ABSTRACT

Large scale urban development projects may profoundly affect erosion and depositional rates in adjacent estuaries, bays and lagoons. The magnitude of such changes, however, are frequently ignored because of a general belief that no reliable parameters exist that will allow differentiation in sediment cores of natural versus man-caused phenomena. Though conversion of forested or agricultural land to commercial or residential use may well cause sediment erosion and depositional rates to be accelerated by up to several orders of magnitude, regulatory agencies and municipal governments have largely avoided entering into litigation with land developers over damage to adjacent water bodies because of a perceived difficulty in quantifying the amount of increased sediment yield.

A marked change in the depositional budget of a watershed, however, does produce a discernible impact upon the sediments. This was especially well seen in core samples collected in D'Olive Bay, Alabama, a small arm of Mobile Bay located adjacent to an area that has undergone extensive change from largely agricultural use to commercial and residential development during the past 15 years. Abrupt changes were observed at the same depth in cores collected in the bay for: (1) sediment size parameters, (2) heavy mineral and clay mineral ratios, (3) sulphur content and (4) zinc, copper and vanadium percentages. Each of these changes reflected the same simultaneous increase in sediment influx into the basin which, in turn, was associated with the onset of urban development in the watershed. Analysis of the core data also permitted accurate estimates to be made for the rate at which the bay is becoming filled, the volume of sediment deposited since the beginning of "impact," the sources within the watershed most responsible for the increased sedimenetation rates and the bay's sediment trap efficiency loss.