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Designing New Water-Smart Parks: Two Stories From California

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 eighth installment in a series of 19 posts.

When it comes to green infrastructure, the easiest parks to work with are new ones — facilities that don’t yet exist and can be specifically designed with stormwater management in mind. Every park is different, from size to geography to the surrounding culture and politics, but when it comes to water-smart parks there are three main issues to be considered:

  • Is the physical relationship of the park to the surrounding community such that a redesign could reduce neighborhood flooding or the pollution of downstream waterways?
  • Does the park have any available space for water flow and storage?
  • Is the composition of the existing soils, water table and underlying rock such that the park can absorb a significant amount of water in the necessary amount of time?

    Tujunga pathway and stream - credit Water Education Foundation

    A recreated streambed at Tujunga Wash in Los Angeles helps revive depleted aquifers. (Water Education Foundation)

There are two overarching methods of slowing water: holding it for conventional treatment, or using natural processes to percolate it through the soil.  A 2014 survey by The Trust for Public Land revealed that 48 major cities have constructed or modified more than 5,000 acres of parkland in one way or another to control stormwater. Detaining stormwater for conventional treatment is only “half green” – it’s a method that helps prevent sewer overflows and flooding, but it still treats runoff as waste by keeping it out of the soil, sending it through traditional physical and chemical treatment processes, and eventually discharging it into a waterway. The “fully green” way of handling stormwater approaches it as a resource, using soil, plants, and microbes, diverting it from the sewer system, and returning it directly to the ground. Mostly, cities use a mix of techniques, complementing lower-cost green infrastructure with more expensive and more familiar gray infrastructure. A large rain garden installation, for instance, will usually have an overflow drain connecting to the traditional storm sewer system in the event of a deluge.

One example is the new Tujunga Wash Greenway in the North Hollywood section of Los Angeles. The wash partially recreates a historic streambed to help revive a depleted aquifer and bring more groundwater to the area. Sixty years after the original wash was obliterated by a concrete flood control channel (allowing developers to build in much of the old floodplain), a new, adjoining artificial stream has been created with water diverted from the channel. The Mountains Recreation and Conservation Authority took advantage of the sandy, well-drained land parallel to the concrete spillway to build a new streambed and, in effect, a new 15 acre, 1.2 mile long park. The recreated stream infiltrates 325,000 gallons of water into the aquifer, enough groundwater to serve more than 3,000 homes, and the surrounding community received a new stream-side trail.

There is seemingly no end of opportunity to convert rain from a problem to a resource. Parks can be a great way for developers to reduce their stormwater management requirements when creating a new neighborhood. The city of Santa Monica even turned a portion of its municipal airport into stormwater-treating parkland. Now, runoff from the airport, which had routinely flooded adjacent streets and polluted Santa Monica Bay, flows into Airport Park. There, it meets 8.5 permeable acres of synthetic soccer fields and a dog park, plus porous parking lots surrounded by infiltration swales. (Artificial turf, even though made of a carpet-like substance from woven green polymer, is nevertheless pervious since the material is punched through with thousands of water holes.)

Airport Park - AHBE

Airport Park in the middle of Santa Monica Airport helps manage runoff from all the surrounding pavement. (AHBE Landscape Architects)

Since these were both new parks, the interventions did not stir up much concern or public opposition. Developing a fresh park is advantageous in a space has no present human users, no recreation history, and no entrenched lobbying blocs to complain. It can be designed to maximize water management features while also selecting park features with synergistic land uses, such as trails and boardwalks. Of course, on the negative side, the land may be expensive, structures may need to be demolished, and pollution may need to be cleaned up.

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