A Cycle of Sedimentation and Erosion in Urban River Channels

Abstract

Historical evidence and contemporary measurements indicate in the Piedmont of Maryland that successive changes in land use have been accompanied by changes in sediment yield and in the behavior of river channels. Sediment yields from forested areas in the pre-farming era appear to have been less than 100 tons/sq.mi/year. Yields from agricultural lands in the same region at a later time range from 300 to 800 t/sq.mi. on large drainage areas. Subsequently, on lands marginal to expanding urban centers, a decline in active farming may be accompanied by a decline in sediment yield. In marked contrast, areas exposed during construction can produce sediment loads in excess of 100,000 t/sq.mi./ year. Small channel systems become clogged with sand during this construction period. While sediment deposited in channels during construction is gradually removed by subsequent clearer flows, rates of removal are slow and hampered by deposition of debris. Increased runoff from urban areas coupled with a decline in sediment yields to values on the order of 50 to 100 t/sq.mi. promote continued bank erosion and channel widening. Maximum observed rates of bank erosion were on the order of 1.0 foot per year. Raw banks adjacent to coarse cobble bars and widespread deposits of flotsam and debris attest to the flood regimen of urban rivers. Canalization in concrete does not eliminate such debris nor does it eliminate deposition of sediment as local changes in gradient, excessive channel width, and debris accumulation foster deposition even in canalized reaches.