Storm Runoff and Baseflow Water Quality Modeling Studies for Austin Creeks

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City of Austin Environmental Resources Management Division

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City of Austin Environmental Resources Management Division


EXECUTIVE SUMMARY: This study updates and summarizes the following City of Austin reports: 1. Stormwater Quality Modeling Study for Austin Creeks 2. Barton Springs Water Quality Trend Analysis 3. Baseflow Water Quality Trend Analysis for Austin Creeks. The objectives of this study are to determine the existing water quality conditions and trends of Austin area creeks, and the effects of urban development on water quality of the creeks. The creeks included in this study are Barton, Bull, Shoal, Boggy, Williamson, Waller, Walnut, Bear, Onion, and Slaughter. The results of the study can be applied to all creeks in the Austin area based on the degree of watershed imperviousness for each creek. The water quality parameters included in this study are solids, organics, nutrients, bacteria, metals, and a few toxic substances. Data (1975-1987)" were obtained mainly from the USGS/City of Austin cooperative monitoring program. The study used statistical methods such as univariate analysis, regression, and analysis of variance. The SAS software was used for the statistical analysis. The analysis consists of rainfall modeling, rainfall to storm runoff to pollutant load/concentration regressions, and baseflow concentration level and time trend studies. The development condition of a watershed is represented by watershed imperviousness. The runoff volumes, pollutant loads, and concentrations for individual storms and for average annual condition were correlated with watershed imperviousness. It was found that in general, both the storm runoff volume and pollutant load increase with increasing percent impervious cover. For several pollutant parameters, the storm event and baseflow mean concentrations also increased with imperviousness. There was either no significant time trend in storm event mean concentrations (EMC) or the EMC data are insufficient for time trend analysis. For most of the creeks and Barton Springs, the time trends of baseflow concentrations are not significant. However, there were significant time trends for some nutrient parameters at a few monitoring stations on Walnut, Williamson, and Onion Creeks. In summary, the water quality of Austin area creeks depends to a large extent on the quality and quantity of stormwater runoff, which in turn depend on percent impervious cover. As percent impervious cover increases, stormwater runoff, pollutant load, and possibly pollutant concentration, increased for any given rainfall. Therefore, the fully urbanized high impervious cover watersheds such as Shoal and Boggy Creeks represent the worst water quality condition. The least developed watershed such as Barton Creek has the best water quality condition. The effect accelerates as the rainfall depth of the storm event increases.


This report parallels information contained in "Stormwater Quality Modeling for Austin Creeks", adding drainage area data, percentage of impervious cover, and runoff data for Waller Creek at 38th street.

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