Report No FR0309



Sept 1992



Application of the procedures contained in this report will enable users to more accurately determine the risk of flooding in a location and so increase confidence in designs for sewerage rehabilitation. The procedures also allow greater confidence in the information on flooding risk supplied to regulators.


To develop methods to assess the risk of occurrence of flooding when designing sewerage rehabilitation works; and methods to assess the risk of occurrence in existing systems from a consideration of historic data.


The Director General of Water Services requires that the number of properties at risk from sewer flooding more frequently than twice in ten years is reported. At present there is uncertainty as to whether current design methods, which are based upon frequency of rainfall events and peak flows, accurately predict the frequency of flooding.


  1. Current methods for assessing the risk of flooding, which are used in the design and analysis of sewer systems can under predict the risk of flooding in certain instances. These are highly dependent on the location of the predicted flooding.
  2. Flood frequencies can be predicted using the methodology given in this report for preparing, ranking and sampling rainfall time series data and using the data with sewer flow simulation models.
  3. Methods of assessing the effect a flood volume predicted by a sewer flow simulation model will have on a particular location are described. This may be used to determine the type of surfaces that will be affected.
  4. The detailed topography of the ground around the sewer will have a great influence on the effect of the flooding. Greater consideration of this factor in the design of new developments could significantly reduce the effect of any sewer flooding in extreme events.


  1. The procedures described in this report should be used in determining the risk of flooding in sewer systems, and in determining the return period associated with the occurrence of reported flooding within a sewer system.
  2. Care should be taken to avoid over-precision in predicted flooding frequencies as they are limited in their accuracy.
  3. When carrying out the design of new developments, greater consideration should be given to the effect of sewer flooding during extreme rainfall.
  4. Existing design methods should be improved to overcome the shortcomings in the use of the synthetic design storms included in the Wallingford Procedure as, these can in some instances significantly under predict the risk of flooding.


This report describes the background to current methods of design and analysis of sewer systems and reviews the various methods of rainfall input available. Work on comparing the results of predictions of flood frequency using the different methods is r eported. Methods of assessing the impact of flooding predicted by a simulation model and methods of assessing the return period of a flooding incident are also described.

Copies of the report are available from FWR, price 25.00, less 20% to FWR Members.