 0.073 113 115 1.7 4.97 5.30 6.6 1.55 1.55 0.0

113 0.047 77 77 0.6 2.86 3.00 5.0 1.50 1.55 3.3

114 0.071 102 105 2.2 4.32 4.50 4.3 1.55 1.55 0.0

115 0.114 143 147 2.4 6.93 7.30 5.3 1.60 1.60 0.0

116 0.032 23 25 10.0 0.72 0.70 2.7 1.55 1.55 0.0

117 0.126 77 81 5.7 2.83 2.80 1.1 1.55 1.60 3.2

118 0.237 105 107 2.1 5.33 5.30 0.5 1.65 1.65 0.0

119 0.086 48 53 10.5 1.93 1.90 1.7 1.60 1.60 0.0

120 0.218 118 125 5.4 5.14 5.20 1.2 1.60 1.60 0.0

120.1 0.263 129 135 5.1 6.77 6.90 1.9 1.65 1.65 0.0

150 0.079 49 52 6.0 1.78 1.80 1.4 1.55 1.55 0.0

151 0.150 83 89 6.7 3.37 3.40 0.8 1.60 1.60 0.0

152 0.183 87 92 6.0 4.11 4.10 0.3 1.65 1.65 0.0

153 0.165 76 81 7.1 3.71 3.70 0.2 1.65 1.65 0.0

Page 1 of 3

Table C-2





154 0.221 111 114 2.1 4.97 5.00 0.7 1.60 1.65 3.1

155 0.124 58 63 8.8 2.79 2.80 0.5 1.60 1.65 3.1

156 0.255 128 130 1.3 5.73 5.70 0.5 1.65 1.65 0.0

157 0.251 149 150 0.5 5.64 5.60 0.7 1.60 1.60 0.0

157.1 0.083 30 34 13.2 1.87 1.90 1.8 1.70 1.70 0.0

158 0.083 47 52 10.4 1.87 1.90 1.8 1.55 1.60 3.2

159 0.061 33 37 12.1 1.37 1.40 2.1 1.60 1.60 0.0

160 0.130 56 61 9.1 2.92 2.90 0.7 1.65 1.65 0.0

161 0.073 40 45 11.3 1.64 1.60 2.5 1.60 1.60 0.0

162 0.024 19 20 6.4 0.54 0.50 7.3 1.50 1.55 3.3

201 0.241 260 264 1.2 14.20 14.90 4.9 1.65 1.65 0.0

202 0.113 153 155 1.0 7.86 8.30 5.6 1.60 1.60 0.0

203 0.054 65 66 0.7 3.76 4.00 6.5 1.60 1.65 3.1

204 0.015 27 27 1.2 1.04 1.10 5.5 1.50 1.50 0.0

205 0.093 127 129 1.3 6.47 6.90 6.7 1.60 1.60 0.0

211 0.184 264 265 0.2 13.94 14.90 6.9 1.60 1.60 0.0

212 0.099 140 140 0.3 7.37 7.90 7.1 1.60 1.60 0.0

213 0.108 153 153 0.0 8.18 8.70 6.4 1.60 1.60 0.0

214 0.187 234 235 0.4 12.77 13.50 5.8 1.60 1.60 0.0

215 0.112 144 142 1.6 9.18 9.80 6.8 1.65 1.65 0.0

216 0.099 139 139 0.0 6.74 7.20 6.8 1.55 1.60 3.2

217 0.208 270 271 0.3 14.14 15.00 6.1 1.60 1.60 0.0

218 0.286 183 188 3.1 9.69 10.00 3.2 1.65 1.65 0.0

219 0.164 85 89 4.7 3.69 3.70 0.4 1.60 1.60 0.0

220 0.108 57 63 8.9 2.43 2.40 1.1 1.60 1.60 0.0

Page 2 of 3

Table C-2





250 0.054 23 27 17.3 1.27 1.30 2.2 1.65 1.65 0.0

251 0.162 77 82 7.1 3.64 3.60 1.1 1.65 1.65 0.0

252 0.110 37 43 15.6 2.71 2.70 0.4 1.75 1.75 0.0

253 0.072 38 42 11.1 1.62 1.60 1.1 1.60 1.60 0.0

254 0.057 34 37 8.1 1.28 1.30 1.5 1.55 1.60 3.2

255 0.110 51 56 9.8 2.47 2.50 1.1 1.60 1.65 3.1

256 0.049 31 33 7.2 1.10 1.10 0.1 1.55 1.55 0.0

257 0.064 51 54 6.4 1.44 1.40 2.7 1.50 1.55 3.3

