IDENTIFICATION OF FUTURE WATER SOURCES

HOLISTIC WATER RESOURCE MANAGEMENT

Conjunctive water use is often implemented in holistic water resource management. Conjunctive use is the coordinated management of surface-water and groundwater supplies to maximize the yield of the overall water resource. An active form of conjunctive use utilizes artificial recharge, where surface water is intentionally percolated or injected into aquifers for later use. A passive method is to simply rely on surface water in wet years and use groundwater in dry years. The success of many of these programs, however, depends on purchasing available surface water from other users (Water Education Foundation, 2006).

Low impact development (LID) is another way to incorporate holistic watershed management. According to the Alabama Cooperative Extension System (ACES), LID is defined as an interdisciplinary systematic approach to stormwater management that, when planned, designed, constructed, and maintained appropriately, can result in improved stormwater quality, improved health of local water bodies, reduced flooding, increased groundwater recharge, more attractive landscapes, wildlife habitat benefits, and improved quality of life. Low impact development minimizes runoff and employs natural processes such as infiltration, evapotranspiration, and storage of stormwater at multiple fine scale locations to be as near to the source of stormwater as possible. Successful implementation of LID recreates a more natural hydrologic cycle in a developed watershed (ADEM, ACES, and Auburn University (AU), 2013).

The ADEM, ACES, and AU developed a Low Impact Development Handbook to provide guidance for LID, stormwater control, green infrastructure, and community planning that promotes holistic management for watersheds in the State of Alabama. The first step in LID is to consider the landscape that will be developed. It is critical to understand local soils, size constraints, groundwater level, native vegetation options, and other potential constraints so that the appropriate LID stormwater control measure practices can be selected to meet project goals. The LID stormwater practice should be designed to effectively store, infiltrate, or spread out stormwater in its landscape setting, ideally working as a system with the other practices in the development and watershed (ADEM, ACES, and AU, 2013). LID practices include bioretention; constructed stormwater wetlands; permeable pavement, grassed swales, infiltration swales, and wet swales; level spreaders and grassed filter strips; rainwater harvesting; green roofs; riparian buffers; rain gardens; curb cuts; and riparian buffers.

Bioretention cells (BRCs) remove pollutants in stormwater runoff through adsorption, filtration, sedimentation, volatilization, ion exchange, and biological decomposition. A BRC is a depression in the landscape that captures and stores runoff for a short time, while providing habitat for native vegetation that is both flood and drought tolerant. BRCs are stormwater control measures (SCMs) that are similar to the homeowner practice of using rain gardens, with the exception that BRCs have an underlying specialized soil media and are designed to meet a desired stormwater quantity treatment storage volume. Peak runoff rates and runoff volumes can be reduced and groundwater can be recharged when bioretention is located in an area with the appropriate soil conditions to provide infiltration (ADEM, ACES, and AU, 2013).

Constructed stormwater wetlands are created wetland areas designed to treat stormwater and function similarly to natural wetlands (fig. 81 [82]). These systems use complex biological, chemical, and physical processes to cycle nutrients and breakdown other pollutants for treatment of stormwater runoff. Constructed stormwater wetlands mimic the filtration and cleansing capabilities of natural wetlands while providing temporary storage of stormwater above the permanent pool elevation and because of this, are often used for water quantity control. These systems are usually large and use shallow pools, complex microtopography, and both aquatic and riparian vegetation to effectively treat stormwater (ADEM, ACES, and AU, 2013).

Permeable pavement is a pervious surface used in place of traditional concrete or asphalt to infiltrate stormwater. Permeable pavement provides a volume reduction of stormwater runoff through temporary storage. It can be used to reduce peak flows and promote stormwater infiltration in urbanizing watersheds. The application of permeable pavement reduces impervious surface area runoff, which has been linked to stream bank erosion, flooding, nonpoint source pollution, and other water quality impairments. Permeable pavement refers to any pavement that is designed to temporarily store stormwater in a gravel base layer. Stormwater is held in the gravel base layer, or subbase, before leaving the system through exfiltration into surrounding soils or through an underdrain. These systems are suitable for residential driveways, walkways, overflow parking areas, and other low traffic areas that might otherwise be paved as an impervious surface (ADEM, ACES, and AU, 2013).

A water quality swale is a shallow, open-channel stabilized with grass or other herbaceous vegetation designed to filter pollutants and convey stormwater. Swales are applicable along roadsides, in parking lots, residential subdivisions, and commercial developments and are well suited to single-family residential and campus type developments. Water quality swales presented in the LID handbook are designed to meet velocity targets for the water quality design storm? [not sure what this means], may be characterized as wet or dry swales, may contain amended soils to infiltrate stormwater runoff, and are generally planted with turf grass or other herbaceous vegetation (ADEM, ACES, and AU, 2013).

