HYDROLOGIC ENGINEERING USING THE HEC-HMS MODULE

1 - Theory and Background

This section consists of a teaching module for undergraduate environmental or civil engineering students.

The module is structured around three illustrative examples. The students are asked to determine the

flood potential of a watershed site located in New Jersey. A step-by-step procedure is presented in the

module in which students are directed to determine rainfall runoff, precipitation, interception/infiltration,

unit hydrograph, and flood routing. Three illustrative examples are given in this module. The first example

is the determination of a 100-year flood hydrograph for a New Jersey site. In the second example, the

students are instructed to route the hydrograph determined in the first example through a given

reservoir/spillover structure. In the third example the first two examples are repeated by dividing the same

watershed into multiple watersheds.

A computer program, "Hydrologic Modeling Systems" (HEC-HMS), developed by the U.S. Army Corps of

Engineers is used in all of the three examples. This program is used in engineering practice to determine

the drainage characteristics of both rural and urban watersheds. The use of this program is given in this

module with instruction on how to prepare the input for the three illustrative examples and the output is

interpreted. A listing of the program input and output is also given in the module.

Hydrologic Modeling System (HEC-HMS) is new generation software for precipitation runoff simulation

that will supersede the HEC-1 Flood Hydrograph Package. HEC-HMS was developed by the U.S. Army

Corps of Engineers and is a Windows version of HEC-1 with significant advances in computer science

and hydrologic engineering.

HEC-HMS contains most of the HEC-1 capabilities, such as flow-frequency curve analysis, snow

accumulation and melt. Hydraulic features of dam capabilities are underway but have not yet been

incorporated. The flood damage analysis will be performed by HEC-FDA software and are not included in

HEC-HMS.

The HEC-HMS computer model has a large number of options, such as multiple basin watersheds, flood

damage analysis, etc. The Soil Conservation Service (SCS) TR 55 approach to the determination of

interception/infiltration and unit hydrographs will be used (TR 55 (1986)). This approach is commonly used

for urban watersheds by the U.S. Army Corps of Engineers.

MODEL USERS BACKGROUND

A description of the HEC-HMS model and its use is given in site examples. There will be little instruction on

the hydrology to determine the computer model input parameters. The user should have taken a first course

in Water Resource Engineering where hydrologic and hydraulic techniques are discussed. A number of

references (Linsley, et. al. (1992), Viessman, et. al. (1989), Hoggan (1989)) are given at the end of this

module for review purposes.

RAINFALL RUNOFF SIMULATION

Simple mathematical relationships are intended to represent model component functions such as

meteorological, hydrologic and hydraulic processes. These processes are divided into precipitation,

interception/infiltration, transformation of precipitation excess to sub-basin outflow, addition of base flow and

flood hydrograph routing. The HEC-HMS model has a number of options for these processes. Refer to the

illustrative examples for the use and application of each processes.

PRECIPITATION

The precipitation model used in the illustrative examples is the frequency-based design storm. This is the PH

record in HEC-1 – hypothetical storms. The 100-year storm frequency is used for all the examples on an

exceed probability of 1 percent. The storm size is the same as the basin area. The series type is either

annual or partial. An annual series has been selected. The duration of the maximum intensity is the smallest

time entered in the duration precipitation list (5 min.) and the storm duration is the longest (24 hours). The

values of the precipitation for the 5 and 15 minute, and 1, 2, 3, 6, 12, and 24 hour 100 year storm was

obtained from charts. By entering the chart in references NOAA (1977) and TP 40 (1961) with the latitude of

41°00’ and the longitude of 74°25’ of Stickle Pond, the precipitation in inches can be determined. Figure 1 is

a typical chart showing the 24 hour 100-year storm.

INTERCEPTION/INFILTRATION

In the HEC-HMS computer model, the land surface interception, depression storage, and infiltration are

referred to as loss rates. The Soil Conservation Service soil classification system will be used here. The

SCS has been able to relate the drainage characteristics of soil groups to a curve number, CN. These CN

values are based on experimentation and experience. The SCS provides information on relating soil group

type to the curve number as a function of soil cover, land use and antecedent moisture conditions at the

onset of a storm. Refer to the illustrative examples for the determination of a CN value.

UNIT HYDROGRAPH

The unit hydrograph technique is used in the runoff component of a rain event to transform rainfall excess to

outflow. A unit hydrograph can be directly input into HEC-HMS or a synthetic unit hydrograph can be

computed from user supplied parameters (TP 40 (1961), TR 55 (1986), Army Corps. Eng. (1990, 1998)).

The SCS synthetic dimensionless unit hydrograph method is used in the illustrative examples. A single

parameter, TLAG, is needed to determine this unit hydrograph. TLAG is equal to the time between the

center of the excess rain and peak of the unit hydrograph where,

TLAG = 0.6 x (Time of Concentration).

An important limitation on the HEC-HMS program is:

∆T < 0.29 x TLAG,

where, ∆T = computation time interval (HMS Control specifications. setup).

