Impacts of channel incision on peak flows and stream processes William H. Renwick & Monica T. Rakovan Miami University Oxford, Ohio, USA
Study Area
1930s Floodplain (now a terrace) Approximate present bankfull elevation Base of alluvium (historic stream bed) Pleistocene till Stream beds are now lower than ever before in the Holocene.
Historic land use changes and stream responses in southwest Ohio Rakovan & Renwick, in press
Factors contributing to sediment supply limitation and stream incision Sediment trapping Sediment What is the effect of stream incision on Channel in impoundments Supply hydrologic regime? Incision Limitation Soil Conservation Is this effect sufficient to generate Reduced Flood appreciable positive feedback, Storage Increased reinforcing incision? Peak Flows Urbanization Climate change
Four Mile Creek 1938 and Today 242 241 1930s floodplain 240 Present bankfull channel 239 238 237 0 5 10 15 20 25 30 35 40 45 50 55 60
Indian Creek 1935 and Today 266 265 1930s floodplain 264 263 262 Present bankfull channel 261 15 20 25 30 35 40 45 50 55 60 65 70 75
Bull Run 1938 and Today 274 272 270 1930s Floodplain 268 266 Present bankfull channel 264 262 20 40 60 80 100 120
Summary of modeling procedures INPUT DATA 10m DEM LiDAR DEM for channels NLCD Land Use/Land Cover SSURGO Soils NOAA Precipitation Frequencies HEC-RAS Flow routing through lower portion of HEC-HMS watershed 1. Pre-incision Runoff hydrograph for 2. Post-incision main channel and tributaries Historic and potential future channel incision Calibration against observed flows
Modeling channel incision Individual channel cross sections were manually adjusted in the Graphic Editor in HEC-RAS to simulate historic incision of 1 m and 2 m and 4 m of future incision. 272 Elevation (m) 271 IC Historic (1m) 270 269 IC Present 268 390 410 430 450 470 490 270 Elevation (m) Distance (m) 268 BR Historic (1930) 266 BR Present 264 262 50 55 60 65 70 75 80 85 90 95 Distance (m) 255 Elevation (m) 251 247 FM Present 243 FM Future Incision (4m) 239 330 340 350 360 370 380 390 Distance (m)
Indian Creek historic and present incision modeled results 100 0 Inflow 80 10 Present Outflow Precipitation (mm) Discharge (m3/S) Historic Outflow 60 20 Precipitation 40 30 20 40 0 50 0 5 10 15 20 25 30 35 40 Hours Averages of all sections in reach Historic Present % change Avg Peak Q (cms) 35.3 37.8 +6.7 Avg Channel Velocity (m/s) 1.2 1.2 +4.3 Avg Stream Power (kg/m S) 0.2 0.3 +28.5 Avg Max Channel Depth (m) 0.9 1.4 +30.6
Bull Run historic and present incision modeled results 20 0 inflow 10 Present Outflow 15 Discharge (m3/S) Precipitation (mm) Historic Outflow 20 Precipitation 10 30 5 40 0 50 0 5 10 15 20 25 30 35 40 Hours Averages of all sections in reach Historic Present % change Avg Peak Q (cms) 10.0 10.1 +1.3 Avg Channel Velocity (m/s) 1.2 1.6 +27.7 Avg Stream Power (kg/m S) 0.4 1.0 +56.3 Avg Max Channel Depth (m) 0.4 0.7 +42.2
Four Mile Creek present and future incision modeled results 100 0 Inflow 80 10 Present Outflow Precipitation (mm) Discharge (m3/S) Future Outflow 60 20 Precipitation 40 30 20 40 0 50 0 5 10 15 20 25 30 35 40 Hours Averages of all sections in reach Present Future % change Avg Peak Q (cms) 76.5 83.6 +9.3 Avg Channel Velocity (m/s) 1.2 1.3 +11.7 Avg Stream Power (kg/m S) 0.3 0.5 +37.4 Avg Max Channel Depth (m) 2.2 2.8 +30.0
Summary of results • Little effect on the magnitude or timing of peak Indian Creek flows (present 1m • Moderate increase of velocity, stream power incision) and flow depth at present • Little effect on the magnitude and timing of Bull Run peak flows because of small drainage area (present 2m • Substantial increase of velocity, stream power incision) and flow depth • Substantial increase in magnitude of peak flow Four Mile Creek (4m and reduction of lag time • Substantial increase of stream power and flow future incision) depth in future incision
Conclusions • Although further model calibration and testing is needed, initial results suggest that incision can contribute to large increases of channel velocity, stream power and flow depth. • These increases generate a positive feedback by that enhances channel incision. This helps explain incision that is unprecedented in post- glacial time.
Thanks! Jonathan Remo, Southern Illinois University Departments of Geography and Geology and Institute of Environmental Sciences, Miami University
Uncertainties and Assumptions • HEC-RAS may not be appropriate for small streams such as these. • Local variations in channel widths & depth may have significant effects on model output. • Modeled incision only; no width changes. • LiDAR (<1m resolution) channel cross- sectional data are not accurate for channels that contained significant water at the time of the survey
Stream and Basin Characteristics Estimated Water- Mean Average Current Incision shed Annual Site Slope since Land Use (% of Size Discharge ratio 1930s watershed) ( m 3 /s)* (km 2 ) (m)** Agricultural: 74 Four Mile 848 0.003 8.9 2-4 Development: 10 Creek 430*** Forest: 16 Agricultural: 74 Indian 270 0.003 2.8 1-2 Development: 9 Creek 215*** Forest: 17 Agricultural: 50 Bull Run 5 0.01 0.05 2-3 Development: 26 Forest: 21 *NHD Plus data; ** estimated based on aerial photos and observations; ***study reach
Sensitivity analysis • Sensitivity analysis of Mannings coefficients (0.025-0.04) of the channel found that differences of Q range from 0-3.5%, channel velocity from 7 to 48%, stream power from 2 to 58% and flow depth from 1 to 13%.
Modeling Procedures Boundary Conditions 10m DEM LiDAR DEM for channels NLCD Land Use/Land Cover SSURGO Soils NOAA Precipitation Freq. Data HEC-GeoHMS HEC-GeoRAS Modify Calibration Sensitivity channel against HEC-HMS HEC-RAS analysis cross observed sections hydrographs Output Output Inflow and tributary Outflow hydrograph flows peak discharge event hydrograph stream power, flow depth & velocity
Modeling effect of incision on flow hydrographs • A reach beginning in mid-catchment and continuing to catchment outlet was selected. • Passage of a ~2-year, 6-hour event was simulated in HEC-RAS. • Peak velocity, stream power, and flow depth were averaged through the model reach
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