BMP Technology in Urban Watersheds: Current and Future Directions

Urban land area in the U.S. more than quadrupled between 1945 and 1997, increasing from 15 to 64 million acres, primarily from conversion of land that was previously forested, pastured, and ranged (USDA, 2000). During this same period, paved road mileage nearly tripled (USEPA, 2000). This urbanization, with the accompanying expansion of impervious surfaces, significantly increased the nation's total amount of developed and paved surface area; reduced the amount of wetlands and other undeveloped land; and decreased our watersheds' ability to mitigate floods, facilitate sediment replenishment, and protect water quality and aquatic health by removing excess nutrients and other chemical contaminants before runoff enters receiving waters. Urban development also causes changes in the hydrologic regime, which can result in habitat loss and other ecological impairments due to more frequent and longer bankful events, lower baseflows, and increased stream channel erosion (Roesner et al., 2001). Significant decreases in biotic measures of aquatic ecologic health also become apparent at low relative impervious areas (Center for Watershed Protection, 1994).

Recently, the City of Milwaukee, Wisconsin embarked on a long-term planning effort based on the Environmental Protection Agency's watershed approach. The use of this integrated approach was prompted by the understanding that if every sanitary sewer overflow was stopped tomorrow, non-point pollution would still cause beaches to close and make fish unfit to eat. The Milwaukee Metropolitan Sewerage District (MMSD) launched this effort to develop an approach to reduce non-point water pollutants. To insure success, MMSD believes it must increase public awareness and public education of the sources of these pollutants in order to reduce non-point pollution. The use of good stormwater management principles by the general public is a major component of this effort.

The pursuit of "sustainability" has emerged in recent years as a progression from previous "environmental protection" endeavors. The ambition of "sustainability" and "sustainable development" is to have lifestyles, and their supporting infrastructure, that can endure indefinitely because they are neither depleting resources nor degrading environmental quality. Land development activities that do not consider sustainability for a growing community can have detrimental impacts on the physical environment, including the health and amenity of water environments.

This chapter contains three sections by U.S. EPA authors that reflect contribution of the Agency's research programs towards protection of watersheds through better understanding of capabilities and beneficial uses of BMP technologies. The topics presented here blend well with those presented by "outsiders," which proves that the EPA keeps up well with the entire scientific community of BMP experts. The three EPA sections range from a state-of-the-art review of BMPs' ability to manage pathogen contamination; a field study, which determined variability of pathogen concentrations in stormwater runoff depending on the land use and on the season of the year; and an ingenious, novel program of economic incentives for those who install BMPs on their properties for detention of excess runoff.

The particle size distributions of stormwater at different locations in an urban area greatly affect the ability of different source area and inlet controls in reducing the discharge of stormwater pollutants. A series of U.S. EPA funded research projects has examined the sources and treatability of urban stormwater pollutants (Pitt, et al. 1995). This research has included particle size analyses of 121 stormwater inlet samples from three states (southern New Jersey, Birmingham, Alabama; and at several cities in Wisconsin) that were not affected by stormwater controls.

Much research has been conducted on the transport, settling, and scour of gross solids in sanitary sewers and in combined sewers, especially in Europe during recent years (Ashley, et al. 1999; 2000; 2002; Butler and Karanaratne 1995; Butler, et al. 1995; Gigana, et al. 1998a, 1998b, 1998c, 1999, 2000, 2001; plus review by Pisano, et al. 1998, amongst others). However, relatively little recent research has been conducted concerning the fate of larger particulates that enter separate stormwater drainage systems. Historical design approaches are intended to minimize particulate deposition in sewerage, and usually present a minimum pipe slope or a minimum velocity objective. If followed, these guidelines usually result in minimal maintenance problems associated with particulate accumulations. However, it is still important to minimize erosion sources in watershed. In addition, catchbasin sumps have also been used to trap the larger particulates that enter storm drainage inlets. Therefore, particulate transport in separate storm drainage has not been considered to be a significant problem for public works managers. However, much more is needed to be known about particulate transport in stormwater drainage systems when conducting stormwater quality investigations, especially when examining the effects of source area controls on outfall quality, especially street cleaning. Prior studies have conducted mass balances in urban drainage systems and have found significant accumulations of solids in the drainage systems. These accumulations are mostly of the largest particulates that enter the drainage system, effectively preventing these from being discharged to the receiving waters. Many source area stormwater quality controls, such as street cleaning and the use of catchbasins, also preferentially remove the largest particulates. Modeling of the benefits of these controls therefore typically leads to inaccurate conclusions concerning reduced discharges of these particulates, and associated pollutants. The pollutants being removed would not likely be effectively transported through the drainage system, but would instead accumulate. In addition, source area and inlet samples frequently indicate much larger amounts of large particulates than would be discharged to the receiving water. This also misleads management strategies pertaining to stormwater quality. This paper presents observations from an urban area mass balance investigation along with some traditional particulate transport approaches in an attempt to explain some of these observations.

