Stormwater is precipitation that does not soak into the ground, but instead runs off its surface and carries and deposit water pollutants (e.g. sediment, nutrients, and pesticides) into surface-water bodies (Jaber, 2008). Pollution from nonpoint sources, including sediment deposition, erosion, contaminated runoff, hydrologic modifications that degrade water quality, and other diffuse sources of water pollution, has been the largest cause of water quality impairment in the United States because of the past and current successes in controlling point sources (EPA, 1995). As population increases, land development for human living spaces has been accelerated and is leading to increased discharge of pollutants from Municipal Separate Storm Sewer Systems (MS4s), because development almost always increases the amount of impervious surfaces (which is a good measure of land use intensity). In particular, urbanization causes fundamental modifications to the hydrological cycle, typically resulting in an increase in the volume of stormwater discharges and associated pollutant loadings. Also, chemical, physical, and thermal changes associated with new development in urban areas can adversely affect receiving waters (EPA, 2002b).

Accordingly, urban activities that reduce these side effects have been required, which include the following approaches: 1) erosion and sediment control; 2) onsite sewage disposal systems (septic tanks); 3) runoff control from development sites; and 4) site-specific activities (e.g. oil and grit separators at gas stations) (EPA, 2001). Among these side-effect water pollutants, sedimentation is one of the most widespread pollutants, affecting assessed rivers and streams, second only to pathogens (bacteria) in construction sites. Sediment runoff rates from construction sites are typically 20 times greater than that of normal land use (Jaber, 2008), 10 to 20 times greater than those from agricultural land, and 1,000 to 2,000 times greater than those from forest land. In a short period of time, construction activity can contribute more sediment to streams than can be deposited over several decades. Accordingly, most erosion and sediment controls require regular maintenance to operate correctly. Accumulated sediments should be removed frequently, and materials should be checked periodically for wear. Also, regular inspections by qualified personnel should be performed after major rain events (EPA, 2005).

Although stormwater discharges are regarded as point sources of pollution, stormwater carries pollutants that are considered as non-point sources of pollution. All point-source discharges, unlike nonpoint sources such as agricultural runoff, are required to be controlled under the Clean Water Act (CWA) to be covered by federally enforceable National Pollutant Discharge Elimination System (NPDES) permits (EPA, 2005; 2012a). Established in 1972, the CWA regulates three types of stormwater activity from MS4s, industrial activities, and construction activities as follows: 1) Medium (population between 100,000 and 249,999) and large (population of 250,000 or more) MS4s; 2) Eleven categories of industrial activity; 3) Construction activity disturbing 5 or more acres of land regulated by Phase I issued in 1990 under the CWA. Phase II was expanded from Phase I and includes additional operators of MS4s in urbanized areas that are not defined as any MS4 not medium or large MS4s and operators of small construction sites that cover between 1 and 5 acres.

In Texas, small MS4s are defined as (1) all small MS4s located in urbanized areas as defined by the Bureau of the Census and (2) small MS4s located outside of the US but designated by Texas Commission on Environmental Quality (TCEQ) (Jaber, 2008). Efforts to improve water quality under the NPDES program traditionally have focused on reducing pollutants in industrial process wastewater and municipal sewage treatment plant discharges. Over time, it has become evident that more diffuse sources of water pollution, such as stormwater runoff from construction sites, are also significant contributors to water quality problems (EPA, 2005).

The small MS4s regulated by Phase II under the NPDES stormwater permits require six minimum control measures and an optional seventh control measure as follows (Jaber, 2008): (1) public education and outreach; (2) public participation/involvement; (3) illicit discharge detection and elimination; (4) construction site runoff control; (5) post-construction runoff control; (6) pollution prevention/good housekeeping; and (7) authorization for municipal construction activities (EPA, 2005). In particular, construction activities require stormwater pollution prevention plans, for which best management practices (BMPs) are erosion and sediment controls and stormwater management controls used during construction (Table 1).

According to the EPA (2002a), most traditional approaches to stormwater management increase downstream property damage and necessitate expensive public works, while more recent approaches to stormwater management seek natural drainage by providing stormwater quantity reduction and quality improvement. Additionally, stormwater is regarded as a resource that can be used for recharging groundwater, supplying freshwater, increasing recreational opportunities, and augmenting drinking water supplies in some cases. Also, stormwater management can lead to the reduction of erosion, flooding, and damage to natural drainage features that provide wildlife habitats.

In this study, the federal urban stormwater regulatory structures and various stormwater control measures are reviewed. The Phase II final rule published by the U.S. EPA is summarized for construction sites, although the publication is outdated because it provides the sight of urban stormwater management. Also, recent BMP application measures for stormwater mitigation are summarized and reviewed.

Generally, urban stormwater management is called BMP. BMPs can be divided into two categories: structural BMPs and non-structural or source-control BMPs. They can be tracked more easily in urban areas than in forest or agricultural areas because there is little change once an area is developed and structural BMPs are installed (EPA, 2001). The structural BMPs are erosion and sediment controls along with runoff, while the non-structural BMPs are the minimization of disturbances, preservation of natural vegetation, and good housekeeping practices. Structural BMPs involve building structures that include ponds, vegetated biofilters, and so on to store stormwater. Ponds generally consist of wet ponds (retention ponds), dry ponds (extended detention ponds), constructed wetland ponds, and infiltration basins. Vegetated biofilters are usually vegetated filter strips, bio-retention cells, or grass swales.

