Water-Sensitive Urban Design (WSUD) is a practical approach that includes both land planning and engineering design to encompass all elements of the urban water cycle into a single design focused on minimizing negative environmental effects while improving aesthetics and recreational opportunities. This combined approach offers both visible and practical rewards for the population as well as less visible but equally important safeguards for urban infrastructure and the local environment. The combination of benefits is expected to provide positive reinforcement and support for the changes from both urban leaders and their constituents.
WSUD includes and integrates each element of the urban water cycle, including stormwater, groundwater, wastewater management, and overall water supply. Integrating management of each of these elements as part of the overarching need to establish a sustainable and resilient water supply is particularly important in these days of climate change and increasing water scarcity.
One of the most important philosophies of WSUD is the focus on urban stormwater runoff as a resource. This is a paradigm shift from the conventional view of stormwater as a nuisance that should be disposed of as quickly as possible, with little regard for the environmental impact. This requires a fundamentally new approach to the planning and design of growing towns and cities and their need to manage both water infrastructure and environmental ecosystems.
The focus of WSUD on the entire urban water cycle means that all streams of water are considered resources, with a multitude of impacts on existing water resources, land, biodiversity, and the community’s enjoyment of and interaction with surface water and waterways. Key principles employed in WSUD include preserving natural systems, protecting water quality, restoring urban water balance, and minimizing potable water demand. We’ll review some of the practical measures adopted by WSUD in this article. It’s encouraging to note that many elements of stormwater harvesting and reuse are already included in modern stormwater management practices.
For communities that have implemented WSUD principles, they have proven to be one of the most effective and environmentally friendly measures to manage the urban water cycle.
WSUD Practices
The range of practical elements involved in WSUD can be roughly divided into three interconnected systems: Streetscapes, or elements that manage water resources (primarily stormwater) conveyed from impermeable surfaces like streets, parking lots, and other urban features. Public open spaces include more natural spaces like parks, greenways, and recreational amenities like fishing spots and scenic areas. Water reuse encompasses elements that focus on water streams as a resource and are instrumental both in reducing stormwater volumes and in easing the pressure on water supplies.
Streetscapes
Bioretention Systems:
Bioretention systems are designed to take advantage of natural filtration and nutrient uptake provided by live plants. Bioretention systems encompass a range of elements, all of which are relatively inexpensive to construct, require less space than other similar measures, and fulfill an effective first-level treatment that can be employed before runoff even reaches street drains.
Vegetation is a defining element in bioretention systems. Plants naturally take up problematic nutrients such as nitrogen and phosphorus that are commonly found in stormwater runoff, and some species are known to be particularly effective in taking up some types of contaminants that may not be filtered out in subsequent steps.
Bioretention systems typically include seven important elements, each of which provides a unique function:
- Grass buffer strips remove suspended solids and reduce the speed and force of runoff
- Vegetation helps reduce the volume of water and removes excess nutrients by absorbing runoff through root systems
- A shallow ponding area stores excess stormwater flow and encourages evaporation. Slow moving water in this area also has an opportunity to drop suspended sediment.
- Organic mulch performs the critical role of fostering biological degradation of petroleum-based pollutants, as well as reducing erosion and filtering other pollutants.
- Engineered soil is a manufactured soil consisting of specified ratios of sand, silt, clay, and organic amendments such as compost and designed for a specific application. In a bioretention system, engineered soil supports the growth of vegetation and include clay which can adsorb pollutants such as hydrocarbons, heavy metals, and nutrients not taken up by vegetation.
- Sand beds enable drainage and maintain soil aeration.
- An underdrain system permits the diversion of excess water to storm drains or surface waters.
In general, the majority of bioretention measures work best in small sites and are therefore ideal for highly urbanized spaces. They’re most effective when they receive runoff as close to its source as possible. Bioretention structures are highly adaptable since they can be designed in a variety of shapes and sizes and can be designed to manage runoff from a variety of areas within a single site. Fully functional bioretention systems can be installed in areas as small as lawns and median strips, and as large as parking lot islands, unused lot areas, and even certain easements.
