While this breakdown was carried out for a particular scheme and variations are expected for
different scenarios, it indicates the relative risks of failure of defence systems with temporary
or demountable parts. Factors which could significantly affect the relative risk of failure
include the sufficiency of lead-in time, reliability of flood forecasting and warning systems,
durability and maintenance of the system components and training of deployment teams.
A review of Figures 3.1 and 3.2 shows that a permanent protection system is potentially safer
than one with temporary and demountable parts as there are no additional operational failure
concerns. As a result of this, temporary and demountable systems should not automatically be
used as alternatives to conventional permanent systems without adequate consideration of
their expected performance, operational issues and adequacy for the proposed scenario. It
should be noted that a temporary or demountable protection system is only functional once the
closure or installation of the barrier is achieved.
The performance objective for a temporary or demountable flood protection system is to make
it as safe as a corresponding permanent system of the same standard. This guide therefore
seeks to minimise the additional risks posed by the operational activities associated with the
deployment of temporary and demountable systems, while seeking to improve its performance
once closure is achieved. The three failure modes are considered further in the following subsections.