FLOOD ROUTING

Flood movement through river reaches and reservoirs is simulated by flood routing. Most of the flood-routing

methods available in HEC-HMS are based on the continuity equation and some relationship between flow

and storage (or stage). A storage versus output table must be provided for a reservoir/spillway as direct input

for the HEC-HMS model. The modified Puls routing method is used in Illustrative Example 2.

2 - Illustrative Examples: Stickle Pond

Three illustrative examples are given here. The first is a determination of the flood hydrograph for an actual

site, Stickle Pond, in New Jersey. The second example is to route this hydrograph through a

reservoir/spillway. A multiple watershed is given in the third example. In all cases, the actual site conditions

and the SCS technique will be used to determine the input parameters to the HEC-HMS computer model.

The examples given are common drainage problems in the determination of flood peaks and

reservoir/detention pond water level elevations for urban watersheds.

Stickle Pond (Lake Kinnelon) is located in Morris County, New Jersey at a latitude 41°00’ and longitude

74°25’. Its surface area is 124 acres, which accounts for 7.4% of the total 1683 acres of its watershed. Refer

to Figures 2 and 3.

The region of New Jersey in which the watershed is located is very rural. Few developed areas dot the site.

For a large part, this is due to the nature of the land - mostly steep rocky forest. Clearing trees, removing

boulders, and building on hillsides with the danger of erosion are all a hindrance to development. Also limiting

the development of the area is the presence of a swampy low region in the watershed.

To the north and east of the pond is a medium-duty and light-duty road system. Any development within the

watershed is located in this general area. Running around the lower or southern half of the pond is an

unimproved dirt road.

PROGRAM STRUCTURE

There are a series of steps to run HEC-HMS.

These steps are:

1. Start a new project; titles,

2. Basin model data; setup the basin network, watershed area, base flow, loss rates, unit hydrograph,

routing data, etc.,

3. Precipitation model data; type in values of the precipitation,

4. Control specifications; starting and ending dates and computation intervals,

5. Create and execute a run,

6. View; tables, graphs, and print results,

7. Exit.

These steps will be described in the Illustrative Examples.

PROGRAM LIMITATIONS AND INSTALLATION

The following is a list of some of the limitations of Version 2.0 HEC-HMS. Up-to-date information on HEC-

HMS execution, features, and limitations can be found on the HEC web site, http://www.hec.usace.army.mil.

The program can also be obtained from this website.

1. The importing of HEC-1 rainfall data files does not always work.

2. When printing tables or graphs from within HEC-HMS, output must be directed to a postscript printer.

3. The currently active screen can be copied to the Windows clipboard by pressing ALT+Print Screen. That

screen can be "posted" into a document.

These limitations should be corrected in later versions of HEC-HMS.

HEC-HMS INPUT PARAMETERS

To compute an SCS synthetic hydrograph for Stickle Pond the following parameters will be determined.

WATERSHED AREA

A topographic map is necessary to delineate a watershed for the study area and calculate its enclosed area.

Generally a detailed topographic map of the site does not exist. Therefore, U.S.G.S. quadrangle maps of the

site must be obtained. These maps may be purchased from map stores in large cities or from the federal

government (Geological Survey). Two quadrangle maps: Boonton, NJ, and Newfoundland, NJ, are

necessary to determine the contributing watershed for Stickle Pond.

Each quadrangle is identified with a name in the upper right hand corner of the map to help locate the sites.

The contour interval is printed at the bottom of the map. For the Boonton, NJ and Newfoundland, NJ

quadrangles, the contour interval is 20 feet.

Figure 2 is a topographic map of the site. The Stickle Pond watershed is delineated in Figure 3. The area of

this watershed may be determined with use of a planimeter.

HYDROLOGIC SOIL CHARACTERISTICS

The SCS curve number loss rate is used to determine the hydrologic soil characteristics for Stickle Pond

watershed. A soil characteristics map of the site is published in SCS books for each county (Morris County

for Stickle Pond (SCS (1981))). This book may be obtained from the SCS office in the state or county of

interest. The soil characteristics map for Stickle Pond watershed is shown in Figure 4. The name and area of

each of the soils making up this watershed are given in Table 1. A hydrologic classification of each soil may

be determined from reference TR 55 (1986).

Antecedent moisture condition (AMC) II was assumed for this analysis. AMC II should be selected unless

rain records of the site are available. The curve numbers in Table 2 apply for AMC II. For dry conditions

(AMC I) or wet conditions (AMC III), equivalent curve numbers can be computed (Veissman, Lewis and

Knapp (1989)).

PRECIPITATION

There are six methods of precipitation models available. The gage data is rarely available for small

watersheds and will not be used for the examples given. The frequency-based storm will be used in the

illustrative examples. This hypothetical storm will be automatically distributed according to the specified

depth/duration data. A triangular precipitation distribution is constructed such that the depth specified for any

duration occurs during the central part of the storm.

The required inputs are:

Exceed Probability; storm frequency, the 100-year storm is used for the examples or 1% probability

Storm Area; storm size will be the same as the watershed area.

