A Stormwater Retrofit Plan for an Urban Creek Subwatershed
In spite of massive public investment in sewerage and drainage infrastructures, runoff pollution loading from urbanized areas continues to have a significant impact on many receiving water bodies. Trends towards the urbanization and industrialization of the Great Lakes watersheds will further stress the environment. The seriousness of these problems is evident in the Remedial Action Plan (RAP) process taking place in the Great Lakes Basin and in the issues facing municipal governments in the basin. Under the RAP program, the Canada--U.S. Great Lakes Water Quality Agreement requires both countries to clean up local water quality in each of the 42 Areas of Concern (AOC). At the seventeen Canadian AOC, urban runoff has been identified as the most important source of water pollution (Schroeter, 1993). Thus, the increased quantities of urban runoff and the corresponding increase in pollutant loadings demand that urban runoff control systems be planned and engineered to effect higher levels of runoff quantity and quality control. However, municipalities within the RAP areas must also address the following physical and financial constraints associated with runoff management in urbanized areas:
1) lack of space
2) integration with existing infrastructure and drainage paths
3) lack of funding
4) lack of proven technologies available for retrofit applications
5) lack of long-term strategy development methodologies
6) safety and liability issues.
These constraints are attributed to different planning principles, timing, and issues between new developments and urbanized areas as illustrated in Table 3.1. One of the many challenges that the RAP municipalities must face, in particular, is the lack of a stormwater retrofit strategy for existing urbanized areas. Stormwater retrofit can be defined as the incorporation and integration of runoff control measures into existing drainage systems.
Recognizing the lack of a stormwater retrofit strategy in the RAP areas, Environment Canada, through the Great Lakes 2000 Cleanup Fund, the Ontario Ministry of Environment and Energy, and the City of Scarborough, commissioned Ryerson Polytechnic University to build on the Stormwater Management Practices Planning and Design Manual (Marshall Macklin & Monaghan, 1994) and develop a planning strategy for stormwater retrofit in urbanized areas (Li, 1997). It is hoped that the stormwater retrofit strategy may allow the RAP co-ordinators and municipal engineers/planners to obtain a planning level estimate of the cost and potential effectiveness of stormwater quality management in their urbanized areas. Additionally, it may assist municipalities in the development of stormwater related capital works and operation programs. However, this strategy cannot answer all the RAP issues and they should be addressed separately.
The stormwater retrofit strategy consists of the following steps:
1) definition of stormwater quality management goals and objectives;
2) identification of feasible retrofit stormwater management practices (RSWMPS);
3) formulation of alternative stormwater retrofit strategies using Step 2;
4) evaluation of alternative strategies; and
5) recommendation of the preferred strategy.
This chapter presents a demonstration case study using the stormwater retrofit strategy. The following sections describe each of the above steps for the Centennial Creek Subwatershed, in Scarborough, Ontario, Canada.
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