Preliminary Findings for Sizing a Bioretention Pond and an Analysis of Urban Stormwater Runoff: San Marcos, TX




Franco, Matthew R.

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The senior engineer for the City of San Marcos has designated an existing Stormwater Control Measure (SCM), a stormwater detention pond at the City of San Marcos City Hall, for improvements to better meet the stormwater quality standards of the Water Quality Protection Plan (WQPP), which has been drafted by the city. The WQPP is part of a larger objective, the Stormwater Master Plan (2018), that is geared toward managing non-point source (NPS) pollutants that accumulate in the stormwater runoff as it flows over impervious surfaces, like parking lots. This report will provide preliminary findings for sizing a bioretention (BR) pond that will replace an existing SCM that handles the runoff from the City Hall parking lot. The research utilizes the City of San Marcos Stormwater Technical Manual (CSM), which is an extension of the WQPP. The manual provides equations to calculate peak discharges, water quality volume (WQV), and the surface area for the BR pond related to Total Suspended Solids (TSS) loads. The WQPP contains equations to estimate Total Phosphorus (TP) and Fecal Coliform (FCOL) loads, which will be included separate from the TSS equations in the CSM. An analysis of urban stormwater runoff is conducted through the use of Event Mean Concentration (EMC) data from the City of Austin, TX, techniques used by the United States Geological Survey (USGS) to estimate pollutant loads and mean concentrations in the stormwater runoff, and the general stormwater quality categories listed in the WQPP. Peak discharges are based on a 2-year storm event (7 cubic feet per second) because they yield about an inch of precipitation, which is the most common type of rainfall that occurs annually and is the threshold to produce stormwater runoff from impervious surfaces. The TSS equations from the CSM estimate WQV to be 8,250 cubic feet, and the surface area is roughly 4,830 ft2. An analysis of EMCs, pollutant loads, and mean concentrations have revealed a wide range of stormwater pollutants that commonly exist in urban runoff. The estimates calculated using techniques from the USGS will affect the WQV and surface area sizing of the SCM because the CSM equations only provide values for WQV and surface area based on TSS. Following this USGS procedure, it is estimated that the proposed bioretention pond will reduce pollutants by 89 percent of the NPS pollutants from the first 1 in. precipitation event and thereby significantly reducing the NPS pollution entering the San Marcos River from this site.



stormwater management, bioretention, event mean concentration (EMC), stormwater control measure (SCM), urban stormwater quality, Applied Geography


Franco, M. (2019). Preliminary findings for sizing a bioretention pond and an analysis of urban stormwater runoff: San Marcos, TX. Master of Applied Geography Degree, Texas State University, San Marcos, TX.


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