About the modelling
The linked 1D/2D model underlying this tool was developed and simulated using Flood Modeller (download a free version from floodmodeller.com). Different versions of the model were developed for the different flood mitigation options. Although this tool presents a fictional example, the costs of the different schemes described within this tool were broadly estimated using cost estimation documents produced by the Environment Agency.
To represent the raised defence options, the bank tops were raised by 0.3, 0.5 and 1.0 m along the river channel. For the upstream storage options, flood flows were removed from the upstream of the town to represent the effect of the flood storage areas, though it is possible to represent reservoirs and storage areas directly in the model. Simulations were performed in Flood Modeller with each combination of the upstream storage and hard channel engineering measures for flood events with an annual exceedance probability of one in 10, 50, 75, 100, 200 and 500.
The average annual damages for each combination of mitigation measure were then calculated using Damage Calculator, a tool within Flood Modeller. Damage Calculator takes modelled water depths for different flood events and property information and calculates average annual damages using depth damage curves which have been provided by Multi-Coloured Manual Online. Floodplain measures representing different property level protection measures were then incorporated as part of Damage Calculator. These measures alter the depth damage curves used to calculate expected damages and represent how the different measures would be expected to reduce damages at given water depths.
Once the annual average damages have been calculated for each of the flood mitigation strategies, the average annual benefit can be calculated by taking the difference between the annual average damages with and without the flood mitigation measures. The benefit was then increased by a factor to represent additional benefits other than those directly related to property damage. This was then multiplied by 29.68 to calculate a lifetime benefit, enabling comparison to the lifetime costs of the options. This value was then divided by the cost of the selected schemes to calculate a benefit cost ratio.