The topography of the site should allow for the design of a drainage channel with sufficient slope and cross-sectional area to maintain a non-erosive flow velocity, generally less than five feet per second. Other factors to be considered when planning for the drainage channel are land availability, maintenance requirements and soil characteristics. This usually occurs during the early growing season and dormant periods. Usually the greatest flow retardance occurs when vegetation is at its maximum growth for the year. Use the maximum expected retardance when checking drainage channel capacity. The two primary considerations when designing a drainage channel are maximizing channel capacity and minimizing erosion. The longitudinal slope of the swale should be as close to zero as possible and not greater than 5%. The topography of the site should allow for the design of a drainage channel with sufficient slope and cross-sectional area to maintain non-erosive flow velocities. When using drainage channels in combination with roadways and sidewalks, it is most appropriate to place the channel between the two impervious covers (e.g., between the sidewalk and roadway). They are also generally not compatible with extensive sidewalk systems. However, the effectiveness of drainage channels may decrease as the number of driveway culverts increases. Drainage channels can also be used in parking lots to break up areas of impervious cover.Īlong the edge of roadways, drainage channels can be used in place of curb and gutter systems. The percentage of impervious cover in the contributing areas must be relatively small. A water quality swale or grass channel must be used in place of the drainage channel when a water quality treatment credit is sought.ĭrainage channels are suitable for residential and institutional areas of low to moderate density. Pollutant removal rates are significantly higher for water quality swales and grass channels. Water quality swales and grass channels, on the other hand, are designed for the required water quality volume and incorporate specific features to enhance their stormwater pollutant removal effectiveness. Therefore, pollutant removal efficiency is typically low for drainage channels. Other than basic channel size and geometry, there are no other design modifications to enhance pollutant removal capabilities. They must be vegetated with grasses to maintain bank and slope integrity. The dimensions (slope and bottom width) of a drainage channel must not exceed a critical erosive velocity during the peak discharge. Drainage channels typically have a cross-section with sufficient hydraulic capacity to handle the peak discharge for the 10-year storm. Drainage channels are designed to have sufficient capacity to convey runoff safely during large storm events without causing erosion. They are a component of a larger stormwater management system and serve to convey runoff from impervious surfaces to or from stormwater treatment BMPs.ĭrainage Channels versus Water Quality Swales The distinction between drainage channels and water quality swales lies in the design and planned use of the open channel conveyance. Use of road salt or other deicers during the winter will necessitate yearly reseeding in the spring.ĭrainage channels cannot be used to meet the Stormwater Management Standards. The first few months after construction and twice a year thereafter. Inspect channels to make sure vegetation is adequate and for signs of rilling and gullying. Pathogens (coliform, e coli) - Insufficient dataĪdapted from the University of New Hampshire.Metals (copper, lead, zinc, cadmium) - Insufficient data.Nutrients (Nitrogen, phosphorus) - Insufficient data.Large area requirements for highly impervious sites.May be impractical in areas with flat grades, steep topography or poorly drained soils.Provides limited pollutant removal compared to water quality swales.Roadside channels are subject to damage from off-street parking and snow removal.Higher degree of maintenance required than for curb and gutter systems.Roadside channels reduce driving hazards by keeping stormwater flows away from street surfaces during storms.Generally less expensive than curb and gutter systems.May be used to achieve temperature reduction for runoff discharging to cold-water fisheries They receive no infiltration or TSS removal credit (Standards 3 and 4).Ībility to Meet Massachusetts Stormwater Management Standards Standard Drainage Channels From the Massachusetts Stormwater Handbook Descriptionĭrainage channels are traditional vegetated open channels that are designed to provide for non-erosive conveyance.
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