Table 3-2: Modelling approaches Flood Modelling Guidance for Responsible Authorities v1.1 19 Model Type Description Uses 0-D / Rapid Flood Spreading 0-D / Rapid 0D models use simple rules to spread water over the floodplain between adjacent depressions. Advantages: Short run times. Limited data requirements. Limitations: Only produce final state of inundation, do not capture flow pathways. Flow velocity not available. Input data requirements: Digital Elevation Model (DEM) or Digital Terrain Model (DTM) To provide a general overview of pluvial flooding when the detailed data required by most 1-D or 2-D models are unavailable.
This may be appropriate for strategic levels studies covering large areas; however for local flood studies the 1D and 2D approaches below are more likely to be appropriate. Flood Spreading 1D - Hydrological Routing 1D routing models use a storage equation to model changes in the shape of the flood hydrograph along a river reach. Parameters in the equation are either calculated from the channel geometry or estimated from observed data. Advantages: Short run times. Limited data requirements (sparse cross sections). Limitations: Loss of accuracy. Levels are not calculated. Input data requirements:
Cross sections or a DTM from which cross sections can be extracted To provide boundary conditions for more detailed river models. Frequently used in flood warning where short run times are required. 1D In 1D models flow is averaged over depth and across defined cross sections. Models can be steady or unsteady. Advantages: Accurate representation of flow and level within channel. Good representation of hydraulic structures. Usually short run times allowing multiple scenarios to be run. Limitations: Complex floodplain flows cannot be represented. Floodplain velocity not available. Input data requirements: Channel sections and structures. Fluvial:
Used for modelling In bank flows, in channel hydraulic structures and narrow well defined floodplains. Pluvial: Used for modelling surface water drainage networks. 2D In 2D models flow is averaged over the flow depth and horizontally over a model grid cell or element. 2D models may be divided into those which solve the full shallow water equations and those which solve a simplified version of the equations to reduce computational effort. Either a regular grid or irregular mesh may be used. Advantages: Accurate representation of floodplain flow. Simple model build. Limitations: Cannot accurately represent structures or in channel flow particularly for narrow watercourses.
Higher resolution models may have long run times. Input data requirements: Digital Elevation Model (DEM) or Digital Terrain Model (DTM). Fluvial: Used for modelling floodplain flows where the channel capacity and transition between in bank and out of bank flow is not important (e.g. larger flood events). Pluvial: Used for modelling overland flow where interaction with the surface water drainage network is not important. Coastal: Used coastal inundation modelling. 1D-2D The channel is modelled in 1D and linked to a 2D model of the floodplain so that they can exchange flow. Advantages: Accurate representation of flow and level within channel and floodplain flow routes. Limitations:
Large time inputs required to set up complex models. Long run times. Potentially costly. Input data requirements: channel cross sections and DEM. Fluvial: Used where there is a need to understand both the channel and floodplain processes. Pluvial: Used where there is need to understand how the surface water drainage network interacts with overland flows e.g. an Integrated Catchment Model (ICM). Coastal: May be used where there is a well-defined drainage network along which flood waters can propagate. 3-D or Physical Models 3d and physical models allow vertical flow to be modelled. Input data requirements: 3d structure information, DEM, bathymetric survey. Advantages:
Can represent vertical variations in flow. Limitations: Costly and complex. Not commonly used in flood studies. They may be necessary for design in some complex cases for example, understanding of flow over, through and around structures. Nearshore wave models Simulate the processes such as energy dissipation due to bottom friction, white capping, or depth induced breaking, growth due to wind, refraction and shoaling. Advantages: Allow nearshore wave conditions to be assessed in the absence of long series of observations. Limitations: Large time inputs required to set up complex models. Long run times. Potentially costly. Input data requirements:
Bathymetric survey and a DTM of the intertidal area. Water levels. Offshore wave conditions. Used in coastal flood studies to transform wave conditions from offshore wave models inshore to produce boundaries for overtopping models. Overtopping Models Empirical and physically based models are available. Advantages: Allow overtopping rates to be assessed. Limitations: Depend on the type of model. Expert judgement may be required to determine the appropriate model in each case. Input data requirements: Nearshore wave conditions and water levels. Crest and toe levels, and defence profile, including information on roughness.
Used in coastal flood studies to predict the rate of water overtopping flood defences, and provide boundaries for 2D inundation models.