Report No FR0309
SEWER FLOODING RISK - FINAL REPORT
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.
- 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.
- 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.
- 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.
- 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
- 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.
- Care should be taken to avoid over-precision in predicted flooding
frequencies as they are limited in their accuracy.
- When carrying out the design of new developments, greater consideration
should be given to the effect of sewer flooding during extreme rainfall.
- 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.
VI RESUME OF CONTENTS
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
Copies of the report are available from FWR, price £25.00, less 20% to FWR Members.