Every new building, parking lot, or plaza changes how rainwater moves across a property. Regulators want to know that these changes have been considered and that the proposed drainage system will manage runoff appropriately. Understanding stormwater management requirements before hiring a designer can help reduce the risk of rejected submissions and costly redesigns.
This guide explains stormwater management in plain language, focusing on Ontario while highlighting considerations that apply elsewhere in Canada. It covers what reviewers typically look for and how to prepare a more complete submission.
Rain Doesn’t Disappear When You Pave Over It
Before development, grass, soil, and trees absorb and retain some rainfall. When rooftops, asphalt, and concrete cover the ground, less water infiltrates into the soil, and more runoff can leave the site quickly. This can contribute to basement flooding, overwhelm municipal storm sewers, and erode the banks of nearby creeks.
Runoff can also carry road salt, oil, sediment, and other pollutants into local waterways. Stormwater management measures help control these impacts and protect surrounding properties and the environment.
Who Sets the Rules? Three Layers of Oversight
Stormwater management in Canada is governed by a combination of provincial, municipal, and, where applicable, conservation authority requirements. There is no single national rulebook that covers every development project.
In Ontario, designers commonly refer to the stormwater management planning and design manual published by the Ministry of the Environment, Conservation and Parks. Municipalities establish additional engineering criteria, while conservation authorities, such as the Toronto and Region Conservation Authority (TRCA), Credit Valley Conservation (CVC), and Lake Simcoe Region Conservation Authority, may establish watershed-specific requirements.
These requirements work together rather than replacing one another. The first step is to identify which provincial guidelines, municipal standards, and conservation authority policies apply to the property.
The Four Goals Every Reviewer Checks
Municipal staff and conservation authority reviewers typically assess stormwater management plans against four key objectives. Each addresses a different aspect of runoff management, and gaps in any area can delay an application.
Quantity: Keep Peak Flows Under Control
Development can increase both the volume and peak rate of stormwater runoff. Depending on local criteria and site conditions, designers may need to control post-development peak flows to meet specified pre-development targets for storms ranging from the 2-year event to the 100-year event.
Common measures include underground storage tanks, dry ponds, rooftop storage, and controlled ponding in parking areas. Outlet structures regulate how quickly stored water is released. Hydrologic modelling helps demonstrate whether the proposed system meets the applicable design criteria.
Quality: Improve Runoff Quality Before Discharge
Ontario guidance establishes stormwater quality objectives based on the receiving watercourse, site conditions, and the applicable level of protection. Enhanced protection commonly targets an 80 percent reduction in total suspended solids (TSS).
Treatment measures may include oil and grit separators, wet ponds, filtration systems, and vegetated swales. Sites such as fuel stations and large parking areas may require particular attention to the pollutants associated with their activities.
Erosion: Protect Creeks and Slopes
Frequent rainfall events can gradually damage stream channels when runoff reaches them too quickly or in excessive volumes. Depending on local requirements, reviewers may request extended detention or other measures to reduce the erosive effects of runoff.
During construction, an erosion and sediment control plan helps prevent soil, sediment, and muddy runoff from entering storm sewers or reaching neighbouring properties and natural areas. These controls protect receiving waterways during construction and help preserve channel stability over the longer term.
Water Balance: Give Rain a Place to Soak In
Many municipalities require or encourage measures that help maintain the natural water balance by promoting infiltration, evapotranspiration, and other processes that manage rainfall close to where it lands.
Bioretention gardens, permeable pavement, and infiltration trenches can help achieve these objectives. However, their suitability depends on site conditions. Soil testing is particularly important in parts of southern Ontario where clay soils may limit infiltration. A geotechnical investigation early in the design process can help identify constraints and guide the selection of appropriate measures.
What Reviewers Expect Inside the Report
Stormwater management report requirements vary by municipality and project type, but many submissions share a common structure.
A report typically includes:
- A description of the site, existing drainage conditions, and relevant planning context.
- Hydrologic modelling, using appropriate tools such as Visual OTTHYMO or PCSWMM, with rainfall data, runoff parameters, and modelling assumptions clearly documented.
- Proposed stormwater storage volumes, outlet sizing, and supporting calculations.
- Overland flow routes and an assessment of how the site will drain during major storm events.
- Erosion and sediment control measures, where applicable.
- An operation and maintenance plan for proposed stormwater management facilities.
- Supporting grading, servicing, and drainage drawings.
The report and drawings should be consistent and demonstrate compliance with the applicable criteria. Where professional engineering is required, the final documents must be reviewed and signed and sealed by an appropriately licensed professional engineer.
Design Standards: Where Goals Become Pipes, Ponds, and Tanks
Municipal engineering criteria translate stormwater management objectives into measurable design requirements. These may cover pipe sizing, minimum cover, outlet controls, pond side slopes, and overland flow routes when the storm sewer system reaches capacity.
Designers should identify the standards used and show how the proposed design meets them. Clear calculations, documented assumptions, and references to applicable criteria help reviewers assess the submission efficiently.
Because municipal requirements can differ, the standards applicable to one property should not automatically be assumed to apply to another.
Low Impact Development: Managing Rain Closer to Where It Falls
Low impact development (LID) uses distributed features to manage rainfall throughout a site rather than relying exclusively on a centralized facility.
Examples include green roofs, permeable pavers, bioretention cells, and soakaways. Depending on site conditions and design requirements, these measures can reduce runoff, support infiltration, and help manage the site’s water balance. They may also reduce the amount of conventional stormwater storage required.
