Merchant, M., Wilcox, B.

The hydrology of landscapes surrounding urban areas is constantly changing due to the expansion of urban boundaries. Established urban and newly developing areas must develop and implement cost effective means for restoring or minimizing impacts from these changes, and planning for future growth (Zhen, et al. 2006; Cheng et al., 2006). Issues from increased urbanization, such as increased loads of non-point source (NPS) pollution in surface runoff place greater demands on stormwater management. Environmental issues associated with stormwater runoff are best controlled by implementing best management practices (BMP). BMPs are methods designed to control stormwater runoff through different structures, such as stormwater detention ponds. A detention pond is a stormwater management facility that includes a permanent pool of water for removing pollutants while having additional capacity above the permanent pool for detaining stormwater runoff (NCDWQ, 2007). The current state of knowledge on the use and siting of stormwater detention ponds is very extensive. Detention ponds have been used widely in North America since the 1970's to manage runoff from urban areas (STEP, 2009). It is well documented that detention ponds are able to simultaneously provide improvement to both stormwater runoff attenuation and water quality (Ferguson, 1991). Detention ponds constructed prior to the 1980's were primarily designed to reduce the peak flows of stormwater runoff, whereas current detention ponds incorporate water quality as well as quantity control (STEP, 2009).

Detention ponds are necessary structures in growing urban areas because increasingly urbanized landscapes are causing flood events with greater magnitude and frequency than in previous years (Perez-Pedini, et al. 2005). Furthermore, impervious surfaces associated with urban areas (e.g., roads, parking lots, roof tops, etc.) prevent the natural infiltration of water into soils (Brabec et al., 2002; Barnes et al., 2002). As a result, hydrological changes caused by urbanization include an increase in the number of drainage slopes and direct flow runoff, while also experiencing decreases in evapotranspiration, groundwater recharge, and base flow (Figure 1). In direct relation to surface flow from storm events, the changes caused by urbanization are known to increase peak flow rates at watershed outlets (Yeh & Labadie, 1997). This makes an increase in sediment loading a critical change associated with stormwater runoff, further illustrating the need for the implementation of detention ponds. As a result, local authorities must take initiative by providing appropriate stormwater management to mitigate the adverse effects of an urbanized watershed.

Current stormwater management initiatives typically involve BMPs such as stormwater detention ponds (Kaini et al., 2007). BMPs constitute a wide variety of appropriate activities and technologies designed to reduce the effects of watershed development on flow regimes without altering riparian morphology (Perez-Pedini et al., 2005). BMPs can be classified into two groups: structural and non-structural. Structural BMPs include engineered and built systems designed to provide water quantity and/or quality control, whereas non-structural BMPs include a range of pollution prevention, education, management and development practices designed to limit the conversion of rainfall into runoff (Martin et al., 2007). BMP sites are selected before construction and must be based on an inventory of pollution source areas and other critical factors such as land availability, geographical conditions, and site-specific legal and jurisdictional considerations in order to be be effective in reducing the negative impacts of urbanization (Zhen et al., 2004). The implementation of BMPs in appropriate locations maximizes stormwater runoff flow attenuation and improvement in local water quality. The effectiveness of a particular series of BMPs is based on the spatial characteristics of the region of interest, and thus should be implemented strategically by authorities (Figure 2). Strategic implementation is also important because urban runoff poses unique challenges with respect to meeting total maximum daily loads (TMDL) (Gardiner et al., 2003). Strategic implementation of stormwater detention ponds involves placing facilities in locations that maximize the pond's effectiveness, such as upstream of the outlet draining into a common collection system (Figure 3). Before implementing a BMP, it is important that its performance, maintenance and environmental advantages and disadvantages are evaluated (Tsihrintzis & Hamid, 1997).

