Series: Ottawa, Ontario Development Approval Series
Slot: Site Servicing & Stormwater : 2:00 PM
Published: September 16, 2026
Service: Stormwater Management Reports, Ottawa, Ontario
A stormwater management report explains how a development will collect, control, treat, and safely release runoff. For Ottawa infill and redevelopment sites, the report must connect hydrologic calculations with the proposed grading, site servicing, storage, outlet control, water quality measures, and overland flow routes.
The objective is not simply to produce calculations. The objective is to create a coordinated design that satisfies City of Ottawa requirements, responds to downstream capacity, and remains practical to construct.
Reliance Engineering prepares stormwater management reports for Ottawa, Ontario with clear calculations, practical controls, municipal experience, clean layouts, and approval support.
Allowable Release Rates Are Site-Specific
The allowable release rate is the maximum flow that may leave a site and enter the receiving system without creating unacceptable downstream impacts.
For an Ottawa property, the controlling rate may depend on:
- The receiving storm sewer or municipal drainage system
- Whether the site discharges to a watercourse
- Existing downstream capacity
- Pre-development drainage conditions
- City of Ottawa stormwater criteria
- Area-specific drainage studies or facility requirements
- Comments from City engineering staff
- Requirements from Mississippi Valley Conservation Authority, Rideau Valley Conservation Authority, or South Nation Conservation
The rate is not automatically the same for every site. A report must explain the source of the target and show how the proposed design achieves it.
A complete allowable release rate analysis should identify:
- The receiving system or watercourse
- Existing and proposed drainage areas
- Runoff coefficients and other calculation assumptions
- The selected design storms
- The allowable release rate
- The proposed outlet control method
- The controlled post-development release rate
In many applications, the design objective is to restrict post-development flows to pre-development levels for the required storm events. However, the applicable requirement must be confirmed for the individual property. Area-specific criteria, downstream constraints, or Conservation Authority requirements may result in a different target.
Pre-Development and Post-Development Drainage Analysis
A professional SWM report begins with a clear understanding of existing drainage conditions.
Pre-development conditions
The analysis typically reviews:
- Existing buildings and hard surfaces
- Existing landscaped and pervious areas
- Existing drainage boundaries
- Existing catchbasins, storm sewers, ditches, or watercourses
- Existing overland flow routes
- Runoff coefficients
- Time of concentration
- Existing peak flows
The assumptions should be traceable and supported by the survey, site inspection, available record information, and applicable municipal criteria.
Post-development conditions
The proposed analysis shows how the building, parking lot, driveways, walkways, landscaping, grading, and servicing infrastructure will change runoff.
The report should clearly connect to the Site Servicing Plans and Site Grading Plans. The proposed stormwater system must be physically achievable on the drawings.
A compliant release rate is not useful if the grading cannot direct runoff to the storage system, if the outlet cannot be accessed for maintenance, or if the proposed storm sewer conflicts with utilities and foundations.
Storage Sizing for Ottawa Infill Sites
After establishing the allowable release rate, the design must provide sufficient storage to temporarily hold excess runoff.
Storage is generally sized by comparing:
- Post-development inflow
- The allowable controlled outflow
- The duration of the design storm
- The available storage volume
- The operation of the outlet control
The report should demonstrate that the proposed system stores the difference between incoming runoff and the permitted release rate. It should also identify the critical storm event and confirm that the system performs for all required design storms.
Typical storage solutions include:
- Underground chambers
- Oversized storm pipes
- Underground tanks
- Rooftop storage
- Controlled roof drains
- Parking lot storage
- Dry ponds
- Bioretention or infiltration systems
- Combined storage and LID measures
Limited footprint solutions
Tight infill lots often have no practical room for a surface pond. In these situations, underground chamber systems can be installed below parking areas, drive aisles, landscaped zones, or other suitable portions of the site.
A pitched roof may make rooftop storage impractical. A flat roof may allow controlled rooftop detention, provided that roof drainage, emergency overflow, structural considerations, access, and maintenance are properly addressed.
An inlet control device, orifice, vortex regulator, or shared outlet control may also be used. The design rationale must be clear. The report should explain why the selected system fits the site and how it will be maintained.
The City of Ottawa Quick Stormwater Management Tool identifies on-site storage and overall release rate requirements for applicable smaller developments based on site area and proposed hard surface coverage. The required storage and discharge method must be shown on the grading and servicing plans.
Where the proposal uses at-grade storage, infiltration, or a combination of measures, a more detailed SWM brief or report may be required.
Outlet Control and Storage Calculations
Storage volume alone does not control runoff. The outlet must be sized to release water at or below the allowable rate.
A clear report should include:
- Storage volume calculations
- Stage-storage information
- Stage-discharge information
- Outlet elevations
- Orifice or control device sizing
- Inlet control calculations, where applicable
- Emergency overflow elevations
- Required and provided storage
- Allowable and proposed release rates
- Maintenance access
- Inspection and operation requirements
The drawings must match the calculations. Pipe sizes, inverts, chamber dimensions, control structures, roof leader connections, and emergency overflows should be coordinated across the civil package.
