Screening LID options

Process for selecting an individual or suite of LID Options

Process Diagram for Screening LID Options

The guidance found on this page is not intended be prohibitive of other LID options, but rather to help reduce the number of LID options in the prelimiary phases of design and alert the designer to LID opportunities unique to the site being assessed.

Site Conditions

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 for:

• road right of way retrofits, see Site Conditions Road Right of Ways
• public lands including parks, churches, or schools, see Link title
• industrial, commercial, and multi-residential, see Link title
• residential, see Link title

Define Design Criteria

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

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. Table 3.4.1

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.

Select LIDs (Conceptual Design)

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 and

QUALHYMO are available.

Assess Effectiveness

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 project can then

proceed to the detailed design stage.

Low Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

refers to the characteristics and functions of a system prior to urban development.

Natural or artificial means of intercepting and removing surface or subsurface water (usually by gravity).

The addition of water to ground water by natural or artificial processes.

The infiltration and movement of surface water into the soil, past the vegetation root zone, to the zone of saturation or water table.

The natural ground material characteristic of or existing by virtue of geographic origin.

The upper surface of the zone of saturation, except where the surface is formed by an impermeable body.

Subsurface water level which is defined by the level below which all the spaces in the soil are filled with water; The entire region below the water table is called the saturated zone.

Credit Valley Conservation

Toronto and Region Conservation Authority

(a) A natural well-defined channel produced wholly or in part by a definite flow of water and through which water flows continuously or intermittently. Also, a ditch, canal, aqueduct, or other artificial channel for the conveyance of water to or away from a given place, as for the draining of a swamp.

(b) A stream or current of water. Legally, a natural stream arising in a given drainage basin but not wholly dependent for its flow on surface drainage in its immediate area, flowing in a channel with a well-defined bed between visible banks or through a definite depression (as a ravine or swamp) in the surrounding land, having a definite and permanent periodic supply of water (the stream may be intermittent), and usually, but not necessarily having a perceptible current in a particular direction and discharging at affixed point into another body of water.

(c) A legal right permitting the use of a flow of a stream (especially of one flowing through one’s land) or the receipt of water discharged upon land belonging to another.

The addition of water to ground water by natural or artificial processes.

The infiltration and movement of surface water into the soil, past the vegetation root zone, to the zone of saturation or water table.

Natural vegetation and vegetative technologies in urban settings such as: urban forests; green roofs; green walls; green spaces; rain gardens; bioswales; community gardens; natural and engineered wetlands and stormwater management ponds; and porous pavement systems. These systems are designed to provide multiple benefits, such as moderate temperatures, clean air and water, and improve aesthetics.

Refers to the conditions in which an organism lives and survives or the conditions in which an organism resides. These conditions can be described as aspects of a “physical”, “social” or an “economic” environment, depending on the perspective perceived by the observer.

State of the art methods or techniques used to manage the quantity and improve the quality of wet weather flow. BMPs include Source, Conveyance and End-Of-Pipe Controls.