Wash Banks Flood Defence Scheme Freiston Environmental Monitoring 2002-2006
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Wash Banks Flood Defence Scheme Freiston Environmental Monitoring 2002-2006 NERC Centre for Ecology and Hydrology (CEH) S.L.Brown1*, A.Pinder1, L.Scott1, J.Bass1, E.Rispin1, S.Brown1, A.Garbutt2, A.Thomson2, T.Spencer3, I.Moller3, S.M. Brooks 4 Report to: The Environment Agency, Peterborough May 2007 Project Leader and Contact: Dr Sue Brown, CEH Dorset, Winfrith Technology Centre, Dorchester, Dorset DT2 8ZD * Current address: c/o FBA, East Stoke, Wareham, Dorset BH20 6BB Tel: 01929 401888 Email: [email protected] CEH Contract No. C01821 1. CEH Dorset, Winfrith Technology Centre, Winfrith Newburgh, Dorchester, Dorset DT2 8ZD 2. CEH Monks Wood, Abbots Ripton, Huntingdon PE28 2LS 3. CCRU, Department of Geography, University of Cambridge, Downing Place, Cambridge CB2 3EN 4. School of Geography, Birkbeck College, University of London, Malet Street, London WC1E 7HX Key Words: Salt marsh, Managed Realignment, Salt marsh creation, Freiston Shore, Accretion, Vegetation, Invertebrates, Fish Acknowlegements: The programme of monitoring was funded jointly by DEFRA and the Environment Agency, and also the NERC CEH Science Budget. We would like to thank the following people for their assistance: Mr Mike Lewis, Governor of Her Majesty’s Prisons (HMP) at North Sea Camp open prison and Mr E. Hewitt, Head of Works Services, for arranging assistance with the installation of accretion plates and marker posts in the realignment site; Mr Roger Appleby (foreman) and an anonymous inmate for helping Dr Sue Brown to install the plates in Spring 2002. Thank you to C. Boffey and D. Leach (CEH Dorset) for helping with some of the field work in September 2005, and D. Freiss (CCRU) for helping A. Thomson with the ground reference data surveys in 2005 and 2006. Particular thanks also go to Dr Susan Brown (the other one!) for stepping in to help with the data presentation and writing of Chapter 6 on invertebrate colonisation. We would also like to thank John Badley of the RSPB for interesting and useful discussions on site and the Freiston Steering group members for discussions during our annual meetings. INTELLECTUAL PROPERTY RIGHTS CONFIDENTIALITY STATEMENT ‘In accordance with our normal practice, this report is for the use only of the party to whom it is addressed, and no responsibility is accepted to any third party for the whole or any part of its contents. Neither the whole nor any part of this report or any reference thereto may be included in any published document, circular or statement, nor published or referred to in any way without our written approval of the form and context in which it may appear.’ EXECUTIVE SUMMARY This report describes the results of four years of monitoring the development of the Freiston managed realignment site since it was breached in August 2002. Comparisons were made with the adjacent salt marsh to assess the progress of the realignment site and to ensure that there were no unexpected adverse effects on the existing salt marsh. The monitoring programme comprised measurements of sedimentation (accretion/erosion); vegetation colonisation, establishment and succession; invertebrate colonisation; and fish utilisation of the realignment site. Accretion/Erosion measurements All vegetated salt marsh sites outside the realignment have shown continued accretion during the monitoring period, ranging between 7mm and 100mm in four years between September 2002 and 2006, depending upon their position on the marsh. The lower pioneer marsh-mudflat transition zone showed fluctuating sediment levels between years, typical of this dynamic area. All sites inside the realignment have accreted sediment since the initial baseline measurements, with 25 out of the 30 sites set up initially building up between 5.5mm and 56mm over four years, depending upon their positions along the transects. Five sites nearest to the central breach showed anomalously high levels of sedimentation (up to 198mm), thought to be due to washed in material from the eroding breach and widening creeks inside the site close to the breach. The elevations of sites inside the realignment were measured in 2005 and ranged from approximately 2.76mODN to 3.26mODN. Sites outside cover a greater elevation range, from 2.09mODN to 3.29mODN. Elevation is of key importance for the amount of accretion on vegetated salt marsh, so to compare summary results for accretion inside and outside the realignment, the sites outside were divided into two elevation categories: >2.75-3.3mODN (equivalent to the realignment site) and <2.75mODN. Mean total accretion inside the realignment site was greatest in the first year (15mm, 2002- 3) and has been lower since (9.3mm, 2003-4; 7.7mm, 2004-5; 8.6mm 2005-6). The few exceptionally high accretion sites had a major influence on mean values, particularly during the first year. When these sites were excluded from the calculations the mean total accretion over the 4 years was 7.9mm, 5.4mm, 5.8mm and 