City of Leander Brushy Creek Regional Detention Study
Finding cost-effective regional solutions for mitigating flood risk
The City of Leander continues to experience rapid growth, and recent development near Brushy Creek has added to the impervious cover and quantity of stormwater runoff. In many cases, developers have not provided onsite detention since they were able to demonstrate through analysis that stormwater storage was not necessary due to peak runoff timing. While this may be true for an individual development on a case-by-case basis, the overall increase to impervious cover within the watershed has caused cumulative peak stormwater runoff and flood risks to increase. As a result, the City retained KFA in 2018-2019 to investigate solutions for regional detention to help mitigate the impacts of rapid development on flood risk.
KFA began by modifying the best available Upper Brushy Creek Water Control and Improvement District (UBCWCID) hydrologic and hydraulic models for this particular area with several large development projects in the planning process with Past (2005), Present (2019) and Future conditions impervious cover calculations based on City of Leander zoning maps, future development plans, and aerial imagery. Through this analysis KFA found that the previous methodology used to assess impervious cover in the UBCWCID HMS model, based on pixel color discretization of aerial imagery, was underrepresenting impervious cover. KFA also determined that the rapid development in the City has indeed increased the cumulative impervious cover in the watershed, increasing peak runoff, and that the cumulative impervious cover is likely to increase further in the near future.
To handle the increase in peak runoff due to the underrepresented impervious cover, KFA developed three potential solutions:
- Constructing a large regional detention pond at the confluence of Brushy Creek and the South Fork of Brushy Creek just west of US 183. This pond would be capable of storing 400 ac-ft and would reduce the 100-year peak flow at Brushy Creek and 183A by 25 percent.
- Constructing a smaller regional detention pond at the confluence of Brushy Creek and the South Fork of Brushy Creek just west of US 183. This smaller pond would be able to store 150 ac-ft and would reduce the 100-year peak flow at Brushy Creek and 183A by 19 percent.
- Constructing an underground culvert diversion system from the South Fork of Brushy Creek beneath N. West Dr. and discharging into UBCWCID Dam 2, capable of reducing 100-year peak flow at Brushy Creek and 183A by 12 percent.
Our team then investigated the feasibility and benefits of each solution, including cost, storage, and flood risk mitigation. This analysis provided the City with vetted mitigation options to consider as rapid development continues.
The project was completed on time and within budget in July 2019. During this study, it became apparent that it would be beneficial for the City to have a citywide model with Atlas 14 rainfall and recent development to eliminate the difficulties associated with developers creating their own individual models to analyze their project impacts. Therefore, following completion of the detention study, KFA was retained to update the Upper Brushy Creek Watershed hydrologic and hydraulic models and floodplain maps. This subsequent project, which was completed in 2021, gave the City a more robust understanding of flood risk for current and ultimate development conditions to help guide development.