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The Mimico Creek Watershed Stormwater Retrofit Plan

A case study of the fully urbanized Mimico Creek watershed in the City of Toronto is used as a demonstration of the stormwater planning methodology. This creek drains an area of 40 square kilometres in the former City of Etobicoke along the western edge of Toronto, and encompasses a spectrum of urban land uses. Selection of a study site committed the project to working with the host municipality and its data and information-technology resources. Etobicoke had previously demonstrated a willingness to participate in stormwater pollution abatement research and was regarded as a typical urban environment in that it has digital geographic records of its municipal infrastructure. Issues associated with conversion and compilation of data records and liaising with municipal officials, in a GIS environment analogous to the situation in many other municipalities, was seen to broaden the applicability of the tool that was developed.

During the period of this study the new City of Toronto was created, combining six former local municipalities and the former Metropolitan Toronto into a single entity. Formerly-disparate responsibilities (such as differentiated Metro and local roads) became consolidated, however, this process had not yet affected the GIS data records used in this study. Etobicoke=s digital geographic data records serve the needs of the Works, Planning and Parks Departments, primarily in terms of mapping. The records are spatially separated (tiled) but thematically aggregated into map series such as Topography (Series 1- trees, buildings, fences, rail lines), Utilities (Series 2 - catch basins, manholes, poles, lights), Contours (Series 3 - lines and spot elevations),) Lot Lines (Series 4 and PIX 1), and Streets (PIX 11 - two-sided roads). The geographic coordinates of the points and linear vectors representing features have been standardized, in units of the three-degree Universal Transverse Mercator projection (3E UTM) , based on the 1927 North American Datum (NAD27). All Works Department records had been created and stored using the Bentley Microstation GIS. Attribute records were not extensive except for roads which had been stored as standard database (dbf) files, but had not been linked to digital geographic representations of their locations. Some additional records were only available as hard copy (paper maps of land use, parks, soils and 13 Akey maps@ of sewersheds) at different scales, to different standards and for diverse purposes. Part of the Mimico Creek basin crosses the Etobicoke boundary with a neighbouring municipality (the City of Mississauga) where the data standards are not the same as in Etobicoke or the new City of Toronto. This was not the ideal situation for geographic analysis, however, it is regarded as typical of municipal records and watershed-based planning, and therefore appropriate for realistically developing the planning tool to be targeted to other Ontario municipalities.

Due to the extensive data conversions and anticipated polygon analyses, Environmental Systems Research Institute (ESRI) products (Arc/Info and ArcView) were selected as the GIS for this project. Other software products may serve these purposes, however this was the GIS platform of choice. These packages have particularly strong capabilities for the geographic data processing tasks involved in this project: for reprojecting data to a common standard, for assessing data integrity , for aggregating polygon features such as sewersheds, soils and land use etc, for isolating geographic features based on their attributes or by overlaying different thematic layers, for ensuring the fidelity of the synthesized features, and for the ease of data extraction for engineering evaluations. The ESRI data structure is complex but permits both the interactive processing of geographic and attribute data (ArcView) and the high-order spatial data analysis (Arc/Info) required for this project. Unlike map-based systems, the ESRI data model enables themes of data (called coverages) to be isolated, and treated as a unit. Roads for instance would be isolated as one coverage, sewersheds as another and outfalls as a third. Logical connectivity (topology) between features is built into the coverages, enabling a high degree of fidelity with the reality of these types of features on the ground.

