Micro-Pool Wet Retention System

Overview:

An apartment complex in Nashville, TN received a Notice of Violation (NOV) stating that their retention pond was out of compliance with the City of Nashville. Northstar was called to inspect the pond and determine the best course of action to resolve the issues identified by the City of Nashville.

Issues:

The ‘retention pond’ was designed to be a micro-pool system with two smaller pre-treatment pools that fed a larger retention basin. The system was not functioning as designed because both slow release orifices on the outlet structure had been plugged, which caused the system to hold water until the water level breached the emergency overflow on the concrete structure. The higher water level kept the incoming stormwater from getting the proper treatment in the pre-treatment pools. The water was bypassing the micro-pools with little to no settling time and dumping all of the sediment and other contaminants into the main retention basin. The result was a large amount of sediment accumulation in the main retention basin, causing poor water quality, excessive aquatic vegetation growth and a breeding ground for mosquitos.

Solution:

Northstar developed a scope of work and timeline to bring the system back into compliance, which was approved by the City of Nashville as well as the property manager. Northstar began the rehabilitation of this system by dewatering the pond. The plugs were removed from the slow release orifices in order to decrease the water level and access the underlying sediment in each basin. Sediment was then removed from both micro pools and the main retention basin. Rip rap overflows were found in between the pre-treatment pools and the main retention basin, and were need of repair. These overflows were designed to remove excess particles from the water as its spilled over into the main basin. The old rip rap was removed and new rip rap was installed in each overflow as well as at each inlet to stabilize the soil of the high energy areas. All bare soil was stabilized with a native grass blend, including the pond basin and the surrounding landscape that had previously been inundated. Northstar developed a management plan for the property manager to ensure the pond continues to function as designed and keep them in compliance.