4.3 Dispersion

Dispersion attempts to minimize the hydrologic changes created by impervious surfaces by restoring the natural drainage patterns of sheet flow and infiltration. Chapter 6 of the 2004 SWMMEW provides design guidance for three types of dispersion BMPs, including:
  • Concentrated Flow Dispersion (BMP F6.40).
  • Sheet Flow Dispersion (BMP F6.41).
  • Full Dispersion (BMP F6.42).

Dispersion is generally cost-effective for projects with sufficient space available on-site, because limited infrastructure needs to be installed and maintained. In some situations, where infiltration is feasible and setback requirements can be met, dispersion may provide adequate runoff treatment and flow control to partially or fully satisfy Ecology’s Core Elements #5 and #6. A brief description of concentrated flow, sheet flow, and fulldispersion is provided below.

Concentrated Flow Dispersion (BMP F6.40)
Concentrated flow dispersion spreads flows from driveways or other pavement types through a vegetated pervious area, attenuates peak flows by slowing entry of the runoff into the conveyance system, allows for some infiltration, and provides some water quality benefits. This BMP can be applied in any situation whereconcentrated flow can be dispersed through vegetation. Dispersion for driveways will generally be most effective for single-family residential lots and in rural development. Urban development typically does not provide sufficient space for effective dispersion of driveway runoff. Figure 4.3.1 shows a concentrated flow dispersion BMP with a concrete and gravel level spreader at the upgradient side to help distribute flows across the grass dispersion area.

Sheet Flow Dispersion (BMP F6.41)
Sheet flow dispersion is among the simplest methods for runoff control. This BMP can be used for any impervious or pervious surface that is graded so as to avoid concentrating flows. Because flows are already dispersed as they leave the runoff-generating surface, they need only traverse a narrow band of adjacent vegetation for effective attenuation and treatment. See Figure 4.3.2 for a schematic illustration of this BMP.

Full Dispersion (BMP F6.42)
Full dispersion routes stormwater runoff from impervious surfaces and cleared areas of commercial, residential and roadway development projects areas of the site that are protected in a natural, vegetative cover condition. The natural vegetation is preserved and maintained in accordance with guidelines provided in Chapter 6 of the 2004 SWMMEW. This BMP is primarily intended for new development, but a “sliding scale” may be used to apply this BMP to other sites. Section 4.3.3: Sizing, provides additional information on use of the sliding scale.

Designs incorporating full dispersion in accordance with BMP F6.42 in the 2004 SWMMEW do not typically require additional runoff control. See the guidelines for dispersion in Chapter 6.5 of the 2004 SWMMEW. Also see the guidelines for dispersion in the 2011 Washington State Department of Transportation Highway Runoff Manual (2011 HRM) for roadway projects, where applicable.

Figure 4.3.1

Engineered dispersion with level spreader. Source: AHBL, Inc.

Figure 4.3.2
Basic dispersion components. Source: CleanWater Services