300 0.147 151 153 1.3 9.60 10.20 6.2 1.65 1.65 0.0

310 0.078 148 146 1.4 5.94 6.30 6.1 1.50 1.50 0.0

320 0.074 102 104 1.6 5.17 5.50 6.4 1.60 1.60 0.0

330 0.063 101 103 2.3 4.40 4.70 6.9 1.55 1.55 0.0

340 0.047 78 79 2.1 3.28 3.50 6.9 1.50 1.55 3.3

350 0.048 35 40 11.3 1.56 1.60 2.5 1.60 1.60 0.0

360 0.016 21 22 2.8 0.77 0.80 3.3 1.50 1.55 3.3

400 0.004 6 6 0.0 0.16 0.20 22.6 1.50 1.55 3.3

410 0.034 54 54 1.2 2.37 2.50 5.4 1.55 1.55 0.0

420 0.051 45 50 9.6 1.43 1.50 4.7 1.50 1.55 3.3

430 0.005 10 10 1.2 0.37 0.40 8.5 1.50 1.50 0.0

440 0.012 21 21 1.2 0.80 0.80 0.0 1.50 1.50 0.0

Page 3 of 3

Appendix D

Watershed Model Channel Routing Comparison

Table D-1

Summary comparison of routing results for the Montoyas Watershed

Routing Routed Peak Q Peak Q Routed Time to Peak Routing Time Step

Reach AHYMO HEC-HMS Differential AHYMO HEC-HMS AHYMO HEC-HMS

ID cfs cfs % hours hours sec sec

RTM4.100 219 213 2.7 1.85 1.70 180 30.6

RTM4.102UP 302 365 21.0 2.20 1.85 180 31.9

RTM4.104UP 318 479 50.8 2.50 1.95 180 46.5

RTM4.106UP 332 573 72.4 2.65 2.05 180 46.1

RTM4.108UP 416 639 53.7 1.95 2.15 180 86.0

RTM4.110UP 475 697 46.6 2.45 2.35 180 87.3

RTM2.100UP 418 413 1.0 1.90 1.75 180 32.4

RTM2.102UP 505 573 13.3 2.10 1.80 180 46.2

RTM2.104UP 544 743 36.5 2.20 1.85 180 48.1

RTM2.106UP 575 904 57.2 2.25 1.90 180 44.2

RTM2.108UP 627 993 58.4 2.20 2.05 180 45.8

RTM2.110TOT 772 1,229 59.2 2.35 2.05 180 105.9

RTM2.112TOT 819 1,370 67.1 2.60 2.15 180 86.9

RTM4.112TOT 1,202 2,100 74.7 2.75 2.20 180 103.6

RTM6.100UP 1,198 2,147 79.2 3.40 2.45 180 88.3

RTM8.100 161 159 1.4 2.10 1.85 180 31.3

RTM8.102UP 222 286 29.0 2.70 2.10 180 45.5

RTM8A.100 254 258 1.6 2.50 2.00 180 31.3

RTM8A.102UP 273 409 49.7 3.35 2.30 180 46.1

RTM8.104 507 790 55.8 3.30 2.60 180 93.3

RTM8C.100 251 255 1.5 2.65 2.05 180 30.8

RTM8C.102UP 309 419 35.3 2.10 2.25 180 96.5

RTM8.106TOT 760 1,297 70.7 3.35 2.40 180 84.2

RTM8.108TOT 1,941 3,502 80.5 3.70 2.55 180 98.3

RTM12.100 307 314 2.1 2.50 2.00 180 31.5

RTM12.102UP 327 502 53.5 3.45 2.35 180 89.3

RTM12a.102UP 561 569 1.4 2.50 2.00 180 31.8

RTM12a.106UP 613 832 35.7 3.05 2.30 180 92.0

RTM10.100TOT 2,357 4,556 93.3 3.70 2.60 180 82.0

RTM14.100UP 2,364 4,553 92.6 3.90 2.70 180 99.9

RTP0.100 272 271 0.2 2.05 1.80 180 31.2

RTP0.105 325 414 27.2 2.15 1.85 180 45.2

RTP0.110 71 77 9.1 1.80 1.65 180 31.8

