Land use data can be derived from land -use maps, being necessary to defferentiate residential (Houses) from non-residential properties (NRPs).
Hydrological/hydraulic data will be needed on the return period of flood events (at least 5), and the mean depth of flooding at each property.
Based on the mean depth of flooding the damages per house or NRPs can be estimated.
Discounting is necessary to calculate the Present value of the damages (PVd). With the 3.5% Test Discount Rate reducing to 3% at year 30 and to 2.5% at year 75, and given the 100-year time horizon, the relevant discount factor is 29.9 (UK HM Treasury 2003).
The costs of the different interventions/ flood risk management measures are generally derived from engineering assessment (for structural measures) or other studies (for non structural measures).
Benefit:Cost Ratio and Benefit minus Cost (or Net Present Value) are two criteria of appraising the economic performance of different options of investments/ flood prevention measures.
Projects are only economically viable if the benefits exceed the costs (i.e. the ratio of benefits to costs is greater than 1.0 or the Net Present Value is greater than 0).
You may download here the CBA calculations tool (xls)