1996-02-06_REVISION - M1977493 (14)

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Chanter 4-Site-Specific 1nAai Storm water BMPs highest flow rate that is expected from a typical storm to determine the most practical size for the swale (in keeping with State or local requirements). The swale should be tilled before grass is planted, and a dense cover of grasses should be planted in the swale. The lation of a road, then the grass chosen should be resistant to the use of de-iici g salts in swale runs next t t northern states). Check dams (i.e., earthen or log structures) may be installed in the swales to reduce runoff speed and increase infiltration. Planners should also consider the design of the outlet at the end of the swale so that the runoff is released from the swale at a low rate (see section on Outlet Protection). Maintenance activities for the swales include those practices needed to maintain healthy, dense vegetation and to retain efficient infiltration and movement of the storm water into and through the swale. Periodic mowing, reseeding, and weed control are required to maintain pollutant removal efficiency. The swale and channel outlet should be kept free from sediment buildup, litter, brush, or fallen tree limbs. Periodic inspections will identify erosion problems or damaged areas. Damaged or eroded areas of the channel should be repaired. Areas with damaged vegetation should be reseeded immediately. Advantages of Grassed Swales • Are easily designed and constructed • Provide moderate removal of sediments if properly constructed and maintained • May provide a wildlife habitat • • Are inexpensive • Can replace curb and gutter systems • Can last for long periods of time if well maintained Disadvantages of Grassed Swales • Cannot control runoff from very large storms • If they do not drain properly between storms, can encourage nuisance problems such as mosquitos, ragweed, dumping, and erosion • Are not capable of removing significant amounts of soluble nutrients • Cannot treat runoff with high sediment loadings • 4-104