7.3mm, respectively, and closer to values from outside the realignment at the same elevation range: 9.0mm, 7.4mm, 5.8mm and 7.7mm, respectively (also highest in the first year). Mean annual rates of accretion were calculated for all the monitoring sites inside and outside the realignment on the vegetated marsh at the equivalent range of elevations (>2.75-3.3mODN). Rates were calculated from data up to 2005 because the markers for two of the upper salt marsh sites were lost to cattle damage in 2006. There was a highly significant inverse relationship (as expected) between accretion rates and elevation outside the realignment, and also inside the site once the major outliers (high accretion sites near the middle breach) were removed. The data overlapped between approximate elevations of i 2.95-3.2mODN, but at lower elevations accretion was higher in the more sheltered conditions of the realignment site than on the more exposed salt marsh outside. The mean annual accretion rate for all sites inside the managed realignment was 10.6mm. This value was reduced to 6.7mm when the outliers were excluded, and was closer to the mean annual accretion rate of 7.3mm outside on the salt marsh at the same elevation range. In the lower portion of the elevation range (2.75-3.0mODN), the mean annual accretion rate was 14.7mm (12.2mm excluding the outliers) inside the realignment, and 9.4mm outside on the salt marsh in this range. At the higher end of the elevation range (above 3mODN) the mean annual accretion rate inside the realignment was reduced substantially from 8.5mm to 4.5mm when the outliers were excluded, and was close to the mean rate outside (5.0mm) at these elevations. The sample median is a useful representative of the data set for sites in the realignment as it is not affected by the high values for the sites close to the central breach. The median annual accretion rate for inside the site overall (elevation range 2.75-3.3mODN) was 7.42mm, close to the median annual accretion rate of 7.33mm outside the site at equivalent elevations. A good agreement was also found for the upper range (3.0-3.3mODN) inside the site (4.23mm) compared with outside (4.87mm), but the median annual accretion rate was still higher inside the sheltered realignment site (13.6mm) than outside on the salt marsh (8.9mm) in the lower portion of the elevation range (2.75-3.0mODN). On average, winter accretion was found to be higher than summer periods both inside and outside the realignment site. Several (pre- and post-depositional) factors may contribute to this including lower levels of suspended sediment in summer, post-depositional dewatering and compaction, and erosion (lifting) of dry cracked surface layers in summer. In conclusion, the data indicated that accretion has been higher inside the realignment at the lowest elevations compared with outside on the salt marsh, but the overall similarities between median annual accretion rates inside and outside the realignment over most of the equivalent elevations, and between the mean and range of total accretion values when the anomalously high sedimentation sites were excluded, indicated that accretion has been occurring at expected levels on the newly created salt marsh. Surface elevation change measurements (SET technique) Measurements of surface elevation change using the Sedimentation Erosion table (SET) technique were made at Freiston Shore between November 2002 and September 2006. Strong spatial and temporal controls on surface elevation change were apparent, both outside and inside the managed realignment site. Patterns of elevation gain and loss were highly dynamic on unvegetated mudflat surfaces outside the managed realignment site. Permanently vegetated salt marsh surfaces outside the site showed long-term (November 2002 – September 2006) mean elevation gains of 4.5-7.8mm at sites north and south of the managed realignment (‘far field’) and 23.6- 24.4mm at sites fronting the site and between the major channels draining the breaches (‘near field’). At the near-field sites the progressive gains in surface elevation seen until June 2004 have been replaced by patterns of seasonal variation in surface elevation change similar to those seen at the far-field sites. ii Sites within the managed realignment site close to breaches showed long-term (November 2002 – September 2006) surface elevation gains of 30–101mm. By comparison, more isolated sites to the rear of the managed realignment site have shown elevation gains (November 2002 – September 2006) of 6.9– 8.9mm. High gains (33.4mm), however, have characterised one internal site close to the head of an excavated channel, suggesting the importance of artificial creek networks in supplying sediments to breach-distant internal locations relatively high in the tidal frame. A site close to, and north, of the central breach and a site in the northwest corner of the site have both shown minimal surface elevation change since April 2005, suggesting that sediment supply to the northern half of the site may have changed adversely since this time.