Deben Options Shortlist

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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.

Top Down Modelling – using historical patterns of estuary change to predict future change; and
Bottom Up Modelling – using the computer model built to simulate the way the estuary behaves at the moment to predict the impact of future management changes.

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 (velocity) and, where applicable, the extra amount of water that would enter and leave the estuary each tide (tidal prism);
erosion; and
water levels.

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 DEBEN ESTUARY
PROPOSED SHORTLIST OPTIONS

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.

No Active Intervention – the government requires us to consider these options as a baseline against which other options must be compared.
Hold the Line – this option presents the existing situation and therefore provides a baseline against which other options can be compared. It was also stated as the preference during the previous rounds of consultation.

(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:

Managed realignment of Kirton Creek Marshes will not have any significant effect on the estuary processes so it is proposed as a shortlisted option.
Realigning over the large area of Falkenham South and Falkenham North Marshes would result in a significant increase in the amount of water entering and leaving the estuary, with associated increase in erosion pressure at the mouth. As such, managed realignment across the whole of these two sites is not proposed. However, managed realignment over a smaller area would allow estuary processes to be managed and is therefore proposed as a shortlisted option. As there is no natural boundary between the two marshes it would be necessary to construct an artificial boundary.
Due to the importance of maintaining the existing estuary mouth, managed realignment over the whole of Felixstowe Marshes is not proposed for shortlisting. However, partial realignment of this site would allow us to manage estuary processes in the upper section of the estuary.

Managed Realignment options in the Upper and Lower Estuary
The effect of managed realignment of one or more of these sites in the Lower Estuary with one or more sites in the Upper Estuary will depend on the overall size and combination of sites selected. To ensure that there is no adverse effect on the mouth of the estuary, the balance of sites proposed needs to ensure that the amount of water entering and leaving the estuary does not increase beyond the capacity of the existing mouth. The effects of individual combinations are discussed in Table 1.

Barrier
Although this option has been shown to have no impact on day-to-day estuary processes, it could have a positive impact during surge events to people and property upstream of the barrier location. A barrier located near the mouth, or at a point upstream towards Ramsholt, is therefore proposed for shortlisting. Due to the highly dynamic nature of the sediments in the estuary mouth around the Knolls, it is not proposed that a barrier is located here.

PROPOSED SHORTLIST OPTIONS
Table 2 - Proposed shortlisted options for the Deben Estuary

Shortlist Options			Indicative Cost
No Active Intervention Options	OPTION A: No Active Intervention throughout the whole Estuary + maintaining existing defences at mouth		LOW
Hold the Line Options	OPTION C: Hold the Line throughout the whole Estuary		HIGH
Managed Realignment Options	OPTION D: Managed Realignment at one or more sites in the Upper Estuary + Hold the Line in the Lower Estuary:		LOW to MEDIUM depending on number of Managed Realignment sites.
	• Ramsholt Marsh • Ramsholt Lodge Marsh • Shottisham Marsh • Pettistree Marsh • Waldringfield South • Waldringfield North	• Kyson Point Marsh • Martlesham Marsh • Methersgate Marsh • Little Sutton Hoo Marsh • Melton Marshes
	OPTION E: Managed Realignment at one or more sites in the Lower Estuary + Hold the Line in the Upper Estuary: • Falkenham North Marshes • Falkenham South Marshes • Kirton Creek Marshes • Partial Felixstowe Marshes		LOW to MEDIUM depending on number of Managed Realignment sites.
	OPTION G: Managed Realignment at one or more sites in the Upper Estuary + partial Managed Realignment at Felixstowe Marshes + Hold the Line Elsewhere		LOW to MEDIUM depending on number of Managed Realignment sites.
	OPTION I: Managed Realignment at one or more sites in the Upper Estuary + Managed realignment at Falkenham North Marshes + Hold the Line Elsewhere		LOW to MEDIUM depending on number of Managed Realignment sites.
	OPTION K: Managed Realignment at one or more sites in the Upper Estuary + Managed Realignment at Kirton Creek Marshes + Hold the Line Elsewhere		LOW to MEDIUM depending on number of Managed Realignment sites.
Advance the Line Options	OPTION L: Barrier at Estuary mouth or upstream towards Ramsholt		HIGH

POTENTIAL IMPLICATIONS OF SELECTED OPTIONS