SHORTLISTING THE DEBEN ESTUARY OPTIONS
The process of shortlisting the options
In order to identify a short list of options from the long list of options shown in Table 1, we have undertaken further detailed analysis of the various options using two modelling techniques.
These techniques have been used to identify how each option will affect the whole estuary under present conditions and scenarios of Sea Level Rise and tidal surges (like the 1953 flood event). This is done by considering how an option affects the following:
Water speeds and erosion are linked in that an increase in water speeds in the estuary will generally result in an increase in erosion pressure on flood defences and intertidal habitats in front of the defences.
Using the results of these model exercises, we can see which options allow us to manage the estuary processes and which options do not. Only options that do not have a significant adverse impact on the estuary processes will be taken forward as ‘Shortlisted Options’ for the estuary. This comprises both options that allow us to minimise the effect of some or all of the existing conditions, such as reducing water speeds, in the Estuary and those options which will have a negligible impact on estuary processes. In contrast, options that do not allow us to manage the estuary processes, for example by immediately increasing erosion pressures, are unlikely to be considered further.
THE SHORTLISTED OPTIONS FOR THE DEBENESTUARY
The options proposed for shortlisting fall into two categories:
(1) Options that must be taken forward for more detailed technical, economic and environmental study regardless of the model results, and
(2) Options that are proposed to be taken forward on the basis of the model results.
(1) Options that must be taken forward
The following options must be taken forward for more detailed study. The reasons for this are set out below. The effect of these options on estuary processes is shown in Table 1.
(2) Options proposed for shortlisting
The following options are proposed for shortlisting based on the model results. The issues associated with each of these options are discussed below. The impact of each option on estuary processes is shown in Table 1.
Managed Realignment options in the Upper Estuary
Managed realignment at any one site in the Upper Estuary in isolation would have a negligible effect on water speeds, water levels and erosion and this is therefore proposed for further consideration. When considering realignment in more than one site in the Upper Estuary care is needed in their selection, for example realignment at Pettistree and Shottisham, could have an adverse effect on estuary processes. This is because managed realignment of several large sites would result in a significant increase in the amount of water entering and leaving the estuary each tide, which would put pressure on the estuary mouth. The exact combination of Upper Estuary sites that are acceptable will be assessed during the next stage of the study.
Managed Realignment options in the Lower Estuary
The analysis of the impact of individual managed realignment sites in the Lower Estuary indicated the following: