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Soil, Stormwater, and City Parks

The following is an excerpt from City Parks, Clean Water, a report by the Trust for Public Land’s Center for City Park Excellence that examines the role of urban parks in managing stormwater. This is the tenth installment in a series of 19 posts.

Across the country, and across all sizes and shapes of parks, there are a few key elements to getting green infrastructure right. Some of them begin in the site survey and design process. These considerations include getting the soil right, creating or ensuring that there is enough space for the planned improvements, and the maintenance of green infrastructure, which can have both pros and cons when compared to traditional “gray” infrastructure. In this week’s post, we will discuss ways in which soils can be modified to improve the water management qualities of a park.

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Pounding feet compacts the soil in parks, often preventing it from performing properly. Amended or otherwise modifed soils can ensure proper filtration and drainage. (Flickr user Humanoide)

What does it take to ensure a park has soil that will absorb, store, clean, and filter runoff? The mere presence of a grassy park does not guarantee water infiltration. Soil in urban parks is often highly compacted because sites have been in-filled with substandard materials packed down during construction by heavy equipment stored on the site. Athletic fields and heavily used lawns can become especially compacted; their runoff rate often resembles that of asphalt, especially during large storms. Thus, soil usually needs to be modified to perform properly and sometimes it takes more than one try. Deficient soil was the culprit in a Seattle project where stormwater was lingering too long in newly installed rain gardens. Those were torn out and successfully rebuilt using a designed mix of amended and native soil that has become the city’s go-to option.

The following are some options to improve the soil are a park.

  • Amended soils, which are pre-existing soils that have been enhanced to meet performance goals, often by mixing in sand or compost. Amendment is the least expensive type of alteration since it doesn’t require a full-fledged replacement.
  • Engineered soils (also called manufactured soils, designed soils, or blended soils), which are combinations of soil, soil components, and soil-like material that are used to replace existing dirt. To manage stormwater, engineered soils usually have higher sand content.
  • Structural soils, which are extreme versions of engineered soils that are designed specifically for strength without compaction. They are able to support plants, and they allow air and water movement even under the great weight of porous pavement and vehicles. They are particularly useful on playing fields and heavily used lawns.

A great example of soil design at work is in Santa Monica’s Beach Green, a former asphalt oceanfront parking lot that now serves double-duty as peak-period car storage and off-peak playing area. Its engineered soil includes not only sand but also plastic mesh, which can increase load-bearing capacity sufficiently to support vehicles without compromising porosity. This formulation, which costs $180,000 per acre, anchors grass while still infiltrating about 560,000 gallons of runoff a year from 1.7 acres of adjoining asphalt.

Beach Green 2 -Mark Tessier Landscape Architect

Santa Monica’s Beach Green does double duty as a park and a parking lot, on busy beach days. (top photo: Mark Tessier Landscape Architects bottom photo: Neal Shapiro)

GreenBeach FullCars tire marks - Neal Shapiro

 

Ultimately, engineering soils for a water-smart park requires balancing the project specifications, materials budget, precipitation patterns, and site limitations – including such factors as native soil porosity, fines content (measured by the percentage of clay and silt), depth to the water table and bedrock, and soil contamination. Sometimes good results can be obtained by mixing sand into existing soils; elsewhere, entirely new soils may be required. Other challenges, such as a high groundwater table or high bedrock, may prove insurmountable. Budgeting early for a technical expert, whether an in-house landscape architect or a consulting stormwater engineer, can reduce headaches and costs.

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