Integrating Water Quality With Water Quantity: City of Austin's Waller Creek Water Quality Retrofit Project
The City of Austin Watershed Protection Division (WPD) retained Freese and Nichols (FNI) to retrofit an existing Central Austin dry stormwater detention facility by integrating it with a new stormwater water quality wet pond. The WPD’s goal was to protect their community by reducing the impact of flooding, maintaining and improving water quality and creating a stable stream system to prevent erosion. FNI’s integration of a wet pond into the existing dry detention pond helped the WPD achieve these goals for stormwater runoff into Waller Creek.
Existing Dry Detention Facility
The existing dry detention pond served a 560-acre watershed comprised of nearly 60-percent impervious cover. The existing facility was designed to allow Waller Creek to flow unobstructed up to the two-year storm event. However, during larger, less frequent storm events, an existing concrete wall created a backwater effect that backs water over two levees into off-line storage basins (north and south of Waller Creek). These basins detained approximately 35 acre-feet of runoff.
Improving Water Quality Using a Wet Pond
The water quality component was added by excavating the southern off-line storage basin to provide extended detention (72 hours) for the “first flush” and creating a permanent stormwater water quality wet pond. Because the existing dry detention facility allowed frequent flows to bypass the off-line basins, a new diversion structure was built in Waller Creek to force the water quality volume to flow into the extended detention basin rather than flowing downstream. Once the extended detention basin is full, the diversion structure allows stream flow to bypass the pond, and the facility works as it was designed.
Wet Pond Features
The integrated stormwater water quality wet pond includes a permanent pool volume of approximately161,100 cubic feet, including a sedimentation forebay of 19,100 cubic feet. The permanent pool includes a clay liner to minimize water loss from the wet pond and a 5,800 square-foot vegetated bench to provide for nearly 1,000 wetland plantings. The extended detention volume of the pond (i.e. the water quality volume) is nearly 289,500 cubic feet and represents the first 0.14 inches of runoff from the watershed. Although this treatment depth is less than that required for new ponds, it meets the City’s goal for retrofit projects. Finally, the stormwater water quality wet pond includes more than 150 western mosquitofish (gambusia affinis) to avoid creation of a mosquito breeding pond.
Why Use Wet Ponds?
There are several reasons wet ponds may prove to be beneficial for your community:
- In a wet pond, pollutants are removed through physical and biological processes.
- Wet ponds have higher level of nutrient removal and water quality
Improving Water Quality in Your Own Existing Wet Ponds
Some existing wet ponds mainly provide aesthetics and/or flood control, and their ability to enhance water quality can be improved. To find out if your existing wet pond’s water quality efforts could be improved, calculate the:
- Water quality volume needed to treat the contributing watershed
- Stream protection volume (if applicable) needed to detain the runoff from the contributing drainage area to protect the adjacent stream or body of water
If these two calculations are larger than the existing volume of the pond, upsizing must be considered to provide enhanced water quality.
If these two calculations are smaller than the existing volume of the pond, no further upsizing is necessary, and you can determine the efficiency of the pollutant removal of the existing pond. However, hydraulic control structures may need to be adjusted to provide enhanced water quality.
For more information, please review the Urban Subwatershed Restoration Manual No. 3: Urban Stormwater Retrofit Practices (Version 1.0).