Series Type; annual or partial.

Duration of Max Intensity; smallest input duration

Storm Duration; maximum input duration.

UNIT HYDROGRAPH

The SCS dimensionless unit hydrograph is used in the illustrative examples. The only parameter necessary

to obtain this unit hydrograph is the Time of Concentration (TC), which is determined by techniques given in

TR-55 (1986). For the Stickle Pond watershed, there are three components of TC: sheet flow, shallow

concentrated flow, and open channel flow. See Table 3 for details in the determination of these values.

EXAMPLE 1: FLOOD HYDROGRAPH

The flood hydrograph for the Stickle Pond watershed is determined in Example 1. As outlined above the

frequency based storm will be used to determine the precipitation data, SCS curve number for the loss

rate, and the SCS unit hydrograph. The determination of these parameters are given in Tables 1, 2 and

3.

EXAMPLE 2: FLOOD HYDROGRAPH ROUTING

The second example is to route a flood hydrograph through a reservoir/spillway complex. The same site,

Stickle Pond, is selected. Water is retained in Stickle Pond by a dam at its southeastern end. Two spillways

are available to prevent overtopping of the dam during periods of excess inflow to the reservoir, see Figure 5.

The main spillway (22.75 feet long) is spanned by a concrete walkway. Three piers support this walkway,

thus subtracting from the spillway length. A secondary spillway (a clear 12 foot long span) is at a higher

elevation, 792.05 feet; than the main spillway at 791.7 feet; so there is a range of water surface elevations

within which overflow occurs only over the main spillway.

The dam is breached (overtopped) when the reservoir water surface reaches an elevation of 792.4 feet. It is

assumed that this constitutes failure of the dam.

The hydrograph determined in Example 1 will be used as the input discharge into the reservoir. The

additional input parameters to compute the resulting reservoir volume and elevation and the output discharge

due to this flood hydrograph are:

FLOOD ROUTING

A storage versus output table is determined for the Stickle Pond reservoir/spillway complex. The Stickle

Pond spillway is shown in Figure 5. As in most routing procedures, the initial routing computation is for a

water elevation at the crest of the spillway. A volume versus elevation table is determined by calculating

the area of each contour enclosing the pond/reservoir. The first area is the area of the pond. See Table

4 for the determination of the storage volumes. The spillway output versus elevation, Table 5, is

determined by the use of the weir equation (King and Brater (1963)) for the Stickle Pond spillways. The

output versus elevation table has been calculated for elevations up to 796 ft. At elevation 792.4 feet, the

dam will overtop. Therefore, there is no need to continue the run. In order to avoid error messages the

dam was extended to an imaginary top of 796 feet even though the dam top is 792.4 feet.

EXAMPLE 3: MULTIPLE WATERSHEDS

Subdivision of the area of a large basin may be necessary because of the size and complexity of the physical

system. A basin with streams and/or a diverse topography may be divided into smaller components. In this

section, the model input preparation for a subdivided basin is presented. The Stickle Pond watershed is

divided as shown in Figure 6.

This watershed may be divided into approximately equal sub-watersheds or sub basins. The watershed

parameter for the sub basins are given in Table 6. These parameters have been described in examples 1

and 2. Since Stickle Pond is a small watershed, one precipitation distribution is assumed for both sub basins.

The base flow will be defined here as the flow which results from releases of water from subsurface storage.

The input parameters starting base flow discharge (ST), threshold discharge (QR), and rate of recession

index (RT) are used to model the base flow in the HEC-HMS model. ST represents the initial flow in the river,

RT is equal to the ratio of the recession limb flow to the recession limb flow occurring one hour earlier and QR

indicates the flow at which an exponential recession begins on the receding limb of the computed

hydrograph.

The base flow record is used to input the base flow into the HEC-HMS model. Using the Stickle Pond case,

there are three streams feeding into the reservoir, and hydrograph output is available from Examples 1 and 2

to estimate the base flow input. The parameters selected will be 5 cfs and 8 cfs for the values of ST for the

north and south sub basins, respectively, 0.05 for QR (i.e. 0.05 times the peak runoff, recommended in the

HEC-1 manual) and 0.9 for RT (a typical recession ratio) for both sub basins.

3 - References

1. Boonton Quadrangle, New Jersey-Morris County, and Newfoundland Quadrangle, New Jersey-Morris County; U.S. Geological Survey, Washington, D.C.

2. "Five to 60 Minute Precipitation Frequency For the Eastern and Central United States"; NOAA Technical Memorandum NWS HYRDO-35, 1977.

