Making Room: Finding the Space in Urban Parks for Stormwater Management
In all considerations of urban stormwater management, space is a factor. A prime difficulty with liberating a stream from a fortified channel is that it then requires a wider footprint – but over the years that historic floodplain has usually been covered with housing, shops or industry. Mountains Recreation and Conservation Authority Landscape Architect Brian Baldauf has calculated that the present 50- to 100-foot-wide riparian corridor of the Los Angeles River and its tributaries (including Tujunga Wash) would need to be five to seven times wider to be naturally resilient against flooding.
The problem is more severe in arid regions with their wider fluctuations between droughts and deluges, but the challenge reaches from coast to coast. In the Charlotte, North Carolina area, which has seen rapid recent development (including a widespread increase in new pavement) and higher water levels, Mecklenburg County has an aggressive program to buy out willing sellers in the flood zone, remove built structures, and turn the land into open space (often resulting in the construction of a natural greenway to be maintained by the parks department). Austin and El Paso similarly have created broad swaths of parkland from former residential neighborhoods decimated by floods.
The space issues manifest in many different situations. Angelyn Chandler, who once headed New York City's Community Parks Initiative, sees the same challenge even with rain gardens. "Rain gardens," she said, "are the cheapest and most visible demonstration of green infrastructure, but they take up a lot of space. In our small community parks, we're weighing the space constraints against the cost constraints, because the more space-efficient elements, like artificial turf and permeable pavers with drains below, are more expensive."
The tradeoff between space and cost has concentrated the most elaborate underground systems in dense cities where there is insufficient open land above ground. High-tech 3.1-acre Canal Park, surrounded by dense development in Washington, D.C., was designed to capture, pre-treat and store in underground tanks the first inch of stormwater falling on a broad swath of surrounding buildings and streetscape. Most of the water is saved for irrigation and the park's toilets; the excess – about 500,000 gallons per year – is pre-treated and released into the sewer system on a delayed basis after a storm. Canal Park not only keeps about 1.5 million gallons of water out of the city's sewage treatment plant every year, but it also supplies 75 to 80 percent of its own irrigation water.
The above 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 twelfth installment in a series of 20 posts.