Establishing Flow Goals for Urban Stream TMDLs, and Implications for LID Design Standards Michael Dietz & John Clausen Low Impact Development Symposium Philadelphia, PA September 25‐28, 2011 Background • Common assumption that urbanization increases discharge • Impervious Cover model assumes impact increases with IC • Schueler says only applies to basins 5‐50 km2 • Schueler also notes IC/hydrology relationship inconclusive Schueler, T., McNeal, L.F., and K. Cappiella. 2009. Is impervious cover still important? Review of recent research. Journal of Hydrologic Engineering. Vol. 14(4), pp. 309‐315. Background • Size LID features to capture 1 inch rainfall event, catch 90% of events, allow large events to pass – aka “Water Quality Volume” Groton, CT Common generalization of urbanization effects What is “normal” runoff? • Mostly forested basins in northern VT – Runoff = 50% of precipitation (20 year period of record) • East Branch Penobscot in northern ME – Runoff = 57% of precipitation (1902‐2006) • Mt. Hope River in northeast CT (USGS index station) – Runoff = 48% of precipitation (1940‐2006) Objective • Determine the long‐term rainfall/runoff relationship for Connecticut basins • Determine what effect IC has on the runoff coefficient Study basins Methods • Choose USGS stations in state – More than 20 years of record • Gather data – Get long‐term runoff from annual reports – Delineate basin for each station (StreamStats) Methods • Obtain long‐term precipitation for basin – Closest NCDC station • Determine percent IC in each basin – NLCD 2006 • Calculate runoff coefficient (runoff/precip) Basin characteristics USGS Basin Period of Average Runoff Basin Name # Record Size (km2) runoff (mm) coefficient % Impervious Broad Brook 01184490 1961-2006 40.1 560.3 0.48 3.7 Coginchaug River 01192883 1981-2006 77.2 722.6 0.54 2.5 Eightmile River East Branch 01194500 1938-2006 57.8 704.3 0.52 0.4 Hockanum River 01192500 1920-2006 190.1 559.1 0.48 14.4 Indian River 01195100 1982-2006 14.7 596.4 0.44 1.6 Mill River 01196620 1969-2006 63.5 697.0 0.52 8.4 Mt. Hope River 01121000 1941-2006 74.1 633.5 0.48 0.7 Natchaug River 01122000 1931-2006 450.7 608.3 0.46 1.3 Naugatuck River 01208500 1928-2006 673.4 700.3 0.56 9.0 Norwalk River 01209700 1962-2006 77.7 643.6 0.52 5.4 Pendleton Hill Brook 01118300 1959-2006 10.4 746.8 0.60 0.5 Quinnipiac River 01195490 1988-2006 45.1 674.1 0.51 24.5 Rooster River 01208873 1977-2006 27.5 531.9 0.47 35.8 Salmon Creek 01199050 1962-2006 76.1 580.1 0.50 0.9 Salmon River 01193500 1929-2006 259.0 643.1 0.48 1.9 Shetucket River 01122500 1929-2006 1046.4 618.0 0.47 1.9 Weekeepeemee River 01203805 1979-2006 69.4 685.5 0.55 0.9 Yantic River 01127500 1931-2006 231.3 641.9 0.48 1.4 Mean 193.6 641.5 0.50 17.3 Median 75.1 642.5 0.49 11.8 Standard deviation 273.7 61.4 0.04 15.1 Basin characteristics Size Average Runoff % (km2) runoff (mm) coefficient Impervious Mean 193.6 641.5 0.50 17.3 Median 75.1 642.5 0.49 11.8 St. dev. 273.7 61.4 0.04 15.1 Percent IC vs. runoff coefficient 0.70 0.60 0.50 y = 0.5072x‐0.008 0.40 R² = 0.02 coefficient 0.30 0.20 Runoff 0.10 0.00 0 10203040 Percent impervious cover (2006) Recent runoff from study basins Does size matter? 0.70 0.60 y = 0.5323x‐0.013 0.50 R² = 0.04 0.40 coefficient 0.30 0.20 Runoff 0.10 0.00 0 500 1000 1500 Watershed size (km2) Watershed size 1400 Lewis 2002 Dietz & Clausen 2011 1200 US USGS Engman 1981 1000 800 600 400 Average runoff (mm) Average 200 0 0 2000 4000 6000 8000 Watershed size (km2) Average daily runoff, CT basins 3 2.5 2 1.5 1 y = 0.0033x - 4.7482 R² = 0.04 0.5 Average discharge (mm/day) Average 0 1900 1920 1940 1960 1980 2000 Year CT annual precipitation 1800 1600 1400 (mm) 1200 1000 800 y = 3.1619x ‐ 5020.4 precipitation 600 R² = 0.16 400 Annual 200 0 1900 1920 1940 1960 1980 2000 Year Average daily runoff for entire USGS gaging station network 1.6 1.4 1.2 1 0.8 0.6 0.4 y = -0.0022x + 5.0821 R² = 0.12 0.2 Average discharge (mm/day) Average 0 1900 1920 1940 1960 1980 2000 Year Caveats • Looks at aggregate runoff and does not take into account shifts in short‐term hydrologic function (e.g. stream ‘flashiness’) – Surface runoff vs. infiltraon→shallow groundwater→baseflow • Doesn’t imply anything about water quality Implications Possible mechanisms • Groundwater/surface water total – Long‐term runoff averages different than short‐ term changes (e.g., “flashiness”) • Bank storage • Storage in downstream network So what? • Generalizations about hydrology are tricky in IC model • Can this be used to guide design? • Flow TMDLs – How is goal determined? Daily discharge at Eagleville Brook 1600000 0 1400000 2 1200000 4 1000000 800000 6 600000 8 400000 Daily discharge (cf) 10 Daily precipitation (in) 200000 0 12 1/25/2010 2/25/2010 3/25/2010 4/25/2010 5/25/2010 6/25/2010 7/25/2010 8/25/2010 9/25/2010 1/25/2011 2/25/2011 3/25/2011 4/25/2011 5/25/2011 6/25/2011 7/25/2011 8/25/2011 11/25/2009 12/25/2009 10/25/2010 11/25/2010 12/25/2010 Runoff coefficient=0.81 Design • 0.8 km2 watershed • 48% IC • High because of high IC (>36%), small basin, or some interaction? • Does it matter? Implications • Changed how I think about water cycling • How does this relate to the 1 inch design method? • Your thoughts? Thanks! [email protected].