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SHORTLISTING THE BLYTH ESTUARY OPTIONS
In order to identify a short list of options from the long list of options shown in Table 1, we have undertaken ‘top down’ and ‘bottom up’ modelling analysis. ‘Top down’ analysis involves using historical patterns of estuary changes to predict what might happen in the future. ‘Bottom up’ analysis involves using a computer model built on the existing conditions in the estuary which is used to predict what will happen in the future under each option.
These modelling 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 pressure; and
  • water levels.

Options that allow us to manage the estuary processes will be taken forward as ‘Shortlisted Options’ for the Estuary. This comprises options that allow us to minimise the effect of some or all of the existing conditions, such as reducing water speeds, in the Estuary. Options that do not allow us to manage the estuary processes, for example by increasing erosion pressures, are unlikely to be considered further.
The effects of each option on estuary processes is contained within the Consultation Document, click here.

THE SHORTLISTED OPTIONS FOR THE BLYTH ESTUARY
The options proposed for shortlisting on the Blyth Estuary are discussed in turn below. A Figure is provided for each option. The image is computer-generated and provides a ‘birds eye’ view of the estuary. The area covered by low tide (the blue areas) in the figures shows the land that will be underwater at all times. The area covered by high tide (brown areas) in the figures shows the land that will be periodically under water, i.e. the intertidal area.
The figures do not show the extent of flooding that would occur under an extreme event; we will investigate this during the next stage of the study. The vertical scale of the images has been deliberately exaggerated so that the shape of the land can be seen more clearly; this does not affect the water level shown. The map is a snapshot in time, and does not show how the estuary would evolve under each option with time.

Option 1: No Active Intervention throughout the Estuary
This option would involve ceasing all maintenance, repair and renewal work on the defences throughout the Blyth Estuary. The defences would continue to be monitored and assessed until they eventually failed or a “Do Something” option was deemed necessary. Areas of land currently protected from tidal flooding would no longer be protected. Figure 1 shows the area over which high spring tide water levels would extend under this option.

Figure 1

Option 2: Hold the Line throughout the Estuary
This option would involve maintaining the existing flood defences on their current alignment throughout the estuary. The change that this option would have on the low and high tides is shown in Figure 2. The low and high tide levels under Option 2 are similar to the current low and high tide levels in the Estuary.

Figure 2

Option 3: No Active Intervention/Managed Realignment Upstream of the Harbour Mouth + Hold the Line of the Harbour Mouth
This option would involve undertaking work on the present harbour arms at the mouth of the estuary (excluding the present Harbour wall upstream). Elsewhere in the estuary No Active Intervention would be applied as in Option 1. The change that this option would have on the low and high tides is shown in Figure 3.

Figure 3

Option 4: No Active Intervention/Managed Realignment at Robinson Marshes + Hold the Line elsewhere
This option would involve an option of No Active Intervention at the stretch of defence that protects Robinson Marshes from flooding and maintaining the defences on their current alignment elsewhere in the estuary. The change that this option would have on the low and high tides is shown in Figure 4.

Figure 4

Options 5a and 5b: Advance the Line by Narrowing the Estuary or Making the Estuary Shallower at the Bailey Bridge + Hold the Line elsewhere
Options 5a and 5b would involve either narrowing the channel or raising the bed of the estuary at the Bailey Footbridge upstream of the present harbour moorings and maintaining the defences on their current alignment elsewhere in the estuary. The change that these options would have on the low and high tides is shown in Figure 5.

Figure 5

Options 6a and 6b: Advance the Line by Narrowing the Estuary or Making the Estuary Shallower at the Bailey Bridge + No Active Intervention/Managed Realignment at Tinkers Marshes + Hold the Line elsewhere
Options 6a and 6b would involve either narrowing the channel or raising the bed of the estuary at the Bailey Bridge in combination with either No Active Intervention or Managed Realignment at Tinkers Marshes and maintaining the defences on their current alignment elsewhere in the estuary. The change that these options would have is shown in Figure 6

Figure 6

NOTES:
Note 1: Do Minimum involves limited amount of maintenance, which will lead to a reduced standard of defence over time due to the effects of sea level rise. In the short term, this option will be the same as the Hold the Line option as it will involve keeping the defences on their current alignment. In the long term, the eventual deterioration of the defences will result in an impact similar to the No Active Intervention option. Do Minimum has not therefore been considered as a separate option for the purposes of the model.

Note 2: Managed Realignment and No Active Intervention are similar options in that they both involve allowing the existing defences to deteriorate (naturally or in a controlled way), with the consequent flooding of the land behind the defences. When looking at whether No Active Intervention or Managed Realignment is more suitable for a site, we need to consider factors such as the topography of the land (e.g. is there naturally rising ground behind the defences) and whether any properties would be flooded further away from the site without any intervention (e.g. a secondary defence). We will investigate the most appropriate solution at sites where these options are considered during the next stage of the strategy study.

Note 3: A barrier is a defence which can be raised and lowered to reduce the flood risk upstream of the defence. A barrier does not affect the day to day estuary use or tidal conditions it only reduces the effects of surge events.

Note 4: A barrage is a permanent defence structure within the estuary. The barrage stops the tide moving upstream of the structure. A barrage provides flood protection to all land upstream of the structure from tidal surges but not fluvial flows. The barrage does not provide any protection to land areas downstream of the barrage.