Managing Storm Water

gain, and revitalizes disused or derelict sites. Plants host myriad pollinator species associated with their flowers and play a central role in the growth of a multitude of plants and crops. Plants are a source of delight and sense of well-being for humans. The presence of plants either as a natural or constructed landscape is a contributory factor to property values and to the overall quality of life and health of communities. The physical and visual presence of plants is now known to improve and speed up the healing process of patients recovering in hospitals.3

The term green infrastructure discussed earlier refers to systems and practices designed by landscape architects and other professionals such as civil engineers that mimic natural processes. One example is the design for handling storm water so as to retain surface water in rain gardens or retention ponds, allowing time for the water to infiltrate the soil and to return the water to the atmosphere either through evaporation or by plants. The designed retention systems hold storm water on the site where it can later be used for irrigation and other purposes (fire control, for example).

Green infrastructure systems are appropriate for a wide range of landscape project types in place of, or in addition to, the traditional stormwater management infrastructure. A landscape architect can employ any one or a combination of the following elements from the green infrastructure tool kit in designing a storm-water management plan:

Bio-swales are vegetated, shallow, slightly sloping, landscaped depressions designed to capture and to treat storm-water runoff as it moves through a select palette of plants across the topography and downstream (see Figure 10.2). Bio-swales are typically sized to accommodate the volume of the water from a prescribed storm event. The initial runoff collected is also known as the “first flush.”

The swales are designed to allow sediment and pollutants to settle out prior to recharging the ground water. The plant species used, in addition to achieving some other functional or aesthetic goals, are selected to absorb targeted pollutants and hold loose sediment in place. Rain gardens are designed to capture, temporarily hold, and allow storm water to infiltrate the soils of a property. They are created with plants in combination with a depressed ground form designed to be an attractive addition to a property, in addition to managing storm water, often in combination with other storm-water management systems (Figure 10.3). Detained water has time to percolate into the soil and to provide moisture for the rain garden plants. In some cases the rain garden itself is designed to slope—in the way that a swale slopes—carrying excess water farther downstream to a larger-capacity retention area.

Figure 10.2 Bio-swale along a city street, City of Burbank, California, Department of Water and Power, by Ahbé Landscape Architects.

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Figure 10.3 A: Rain garden, Burbank, CA, by Ahbé Landscape Architects; B: Concrete pavers imbedded in lawn area allow for some foot traffic and surface water runoff to infiltrate soil, Sichuan Agricultural University, Chengdu, China.

Retention and detention ponds are constructed ponds or depressions designed to hold or slow down storm water collected before the water moves downstream within a site or later off-site to a water body such as a stream or wetlands. The intent of creating these ponds is to reduce flooding on a property and to allow sediment and pollutants to settle out or be absorb by plants.

A retention pond is designed to allow excess storm water to move further downstream when the pond is connected to a constructed drainage swale or where the topography slopes toward a potential watercourse (Figure 10.4). The purpose of a detention pond is to slow down and temporarily hold water before it moves downstream while a retention pond is designed to retain a certain percentage of the storm water. These ponds can be a single body of water or a series of ponds connected by a swale. The first or upstream pond serves the purpose of slowing the water flow, allowing solids (sediment and even trash) to settle out.

The subsequent ponds in the chain serve to act as cleaners to remove undesirable chemicals (pesticides and fertilizer products, for instance) so that the water exiting the designed system and before it enters a stream or wetlands is of higher quality than the water entering the system. Water detention ponds in combination with swales serve to slow down storm water and store it for later use. The combination can be graded and planted so that it is also a site amenity, adding visual interest, providing recreation opportunities, and increasing biodiversity. Detention ponds with swales are also designed to serve their intended purpose but appear more integrated into a larger landscape as in the examples in Figure 10.5.

A wide drainage swale was created in two parks, allowing for the detention of extreme storm water during heavy rains. Both were designed with a minimum slope to slow the movement of the water so that it can penetrate the soil and eventually dry out. When dry, the feature allows for informal park uses. A trail of boulders, seen in Figure 10.5A, provides passage across the swale during

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Figure 10.4 A: The retention pond is situated in a residential neighborhood in Delft, the Netherlands, and was designed as a part of a park-like greenway system with trails and passive recreation elements; B: The retention pond was designed to create a more natural-like setting in Zud Park in Rotterdam, the Netherlands.

Figure 10.5 A: Boulder trail crossing a drainage swale in Zud (South) Park, City of Rotterdam, the Netherlands; B: Limestone plinths used for trail crossing of a bio-swale in Manzanares Park, Madrid, by West 8 landscape architects.

