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Polders and 'Warping' - the Schleswig - Holstein Method
This is a process much used in Holland and Germany to create saltmarsh on tidal
flats. It involves the construction of 'sediment fields' (polders)
designed to trap sediment. Initially simple earth dykes were used to promote
natural colonisation. This approach was enhanced by digging trenches ('gripping')
the area, piling the excavated mud between the trenches. This helped to enhance
plant colonisation on the raised areas and trap sediment in the lower excavated
areas. A system of squares was developed using brushwood groynes to enclose
the earth embankments. Within these structures sediment deposition was also
encouraged by planting Spartina and other
saltmarsh plants. Examples can be found in the vicinity of Vlissingen in south
west Holland (Beeftink 1977 page 106) and
extensively in the Wadden Sea.
The technique evolved into what has become known as the Schleswig Holstein
Method. In this method sediment fields up to 400m square are place on mudflats
and enclosed with brushwood fences. Ditches are dug either side of the fences
and material used to strengthen them. The sediment fields themselves are 'gripped'
to enhance accretion by retaining 'warp' (the accreted sediment) within the
enclosure. Even when successful it takes thirty to forty years for saltmarsh
to become fully established (Wagret 1968).
Generally this method has been more or less abandoned. Labour costs and uncertainties
surrounding the long term viability of such schemes (especially where sediment
availability is restricted) are two of the factors influencing the decline in
this approach.
Comment: The technique has been applied at several sites in Essex,
such as at Cudmore
Grove, and in conjunction with sunken barges along the Dengie
coastline. An analysis of the results of these trials suggests that they have
had mixed success (Holder & Burd 1990).
Since these trials took place similar approaches have been very limited and
more effort is being given to managed
realignment schemes.
References
Beeftink, W.G.
1977.
Salt-marshes.
In: The Coastline, ed.Barnes, R.S.K.John Wiley & Sons, Chichester, 93-122.
Holder, C.L. & Burd, F.
1990.
Overview of Saltmarsh Restoration Sites in Essex: an Interim Report.
Nature Conservancy Council.
Wagret, P.
1968.
Polderlands. Methuen and Co., London.
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