Stormwater Management Report: Key Requirements for Developers

A stormwater management (SWM) report is a core requirement for most development applications in Ontario, including Site Plan Approval, Zoning By-law Amendments, and Subdivision approvals.

It demonstrates how a site will manage rainfall runoff without negatively impacting municipal infrastructure or adjacent properties. Done correctly, it supports approvals. Done poorly, it is one of the most common causes of delay.

Municipalities typically assess three components:

1. Quantity Control

Post-development runoff must not exceed pre-development conditions.
This is typically evaluated for major storm events (e.g., 2-year to 100-year storms) and achieved through:

  • Underground storage tanks

  • Oversized storm piping

  • Rooftop detention systems

2. Quality Control

Runoff must be treated prior to discharge to remove pollutants.
Common requirements include:

  • Oil/grit separators

  • Enhanced treatment levels depending on land use

3. Water Balance (Infiltration)

Where feasible, developments must maintain natural groundwater recharge.
This may require:

  • Infiltration galleries or soakaway systems

  • Low Impact Development (LID) measures

Soil conditions are critical and often limit feasibility.

Key Considerations for Developers

Stormwater management directly impacts:

  • Site layout (space for storage systems)

  • Construction costs (often significant for constrained sites)

  • Approval timelines (frequent source of resubmissions)

Early coordination between civil, architectural, and planning teams is essential.

Common Risks

Projects are often delayed due to:

  • Inadequate storage sizing

  • Lack of confirmed outlet or system capacity

  • Unsupported infiltration assumptions

  • Conflicts with underground infrastructure