The initial tasks involved isolation of the available Microstation (.dgn) records from municipal files and supplementing these with data digitised specifically for this project (also using Etobicoke=s Microstation software, and Etobicoke=s standard projection and datum). These map-sheet files were clipped to the limits of the Mimico Creek watershed, then exported and converted to feature-based and topologically-structured Arc/Info coverages. Most themes required joining into single coverages (e.g. combining of Etobicoke and Mississauga data, merging of the 13 Storm Key Plans, etc). Tables of feature attributes were subsequently linked to these GIS files. The specific data coverages used in this study included:
C sewersheds - heads-up digitised from Storm Key Plans, keyed-in runoff?coefficient attribute
C outfalls - 106 points heads-up digitised from Storm Key Plans, attribute fields keyed in
C roads - PIX 11 heads-up digitised, sewershed-delimited topologically-structured centre-lines; linking of Aroads@ in the attribute database to road segments
C watercourses - Series 1 (Topography) heads-up digitised, topologically-structured
C open spaces - heads-up digitised, topologically-structured; attributes keyed in
C utilities easements - Series 1 & 2, topologically-structured; attributes keyed in
C buildings - Series 1 (Topography), topologically-structured polygons
C soils - scanned then topologically-structured, texture attribute keyed in
C elevation - conversion of approximately 40 000 spot elevations
C land use - heads-up digitised, topologically-structured
C lots (PIX 1) converted to topologically-structured
C downspout areas heads-up digitised, topologically-structured

The analytical capability of GIS comes at the cost of the efforts required to assure data fidelity and consistency (Banting, 1992). The exercise of data conversion, compilation, topological structuring and integrity checking represents a significant up-front set of endeavours. The benefit, however is that GIS represents the Mimico Creek basin stormwater management system, enabling evaluation of the alternative RSWMP scenarios for meeting the retrofit goals and objectives.

Step 1 - Definition of Storm Water Retrofit Goals and Objectives

In the Mimico Creek watershed, the environmental and economic goals of storm water quality management were defined as follows:
C Environmental goals
$ to rehabilitate and enhance the existing hydrologic cycle; and
$ to rehabilitate and improve the existing runoff quality.
C Economic goals
$ to integrate the storm water quality management strategy with municipal capital works and maintenance programs; and
$ to minimize the cost of storm water quality management in urbanized areas.

As there has been no comprehensive watershed plan developed for this watershed, no numerical storm water control targets were set for the analysis. The purpose of this case study was to determine the achievable control targets and their associated costs. The environmental and economic goals were then defined by the following objectives:
$ to reduce the existing runoff volume;
$ to reduce the existing total suspended solids loading; and
$ to use cost-effective RSWMP=s.

Step 2 - Identification of Appropriate RSWMP=s

A number of RSWMP=s have been tested or proposed in the Greater Toronto Areas: downspout disconnection (MOEE, 1994), oil/grit separators (MOEE, 1994), storm water exfiltration systems (Li et. al., 1997a), swales and ditches (Li et al., 1998), retrofit quantity ponds, storm water quality ponds (MOEE, 1994), and off-shore flow balancing systems (Aquafor, 1994).

Li et al. (1997a) describe the conditions under which various RSWMP=s are considered feasible. Downspout disconnection for lot level runoff disposal is considered potentially effective if slope gradients are gentle, soils are permeable, the groundwater table is deep, and there is open space available on individual lots. For oil/grit separators to be contemplated as an RSWMP option, the land use should be commercial or industrial; for cost effectiveness existing or proposed storm sewers should have construction impending. Storm water exfiltration systems can be considered for local roads in residential areas where the soil is permeable. Swales and ditches (Li et al., 1998) may replace storm sewers where there is sufficient right of way and maintenance of existing swales and ditches can be considered to be a stormwater quality measure. Retrofit of storm water quantity ponds is only feasible where they currently exist, where there is sufficient space to accommodate the water quality function, where public awareness can be assured and where access for maintenance can be provided. No quantity ponds were found in the study area so this option was eliminated from further consideration. New storm water quality ponds require open space, with sufficient area off-line to capture the first flush of runoff events. Concerns regarding compatibility with adjacent lands are similar to those of retrofit ponds. An off-shore flow balancing system (Aquafor, 1994) requires specific environmental conditions in the receiving waters of the watershed. A sheltered embayment devoid of significant aquatic ecosystem function is required. The downstream position and other concerns eliminated this option from further consideration for Mimico Creek.