RTP0.115 82 117 42.5 2.05 1.70 180 31.4

RTAPP0.130TO 399 554 38.8 2.60 2.00 180 48.0

RTP0.120 72 76 5.3 1.75 1.65 180 22.8

RTP0.125UP 106 146 38.5 2.25 1.80 180 31.1

RTP2.100 76 82 8.7 2.00 1.75 180 31.2

Page 1 of 3

Table D-1





RTP2.105 60 64 5.8 1.80 1.65 180 22.6

RTP2.110 124 221 78.9 1.70 1.75 180 30.9

RTP2.120 185 285 53.4 1.85 1.75 180 31.7

RTP2.125UP 209 336 60.9 2.30 1.85 180 30.4

RTP10.100TOT 670 1,129 68.6 2.60 2.20 180 47.3

RTP10C.100UP 778 1,360 74.7 2.60 2.25 180 89.1

RTP4.100 69 74 7.5 2.00 1.85 180 30.7

RTP4.135 84 110 30.0 2.30 1.95 180 46.6

RTP4.105 160 156 2.3 1.80 1.70 180 30.4

RTP4.130 199 211 6.1 1.80 1.75 180 32.4

RTP4.115 169 167 1.1 1.80 1.70 180 33.4

RTP4.110 302 336 11.4 1.80 1.70 180 31.7

RTP4.125 366 419 14.7 1.85 1.75 180 30.4

RTP4.145 569 667 17.1 1.95 1.80 180 43.7

RTP4.150 661 823 24.4 2.00 1.80 180 48.2

RTP4.160 687 909 32.3 2.15 1.85 180 52.1

RTP4.165 691 970 40.4 2.25 1.90 180 45.0

RTP4.170 701 1,031 47.1 2.35 1.95 180 46.8

RTP4.175UP 689 1,066 54.8 2.95 2.15 180 90.6

RTP10B.100UP 681 1,117 64.1 3.25 2.25 180 99.1

RTP10.104TOT 1,217 2,492 104.8 3.70 2.45 180 93.1

RTP6.140 68 71 5.5 1.75 1.65 180 23.0

RTP6.145 110 130 17.5 1.75 1.65 180 33.5

RTP6.100 139 137 1.2 1.70 1.70 180 30.0

RTP6.105 253 243 4.0 1.85 1.75 180 31.7

RTP6.115 277 316 14.1 1.95 1.80 180 31.0

RTP6.125 286 351 22.6 2.05 1.85 180 44.2

RTP6.110 85 90 5.5 1.80 1.65 180 23.3

RTP6.120 139 212 52.9 2.10 1.70 180 30.6

RTP6.130 443 595 34.5 2.20 1.85 180 49.6

RTP6.150TOT 473 759 60.4 2.55 1.95 180 45.3

RTP6.155 61 65 7.3 1.65 1.65 180 29.4

RTP6.160 39 44 12.0 1.80 1.60 180 22.9

RTP6.180TOT 100 149 48.1 2.25 1.80 180 31.0

RTP12.100TOT 584 1,118 91.5 3.00 2.10 180 46.4

RTP12.102UP 609 1,304 114.1 3.10 2.15 180 80.7

RTP12A.100 323 337 4.4 2.75 2.10 180 31.6

RTP10.106TOT 1,872 3,819 104.0 3.55 2.50 180 89.6

Page 2 of 3

Table D-1





RTM14.102TOT 4,070 7,964 95.7 3.80 2.70 180 73.2

RTM16.100UP 4,075 7,976 95.8 3.90 2.75 180 100.8

RTM18.100UP 338 339 0.4 2.00 1.80 180 32.2

RTM18.102TOT 461 604 31.1 2.15 1.85 180 49.5

RTM18.104UP 538 754 40.2 1.90 1.90 180 48.4

RTM18C.100UP 972 1,172 20.6 2.10 2.00 180 50.9

RTM16.102TOT 4,164 8,372 101.0 4.00 2.80 180 103.3

RTM20.100TOT 4,168 8,373 100.9 4.05 2.90 180 102.7

RTM20.102TOT 4,208 8,533 102.8 4.20 2.90 180 89.3

RTM20.104TOT 4,219 8,582 103.4 4.40 3.00 180 87.4

RTM20.108UP 4,226 8,587 103.2 4.40 3.00 180 47.3

