Low Impact Development

Low impact development (LID) practices are ways of dealing with stormwater runoff in cities to prevent issues such as floods, erosion, sedimentation, and pollution.

runoff pours into city storm drain

Unlike conventional stormwater management methods, LID mimics natural water cycles by increasing the infiltration of stormwater into the soil, where it gets absorbed by plants or filtered into underground aquifers.

Understanding LID: The Hydrological Cycle

The hydrological cycle involves the movement of water between the atmosphere, land, and oceans.

It moves through the processes of precipitation, surface and ground water infiltration, runoff, and evapotranspiration.

illustration of the natural hydrological cycle

Precipitation infiltrates directly into the soil in natural landscapes with permeable surfaces. Precipitation can also be intercepted and evaporated by surface vegetation cover.

raindrops on leaves

Runoff only happens when the soil’s storage capacity is exceeded, or if the intensity of precipitation exceeds the maximum rate at which it can infiltrate the soil.

The timing, quantity, and quality of water movement through the natural landscape supports the terrestrial and aquatic ecosystems it flows through.

Why LID Matters

street lights shine on wet city street and sidewalk on a winter night
city street beneath a concrete overpass

Urbanization and development replace permeable surfaces with impermeable ones such as roads and parking lots, reducing natural infiltration and increasing runoff. This leads to pollution entering waterways.

LID methods counteract these effects.

Benefits of Low Impact Development

Low impact development combines practices and technologies that mimic natural water processes, lessening the impact of stormwater.

LID increases stormwater infiltration into the ground, and supports evapotranspiration and filtration.

LID methods include:

TRCA LID Projects

Low Impact Development Treatment Train Tool (LID TTT)

Developed by Toronto and Region Conservation Authority (TRCA), Lake Simcoe Region Conservation Authority (LSRCA), and Credit Valley Conservation (CVC), the LID TTT helps planners, developers, consultants, municipalities, and others adopt sustainable stormwater management practices.

Low Impact Development Treatment Train Tool

The tool helps users assess how LID technologies can restore a site’s water balance, peak flows, and water quality to its predevelopment state.

The LID TTT is built upon the open-source EPA SWMM 5 model.

Functionalities include:

  • Analyzing annual and event-based runoff volumes and pollutant load removal using (LID) techniques
  • Providing preliminary water budget analyses (i.e., surface evapotranspiration, surface runoff, infiltration to soil) and pollutant load removal estimates for pre- and post-development scenarios.



Modelling Impact of Trees on Site Water Balance:
We have added the ability to model the impact that trees have on the water balance of a site. Users can now model both trees planted in ground and in soil cells. This functionality provides valuable insights that can guide tree planting efforts. No other stormwater model currently provides this level of detailed tree modeling alongside other SWMM 5 functionalities.

Estimating Costs for LIDs and Green Infrastructure:
We have introduced a costing functionality to estimate the capital and 25-year maintenance costs of LIDs. Based on the STEP Low Impact Development Life Cycle Costing Tool, this feature allows users to accurately assess the financial implications of their stormwater management plans. We have developed costing curves that establish a relationship between the specific project and the cost for a standard design. In cases where there are significant design differences, separate curves have been developed (e.g., bioretention with and without underdrains).

Low Impact Development Life Cycle Costing Tool

Developed by TRCA through the Sustainable Technologies Evaluation Program (STEP), this tool helps to assess LID costs accurately, taking into account construction, inspection, and maintenance.

This allows users to determine if implementing LID is within budget, compare the financial differences between LID approaches, and determine if estimates provided by contractors are in line with industry standards.

The costing tool evaluates practices such as bioretention cells, permeable pavement, porous asphalt, infiltration trenches and chambers, enhanced swales, rainwater harvesting cisterns, vegetated filter strips, and green roofs.


Low Impact Development Stormwater Management Planning and Design Guide Wiki

The STEP partnership between TRCA, LSRCA, and CVC offers a comprehensive wiki on LID planning and design, including stormwater management, LID types, case studies, and details about the LID TTT and Life Cycle Costing Tool.


Green Infrastructure Opportunity Assessment

This proof-of-concept study evaluated LIDs for corporate sites in Peel Region, estimating the capital investments required for their implementation.



TRCA Watershed Planning and Ecosystem Science: wpes@trca.ca