To identify suitable RSWMP=s, values for each of the screening criteria were derived through GIS processing of the thematic coverages. Figure X.2 to X.5 illustrates the screening procedure of downspout disconnection, oil/grit separators, exfiltration systems, and water quality ponds. Prominent among these processing tasks were the Aquery by attribute@, Aoverlay@, and Acalculator@ functions. Query by attribute selects records from a thematic coverage based on specified value ranges for a set of attributes, for instance all polygons with a residential land use. Overlay combines two topologically-structured coverages, merging the features of each into new features. Overlay of buildings onto lots creates a new coverage with built and open portions distinguished on a lot by lot basis. Attributes of the original coverages are also joined. Calculator functions permit aggregation, measurement and export of derived values, so that for each RSWMP, the number of lots, drainage areas or length of roads that are suitable for its use are identified and measured.

Downspout Disconnection. For the downspout disconnection RSWMP, Li et al., (1997a) indicate that the roof to lot area ratio be less than 0.5, the land use residential, and the soil sandy. Using Arc/Info, the buildings were overlaid on lots, preserving each coverage=s Apolygon area@ attribute. (Arc/Info automatically determines the area of polygon features and stores this as an attribute field.) For lots with multiple buildings the calculator function summed buildings within the lot and posted this sum to the attribute table. A new field was then created indicating the ratio of building area to lot size. Lots were then overlaid onto land use and soils polygons, as well as onto sewersheds. The resulting coverage consisted of 830 polygons, each with the required attribute fields -- building to lot area, land use and soil texture, as well as a link to the sewershed coverage. The ArcView query function then isolated the 69 sewersheds which met all of the prescribed criteria, and generated an export table summarizing drainage area, roof area and the number of lots in the area identified as suitable for downspout disconnection.

Oil/Grit Separator. This RSWMP is suitable in commercial and industrial areas, where a storm sewer is present. Among the attributes of roads is a classification field which identifies the presence of storm sewers. Pre-processing had redefined road features as the segment contained within an individual sewershed. Aggregation of all road segments within sewersheds enabled use of the calculator function to proportionalise the sewershed areas suitable and unsuitable for separator treatment. Overlay of sewersheds with land use isolated commercial and industrial areas. For each sewershed, the commercial and industrial drainage areas suitable for oil/grit separators were calculated and tabulated.

Exfiltration Systems. Identification of areas in which storm water exfiltration systems can be considered suitable was based on a series of overlays and queries. The ArcView query function was initially used to isolate from the roads database, those segments requiring reconstruction in the next 5 years (Apoor roads@), then constraints regarding width (2 or fewer lanes), land use (in residential areas), and soil (sand) were imposed. Then overlay and the calculator function enabled creation of export tables of sewershed-based summaries. These were comprised of the length of suitable roads and the proportionalised suitable areas where exfiltration systems could be considered.

New Quality Pond. Sites at storm-sewer outfalls were targeted for the construction of a new stormwater quality pond. Suitable sites were regarded as those where the outfall diameter was over 600mm, and the drainage area exceeded 5 hectares. Each of the outfalls had been linked to its sewersheds via an attribute field created during data capture. By aggregating sewersheds using the calculator function, total drainage area was determined and used as the basis of an ArcView query. Subsequent querying by the diameter criterion isolated 60 of the 106 outfalls. These were plotted over the open space and utilities easements coverages. By zooming to the areas immediately adjacent to each selected outfall site, details of sites were inspected for incompatible conditions -- 100-year flood line restrictions, proximity to open space uses such as recreation facilities (e.g. tennis courts, picnic areas), naturalised areas and wildlife habitat, and to potentially conflicting neighbouring lands (e.g. provincial highways, residential areas). Seven sites were identified for further on-site evaluations. For each of these sites, Arc/Info overlay and the ArcView query function enabled determination of residential, commercial and industrial areas served by each pond.

Other RSWMPs cited above were found to be infeasible for this watershed and no further investigation was conduction.

