Designs that move toward more naturalized stormwater management can help counterbalance impervious urban development.
By Gerald DeFelicis, Jr., LLA, RLA, PP, with Maraliese Beveridge
Managing surface runoff is a challenge that is addressed using many different approaches depending on the setting. Whether the environment is natural terrain, paved cityscape, or one that runs along another body of water, each situation needs to be addressed uniquely. Two things you can count on are that water always finds its way to the lowest point, and nature doesn’t always put it where we’d like it, when we want it.
A little history
In the wake of the last 100 years of build-out, redevelopment, and overdevelopment, installation of impervious products has dominated the base plane, making stormwater management all the more challenging. Asphalt mixed with any number of elements, including crushed brick, stone, sand, gravel, shells, cement, slag, geosynthetic, and other composite aggregates, consistently prevent water penetration. While projects requiring these surfaces have a drainage component designed into them, they are only required to manage normal rainfall amounts and don’t always calculate the advantages of green methods to dispose of their water overages. The mission of green infrastructure is to channel water back into the ground more directly to prevent flooding rather than having it run off and dealt with in a series of retention and detention scenarios.
Eighty percent of all rain events in the Northeast yield 1 inch of rain or less per hour. In most instances, any rainfall above that becomes problematic. Since the success of managing stormwater has traditionally been based on how quickly we can drain runoff, using this equation we are left with a giant problem handling excess volume. It has been the intent of most green stormwater systems to evaluate the possibility of intercepting the initial 1 inch to 1-1/2 inches of rainfall and divert it to an alternative drainage system that can infiltrate or store the potential runoff. In a perfect world — if green means alone could handle that amount — there would be a drastic reduction to the potential problems.
Unfortunately, the term 100-year-flood doesn’t mean it has the potential to occur once every 100 years. Rather, it’s a flood that statistically has a 1 percent chance of occurring in any given year (USGS). Superstorm Sandy was the hurricane that broke the camel’s back. According to the National Weather Service, “In more than 100 years, there is not a single storm that came up the East Coast and turned west.” Instead of heading out to sea, she caused $50 billion in damages in the U.S., second only to Hurricane Katrina (NOAA).
Sandy was the exception to all the rules and catalyst to changes in building codes designed to protect structures and shorelines by making them more resilient. She also shed a light on the types of damage that could occur further inland that had never been experienced before, causing public-sector entities to take a closer look at local infrastructure that was formerly considered safe from this type of severe weather event.
To complicate matters, stormwater runoff mixes with man-made contaminants in all of these settings during a storm event, so the very least we can do is provide systems that can mitigate and separate it within our environment as efficiently as possible. Since the notion of global warming has increasingly been accepted and extreme weather events kicked-off by Sandy have illustrated how devastating water can be, corporate, public, and private agencies have become more receptive to the notion of going green, or at least adding green components, to help offset existing hard infrastructure systems.
Of course, urban settings have been the most problematic, since many of them over the years have evolved into asphalt jungles. Myriad methods have been devised to channel runoff from pavement on roadways, walkways, and parking lots through a system of drainage basins, bioretention ponds, and pipelines.
Some cities have another problem to deal with using underground combined sewer overflow (CSO) systems that collect rainwater runoff, domestic sewage, and industrial wastewater into one pipe. Combined sewer systems were originally designed along the eastern seaboard as a means of successfully flushing waste, which under normal conditions is transported to a sewage treatment plant without incident. However, during heavy rainfall or snow melt, when this volume exceeds the treatment plant capacity, untreated water discharges directly to nearby streams, rivers, and other water bodies.
Green street has become a term synonymous with the introduction pervious surfaces, use of native vegetation, and adaptation of substrate to facilitate absorption of stormwater. The definition of green streets has expanded over time to recognize the adaptability of various urban spaces, and in an aesthetically pleasing manner.
Many urban areas do not have the luxury of having large spaces of land that could be used for traditional drainage basin structures, so other means to manage runoff have been developed. In the early 2000s, many older Northeast cities that have combined sewer systems led the way to development of new, innovative methods of green infrastructure. In one instance, runoff was redirected in the higher ends of drainage sheds, thereby reducing the overall volume flowing into the combined sewer system. Other methods introduced included stormwater trenches combined with raingardens and other planted areas.
One raingarden we designed was constructed to divert two city blocks of stormwater into a storage and infiltration bed with native plantings. Some combined sewer systems have been retrofitted with green inlets where trees take in water directly to the root zone and porous piping combined with stone trenches. Depending on soil conditions, these systems enable storage of stormwater volumes to be slow-released back into combined sewer systems at non-peak times or when able systems can serve as infiltrate beds to return water directly into the ground.
There are other urban situations that address stormwater runoff in areas where it may not previously have been anticipated to be a problem. In one example, a water utility’s staging and storage yard was the subject of a green makeover. Previously, when an excessive grade or slope was confronted, or an area was difficult to maintain, the practice was to pave the area to prevent erosion. While this offered a short-term solution, it only contributed to the problem of runoff. The storage yard sat above the surrounding street grade, therefore, the logical green solution was to remove the concrete from the slopes and, through a system of double weir inlets, collect the initial runoff and direct it to stilling wells in level trenches to provide infiltration and to water native plant materials surrounding and greening the entire area.
Trees do more than just provide shade. They soak up gallons of water, reducing stormwater runoff, act as a filtration system for pollutants in soil and water, help regulate water flow, recharge groundwater, improve soil microbiology, produce oxygen, and reduce erosion, to name a few!
