Ashland Palomino Ridge Detention Basin Analysis
CM Archer assisted the City of Ashland in the review of the existing detention facilities located at the northern extent of the Palomino Ridge Subdivision due to periodic localized flooding that has occurred. Complaints had been received from residents regarding the failure of the detention facilities to detain appropriate storm events.
The development plans for the project, issued in 2001, show that no permanent detention basin was planned for the site. However, the land disturbance plans indicate the required construction of a sediment trap located on the northern extent of Caspian Circle, adjacent to the existing subdivision sanitary sewer lift station.
Sometime after the commencement of construction of the proposed infrastructure improvements for the subdivision, the decision was made to install a permanent detention basin on a lot which was dedicated to the City as green space.
It is believed that the structure was originally designed to be a retention basin which was a total depth of 8-ft, measured from the floor of the basin to the top of the berm. The outlet structure consisted of one 36-inch diameter HDPE pipe which penetrated the basin berm on the east side of the structure and discharged into the adjacent lot. The outlet structure was constructed such that its flowline was located approximately 4-ft above the bottom of the basin. The basin therefore retained 4-ft of water after each rain event, until such a time as the water evaporated or flowed out of the basin by means of infiltration.
Over time, the basin filled with sediment, to the extent that the basin depth, as measured from the bottom of the basin to the top of the berm is now approximately 4-ft deep.
Archer reviewed analysis results and made several conclusions regarding the existing detention basin and recommended specific improvements to bring the existing detention facilities in conformance with current regulations and to reduce the potential for localized flooding downstream of the basin.