Innovative Urban Wet-Weather Flow Management Systems

Abstract

This research project describes innovative methods to develop improved wet weather flow (WWF) management systems for urban developments of the 21st century. This document addresses the competing objectives of providing drainage services at the same time as decreasing stormwater pollutant discharges. Water quality aspects of WWF discharges and associated receiving water problems have only been studied for a relatively short period (a few decades), compared to conventional drainage designs (a few centuries), and few large-scale drainage systems adequately address both of these suitable objectives.

General principles of urban water management are presented that might permit the development of more sustainable systems by integrating the traditionally separate functions of providing water supply, collecting, treating, and disposing of wastewater, and handling urban WWF. Integration can be achieved by designing neighborhood scale, integrated infrastructure systems wherein treated wastewater and stormwater are reused for nonpotable purposes such as lawn watering and toilet flushing. The automobile is seen to have caused major changes in urban land use in the 20th century. For the average urban family, the area devoted to streets and parking in their neighborhood exceeds the area devoted to living. Similarly, more area is devoted to parking than to office and commercial space in urban areas. The net result of the large scale changes to accommodate the automobile in cities is about a two to three fold increase in impervious area per family and business activity.

The physical, chemical, and biological water quality characteristics of urban runoff are evaluated and summarized. Then, the impacts of urban WWF on receiving waters are evaluated. These impacts on surface and groundwater are complex and difficult to evaluate. Physical changes in smaller urban streams can be detected in terms of degraded channels from higher peak flows. Also, sediment transport characteristics change with urbanization. Toxic effects on aquatic organisms have been detected.

Traditionally, wet-weather collection systems were designed to move stormwater from the urban area as quickly as possible. This design approach often simply transferred the problem from upstream to downstream areas. More recently, restrictions on the allowable maximum rate of runoff have forced developing areas to include onsite storage in detention ponds to control these peak rates of runoff. On-site detention also allows smaller pipe sizes downstream. In the early part of the 20th century, communities relied on combined sewers. Later, separate storm and sanitary sewers became accepted practice. However, as the need to treat more contaminated storm water becomes more apparent, it is necessary to take a fresh look at combined sewers. However, because of the strong trend to lower density urban development to accommodate the automobile, the quantity of urban runoff per family is two to three times what it was with higher density developments. Most of the traffic flow in cities occurs on a relatively small percentage of streets, about 10-20%. Also, most parking areas are underutilized. Thus, it may be possible to focus WWF treatment on these more intensively used areas including commercial and industrial areas. This finding suggests that hybrid collection systems may be attractive alternatives for 21st century collection systems. Another innovative option is to oversize sewer systems and utilize storage in the sewers as part of a real-time control system.

Extensive discussions regarding the effectiveness of a wide variety of WWF controls are presented in two chapters. These descriptions include design guidelines. Source controls as well as downstream controls are included. Source area controls, especially biofiltration practices that can be easily implemented with simple grading, may be appropriate in newly developing areas. In addition, critical source areas (such as vehicle service facilities) may require more extensive onsite treatment strategies. An innovative approach is to reuse stormwater within the same service areas for irrigation, toilet flushing, and other nonpotable purposes. More aggressive stormwater reuse systems would capture roof runoff in cisterns, treat this water, and use it for potable purposes. Monthly water budgets for cities throughout the United States indicates that sufficient quantities of precipitation are generated, except in the arid southwestern United States, to make such systems technically feasible. The cost of providing for water infrastructure is summarized. The traditional problem of finding the optimal size of service area for water supply is addressed by finding the minimum sum of the costs of source acquisition, treatment, and distribution. For wastewater and stormwater, the minimum total cost is the sum of collection, treatment, and disposal. These costs per residence have grown substantially as development densities have decreased. Also, if wastewater and stormwater reuse are included, then the optimal size of infrastructure system may be at the neighborhood scale since piping costs remain the largest single cost in urban water infrastructure.

Lastly, institutional arrangements need to change in order to successfully implement changes in how urban water infrastructure is managed. Privatization, moving from large centralizes systems to neighborhood based systems, and other projected changes required innovative changes in the governing institutions.

This report was submitted in fulfillment of Cooperative Agreement Nos. CX824932 and CX824933 by American Society of Civil Engineers (University of Colorado and the Urban Water Resources Research Council) and the University of Alabama at Birmingham, respectively under the sponsorship of the U.S. Environmental Protection Agency. This report covers a period from May 1996 to August 1998, and work was completed as of August 1998.

This document is also available throught the National Technical Information Service - NTIS web site.

EPA# EPA/600/R-99/029
NTIS# PB99-151359INZ