Agenda Reports Pack (Public) 15/10/2012, 19.00 - Meetings ...

as from information provided by residents and businesses that have sufferedfrom flooding in the past, in order to map and model surface water flooding, andhence identify high risk flooding areas. The Task Group were concerned that thisdata was insufficient as it was not a true reflection of the actual flooding risk in theborough. This concern arose from the fact that the Task Group were made awarethat many people affected by flooding chose not to report it, primarily but notsolely because of concerns relating to the potential impact on property prices andinsurance premiums.1.4. The Pitt Review commissioned analysis of a number of different approaches tomodelling surface water flooding; examining their effectiveness and undertaking abasic cost-benefit analysis. The results of this work are set out below.1.5. The five approaches considered by the Pitt Review were:iiiiiiivvTopographic index analysis – This is a basic terrain model with no rainfallinput. There is no correlation between the model’s outputs and areas ofknown flooding, and so it would be of little use.2D overland routing of uniform rainfall event – This model makes noallowance for differences in rainfall, and assumes that every area has auniform capacity to drain water. It could be used for high level analysis butsignificantly overestimates the extent of flooding.Decoupled sewer model and 1D overland routing – This model takesaccount of the effect of drainage by using a detailed sewerage network model.It is the most accurate method of identifying properties on water companyregisters but underestimates the spatial extent of flooding.Decoupled sewer model and 2D overland routing – This model includes2D surface runoff data and detailed sewerage network data, but does notinclude assessment of below-ground flooding mechanisms. It produces amuch better estimate of the spatial extent of flooding but fails to identify someproperties on water company registers.Coupled sewer model and 2D overland routing – This model combinessurface runoff data, detailed sewerage network data and a full 2D model ofabove-ground flooding. It does not include below-ground floodingmechanisms but this could be added. It gives a very accurate assessment ofthe spatial extent of flooding but fails to identify some properties on the watercompany registers.1.6. The Pitt Review made a number of comments regarding the modelling approaches:i. Tools exist that can reliably and accurately model surface water flooding inurban areas.ii. The cost of the different models can vary widely owing to the information anddetail required. Additionally the cost of accurate modelling can be high ifmodels of sewerage networks have to be built from scratch.iii. Simplified modelling is possible at relatively low cost but is far less reliableand probably only suitable for high level risk assessments on an area widebasis. Such approaches are not suitable for assessment at the level of detailof individual streets or for producing solutions to flooding.iv. Surface water flooding can be accurately modelled and mapped but furtherwork is required to understand user needs and the costs associated withmeeting those needs.- 14 -Page 173