5.2 Selection of Principal Options
This stage involves the review of the hydraulic characteristics of the catchment such as the
shape, magnitude and timing of level hydrographs. This information would be obtained from
hydrological studies, calibrated hydraulic model runs and historical catchment knowledge and
records.
The reliability of this information is very important, as the results of subsequent processes
would depend on its accuracy. Once the catchment hydraulics has been reviewed for a range
of flooding probabilities, the next step involves the determination of the lead-in time between
the flood warning trigger or action level and the lowest safe permanent level. These levels are
as described in Section 3.4 and Figure 3.4. The lead-in time is made up of two separate
processes that lead onto each other, namely, mobilisation and closure. The mobilisation
period includes the call-out and transportation of all required personnel, plant and materials to
site. Its duration will depend on the call-out system, remoteness of site from the resources,
including storage of the temporary barrier or demountable section and any required materials.
Transportation and access can also affect the mobilisation period. The closure is the process
of sealing the gap in the flood protection or erecting the temporary system. This could involve
erection of a temporary barrier or demountable section, manual closure of a pre-installed
barrier, automatic closure based on receiving electronic signals or direct closure though
hydraulic connection of a moveable barrier to the watercourse. Any required site preparation
is also included. The closure period would depend on the type of protection system, the length
of the moveable sections or barrier and the resources available for its closure.
The time available for the mobilisation and closure will be obtained from the hydrograph.
This is governed by the steepness of the hydrograph, the character of the trigger mechanism or
action procedure in relation to the location of the defence, and the difference in level between
the flood warning trigger or action level and the lowest safe permanent level.
A decision on the principal type of temporary or demountable flood protection can be made
from the available lead-in time using the chart given as Figure 5.2.