Site-Specific Flood Assessment

Scope note. The free FloodMap.nz layer is overland flow paths only — it shows where surface water would route across bare earth. A site-specific flood assessment adds depth, velocity, hazard category, and rainfall-driven simulation through HEC-RAS 2D. That's not currently a productised offering on FloodMap.nz; it's engineering work scoped per site through SAE Ltd. This page describes what such an assessment typically covers.

What's typically covered

Inside a site-specific flood assessment.

Modelling

HEC-RAS 2D unsteady

Solver-grade hydraulic model on a 2–5 m mesh, sized to the catchment. TP108 design rainfall (or regional equivalent via NIWA HIRDS v4) at 10%, 2% and 1% AEP, 24 hr duration.

Rasters

Depth, velocity, hazard

Maximum depth raster, peak velocity raster, depth × velocity hazard product, and the standard NZ flood hazard category grid for the site and immediate surrounds.

Boundary

AEP depth at site boundary

Tabulated 1%, 2% and 10% AEP water levels at each property boundary and at proposed building platforms — what councils want to see for FFL setting.

Earthworks

Before/after comparison

Run with proposed earthworks burned into the DEM. Depth-difference rasters and a written assessment of upstream/downstream impact, suitable for hydraulic-neutrality demonstration.

Inputs

Documented assumptions

LiDAR vintage and survey ID, LCDB land cover classes, FSL/S-Map soil drainage class, Manning's n table per SWCoP v4, rainfall return periods.

Sign-off

CPEng-signed PDF

Final report signed by Andre Magdich CPEng (237893). Suitable for resource consent or building consent submission.

When it's worth commissioning

Cases site-specific assessment is built for.

Resource consent for subdivision or land use — when council requires a site-specific flood assessment, or when district plan flood overlays are too coarse to demonstrate compliance.
Building consent on an unmapped site — when the council viewer doesn't show a flood layer for your site but a stream or overland flow path is visible on aerial imagery or in the field.
Hydraulic neutrality for earthworks — when proposed cut and fill could redirect overland flow and you need to demonstrate no upstream/downstream impact.
Pre-purchase due diligence on a high-value site — when the public OFP map shows significant flow paths and the deal warrants a full hydraulic study before settlement.
Insurance dispute or property valuation — when an insurer or bank requires independent CPEng-signed flood evidence at property level.
RFI response on an existing application — when council has come back asking for HEC-RAS modelling rather than the rational-method calc you submitted.

Specifications

Technical approach.

The methodology aligns with the broader FloodMap.nz pipeline — see /methodology for the full nationwide approach. Site-specific assessments apply the same standards at higher resolution.

Solver	HEC-RAS 6.7 implicit 2D, unsteady
Mesh resolution	2–5 m, sized to the catchment area
Source DEM	LINZ 1m LiDAR composite (L121859), latest survey for the region
Rainfall	TP108 (Auckland) or NIWA HIRDS v4 design storm, 10% / 2% / 1% AEP, 24 hr
Land cover	LCDB v5 mapped to Manning's n per SWCoP v4 (sealed 0.015, grass 0.030, forest 0.100, buildings 0.150)
Soils	FSL Hydrologic Soil Group + S-Map where published, mapped through TR-55 AMC II
Impervious surfaces	Aerial-derived ExG threshold, masked by LCDB urban classes
Outputs	Depth raster, velocity raster, hazard grid, depth × velocity, AEP depth at boundary, before/after earthworks if applicable
Standards aligned	TP108, SWCoP v4, NZS4404:2010, AUP E36, NRCS TR-55, HEC-RAS 2D User Guide (USACE)

Need a site-specific assessment?

Site-specific HEC-RAS 2D flood assessments are scoped and delivered through SAE Ltd. Get in touch with what you're working on and we'll come back with what's involved.

Get in touch → SAE Ltd ↗

HEC-RAS 2D flood assessment, on commission.