3.4.2 FAILURE OF THE FLOOD WARNING SYSTEM
Once the design flood hydrograph (see Figure 3.4) or other reliable indication of threshold
flood level has been determined, a flood warning system is required to trigger the mobilisation
phase of the closure operation. Flood warning can be based on forecasting from rainfall, flow
or water level measurements. These systems can be manually read, electronically recorded or
monitored and simulated in real-time. The actual warning process could be automated,
manual call-out or a combination of both. For a local community, they could be actioned by a
more general warning by the Environment Agency through “Floodline”.
The failure of a flood warning system will occur either by the failure of the technical part of
the system such as instrument malfunction or human failure of the interpretation and action
followed upon a flood warning to initiate mobilisation.
A reliable flood warning system will need to be manageable by the size and skills of the
operating body. The mechanics of forecasting and monitoring will need to be reliable and
have a ready back-up system. The reliability of the forecasting and monitoring system will be
low for one relying on weather forecasts or rain fall/runoff read-outs only and high for realtime
monitoring of the upstream catchment.
A reliable flood warning system is a pre-requisite to the successful use of a temporary or
demountable flood protection system.