Screening LID options

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Process for selecting an individual or suite of LID Options[edit]

Process Diagram for Screening LID Options

The guidance found on this page is not intended be prohibitive of LID options, but rather to help reduce the number of LID options in the preliminary phases of design and alert the designer to LID opportunities

unique to the site being assessed. When going through the process of screening LID options, it is important to have a larger contextual understanding of Integrated water management, Site design strategies,

Siting and layout of development, and Link title.

Site Conditions[edit]

Complete definition of pre-development site conditions is essential prior to screening of potential stormwater BMPs. The designer should prepare maps describing site

conditions to ensure that all environmental features and functions that need consideration in accordance with provincial, municipal and conservation authority

development regulations are identified. This includes watercourses and small drainage features, floodplains, important recharge areas, steep slopes, wetlands, natural heritage

conservation areas and significant wildlife habitats. In addition, information regarding native soil types, infiltration capacity and depth to water table must be determined.


In retrofit scenarios, site conditions will vary and can be explicit with land use type such as road right of ways, parks, residential, industrial, commercial and/or institutional should be noted.

For additional information on site conditions to note for:

Define Design Criteria[edit]

A detailed description of the design criteria that need to be defined is provided in the respective CVC and TRCA Stormwater Management Criteria documents. If outside jurisdictional

boundaries, criteria developed should have requirements to:

  • preserve groundwater and baseflow characteristics;
  • prevent undesirable and costly geomorphic changes in the watercourse;
  • prevent any increases in flood risk potential;
  • protect water quality; and ultimately,
  • maintain an appropriate diversity of aquatic life and opportunities for human uses

The design criteria required to protect, enhance or restore the environmental resources can be grouped under the following five categories.

  • Flood Protection;
  • Water Quality;
  • Erosion Control;
  • Recharge; and
  • Natural Heritage Systems or green infrastructure


Screen Potential LID Options[edit]

A number of factors need to be considered when screening the suitability of a given location within a development site for application of stormwater BMPs. LID Site Criteria and Constraints

summarizes site constraints associated with some general types of structural LID practices for stormwater management that should be considered. Further

information regarding constraints to the design of various end-of-pipe BMPs can be found in the Ontario Ministry of the Environment Stormwater Management Planning and

Design Manual (2003). The use of LID BMPs should be considered first to meet the design criteria before the use of end-of-pipe BMPs. Other considerations for screening potential LID options is detailed below.


Performance Requirements[edit]

Resources on LID performance studies are widely available. Performance of LID within Ontario can be found on

the Sustainable Technologies Evaluation Program and

Credit Valley Conservation's LID Monitoring program. To garner a global perspective of LID performance across various

geographical regions, check out the International Stormwater BMP Database, performance data can be downloaded and statistical analysis tools are provided.

Operations and Maintenance Requirements[edit]

LID Inspection and Maintenance Guide

Life Cycle Costing Requirements[edit]

LID Life Cycle Costing Tool

Selection of Suite of LIDs[edit]

In order to assess if the selected suite of BMPs effectively meet the design criteria either computer models or simple spreadsheet models should be used. Model selection

will be based on the size and type of development. A wide range of simple to complex computer models such as Visual OTTHYMO, SWMM, SWMMHYNO, HSP-F andQUALHYMO are available.

Recently, the Low Impact Development Treatment Train Tool (LID TTT) has been developed by Lake Simcoe Region Conservation Authority (LSRCA), Credit Valley Conservation (CVC) and Toronto and Region Conservation Authority

(TRCA) as a tool to help developers, consultants, municipalities and landowners understand and implement more sustainable stormwater management planning and design practices in their watersheds. The purpose of the tool is

to analyze annual and event based runoff volumes and pollutant load removal by the use of Best Management Practices (BMP)’s and Low Impact Development (LID) techniques. The LID TTT provides preliminary water budget analysis

(i.e. surface ET, surface runoff, infiltration to soil) and pollutant load removal estimates for pre- and post-development scenarios. The tool is built upon the open source EPA SWMM5 model providing a user-friendly

interface for novice modelers and cross-compatibility with SWMM5 for further model development. To download the beta version, please click

LID Treatment Train Tool

Assess Effectiveness[edit]

Once the suite of best management practices have been selected and the models have been run, a comparison of the results and the environmental design criteria can be made. An iterative approach, which involves adjusting the

size or adding/deleting BMPs should be used until the environmental design criteria are met. The LID Treatment Train Tool can help with

adjusting the size of the features to see if criteria are met. Once met, the project can then proceed to the detailed design stage.