Post-restoration evaluation of two urban streams in Austin, Texas, USA

Abstract

Rapid urban growth of Austin, Texas, has resulted in significant alteration of the surface characteristics of the Colorado River Watershed. These changes have increased the runoff and accelerated erosion of the banks of stream channels. To minimize the threat of stream erosion to real estate and infrastructure, the City of Austin began restoring unstable channel reaches through the placement of rock armor on the banks, construction of rock grade controls, and planting of riparian vegetation. Since the late 1990s, approximately thirty channel reaches have been restored in the Austin area. Considerable discussion is taking place regarding the true impact of restoration on streams. Few studies have attempted to conduct post-project evaluation to assess the impact of restoration efforts. Because it has been several years since steps were taken to stabilize these streams, a sufficient time period for stabilization to occur has passed. Thus, we believe these projects now can be assessed for the temporal impact of restoration on these streams. We studied the restored and natural reaches of two of these streams. The natural reaches served as ergodic surrogates for temporal channel development of the restored reaches. We used Rosgen?s (2001) methodology of channel stability assessment and repeat ground photography (Graf, 1985) to evaluate the stability of Waller Creek and Tannehill Branch. Variables of channel morphology analyzed included riparian vegetation cover, vertical stability, scour/deposition potential, and bed sediment composition. From our analysis, restoration enlarged stream channels, decreased bank height ratios and reduced flood prone width. Bed sediment analysis revealed that pools contain a higher percentage of fines whereas riffles are coarser in restored reaches than pre-restoration reaches. Visual examination of ground photographs and scores from the Pfankuch channel stability evaluation indicate that restoration increased vegetative cover and deposition. Thus, restoration efforts worked on these two streams. Data from the assessments of stream channel stability provide the basis upon which longer-term monitoring and evaluation can be conducted. Knowledge gained from long-term monitoring can be used to improve the effectiveness of the current and future restoration projects in Texas and elsewhere.