General nonstructural or source-control BMPs limit pesticide use in agricultural areas or retaining rainwater on residential lots (EPA, 2002a). In the past, BMP models considered only hydrology.

However, at present, three components of hydrology, sediment transport, and water quality are integrated and considered together. In particular, many receiving water quality issues are due to the high level of contaminants generated by three types of urban wet-weather flow (WWF) discharge: stormwater, combined sewer overflow (CSO), and sanitary-sewer overflow (SSO) (EPA, 2012c). Also, low flow or dry-weather flow originating from sanitary wastewater, industrial and commercial pollutant entries, and septic tank systems can cause significant pollutant loading to receiving water in urban drainage systems.

To implement BMPs in the planning of stormwater management, various stormwater models have been developed. The EPA has developed an Integrated Stormwater Management Decision Support Framework (ISMDSF), Urban Stormwater Treatment and Analysis Integration (SUSTAIN), and a Storm Water Management Model (SWMM). The ISMDSF is a decision support system for developing, evaluating, selecting, and placing BMP options based on cost and effectiveness (Lai et al., 2005). The SUSTAIN was designed for use by watershed and stormwater practitioners to develop, evaluate, and select optimal BMP combinations at various urban watershed scales based on user-defined cost and effectiveness criteria (EPA, 2012b). The SWMM is a dynamic rainfall-runoff simulation model used for single-event or long-term (continuous) simulation of runoff quantity and quality from primarily urban areas (Huber and Dickson, 1988; Rossman, 2009).

Recently, several traditional and newly developed BMPs have been used in urban stormwater management: conventional retention ponds with a floating treatment wetland for water pollutants such as TSS, copper, and zinc (Borne et al., 2013; Winston et al., 2013), field-scale rain gardens for nitrate, phosphate and others (Yang et al., 2013), rain barrels/cisterns, and porous pavement (Ahiablame et al., 2013), bioretention (Brown et al., 2013), bioretention/porous pavement (Lee et al., 2012), detention basins (Park and Roesner, 2012), bioinfiltration rain gardens (Komlos and Traver, 2012; (Flynn and Traver, 2013), pervious concrete infiltration basins (Horst et al., 2011), irrigated green roof systems (Hardin et al., 2012), rainwater harvesting and permeable pavements (Damodaram and Zechman, 2013), and wetlands (O’Connor et al., 2012). Also, environmental indicators of urban BMP implementation for runoff control and pollution prevention are provided (Table 2) for understanding the role of BMPs in urban stormwater management, along with management measures (MMs), which are economically achievable measures to control the addition of pollutants (EPA, 1993, 2001; Clayton and Brown, 1996).

While certain structural control measures are effective in certain circumstances, their measures may not be available for previous developed land for implementation. In these circumstances, the EPA (2012a) explains examples of improper structural measures, which are first flush diversion systems, detention/infiltration basins, retention basins, extended detention basins, infiltration trenches, porous pavement, grass swales, and swirl concentrators. Accordingly, the following non-structural or source-control BMPs can be considered as alternative practices when land is limited or unavailable: erosion control, stream bank management techniques, street cleaning operations, vegetation/lawn maintenance controls, debris removal, road salt application management, public outreach, education, and awareness. As shown in Table 3, water quantity (e.g. storm flow, peak flow, and runoff) and water quality (e.g. TP, TN, CU, and so on) BMP measures are applied in various locations of urban or laboratory.

Preventing the generation of polluted runoff costs less than treatment. Accordingly, many municipalities have been implementing non-structural or source-control BMPs to prevent runoff, as well as utilizing structural BMPs in construction sites or redevelopment sites (EPA, 2002a). Construction sites commonly discharge sediment, solid and sanitary waste, phosphorous, nitrogen, pesticides, oil and grease, concrete truck washout, construction chemicals and debris pollutants. These pollutants cause physical, chemical, and biological harm to water resources. In particular, within a short period of time, construction sites can contribute more sediment to streams than can be deposited naturally over several decades.

Several site-specific BMPs are required to prevent stormwater pollutants in traditional stormwater management controls in construction. These BMPs are used to reduce the amount of pollutants contaminating surface water bodies, prevent erosion, and redirect stormwater flow. These controls can be used after actual BMP construction to prevent pollution due to stormwater runoff (EPA, 2005). Also, the BMPs can be divided into pre- and post-construction BMPs, depending on the point in construction. The pre-construction BMP control measures are erosion and sediment controls by temporary seeding, permanent seeding, and mulching, as well as pollutant prevention by limiting the amount of water flow, or changing the water direction with earth dikes, silt fences, sediment traps, and sediment basins (Jaber, 2008). The post-construction BMP control measures include retention ponds, detention ponds, infiltration measures, vegetated swells, and natural depressions. Retention ponds store stormwater runoff, with runoff removed only through evaporation, infiltration, or emergency bypass. In detention ponds, all sediments are allowed to settle, and then the retained water is slowly released.