3. "HEC-1 Flood Hydrograph Package"; U.S. Army Corps of Engineers, Davis, California, 1990. 4. “HEC-HMS Hydrologic Modeling System,” U.S. Army Corps of Engineers, Davis, California, 1998 5. Hoggan, D.; "Computer Assisted Floodplain Hydrology and Hydaulics", McGraw-Hill, New York, 1989. 6. King, H and Brater, E., "Handbook of Hydraulics" McGraw-Hill, New York, 1963. 7. Linsley, R., Franzini, J., Freyberg, D., and Tchobanoglous, G.: “Water Resource Engineering”,

McGraw-Hill, New York, 1992. 8. "Rainfall Frequency Atlas of the United States, Technical Paper 40"; Department of Commerce;

Washington, D.C., 1961. 9. “Soil Survey of Morris County, New Jersey”; Department of Commerce; Washington, D.C., 1981. 10. "Urban Hydrology for Small Watersheds", Technical Release 55; Department of Commerce; Washington

D.C., 1986 11. Viessman, W., Lewis G., and Knapp, J.; "Introduction to Hydrology"; Harper and Row, New York, 1989.

TABLE 1

SOIL TYPES AND CURVE NUMBERS COMPUTATION SHEET

Soil Description Area* CN Weighted CN (Area x CN)

WATER Stickle Pond, Hoot Owl Lake, New Pond 28 100 2800 RsC Rockaway: Rock Outcrop 71.5 75 5362.5 RvF Rock Outcrop: Rockaway 31.8 90 2862 HbC Hibernia: Stony Loam 27.5 70 1925 RsD Rockaway: Rock Outcrop 23.25 89 2069.25 RgA Ridgebury: Very Stony Loam 18.5 80 1480 RpC Rockaway: Very Stony Sandy Loam 17.25 72 1242 RiB Ridgebury: Extremely Stony Loam 12 81 972 RrD Rockaway: Extremely Stony Sandy Loam 9.5 76 722 Cm Carisle Muck 7.75 82 635.5 Ad Adrian Muck 3 80 240 PeC Parker-Edneyville:Extremely Stony Sandy

Loam 2 70 140

Wm Whitman: Very Stony Loam 1.5 80 120 RsC Rockaway: Rock Outcrop 1.5 83 124.5 RpC Rockaway: Very Stony Sandy Loam 0.75 72 54

* Note: Area in arbitrary units Total = 255.8 Total = 20748.75 (Total Weighted CN)/(Total Area) = CN = 81.11

TABLE 2

RUNOFF CURVE NUMBERS

COVER DESCRIPTION CURVE NUMBERS FOR HYDROLOGIC SOIL GROUP

COVER TYPE HYDROLOGIC CONDITION

A B C D

Pasture, grassland, or range-- continuous forage Poor 68 79 86 89 Fair 49 69 79 84 Good 39 61 74 80 Meadow, continuous grass, protected from grazing and generally mowed for hay.

--- 30 58 71 78

Brush--brush-weed-grass mixture with brush the major element. Poor 48 67 77 83 Fair 35 56 70 77 Good 30 48 65 73 Woods--grass combination (orchard or tree farm). Poor 57 73 82 86 Fair 43 65 76 82 Good 32 58 72 79 Woods. Poor 45 66 77 83 Fair 36 60 73 79 Good 30 55 70 77 Farmsteads--buildings, lanes, driveways and surrounding lots. ---- 59 74 82 86

TABLE 3

COMPUTATION SHEET FOR TC, TIME OF CONCENTRATION

Because of the varied shape and topography of the Stickle Pond watershed, three paths of flow were

selected to determine TC, see Figure 2. These three paths are located in the top, middle and bottom of

the watershed. The TR-55 reference 1 is used to determine TC.

There are three components of TC:

1. Sheet flow TC1 = .007 (nl).8 / (P2).5 s.4

Where, n = 0.4 woods, L = 150 (L maximum of 300 feet, a smaller value is recommended),

P2 = 3.25 inches (2 year, 24 hour rain), S = slope of watershed at the divide.

2. Shallow concentrated flow TC2 = L / 3600v where, v = 16.134s1/2; s = water course slope.

3. Open channel flow TC3 = L / 3600v where, v = (1.49r2/3 s1/2 )/n

where n = .03 from site visit, TR-55 recommends .05; s = channel slope;

r = hydraulic radius = .4(TR55).

4. Total TC = TC1 + TC2 + TC3

Path S TC1 L (ft.) DH(ft.) S TC2 L (ft.) DH(ft.) S TC3 Total TC Top .060 .317 1600 80 .050 .123 4400 78.3 .0178 .339 .779 Middle .0313 .411 4900 125 .026 .523 2000 68.3 .0342 .111 1.045 Bottom .224 .187 3400 272 .080 .207 3200 19.3 .0060 .425 .819 The largest value of the time of travel is usually selected for TC. But because of the number of

assumptions made in selecting the parameters, an average TC will be used.

