LID Suitability

Last data update: January 14, 2020 The purpose of this layer is to help guide infiltration based Low Impact Development (LID) placement within the East Holland River Subwatershed by ranking the subwatershed based on its suitability for infiltration. This layer was developed from interpretations of two input criteria; Ontario Geologic Survey (OGS) surficial geology enhanced by local interpretations (GeolRank attribute field), and depth to water table (WLRank attribute field). Infiltration based LID ranking information is stored in the FinalRank field. Infiltration based LID Ranking information is stored in the FinalRank field. A high ranking indicates locations considered to be highly suitable for infiltration, Medium identifies those slightly less suitable for infiltration. A low ranking identifies locations considered the lowest suitability and that require further investigation before selection. As a result, the final GIS layer could prove to be useful in prioritizing infiltration based LID placement in situations where multiple locations for LID placement are available. The GIS layer is meant as a first look to help evaluate possible locations for infiltration based LID retrofits and should be combined with further site investigations to confirm suitability.

It is important to remember when utilizing and interpreting this layer that the analysis for determining infiltration based LID suitability does not consider the water quality of infiltrating water, the presence and influence of geologic fill throughout much of the subwatershed, or the land use and policies within the area; that the depth to water interpretation is based off an average approximate water level and therefore site specific water levels may vary up 1 to 2 m; and that areas ranked as low suitability are not necessarily unsuitable for infiltration based LID implementation but instead are areas where further, site-specific investigations should be made before infiltration project commence. It is also important to note that those locations identified as lower suitability only considered the suitability for infiltration based LID practices and that LID provides a wide range of practices that can help offset various negative impacts of Urban Stormwater beyond reduced infiltration. Other LID practices could be highly suitable in many more locations than identified for infiltration based LID practices in this report.
Further investigation into the suitability of a location should first start with a look at the attributes table, followed by site specific field investigations to confirm suitability. Since three different input criteria are considered in the final infiltration based LID location suitability layer it may be that only one of these input criteria are considered ‘unsuitable’ for infiltration and the other two are favourable. The only input criteria that may physically limit the ability to implement an LID project is the depth to water table, as shallow water levels (< 1 mbgs) are much more difficult to offset through design considerations than geologic and land use / policy limitations.

In order to determine which input criteria is the limiting factor, data in the attribute table can be reviewed. The attribute field called ‘LimitFactr’ identifies the criteria(s) that are considered the limiting factor. In addition, each input criteria has an attribute field that indicates the infiltration based LID suitability / ranking based purely on that criteria alone. The geology suitability / ranking can be found under the GeolRank field while the depth to water table suitability / ranking can be found under the WLRank field. Each of these fields indicates the suitability for infiltration based LID implementation.

The attribute fields also provide information on the presence or absence of a waterbodies, aggregate pits, flood plains, wetlands, ESGRA / SGRA, issue contributing areas (ICA), or expected growth locations. This information is often useful to the user to identify locations that are clearly suitable or unsuitable locations for infiltration based LIDs based on other factors not considered in the final ranking. In addition, these attribute fields indicate the current land use categorization, the water level depth range, and the surficial geology. Descriptions of the attribute fields can be found in the final report.

Before using the final layer to help guide selection of a site for an infiltration based LID project it is important to remember that this GIS layer is meant as a guide and a ‘first-look’ at possible locations for infiltration based LID retrofits and requires confirmation through in-situ field investigations. Further, the final report outlines limitations and assumptions of the methodology and analysis, which includes limitations such as; water quality, which was not considered when ranking the subwatershed for infiltration; the depth to water estimates are long term averages and may not represent the season high at a site; and geologic interpretations do not consider the presence or influence of possible fill materials.

Further details about the development of this layer can be found in the final report "LIDEastHollandSuitabilityReport2019.docx", which can be obtained upon request.