2. 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

3. 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
: Off-channel basin constructed
First, some history…

4. Sierra College Boulevard, 1995

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

6. 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.

7. Available Models HEC-1 models of the Dry Creek watershed
HEC-RAS models converted from HEC-2

8. 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

9. 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.

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

11. 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.

12. 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.

13. 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

14. HEC-HMS from HEC-1
This project was done in HEC-HMS (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.

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

16. HEC-1 Definition Typical on Miners Ravine
KK MR30 ROUTE MRC6 TO MR30
RS 15 FLOW -1 SV SQ
KK MR30
BA 1.75 BF
PT 8
PR 8 LU US
KK MRC7 - COMBINE MR30 WITH ROUTED MRC6
HC 2
KK MR31 ROUTE MRC7 TO MR31 SV
Note that there was no storage value for a zero discharge in the SV/SQ records

17. Adding Zeros…

18. 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.

19. Detailed Baseline Unsteady Model

20. Refined Model Simulates Pre-Project Weir Flow

21. Detailed Proposed System Unsteady Model

22. Hydraulic Model Shows Benefit Near the Site

23. HEC-RAS Results Used in HEC-HMS

24. 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.

25. Each Run Required a Basin Model

26. HEC-HMS Provided Results for Flow Reduction Calculation

27. 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.

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

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

30. Modeling Effort Demonstrated Benefit of Control Structure

31. Process Led to Project Construction!