TC = 0.88 hours and, TLAG = 0.6 x TC = 0.53 hours

TABLE 4

STORAGE CAPACITY

Drainage area: 1682.5 ac Stickle Pond area: 124ac

Small islands in Pond: 2.8 ac (included in Pond area)

New Pond & Hoot Owl Lake: 18 ac Area of 800 ft. contour: 204 ac

Assuming linear spread from 791.7 Pond elevation to 800 ft. contour. Therefore,

204 - 124 = 9.639 ac/ft, and the volume = ∆Z (A1 + A2)

800 - 791.7 2

ELEVATION AREA VOLUME CUM. VOLUME (ft) (ac) (ac-ft) (ac-ft) 791.7 124.0 0.0 0.0 791.8 125.0 12.5 12.5 791.9 125.9 12.5 25.0 792.0 126.9 12.6 37.6 793.0 136.5 131.7 169.3 794.0 146.1 141.3 310.6 795.0 155.8 151.0 461.6 796.0 165.4 160.6 622.2

TABLE 5

SPILLWAY CAPACITY

Use the weir equation Q = CL H1.5

where Q = discharge over the spillway

L = length of spillway (adjusted for pier-edge effects)

H = water height above the spillway crest

Take C = 3.39 (H:V 5:1) (King and Brader (1963) Table 5.11)

1. Primary spillway Crest 791.7 ft. el.; length = 22.75 ft. with 3 piers

L = 22.75 - 0.8(H)

2. Secondary Crest 792.05 ft. el.; length = 12 ft.

L = 12 - 0.2H

3. Reservoir Crest 792.4 ft. el.

Primary Spillway Secondary Spillway Elevation H L Q H L Q Total Q [ft.] [ft.] [cfs] [ft.] [ft.] [cfs] [cfs] 791.7 0.0 22.75 0.00 0.00 12.00 0.00 0.00 792.0 0.3 22.51 12.54 0.00 12.00 0.00 12.54 793.0 1.3 21.71 109.09 0.95 11.81 37.07 146.16

794.0 2.3 20.91 247.26 1.95 11.61 107.17 354.43 795.0 3.3 20.11 408.68 2.95 11.41 195.98 604.66 796.0 4.3 19.31 583.69 3.95 11.21 298.33 882.02

TABLE 6

SUB BASIN CHARACTERISTICS

Sub Basin Tc (hours) Area (Square miles) CN 100 yr Precip (in) North 0.77 0.771 85.2 7.5 South 0.9245 1.859 79.4 7.5

Figure 1: One-hundred year, 24-hour rainfall

Figure 2: USGS Topographic Map of Stickle Pond

Figure 3: Stickle Pond Watershed & TC Computation Paths

A

A

5’-7” 6 ft. 1 in.5’-6”5’-7”

51 SECTION A - A

El. 791.7ft.

TOP DAM El. 792.4ft.

Figure 5: Diagram and Cross Section of Primary Spillway

Figure 4: Soil Survey of Stickle Pond Watershed

Figure 6: Stickle Pond Sub basins & TC Computation Paths

4 - Example 1: Flood Hydrograph

A flood hydrograph will be determined for a rural watershed in this example.

1. Start HEC-HMS by choosing HMS from the Programs item on the Windows Start Menu. The main

HEC-HMS windows will appear on screen (Figure 7).

2. Using the mouse select New Project from the File pull down menu in the main HEC-HMS window

(Figure 7).

Figure 7: HEC-HMS Main Window

3. Enter a name for the project and an optional description. The program will automatically choose a

location for the project files, although this may be changed by clicking the Directory Select button.

For this example, the name of the project is Stickle Pond. Press OK to continue, or Cancel to start

over.

4. The program defaults are selected during installation. This example uses the English system of units.

Verify that the English system is selected by choosing Default Preferences from the Options pull

down menu in the main HEC-HMS window, and selecting English in the Units section. Choose OK to

continue.

5. Choose Basin Model from the Component pull down menu in the main HEC-HMS windows, then

select New. The New Basin Model dialog box will appear on screen. Press OK to accept the default

name for the basin. Optionally, you may enter your own name and a description in this dialog box

before pressing OK. The HMS Basin Schematic window will appear on screen.

6. Create a subbasin by clicking on the Subbasin button, and while still holding down the mouse button,

dragging the cursor to the center of the Basin Schematic window and releasing the mouse button. A

subbasin icon will be created in the Basin Schematic window (Figure 8).

Figure 8: Basin Schematic Window

7. Edit the characteristics of the subbasin by double clicking the subbasin icon just created. The

Subbasin Editor window will appear (Figure 9). Enter 2.629 sq. mi. as the subbasin area. In the Loss

Rate tab, change the Method to SCS Curve No. Enter 0 for Initial Loss (assuming no loss for this

example) and % Impervious and 81.11 for SCS Curve No.

Figure 9: Subbasin Editor Window

8. Select the Transform tab in the Subbasin Editor window and change the Method to SCS. Enter 0.53

hours for the SCS Lag (change the Lag time units from the pull down menu). Choose OK to accept

and save the data. You will now be returned to the Basin Schematic window. The subbasin editor

will then prompt you to confirm the changes made to the old Loss Rate method. Click OK and then

continue.

9. Choose Close from the File pull down menu. You will be prompted to save changes. Accept the data

by choosing Save Changes. You will be returned to the main HEC-HMS window.

