Modeling Post-Development Runoff and Channel Impacts from Hydromodification: Practical Tools for Hydromodification Assessment

Transcription

1 Modeling Post-Development Runoff and Channel Impacts from Hydromodification: Practical Tools for Hydromodification Assessment Chris Bowles, Ph.D. Andy Collison, Ph.D. Matt Wickland, M.S.

2 Emerging 3 Step HydroMod Approach Project Proposal Existing condition Proposed conditions Practical Tools 1) Does Project Maintain or Reduce Existing Impervious Area? Increase Maintain or Reduce? No Action Continuous Rainfall-Runoff Modeling (HMS, HSPF, SWMM) 2) Do BMPs Enable Project to Match Pre- and Post-Development Flow Duration Curve?? Match No Action Channel Vulnerability Modeling (Geomorphic approaches, E p, Sediment Transport) 3) How Vulnerable is Receiving Channel To Increased Flows? Partially match or exceed flows Low No Action? Medium or High In Channel Mitigation

3 Continuous Rainfall-Runoff Modeling Rationale for using CRRM to assess channel vulnerability Example of CRRM application from Santa Clara County HMP (PWA, Geosyntec, Balance Hydrologics) Modeling pre- and post-development flow duration curves in selected watersheds using continuous HEC-HMS modeling to assess impact of urbanization on runoff, and linkage to channel degradation Or how to use essentially an event based model for CRRM

4 Rationale for using CRRM to assess vulnerability Erosion occurs across a wide range of flows Need to assess all flows to determine erosive impact A range of flows are responsible for erosion Entrainment threshold - flows below this do no work Above a certain level flows are infrequent enough to have little effect One goal of Santa Clara HMP was to quantify these ranges

5 Assessing Effect of Past Urbanization of Rainfall-Runoff Objective assess past effects of urbanization to predict future sensitivity Adobe Creek watershed Located in western Santa Clara Valley Watershed area of 11 mi 2

6 Watershed context Lower watershed developed Upper watershed still fairly rural

7 Data requirements GIS development Land use, soils, topography, sub-basin delineation Rainfall 50-year rainfall record from San Jose Airport

8 Modeling approach Model construction Past and Existing Conditions Model calibration Calibration of rural areas (past conditions) and urban areas (existing conditions) using flood frequency values and flow gage data

9 Assumptions and Limitations Soil Moisture Accounting Balance between physical conditions and model representation of soil and groundwater layers in calibrated model Post-Processing Long run times Model was split into 10-year increments to manage runtimes and ease data handling but required significant postprocessing in Matlab

10 Model results Long term flow results Used to develop past and existing conditions flood frequency and flow duration curves

11 Model results Develop flow duration curves for pre- and postdevelopment conditions Geomorphic assessment to predict what actual receiving channel impacts were and set thresholds Hypothetical future flow Upper threshold Q10 Entrainment threshold approx 0.1Q2 Most erosive flows

12 HEC-HMS Continuous Modeling Conclusions Pros HMS event-based model is often required by other agencies (e.g. County PWDs for Capital Flood analysis) so efficient to use for both purposes Easy to adapt event-based HMS model to continuous model when required General Calibration to actual flows is essential to successful applications Ability to predict relative difference (pre- and postdevelopment) is much higher than absolute value prediction Cons Continuous HMS model (as used in 2003) was a new application without extensive user experience or parameterization guidance Hard to implement post-project conditions (need to incorporate IMPs and BMPs beyond just detention basins) unlike BAHM, WWHM etc Arduous post-processing requirements

13 Acknowledgements Dipankar Sen and Robert Van Den Berg, Santa Clara Valley Water District Matt Wickland, Jeff Haltiner, Christie Beeman, PWA

14 Modeling approaches to assess channel vulnerability to Hydromod: Or: How to do sediment transport modeling with no data

15 Emerging 3 Step HydroMod Approach Project Proposal Existing condition Proposed conditions Practical Tools 1) Does Project Maintain or Reduce Existing Impervious Area? Increase Maintain or Reduce? No Action Continuous Rainfall-Runoff Modeling (HMS, HSPF, SWMM) 2) Do BMPs Enable Project to Match Pre- and Post-Development Flow Duration Curve?? Match No Action Channel Vulnerability Modeling (Geomorphic approaches, E p, Sediment Transport) 3) How Vulnerable is Receiving Channel To Increased Flows? Partially match or exceed flows Low No Action? Medium or High In Channel Mitigation

16 Assess stream vulnerability to erosion Project larger than 20 acres? Yes Comprehensive geomorphic assessment No Is channel continuously hardened, tidal or depositional between outlet and SF Bay? No Yes Basic geomorphic assessment High Medium Low Comprehensive analysis; possible restoration plan More detailed analysis; In-stream mitigation plan Exempt from HMP Risk of Increased Erosion and Impacts Management action: allows in-stream measures in lieu of on-site mitigation

17 Defining High, medium and low vulnerability Field Evaluation & Review Of Available Data Risk of Increased Erosion and Impacts High Medium Low Develop a scientific basis for discriminating medium and high. Must be objective, repeatable, easy for applicant to implement and for permit grantor to check. Armored flood control channels, channels within the tidal zone Require basic on-site BMPs (to MEP)

18 Channel geomorphology 101 Over time channel geometry (width, depth, gradient) adjusts to be in equilibrium with water and sediment load

19 Channel geomorphology 101 HydroMod increases peak flow and reduces sediment load -Result is channel incision, expansion and slope flattening -The question is how much?

