Hydrograph Reshaping for Hydromodification Control

2. Ongoing Research • Skunk Works project • Started before hydromod was mainstream • Hydromod only • Engineering challenge to solve agency processing problem for clients • Approach reflects timing – private development driven • Idea conceived prior to agency response

3. Development - Watershed Changes • Urbanization changes watershed physiography • Increased impervious area • Connected surfaces • Stormwater conveyance utilities • The watershed becomes a freeway for the excess surface water

4. Hydromodification • Change in watershed rainfall-runoff response caused by alterations to watershed physiography • Reduced time to peak • Higher peak flows • Increased runoff volume • More frequent flow events

5. Storm Response • Basin response to a storm (hydrograph) is altered

6. Quantifying The Changes • Hydromod effects are cumulative over time • Commonly expressed as long term flow frequency diagram

7. The Problem • Change to runoff pattern changes stream sediment transport and stream stability

8. Fix It Attempts • MS4s in California have measures aimed at reducing hydromodification • General measures – low impact development • Specific measures – volume standards • Smaller projects are easier – set standards • Larger projects are harder – do a study • LID, LID, LID, LID, LID ...

9. Development Approach 1

10. Development Approach 2

11. Property Owner Perspective • New MS4 requirements can be onerous • Retention takes space • LID can drive property use • LID implementation standard vague • Additional approval cycles • It is all new • No more studies please

12. Middle Ground • Property owners are adjusting to new requirements • Development plans altered mid-stream • Uncertainty likely to persist even for properties at planning stages • Implementations may be awkward for now

13. Hydrologic Wishful Thinking • Hydromod process is complicated on a regional scale • MS4 approaches seek to reverse or reduce physiographic changes to individual properties • Effect on watershed scale is not certain – probably better, but still modified

14. Engineer’s Approach • Solve a problem for the client • Property development changes watershed subarea • Runoff hydrograph is altered • Restore original runoff hydrograph • Use off the shelf ideas • Catalytic converter for a property

15. Hydrograph Plastic Surgery • Sculpt a post development hydrograph back into the shape of the pre development hydrograph

16. Increased Runoff • Permanent storage is necessary

17. Ponds and Wetlands • Ponds and Wetlands provide the basic structure for hydromod control

18. Necessary Elements and Beneficial Features • Retention (volume control) • Detention (shape control) • Aesthetic feature • Structurally isolated • Water quality • Retrofit potential • Added value

19. Storage Configuration • Pre Post hydrograph volume difference is stored • Sub invert storage chops off front of hydrograph

20. Hydrograph Shaping Problem • Once front part of incoming hydrograph is stored problem is reduced to reshaping

21. Hydrograph Reshaping – What We Know • At any time we know … • Incoming flow (chopped post development hydrograph) • Target outflow (pre development hydrograph) • Volume stored • Basin geometry (WSEL)

22. Discretized Approach • Discretize pre development hydrograph (target points) • Match pre development storage and flowrate (outflow) at target points

23. Outlet Works Driven

24. Pond/Wetland Tuning • Outlet works is adjusted to match pre development flow at each target point • Rows of orifices added at each target WSEL to adjust total pond outflow • Orifices not mandatory – may use weirs • Tuning approach works great provided that pre development hydrograph is rising

25. Going Up Is Easy • Pond volume (WSEL) is increasing • Pre development flow is increasing • Add more orifices

26. Engineering Decision Point • Match pre development hydrograph on the way up • Cross our fingers on the way down • Examine cumulative process • Normal approach

27. Normal Approach • Regard Pond or Wetland as treatment control • Tune pond or wetland for fundamental storm • High frequency event • Stream changing effects • Adjust to obtain desired long term results • Continuous simulation

28. Out of The Box Approach (Research) • Seek to continue hydrograph sculpting on falling limb of pre development hydrograph • Requires additional design element • Idea conceived for wetlands diversion project

29. Latched Outlet Works • Use a float mechanism to activate flapgate • Open up or close off a portion of the outlet works • Alter outlet works configuration • Contour hydrograph on falling limb • Hydrograph matching • Agency approval may take work initially

30. Hydrograph Matching • Discretized approach (target points) allows the discharge from the pond or wetland to closely match pre development hydrograph • Moot point since hydrograph is matched for the fundamental storm • Cumulative effects, long term simulation, and tuning is still required

31. Numerical Process • Unit hydrograph convolution • R1 = U1 *P1 • R2 = U2*P1 + U1*P2 • R3 = … • Pond WSEL and pond geometry provide means to predict pre existing outflow for any storm

32. Hydrologic Undo • Pond configuration maps incoming flow to project watershed precipitation pattern • Pre development unit hydrograph is used to convert rain pattern to pre existing runoff which provides target outflow • Goal is to convert post development project to pre development hydrology at the D/S end

33. Future World • Goal is to match hydrographs for any storm • Eliminate hydromod • Ultra low impact development • Agency rubber stamp • Everyone happy