4.3.2 Design

This section provides considerations for design of the geometry, soils, and vegetation used in dispersion facilities. Additional design guidance is provided in the Sizing section below.

Geometry
See the discussion in Section 4.3.3: Sizing, for guidelines on how to size the length, width, depth, and slopes of dispersion facilities for effective flow control and runoff treatment.

Soil Amendment/Mulch

Dispersion areas can either contain natural soils or amended soils as necessary. Amended soils with appropriate organic content may enhance infiltration and plant establishment and growth, and reduce the need for summer irrigation and fertilizers. Soil requirements will vary depending on the selected plants and site conditions; however they shall be sufficient to maintain vegetated cover. Refer to Section 4.2: Amending Construction Site Soils, for additional guidance.

Vegetation

The dispersion area is planted or seeded with a mix of grasses, wildflowers, and/or groundcovers well-suited for moist to semi-arid soil conditions. Trees are generally not encouraged in dispersion areas, as they may affect the level spreading of flows across the surface. If soil amendments are used, native plants may not be appropriate and plants should be selected accordingly. Because unplanted areas may decrease infiltration and promote erosion, the entire dispersion area should have mature vegetation coverage by the end of the establishment period.

Plant selection should focus on species that require little maintenance after establishment. Native plants are encouraged, but adapted, non-invasive ornamentals may be acceptable for added aesthetic and functional value. The site’s micro-climate and soil conditions should be factored into plant selection. Consult with the local jurisdiction and/or Landscape Architect as appropriate. The reference plant lists in Appendix D provide examples of appropriate plantings.


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