What Does a Flood Hazard Assessment Report Actually Include?

A resource consent condition requiring a "flood hazard assessment" sounds straightforward, until you receive a quote for $15,000 and don't know what you're getting. A proper flood hazard assessment covers more than a desktop LiDAR review. It includes hydrological analysis, hydraulic modelling, climate change uplift, and a determination of flood depths and velocities at your specific site. This post breaks down exactly what goes into one and what the outputs mean for your development.

Why councils require them

Councils require flood hazard assessments when a proposed development is located within or near an identified flood hazard area. This could be a mapped floodplain, a site adjacent to a river or stream, or land that council records indicate has been affected by historical flooding. The purpose is to establish, with engineering rigour, whether the site can be safely developed and what conditions (such as minimum floor levels or building setbacks) are needed to manage the flood risk.

The assessment must be prepared or reviewed by a suitably qualified engineer. In most cases, this means a Chartered Professional Engineer (CPEng) with experience in hydrology and hydraulic modelling.

Component 1: Desktop review and data collection

Every flood hazard assessment starts with a desktop review. This includes:

• LiDAR terrain data. LiDAR (Light Detection and Ranging) provides a detailed ground surface model, typically at 1-metre resolution. This is the foundation for the hydraulic model. In areas where LiDAR is not available, surveyed cross-sections or photogrammetry may be used instead.
• Council flood hazard maps. Most councils maintain flood hazard mapping from previous regional or catchment-scale studies. These maps identify areas subject to flooding but are often coarse-scale and may not reflect current conditions or climate change projections.
• Historical flood records. Recorded flood levels, photographs, and anecdotal evidence of past flooding provide calibration data for the hydraulic model. If the 2023 Cyclone Gabrielle flood levels were recorded near your site, these become a critical validation point.
• Site survey. A topographic survey of the site and its surroundings, including channel cross-sections, culvert dimensions, bridge openings, and any existing flood protection structures.

Component 2: Hydrological analysis

The hydrological analysis determines how much water arrives at the site during a design storm event. This involves:

• Catchment delineation. Identifying the full upstream catchment area that contributes flow to the site. For a river or stream assessment, this could be tens or hundreds of square kilometres.
• Rainfall analysis. Design rainfall depths and intensities from HIRDS v4 for the relevant return periods (typically 10-year, 50-year, and 100-year ARI).
• Flood frequency analysis. Where stream gauge records exist, statistical analysis of recorded peak flows provides an independent check on the rainfall-runoff estimates.
• Design flood flows. The peak flow rates for each return period, which become the inputs to the hydraulic model.

Climate change uplift is applied to the design rainfall or design flows. The standard approach in NZ is to apply the Ministry for the Environment's climate change guidance, which specifies percentage increases based on the Representative Concentration Pathway (RCP) scenario and the time horizon.

Component 3: Hydraulic modelling

This is the core of the assessment and where most of the engineering effort is concentrated. The hydraulic model simulates how floodwater moves across the landscape at the site. There are two main approaches:

• 1D modelling. Suitable for well-defined channels where flow stays within the banks or follows a clear path. Software such as HEC-RAS 1D models flow along cross-sections perpendicular to the channel. This is appropriate for river reaches where overbank flow is limited.
• 2D modelling. Required where floodwater spreads across a floodplain, interacts with structures, or follows complex flow paths. HEC-RAS 2D or TUFLOW models flow across a grid of cells, producing spatially distributed flood depth and velocity maps. This is the standard approach for most flood hazard assessments in NZ.

At the Mt Herbert Road, Tukituki assessment, we used HEC-RAS 2D to model flood behaviour across the site and surrounding floodplain for the 10-year, 50-year, and 100-year ARI events. The model was calibrated against recorded Cyclone Gabrielle flood levels at nearby benchmarks. At Glendale Road, Auckland, the assessment required modelling of both riverine and overland flow paths that interacted with the existing urban stormwater network.

Component 4: Climate change scenarios

Current best practice requires flood hazard assessments to include climate change sensitivity analysis. This typically means running the hydraulic model for at least two scenarios:

1. Current climate. Design flows based on current HIRDS v4 data without uplift.
2. Future climate (RCP 8.5, 2081-2100). Design flows increased by the appropriate climate change factor. For most North Island locations, this is a 16-22% increase in rainfall depth, which translates to a larger increase in peak flow (because the relationship between rainfall and runoff is non-linear).

Some councils also require an intermediate scenario (RCP 4.5 or mid-century RCP 8.5) and a sea level rise scenario for coastal or tidally influenced sites.

Component 5: Outputs and recommendations

The assessment report delivers specific outputs that the council, planner, and developer use to make decisions:

• Flood depth maps. Spatial maps showing the depth of flooding at the site for each return period and climate scenario. These are typically presented as colour-coded GIS layers over an aerial photograph.
• Flood velocity maps. Velocity is critical for assessing the safety risk from floodwater. The NZ Building Code and many district plans use a depth-velocity product (d x v) threshold to determine whether a site is suitable for habitable buildings.
• Minimum floor levels. The recommended floor level for habitable buildings, typically set at the 100-year ARI flood level plus a freeboard allowance (commonly 500mm) plus the climate change uplift.
• Flood hazard classification. Whether the site is classified as having a low, medium, or high flood hazard, based on the depth-velocity product and the frequency of flooding.
• Development recommendations. Specific conditions under which the site can be safely developed, including building setbacks from watercourses, fill requirements, and any restrictions on building use or occupancy.

What drives the cost

The cost of a flood hazard assessment depends primarily on the complexity of the hydraulic modelling. Factors that increase cost include:

• Large upstream catchment area requiring detailed hydrological analysis
• Complex floodplain geometry requiring 2D modelling
• Absence of LiDAR data, requiring field survey
• Need for model calibration against recorded flood events
• Multiple climate change and sea level rise scenarios
• Peer review requirements (some councils require independent review of the hydraulic model)

A straightforward assessment for a single residential lot adjacent to a small stream might cost $5,000-$10,000. A complex assessment for a large site on a major river floodplain with multiple scenarios and peer review requirements can exceed $30,000. The $15,000 quote that prompted this post sits in the middle range and is typical for a moderately complex site requiring 2D modelling.