Rooftops can generate significant amounts of runoff volume during a storm; however, a Rain Water Harvesting System can capture this runoff for a future, beneficial use.
The most common use of harvested rainwater is landscape irrigation. For commercial developments, rain water harvesting systems can be used as an innovative water quality control and are eligible for a full or partial water quality credit. The objective of the water quality credit system is to provide flexibility for meeting the City’s water quality requirements. The amount of credit earned can be applied as either a reduction in the size of a water quality control or a reduction in the water quality fee-in-lieu cost.
Rainwater harvesting systems can provide equivalent treatment to a more common sedimentation/filtration system and can be used in the Barton Springs Zone. The City of Austin has two design options for rain water harvesting, called Option A and Option B.
Design Option A has the captured runoff discharged to a vegetated area for infiltration:
There are requirements to be eligible for water quality credit the vegetated area must meet the following criteria per the City Of Austin Environmental Criteria Manual:
- The length (dimension in direction of flow) of the vegetative area should be at least 15 feet.
- The average slope of the vegetative area must be between 1% and 10% with no portion exceeding 15%.
- The hydraulic loading rate should not exceed 0.05 cfs per ft. width for the maximum flowrate applied to the vegetated area. Higher hydraulic loading rates are allowed, but will reduce water quality credit. In this case, a maximum allowable rate of 0.15 cfs per ft width is allowed.
- The soil depth should be a minimum of twelve inches
- The vegetated area should have dense vegetative cover (minimum 95% coverage as measure at the base of the vegetation). The use of native grasses is strongly recommended due to their resource efficiency and their ability to enhance soil infiltration. In the case of natural wooded areas where 95% vegetative cover is not present, a minimum of four inches of leaf litter or mulch must be in place.
- An irrigation plan is also required.
Design Option B has captured runoff used to irrigate a vegetated area:
The required water quality volume must be provided by the designer and the maximum drawdown time that can be used is 120 hours. The system should be designed according to the retention/irrigation criteria in Section 1.6.7.A in the Environmental Criteria Manual.