A Stormwater Management (SWM) Report is required for most Ontario development applications where the proposed development materially changes the hydrological characteristics of the site. Understanding what reviewers are looking for — and what triggers comment letters — is the first step toward a report that passes on first review.
The Three Pillars of Ontario SWM
Ontario's SWM framework, established through the Ministry of Environment, Conservation and Parks guidelines and refined through municipal and conservation authority policy, addresses three distinct concerns:
1. Quantity Control
Post-development peak flows must not exceed pre-development conditions for the design storm events required by the jurisdiction. In most Ontario municipalities, this means controlling peak flows for the 2-year, 10-year, 25-year, and 100-year storms. The analysis must demonstrate that the proposed detention or infiltration measures are sufficient to meet these targets for each event.
Common deficiencies in quantity control analysis: using incorrect pre-development conditions (particularly on previously developed sites), incorrect assessment of impervious cover in the post-development scenario, or detention sizing that works for some events but not others.
2. Quality Control
Total Suspended Solids (TSS) removal must meet the targets specified by the municipality or conservation authority — typically 70-80% removal for Level 1 protection. The proposed treatment measures must be demonstrated to achieve these targets using recognized performance data.
Common deficiencies: selecting treatment measures based on cost rather than demonstrated performance, failing to confirm that the available land area is sufficient for the proposed facility size, or omitting maintenance access and protocol documentation that many reviewers now require.
3. Water Balance and Groundwater Recharge
Increasingly, Ontario municipalities and conservation authorities require a water balance analysis demonstrating that the proposed development does not materially reduce groundwater recharge relative to pre-development conditions. Low Impact Development (LID) measures — bioretention, infiltration galleries, permeable paving — are typically required to achieve this target.
Water balance requirements vary significantly between jurisdictions. Some municipalities have adopted specific numeric targets while others require LID to be implemented to the extent feasible. Confirm the applicable requirement for your specific jurisdiction before finalizing the SWM strategy.
Common Delay Factors in SWM Review
- Outdated reference data: Using IDF curves, design storms, or pre-development hydrology models that don't reflect current municipal standards
- Incomplete LID analysis: Stating that LID is not feasible without demonstrating that feasible LID options were evaluated and why they were rejected
- Missing maintenance protocols: Reviewers increasingly require a maintenance protocol for SWM facilities as a condition of approval
- Off-site drainage omissions: Failing to address how the site handles drainage from adjacent properties that currently flows through the site
- Uncoordinated SWM and grading: Stormwater routing assumptions in the SWM report that are inconsistent with the grading plan
A SWM report that anticipates and addresses these issues before submission will clear review more quickly than one that waits for comments to identify them.