1. 1 Quantifying Hydromodification Impacts and Developing Mitigation Using a Four Factor Approach Judd Goodman jgoodman@geosyntec.com CASQA Conference November 2, 2010

2. 2Objectives How are geomorphic impacts modeled? f(f( ) = What is geomorphology? Why should I care about it?

3. 3 What is geomorphology? Geomorphology = the scientific study of landforms and the processes that shape them Fluvial = of, relating to, or occurring in a river or stream Geomorphic Impact = Changes in landforms and the processes that shape them. Often caused by land use change. Restoration vs. Hydromod Management fix an existing geomorphic impact prevent a future geomorphic impact

4. Pre-Development flow flow 4 Why care about geomorphology? Example of geomorphic impact: flow 140-ft flow Images from GoogleEarth Include watershed changes and outfall undermining Post-Development 17-ft Judd 300-ft ~290 acre watershed Hydromod!

5. 5 Qualitative: f(f( ) = Quantitative: Lane (1955) Source: Rosgen (1996), From Lane, 1955. Reprinted with permissions How are geomorphic impacts modeled?

6. 6 Hydrologic Models are applied to simulate the hydrologic response of catchments under pre- and post-developed conditions for a continuous period of record. Pre-Urban Post- Urban Time Discharge Hydrologic Inputs : Rainfall Catchment Delineation Soils % Imperviousness Lag Time In-stream Infiltration Evapotranspiration flow

7. 7 Flow output from hydrologic model is used to generate flow duration curves. Currently Hydromodification Management focuses on State of the practice is flow-duration matching.

8. 8 FACTORATTRIBUTE NATURAL CONDITION DEVELOPED CONDITION HYDROLOGY Drainage Area~286 acres~296 acres Average Annual Rainfall 18.4 inches Hydrologic Soil Group 88% Type B, 10% Type C, 2% Type A Imperviousness0 to 1%33% Peak Flow600 cfs731 cfs Runoff Coefficient0.0330.349

9. 9 Cross-sections and longitudinal profiles of the active channel are surveyed at strategic locations during the geomorphic field assessment. flow Plan View

10. 10 FACTORATTRIBUTE NATURAL CONDITION DEVELOPED CONDITION CHANNEL GEOMETRY Longitudinal Slope3.0%2.1% Plan Form Multiple braided channels Single incised channel cutoff from the floodplain. Severe meanders at the downstream end cut into hill slope toe. Channel Depth0.5 to 2 feet12 to 18 feet Channel Width5 to 30 feet50 to 140 feet Hydraulic Roughness Unvegetated: n = 0.035 Vegetated: n = 0.06 Unvegetated: n = 0.035 Vegetated: n=0.05

11. 11 For each cross-section surveyed, a measure of critical shear stress is obtained on the bed and bank material. Non-cohesive bed: Wolman Pebble Count and/or Sieve Analysis Cohesive bed and bank: Jet Test or Characterize using Tables Vegetated bank: Characterize using Tables Bank Material Type Critical Shear Stress (  c lbs/ft 2 ) ASCE Manual No. 77 Hardpans, Duripans0.67 Compacted Clays0.50 Graded Loams with Cobble 0.38 Stiff Clays0.32

12. 12 FACTORATTRIBUTENATURAL CONDITIONDEVELOPED CONDITION BED AND BANK MATERIAL Median Grain Size of Bed (D50) 1 to 2 mm Bed Material Sand bed with interspersed gravel and cobble. Sand bed with abundant granitic cobble and gravel. Cobble and gravel is absent further downstream. Bank Material Consolidated sandy silt banks embedded with gravel and cobble. Heavily vegetated. Consolidated sandy silt banks with embedded gravel and cobble. Little vegetation.

13. 13 Sediment yields are calculated using a GIS raster based analysis. Sediment generated from developed land and areas tributary to detention facilities are removed in the post-project condition. Sediment Yield Map % Sed Reduction = [ Yield (pre) - Yield (post) ] Yield (pre)

14. 14 FACTORATTRIBUTENATURAL CONDITIONDEVELOPED CONDITION SEDIMENT SUPPLY Sediment Yield 0.5 to 1.0 ac-ft/sq-mi/yr ~0 ac-ft/sq-mi/yr

15. 15 Model Summary

16. 16 Work for Consolidated Material: Estimating Geomorphic Impact

17. 17 Ratio of Transport is calculated by comparing relative changes in total work/transport capacity in the pre- and post-development conditions: Transport post Transport pre Estimating Geomorphic Impact

18. 18 For each cross-section location the Ratio of Transport is compared to the Ratio of Sediment Supply (Target Ratio) to get a Probability of Instability. Estimating Geomorphic Impact Target Ratio of Transport = Ratio of Transport ( Target = 1.0 ) Study in Bay Area:

19. 19 Statistical Relationship 40 Cross Sections:  Thompson Creek  Ross Creek  San Tomas Creek Santa Clara Valley HMP

20. 20 Out-of-Stream Mitigation

21. 21 Out-of-Stream Mitigation Route post-development runoff through flow duration control facilities to mimic pre-development hydrology. Detention Basin Bio-Retention

22. 22 In-Stream Mitigation

23. 23 In-Stream Mitigation Reduce longitudinal slope with in-stream grade control structures to mimic pre-development work/sediment transport.

24. 24 Project Solution? Equilibrium Slope = 0.2% may not be feasible to construct. Outfall structure needs to be retrofit to properly dissipate energy. Opportunity for combined flow control and grade control mitigation.

25. 25 Thank You! Questions ? f(f( ) =

26. 26 Watershed Scale Longitudinal Profile