Ontario municipalities and conservation authorities may require or encourage LID measures as part of their stormwater management criteria. The specific approach depends on local policies, soil conditions, groundwater levels, and other site constraints.
Planning for Heavier Storms
Intense rainfall can place additional demands on drainage systems, making it important to use current rainfall information and account for major storm events.
Designers should consult applicable stormwater management guidelines and the relevant rainfall data published by Environment and Climate Change Canada. Municipal criteria may also specify rainfall scenarios, updated intensity-duration-frequency data, or climate change considerations that must be addressed.
Assessing major storm events and safe overland flow routes during the design process can help reduce flood risks and identify potential vulnerabilities before construction begins.
Every Site Type Has Its Own Stormwater Requirements
The stormwater management approach depends on the site’s proposed use, size, layout, and surrounding environment.
- Gas stations: Fuel handling and vehicle activity can create additional water-quality concerns. Applicable requirements may include spill management and appropriate treatment measures.
- Industrial properties: Large paved areas and site-specific operations can affect runoff volumes, pollutant loads, and storage requirements.
- Residential subdivisions: Designs often coordinate road drainage, storm sewers, shared stormwater management facilities, and overland flow routes.
- Small infill developments: Limited space may make rooftop storage, permeable surfaces, and other distributed measures relevant options.
Identifying the type of development early helps determine which municipal standards, environmental considerations, and stormwater management measures need to be addressed.
Mistakes That Can Delay a Stormwater Report
Incomplete or inconsistent submissions can lead to additional review comments and revisions. Common issues include:
- Outdated or inappropriate rainfall data.
- Missing downstream drainage or capacity assessments where required.
- Outlet details that conflict with the grading plan.
- Inconsistencies between stormwater calculations and architectural or servicing drawings.
- Unaddressed conservation authority requirements.
- Missing maintenance details or insufficient documentation of design assumptions.
Checking the report against local stormwater management report requirements before submission can help identify these issues early.
Maintenance: A Design Is Only as Good as Its Upkeep
Stormwater management facilities require ongoing maintenance. Ponds can accumulate sediment, separators need servicing, and catch basins can collect leaves, grit, and litter.
Approvals may require an operation and maintenance plan that identifies inspection schedules, maintenance responsibilities, and procedures for managing collected sediment and other materials.
The plan should be practical for the property owner or operator, with clearly defined tasks and realistic inspection intervals. Regular maintenance helps preserve system performance and supports compliance with applicable requirements.
Working Across Canada: Why One Template Never Fits
Stormwater management requirements vary across Canada. British Columbia, Alberta, Quebec, and the Atlantic provinces have their own provincial guidance, rainfall data, approval processes, and environmental policies.
A report prepared for an Ontario project may need substantial revisions before it can be used elsewhere. Requirements can also differ between Ontario municipalities, including Toronto, Ottawa, Barrie, and smaller communities.
Confirming the applicable local criteria before beginning the design helps ensure the report addresses the right requirements from the outset.
Need Help Understanding Your Stormwater Management Requirements?
Preparing a stormwater management report involves coordinating provincial guidance, municipal engineering criteria, site conditions, and any applicable conservation authority requirements.
n Engineering Inc. is a civil and traffic engineering firm based in Richmond Hill, Ontario. Its services include stormwater management, site servicing, traffic studies, road design, and septic systems for development projects across Ontario.
For information about stormwater management services or the requirements that may apply to your project, contact n Engineering Inc. at (416) 256 – 9741 or info@nengineering.com.
Frequently Asked Questions
1. Who is allowed to prepare and sign a stormwater management report?
Requirements depend on the project and the reviewing authority. Where professional engineering services are required, the report must be prepared or reviewed, signed, and sealed by an appropriately licensed professional engineer in accordance with applicable requirements. The engineer should be familiar with the relevant provincial guidance and municipal criteria.
2. Do I need conservation authority approval as well as municipal approval?
Possibly. Properties near watercourses, wetlands, valleys, or floodplains may be subject to conservation authority permitting requirements. The need for a permit depends on the property’s location, the proposed work, and the applicable regulations. Conservation authorities may also provide technical comments on stormwater management. Confirm the requirements for the specific property.
3. What is low impact development, and is it mandatory?
Low impact development uses features such as bioretention cells, permeable pavers, and infiltration galleries to manage rainfall closer to where it falls. Ontario municipalities may require or encourage these measures, depending on local policies and site conditions. The applicable criteria determine which measures are necessary.
4. Which rainfall events must the design address?
Many municipal criteria require assessment of multiple design storms, potentially ranging from the 2-year event through the 100-year event. Designers may also need to assess major system performance, overland flow routes, or a Regional Storm, depending on the location and applicable requirements. The required events and rainfall data should be confirmed with the reviewing authority.
5. Does a small project still need a stormwater management report?
It depends on the scope of work and the municipality’s requirements. Site plan applications, subdivisions, and developments that increase impervious surfaces may require a stormwater management report or supporting drainage calculations. Smaller projects may be eligible for a simplified assessment. Contact the municipal planning or engineering department to confirm the applicable submission requirements.
6. How is a stormwater management report different from a grading plan?
A grading plan shows proposed elevations and how surface water is intended to flow across a property. A stormwater management report explains the drainage design, supporting calculations, assumptions, and how the proposed system meets applicable criteria. The two documents should be coordinated and consistent.