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New flood defences were created at the rear of the 54ha Welwick site to a minimum height of 6.1m
ODN and were designed to withstand a 1 in 50 year design event. A strip of saltmarsh was expected to develop in front of the new defences. The 70,000m3 of material needed for this defence was obtained from within the site from a combination of reprofiling and creation of temporary borrow pits. The new embankment was seeded and left to stabilise for one year.
The existing seawall was removed over a length of 1,400m, and the approximately 20,000m3 of the material gained was used to fill the temporary borrow pits. The wholesale removal, rather than the creation of solitary breaches, was chosen for a number of reasons:
(1) Removal of the rear of the embankment;
(2) Removal of the rock gabions; and
(3) Undertaking an overall lowering of the embankment.
Following this, breaches were created in the existing saltmarsh in front of site. These were required as the fronting marsh is designated, and could thus not be removed completely to increase wave energy even further. As the typical elevation of this marsh was 3.2mODN, which coincides with the MHWS level, these breaches were necessary to allow the site to flood and drain sufficiently. The location of the breaches was chosen to minimise marsh losses (approximately 0.4ha). Their width had been assessed by calculating the discharge and considering the critical threshold for erosion of sediment. The suggested breach size was considered large enough for the velocities to be below the critical threshold for erosion.
Status of the measure
This measure was breached in June2006.
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Welwick
Table of content
- 1. Measure description
- 1a. Description of the issue and measure
- 1b. Monitoring
- 1c. Monitoring results
- 2. Execution of main effectiveness criteria
- 2a. Effectiveness according to development targets of measure
- 2b. Impact on ecosystem services
- 2c. Degree of synergistic effects and conflicts according to uses
- 3. Additional evaluation criteria in view of EU environmental law
- 3a. Degree of synergistic effects and conflicts according to WFD aims
- 3b. Degree of synergistic effects according to Natura 2000 aims
- 4. Crux of the matter
Description of the issue and measure
Welwick was undertaken for the same purpose as another realignment on the Humber, Chowder Ness, which is presented as a separate First Analysis Step (FAS) Report. Both schemes were designed and implemented by the same organisations (Associated British Ports (ABP) and ABPmer), and to very similar timescales and principles. To inform the final design of these sites, numerical modelling was undertaken based on LiDAR elevation data. This was designed to ensure the correct balance of habitats would be achieved. As mudflat creation was the main objective of the schemes, and as the sites were largely too high for this to occur, the land was re-profiled to increase the extent of lower areas where mudflat could develop (i.e. below Mean High Water Neap (MHWN)) (see Figures 2 and 3 for an illustration of the design steps undertaken). Prior to these works, the land at Welwick had an approximate elevation of 2.8m Ordnance Datum Newlyn (ODN), some 0.4m below the level of the Mean High Water Spring (MHWS) tides. The reprofiling included the creation of a gentle slope from the fronting, existing, mudflats to the rear of the sites.New flood defences were created at the rear of the 54ha Welwick site to a minimum height of 6.1m
ODN and were designed to withstand a 1 in 50 year design event. A strip of saltmarsh was expected to develop in front of the new defences. The 70,000m3 of material needed for this defence was obtained from within the site from a combination of reprofiling and creation of temporary borrow pits. The new embankment was seeded and left to stabilise for one year.
The existing seawall was removed over a length of 1,400m, and the approximately 20,000m3 of the material gained was used to fill the temporary borrow pits. The wholesale removal, rather than the creation of solitary breaches, was chosen for a number of reasons:
- It improves connectivity with the wider estuary;
- It more closely recreates the type of environments that existed prior to the land claim;
- It enables the whole cross sectional area of estuary including the realignment site, to respond to estuary wide changes; and
- It increases energy levels within the site, thereby improving the likelihood that mudflat habitat will be maintained.
(1) Removal of the rear of the embankment;
(2) Removal of the rock gabions; and
(3) Undertaking an overall lowering of the embankment.
Following this, breaches were created in the existing saltmarsh in front of site. These were required as the fronting marsh is designated, and could thus not be removed completely to increase wave energy even further. As the typical elevation of this marsh was 3.2mODN, which coincides with the MHWS level, these breaches were necessary to allow the site to flood and drain sufficiently. The location of the breaches was chosen to minimise marsh losses (approximately 0.4ha). Their width had been assessed by calculating the discharge and considering the critical threshold for erosion of sediment. The suggested breach size was considered large enough for the velocities to be below the critical threshold for erosion.
Status of the measure
This measure was breached in June2006.