This chapter emphasizes the value of considering the particle size distribution of solids and their associated pollutants when designing and analyzing stormwater treatment projects. Studies show particle size affects the type of treatment techniques best suited for removing pollutants and indicates a series of practices may be necessary to meet the more stringent goals required by Total Maximum Daily Loads (TMDL) and the National Pollutant Discharge Elimination System (NPDES) regulations. Also emphasized in this chapter are methods to calculate particle transport, calculations for including gross solids in efficiency calculations, and impact of the size distribution in designing filter systems to remove particles. The problems associated with collecting representative samples that include the variety of stormwater particle sizes, including the largest sizes, in studies conducted using automatic water quality samplers are also discussed. The contribution of gross solids (>100 um) has been overlooked in most stormwater monitoring efforts.

Stormwater managers are increasingly faced with the need to address many potentially-conflicting issues as part of their activities. First, they need to control the potential flooding waters generated after urbanization. This has led to the use of detention facilities and control of peak flow rates to pre-development levels. However, these designs that focused on flow control for larger storms have been providing inconsistent performance for the control of other pollutants. Many other techniques, in addition to detention/sedimentation, have been proposed to alleviate these pollutant concerns. Stormwater ordinances/regulations and the development of TMDLs (with its subsequent need to control loadings to a stream) has driven the improvement of existing treatment technologies (often referred to as structural best management practices [BMPs]) and the development of new ones. Assumptions have made regarding the range of performance of these devices, based on the field data that currently exists, and as new data is submitted for publication, these new insights are added into the body of knowledge on BMP design and performance, specifically on the design needed to provide a specific performance.

There are many models available that incorporate steady-state dynamic simulation of mass transport and water quality processes in all types of surface water environments, including overland flow, small creeks, rivers, lakes, estuaries, coastal embayments, and offshore areas. As the practice of stormwater management moves toward addressing runoff volume and quality, new design practices and innovative technologies are being explored. Similarly, the requirement for the development of total maximum daily loads (TMDLs) has caused a significant increase in the need for sophisticated modeling approaches to predict the outcome of water quantity and quality controls. Also, the U.S. Environmental Protection Agency determined that it is necessary to establish a level of consistency and defensibility for TMDL modeling tools.

The past two decades have witnessed a great deal of progress in the application of BMP technology for nonpoint source pollution control in the United States and other parts of the world. In the Pacific Rim Region, the importance of nonpoint pollution control has been gaining widespread recognition in recent years as a result of more point source pollution being gradually put under control. In Taiwan, for example, field-testing of BMPs began in early 1990s and a few BMP guidebooks have been written. However, the implementation of BMPs is still voluntary because there is no regulatory framework dealing with nonpoint or stormwater runoff pollution at the present time. In China, on the other hand, there has been a strong movement towards mandating nonpoint pollution control since a few years ago. The phenomenal economic growth and urban-industrial development in China have made worse the environmental situations, but at the same time, made more resources available for mitigating the environmental impacts of progress.

Best management practices for controlling urban runoff pollution are normally designed using the design-storm approach at the site level. In general, the design criteria specify that the post-development peak discharge should be attenuated to the pre development rate, and that a certain water quality volume of runoff should be treated for a selected design storm. The design storm approach is widely accepted and employed because of its simplicity and ease of implementation However, BMPs designed using the design storm approach are not guaranteed for their long-term effectiveness. Also, their water quality and flood control benefits at the watershed scale are often in question. In recent years, owing to the extraordinary advances in computer technology, process-based BMP simulation models have been developed and are becoming an important tool in evaluating BMP performance, as well as facilitating the planning and design of BMPs. When coupled with a watershed model, BMP simulation models can be used not only at the site level, but also for evaluating BMP effectiveness at the watershed scale. In addition, optimization techniques can be applied together with BMP and watershed models to facilitate BMP placement analysis and design for achieving cost-effectiveness.