2002-06-26 CC Packet - Laserfiche WebLink

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. Retention_Svstems - "W'erponc1s percent (SEWRPC, 1991). Seepage through the embankment can also affect the stability of the structure. Seep- age can generally be minimized by adding drains, anti seepage collars, and core trenches. The embankment side slopes can be protected from erosion by using minimum side slopes of3:] and by covering the embankment with vegetation or riprap. The embankment should also have a minimum top width of2 meters (6 feet) to aid in mainte- nance. Scour Control: Scour is the erosion of pond bottom or bank material due to high flow velocities. Scour control is important to maintain the function of the pond and reduce erosion, especially near the inlet. Inlet areas and inlet structures should be designed to control velocities at the inlet whether from large or small storm events. Flow-diffusion devices, including plunge pools, directional berms or other specially created dissipation structures, are often recommended. For annual events, the velocity leaving the inlet area and entering the main treatment area should be less than I foot per second (fps). Decreasing velocity reduces scour and more importantly reduces mixing currents that reduce treatment efficiency. The MPCA recommends that the following design storms be considered in the pond design (MPCA, 2000). Scour prevention can be achieved if velocities through the main treatment area are limited to the following maximums: I foot per second for the I-yr 2.4" event: 3 feet per second for the 2-yr, 2.8" event: 5 feet per second for the 10-yr, 4.0" event and the 100-yr, 6.0" event Velocities are calculated as outflow divided by the area of the critical cross-section. All events are considered to be the 24-hour NRCS distribution event. . Water Quantity Control Requirements: Wet ponds should be designed to meet both storm water quality and quantity control requirements. Storm water quantity requirements are typically met by designing the pond to control post-development peak discharge rates to pre-development levels. Usually the pond is designed to control multiple design storms (e.g. 2- and/or 10-year storms) and safely pass the 100-year storm event. However, the design storm may vary depending on local conditions and requirements. Design for Winter-Runoff Conditions During the winter period, the design volume of the wet pond can be less than desired. Ground freezing throughout the tributary watershed effectively increases the watershed's imperviousness, which increases the fraction of precipitation that reaches the pond. Moreover, the available volume in the permanent pool can be reduced by the formation of ice. Fortunately, winter rainfall and snowmelt events (as opposed to spring) typically produce lower volumes of runoff than summer storms and so most events will continue to be captured entirely (e.g, the volume of runoff will be less than the reduced volume of the wet pond). However, temperature regimes in the northern regions of Minnesota are such that ice cover may persist into the spring period when runoff rates and contaminant washoff rates are higher. In these areas, some authorities recommend that the wet pond volume be increased by an amount equal to the expected volume ofthe ice cover (Ontario Ministry of the Environment, 1999). Metropolitan Council! Barr Engineering Co. 3-261