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U.S. Department of the Interior U.S. Geological Survey Prepared in cooperation with the Johnson County Stormwater Management Program Casey J. Lee U.S.

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Presentation on theme: "U.S. Department of the Interior U.S. Geological Survey Prepared in cooperation with the Johnson County Stormwater Management Program Casey J. Lee U.S."— Presentation transcript:

1 U.S. Department of the Interior U.S. Geological Survey Prepared in cooperation with the Johnson County Stormwater Management Program Casey J. Lee U.S. Geological Survey, Lawrence, Kansas Sediment Transport from Urban, Urbanizing, and Rural Areas in Johnson County, Kansas,

2 Background Increased erosion and sediment transport from construction sites is well documented

3 Background Increased erosion and sediment transport from construction sites is well documented Sediment deposition and stream-channel erosion complicate relations between urbanization and sediment transport in larger basins

4 Objective Characterize how urbanization affects suspended- sediment transport across basin scales Small (5-11 mi 2 ) and large (45-65 mi 2 ) basins with similar natural features, but varied stages of urbanization were monitored Turbidity sensors were used as a surrogate for suspended-sediment concentration and were coupled with stage to compute sediment loads across space and time

5 Continuous turbidity monitoring

6 Study area Range of land- use and watershed sizes Similar topography and soils across county Urban construction quantified using annual changes in impervious surface Lee and others, 2009; Rasmussen and others, 2008) Monitoring site Urban Rural Urban Urbanizing

7 Turbidity is an effective surrogate for suspended sediment

8 Stormflow hydrograph at small urban and construction-affected basins More stormflow, and larger peak-flows at urban site

9 Sediment transport limited by available supply in urban basin

10 Typical construction-affected turbidigraph – transport limited Larger, extended, turbidity conditions at construction- affected site

11 Relatively dry year, small stormflow and sediment yields n = 1n = 2n = 3 Mean precipitation – 40.2 in. Stormflow yield Sediment yield More flow from urban basin, Similar sediment yields

12 Wet year, more flow/sediment n = 1n = 2n = 3 Mean precipitation in. Mean precipitation in. Stormflow yield Sediment yield Construction results in larger downstream sediment loads

13 2 nd wet year, less sediment at urbanizing sites n = 1n = 2n = 3 Mean precipitation in. Stormflow yield Sediment yield

14 Mean annual yield from Large urban basin transports most flow and sediment n = 1n = 3n = 1 n = 3n = 2 Stormflow yield Sediment yield

15 General conclusions Construction activity was a substantial source of suspended-sediment in small (5-11 mi 2 ) basins Despite improved management practice and dilution from intermediate basin Sediment concentrations at construction-affected sites were larger among both stormflow and low-flow conditions Among larger (45-65 mi 2 ) basins, the oldest urban basin had 2-10 times larger sediment yields than urbanizing or rural basins Sediment deposition, resuspension, and channel erosion have more influence on sediment transport

16 Report/Contact Info Report available online Casey Lee – USGS

17 Extra slides

18 Reservoirs trap suspended sediment

19 Sediment concentrations at construction-affected site throughout observed streamflow conditions Site downstream from most construction activity (5.5 mi 2 ) Median turbidity-computed sediment concentrations compared relative to streamflow exceedance Site downstream from stable urban land (8.8 mi 2 ) ~1,500 turbidity-computed SSC values for each percentage of streamflow exceedance

20 Sediment concentrations at construction-affected site also larger during low-flow conditions Median, 10 th percentile, and 90 th percentile of turbidity-computed sediment concentrations compared relative to streamflow exceedance 10 th percentile 90 th percentile Median Site downstream of most construction activity (5.5 mi 2 ) Site downstream of stable urban land (8.8 mi 2 ) ~1,500 turbidity-computed SSC values for each percentage of streamflow exceedance

21 Construction an immediate source of sediment in smaller basins

22 Relation more complicated with increasing basin size


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