The operational stages from flood warning through to demobilisation and clean-up have been
outlined separately to assist with the proper consideration of the processes within them. The
actual process required for the operation of a system would depend on the type of system and
the organisation or people responsible for its deployment.
A temporary system or demountable section of a flood protection system should be fully in
place to form a closed barrier before time T2. T2 is the time when the lowest safe level of the
riverbank or permanent part of the protection is reached. This refers to the flood water level
that can be permitted within the river or floodplain without the risk of flood water passing the
line of the barrier. This level is lower than the lowest permanent level due to the effect of
wave run up.
T1 is the time at which the actual closure of the barrier commences. It is determined by
working back from T2, having allowed adequate time for closure. At T1 all resources required
for the closure of the barrier would already be on site ready to commence closure. The closure
period is dependent on the type of operation required to close the barrier and the extent of the
temporary or demountable parts. This could vary from a few minutes for automatic operation
of a barrier or gate to over 24 hours for the erection of long lengths of barriers requiring use of
heavy plant and materials.
T0 is the trigger level at which the call-out procedure is instigated and mobilisation of all
resources commences. The maximum value of T0 is determined by working back from T1,
having allowed adequate time for mobilisation. The actual trigger (action) level is then
decided, based on the need to provide adequate time for mobilisation and minimising the risk
carrying out abortive emergency operations. An unacceptable level of abortive warnings and
operations would be counter-productive. The mobilisation period depends on the call-out
procedure of the organisation of people responsible for the operation of the defences and the
transportation and access requirements to get all resources to site. The mobilisation period
could range from zero for automated sites to over 12 hours for isolated sites where storage of
the demountable sections or temporary barrier units are not near the proposed deployment
area.
The length of time T2 – T0 is the period required between the water reaching a pre-determined
trigger or action level and achieving full closure. This is the most important factor in
determining whether an area at risk from flooding can be protected with a temporary or
demountable protection system and which types of systems would be suitable. While atemporary or demountable system which requires significant mobilisation and closure time
could be suitable for areas at the downstream end of a large river catchment, a demountable
system that is automatically operated by water level sensors may be the only viable nonpermanent
option for protection of an area at the upstream end of a catchment in which river
levels rise quickly following a storm.
After reaching the peak level, the flood level starts to fall with time until it reaches a level at
time T3 corresponding to the barrier opening level. This level is usually the same as the lowest
(safe) permanent protection level (the level at time T2 as earlier discussed). The barrier
opening or dismantling will normally commence at T3 unless a second peak in level is
forecast.
The failure of closure of a flood protection system will occur when the river level is higher
than the lowest permanent part of the river bank or system and the temporary barrier or
demountable section of the defence is not fully in position. Using the principles laid out in
Figure 3.4, the following figure shows a simplified failure tree for non closure of a flood
protection system.