Is your site near a river or creek? Our two-stage floodplain analysis — a Stage 1 screening review, then Stage 2 detailed modelling — establishes where you can build and to what elevation.
If your property is near a watercourse, a conservation authority regulates what you can build and where. Floodplain analysis establishes whether the site is within the regulatory floodplain, where the flood limit falls, and — if development is possible — the elevation you must build and floodproof to.
We approach floodplain analysis in two stages so that the level of technical effort is matched to the site constraints and approval requirements: a Stage 1 screening review first, then Stage 2 detailed modelling if the site warrants it.
| Stage 1 — Screening reviewFeasibility | Stage 2 — Detailed modellingDesign & approval | |
|---|---|---|
| What it is | A desktop review against the conservation authority's existing floodplain mapping and regulated-area limits | Site-specific hydrologic and hydraulic modelling of the watercourse |
| Method | Review CA regulatory mapping, regulated limits, and known constraints; confirm whether the site is in or near the floodplain | A detailed topographic survey of the study area tied to the floodplain survey benchmark, then hydrology (e.g. Visual OTTHYMO, PCSWMM, or HEC-HMS) to derive flows and hydraulics (e.g. HEC-RAS 1D/2D, or Mike+) to compute water levels |
| Output | Whether the site is affected, the constraints, and whether Stage 2 is required; if minimally affected, whether a low-impact cut-and-fill balance is an option | Refined flood lines, the regulatory flood elevation, floodproofing levels, and a feasible cut-and-fill balance |
| Purpose | Feasibility and scoping | Detailed design and approval |
Ontario floodplains are mapped to a regulatory flood — generally the greater of the 100-year flood or the applicable Regional Storm. Across most of the Hurricane-Hazel-affected GTA, the Hurricane Hazel storm (about 285 mm of rain over roughly 48 hours) governs and produces a larger floodplain than the 100-year event.
Hazel is not the only one. Ontario also applies the Timmins Storm — a shorter, more intense event of roughly 193 mm in about 12 hours — which is the governing Regional Storm in other parts of the province, including areas such as Scugog. Which storm applies depends on the watershed, so the first thing to confirm is which standard your conservation authority uses.
Whether new development is possible depends on the authority's one-zone or two-zone policy — and, in some designated areas, Special Policy Area rules. Safe access and upstream/downstream spill also have to be considered.
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How we help
We confirm the regulatory flood elevation and floodplain limit, then advise whether your building sits safely above it — and design low-impact fill where area needs to be recovered.
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We deliver the floodplain analysis the authority requires — Stage 1 screening or Stage 2 HEC-RAS modelling — and carry it through their review.
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We map the flood hazard against your lots and blocks so you know the true developable area early, with the modelling planning staff need to approve it.
← BackStage 1 is a desktop screening review against the conservation authority's existing floodplain mapping to see whether your site is affected and what the constraints are. Stage 2 is detailed hydrologic and hydraulic modelling that refines the flood lines and elevations for design and approval.
The regulatory flood — generally the greater of the 100-year flood or the applicable Regional Storm. In most of the Toronto region the Hurricane Hazel storm (about 285 mm of rain over roughly 48 hours) governs. Elsewhere in Ontario the Timmins Storm (roughly 193 mm in about 12 hours) is the governing Regional Storm — the Scugog area, for example. The applicable standard varies by watershed, so confirm with the conservation authority.
It depends on the authority's policy. Under a one-zone approach the entire floodplain is treated as floodway and new development is generally not permitted. Under a two-zone approach, development in the flood fringe may be allowed if it is floodproofed to the regulatory flood elevation.
Hydrologic models such as Visual OTTHYMO, PCSWMM, or HEC-HMS generate the design flows, and a hydraulic model such as HEC-RAS (1D or 2D) or Mike+ computes the water surface elevations and flood limits.
Yes — Stage 1 screening reviews and Stage 2 detailed hydrologic/hydraulic modelling, floodproofing, and cut-and-fill balance, for conservation-authority and municipal approvals across the GTA and Ontario.
General educational information only — not engineering advice for a specific property, and no substitute for the current regulations or the requirements of the conservation authority or approval body having jurisdiction. Standards, allowances, and criteria vary by watershed and authority; confirm for your site with a qualified professional.
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