Step 3 - Formulation of Alternative Storm Water Retrofit Strategies

The alternative storm water quality management strategies for the Mimico watershed were formulated by combining various mixes of appropriate RSWMP=s in accordance with a preferred hierarchy of RSWMP=s. This hierarchy emphasizes the use of source and drainage system controls before downstream water quality ponds. Alternative strategies also reflect, the experience and knowledge of the RSWMP=s, and both short and long-term implementation constraints. Based on these principles, alternative storm water retrofit strategies were selected as indicated in Table 1. It is noted that runoff quality control strategies have already included runoff volume control strategies.

Step 4 - Evaluation of Alternative Retrofit Strategies

Alternative storm water quality management strategies were evaluated with respect to their achievement of environmental and economic objectives. Analytical probabilistic models (Adams and Bontje, 1983) and a multi-efficiency model (Weatherbe, 1995) were selected for this study because the required data and the level of accuracy are suitable for the preliminary planning of RSWMP=s. Using derived probability theory, the analytical probabilistic models transform long-term rainfall statistics (e.g., average rainfall event volume, duration, inter-event time, and average annual number of rainfall event) into runoff statistics (e.g., average runoff event volume and the average annual runoff volume) in an urbanized catchment. Assuming a constant concentration of total suspended solids, the average annual suspended solid loadings from an urbanized catchment can be determined. The multi-efficiency model assumes the cumulative control performance of RSWMP=s can be determined in a manner similar to the cumulative treatment efficiency of a series of treatment systems. The analytical and multi-efficiency models have been coded into a RSWMP analysis spreadsheet (see Section X.1.3) and were used to evaluate alternative storm water retrofit strategies. The cost-effectiveness of RSWMP=s was also investigated by comparing the marginal costs of quantity and quality controls. In the Mimico watershed, the descending order of cost-effectiveness is: (1) downspout disconnection; (2) water quality ponds; (3) storm water exfiltration systems; and (4) oil/grit separators.

Step 5 - Selection of a Retrofit Strategy

The maximum runoff volume reduction and solids loading reduction that can be achieved in the Mimico watershed are about 14% (Strategy S3a) and 18% (Strategy S3b) and the associated costs are $7.8 million and $10 million respectively over the next 25 years. These strategies assume:
C All the appropriate areas for downspout disconnection will be completed in 25 years;
C Thirty percent of appropriate areas for oil/grit separation application will be installed in 25 years;
C All the appropriate roads which are in poor condition now will be retrofitted with exfiltration systems in 25 years.
C Twenty percent of the appropriate roads which are in good condition now but will be deteriorated in the next 25 years will be retrofitted with exfiltration systems.
C Six potential water quality ponds will be constructed in the next 25 years as capital projects or redevelopment water quality controls.
If the existing swales and ditches in the watershed are assumed to contribute to the overall runoff volume and solids loadingd reduction, the maximum runoff volume and solids loading reduction that can be achieved in the watershed can be increased to about 17% and 33% respectively. Thus, maintenance of existing swales and ditches may be an effective stormwater management measure. operational controls such as improved street sweeping practices and increased catch basin sump maintenance and sewer flushing are also recommended to complement the selected strategy and improved the solids loading reduction.

Conclusions And Recommendations

The stormwater planning tool provides a systematic methodology for municipalities to develop storm water quality management strategies in urbanized watersheds. It allows municipal planners and engineers to develop numerical control objectives and evaluate the cost-effectiveness of alternative management strategies. Though GIS enables digital processing of geographically-distributed data, its value for standardising data compilation is preferred to the assembly of paper maps. The value of GIS was clearly demonstrated in the repetition of processing tasks such as the querying, overlaying and calculation of summary data for each RSWMP. The use of GIS was not intended to automate the processes of site selection and evaluation of alternatives, but to foster the evaluation of various scenarios for retrofitting the stormwater management system. Additional scenarios have been suggested by the case study and will be the subject of continuing research initiatives. With continued refinement of the tool, a higher degree of integration of the engineering and geographic analyses could be achieved, further streamlining the identification, evaluation and selection functions.


This research study was initiated and funded by Environment Canada, the Ontario Ministry of the Environment, and the City of Toronto. Throughout the course of the study, the Study Steering Committee members provided excellent guidance, advice, and data to the Study Team.


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