RTM20.110UP 4,230 8,594 103.2 4.60 3.10 180 90.3

RTM20.112UP 4,233 8,574 102.5 4.65 3.15 180 91.1

RTM20.114UP 4,246 8,581 102.1 4.70 3.15 180 55.9

RTM20.116UP 4,250 8,562 101.5 4.80 3.25 180 88.9

RTM20.118TOT 4,409 8,769 98.9 2.85 3.30 180 87.4

RTM20D.100 668 682 2.1 2.35 2.00 180 92.6

RTL2.100 335 344 2.7 2.60 2.05 180 45.9

RTL2.102TOT 608 797 31.0 2.10 2.15 180 90.4

RTL2.104TOT 753 925 22.8 2.35 2.30 180 86.7

RTL2B.100 152 151 0.9 2.15 1.90 180 30.6

RTL2.106TOT 908 1,138 25.3 2.50 2.05 180 103.7

RTL2.107UP1 922 1,188 28.9 2.55 2.10 180 105.1

RTL2.107UP2 921 1,171 27.1 2.65 2.20 180 92.8

RTL2A.1081 318 324 2.0 1.75 1.75 180 85.0

RTL2A.1082 311 317 2.0 1.85 1.80 180 96.1

RTTOTFLOW 5,577 9,250 65.9 2.90 3.30 180 163.0

Page 3 of 3

Table D-2

Summary comparison of routing results for the Black Watershed




101.9 163 156 4.2 1.70 1.65 180 37.3

102.91 289 282 2.7 1.70 1.70 180 31.9

104.91 555 554 0.3 1.80 1.80 180 95.6

105.91 685 680 0.8 1.80 1.80 180 44.8

106.91 929 961 3.5 1.85 1.80 180 55.8

108.92 1,066 1,157 8.6 1.90 1.80 180 82.4

110.91 195 196 0.1 1.60 1.60 180 55.3

111.91 1,127 1,237 9.8 1.95 1.85 180 75.5

112.91 339 408 20.6 1.80 1.65 180 84.5

113.91 274 308 12.3 1.70 1.65 180 83.9

114.9 101 102 0.6 1.70 1.60 180 86.7

117.92 1,415 1,590 12.4 1.95 1.80 180 134.2

118.9 102 105 3.2 2.05 1.75 180 30.7

119.91 1,427 1,618 13.4 2.05 1.80 180 93.9

150.9 48 52 6.8 1.95 1.75 180 22.6

151.9 82 86 4.8 1.70 1.65 180 35.3

152.91 89 133 48.8 1.90 1.70 180 30.8

152.93 165 206 24.8 1.75 1.70 180 61.3

153.9 76 81 6.4 1.70 1.70 180 31.9

153.92 237 281 18.5 1.85 1.75 180 47.9

154.91 298 368 23.1 1.90 1.80 180 43.8

156.9 128 128 0.5 1.85 1.75 180 30.7

157.91 193 235 21.9 1.85 1.75 180 31.4

158.91 47 51 9.4 1.70 1.65 180 28.8

158.93 2,295 2,668 16.2 2.05 1.85 180 37.7

159.92 2,279 2,660 16.7 2.00 1.85 180 99.0

160.92 2,456 2,915 18.7 2.05 1.85 180 33.5

160.94 2,464 2,933 19.1 2.05 1.90 180 31.1

201.9 258 263 1.7 1.70 1.65 180 69.7

202.9 150 153 1.7 1.85 1.70 180 100.8

203.91 547 606 10.7 1.95 1.75 180 101.2

205.91 366 396 8.3 1.75 1.65 180 47.5

211.91 260 259 0.4 1.75 1.65 180 89.1

213.91 149 151 1.2 1.60 1.60 180 23.8

214.9 230 234 1.6 1.75 1.65 180 90.2

215.9 144 140 2.7 1.70 1.70 180 92.0

217.91 1,099 1,520 38.2 1.95 1.80 180 102.1

218.91 1,206 1,646 36.5 1.95 1.85 180 79.6

219.91 1,232 1,673 35.8 2.15 1.95 180 92.0

