Understanding Hydrologic Process for Better Stormwater Management

Transcription

1 Understanding Hydrologic Process for Better Stormwater Management Scott A. Brown, P.E. Larry A.J. Fennessey, Ph.D., P.E. Gary W. Peterson, Ph.D., P.E. Civil and Environmental Engineering

2 GOAL to project an understanding of how surface cover characteristics, topography / landscape position, soils and geology all play a key role in how water moves over and under the surface of a site; the difference between surface runoff response from a natural watershed and that from a small land area or hypothetical watershed; and the limitations of available stormwater modeling tools when applied to hypothetical watersheds.

3 Hydrologic Process a lay perspective

4 Surface and Subsurface Hydrologic Process: Modified From: Controlling Urban runoff, Schueler, 1987 Infiltration Soil Water Infiltration Recharge Soil Water Recharge Interflow Soil Water Infiltration Increased Peak Rate, Volume, and Frequency of Runoff.

5 Hillslope hydrologic Processes Return Flow/ Effective Surface Runoff Baseflow in Effluent Stream 7 5 Pan Induced Exfiltration Saturated Throughflow Hortonian Surface Runoff 3b 4b 4c 3a 2 4a Throughflow 4c Saturated Areas Saturation Excess Surface Runoff 4a 1 5 Pan 3a Ridge Unsaturated/ Soil Storage Deep Percolation 6 Water Table Bedrock Groundwater Storage 1 5

6 Factors Affecting the Movement of Gravitational Water Through Soil 1. Texture 2. Structure 3. Layering 4. Preferential Flow Paths

7 Factors Affecting the Movement of Gravitational Water Through Soil 1. Texture Clay Loam Sandy Loam

8 Factors Affecting the Movement of Gravitational Water Through Soil 1. Texture

9 Factors Affecting the Movement of Gravitational Water Through Soil 1. Texture Clay Loam Sandy Loam

10 Factors Affecting the Movement of Gravitational Water Through Soil 2. Structure Round Blocky Angular Platy

11 Factors Affecting the Movement of Gravitational Water Through Soil 2. Structure Round / Angular Platy

12 Factors Affecting the Movement of Gravitational Water Through Soil 3. Soil Layering TIME

13 Factors Affecting the Movement of Gravitational Water Through Soil 3. Soil Layering Restrictive Soil Layers Fragipan

14 Factors Affecting the Movement of Gravitational Water Through Soil 4. Preferential Flow Paths in the soil Rainfall Macropore (Crack) Soil Matrix Macropore (Animal burrow) Macropore (Root channel)

15 Factors Affecting the Movement of Gravitational Water Through Soil 4. Preferential Flow Paths in shallow bedrock

16 Factors Affecting the Movement of Gravitational Water Through Soil 4. Preferential Flow Paths in shallow bedrock

17 Factors Affecting the Movement of Gravitational Water Through Soil 4. Preferential Flow Paths in shallow bedrock

18 The Net Affect. 1) Subsurface seepage through macropore networks in subsoils 2) Lateral flow through the interface between A and B horizons A Bw Stream 3) Return flow at footslope and toeslope during snow melts or large storms O A Bt Btx O A Bw or Bt C/R Backslope (Moderately Wet or Moderately Dry Site) 4) Flow at the soil-bedrock interface C/R Hilltop (Dry Site) Shale Hills Watershed Huntingdon Co., PA C Valley Floor or Swale Bottom (Wet Site)

19 Watershed Response Processes Surface Runoff Stream Saturated Area Unsaturated Saturated Areas Bedrock Bedrock

20 Watershed Response Processes Partial (Variable) source Area Saturated Area Watershed Outlet 1st-Order Watershed Boundary An example of saturated watershed areas at the beginning of a storm in a humid region

21 Watershed Response Processes Partial (Variable) source Area Saturated Area Expanding Watershed Outlet 1st- Order Watershed Boundary An example of saturated watershed areas near the end of a storm in a humid region

22 Watershed Response Processes Partial (Variable) source Area Saturated Area Expanding More Watershed Outlet 1st-Order Watershed Boundary An example of saturated watershed areas during a major storm in a humid region

23 Watershed Response the standard of engineering practice Ridge Surface Runoff Water Table Groundwater Flow Hortonian Overland Flow Model

24 Watershed Response what really happens. Process is complex understand site specific hydrologic response Pan Induced Exfiltration Saturation Excess Surface Runoff Ridge Return Flow/ Effective Surface Runoff Saturated Throughflow Hortonian Surface Runoff Unsaturated/ Soil Storage Baseflow in Effluent Stream Pan Deep Percolation Throughflow Groundwater Ridging Saturated Areas Bedrock Groundwater Storage Water Table Partial Components of Hillslope Hydrology Adapted from Chorley (1978) Hillslope Hydrologic Processes

25 Also. Hydrologic response is not uniform and some areas of a watershed will produce more runoff than others. Saturated Area Watershed Outlet 1st-Order Watershed Boundary

26 And Hydrologic models in common use today are simplistic representations of average watershed response and will over predict runoff in some areas, and under predict runoff in others. Models Over-predict Models under-predict

27 So what does all this mean

28 Consider a development centered in upland ridge areas.

29 Pre-Post Hydrologic Analysis

30 Hydrologic Process And The Placement Of Infiltration Facilities Saturated Area Ridge Unsaturated/ Soil Storage Baseflow in Effluent Stream Pan Deep Percolation Throughflow Groundwater Ridging Saturated Areas Bedrock Groundwater Storage Partial Components of Hillslope Hydrology Adapted from Chorley (1978)

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33 Remember to consider Down gradient impacts from concentrated infiltration practices Rainfall Rainfall Meadow Small Seep / swale Pasture Confining layer Natural Meadow Soils have high infiltration characteristics

34 Remember to consider Down gradient impacts from concentrated infiltration practices Rainfall Rainfall Wet Basement Storm Runoff from Impervious Areas Infiltration BMP Wet Pasture Confining layer.. Removing these deep soils and vegetation and replacing them with a structural feature concentrating infiltration can cause adverse down-gradient impacts.

35 Remember to consider And what a about impacts in Karst Areas?? Rainfall Rainfall Surface Runoff Infiltration BMP New Development Unsaturated Restricting layer Saturated Bedrock

36 Concluding Statements 1. Surface and subsurface flow characteristics at a watershed and land development scale are defined by a complex interaction of a surface and subsurface physical characteristics. 2. Available computational tools for assessing runoff response are based on simplified conceptual models that don t adequately simulate development scale runoff response; 3. Responsible design of stormwater mitigation measures requiries an understanding of hydrologic process, and consideration of site specific characteristics driving the runoff response mechanisms. It can not rely on blind application of available hydrologic models.