Urban Channel Erosion Quantification in Upland Coastal Zone Streams of Virginia, USA

Abstract

To quantify sediment contributions due to urban channel enlargement, 50 study sites were selected on 1st- through 3rd-order streams, in watersheds with varying levels of urbanization, in two Physiographic Regions (Coastal Plain and Piedmont), and in the Coastal Zone Management Area of Virginia. At each site, riffle cross-sectional and longitudinal surveys were conducted to measure the channel morphology. Enlargement ratios for bankfull cross-sectional parameters were calculated to quantify channel change relative to stable streams (from regional curves). Relationships between dependent, channel characteristics and watershed-scale, independent variables were assessed. The main objectives were to: 1) test for differences in the morphological features between Coastal Plain and Piedmont streams; 2) develop relationships between watershed-level, urbanization characteristics and stream morphological features; and, 3) determine if relationships exist between watershed urbanization and channel enlargement ratios to estimate sediment loading from urban streams for use in statewide nonpoint source pollution assessment activities.

It was determined that: 1) for a given watershed area, streams in the Piedmont tended to be larger than those in the Coastal Plain Region (for regional curve streams and for project streams); 2) among all project sites and sites in the Piedmont, watershed area was the best indicator of channel morphology, but among the Coastal Plain sites, the number of road crossings over streams was the best indicator of channel morphology; and, 3) few significant relationships between enlargement ratios and watershed urbanization variables existed; however, one commonality observed across all sites was an inverse relationship between watershed area and channel enlargement ratios.