According to Vincent Cotrone, urban forester, Penn State Extension, Northeast Region, “In urban and suburban settings, a single deciduous tree can intercept from 500 to 760 gallons per year; and a mature evergreen can intercept more than 4,000 gallons per year. Even young, small trees help. In a recent Forest Service study, a single small tree that was only nine years old was able to intercept 58 gallons of stormwater from a 1/2-inch rain event — 67 percent of the rain that fell within the canopy.” (https://extension.psu.edu/the-role-of-trees-and-forests-in-healthy-watersheds).
Introducing trees as part of green infrastructure design can be a formidable solution for helping to manage stormwater in some situations. The cost of the complete replacement of the existing subsurface infrastructure system is not a likely solution. But in some cases, integrating trees into key areas within an existing system can substantially help reduce runoff volume before it enters the treatment plant. Trees can also be useful installed in areas that don’t normally flood by acting as a preventative measure.
A perfect project
The Pleasant Hill Fish Hatchery in Philadelphia is an exemplary project illustrating a complete transition of an existing park with impervious surfaces and invasive vegetation species to a green space designed to manage stormwater runoff naturally while reflecting the environment along the Delaware River. The site has hatchery ponds that are fed from secondarily treated water and act as filtration prior to release into the river further downstream.
Redesign of the park was intended to disconnect a majority of the existing hard surfaces by installation of pervious pavement, porous concrete, and stabilized earth walkways that direct stormwater runoff over an equivalent flow length of pervious area.
The hatchery ponds were dredged, deepened, and stabilized with a rock base lining, and the water edges were planted with terraces of native wetland species adapted to this environment. The surrounding uplands have been re-naturalized with species found within the area and the entire park has become reintegrated to the natural riverfront of the Delaware River.
The native plant materials that have been reintroduced around the grounds are more appropriate for the weather, soils, and USDA Plant Hardiness Zone. These plants readily adapt to the environment and, once established, allow natural processes of green infrastructure that doesn’t require false or artificial maintenance such as irrigation and reestablish soil profiles in areas that have been harmed or scarified.
Some areas that were formerly mowed were reintroduced to trees and other native species, creating open areas in which birds will naturally redistribute seed, encouraging additional native flora and fauna habitats. This newly created environment more accurately reflects nature for this region and historic seasonal changes. In colonial times, during the second week of April each year, the native Shadblow serviceberry (Amelanchier canadensis), bloomed along the Delaware River when the Shad fish came in. This was how the community was signaled that it was time to fish.
Shorelines and wetlands
Perhaps the area in which green infrastructure has advanced the most is along shorelines and wetlands, integrating more natural ways to protect our coastal areas using increasingly innovative protectants. The green infrastructure measures for shore communities include both means of mitigating potential flooding through introduction of good planning and design practices, and an encouragement of the use of native materials (particularly plants).
Wetlands are an important resource that cannot be overlooked. They help protect and improve water quality, store flood waters, maintain surface water flow, provide a habitat for fish and wildlife, suppress tidal forces, and improve marshland health. Loss of wetlands is a result of a combination of encroaching development, constriction of the flow of water, rising water levels, weather-related circumstances, and associated landfills and dredge spoil sites. Wetland health can be checked through aerial imagery and mapping, while pollution can be monitored through scientific testing and visual assessment. Either way, regular monitoring and associated repair or replacement is vital to a healthy ecosystem.
Tidal marshes are marine landscapes that contain wetlands along the coasts of tidal basins, including estuaries, which are frequently inundated by flooding from the daily tidal flow of the adjacent ocean or major water body. Living Shorelines provide an infiltration buffer between the water and land and can be designed in many ways to mimic the natural process while helping to control flooding. Many have a breakwater of rocks, a strand of coastal wetlands and beach, then a bank and an upland buffer. When properly maintained, shorelines and wetlands with soft edges, marsh, and natural structures are better at absorbing the impact of floodwater and more resilient than water edges created with rip rap and bulkheads, particularly as a long-term solution.
We must face that we, and those before us, have done almost everything possible to prevent terrestrial hydrology from occurring naturally. Flooding, of any amount, in cities, parks, and shorelines has been a long-time problem that has only gotten worse as we continue to develop land — which is why it is essential to continue to create alternate and proactive methods of dealing with flood protection.
Water is our most precious resource, and although we predictably can’t stop every flood from affecting the built environment, as we move forward, our approach needs to migrate through design and planning toward more naturalized stormwater management. Ongoing research is constantly finding green solutions, such as living roofs that mitigate water at the top of the chain before it becomes runoff, and aquatic mosses that can quickly remove contaminants from water (www.innovatorsmag.com/magic-moss-fixes-contaminated-water).
Green infrastructure is not a fix-all solution in every situation, but it can be very effective when integrated as a component within an overall system. Green systems are not sustainable by themselves; however, the maintenance required and their ability to work in tandem with other hardscape systems to help prevent flooding, filtrate contaminants, and facilitate groundwater recharge is a long-term benefit. While we can’t completely tame the asphalt jungle, we can counterbalance future installations by integrating design that creates an equilibrium through a combination of viable, natural working solutions that are more in sync with nature to achieve the best results.
Gerald DeFelicis, Jr., LLA, RLA, PP, senior project manager at Maser Consulting PA (www.maserconsulting.com), has more than 30 years of varied experience within the fields of landscape architecture and planning. His expertise lies in educational and recreational design, and he is fully versed in the management and design of projects, including site analysis, conceptual and final design, design development, construction document preparation, and project site supervision. DeFelicis’ current responsibilities include coordination, design, and overall management of recreation facility projects and the provision of landscape architectural services in the Philadelphia and southern New Jersey region. He has been instrumental in creation and development of several stormwater management design initiatives within the City of Philadelphia.
Maraliese Beveridge, senior technical writer and public relations specialist for Maser Consulting PA, has more than 25 years of experience in journalism and is a nationally published writer within the engineering industry. Her expertise is focused on transforming complex technical ideas into comprehensible articles on trending subjects.