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low-water events while providing a connection to a paved walking trail on the higher ground. A narrower and deeper swale could have been constructed, providing for a similar flood capacity, but that would limit the use of the area for recreation. The narrower swale would create a physical and visual barrier that would unnece ssarily divide the park. Another trail crossing design to continue a park trail through a drainage bio-swale is shown in Figure 10.5B. Constructed wetlands are designed with three main purposes.

They are artificial wetlands created as new habitat for native and migratory wildlife, wetlands constructed to manage wastewater either as the primary treatment before water flows downstream or

as the final stage of treatment termed polishing.

Constructed wetlands are also created as part of a larger strategy for a community to manage storm water and for storm protection. Permeable modular and pervious paving surfaces for roads, parking lots, and just about any paved surface can be designed to allow surface water to infiltrate the soil below (Figure 10.6).

Modular units are made in a factory. They are installed by placing the units on a bed of sand.

The sand base, in turn, is constructed with a compacted crushed rock base. Rain and surface water seeps into the space between the units, passing through the layers of sand and rock to the soil below. Pervious paving is similar to traditional concrete and asphalt but manufactured in a plant with material additives that leave void spaces after the concrete sets or the asphalt hardens. Rain and surface water can flow through the pavement to infiltrate the soil below. Rock reed or plant filters are a sub-surface cavity constructed beneath the soil surface and filled with organic materials (plant materials such as branches and graded rock material). They are located in areas where soils are not suitable for absorption fields or will not allow water to percolate to the aquifer below (Figure 10.7). Green roofs are an old idea4 and technology applied to mitigate against cold weather in the form of roofs covered in sod (grass) to insulate a building in cold climes (Figure 10.8). Green or landscaped roofs are used to mitigate contemporary undesirable urban environmental conditions such as to reduce the heat island effect caused by the large percentage of pavement in urban cities, as means of collecting and cleaning rainwater for use on site, and by providing an added layer of roofing insulation to moderate the internal climate control of a building. A roof garden enhances the aesthetic and economic value of a building and at the same time is a place for the enjoyment of a building’s inhabitants just as any outdoor garden would be. The installation of a green roof is suitable for an old or new residence, commercial or industrial building, or any structure with a roof.

The design of a garden roof requires specialized knowledge of materials and infrastructure support (leak-resistant membrane materials, light-weight soil design, and a drainage system, for instance). There are a growing number of

Figure 10.6 Permeable surfacing for the campus courtyard consisting of concrete modular units set in sand for walks and compacted crushed granite under picnic table, by Ahbé Landscape Architects.

Figure 10.7 Rock reed filter at Bluebonnet Swamp parking lot, Baton Rouge, LA, by Ted Jack, PLA.

Figure 10.8 Library with sod roof, Technical University of Delft, the Netherlands.

landscape architects who have developed green roof design into an area of specialized practice. Low impact development (LID) is an approach to managing storm water applied to new land development or the redevelopment of old urbanized areas. The approach values storm water as a resource to be used on-site rather than as a waste product that needs to be removed as quickly as possible (Figure 10.9). LID applies a diverse range of designed topographic, landscape, and infrastructure features that mimic how natural landscape systems manage storm water, keeping the water as close to its source as possible. LID employs principles such as preserving and reconstructing natural landscape features (such as wetlands), minimizing the application of impervious paving surfaces, gathering and storing surface water, and other design devices. The aim of

LID design is to create functional and appealing ways of handling storm water that treat the water as a resource rather than a waste product to be removed. There are many systems designed by landscape architects that have been used to adhere to these principles, such as building bio-retention ponds, constructed wetlands, rain gardens, vegetated rooftops, introducing cisterns for water storage, and permeable pavements. By implementing LID principles and practices, water can be managed in a way that reduces the potential for flooding and promotes the natural movement of water within a project site toward an adjacent ecosystems or watershed. Applied on a broad scale, LID can maintain or restore a watershed’s hydrologic and ecological functions that have been altered by previous land development that employed storm-water disposal infrastructure. LID is appropriate and effective for new development or to retrofit an existing development for a range of land uses from high-density, ultra-urban settings to low-density development. A well-planned development employing LID principles adds economic value to a project, increases the quality of life

Figure 10.9 Storm water directed from street and sidewalk to depressed planting area designed to detain water and allow time to penetrate into soil. LID design has added benefit to supplement irrigation of landscape. Figure 10.10 Comprehensive low impact storm-water management design for a condominium complex in Amersfoort, the Netherlands.