10. Select Meterelogical Model from the Component pull down menu in the main HEC-HMS windows,

then select New. The New Precipitation Model dialog box will appear on screen. Press OK to accept

the default name for the model. Optionally, you may enter your own name and a description in this

dialog box before pressing OK.

11. Next you will be prompted to confirm your Meterelogical Model selected from the subbasin distribution

window. This can be done by clicking OK in the pop up window. A Meterelogical window will now

appear. Select Frequency Storm from the Method pull down menu, then choose OK to continue. The

Precipitation Model data window will appear on screen (Figure 10). The elements of this window will

be substantially different for each of the different precipitation model methods (for example, Standard

Project Storm – Eastern US and User-specified Gage Weighting).

12. The storm used in this example has an Exceedence Probability of 1% (100-year storm), a storm area

equal to the basin area (2.629 sq. mi.), a 5 minute duration of maximum intensity and a storm

duration of 24 hours. For storms with an exceedence probability of less than 10%, the Series Type

may not be selected. For an exceedence probability of 10% or more, the user may select Annual or

Partial Duration series. The storm precipitation at the 5 and 15 minute and 1, 2, 3, 6, 12 and 24 hour

intervals are 0.77, 1.61, 3.05, 3.75, 4.3, 5.2, 6.7 and 7.5 inches, respectively. Enter this data as

shown in Figure 10. Precipitation cannot be entered for time periods less then the duration of

maximum intensity or greater than the storm duration. For example, if the storm duration is 12 hours

with a maximum intensity of 15 minutes, it would not be possible to enter data for 5 minutes or 24

hours or greater. Choose OK to accept and save the data. You will be returned to the main HEC-

HMS window.

13. Choose Control Specifications from the Component pull down menu in the main HEC-HMS windows,

then select New. The New Control Specifications dialog box will appear on screen. Press OK to

accept the default name for the model. Optionally, you may enter your own name and a description

in this dialog box before pressing OK.

14. The Control Specifications Setup dialog box will appear on screen (Figure 11). Enter the starting and

ending dates and times and a computation time interval. For this example, the starting date and time

is 1 Jun 1998, 00:00 and the ending date and time is 2 Jun 1998, 00:00. The computation interval

(∆T, Section 1) is 6 minutes. After entering the data, choose OK to continue. The computation time

interval must divide the time interval between the starting time and date and the ending time and date

by an integral number, or an error will result during computation and the run will stop. For example,

the 24 hour time interval used in this example is evenly divided by computation time intervals of 5

minutes (288 intervals), 6 minutes (240 intervals), 10 minutes (144 intervals), etc., but not by 7

minutes (205.7 intervals).

Figure 10: Completed Precipitation Data Window

Figure 11: Completed Control Specifications Dialog Box

15. You will be returned to the main HEC-HMS window. Save the project by selecting Save Project from

the File pull down menu.

16. To execute a run, double click on Basin 1 in the main window. The Basin Schematic window of

Figure 8 will appear. Choose the components to be analyzed by selecting Run Manager from the

Simulate pull down menu. The Run Manager dialog box will appear on screen (Figure 12). Click on

Run Configuration from the Simulate pull down menu and the Run Configuration dialog box will

appear (Figure 13). Choose Basin 1, Precip 1 and Control 1 from the Basin ID, Precip ID and Control

ID lists. The program automatically assigns a Run ID, however the user can optionally change the

Run ID. For this example, the default value of Run 1 will be kept. Click Apply and Close to accept

the components and return to the Simulation Manager dialog box (Figure 12). For this example, there

is only one basin model, one precipitation model and one control specification and the program

automatically selects these as defaults. Therefore, this step is performed automatically by the

program and is not actually necessary. However, the program is capable of storing more than one

basin model, precipitation model or control specification per project. The user can mix and match

among the different models, however each run MUST have one basin model, one precipitation model

and one set of control specifications selected.

17. Execute the run by clicking on the Compute button (Figure 12). The program will then perform the

analysis. The HMS Compute dialog box will appear, informing you of the progress of the run. When

complete, it will display “Compute Successful.” You can view a log of any errors, warnings and notes

by pressing View Log. Press Close to continue. You will be returned to the Simulation Manager

dialog box. Press Close again to be returned to the Basin Schematic window.

Figure 12: Simulation Manager Dialog Box

Figure 13: HMS Run Configuration Dialog Box

18. To view the computation results, click and hold the right mouse button while the cursor is positioned

over an element in the Basin Schematic window (for this example, there is only one element,

Subbasin 1). A pop-up menu will appear on screen. While still holding the right mouse button, move

the cursor down to View Results on the pop-up menu. A new pop-up menu will appear (Figure 14).

When this menu appears, release the right mouse button and move the cursor on to the new menu.

19. Select Graph from the pop up menu to view the hydrograph for the subbasin. The hydrograph will

appear on screen (Figure 15). The hydrograph may be printed by clicking the Print button. When

done viewing the hydrograph, press Close to return to the Basin Schematic window.

20. Repeat Step 18, then select Summary Table on the pop-up menu. A summary of the run results will

appear (Figure 16). Click on Print to print out the results and Close to return to the Basin Schematic

Window.