Infiltration measures include trenches, basins, and dry wells used to allow water to percolate from the surface into the soil below. Vegetated swells and natural depressions are lined with vegetation (usually grass), which removes sediments from runoff and allows water to infiltrate into subsurface soil (Jaber, 2008). Brush barriers, filter strips, silt fencing, vegetated channels, and inlet protections (EPA, 2001) can be used for implementing the construction site erosion and sediment control management measures. Often, the effectiveness of structural stormwater controls, especially detention and retention basins and infiltrations devices, is limited by a lack of maintenance. Accordingly, catch basins and drainage channels require regular maintenance (EPA, 2002b). Additionally, redevelopment projects – one kind of post-construction measure – are done to alter the footprint of an existing site or building in such a way that there is a disturbance greater than or equal to 1 acre of land. Because redevelopment projects may have site constraints that are not found on new development sites, the U.S. EPA Phase II Final Rule provides flexibility for implementing postconstruction controls on redevelopment sites that consider these constraints.

Utilization of BMPs can increase the availability of the water supply and convert stormwater and water pollutant problems into sustainable water sources. Also, prior planning and design for the minimization of pollutants in postconstruction stormwater discharges can be the most costeffective approach to stormwater quality management. However, the EPA (2005) indicates two forms of substantial impacts of post-construction runoff: an increase in the type and quantity of pollutants in stormwater runoff and runoff increase by the quantity of water delivered to the water body during storms. Table 4 lists the measurable parameters for pre- and post-construction site stormwater runoff controls.

Operators can develop and implement strategies that include a combination of structural or non-structural BMPs, and implement post-construction runoff controls corresponding to ordinances. The EPA (2005) recommends that small MS4 operators develop and implement structural and non-structural or source-control measures. The structural BMPs are as follows: 1) stormwater retention/detention ponds for both controlling stormwater volume and settling out particulates for pollutant removal; 2) infiltration BMPs (infiltration basins/trenches, dry wells, and porous pavement) that facilitate the percolation of runoff through the soil to ground water, resulting in reduced stormwater runoff quantity and reduced mobilization of pollutants; and 3) vegetative BMPs (grassy swales, filter strips, artificial wetlands, and rain gardens) that remove pollutants and facilitate percolation of runoff, thereby maintaining natural site hydrology, promoting healthier habitats, and increasing aesthetic appeal. The non-structural BMPs are planning procedures and site-based BMPs (buffer strip and riparian zone preservation, minimization of disturbance and imperviousness, and maximization of open space). The non-structural or source-control BMPs (EPA, 2002a) are a type of low impact development (LID) that can help to meet NPDES requirements and offer construction-cost savings along with other benefits compared with the traditional practices, such as detention ponds and retention basins. The focus of LID is on emulating the functions of natural systems to reintegrate rainfall into the water cycle rather than disposing of it as a waste product. LID can be integrated into municipal stormwater programs that help communities meet NPDES permit requirements. LID BMPs can reduce runoff and associated pollutants across a watershed through 1) disconnected impervious surfaces; 2) preservation of open space/natural features; 3) bioretention cells (rain gardens); 4) flow-through planters and tree boxes; 5) porous pavement; 6) water harvesting (rain barrels, cisterns); 7) ecoroofs; 8) low-input landscaping; and 9) grassed swales.

Sometimes, LID is considered as a type of environmental sensitive development (ESD) with features such as better site design, green infrastructure, conservation design, integrated site design, sustainable development, and smart growth. ESD has a number of advantages over traditional and engineered stormwater drainage approaches: 1) stormwater at its sources; 2) more protective of streams and watersheds; 3) promotion of groundwater recharge; 4) more flexible site layouts; 5) enhanced aesthetics and public access/use; 6) cost savings; 7) groundwater saving; 8) reduction of impervious surfaces and runoff (peak flow volume and rate); and 9) improvement of water quantity (e.g. reduced risk of flooding) and quality (EPA, 2009).

Stormwater derived from precipitation carries water pollutants that are regarded as a Non-Point Source (NPS), although stormwater discharges are considered as point sources of pollution, because the U.S. EPA has regulated all point source discharge (e.g. stormwater) from MS4s to be covered by NPDES since 1972. Also, Phases I and II under the CWA have regulated the MS4s as small, medium, and large sizes based on population, construction activity, industrial activity categories, and designation. In this study, the pre- and post-construction BMPs were focused on and separated into two categories: 1) structural BMPs and non-structural or source-control BMPs. The structural BMPs are generally constructed to reduce the runoff and water pollutants from runoff, while the non-structural BMPs are institutional and educational measures to limit the stormwater amount and water pollutants derived from landscapes. The recent literature on BMP applications to various locations have been reviewed and summarized based on two categories: water quantity applications and water quantity, quality and pollutant applications. The summary and review of urban stormwater management provide understanding of BMPs and beneficial knowledge to model operators and users.