Stormwater Runoff Quality and Quantity from Small Watersheds in Austin, TX: Updated through 2008

This report contains a variety of water quality data from several Waller Creek locations.EXECUTIVE SUMMARY: Almost all stormwater quality activities rely upon monitoring as their foundation to one degree or another. Design and construction of water quality controls or other best management practices (BMPs) are, or should be, based on monitoring data to ensure the BMP meets the desired goals. Rules and regulations that are not based on monitoring data may reflect the desire of the rule maker more than the science of the physical world. Modeling, which may be used to develop rules and design guidelines, is dependent on monitoring to first develop the stochastic or physical theories on which the model is based and then to calibrate the model for a specific location. The City of Austin (COA) engages in all of the above activities; proposing and enforcing development rules and regulation, developing design guidelines for and construction of BMPs, and modeling small and large watersheds. These activities are all based on a solid foundation of stormwater monitoring that has encompassed more than twenty-five years. The City participated in the Nationwide Urban Runoff Program (NURP) in 1981 (Engineering Science and COA, 1983) and included monitoring of two water quality control systems in their 1983-84 cooperative monitoring program agreement with the U.S. Geological Survey (USGS). These two monitoring projects were limited in both scope and duration (COA, 1984; USGS, 1987). In the mid-1980s, COA initiated a more comprehensive monitoring program to collect data to support a series of watershed management ordinances adopted by the City (COA, 1985). The original plan was to monitor eleven sites including seven water quality controls over a five-year period. The longer monitoring period was supposed to allow for monitoring that better reflected the local rainfall and runoff patterns since the earlier programs focused mainly on smaller events. The data from this program were the basis for much of the quality and quantity information in the current COA Environmental Criteria Manual (ECM) as well as initial discussions on the first-flush phenomena and design criteria for the Austin sand filter design. In 1990 COA started a comprehensive monitoring program to meet the City's ongoing stormwater monitoring needs (COA, 1996). These needs include evaluating the design and iii performance of different types of structural BMPs, evaluating effectiveness of education programs, evaluating and refining quality and quantity of runoff from different types of land use and meeting the requirements of the City's MS4 discharge permit under the National Pollution Discharge Elimination System (NPDES) and Texas Pollution Discharge Elimination System (TPDES) portions of the Clean Water Act. Through 2008, the Stormwater Quality Evaluation (SQE) Section of the Watershed Protection Department has collected runoff quality and quantity data from more than one hundred monitoring locations including twenty-eight BMPs and ten watersheds greater than five hundred acres. This report is intended to summarize the runoff quality and quantity data collected by the city of since 1981. During the preceding thirty years collection techniques, equipment and personnel have changed, all having an impact on data quality. However, the data used in this report represent a unique dataset in both scope and duration. While far from an exhaustive examination of the data, this report does verify some existing hypotheses and also challenges some existing assumptions. The relationship between total impervious cover (TIC) and Rv found in this report differs significantly from that found in the COA ECM (2009). If the relationship found in this report is adopted there will be no changes in capture volume requirements for BMPs currently found in the COA ECM except wet ponds which would be larger for most cases. There could be impacts on the designs for alternative controls as well. An earlier COA study (2006) found no difference between the runoff from recharge and non-recharge areas, so only one relationship is presented here. It was demonstrated that some mean pollutant concentrations changed with development conditions. Ammonia (NH3), lead (Pb) and zinc (Zn) increased exponentially with impervious cover. Total phosphorus (TP), dissolved phosphorus (DP), total Kjeldahl nitrogen (TKN) and total nitrogen (TN) increased as the fraction of non-urban land decreased. Chemical oxygen demand (COD), 5-day biochemical oxygen demand (BOD), cadmium (Cd) and copper (Cu) increased linearly as total impervious cover increased. Fecal coliform (FCOL) increased as the fraction of single-family residential (SFR) land use increased while volatile suspended solids (VSS) varied with changes in SFR and commercial land uses. Nitrate + nitrite (NO3+NO2) iv concentrations were different between developed and undeveloped areas but there were no significant relationships with impervious cover or land use. Fecal streptococci (FSTR), total organic carbon (TOC) and total suspended solids (TSS) were not significantly related to any changes in development condition tested in this report. A table was prepared to replace the existing COA ECM (2009) stormwater concentration assumption in Tables 1.10 and 1.11. This change would have no impact on existing BMP designs but would impact the design of alternative controls. It was found that using disconnected impervious area (DCIA) instead of TIC did not result in improved predictions of mean concentrations or runoff-rainfall ratios, Rv. DCIA was estimated in this report based on empirical relationships developed elsewhere. If local relationships are developed or if DCIA were actually measured, this conclusion may be different. Significant relationships were developed to predict event mean concentrations (EMCs) for the pollutants studied and four classes of development. The models used one or more of the following as predictive variables: preceding dry time, 15-minute peak rainfall intensity and total rainfall. While these models were statistically significant, most models resulted in predictions that were no better than using the mean of the observed values. Better physical models are needed to predict EMCs, rather than relying on stochastic relationships. The analyses confirmed results of earlier studies that indicated runoff concentrations are not constant during a runoff event in small watersheds with moderate to high impervious cover. The first-flush effect was less pronounced (even non-existent for some pollutants) in undeveloped areas. While other studies focused solely on impervious cover, this report also examined the type of land use associated with the impervious cover. It was found that in SFR areas, nutrients, especially dissolved nutrients, exhibited a last-flush with pollutant concentrations increasing rather than decreasing as runoff volume increased. This effect may have a substantial impact future BMP design. Testing of proposed modifications to the NRCS curve number method found a slight improvement over the currently accepted method but it still under predicts runoff volumes for v smaller events: those of most concern for water quality design. While the curve number method may still be used for flood design, models based on physical processes should be employed when attempting to perform continuous simulations for water quality design.Waller Creek Working Grou

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