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 First, some history…

Sierra College Boulevard, 1995

Project and Dry Creek Watersheds Point out project location and the objective to quantify flow reduction benefit between Folsom Rd and Vernon St

Project Site The site was bounded to the south by Miners Ravine. There was a berm between the creek and the historic sewage treatment area. It was determined that due to habitat, geotechnical and geometric constraints, it was not feasible to use the western portion of the site as had been anticipated in the District’s feasibility study. Our goal was to demonstrate that an effective project could be constructed on the available land.

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 These showed the initial flow to be similar in the 2- to 100-year events and the 10-year initial flow to be higher that the 100-year initial flow. This didn’t match baseflow. The initial flow in the 2-year event was greater than the falling limb flow. This didn’t make sense.

Pre-Project Site in the Floodplain provided some Peak Flow Reduction

Original HEC-RAS Modeled Project Area as Ineffective Flow Effect of existing embankment was not considered in the pre-RBF models. Also, the bridge definition precluded weir flow over Sierra College Boulevard.

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-HMS from HEC-1 This project was done in HEC-HMS 2.2.2. (note: Due to the new data management system, it isn’t a simple matter to convert to HEC-HMS 3.2.) HEC-HMS wouldn’t run due to missing values.

Miners Ravine in HEC-HMS The project site is just below the SCB MRC junction. Miners Ravine in HEC-HMS

HEC-1 Definition Typical on Miners Ravine KK MR30 ROUTE MRC6 TO MR30 RS 15 FLOW -1 SV 47 101 185 267 348 427 SQ 500 1000 2000 3000 4000 5000 KK MR30 BA 1.75 BF -23 -0.08 1.01 PT 8 PR 8 LU .10 .09 11 US 1.31 .60 KK MRC7 - COMBINE MR30 WITH ROUTED MRC6 HC 2 KK MR31 ROUTE MRC7 TO MR31 SV 31 66 133 210 283 330 Note that there was no storage value for a zero discharge in the SV/SQ records

Adding Zeros…

Makes a Difference Fixing the low flow point in the Modified Puls model not only made a difference in the low flow portion of the hydrograph, major event peak discharges were also significantly lower with the corrected routing. These flows were used (along with 5-, 25- and 50- year hydrographs) as upstream from hydrographs for the unsteady RAS evaluation of the project site.

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 Results Used in HEC-HMS

HEC-HMS Routing Replaced by Gage Unsteady-state HEC-RAS flow results entered as a gage hydrograph to an HEC-HMS model with routing above the project site removed.

Each Run Required a Basin Model

HEC-HMS Provided Results for Flow Reduction Calculation

Compared Results for Numerous Options and Recurrence Intervals Numerous scenarios were evaluated and the preferred configuration further refined. Note that flow reduction for the 25-year event is lower than the reduction for the 10-year of 50-year events because the pre-project embankment configuration was reasonable effective at attenuating 25-year runoff.

Stage Reduction from Rating Curves from Steady-State HEC-RAS

Stage Reduction from HEC-HMS Flows and HEC-RAS Rating Curves

Modeling Effort Demonstrated Benefit of Control Structure

Process Led to Project Construction!