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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 68 (1984)

Issue: 9. (September)

First Page: 1213

Last Page: 1213

Title: Identification of Short-Term Changes in Sediment Depositional Rates--Importance in Environmental Analysis and Impact: ABSTRACT

Author(s): Wayne C. Isphording, F. Dewayne Imsand, Gregory W. Isphording


Large-scale urban development projects may profoundly affect erosion and depositional rates in adjacent estuaries, bays, and lagoons. The magnitude of such changes, however, is commonly 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 on the sediments. This is especially apparent 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 yr. In cores collected in the bay, abrupt changes in (1) sediment size parameters, (2) heavy mineral and clay mineral ratios, (3) sulfur content, and (4) zinc, copper, and vanadium percentages were noted. Each of these changes occurred at the same depth and reflected a simultaneous increase in sediment influx into the basin and the onset of urban development in the watershed. Analysis of the core data also permi ted 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 sedimentation rates, and the efficiency loss of the bay's sediment trap.
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