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The authors are solely responsible for the content of this report. Material included herein does not represent the opinion of the European Community, and the European Community is not responsible for any use that might be made of it.
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In summary, additional new habitat to compensate for losses and address rising sea levels has been created at Paull Holme Strays. Vegetation cover has been increasing inside the realignment site each year and some common species outside that were initially slow to establish inside the site are now thriving at their appropriate elevations. Both intertidal salt marsh and mudflat have developed on the site, but the expected outcome in terms of area of compensatory salt marsh and mudflat habitat is proving to be different from that anticipated before the breach.
Overall, the monitoring has found an accretionary trend over the survey period between 2004 and 2010 resulting in a trend of increase in elevation. The change in elevation in subsequent years has decreased, although increases of up to 40cm have still been observed at some locations across the site between the 2009 and 2011 surveys. Over the survey period, the majority of change in elevation (accretion) has occurred in lowest (seaward) parts of the site, which were around 1.5 to 1.75mODN in 2007 - with changes in elevation of the order of 10-40cm.
Saltmarsh vegetation has generally established in areas above 2.5 to 3mODN (2.5mODN is approximately 50cm above the MHWN level). Most of the mudflat areas below 2mODN remain unvegetated.
The high rate of accretion is thought to be due to the degree of shelter and the upper shore location of realignment sites and has been recorded from similarly sheltered areas in the middle region of the Humber (Mazik, 2004; Boyes & Allen, 2007). This is likely to be due to lower wave energy and possibly longer residence time of water, due to protection from the remaining sea wall. However this has not been formally tested.
Based on the findings at this site, sites with an initial elevation of >2.2-2.4 m and sites where accretion is likely to lead to an increase in elevation above 2.2-2.4 m, are unlikely to develop into compensatory habitat for mudflat in the long term. However, there is a strong likelihood that marsh may develop suggesting that compensation for loss of this habitat could be highly effective.
Furthermore, sites should be designed to limit accretion rates (e.g. exposing sites to tidal exchange and wave action by removing the maximum length of sea wall possible) in the long term to prevent large increases in elevation and subsequent colonisation by saltmarsh. Similarly, these considerations are necessary if saltmarsh creation is the primary objective. High (>2.2-2.4 m in the context of this site) elevation sites in the Humber are unlikely to provide suitable direct compensatory habitat (e.g. for mudflat) in the long term, and instead will provide additional saltmarsh.
This raises questions about the possibility of creating sustainable mudflat in areas like the Humber estuary with high suspended sediment loads, and removal of the entire sea defences may be necessary in front of a realigment to more closely reproduce the exposure conditions on natural mudflats. Full embankment removal would not be a viable option at Paull Holme Strays as the gradient of the site between the breaches slopes downwards in a landwards direction from behind the sea defences which are fronted by a higher cliffed salt marsh. Attempting to create mudflat behind higher areas of salt marsh is unlikely to succeed in the long term. Furthermore, on the north side of the middle estuary a wide expanse of mudflat is now at an elevation suitable for Spartina colonisation and plants are spreading across the area. It may therefore be necessary to view any creation of compensatory mudflat, in this part of the estuary at least, as a temporary solution which may persist for a decade or two, depending on site-specific conditions.
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Paull Holme Strays Managed Realignment
Table of content
- 1. Measure description
- 1a. Measure description
- 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
- 5. Literature
Crux of the matter
The “crux of the matter“ refers to the basic, central or critical point of an issue. For example, in this context, the main issues relating to the development and progression of the specific measure detailed within this FAS Repost represent the crux of the matter.In summary, additional new habitat to compensate for losses and address rising sea levels has been created at Paull Holme Strays. Vegetation cover has been increasing inside the realignment site each year and some common species outside that were initially slow to establish inside the site are now thriving at their appropriate elevations. Both intertidal salt marsh and mudflat have developed on the site, but the expected outcome in terms of area of compensatory salt marsh and mudflat habitat is proving to be different from that anticipated before the breach.
Overall, the monitoring has found an accretionary trend over the survey period between 2004 and 2010 resulting in a trend of increase in elevation. The change in elevation in subsequent years has decreased, although increases of up to 40cm have still been observed at some locations across the site between the 2009 and 2011 surveys. Over the survey period, the majority of change in elevation (accretion) has occurred in lowest (seaward) parts of the site, which were around 1.5 to 1.75mODN in 2007 - with changes in elevation of the order of 10-40cm.
Saltmarsh vegetation has generally established in areas above 2.5 to 3mODN (2.5mODN is approximately 50cm above the MHWN level). Most of the mudflat areas below 2mODN remain unvegetated.
The high rate of accretion is thought to be due to the degree of shelter and the upper shore location of realignment sites and has been recorded from similarly sheltered areas in the middle region of the Humber (Mazik, 2004; Boyes & Allen, 2007). This is likely to be due to lower wave energy and possibly longer residence time of water, due to protection from the remaining sea wall. However this has not been formally tested.
Based on the findings at this site, sites with an initial elevation of >2.2-2.4 m and sites where accretion is likely to lead to an increase in elevation above 2.2-2.4 m, are unlikely to develop into compensatory habitat for mudflat in the long term. However, there is a strong likelihood that marsh may develop suggesting that compensation for loss of this habitat could be highly effective.
Furthermore, sites should be designed to limit accretion rates (e.g. exposing sites to tidal exchange and wave action by removing the maximum length of sea wall possible) in the long term to prevent large increases in elevation and subsequent colonisation by saltmarsh. Similarly, these considerations are necessary if saltmarsh creation is the primary objective. High (>2.2-2.4 m in the context of this site) elevation sites in the Humber are unlikely to provide suitable direct compensatory habitat (e.g. for mudflat) in the long term, and instead will provide additional saltmarsh.
This raises questions about the possibility of creating sustainable mudflat in areas like the Humber estuary with high suspended sediment loads, and removal of the entire sea defences may be necessary in front of a realigment to more closely reproduce the exposure conditions on natural mudflats. Full embankment removal would not be a viable option at Paull Holme Strays as the gradient of the site between the breaches slopes downwards in a landwards direction from behind the sea defences which are fronted by a higher cliffed salt marsh. Attempting to create mudflat behind higher areas of salt marsh is unlikely to succeed in the long term. Furthermore, on the north side of the middle estuary a wide expanse of mudflat is now at an elevation suitable for Spartina colonisation and plants are spreading across the area. It may therefore be necessary to view any creation of compensatory mudflat, in this part of the estuary at least, as a temporary solution which may persist for a decade or two, depending on site-specific conditions.