250.9 23 27 18.5 1.85 1.70 180 91.6

251.92 341 363 6.4 1.75 1.70 180 37.8

252.9 37 43 15.0 2.15 1.90 180 85.9

253.9 37 42 12.5 1.75 1.65 180 33.0

253.92 393 431 9.6 1.90 1.75 180 50.4

254.91 361 400 10.8 1.80 1.70 180 32.8

Page 1 of 2

Table D-2





255.91 417 477 14.5 1.95 1.75 180 75.3

256.91 1,542 2,086 35.3 2.10 1.95 180 51.9

300.1 151 152 0.4 1.70 1.70 180 90.7

300.2 151 151 0.0 1.70 1.70 180

320.1 92 94 2.1 1.60 1.60 180

320.4 86 87 0.9 1.65 1.65 180

350.1 35 38 8.6 1.60 1.65 180

350.2 28 31 12.5 1.75 1.75 180

360.99 387 335 13.4 2.00 1.80 180

400.3 125 128 1.8 2.05 2.10 180

400.4 125 128 1.9 2.05 2.10 180

410.3 144 150 4.0 1.70 1.70 180

430.3 170 97 43.0 1.60 1.80 180

430.4 166 97 41.9 1.60 1.80 180

440.2 14 14 2.1 1.60 1.60 180

440.3 13 13 1.1 1.65 1.65 180

440.5 24 27 9.1 1.65 1.65 180440.6 23 25 9.1 1.70 1.70 180

Page 2 of 2

Table D-3


Routing time step of 0.01 hours

Sediment bulking

Modlified AHYMO executable

Routing Routed Peak Q Peak Q Routed Time to Peak

Reach AHYMO HEC-HMS Differential AHYMO HEC-HMS

ID cfs cfs % hours hours

RTM4.100 264 254 3.6 1.70 1.70

RTM4.102UP 453 428 5.7 1.90 1.85

RTM4.104UP 591 582 1.6 2.00 1.90

RTM4.106UP 681 686 -0.7 2.10 2.00

RTM4.108UP 743 775 -4.4 2.20 2.10

RTM4.110UP 785 854 -8.7 2.10 2.30

RTM2.100UP 496 489 1.4 1.75 1.75

RTM2.102UP 697 681 2.3 1.80 1.80

RTM2.104UP 902 883 2.1 1.80 1.85

RTM2.106UP 1,094 1,093 0.1 1.85 1.85

RTM2.108UP 1,190 1,208 -1.5 1.95 2.00

RTM2.110TOT 1,516 1,504 0.8 2.00 2.05

RTM2.112TOT 1,682 1,675 0.4 2.10 2.10

RTM4.112TOT 2,494 2,587 -3.7 2.15 2.20

RTM6.100UP 2,526 2,648 -4.8 2.40 2.40

RTM8.100 192 189 1.2 2.00 1.85

RTM8.102UP 358 345 3.6 2.20 2.05

RTM8a.100 310 305 1.5 2.10 1.95

RTM8a.102UP 481 463 3.8 2.40 2.25

RTM8.104 953 967 -1.5 2.40 2.55

RTM8c.100 307 298 3.1 2.10 2.00

RTM8c.102UP 486 496 -2.0 2.25 2.20

RTM8.106TOT 1,577 1,590 -0.9 2.40 2.35

RTM8.108TOT 4,138 4,314 -4.3 2.50 2.50

RTM12.100 373 367 1.8 2.10 2.00

RTM12.102UP 537 514 4.4 2.50 2.35

RTM12a.102UP 690 678 1.6 2.10 2.00

RTM12a.106UP 985 982 0.3 2.30 2.25

RTM10.100TOT 5,271 5,603 -6.3 2.60 2.50

RTM14.100UP 5,263 5,652 -7.4 2.75 2.65

RTP0.100 328 322 1.8 1.90 1.80

RTP0.105 507 489 3.5 1.90 1.85

RTP0.110 85 91 -7.8 1.65 1.65

RTP0.115 127 139 -9.3 1.70 1.70

RTAPP0.130TO 647 652 -0.7 2.05 2.00

RTP0.120 84 89 -6.1 1.60 1.60

RTP0.125UP 151 175 -15.9 2.00 1.80

