Design and Implementation Resources

Measure soil infiltration rates:  By determining the infiltration rate of site soils before beginning the design of stormwater controls, designers can avert unwelcome and costly surprises.  Guidance materials generally recommend that the infiltration rate of native soils beneath infiltration practices be greater than 0.25 – 0.5 inches/hour. For a cautionary tale on the dangers of not incorporating measured infiltration rates into rain garden design, read about Seattle Public Utilities' experience with the Ballard Roadside Rain Garden pilot project (PDF) (10 pp, 318K, About PDF).
Amend soils beneath infiltration practices: Amending clay soils with compost or other organic matter can increase soil infiltration rates, while improving soil fertility and improving the ability of the soil to remove pollutants.  The Low Impact Development (LID) Center has developed a design specification for soil amended with compost.

Plant deep-rooted vegetation: Deep-rooted vegetation enhances soil infiltration rates by creating small conduits for water to infiltrate and increasing biological activity in the soil.  A 2010 report (PDF) (82 pp, 5.3MB, About PDF) by the U.S. Geologic Survey found that the median infiltration rate of a clay soil planted with prairie species (0.88 inches/hour) was more than three times the median infiltrate rate of a clay soil planted in turf (0.28 inches/hour).

Expand the storage layer and include an underdrain: Even when soil infiltration rates limit the volume of stormwater that can percolate into the groundwater, including a larger storage layer with an underdrain can significantly slow peak flows, protecting stream banks and potentially reducing combined sewer overflows.

Use practices that do not require infiltration: Rainwater harvesting, green roofs, vegetated swales, and practices designed for peak flow attenuation do not require soils with high infiltration rates.  These practices may be integrated into treatment drains to mitigate stormwater impacts. 

Evaluate and recondition urban soils: Evaluate and recondition urban soils: Reconditioning methods for improving poor quality soils will vary depending on soil conditions and the intended use of the site. Sites intended for urban agriculture might require extensive reconditioning, whereas sites intended for recreation might require only moderate improvement. In 2011, EPA completed a report addressing the Evaluation of Urban Soils (26 pp, 3.2MB, About PDF). This report provides a concise, practical, and scientifically-based overview of the typical conditions of urban soils, and offers recommendations for how such soils can be rehabilitated or reconditioned to support green infrastructure or urban agriculture.

Include a mulch layer: Mulch acts as a filter for the sediments carried in stormwater. By including a mulch layer above infiltration practices and replacing this layer when it is filled, the soil and gravel layers below can be protected from sedimentation.

Include a sediment trap: A sediment trap is a small depression bordered by a small berm that captures and collects sediment at the entrance to a bioretention area. Traps can be used at the inflow of many green infrastructure features to facilitate the removal of accumulated sediment and prevent the feature from becoming clogged.

Perform periodic maintenance: If a mulch layer or sediment trap is included, the accumulated sediment must be regularly removed to maintain the function of the stormwater management practice.

Create a plan: The first step in designing landscape features that can remain healthy and attractive with limited irrigation is to create a plan balancing water supply and demand. Annual water budgets will generally be appropriate for landscapes consisting of native plants at native densities, while monthly water budgets will be more effective for landscapes with exotic plants at higher plant densities.

Use low water use plants: Native and drought tolerant plants can drastically reduce, if not eliminate, the irrigation requirements of green infrastructure practices.

Use efficient irrigation systems: Irrigation systems will be most efficient when plants are grouped according to their water needs, and when the frequency and depth of irrigation is adjusted according to plant type, plant maturity, and season.

Consider soil amendments: Healthy soils are essential to retain soil moisture, sustain vegetation, and treat stormwater runoff. If site soils are poor, soils can be amended with organic material.

Use mulches: Organic mulch can increase water retention and pollutant removal while building soil structure and suppressing weeds. Note, however, that many desert trees and shrubs react poorly when their trunks come in contact with mulch.

Maintenance: All landscapes require maintenance, and xeriscaping is no exception.

Use features that serve multiple purposes: Swales and bioretention areas can be integrated into landscaped areas, medians, or parking strips. Similarly, permeable pavements provide volume reduction and water quality treatment without requiring any additional space.

Use features that fit into small spaces: Planter boxes and tree pits are examples of green infrastructure features that may be designed to fit into small spaces.

Subsurface storage or infiltration: Subsurface storage or infiltration tanks provide an alternative when space is too limited for any surface practices.