WSUD Practices: Public Open Spaces
Sedimentation basins
Sediment basins are frequently used in construction sites to manage the movement of sediment to nearby waterways. They operate in much the same manner as WSUD sediment basins but are employed in very different circumstances. WSUD sedimentation basins are simple depressions that slow and detain stormwater as it flows across the ground. In contrast to bioretention systems, their ultimate function is not to store water, but to reduce its velocity and drop some of its sediment load. As stormwater slows, it sheds energy and is much less likely to cause damaging erosion. In slow moving water, coarse to medium sized sediments have an opportunity to settle out of the water column, which reflects an immediate improvement in water quality. Removal of this type of coarse particles protects downstream vegetation from being smothered under loads of sediment and allows downstream systems to focus on smaller particles and other pollutants.
Sedimentation basins are relatively simple and inexpensive to construct and are often one of the first steps in a WSUD treatment system.
Constructed Wetlands
Constructed wetlands are essentially manmade versions of swamps, marshes, estuaries and floodplains, where the land is saturated and submerged in water either seasonally or permanently. In nature, wetlands are highly effective natural flood control and filtering mechanisms. Constructed wetlands fulfill an important role in stormwater management and WSUD practices. When properly designed and built, constructed wetlands can remove a wide range of pollutants associated with urban stormwater and even some municipal and industrial wastewater. Microorganisms that create BOD (biological oxygen demand), suspended solids, excess nutrients, hydrocarbons and even toxic metals can be difficult to treat in traditional wastewater treatment plants but are efficiently removed in constructed wetlands.
Constructed wetlands are shallow bodies of water that are rich with vegetation. The types and density of vegetation provide enhanced sedimentation (removing smaller suspended solids from the water column), and fine filtration. The types of plants included in a wetlands design include those that are particularly suited to take up pollutants through adsorption and biological transformation.
Constructed wetlands are usually designed with three distinct zones.
- The inlet zone is essentially a sedimentation basin where water is slowed until coarse particles can settle to the bottom.
- The macrophyte zone is densely planted and well suited to take up soluble pollutants and filter even finer particles from the water column.
- A bypass channel to protect the macrophyte zone when water flow is exceptionally heavy.
Like natural wetlands, constructed wetlands can accommodate rising water during storm events and then release the excess water over time. This ability mimics the extremely valuable function provided by natural wetlands: flood control. When wetlands are able to accept and store large amounts of water with no more negative effect than increased depth, that protects developed areas from potentially damaging floods. Over time, the excess water is not only released but it exits the wetland environment as much higher quality water than when it arrived.
Swales and Buffer Strips
Swales installed in public open spaces are typically designed to move stormwater toward a release point, such as a constructed wetland or other surface waters instead of retaining it for an extended time. In these cases, swales are employed to replace underground pipes and provide an important buffer zone between impervious catchment areas and the waterway.
Ponds and lakes
Artificial ponds and lakes are features designed to store relatively large amounts of water, usually on a permanent basis. Man-made ponds and lakes can accept runoff and accomplish major treatment steps during extended periods of detention such as sedimentation, nutrient removal, and even a certain amount of UV disinfection at the water’s surface.
Artificial lakes and ponds of all sizes can also offer direct benefit to local populations with popular recreation opportunities like swimming, fishing, and boating. Wildlife habitats add to the aesthetics of the area and the stored water can be a valuable resource providing resiliency to the community in the case of drought.
WSUD Practices: Water Reuse
Treating all available water streams as a resource is a fundamental principle of WSUD, and a key focus is the mission to reuse existing water streams whenever possible. In principle, if water can be used safely in its captured state, without additional, energy-intensive treatment, that is the best sustainable use. Constructed features like wetlands, basins, and rain gardens, can provide a certain level of passive treatment through sedimentation, vegetative filtration, etc. Water stored at that point is higher quality than raw stormwater, but it is certainly not potable. The mission at this point is to identify beneficial uses for stored water that reduce the demand on municipal water supplies, satisfy demands for non-potable water, and promote aquifer recharging.
Rainwater Harvesting and Rainwater Tanks
Rainwater tanks are used to store water collected from rooftops for future use. Rainwater is expected to be less polluted in general than stormwater runoff, but it is not safe for human consumption without additional filtration and disinfection. Rainwater harvesting is intended to reduce the demand on municipal water utilities and potable water supplies by using collected rainwater to satisfy needs that don’t require drinking water quality. By reducing the current demand for potable water, utilities are able to slowly ease the pressure on aquifers and surface water sources. Expenses and energy use associated with treating water to potable grade when it will ultimately be used for flushing toilets and watering the lawn can be eliminated.
Captured rainwater can be used for tasks like toilet flushing, laundry, watering the lawn and garden, and car washing. Even with these forgiving uses, some contamination picked up on rooftops may still require active management, including animal droppings, atmospheric pollution, mosquito larvae and other insects, paint, detergent and roofing materials.
Stormwater Harvesting and Reuse
Rainwater harvesting is defined as a practice where precipitation is collected directly from rooftops before it contacts any other surface. Stormwater harvesting encompasses collecting water from a variety of sources. The stormwater resource stream encompasses water that has contacted multiple surfaces, increasing the risk and degree of contamination the further it has traveled. These potential sources of contamination can include urban environments like parks, pavement, sidewalks, sports fields, and gardens. Other sources like runoff from creeks, seasonal streams, and underground pipes also represent different types of contamination.
Stormwater harvesting is a key element of most WSUD and Stormwater BMPs guidelines and these projects typically have a range of objectives, including preventing contaminated runoff into vulnerable surface waters, supporting groundwater recharge, and directing lower quality water sources to the uses where they can be employed to the best effect.
Because stormwater can have a myriad of contaminants from dozens (or more) of sources, removal of a veritable soup of pollutants is critical so users have confidence that the water is safe for its intended use.
Collection
Collecting stormwater seems like a simple matter of directing flowing water during a storm event into waiting storage structures. It’s never quite that simple, though. Two types of storage can be employed, referred to as online storage and offline storage. Online storage is the traditional practice of pulling stormwater directly from waterways or storm drains to fill up storage facilities. Offline storage isn’t integrated directly into the drainage system and therefore requires additional work to transfer water from waterways into the storage structure. Aside from directing several levels of treated water for reuse, stormwater collection can help mitigate urban runoff and reduce the risk of flooding.
End Use
As water resources dry up or become contaminated while the human population continues to expand, robust systems need to be installed that will minimize water consumption across the board, beginning with the wasteful use of high-quality potable water. Lower quality water can be used for most graywater purposes such as firefighting, irrigation, gardening, and so on, while certain stormwater emerging from pollution hotspots may be unsuited for virtually any use. Ultimately, the end use of any stream of collected water will dictate the amount of treatment necessary.
Concerns
Stormwater harvesting projects face a typical range of challenges that come with any fundamentally new solution to existing problems. Concerns like capital costs, cost effectiveness, quality and quantity of the end product and reliability of the system are considered repeatedly, every time a new community broaches the subject. In some cases, the cost of stormwater harvesting has proven to be significantly higher than other potable water alternatives. It’s true that established urban infrastructure can generate an enormous budget when the need to construct a third pipe network emerges. It’s necessary, however, to consider the reality of current trends in water scarcity and global population growth. Even the largest alternative potable water sources can’t be depended upon in the long term, and stormwater reuse won’t be fully adopted until that third pipe infrastructure is available.