RTP2.100 93 96 -3.8 1.90 1.75

RTP2.105 70 75 -6.1 1.75 1.65

RTP2.110 231 265 -14.8 1.90 1.70

RTP2.120 291 347 -19.5 1.65 1.75

RTP2.125UP 348 412 -18.6 1.95 1.85

RTP10.100TOT 1,294 1,384 -6.9 2.25 2.15

RTP10C.100UP 1,532 1,673 -9.2 2.30 2.20

RTP4.100 82 87 -6.4 1.85 1.85

RTP4.135 123 130 -5.9 1.90 1.90

Page 1 of 3

Table D-3



Sediment bulking





RTP4.105 188 185 1.9 1.70 1.70

RTP4.130 249 250 -0.6 1.70 1.75

RTP4.115 201 197 1.5 1.70 1.70

RTP4.110 396 400 -1.0 1.70 1.70

RTP4.125 485 497 -2.4 1.70 1.75

RTP4.145 777 788 -1.4 1.75 1.80

RTP4.150 949 989 -4.2 1.80 1.80

RTP4.160 1,033 1,091 -5.6 1.85 1.85

RTP4.165 1,096 1,163 -6.1 1.90 1.90

RTP4.170 1,174 1,249 -6.4 1.90 1.90

RTP4.175UP 1,201 1,290 -7.4 2.15 2.10

RTP10b.100UP 1,258 1,356 -7.8 2.25 2.20

RTP10.104TOT 2,784 3,055 -9.7 2.50 2.35

RTP6.140 80 84 -3.9 1.60 1.65

RTP6.145 144 153 -6.8 1.65 1.65

RTP6.100 162 161 0.4 1.65 1.70

RTP6.105 290 286 1.4 1.70 1.75

RTP6.115 367 375 -2.3 1.75 1.80

RTP6.125 404 420 -3.8 1.85 1.85

RTP6.110 99 105 -6.5 1.75 1.65

RTP6.120 247 256 -3.7 1.80 1.70

RTP6.130 672 734 -9.1 1.90 1.85

RTP6.150TOT 829 935 -12.8 1.95 1.90

RTP6.155 72 77 -6.9 1.65 1.65

RTP6.160 47 52 -10.2 1.65 1.60

RTP6.180TOT 173 179 -3.4 2.05 1.80

RTP12.100TOT 1,250 1,372 -9.8 2.20 2.05

RTP12.102UP 1,469 1,597 -8.7 2.25 2.10

RTP12A.100 404 396 1.9 2.05 2.05

RTP10.106TOT 4,204 4,758 -13.2 2.55 2.45

RTM14.102TOT 8,567 9,858 -15.1 2.75 2.65

RTPOND1 152 187 -22.8 1.80 1.75

RTM16.100UP 8,572 9,891 -15.4 2.85 2.70

RTM18.100UP 405 400 1.2 1.80 1.80

RTM18.102TOT 732 715 2.4 1.85 1.85

RTM18.104UP 915 914 0.1 1.95 1.90

RTM18c.100UP 1,446 1,424 1.6 2.00 1.95

RTM16.102TOT 8,914 10,428 -17.0 2.90 2.75

RTM20.100TOT 8,902 10,421 -17.1 3.00 2.80

RTM20.102TOT 9,007 10,636 -18.1 3.05 2.85

RTM20.104TOT 9,044 10,705 -18.4 3.15 2.90

RTM20.108UP 9,037 10,723 -18.7 3.15 2.90

RTM20.110UP 9,055 10,724 -18.4 3.25 3.00

RTM20.112UP 9,052 10,711 -18.3 3.30 3.05

RTM20.114UP 9,070 10,748 -18.5 3.30 3.05

RTM20.116UP 9,075 10,734 -18.3 3.45 3.15

Page 2 of 3

Table D-3



Sediment bulking





RTM20.118TOT 9,268 10,944 -18.1 3.45 3.15

RTM20d.100 803 800 0.3 2.00 2.00

RTL2.100 408 404 1.0 2.15 2.05

RTL2.102TOT 984 962 2.3 2.20 2.10

RTL2.104TOT 1,133 1,134 -0.1 2.00 2.25

RTL2B.100 180 177 1.4 2.05 1.90

RTL2.106TOT 1,384 1,351 2.4 2.10 2.05

RTL2.107UP1 1,430 1,418 0.8 2.15 2.05

