Planted Detention Bays
During larger precipitation events, our porous parking bays overflow to two detention bays planted with native wetland plants. They offer additional water storage while filtering and slowing any water that escapes.
In a perfect world, every drop of water that falls on our parking lot would filter through the gravel basins, soaking into the subsoil to eventually become groundwater. Our region's soils are characterized by a dense, clay and silt rich layer called fragipan (not to be confused with delicious frangipane). This acts like a barrier, causing stormwater to perch above the subsoil for considerable time before finding its way to groundwater.
This means enough rainfall can fill the storage areas beneath permeable pavements completely, necessitating an overflow system. We chose to send our parking lot overflow to small depressions in the earth known as detention basisns or settling ponds. These are further optimized by adding a variety of wetland plants like blue flag iris, cardinal flower, grasses, sedges, and bulrush. The addition of plants, their roots, and the soil media required to grow them increases the amount of filtration the bays perform. Microbial processes at the root zone work to remove excess nutrients and contaminants from water before it either runs off or sinks into the ground. The plants also help to disperse water by moving it through their tissues using a process called evapotranspiration.