Indicative flood plain and flood risk assessment

To calculate the extent of flooding likely to take place under each climate change scenario, the game estimates the effects of each of the climate change parameters on river and coastal flows. It does this using a precipitation model to derive flows into a drainage network. Extreme events such as rainstorms and storm surges are introduced throughout each decade.

The precipitation model is based on delivering varying amounts of rainfall into the upland and lowland areas of the landscape according to the different climate change scenarios. The runoff is determined by the permeability of the underlying geology and is influenced by a north-south gradient. Together these represent the water flowing into the drainage network. Rainstorm events of different sizes and frequency add to the amount of rain delivered into the catchment within each decade. Similarly, tidal height, sea level rise, coastal subsidence and storm surges add to the volume of water entering the catchment. All four contribute to the calculation in the flood model of the indicative flood plain, and assessment of risk of coastal flooding and risk of river flood.

• precipitation model: the average annual rainfall over each decade and modified by altitude and the north-south gradient is allowed to drop onto each grid square.
• runoff: the amount of water that flows down each section of the river is affected by the permeability of the underlying geology. The scale is normalised from peat (0.1) to clay (0.3) to Carboniferous limestone (0.8)
• drainage network: this is created by accumulating flow from each grid square in the elevation model. Flow always follows the steepest path to the sea. If the water reaches an area where there is no immediate downward flow, then it accumulates until the height of water in the depression is enough to flow downwards again.
  
• indicative flood plain: this is calculated using the volume of water accumulated in the drainage network. At each point in the network, the volume of water present is distributed sideways, so that when there is more water it extends further outwards. The colour scale shows the event horizon or frequency (e.g. a 1 in 100 year event) of the sideways spread; red is 1:5 and blue is 1:100.
• rainstorm events: the size and frequency depend on the climate change scenario. For Medium-High and High the frequency increases threefold over the decade and the event scales with the increase in average annual precipitation. For Low and Low-Medium the frequency increases twofold. The events are sampled randomly over the decade for 1:5, 1:10, 1:20, 1:30, 1:50, 1:75, 1:100.
• storm surges: the size and frequency depend on the climate change scenario. For Medium-High and High the frequency increases threefold over the decade and the event scales in the same way as rainstorm events. For Low and Low-Medium the frequency increases twofold. [No other evidence on storm surge scaling is currently available]. The events are sampled randomly over the decade for 1:5, 1:10, 1:20, 1:30, 1:50, 1:75, 1:100, 1:150, 1:200.
• sea-level rise: sea-level rise changes according to the climate change scenario and adds to tidal height. The maximum value of 1m occurs under the High scenario.
• subsidence of the landscape: current calculations of the natural rates of tectonically induced sea-level rise resulting from post-glacial subsidence are estimated at 1.8mm/year.