Dry and Wet SwalesDesign Criteria: Soil Bed

• Dry Swale

  • moderately permeable soils (USCS ML, SM, or SC)
  • 30” deep with gravel/pipe underdrain system

• Wet Swale

  • undisturbed soils
  • no underdrain

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The soil bed below the dry swale should consist of a moderately permeable soil material with a high level of organic matter. The soil bed should be 30 inches deep, and should be accompanied by a gravel/pipe underdrain system. This soil mixture is necessary in residential areas to ensure drainage of the swale system within a moderately short time period to avoid safety and nuisance concerns. The soil/gravel interface should be roto-tilled to have an approximate six inch mixing zone of fine sand, soil, and gravel to augment the filtering capability of the practice.

The soil bed below the wet swale should consist of undisturbed soils. This area may be periodically inundated and remain wet for long periods of time, and is therefore not appropriate for residential land uses.