Bioretention Basins and Swales
Bioretention basins and swales are shallow basins or landscaped depressions placed to gather stormwater runoff. Basins and swales are primarily differentiated by their configuration: swales are typically longer and narrower and therefore better suited for on-street placement, while basins work well in parking lots. In both cases, the depressions slow incoming water and allow it to percolate through a variety of physical, chemical, and biological processes provided by live vegetation. Having moved through the basin or swale, clean, slow-moving water is allowed to infiltrate the soil or may be directed to nearby stormwater drains or surface waters. If necessary, excessive inflow that exceeds the structure’s ability to store is typically directed to stormwater drains.
In areas with large areas of impervious surface, such as large buildings and parking lots, or in residential developments where large lawns and natural areas are becoming increasingly rare, bioretention basins and swales can be incorporated in relatively small areas where the islands can become attractive features valued by residents and tenants.
Long, narrow swales work especially well in low traffic areas such as residential developments where they run alongside streets, shared-use paths, medians, roundabouts, and other unused right-of-way areas. Swales usually have virtually no vertical separation between the sidewalk and the street, so traffic density is an important consideration. Even so, certain traffic calming devices such as curb bulbs may serve as excellent locations for bioswales. Swales can also serve as valuable locations for an assortment of street trees, which add aesthetic and monetary value to most developed areas.
In situations where existing urban and residential developments are required to retrofit to comply with evolving stormwater management requirements, the addition of bioretention basins is often a relatively easy and inexpensive solution. For example, relatively simple makeovers of parking lot islands or adjacent landscaped areas can satisfy stormwater requirements while adding valuable visual appeal. A wide variety of vegetation can be incorporated into a bioretention basin, making it easy for basins to harmonize with existing landscape styles.
Bioretention systems are extremely effective elements in stormwater management, green infrastructure and WSUD. In one example, the American Society of Landscape Architects (ASLA), calculated that a 4-meter bioswale can reduce about 25% of total rainfall runoff on a typical road.
Rain Gardens
Rain gardens are another bioretention tool, similar to bioswales, but are generally more suited to residential and other private properties. It’s worthwhile noting, however, that the terms for each of these elements are frequently interchanged.
Like basins and swales, rain gardens are shallow, vegetated basins that capture and hold stormwater runoff to support the garden’s plantings, then allow excess water to infiltrate the soil, finally moving the water that is not absorbed into the larger stormwater management system.
Rain gardens are usually designed and planted for aesthetic appeal, using grasses and flowering perennials, with a focus on native plants. Rain gardens perform many of the same functions as other bioretention systems, filtering out about 90% of harmful metal contaminants such as copper, lead, and zinc, as well as nutrients, where they typically remove about 50% of nitrogen and 65% of phosphorus. They’re also effective at limiting stormwater volume: typically, about 30% of incoming runoff infiltrates the ground with these setups. It’s helpful to remember that rain gardens operate most efficiently when they’re integrated into the larger stormwater management system.
A particularly attractive feature of rain gardens sited in residential neighborhoods is the wildlife habitat they can provide. Butterflies, birds and other creatures find food and shelter within the planted areas and provide valuable support for vulnerable ecosystems.
Rain gardens can also be established on private property, in the yards of residents for example, where private rain gardens can reduce flooding, mitigate drainage problems, and reduce the burden of excess runoff on public stormwater systems.
Stormwater Planters
Perhaps the smallest unit of WSUD practices for streetscapes is the stormwater planter. In tightly packed urban commercial areas, for example, it may be difficult to devote sizeable areas to bioretention swales or even rain gardens. Stormwater planters are relatively easy to place and provide valuable aesthetic appeal to pedestrian throughways.
Stormwater planters include no-infiltration or filtration-only types. Some types are ideal for placing over existing underground infrastructure or where soils are vulnerable to subsidence, as well as sites that are considered pollution hotspots, where the collected water can’t be allowed to enter the environment. In these cases, water that percolates through the planter is captured in an impermeable underdrain and diverted to appropriate stormwater channels for treatment.
The unique value of stormwater planters is that they can be squeezed into spaces alongside buildings, spaced along sidewalks, or dispersed around pedestrian plazas. They’re frequently used to plant trees, which are valuable additions to the urban landscape which often struggle to survive when planted directly into the ground, surrounded by impervious pavement. Stormwater planters can also be situated to capture a share of rainwater that flows from rooftops.