Level spreaders are devices that create diffused or sheet flow that is evenly distributed or dispersed to decrease flow velocity and discourage erosive forces associated with concentrated flows. Most commonly, level spreaders are paired with grassed filter strips, riparian buffers, or a combination of the two to provide pollutant removal. The primary purpose of a level spreader is to disconnect impervious surfaces by creating non-erosive stormwater connectivity with grassed filter strips. A grassed filter strip is a linear strip of dense vegetation that receives sheet flow of stormwater runoff from a nearby impervious surface or level spreader in order to reduce peak discharge rates, encourage sediment deposition, and provide limited infiltration. Grassed filter strips are planted with turf grass, which is easy to maintain and blends seamlessly into urban landscapes. Grassed filter strips are most effective when combined with level spreaders (ADEM, ACES, and AU, 2013).

Rainwater harvesting is the collection of rainwater for reuse, typically from a rooftop, and can be used as a form of rooftop runoff management to reduce runoff from impervious surfaces. Rooftop systems typically collect stormwater through a connection to a rain gutter system. Rainwater harvesting systems may be above or below ground systems and can be large or small depending on the site, application, and intended use. When designed and used properly, these systems are an excellent way of saving water, energy, and money. Rain barrels are systems used for small-scale (50-60 gallons) applications such as residential areas and cisterns are larger storage tanks (100-10,000 gallons) that are better suited to residential or agricultural settings where large volumes of water are needed (ADEM, ACES, and AU, 2013).

Green roofs are landscaped roofs that use a specialized growing substrate, storage, drainage mat, and vegetation that is tolerant of extreme climates experienced on rooftops (fig. 82). Green roofs mitigate stormwater runoff, reduce the heat island effect of impervious surfaces from rooftops, extend roof membrane life, conserve energy, reduce noise and air pollution, provide wildlife habitat in urbanized settings, and improve fire resistance of buildings. These systems have been used in Europe for decades and are becoming more prevalent in the U.S. as stormwater retention practices that provide aesthetic value. As a stormwater control measure, green roofs are more effective at reducing runoff volumes resulting from small storms rather than providing pollutant load reductions from impervious surface runoff (ADEM, ACES, and AU, 2013).

Riparian buffers are permanently vegetated transition zones that connect upland areas to streams. Prior to development, most streams in the Southeast had naturally occurring riparian buffers. These streamside forests slow runoff velocity, create diffuse flow, and reduce nonpoint source pollution concentrations before runoff enters nearby streams or other water bodies. Buffers filter pollutants from agricultural, urban, suburban, and other land cover through natural processes such as deposition, infiltration, adsorption, filtration, biodegradation, and plant uptake. Riparian buffers also stabilize stream banks and provide food and shelter to wildlife to connect otherwise fragmented wildlife communities in a watershed. Riparian buffers are often recommended as part of a holistic watershed management plan aimed at reducing nonpoint source pollution (ADEM, ACES, and AU, 2013).

Rain gardens are shallow depressions in a landscape that capture water and hold it for a short period of time to allow for infiltration, filtration of pollutants, habitat for native plants, and effective stormwater treatment for small-scale residential or commercial drainage areas. Rain gardens use native plants, mulch, and soil to filter runoff. As urbanization increases and pervious surfaces decrease, rain gardens are an excellent practice to promote infiltration of up to 30% more stormwater than traditional lawns. Residential stormwater management can often help homeowners save money on lawn irrigation when lawns are converted to rain gardens. These areas are designed to capture 3 to 6 inches of runoff after a storm, which allows water to infiltrate and return to groundwater, rather than being discharged to a stormwater conveyance system (ADEM, ACES, and AU, 2013).

Curb cuts convey stormwater into vegetated areas such as roadside swales, parking lot islands, rain gardens, or bioretention areas. Curb cuts are an easy retrofit that can be used in residential or commercial land use areas and are effective in moving stormwater to landscaped areas. Curb cuts are often used to convey stormwater into another LID practice. Curb cuts do not perform any pretreatment, but can minimize erosion by creating diffuse flow into other SCMs. Curb cuts can also be installed to redirect stormwater into a grassy field. While this is not directly considered a LID practice, it does reduce stormwater quantity in the receiving water body. Roadside curb cuts usually intercept perpendicular stormwater flow and in many cases multiple curb cuts are needed to adequately collect and move stormwater (ADEM, ACES, and AU, 2013).

Disconnected downspouts can direct rooftop runoff to vegetated areas through the disconnection of rooftop downspouts. By redirecting rooftop runoff, stormwater entering the stormwater conveyance network is reduced and groundwater recharge and runoff infiltration is increased. Disconnected downspouts are often used in conjunction with other stormwater infiltration practices by directing runoff to practices such as rain gardens, bioretention areas, and grassed swales. In doing so, the need for curbs, gutters, and conventional collection or conveyance of stormwater can be reduced (ADEM, ACES, and AU, 2013).

These practices may be found in detail in the Low Impact Development Handbook for the State of Alabama. The handbook includes site selection strategies; design guidance, formulas, and examples; construction activities; vegetation design guidelines and examples; maintenance schedules; pollutant removal tables; and references for each practice listed.