Stormwater management authorities have started to follow specific criteria requirements when siting detention ponds, such as drainage area, slope, soils, and topography, in order to maximize the effectiveness of the structure (USEPA, 2006). For example, an effective detention pond will have a high trap efficiency (i.e. the ratio of amount of runoff retained in a pond to the amount flowing in). High trap efficiency allows for algae and microorganisms to degrade and remove metals, nutrients, and organic chemicals from the urban landscape runoff (Ferguson, 1998). The removal efficiency of detention ponds for suspended solids has been reported to range from 10-85% depending on basin design, outlet structure, detention time, and base-flow concentrations (Scherger & Davis, 1982). Furthermore, water quality associated with urban runoff is an important component to consider in stormwater management because it depends on factors that determine flow rate magnitude, time distributing, and pollutant concentration (Tsihrintzis & Hamid, 1997). Urban water quality is in turn affected by rainfall patterns, volume, intensity, traffic volume, dry days, land use, maintenance practices, geographic and geologic characteristics of the region, and drainage system configuration (Tsihrintzis & Hamid, 1997). Water quality treatment is possible with stormwater detention ponds because pollutants that are transported from stormwater runoff into pond basins settle out and attach to sediment particles (Ferguson, 1998; Whipple & Hunter, 1981). Attention is also given to less important criteria, such as siting a detention pond where it can function as an aesthetic amenity (CSQA, 2003).

When siting stormwater detention ponds with the use of a GIS tool, the scale of implementation must be taken into account. BMPs are often applied at a local, point source scale. Literature however, suggests that the best way to implement BMPs is at a watershed or subcatchment scale (Yeh & Labadie, 1997). An example of this is in the county of Los Angeles, which has developed a BMP prioritization methodology at watershed scale using a GIS platform (Figure 4). The first step in this methodology involves performing a catchment prioritization, which is based on the following criteria: pollutants of concern, pollutant loadings, impairments, and regulatory requirements (Susilo et al., 2006). The second step involves identifying opportunities for BMP implementation, which is based on the following criteria: available space, land ownership, slopes, liquefaction, environmentally sensitive areas, and infrastructure. Osbourne (2000) developed a geographic information systems (GIS) tool for incorporating NPS and best management practices for the city of Austin, Texas. The criteria associated with this tool took into account terrain (i.e. elevation and slope), urban stream networks, precipitation, and land use. Land use is an important factor to consider for the heavy metal pollution of ponds. This is because within a given type of land use, heavy metal concentrations in pond sediments increase with age (Liebens, 2001). System for Urban Stormwater Treatment and Analysis Integration (SUSTAIN) is another decision-support GIS tool developed by the U.S. Environmental Protection Agency (USEPA) for strategically placing BMPs in urban watersheds in order to help develop, evaluate, select, and place BMP options based on cost and effectiveness (Lai et al., 2007).

GIS applications have proven to be very beneficial in locating suitable sites for BMPs. GIS enhances and supports the ability to make decisions concerning the spatial distribution of processes and structures existing within a watershed or catchment (Tsihrintzis et al., 1996). In addition to multi-criteria evaluations (MCE), genetic algorithms (GAs) are commonly used with GIS applications for siting BMPs (Zhen et al., 2004). Dorn et al. (1995) developed a GA for the placement of a wet detention basin system, indicating that GAs can be succesfully applied to GIS. Young et al., (2011) also developed a GA by creating a mathematically based software with a GIS input to help locate BMP sites in Blacksburg, Virginia. Although MCE and GA applications in GIS have shown success to date, model effectiveness and appropriate input parameters from validation studies on watersheds of various sizes and characteristics need to be reported in order to have a better understanding of relevant processes, and to apply them more effectively in environmental impact studies (Hamid et al., 1995).

With stormwater runoff being a significant mobilization of NPS pollution in urban areas, GIS applications are very useful for planning urban control efforts as they are able to incorporate multiple criteria, such as land use and slope, into a decision making framework (Ventura & Kim, 1993). Thus it is important to use GIS to site stormwater detention ponds in areas with disproportionately large pollutant loadings by delineating boundaries of different land uses. Several studies have included this task by determining pollutant loading rates which then allow for the prioritizing and identification of critical areas (Tsihrintzis et al., 1996; Cheng et al., 2009). Determining areas of high NPS pollutant loadings is best performed using GIS. This is accomplished by estimating impervious surfaces in an urban watershed and then using a GIS application to model watershed runoff flow (Rootes-Murdy, 2011; Ventura & Kim, 1993). Some studies have also developed GIS models for locating BMPs based on low impact development (LID) site design criteria and hydrology principles (Eslami, 2010).