Stormwater Demonstration Project
Purpose of the Project
The demonstration project allows stormwater drainage from Ustick and Maple Grove Roads be conveyed to the wetland where it will pass through a vegetated sand filter to remove oil, grease, and other pollutants produced from roadway and roadway tributary areas. The water quality entering the wetlands after it passes through the filter is anticipated to be clean, free of sediment, oil, grease, other pollutants, and should not negatively impact the wetlands. The treated stormwater will then be discharged to the Thurman Mill Drain prior to reaching the Boise River.
A vegetated sand filter has a grass surface underlain by 6 inches of topsoil, 18 inches of sand, and 12 inches of crushed drain rock. The filter is sized to treat stormwater runoff from approximately 80% of all the precipitation events occurring within the drainage basin. Twenty percent of storms will be too large for complete treatment. What can't be treated through the filters will be discharged into the wetland. Vegetated sand filters will remove oil, grease, heavy metals, bacteria, and other pollutants that may be contained in stormwater.
Specially selected grasses are placed on top of the filters supported by a layer of topsoil. The selected grasses not only tolerate both wet and dry conditions, but also have extensive roots systems that maintain soil porosity and maximize the grass’s ability to absorb nutrients contained in stormwater. The grass and topsoil also help remove smaller sediment particles that enter the filters.
Beneath the topsoil is a layer of sand. The sand layer removes nutrients, bacteria, and metals. After the stormwater passes through the sand, it enters a layer of crushed rock which conveys treated stormwater to the wetland. The water quality entering the wetland after it passes through the filters is anticipated to be clear, free of sediment, oil, grease, other pollutants, and will not be detrimental to the function of the existing wetland.
This project would not have happened without support from different entities. The Ada County Highway District (ACHD) is a key partner and developed additional wetlands on an adjacent 10-acre parcel they purchased, which they donated to the City to expand the Reserve. Boise City and ACHD have worked together to incorporate stormwater treatment and storage into the project, and Settlers Irrigation District partnered to provide irrigation water during the dry season while receiving water storage benefits. Cooperative approaches, such as this, allows project dollars to stretch farther and to resolve multiple problems at once.