EHMH · Hydraulic and hydrological engineering
Flood Risk Assessment for Informed Project Decisions
Assess how flooding and drainage constraints can affect your project. Connect catchment evidence and modelling to site selection, design decisions and mitigation priorities.
01 / Before investing, expanding or building
Understand the water before defining the works
A site can receive runoff from a wider catchment, be affected by river levels or discharge into a drainage system with limited capacity. Looking only within the property boundary can miss the conditions that control its performance.
Hydrological risk is a broad term. Here, the focus is flooding and drainage: hazard describes event likelihood and intensity, exposure identifies the people, assets and activities in its path, and vulnerability describes their susceptibility to harm. Preparedness and response capacity also influence consequences. A flood map is an essential input, but not a complete risk assessment.
River overbank flows, rainfall-generated surface runoff and restricted outfalls may each contribute to flooding. Their relative importance must be assessed for the site.
Site selection and feasibility
Identify water-related constraints before acquiring land or defining the layout of an urban, industrial or logistics development.
Design and alternatives
Compare drainage, storage and protection options, considering catchment behaviour and downstream effects.
Operational continuity
Identify exposed access routes, equipment and operational areas to prioritise preventive measures and investment.
02 / Match the study to the decision
What a flood risk assessment can cover
The physical system
- Catchment characteristics, rainfall records, historical floods and existing drainage.
- Topographic surveys and the geometry of relevant channels and structures.
- Hydrological modelling of runoff and hydraulic modelling of water levels and flow paths.
- Flood extent, depth and velocity mapping where supported by the data and model.
The consequences
- Exposure of buildings, access routes, utilities and operational areas.
- Existing and proposed development scenarios with explicit assumptions.
- Comparison of drainage, storage, protection and non-structural measures.
- Residual risk, uncertainty, maintenance needs and additional investigations.
This page focuses on flooding and drainage. Water availability, water quality and debris-flow processes may require separate studies; a flood assessment does not resolve them.
03 / Evidence and engineering judgement
How EHMH approaches the assessment
- Frame the decision. Establish the site, project stage, assets and operational constraints; identify the applicable authority requirements and assessment criteria.
- Review the evidence. Check survey accuracy, drainage connectivity and historical observations. Make data gaps visible before selecting the modelling approach.
- Test scenarios. Define rainfall and flow inputs, boundary conditions and development assumptions. Calibrate or validate against suitable observations when available, and examine sensitivity.
- Compare responses. Evaluate the effects of mitigation within and beyond the site, including downstream impacts, maintenance requirements and remaining exposure.
- Support the next decision. Communicate findings, limitations and practical requirements for further design or risk management.
04 / From analysis to usable evidence
Typical outputs, agreed for each project
Assessment report
Documented sources, assumptions, methods, scenarios and conclusions, with the limits of their application.
Maps and model results
Plans and scenario maps. Model files and GIS/CAD deliverables can be included where specified in the agreed scope.
Options and priorities
Criteria for comparing measures, information requirements and recommendations for design or operational planning.
Deliverables and assessment boundaries are established in the technical proposal. Regulatory acceptance remains subject to the relevant authority’s requirements and review.
05 / Frequently asked questions
Reading flood-risk results responsibly
What information is needed to get started?
The site location and area, its intended use and the project stage. Plans, topographic surveys, flood history and outfall information are also useful. We review their quality before identifying any additional survey or data requirements.
Is flood mapping the same as assessing risk?
No. Flood mapping characterises the hazard under defined scenarios. Risk assessment also considers what is exposed and how it may be affected. The level of detail should match the decision and the available evidence.
What does a 100-year flood mean?
Under stationary statistical assumptions, it corresponds to a 1% annual exceedance probability. It does not mean that a flood occurs only once every hundred years, and it does not prevent successive events.
Can publicly available terrain data replace a site survey?
It may support initial screening, but its resolution and vertical accuracy may not support detailed design or floor-level decisions. Survey needs should follow from the scale of the problem and the consequences of error.
Can mitigation guarantee that a site will never flood?
No. Measures retain residual risk and depend on operating and maintenance conditions. Events beyond the assessed scenarios, blockage or changes elsewhere in the catchment can alter outcomes.
Is hydrological modelling the same as hydraulic modelling?
No. Hydrological modelling estimates catchment runoff and flow hydrographs. Hydraulic modelling represents how those flows move through channels, floodplains and drainage systems. A study may need both, depending on the question.
How should climate uncertainty be considered?
Climate and land-use scenarios can be examined where relevant and supported by appropriate evidence. Assumptions, sensitivity and limits must be explicit; a single scenario should not be presented as a precise prediction of future conditions.
Apply the assessment to your site
Turn a water-related concern into a defined study
Tell us where the site is, what you plan to develop and which decision the assessment needs to support. We can review the available information and define an appropriate technical scope.