Quantifying Flow Reduction Benefit of the Miners Ravine Off-Channel Detention Basin

Using HEC-HMS

Or What Happens When a Project Requires Use of an HEC-1 Model to Evaluate Regional Benefits and an Unsteady HEC-RAS to Model Project Function

History of Miners Ravine Off-Channel Detention

Basin• 1992: Flood Control Study called for

on-channel basin• 1998: On-channel basin concept rejected• Off-channel basin concept pursued• 2006-2007: Off-channel basin constructed

Sierra College Boulevard, 1995

Project and Dry Creek

Watersheds

Project Site

Available Models

• HEC-1 models of the Dry Creek watershed

• HEC-RAS models converted from HEC-2

Modeling Challenges

• HEC-1 models did not provide reasonable hydrographs necessary to evaluate basin function from 2- to 100-year events

• HEC-RAS models did not reflect the pre-project flood attenuation function of the site

HEC-1 Flow Hydrographs

Pre-Project Site in the Floodplain

provided some Peak Flow Reduction

Original HEC-RAS Modeled Project Area as

Ineffective Flow

Modeling Goals

• Provide a consistent basis for evaluation of project alternative

• Evaluate local flow and stage reduction• Estimate benefits at locations about 3.5

miles downstream, below major confluences.

Steps to Make an Apples to Apples Assessment of

Basin Benefit• Create HEC-HMS from HEC-1• Use HMS results as input to RAS• Create detailed unsteady RAS model of existing site• Create unsteady RAS model of project alternative

configurations

• Use unsteady RAS results as input to HMS model• Use HMS model results and steady-state RAS rating

curves to quantify downstream benefit

HEC-HMSfrom HEC-1

Miners Ravinein HEC-HMS

HEC-1 Definition Typical on Miners

RavineKK MR30 ROUTE MRC6 TO MR30RS 15 FLOW -1SV 47 101 185 267 348 427SQ 500 1000 2000 3000 4000 5000KK MR30BA 1.75BF -23 -0.08 1.01PT 8PR 8LU .10 .09 11US 1.31 .60KK MRC7 - COMBINE MR30 WITH ROUTED MRC6HC 2KK MR31 ROUTE MRC7 TO MR31RS 15 FLOW -1SV 31 66 133 210 283 330SQ 500 1000 2000 3000 4000 5000

Adding Zeros…

Makes a Difference

Detailed Baseline Unsteady Model

Refined Model Simulates

Pre-Project Weir Flow

Detailed Proposed System Unsteady

Model

Hydraulic Model Shows

Benefit Near the Site

HEC-RAS ResultsUsed in HEC-HMS

HEC-HMS RoutingReplaced by Gage

Each Run Required

a Basin Model

Table A: Baseline Discharges (cubic feet per second)

Recurrence Interval (years) Location 2 5 10 25 50 100

Secret Ravine 757 1,277 1,680 2,544 3,385 3,788 Harding Blvd 1,654 2,875 3,872 5,595 7,012 8,024 Folsom Rd 2,390 4,267 5,125 7,465 9,699 10,718 Vernon St 3,371 5,828 6,974 10,013 13,004 14,344 Table B: Design Configuration Discharges (cubic feet per second)

Recurrence Interval (years) Location 2 5 10 25 50 100

Secret Ravine 749 1202 1594 2496 3148 3374 Harding Blvd 1654 2769 3778 5535 6747 7587 Folsom Rd 2389 4161 5044 7381 9422 10475 Vernon St 3370 5701 6884 9934 12731 14081 Table C: Design Configuration Flow Reduction (cubic feet per second)

Recurrence Interval (years) Location 2 5 10 25 50 100

Secret Ravine 8 75 86 49 236 414 Harding Blvd 1 106 94 60 265 437 Folsom Rd 0 106 81 84 277 243 Vernon St 1 127 90 80 273 263

HEC-HMS Provided Results for Flow

Reduction Calculation

Table A: Option A

Recurrence Interval (years) Location 2 5 10 25 50 100

Secret Ravine 5 91 98 25 26 85 Harding Blvd -1 59 108 25 83 138 Folsom Rd 0 67 69 31 207 197 Vernon St -1 89 93 34 151 126 Table B: Option B

Recurrence Interval (years) Location 2 5 10 25 50 100

Secret Ravine 8 6 49 118 328 347 Harding Blvd 4 8 45 131 303 395 Folsom Rd 0 7 48 182 147 104 Vernon St 2 9 44 171 271 203 Table C: Option C

Recurrence Interval (years) Location 2 5 10 25 50 100

Secret Ravine 8 39 68 13 227 396 Harding Blvd 4 46 53 29 258 449 Folsom Rd 0 47 78 78 288 245 Vernon St 2 49 60 73 280 269

Compared Results for Numerous Options and Recurrence Intervals

Stage Reduction from Rating Curves from Steady-

State HEC-RASRating Curve Upstream from Folsom Street

130

135

140

145

150

155

0 4000 8000 12000 16000

Discharge (cfs)

Sta

ge

(fee

t)

Table D: Design Configuration Stage Reduction (feet) Recurrence Interval (years) Location

2 5 10 25 50 100 Secret Ravine 0.03 0.09 0.09 0.04 0.16 0.28 Harding Blvd 0.00 0.11 0.09 0.05 0.27 0.44 Folsom Rd 0.00 0.14 0.07 0.08 0.23 0.20 Vernon St 0.00 0.11 0.06 0.05 0.18 0.18

Stage Reduction from HEC-HMS Flows and HEC-

RAS Rating Curves

Modeling Effort Demonstrated

Benefit of Control Structure

Process Led to Project Construction!