EVALUATION OF SALT MARSH RESTORATION BY MEANS OF SELF-REGULATING TIDAL GATE – AVON 2 ESTUARY

7 8 Salt marshes provide important regulating ecosystem services, including natural flood defence and 9 carbon sequestration, which adds value to restoration and biodiversity offsetting schemes. This study 10 evaluates the success of salt marsh restoration using a Regulated Tidal Exchange (RTE) system in SW 11 England, i.e. a self-regulating tidal gate (SRT), in controlling the partial saline inundation of a 14-ha 12 area of former salt marsh reclaimed for agriculture in 1760. A combination of (a) direct hydrodynamic 13 monitoring of water and sediment flux and (b) repeat surveys to evaluate morphological and 14 ecological (plants and foraminifera) changes over a 5-year period, was implemented immediately 15 following SRT commissioning. Morphological changes were limited to the proximity of the SRT system 16 due to limited sediment influx yielding sedimentation rates that were an order of magnitude below a 17 nearby natural marsh. Ecological change to an ephemeral salt marsh community was only detected 18 after 5 years of inundation cycles, with the delayed response attributed to (a) an initial limited tidal 19 inundation due to conservative SRT settings, followed by (b) excessive inundation due to excessive 20 rainfall and recurring SRT failure in an open position, and (c) a lack of sediment and propagule supply 21 caused by (a) & (b) and the relatively narrow inlet pipe used in the SRT system. While the ecological 22 response under optimum SRT settings was encouraging, the lack of perennial plants and limited 23 foraminifera abundance demonstrated that the marsh was far from reaching natural status. We 24 surmise that this is primarily due to inundation being more rapid than drainage leading to excessive 25 submergence during a tidal cycle. Our study shows that the design of tidal inundation schemes 26 requires a synergistic understanding of core ecological and geomorphological approaches to assess 27 viability and success. We conclude that SRT can be a useful technique for intertidal habitat creation 28 where there are significant site constraints (especially flood risk), but we need to be realistic in our 29 expectations of what it can achieve in terms of delivering a perennial salt marsh community. 30 31

[1]  Gemma L. Harvey,et al.  Understanding system disturbance and ecosystem services in restored saltmarshes: Integrating physical and biogeochemical processes , 2012 .

[2]  J. S. Rodwell,et al.  British Plant Communities: Volume 5, Maritime Communities and Vegetation of Open Habitats , 2000 .

[3]  A. Grant,et al.  Does managed coastal realignment create saltmarshes with ‘equivalent biological characteristics’ to natural reference sites? , 2012 .

[4]  Anthony J. Davy,et al.  Drainage and Elevation as Factors in the Restoration of Salt Marsh in Britain , 2002 .

[5]  Matthias Kudella,et al.  Wave attenuation over coastal salt marshes under storm surge conditions , 2014 .

[6]  P. Bernatchez,et al.  Sediment transport in a cold climate salt marsh (St. Lawrence Estuary, Canada), the importance of vegetation and waves , 2012 .

[7]  D. Charman,et al.  Foraminifera, testate amoebae and diatoms as sea‐level indicators in UK saltmarshes: a quantitative multiproxy approach , 2001 .

[8]  D. Walling,et al.  Floodplain Sedimentation: A Preliminary Investigation of Contemporary Deposition within the Lower Reaches of the River Culm, Devon, UK , 1987 .

[9]  S. Stubbles.  Responses of recent benthic foraminifera to metal pollution in south west England estuaries : a study of impact and change , 1999 .

[10]  J. Bakker,et al.  What factors determined restoration success of a salt marsh ten years after de-embankment? , 2016 .

[11]  R. Morris.  Managed realignment: A sediment management perspective , 2012 .

[12]  J. Bakker,et al.  Salt-marsh restoration: evaluating the success of de-embankments in north-west Europe , 2005 .

[13]  P. W. French.  Managed realignment The developing story of a comparatively new approach to soft engineering , 2006 .

[14]  E. Barbier,et al.  The present and future role of coastal wetland vegetation in protecting shorelines: answering recent challenges to the paradigm , 2011 .

[15]  A. Grant,et al.  Constraints on Salt Marsh Development Following Managed Coastal Realignment: Dispersal Limitation or Environmental Tolerance? , 2012 .

[16]  D. Reed,et al.  Effect of hydrologic management on marsh surface sediment deposition in coastal Louisiana , 1997 .

[17]  D. Charman,et al.  AN INTERTIDAL FORAMINIFERA-BASED TRANSFER FUNCTION FOR RECONSTRUCTING HOLOCENE SEA-LEVEL CHANGE IN SOUTHWEST ENGLAND , 2006 .

[18]  M. Hudson,et al.  Intertidal mudflat and saltmarsh conservation and sustainable use in the UK: a review. , 2013, Journal of environmental management.

[19]  R. Wilby,et al.  Stormiest winter on record for Ireland and UK , 2014 .

[20]  D. A. Ratcliffe,et al.  British plant communities. Volume 3. Grasslands and montane communities. , 1994 .

[21]  Anthony J. Davy,et al.  Colonization of a newly developing salt marsh: disentangling independent effects of elevation and redox potential on halophytes , 2011 .