With the hydrograph established,
the point at which a reliable forecast can be achieved
can then be determined. This is the earliest point at which the alert can be issued.
The next step is to determine the lowest (safe) permanent protection level. Firstly, one
needs to establish the lowest permanent protection level. This is the lowest level of the
permanent flood defence where one exists or the lowest point along the line of defence
through which a pathway of water can be created into the protected area. Using the
level hydrograph information and the lowest permanent protection level, the lowest
(safe) permanent protection level is then determined as the level of water above which
safe closure of any temporary or demountable flood protection would not be possible.
This level would either be the same or lower than the lowest permanent flood
protection level. These levels and their interaction with the flood hydrograph,
mobilisation and closure processes are illustrated in Figure 4.1.