Flood Risk Assessment NSW

If your land is mapped as flood affected — or council merely suspects it might be — your development application will not be assessed on its merits until the flood question is answered. A flood risk assessment (sometimes called a flood impact assessment, flood report or site-specific flood study) establishes how deep and how fast floodwater gets across your site, what floor levels and access arrangements are needed to keep future occupants safe, and whether your proposal makes flooding worse for anyone else. It is a risk document as much as an engineering one, and councils read it that way.

Contrive Consultants prepares flood risk assessments for sites across NSW: mainstream riverine flooding in the Hawkesbury-Nepean and other valley floodplains, local overland flow in Sydney’s built-up catchments, and everything from single dwellings to industrial estates and subdivisions. Working from Rouse Hill, on the edge of one of Australia’s most scrutinised floodplains, we understand how NSW councils apply their flood policies in practice — and we write reports that give assessment officers what they need to sign off, rather than reasons to refer the file upward.

When council requires it

Flood-affected land at DA stage — if any part of the site falls within council’s flood planning area, or is tagged flood-affected on the planning certificate (Section 10.7), a flood risk assessment is typically a mandatory DA lodgement document.
Overland flow paths — many urban sites with no river in sight are still crossed by mapped or unmapped overland flow. Councils increasingly require an overland flow assessment before accepting floor levels, driveway grades or fencing that could obstruct flow.
CDC exclusions — complying development is generally not available on flood control lots without supporting flood information; a flood report can determine whether the CDC pathway remains open or the project must proceed by DA.
Sensitive and vulnerable uses — childcare centres, seniors housing, hospitals and emergency infrastructure attract stricter flood criteria, often requiring assessment against rarer events up to the probable maximum flood (PMF).
Consent conditions — approvals on flood-affected land routinely condition minimum floor levels, flood-compatible materials, evacuation arrangements and no-worsening demonstrations to be verified before CC or occupation.

What we deliver

Site-specific flood risk assessment reports formatted for DA lodgement and council assessment
Flood level, depth, velocity and hazard analysis for the design events council nominates — commonly the 1% AEP (1-in-100-chance-per-year) event, plus checks up to the PMF where required
Flood planning level (FPL) determination, applying the council’s adopted freeboard to derive minimum habitable and non-habitable floor levels
Overland flow path assessment and modelling for local catchment flooding
Flood impact (afflux) assessment demonstrating the development does not adversely raise flood levels on neighbouring land
Hydraulic hazard categorisation and commentary on structural adequacy, flood-compatible materials and fencing
Evacuation and emergency response commentary consistent with the relevant flood emergency arrangements
Interpretation of council flood study data, and liaison with council flood engineers where site data is incomplete

Software & standards

Flood assessment in NSW sits on two foundations: the hydrology of Australian Rainfall and Runoff 2019, which governs design rainfall, losses and the treatment of event probability, and the state’s flood risk management framework as applied through each council DCP, LEP flood clause and adopted floodplain risk management study. Where mainstream flood levels have already been established by a council-adopted flood study, we build on that data rather than reinventing it. For local catchments and overland flow we undertake hydrologic and hydraulic analysis using DRAINS and complementary hydraulic methods appropriate to the scale of the problem, and document site drainage interfaces to AS/NZS 3500.3:2021. Road and access considerations draw on Austroads guidance. Because freeboards, flood planning levels and PMF requirements are set locally, we always confirm the current criteria with your consent authority.

Our process

Brief and data gathering — we review the planning certificate, council flood mapping and any adopted flood study, and request council’s flood information for the site where available.
Site and catchment appraisal — survey review and catchment analysis to establish how water actually reaches, crosses and leaves the land, including unmapped overland flow.
Analysis and modelling — flood levels, depths, velocities and hazard across the required events, from frequent storms to the 1% AEP and, where required, the PMF.
Design advice — recommended floor levels, driveway and access levels, structural and materials advice, and layout changes that reduce risk while the design is still flexible.
Reporting and assessment support — a clear, defensible report lodged with your DA, followed through referrals, RFIs and any conditions requiring certification at CC or occupation stage.

Why Contrive

Approvals-focused reporting — we answer the questions council’s flood engineer must tick off, in the order they must tick them off.
Integrated with your wider application — through the Contrive group we coordinate flood advice with ARQI‘s architecture, URBA‘s planning submissions and Sweptpaths‘ access design, so the flood report supports the scheme instead of contradicting it.
NSW-wide capability — riverine floodplains, coastal catchments and urban overland flow, for councils across Sydney and regional NSW.
Senior-led judgement — flood risk involves interpretation, not just computation; every assessment is directed and reviewed by a senior engineer.

Frequently asked questions

What does 1% AEP actually mean?

AEP is annual exceedance probability. A 1% AEP flood has a one-in-a-hundred chance of being equalled or exceeded in any given year — it is not a flood that happens once a century, and a bigger event can occur in consecutive years. Councils typically use the 1% AEP event as the basis for the flood planning level, with freeboard added, while rarer events up to the PMF are considered for evacuation and for vulnerable land uses.

What is the difference between a flood study and a flood risk assessment?

A flood study is usually a catchment-wide investigation, most often commissioned by council, that establishes flood behaviour across a whole floodplain. A flood risk assessment is site-specific: it applies that catchment knowledge (or generates local modelling where none exists) to your land and your proposal. Most development applications need the site-specific document; we prepare those and, where council data is thin, undertake the local modelling to fill the gap.

My 10.7 certificate says the land is flood affected — can I still build?

Usually, yes. Flood affectation is a design constraint, not a prohibition, on the majority of sites. The assessment establishes the flood planning level and hazard category, and the design responds — raised floor levels, flood-compatible construction below the FPL, open-style fencing across flow paths, and safe access. Genuinely undevelopable cases exist, but they are the exception, and a proper assessment tells you early either way.

How do flood planning levels (FPLs) get set?

The FPL is generally the adopted design flood level — most commonly the 1% AEP level — plus a freeboard set by council policy to cover uncertainty and local effects such as wave action. Habitable floors must sit at or above the FPL, and some councils apply different levels for garages, commercial floors or critical facilities. Because both the design event and the freeboard are set locally and are periodically reviewed, we confirm current figures with your council rather than quoting generic numbers.

Related services

Flood affected and need answers before you commit? Call Contrive Consultants on +61 497 848 111, email info@contriveconsultants.com.au or send your planning certificate through the contact page and we will scope the assessment your site actually needs.