A Flood Risk Assessment should be completed as early on as possible in the planning process. One of the main goals of The Planning System and Flood Risk Management Guidelines for Planning Authorities is to ensure that flood risk management is a key consideration in development planning and the assessment of planning applications.
If your site is near a watercourse and/or along the coast, then a flood model is likely required as part of the Flood Risk Assessment. The purpose of the flood model is to determine, as accurately as possible, the flood extents and water levels that would result from a future river and/or coastal flood.
A flood model that over-predicts the flood extents and water levels could lead to over-design and poor use of space. In contrast, one that under-predicts could leave your future development susceptible to flooding and property damage.
Recently, I was chatting with my co-worker about the accuracy of flood models. He reminded me of an old saying: a model’s output is only as good as its input. In other words, if the model was built using poor-quality data, the predicted flood extents and water levels will be inaccurate.
Does this mean you shouldn’t trust any flood model? Well, no.
This means you should hire a consultant you trust to build an accurate flood model and carry out a high-quality Flood Risk Assessment.
Some key data requirements include:
- Cross-section data at several points along the river
- LiDAR data (to represent the floodplain in more detail, if necessary)
- Dimensions and properties of all structures (culverts, bridges, flood defences, etc.)
- “Roughness values” for the river, riverbanks, and floodplain (to define the amount of flow resistance in each region)
- Catchment data (used to estimate river flows and/or sea levels for the flood events to be modelled)
TOBIN has an experienced surveyor who can accurately record river cross-section data and structure dimensions. The engineer/hydrologist responsible for building the flood model (either myself or my co-worker) also does a walkover survey of the site. This allows us to determine roughness values and better understand the area we will be modelling. If required, we purchase LiDAR data from a trusted provider.
Finally, we use the newest version of HEC-RAS software to build our flood models. HEC-RAS was first released in 1995 by the US Army Corps of Engineers, and is used globally for river and floodplain modelling.
In short, the Water Services team at TOBIN will ensure the flood model and Flood Risk Assessment you require are carried out to the highest professional standards.
