Stormwater Quality Modeling for Austin Creeks

This report "analyze(s) the hydrologic and water quality that are available from various monitoring programs." One such monitoring program exists at Waller Creek. As such, the report includes information about Waller Creek's surface area, including a breakdown of whether the area is residential, commercial, industrial, street, public, or undeveloped. Lastly, the report includes runoff coefficients for Waller Creek above 23rd Avenue.EXECUTIVE SUMMARY: This study is one of several modeling studies which were initiated to compile and analyze the hydrologic and water quality data that are available from various monitoring programs. The objective of this study was to develop mathematical models which describe stormwater runoff and pollutant load for Austin creeks. The watersheds of the creeks have varying degrees of multiple land use development. The creeks included in this study are Barton, Bull, Shoal, Boggy, Bear, Walnut, Waller, and Williamson. The pollutant parameters included are fecal coliform (FC), total suspended solid (TSS), Nitrate (NO3), Ammonia (NH3), Total Kjeldahl Nitrogen (TKN), Total Phosphorous (TP), Total Dissolved Solid (TDS), Biological Oxygen Demand (BOD5), and Total Organic Carbon (TOC). Data for this study were obtained mainly from the USGS/City of Austin Cooperative Monitoring Program. Rainfall to runoff to .pollutant load relationships were developed for various pollutant parameters in Bull, Boggy, Shoal, and Barton Creeks, using monitoring data from significant storms occurring between 1976 and 1982. For each watershed the runoff volume versus rainfall depth and runoff pollutant load versus runoff volume relationships were fitted to linear or geometric equations using the least square methods. In general, variability measures indicated that these equations provided a good representation of the actual situation. Using the equations developed in this study and available rainfall data, the runoff volume, and in turn the pollutant loads, were computed for all rainstorms within a specific historical period. The monthly pollutant load was estimated by summing the loads generated from the individual storms. The estimated loads for one-month periods were averaged over time to yield pollutant load for a particular calendar month. The sum over all twelve (12) months of pollutant loads for a particular pollutant parameter was used to produce the predicted annual load for each watershed. It was found that in general, both the rainfall runoff and pollutant load - increased with increasing percent impervious cover, an indication of urban development. Thus, as impervious cover increased, a given rainfall depth produced greater runoff depth, and in turn, greater pollutant load. The runoff and pollutant load to impervious relationships for storm-event conditions were developed. Pollutant concentrations also increased with imperviousness in most cases. Baseline water quality conditions were evaluated for Bull, Barton, Shoal, Boggy, Bear, Walnut and Williamson Creeks. The storm-event and baseline average pollutant concentrations for the study watersheds are presented. In summary, the water quality of Austin area creeks depends on the quantity of stormwater runoff, which in turn depends on percent impervious cover. As percent impervious cover increases, pollutant load increases for a given rainfall. The effect generally accelerates as the rainfall depth of the storm event increases. Monthly and annual pollutant loads predicted for Austin area creeks may be useful for planning and design purposes. This report supersedes the interim report on this subject completed January 1983.Waller Creek Working Grou