21. Repeat Step 18, selecting Time Series Table on the pop-up menu. A table detailing the precipitation,

loss, excess, direct flow due to precipitation, baseflow (0 for this example, see Example 3 for a run

with baseflow) and total flow for each interval appears (Figure 17). Click on Print to print out the

results and Close to return to the Basin Schematic Window.

Figure 14: Basin Schematic Window with pop-up menus

Figure 15: Output Hydrograph

Figure 16: Summary Table

Figure 17: Time Series Table

The peak discharge is 4378.5 cfs, occurring 12.6 hours after the start of the storm as given in the

Summary of Results Table (Figure 16). The runoff hydrograph is given in Figure 15 with a graph of

the precipitation in gray and loss in inches in black. The difference between the precipitation and loss

is the excess runoff in inches. A portion of a table of precipitation, loss, excess runoff, direct Q,

baseflow (no baseflow input for Example 1) and total Q (the sum of the direct Q and baseflow values

are plotted in the hydrograph, Figure 15) is given in Figure 17 for 6 minute intervals. The remaining

part of this table is basically the same (Figure 17).

5 - Example 2: Flood Hydrograph with Routing

In Example 2, the flood hydrograph from Example 1 will be routed through a reservoir spillway structure.

There will be two methods shown in example 2. The first is to assume Example 1 has been run. The

second assumes no data has been entered for the basin.

METHOD 1

1. In the main HEC-HMS window, choose Open Project from the File menu and open the project

created in Example 1: Flood Hydrograph. The Open Project dialog box will appear. Click on the

project created in Example 1, then click on the Open button. Double click on Basin 1 under Basin

Model to open the basin model. The Basin Schematic window will appear. Choose Save Basin

Model As from the File pull down menu. Enter Basin 2 in the New Basin Model field and press OK.

The Basin Schematic window will appear on screen, set to Basin 2, which is an exact copy of Basin 1.

2. In the Basin Schematic window, select Basin Model Attributes from the File pull down menu. The

Basin Model Attributes dialog box will appear on screen. If the Channel Routing field in Default

Methods is not set Modified Puls, click the down arrow next to the field and select Modified Puls.

Press OK to be returned to the Basin Schematic window.

3. In the Basin Schematic window, click on Reservoir button and, while still holding the mouse button,

drag the cursor to a point below Subbasin-1, and release. A reservoir icon will be created in the

Basin Schematic window.

Figure 18: Basin Schematic Window with Routing

4. Click on the Subbasin-1 icon. Then click and hold the right mouse button while the cursor is

positioned over the Subbasin-1 icon. A pop up menu will appear on screen. While still holding the

right mouse button, move the cursor down the menu to Connect Downstream, then release the

button. The cursor will become a set of crosshairs. Click on Reservoir-1. A line will be drawn

between Subbasin 1 and Reservoir-1. By this procedure, the direction of flow will be from Subbasin–

1 to the reservoir (Figure 18).

5. The characteristics of the reservoir are edited by double clicking the Reservoir-1 icon. The Reservoir

Routing window will appear on screen (Figure 19). Under Input Options, select Elevation-Storage-

Outflow. You will be prompted as to whether you want to change the table type. Press OK when

prompted.

6. Enter the elevation, storage and outflow data as indicated in Tables 4 and 5 (Figure 19).

Figure 19: Completed Reservoir Routing Window

7. Press OK. You will be returned to the Basin Schematic window.

8. Select Run Manager from the Simulate pull down menu. Next, click on Run Configuration from the

Simulate pull down menu and the HMS Run Configuration dialog box will appear on screen (Figure

20). Choose Basin 2, Precip 1 and Control 1 from the Basin ID, Precip ID and Control ID lists. Click

Add and then Close to be returned to the Simulation Manager. Click on Run 2 under Run ID. Click

on the Compute button to complete the run.

9. Display the results for Subbasin-1 by clicking on Subbasin-1 and then following steps 18 through 21

from Example 1: Flood Hydrograph.

10. To review results, repeat step 9 for Reservoir-1.

Figure 20: HMS Run Configuration dialog box

METHOD 2

1. Repeat steps 1 through 12 from Example 1.

2. Repeat steps 3 through 8 from Method 1 in this example.

3. To review results, repeat steps 18 through 21 from Example 1.

4. To review results, click on the Reservoir-1 icon in the Schematic window and repeat steps 18

through 21 from Example 1.

The inflow hydrograph is the same as Example 1 since all the input variables are the same. The peak

outflow is 599.72 cfs and peaks at 14.7 hours into the storm, or 2.1 hours after the peak inflow (Figure

21). The elevation in the pond peaked at 794.98 feet (Figure 21). The time of the maximum storage was

14.7 hours, the same time as the maximum outflow. Since the calculated maximum elevation (794.98

feet) is greater than the elevation of the top of the dam (792.4 feet), the dam will overtop. The inflow and

outflow hydrographs are given at the bottom of Figure 22 and the reservoir surface water elevation and

storage volume at the top of Figure 22. A portion of the table with the reservoir storage, elevation, inflow

and outflow every 6 minutes is given in Figure 23.

