Effects of Urbanization on Streamflow and Sediment Transport in the Rock Creek and Anacostia River Basins, Montgomery County, Maryland, 1962-74

Yorke, Thomas; Herb, William

Land use/land cover, precipitation, streamflow, and sediment data were collected from nine drainage subbasins in a 32-square-mile (83-square-kilometer) area north of Washington, D.C., in Montgomery County, Md. This study was begun in 1962 to define urban runoff and sediment problems and was expanded in 1966 to evaluate response to sedimentcontrol practices in areas undergoing urban development. Land use/land cover varied considerably in the study subbasins, which ranged in size from 0.35 to 21.1 square miles (0.91 to 54.6 square kilometers). Three subbasins remained virtually rural, while the others underwent urban development. In 1974, urban land represented from 0 to 60 percent of the land use in the nine subbasins. Urbanization did not affect median and low flows, but did increase storm runoff and peak discharges. Suspended sediment transported from one of the basins that underwent urban development, the 21.1 square mile (54.6 square kilometer) Anacostia River basin, averaged 14,800 tons (13,400 tonnes) per year between 1962 and 1974, while urban construction averaged about 3 percent of the drainage area during this period of time. Bedload was estimated to be 6 to 13 percent of the total sediment load. Most of the suspended load was transported during storms, and varied from storm to storm and seasonally. High loads were generally associated with high runoff in the spring and intense thunderstorms in the summer. Cropland, urban land, and construction sites were the major sources of sediment. Annual suspended-sediment yields from land under cultivation ranged from 0.65 to 4.3 tons per acre (1.5 to 9.6 tonnes per square hectometer), compared to estimated yields from forest and grasslands ranging from 0.03 to 0.2 ton per acre (0.07 to 0.45 tonne per square hectometer). Yields from urban land were about 3.7 tons per acre (8.3 tonnes per square hectometer), most of which came from stream-channel erosion. Average annual suspended-sediment yields computed for urban construction sites ranged from 7 to 100 tons per acre (16 to 224 tonnes per square hectometer). The magnitude of the yield from construction sites was significantly affected by (1) the average slope of the sites, (2) the proximity to stream channels, (3) the existence of buffer zones of natural vegetation, and (4) the use of sediment-control meas,ures. Sediment controls, particularly those enforced under a 1971 sediment-control ordinance, decreased construction-site suspended- sediment yields by 60 to 80 percent. It was estimated that the suspended-sediment load in the Anacostia River basin between 1962 and 1974 would have been reduced by 50 percent if strictly enforced sediment controls had been used throughout the period. Based on a reported cost of $1,030 per acre (0.4 square hectometer), the cost of sediment controls on the 1,900 acres (769 square hectometers) developed during the period would have been $19 per ton ($21 per tonne).

Effects of Urbanization on Streamflow and Sediment Transport in the Rock Creek and Anacostia River Basins, Montgomery County, Maryland, 1962-74

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