Flood Modelling

What is Flood Modelling?

Floods are a significant natural hazard that can lead to loss of life and income and have adverse economic and social impacts. Flood risk management is the process of reducing these negative impacts.

In Ireland, Flood Risk Assessments (FRA) are carried out in accordance with The Planning System and Flood Risk Management Guidelines for Planning Authorities 2009. These guidelines recommend that FRA’s are separated into three distinct stages:

• Stage 1 – Flood risk identification
• Stage 2 – Initial flood risk assessment
• Stage 3 – Detailed flood risk assessment

A Stage 3 assessment is a quantitative assessment of potential flood events. It is undertaken if the previous stages of the flood risk assessment determine the need for quantitative analysis to assess flood risk issues relating to a specified site and will often include flood modelling.

Figure 1 – Flooding to Main Square, Bantry, Co. Cork, Oct 2020

Flood modelling is the use of software and the computational power of modern technology to solve equations which predict water levels, flows in channels/systems/networks and flood depths on flood plains for a range of different scenarios, which can include rainfall, extreme flows/tides, inclusion/exclusion of flood defence etc., for present conditions and takes future climate change into consideration.

Flood modelling addresses the uncertainty involved with flooding and can be used to assess different scenarios and analyses. It’s important to note that flood modelling approximates real-world physics, and models can range from a simple horizontal projection of a water level in a channel or sea cross flood plain to a complete solution method.

Flood risk management in a modelling context involves a range of interacting activities i.e.

• Defining warning areas and forecasting water levels for Flood Incident Management – the process of steps prior to or during flood events can then be developed.
• Appraisal and design phase of flood assets – setting required crest levels for flood defence (e.g. flood walls, embankments, walls, sluices, barriers etc.) or an appraisal to work out damages and how these could change with different types of options and work out where the best application of a budget would be.
• Guiding appropriateness of property-level responses – steps to keep water out of properties and enable speedy recovery after flooding.
• Zoning of floodplains to guide appropriate development – spatial planning and management to guide development towards areas with lower flood risk. Visual aids generated by the model usually make it easier for planners/developers to understand where different types of development are most appropriate.
• Flood mapping to raise community awareness – understanding what the risk is and encouraging more uptake from community where stakeholders can visualise different likelihoods on a floodplain.
• Assess benefits of natural flood management options – for example, different options in the headwater of a catchment to slow down the flow of water so downstream areas will benefit.

Figure 2 – An example of the Flood Modeller (ISIS) Software graphical display

The flood modelling process usually includes:

• Selection of best modelling approach – selecting the most appropriate approach for a particular problem and catchment is the most important step.
• Obtain data and build an initial model.
• Test, calibrate and validate – for example, against a historical event, i.e. does the model actually represent real-world conditions as much as possible.
• Production Runs – quality-controlled outputs – types of production runs depend on what activity the flood modelling supports, what types of options/interventions change water level and flood extent or could be used to produce maps for different conditions, flood probabilities or climate change conditions.

When carrying out flood modelling, different points should be kept in mind:

• Objectives – These are fundamental in a modelling project. For example, is there a need to model the flood plain or will modelling the river channel be enough? Do we need to look at the whole system of will part of the river channel be enough?
• Flood Characteristics – How the water moves through the river channels and onto the flood plain.
• Skills – how complex and time-consuming model set-up will be.
• Data – cross sections, point details, topography, bathymetry, roughness, boundary conditions etc.
• Software – does the choice of software fulfil determined objectives?
• Budget – i.e. need to allocate expenses appropriately with respect to project set up and also running the model accordingly.
• Timescale – depending on the type of flood model utilised, run times can vary greatly.

Figure 3 – Cross section viewer displayed on Flood Modeller (ISIS) software

Generally, two main types of flood modelling are utilised, 1D and 2D modelling.

1D modelling solves one-dimensional flow equations in a channel. These equations consist of conservation of mass and momentum. 1D modelling consists of averaging all parameters at one location. For example, cross-sectional flow at a point using only the average value.

1D models are usually quick to run, good at representing in-channel water levels and flows, good at bridges/weirs/sluices and other ‘point’ features, and can sometimes be suitable enough to cover floodplain flow/flood mapping. The downside to 1D modelling is the need to identify major flow routes to set up the model. Complex systems can take time to set up, can be poor at flood routes in urban areas and have no velocity distribution on a flood plain.

2D models solve the same fundamental equations as 1D models but solve these in two dimensions.

2D models have no need to predefine flow routes, are easy to set up (square grid can be more difficult for mesh), are likely to be more accurate, and have velocity variation on a floodplain with flood maps and depth grids being direct output. However, it can be very slow to run in comparison to 1D models, poor at point features such as bridges/weirs/sluices etc., and there can sometimes be a need for a fine grid/mesh for some river channels.

The model choice depends on numerous factors (i.e. model turnaround times, breach analysis, budget, design for point features etc.). Linking 1D-2D together is generally the most popular option as this uses the best aspects from both the 1D and 2D modelling systems i.e. uses 1D for channel and point features and then a 2D approach on the flood plain.

ORS environmental consultants provide specialist flood risk assessments, site surveys and flood modelling. If you require a design phase FRA or need to respond to a ‘Further Information’ request from a planning authority, speak to a member of our team today. For further resources on the benefits of incorporating Flood Risk Assessments into your building process, click here.