20 How vulnerable is the stream to HydroMod? The degree of channel response is sensitive to internal and external factors: External Factors Magnitude of flow increase Magnitude of sediment reduction Internal Factors Channel geometry Prior channel degradation (e.g. past incision concentrates new flows) Channel resistance (channel material and vegetation)

21 External factors increased water Key problem is increases in duration of flow above erosion threshold of channel sediment. Can be assessed as Erosion Potential (E p ) a measure of the sediment transport capacity. E p = Cumulative sediment transport after development Cumulative sediment transport before development If E p > 1.2 channel erosion is likely (Source: Geosyntec)

22 External factors reduced sediment In the land of hydromod, sediment is your friend More sediment = steeper channels, fewer drop structures, less mitigation There is a conflict between flow control and sediment supply - Need to maintain some sediment transport if possible

23 Trading off sediment supply for steeper channels using SAM or HEC-RAS Stable channel gradient (USACE SAM model) SAM calculated slope 5.0% 4.0% 3.0% 2.0% 1.0% 0.0% e.g. with 100% sediment delivery stable channel gradient is 3% 1% initial slope 2% initial slope 3% initial slope 4% initial slope Power (1% initial slope) Power (2% initial slope) Power (3% initial slope) Power (4% initial slope) y = x y = x y = x y = x Stable channel gradient Proportion of sediment capacity Proportion of original sediment delivery 4% 3% 2% 1%

24 Trading off sediment supply for steeper channels Stable channel gradient (USACE SAM model) SAM calculated slope 5.0% 4.0% 3.0% 2.0% 1.0% 0.0% 1% initial slope if we 2% initial cut slope off 50% of sediment 3% initial slope to a 3% channel the new 4% initial slope stable Power (1% grade initial slope) will be 1.8%. Power (2% initial slope) At Power 30% (3% initial sediment slope) slope will be Power (4% initial slope) 1.3% y = x y = x y = x y = x Stable channel gradient Proportion of sediment capacity Proportion of original sediment delivery 4% 3% 2% 1%

25 Internal factors affecting vulnerability Stream erosion is most likely to follow HydroMod where: Small increases in flow lead to large increases in shear stress (shear stress sensitivity) Small increases in shear stress lead to large increases in erosion (low channel resistance)

26 Conceptual approach Wide shallow channel little increase in shear stress with Q. Q5 dissipates over floodplain Narrow deep channel large increase in shear stress with Q. Q5 confined in channel. Increasing vulnerability

27 Conceptual approach Coarse sediment and vegetated channel less erosion-prone Fine sediment and unvegetated channel more erosion-prone Increasing channel vulnerability

28 Conceptual approach resistant sediment, not very entrenched resistant sediment, highly entrenched non resistant sediment, not very entrenched non resistant sediment, highly entrenched Increasing vulnerability Increasing channel vulnerability

29 Turning concepts into measurable attributes Increasing vulnerability Increasing channel vulnerability

30 Turning concepts into measurable attributes Increasing vulnerability Increasing channel vulnerability

31 Turning concept into measurable attributes Entrenchment Ratio = (Floodprone Area Width*) / (Bankfull Width) *Floodprone width = width at 2 x bankull depth (Q5 in Southern California) Note: Rosgen definitions of degree of entrenchment differ from those used here Floodprone width Bankfull width Bankfull depth ER > 1.6 channel is non entrenched Floodprone width Bankfull width Bankfull depth ER < 1.6 channel is entrenched

32 Turning concept into measurable attributes Entrainment ratio = critical diameter for entrainment/bed diameter If d c > size class of bed, channel is non-resistant If d c < size class of bed, channel is resistant Critical bed diameter for entrainment, d c = 13.7 x depth x slope

33 Primary and secondary vulnerability criteria Vulnerability Entrenchment Ratio Entrainment Ratio Width to Depth Ratio Schumm State Class Medium > 1.6 < 2.0 > 12 1, 5 & 6 Primary Criteria High < 1.6 > 2.0 Secondary Criteria If both primary criteria indicate the same vulnerability class, Confinement Class UC WC or MC that class is adopted. Active Bank Erosion Class If primary criteria disagree, use preponderance Low of Moderate or High Active secondary Sedimentation criteria. Class varies varies < 12 2, 3 & 4

34 Primary and secondary vulnerability criteria Vulnerability Medium Primary Criteria High Entrenchment Ratio Entrainment Ratio > 1.6 < 2.0 < 1.6 > 2.0 Confinement Class Active Bank Erosion Class UC Low Secondary Criteria WC or MC Moderate or High Area of Sediment Cut Off Width to Depth Ratio Schumm State Class <10% > 12 1, 5 & 6 >10% < 12 2, 3 & 4

35 Contra Costa Channel Vulnerability Tool

36 Example field sheets

37 Summary For small developments (<20 acres) relatively simple field indicators can be used to quickly classify the majority of streams into risk categories Larger developments or more complex stream systems require more sophisticated predictive approaches Mitigation should address the underlying cause of erosion, avoid tendency to harden as mitigation However, there are not good long term studies of creeks before and after HydroMod to validate any of the approaches out there

38 Questions? Chris Bowles (916) Andy Collison (415)

39 Determine post HydroMod stresses and develop appropriate stabilization Source:

40 Channel Response to HydroMod Top of bank Stable gradient for original water and sediment mix Stable gradient for original water and sediment mix Stable bank height

41 Channel Response to HydroMod Top of bank Stable bank height Stable gradient for original water and sediment mix New stable gradient for water and sediment mix after HydroMod

42 Channel Response to HydroMod Top of bank Stable bank height Stable gradient for original water and sediment mix headcut New stable gradient for water and sediment mix after HydroMod

43 Channel Response to HydroMod Top of bank Stable bank height Unstable bank height Stable gradient for original water and sediment mix New stable gradient for water and sediment mix after HydroMod