How much should the freeboard allowance be?
Freeboard takes into account uncertainty in modelling predictions of flood risk (see What is freeboard? for more information). The greater the uncertainty, the greater the freeboard allowance should be. For example, in South Essex, some flood cells or embayments are very large and, in an extreme flood event, flood levels predictions within the embayment would not vary very significantly given the uncertainty of other factors. A freeboard value of say 200/300 mm above an extreme event flood level might be appropriate. Another example could be a small tributary of the River Thames which is ungauged, in a narrow valley and with a complex upstream urban catchment. In such a case, uncertainty in the model predictions could be higher and a greater freeboard allowance should be afforded (say 500/600 mm or higher). The freeboard allowance should therefore be commensurate with the uncertainty in water level prediction.