RTL2.107UP2 1,407 1,402 0.3 2.25 2.15

RTL2a.1081 374 380 -1.8 1.75 1.75

RTL2a.1082 369 375 -1.6 1.80 1.80

RTTOTFLOW 9,702 11,641 -20.0 3.50 3.20

Page 3 of 3

Table D-4



Sediment bulking





101.9 189 183 3.4 1.65 1.65

102.91 340 331 2.6 1.70 1.70

104.91 639 641 -0.3 1.75 1.80

105.91 766 778 -1.6 1.80 1.80

106.91 1,087 1,093 -0.5 1.75 1.80

110.91 206 210 -1.7 1.60 1.60

108.92 1,290 1,318 -2.2 1.80 1.80

111.91 1,370 1,399 -2.1 1.80 1.80

114.9 107 109 -1.6 1.60 1.60

113.91 320 331 -3.4 1.60 1.65

112.91 420 439 -4.5 1.65 1.65

117.92 1,714 1,779 -3.8 1.80 1.80

119.91 1,732 1,808 -4.4 1.85 1.85

118.9 121 124 -2.2 1.75 1.75

300.1 160 162 -1.6 1.70 1.50

300.2 159 162 -1.6 1.72 1.60

320.4 88 90 -2.6 1.63 1.55

400.3 128 131 -2.1 2.07 2.10

400.4 128 131 -2.1 2.08 2.10

410.3 148 155 -4.7 1.69 1.60

320.1 96 102 -6.3 1.59 1.60

430.3 177 187 -5.6 1.58 1.65

430.4 171 182 -6.1 1.61 1.50

440.2 16 18 -9.7 1.61 1.60

440.3 15 17 -10.4 1.63 1.65

440.5 28 31 -10.4 1.63 1.65

440.6 26 30 -12.2 1.67 1.70

350.1 39 43 -9.8 1.61 1.55

350.2 30 35 -15.9 1.72 1.70

360.99 418 443 -5.9 1.74 1.60

153.9 88 94 -7.2 1.70 1.70

150.9 55 61 -9.6 1.70 1.70

152.91 151 158 -4.6 1.70 1.70

151.9 96 100 -4.8 1.60 1.65

152.93 231 244 -5.5 1.70 1.70

153.92 313 333 -6.4 1.75 1.75

154.91 413 435 -5.2 1.80 1.75

159.92 2,943 3,081 -4.7 1.82 1.85

158.91 54 60 -10.5 1.60 1.85

158.93 2,974 3,103 -4.3 1.84 1.80

156.9 149 150 -0.7 1.75 1.70

157.91 278 278 0.0 1.75 1.65

160.92 3,290 3,402 -3.4 1.84 1.90

160.94 3,310 3,402 -2.8 1.85 1.60

201.9 279 281 -0.8 1.65 1.70

Page 1 of 2

Table D-4



Sediment bulking





205.91 417 425 -2.0 1.65 1.60

202.9 161 163 -1.4 1.70 1.65

203.91 636 649 -2.0 1.75 1.65

213.91 162 162 -0.2 1.60 1.65

214.9 247 250 -0.8 1.65 1.70

215.9 152 150 1.4 1.70 1.65

217.91 1,600 1,634 -2.1 1.80 1.65

218.91 1,739 1,782 -2.5 1.85 1.85

219.91 1,759 1,816 -3.2 1.90 1.90

250.9 27 32 -17.5 1.70 1.65

211.91 279 280 -0.5 1.65 1.75

251.92 387 400 -3.3 1.70 1.90

252.9 44 50 -14.9 1.90 1.60

254.91 416 447 -7.3 1.70 1.70

253.9 44 49 -11.3 1.65 1.75

253.92 452 482 -6.8 1.70 1.70

255.91 504 539 -7.0 1.75 1.75256.91 2,173 2,273 -4.6 1.95 1.95

Page 2 of 2

Appendix E

Digital Files

Documents

Technical Documentation for use of HEC-HMS with …sscafca.org/development/documents/DPM/Technical...Technical Documentation for use of HEC-HMS with the Development Process Manual