The following is the second installment of the two-part series excerpted from the Trust for Public Land’s (TPL) Center for City Park Excellence (CCPE) publication, City Parks, Clean Water: Making Great Places Using Green Infrastructure. To view Part I, click here.
Different Solutions and How They Actually Work
There is no simple formula for green infrastructure in parks. For one thing, geography alone dictates that there are dozens of different kinds of urban parks, from narrow stream side greenways to large flat forestland, from stepped brick plazas to lush community gardens, and from windswept hilltop viewpoints to massive sports complexes. But when it comes to water-smart parks, there are three principal issues:
- 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?
Creating New Parks
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. Depending on available financial resources, the land is regraded to the optimal slope and shape, a proper sub-base is installed, engineered soil is added to increase absorption, and plant materials are selected specifically to manage stormwater.
The best of these new projects find inspiration from the original geography and natural history of the site. Railroad Park, 19 acres of former tracks in Birmingham, Alabama, is located in a one-time marsh that drained all of the present-day downtown. Not surprisingly, local flooding had plagued the subsequent industrial landscape. When it was finally converted into a park, the designers prioritized stormwater management, creating a stream system to circulate runoff through wetlands and a one-acre lake that doubles as a detention basin. With about five acres of green infrastructure, the park filters 100 percent of the site’s runoff; after a storm, the water is released to a traditional wastewater management facility. The park has greatly reduced flooding, and it stimulated a multi-million-dollar construction boom in a formerly decrepit, park-poor neighborhood.
Similarly, the new Tujunga Wash Greenway partially recreates a historic streambed in the North Hollywood section of Los Angeles. 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. Although water flow studies by the Los Angeles Bureau of Sanitation determined that the channel couldn’t be removed, 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 create a new 15-acre/1.2-mile-long park. The recreated stream infiltrates 325,000 gallons of water into the depleted aquifer – enough groundwater to serve more than 3,000 homes.
Developing a fresh park has the advantage that the space has no present human users, no recreation history, and no entrenched lobbying blocs to complain. In contrast, making changes to venerable, beloved landscapes can be more complicated. Some compromises may be required, but water features – even those that might fluctuate in depth and spread – can be great spaces if they are sensitively designed, properly maintained, and appropriately programmed.
Getting the Soil Right
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. 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.
Keeping Green Infrastructure Green
Swales, rain gardens, and detention ponds are critical components for stormwater management. Long-term aesthetics may take a back seat, especially for wastewater utility staff focused primarily on regulatory compliance. Rain gardens, swales, and basins are beautiful in design renderings by landscape architects but over time some may start to look mangy. To keep these park areas attractive, experts choose plants carefully and support good maintenance.
Susannah Drake, a landscape architect at DLANDstudio, states “The trick to keeping green infrastructure installations beautiful is not only maintenance; it’s also smart planting design. It’s important to have some woody and evergreen plants to help maintain structure upon which the perennial plantings can shine.”
Managing water in a park requires commitment even after construction ends. The good news about maintenance is that green infrastructure is easily accessible, not somewhere deep below a utility cover. The bad news is that, compared to other park spaces, it often requires more attention because of specialized plant material and because sediment buildup can become debilitating. Infiltration basins require annual or semi-annual mowing, weeding, and removal of debris and dead plants.
Public Fears and Legal Liability
There’s no question that standing water has its drawbacks, and hydraulic engineers walk a fine line between holding water back for too little or too much time. Shorter retention periods and the sewage plants get overwhelmed; longer retention, and the water may allow mosquitoes to breed and algae to grow. Standing water in parks may cause concern about the safety of children and other visitors. However, this issue is standard for virtually all park departments and it should not require any special rules since every park system already has rivers, streams, ponds, lakes, or ocean beaches to consider. For example, when a stormwater treatment wetland in Shoreline, Washington seemed hazardous for children because of its steep sides and long drop from an adjacent walking path, the city eventually installed safety cables. In New York City, maintaining water depressions at a maximum of six inches has assisted in avoiding problems within neighborhood parks.
Using parks as infrastructure may be a time-honored tradition, but it also requires new technologies and new practices. In some cases it entails economic costs; in others it provides savings and the ability to share an expense between multiple agencies. Practitioners need to be realistic about what green infrastructure can accomplish, what it costs to create and maintain, and how it impacts other facets of park life. In some cases, providing water management will improve a park, or will be a barely noticed constraint on usable park space. In other cases, fields or trails may be soggy or covered with debris for a few extra days. In many situations, this burden will be minor and easily tolerated where others may be too injurious and intrusive. In all cases there should be recompensing actions, policies, and developments that result in synergies and improvements for park users.
Perhaps the most important recognition is that green infrastructure needs human attention. While true wilderness can be self-sustaining, naturalized areas in the urban environment demand maintenance, especially if they are designed to manage large volumes of water from surrounding areas. Pervious pavement needs sweeping and vacuuming, catch basins require cleaning and emptying, swales obligate weeding and replanting, high-water debris needs removal. Utilizing green infrastructure does not mean getting a free ride. In fact, like the grey infrastructure it replaces, it carries a maintenance obligation. Keeping up with that maintenance almost certainly requires more day-to-day staffing in parks and greater park budgets in the future.
But this can be a good thing. More park workers mean more inviting parks and more users. More users mean more safety, more tourists, and more health benefits for local residents. And parks that serve multiple functions relating to both water and recreation become much more important governmental facilities in the public’s mind – facilities that deserve higher levels of funding and more staff, whether paid out of the parks budget, the water department budget, or both.
And the logic of the parks/water link is inescapable.
by Peter Harnik, Director and Abby Martin, Assistant Director, The Trust for Public Land’s Center for City Park Excellence, based in Washington, D.C.