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Stormwater Management System at East Wayne/Long Branch Local Park |
Over the past 50 years, urbanization has resulted in a substantial increase in the amount of impervious surfaces in the heavily developed Sligo Creek watershed. This has caused a decrease in groundwater levels, and an increase in stormwater runoff resulting in extensive damage to the creek. One way to correct these problems is by installing stormwater management retrofits.
In 2004 and 2005, MN-CPPC implemented a stream bank protection project in Long Branch. As part of that effort in the spring of 2005, M-NCPPC installed a Stormwater Management Retrofit project in the Long Branch watershed. The project consists of cutting into existing storm drainage systems that serve the park and a small portion of University Blvd., re-grading the ground, installing micro-pools and vegetated swales, and excavating and installing a 40-foot by 18-foot Surface Sand Filter, and installing a riser structure to control storm flows. The system provides stormwater management for approximately 4.5 acres. The system is designed to provide groundwater recharging, stormwater quality treatment, and stream channel protection.
See Surface Sand Filter System Diagram below. Stormwater runoff is initially slowed down through the micro-pools and vegetated swales. As the stormwater slows, sediment in the stormwater settles out. The plants and sand bed act as a bio-filter that absorbs nutrients and other contaminants. The initial rain, after it goes through the bio-filter, collects in the bottom of the Surface Sand Filter system (below a 6" perforated drain pipe) where it is allowed to slowly infiltrate or percolate into the soil and replenish the groundwater. Water that collects above the perforated pipe slowly goes into the perforated pipe and then re-enters the existing storm drainage system. This allows capture and treatment of the first inch of rain from the surrounding impervious areas. The first inch of rain, also called the first flush, is the most contaminated runoff. After the initial 1" of rain, the stormwater system is designed to limit stormwater flow into the creek by retaining water in a detention basin to provide channel protection. A slot built into a stormwater riser acts as an overflow.
Surface Sand Filter System: The Sand Filter System is a stormwater Best Management Practice (BMP) that captures and temporarily stores stormwater and passes through a filter bed of sand. Filtered stormwater runoff is then either returned to the conveyance system (remaining storm drainage system in this project) or allowed to slowly infiltrate into the soil.
Groundwater Recharge: Groundwater recharge is the slow infiltration of rainfall through the soil. Groundwater recharge is critical for replenishing groundwater levels (water table). The health of Sligo Creek's aquatic systems depends of steady groundwater discharge into the creek. For example, during periods of dry weather, groundwater-fed springs maintain water flow in the creek.