Minor and Major System Drainage
Ottawa stormwater design must consider both the minor and major systems.
The minor system generally includes:
- Catchbasins
- Storm sewers
- Roof leaders
- Lot grading
- Swales
- Ditches
- Foundation drainage
- Inlets and related collection infrastructure
The major system conveys runoff when the minor system is exceeded. It may include:
- Roadways
- Overland channels
- Swales
- Open spaces
- Emergency spill routes
- Ponds
- Natural drainage features
A complete report should assess the major system, including the 100-year event where required. The site must have a safe and constructible overland flow route that protects buildings, entrances, adjacent properties, public rights-of-way, and critical infrastructure.
The design should confirm:
- Water is directed away from building openings
- No uncontrolled drainage reaches neighbouring properties
- Emergency overflow routes are defined
- Ponding depths remain acceptable
- Parking and drive aisles can convey major flows safely
- Major system flows do not conflict with pedestrian or vehicle access
Water Quality Controls and LID Review
Quantity control is only one part of a complete stormwater strategy.
Depending on the site, receiving environment, municipal criteria, and Conservation Authority requirements, the design may need to address water quality treatment and low impact development.
Potential treatment measures include:
- Oil-grit separators
- Sediment forebays
- Wet ponds
- Extended detention
- Bioretention areas
- Vegetated swales
- Infiltration systems
- Filtration systems
- Approved proprietary treatment devices
An Enhanced Level of Protection is commonly associated with approximately 80% long-term average total suspended solids removal. The applicable performance requirement should be confirmed for the project and documented in the report.
LID measures may reduce runoff volume, support infiltration, improve water balance, and reduce the storage demand placed on downstream systems. However, LID is not a substitute for practical design documentation. Soil conditions, groundwater, maintenance, overflow, grading, and constructability must be reviewed.
Commercial and industrial sites may also require additional consideration for sediment, oil, hydrocarbons, and spill control.
Construction-phase protection should be coordinated through an Erosion and Sediment Control Plan. Sediment entering catchbasins or stormwater facilities can block controls, damage municipal infrastructure, and affect downstream watercourses.
Conservation Authority and Downstream Review
Conservation Authority review may be required where a property affects a regulated watercourse, wetland, floodplain, valley, outlet, or natural feature.
In Ottawa, review may involve:
- Mississippi Valley Conservation Authority
- Rideau Valley Conservation Authority
- South Nation Conservation
Additional information may be requested regarding:
- Flood hazards
- Erosion risks
- Water balance
- Groundwater recharge
- Wetland hydrology
- Outlet stability
- Watercourse protection
- Regulatory setbacks
- Natural heritage features
- Downstream impacts
Municipal approval and Conservation Authority approval are separate considerations. The receiving environment should be confirmed early, particularly where the site discharges directly to a watercourse or regulated drainage feature.
What a Complete Ottawa SWM Report Includes
A practical stormwater management report commonly includes:
- Design criteria and source documents
- Existing conditions and drainage areas
- Proposed conditions and drainage areas
- Runoff coefficient and peak flow calculations
- Allowable release rate calculations
- Quantity control storage sizing
- Orifice, inlet control, and outlet calculations
- Water quality treatment strategy
- LID review, where required
- Overland flow and major system assessment
- Erosion and sediment control coordination
- SWM summary tables
- Maintenance and design recommendations
The report should support the City of Ottawa Site Plan Control process and coordinate with the full civil submission.
It may also form part of an Ottawa Functional Servicing Report, Site Plan Approval Drawing, OPA/ZBA Drawing, Severance or Consent Application Drawing, or Building Permit Drawing.
Start the Stormwater Strategy Before Finalizing the Layout
Stormwater management should be reviewed before the building, parking, landscaping, and servicing layouts are fixed.
Early coordination helps determine:
- Whether storage can fit on the property
- Whether underground chambers or rooftop storage are practical
- Whether the receiving system has capacity
- Whether a legal outlet is available
- Whether LID measures are feasible
- Whether the major system route is safe
- Whether Conservation Authority review may apply
- Whether additional municipal studies are required
Reliance Engineering prepares clear stormwater management reports, Ottawa site servicing plans, functional servicing reports, and coordinated civil drawings for municipal approvals across Ontario.
Contact Reliance Engineering
Naresh Ochani, P.Eng. M.Eng.
PEO Certificate of Authorization: 100548882
Reliance Engineering
6850 Millcreek Dr, Mississauga, ON L5N 2H4
Phone: 647-385-6418
Email: [email protected]
Office hours:
Monday–Friday: 9:00 AM–5:00 PM
Saturday: 12:00 PM–2:00 PM
Sunday: Closed
Request a consultation for your Ottawa stormwater management, site servicing, Site Plan Control, severance, zoning, or building permit project.
