4.3.3 Sizing

Concentrated Flow Dispersion
Concentrated flow dispersion can be used in situations where concentrated flow can be dispersed through vegetation. The following sizing criteria apply:
  • A vegetated flow path of at least 50 feet should be maintained between the discharge point and any property line, structure, steep slope, stream, lake, wetland, lake, or other impervious surface.
  • A maximum of 700 square feet of impervious area should drain to each dispersion BMP.
  • A pad of crushed rock (2 feet wide by 3 feet long by 6 inches deep) should be placed at each discharge point.
  • No erosion or flooding of downstream properties may result.
  • Runoff discharge towards landslide hazard areas must be evaluated by a geotechnical engineer or qualified geologist. The discharge point shall not be placed on or above slopes greater than 6:1 or above erosion hazard areas without evaluation by a geotechnical engineer or qualified geologist and approval by the local jurisdiction.
  • For sites with septic systems, the discharge point should be downslope of the drain field primary and reserve areas. This requirement may be waived by the local jurisdiction if site topography clearly prohibits flows from intersecting the drain field.
See BMP F6.40 in the 2004 SWMMEW for additional design guidance.

Sheet Flow Dispersion
Sheet flow dispersion sizing criteria, based on the 2004 SWMMEW and the 2011 HRM, are as follows:
  • A 2-foot-wide transition zone to discourage channeling should be provided between the edge of the pavement and the downslope vegetation.
  • A vegetated buffer width of 10 feet of vegetation should be provided for up to 20 feet of width of paved or impervious surface. An additional 5 feet of width should be added for each additional 20 feet of width or fraction thereof.
  • A vegetated buffer width of 25 feet of vegetation should be provided for up to 150 feet of contributing cleared area (e.g., bare soil, non-native landscaping, lawn, and/or pasture).
  • Slopes within the dispersion area should be no steeper than 8 percent. If this slope criterion cannot be met, the flow length in the dispersion area should be increased by 1.5 feet for each percent increase in slope above 8 percent.
  • The resultant slope from the contributing pavement should be less than or equal to 9.4 percent, calculated as follows:
      SCFS = (G2 + e2).5

      Where:
        SCFS = resultant slope of the lateral and longitudinal slopes.
        e = lateral slope (e.g., superelevation of roadway or driveway) (%).
        G = longitudinal slope (grade) (%).
    Steeper slopes for contributing pavement areas may be allowed if gravel level spreaders are used between the pavement edge and the dispersion BMP and if the existing slopes are well vegetated and show no signs of erosion.
  • If evidence of channelized flow (rills or gullies) is present, a flow spreading device should be used before those flows are allowed to enter the dispersion area.
  • No erosion or flooding of downstream properties may result.
  • Runoff discharge towards landslide hazard areas must be evaluated by a geotechnical engineer or qualified geologist. The discharge point shall not be placed on or above slopes greater than 20 percent or above erosion hazard areas without evaluation by a geotechnical engineer or qualified geologist and approval by the local jurisdiction.
  • For sites with septic systems, the discharge point should be downslope of the drain field primary and reserve areas. This requirement may be waived by the local jurisdiction if site topography clearly prohibits flows from intersecting the drain field.
See BMP F6.41 in the 2004 SWMMEW and Chapter 5 of the 2011 HRM for additional design guidance.

Full Dispersion

Full dispersion is primarily intended for areas of new development. However, the 2004 SWMMEW provides a “sliding scale” to allow application to other sites (see Table 4.3.1). The preserved area should be situated to minimize the clearing of existing natural vegetative cover, to maximize the preservation of wetlands, and to buffer stream corridors. The preserved area should also be placed in a separate tract or protected through recorded easements for individual lots.

Table4-3-1

Areas of residential developments can meet treatment and flow control requirements by distributing runoff into native vegetation areas that meet the limitations and design guidelines below if the ratio of impervious area to native vegetation area does not exceed 15 percent. Vegetation must be preserved and maintained according to the following requirements:
  • If feasible, the preserved area should be located downslope from the building sites, since flow control and water quality are enhanced by flow dispersion through undisturbed soils and native vegetation.
  • The preserved area should be shown on all property maps and should be clearly marked during clearing and construction on the site.
  • Vegetation and trees should not be removed from the natural growth retention area, except for the removal of dangerous and diseased trees. See BMP F6.42 in the 2004 SWMMEW for additional design guidelines for full dispersion from roof downspouts, driveways, roadways, and cleared areas.


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