Figure 21: Summary Table

Figure 22: Hydrograph for Reservoir-1

Figure 23: Time series table

6 - Example 3: Multiple Watersheds

The same watershed given in Example 2 will be divided to create multiple watersheds discharging into a

reservoir. This flow will also be routed over the same spillway given in example 2.

1. In the main HEC-HMS window, choose Open Project from the File menu and open the project

created in Example 1: Flood Hydrograph. Choose Basin Model from the Edit pull down menu in the

main HEC-HMS windows, then select New. The New Basin Model dialog box will appear on screen.

Press OK to accept the default name for the new basin model, which should be Basin 3. The HMS

Basin Schematic window will appear on screen.

2. Create the first subbasin by clicking on the Subbasin button, and while still holding down the mouse

button, dragging the cursor near the center of the Basin Schematic window and releasing the mouse

button. A subbasin icon, named Subbasin-1, will be created in the Basin Schematic window.

3. Repeat step 2, placing Subbasin-2 to the right of Subbasin-1.

4. Create a junction by clicking on the Junction button, and while still holding down the mouse button,

dragging the cursor below the two subbasins and releasing the button.

5. Create a reservoir by clicking on the Reservoir button, and while still holding down the mouse button,

dragging the cursor below Junction-1 and releasing the button.

6. Click on the Subbasin-1 icon. Then click and hold the right mouse button while the cursor is

positioned over the Subbasin-1 icon. A pop up menu will appear on screen. While still holding the

right mouse button, move the cursor down the menu to Connect Downstream, then release the

button. The cursor will become a set of crosshairs. Click on Junction-1. A line will be drawn

between Subbasin 1 and Junction-1.

7. Repeat step 6, connecting Subbasin-2 to Junction-1.

8. Repeat step 6, connection Junction-1 to Reservoir-1. The Basin Schematic window should look like

that pictured in Figure 24 (the names of the basins will be changed in steps 9 and 12). Using this

procedure, the flow direction goes from the subbasins to the juncture to the reservoir. The reason to

have a juncture in this example is to have a separate record of the combined flows. The junction can

be eliminated by directing the flow from the subbasins to the reservoir.

9. Double click on Subbasin-1. The Subbasin Editor window will appear. Change the name of

Subbasin-1 to North. Enter 0.771 sq. mi. as the subbasin area. In the Loss Rate tab, change the

Method to SCS Curve No. Enter 0 for Initial Loss and % Impervious and 85.2 for SCS Curve No. You

will now be returned to the Basin Schematic window. The Subbasin Editor will then prompt you to

confirm the changes made to the old Loss Rate method. Click OK and then continue.

10. Select the Transform tab in the Subbasin Editor window and change the Method to SCS. Enter 0.462

hours for the SCS Lag (change the Lag time units from the pull down menu).

11. Select the Baseflow tab from the Parameters tab and change the Method to Recession if it is not

already selected. Enter the Initial Q of 5 cfs, Recession Ratio of 0.9 and Threshold Q of 0.05 cfs in

the appropriate fields as shown in Figure 25. Choose OK to accept and save the data. You will be

returned to the Basin Schematic window.

12. Repeat steps 9, 10 and 11 for Subbasin-2. Change the name of Subbasin-2 to South. The basin

area for the South basin is 1.859 sq. mi., the Initial Loss and % Impervious is 0, the SCS Curve No. is

79.4, the SCS Lag is 0.5547 hours, and for the baseflow, the Initial Q is 8 cfs, the Recession Ratio is

0.9 and the Threshold Q is 0.05 cfs.

Figure 24: Basin Schematic Window with Multiple Watersheds

Figure 25: Baseflow Tab of Subbasin Editor

13. Repeat steps 5 through 7 from Example 2 for the reservoir.

14. Select Run Manager from the Simulate pull down menu. Click on Run Configuration from the

Simulate menu and the HMS Run Configuration dialog box will appear on screen (Figure 26).

Choose Basin 3, Precip 1 and Control 1 from the Basin ID, Precip ID and Control ID lists. Click Apply

and then Close to be returned to the Simulation Manager. Click on Run 3 under Run ID. Click on the

Compute button to complete the run.

Figure 26: HMS Run Configuration dialog box

15. Display the results for the North basin by clicking on North and then following steps 18 through 21 (to

view results) from Example 1: Flood Hydrograph.

16. To review results, repeat step 15 for the South basin, Junction-1 and Reservoir-1.

Figure 27 shows the three inflow hydrographs, North basin, South basin and their combined flows.

The outflow hydrograph and storage elevation is not shown because it is basically the same as in

Example 2. The only difference in the format of the output in Example 3 is the time series table at the

juncture (Figure 28), showing the inflow for the North and South basins with the combined flow. Note,

the inflows include the baseflow for each basin.

Figure 27: Outflow Hydrographs

Figure 28: Time series table at juncture