Low impact development (LID) practices are ways of dealing with stormwater runoff in cities to prevent issues such as floods, erosion, sedimentation, and pollution.
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.
Precipitation infiltrates directly into the soil in natural landscapes with permeable surfaces. Precipitation can also be intercepted and evaporated by surface vegetation cover.
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
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 LID
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.
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.
- Estimating costs for LIDs and Green Infrastructure.
The LID TTT: NEW IN VERSION 3 (Click here to view)
LID Treatment Train Tool Version 3.0: Climate-Adjusted Design Storm Update
LID Treatment Train Tool Version 3.0 is now available with new options to select both historical and projected climate design storms directly within the tool interface.
Users can now retrieve historical IDF-based storm depths and compare them with projected climate storm depths for representative climate stations across Southern Ontario. Projected climate storm options currently include IPCC Climate scenarios SSP2-4.5 and SSP5-8.5 for the 2050s.
This update helps users quickly screen how LID designs may perform under future precipitation conditions, supporting more climate-resilient stormwater planning and design. The tool also provides high-level cost estimates, allowing users to explore the potential design and cost implications of adapting to future storm conditions.
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.
VIEW & DOWNLOAD THE COSTING TOOL
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.
Contact
TRCA Watershed Planning and Ecosystem Science: wpes@trca.ca