The effect of urbanization on stream-flow patterns on big creek watershed, alpharetta, georgia

Abstract

The natural flow regime, defined as the characteristic pattern of discharge variation in streams and rivers not altered by humans, plays an important role in the ecosystem. The increase in urban development in the U.S. is an important environmental change that alters the landscape and affects the flow regime of streams and rivers. Previous hydrological studies have used different methods to investigate the effects of urbanization on stream flow; these studies have found that urbanization can often lead to an increase in peak flow (largest instantaneous magnitude occurring in a water year), increased flashiness (rapid change in magnitude), and reduced baseflow (relatively steady stream-flow primarily maintained by groundwater between storm runoff events). With increasing urbanization in the southern U.S., my study focused on Big Creek watershed, located in one of the most populated cities, Metro-Atlanta. Several studies have investigated urbanization effects on stream-flow in Metro-Atlanta, but these studies have focused on the Peachtree Creek watershed, a highly urbanized watershed located near downtown Atlanta. In contrast, the Big Creek watershed is a less intensively developed area about 20-35 miles from downtown Atlanta. I used a paired watershed approach with a nearly 60-year period of record to characterize Big Creek discharge before and after urban development and to compare changes in Big Creek to a non-urban reference watershed, Snake Creek. Several hydrological trend analyses and pre- versus post-urbanization comparisons revealed that urbanization altered some features of the flow regime. The post-urban period in Big Creek resulted in an 11% increase in high flow (defined as 5% annual exceedance probability), 25% decrease in low flow (90% annual exceedance probability), 27% decrease in average peak flow, 14% decrease in average baseflow, and a flashier discharge. Current stormwater management strategies within Big Creek target minimizing peak flow events by requiring facilities to control for the 2-year to 100-year flood event. The findings indicate that stormwater management strategies should also aim to restore baseflow and reduce flashiness within the watershed.