|Met het tij mee: over de ontwikkelingen in het Sieperdaschor|
Stikvoort, E. (2000). Met het tij mee: over de ontwikkelingen in het Sieperdaschor. Rapport RIKZ = Report RIKZ, 2000(46). Rijkswaterstaat. Rijksinstituut voor Kust en Zee: Middelburg. 50 pp.
Part of: Rapport RIKZ = Report RIKZ. Rijksinstituut voor Kust en Zee (RIKZ): s-Gravenhage. ISSN 0927-3980, more
A severe storm in late February 1990 caused a breach of the summer dike of the Selenapolder, an elongated and narrow former salt marsh along the Scheldt estuary. The damage to the dike was so extensive, that repairing became too costly. It was decided to allow a permanent return of tidal influence into the area, restoring the polder into a brackish salt marsh. Researchers at the National Institute for Coastal and Marine Management (RIKZ) were aware of the research opportunities of this ‘natural experiment’. In 1994 - the Selenapolder was meanwhile renamed Sieperdaschor - a monitoring programme started, commissioned to RIKZ by the estuary management authority. The ‘Monitoringplan Sieperdaschor’ was established to monitor the physical, chemical and biological developments. At the time the establishment of natural flooding areas by managed-retreat was seen as a contribution to fight rising water levels, as a result of a continuous dredging of shipping channels and a gradual sealevel rise. Managed-retreat, preferably in the brackish or freshwater part of the estuary, could create extra channel detention. The Sieperdaschor is situated in the brackish zone of the estuary. Some years later a new argument for managed-retreat surfaced. The second deepening and widening of the shipping channel in the Western Scheldt was near. Due to new legislation negative effects of this measure should be compensated. Managed retreat formed a major initiative of the proposed compensating measures (1996). At present managed retreat is considered as one of the possible solutions in discussions between the Dutch and Flemish authorities about a sustainable use of the Scheldt estuary. The Sieperdaschor lessons will be useful for future management planning. Ten years of monitoring has demonstrated that the former polder quickly obtained the appearance of a salt marsh. However, some distinct zones reflecting differences in former land use can still be observed in the marsh: the grazed western part, the former crop fields (the major part of the eastern half) and the never levelled far eastern part that is partly grazed. Due to the elongated and narrow shape of the area the tide rarely penetrates the entire marsh. The western half, which is furthest away from the river, does not have a natural drainage system. Creeks are absent. The former crop fields in the central part of the Sieperdaschor changed initially into a mudflat. The repositioning of the main gully in 1993 enhanced the development of a creek system in the eastern part. From that moment the vegetation expanded explosively on the mudflats. Since 1999 there is hardly any uncovered area left. Instead there is a mosaic of reed marsh and high Scirpus maritimus and Aster tripolium. The organisms of the area are typical for brackish saltmarshes. Cattle grazing leads to a low vegetation of Puccinelia maritime. The population of birds changed as a result of the wetter conditions. In the grazed parts, meadow- and shorebirds found new opportunities to breed, to forage or to rest. When the mudflats became covered with lush vegetation, the population of birds changed. Exposed mudflats disappeared, which led to a minimisation in numbers of birds that prey upon little bottom-dwelling fauna. The lush vegetation and reed marshes offered food for grazing and seed eating birds and a breeding site for marsh birds. The easternmost part of the Sieperdaschor always had the character of a marshland and therefore has not changed much. The vegetation and bird population of the grazed parts in this section of the marsh are very comparable to those in the western part. The ungrazed parts show a vegetation and bird population that is comparable to that of the former fields. A comparative study of the experiences with managed retreat showed that Sieperdaschor is one of the largest marshes restored by managed retreat and has the added advantage of being situated in the brackish part of an estuary. Most other examples were located in the marine zone. Furthermore, this area was a polder for a relatively short time (24 years). That fact has probably been beneficial to the rapid restoration of marsh characteristics in the former polder. The extremely elongated and narrow shape of the marsh being orientated almost perpendicular to the river is quite a distinct feature. This explains the situation in the westernmost part, with no erosion and hardly any sedimentation. For the development of marsh vegetation the elevation of the soil was favourable, although the drainage pattern of the creeks will not get the branching appearance of a natural marsh. Though not unique, the bottom dwelling fauna is remarkably abundant, both in density and diversity. In other restored marshes this fauna recovered with difficulty, probably as a result of the changed chemical characteristics of the soil during the period the area served as a polder. If the management of the marsh remains unchanged, it will develop as follows. The grazed, western part will by and large keep its character: an open landscape with low vegetation of Puccinellia maritime and pioneer species. The low lying parts will be swampy due to trampling by cattle. The ponds and trenches will be getting shallower until they are filled up and fully covered by vegetation. The eastern part with its high vegetation will expand and develop further into one large marsh, dominated by Phragmites australis. Marsh birds will thrive in this environment, but species diversity will decrease. In the easternmost part of the marsh the grazed and ungrazed parts will face a similar development. Summarising, Sieperdaschor will get more uniform and will lose some biodiversity. Regarding long-term management objectives for the Scheldt Estuary managed-retreat seems a possible option. The spontaneous Sieperdaschor marsh restoration project can be seen as a successful and valuable experiment. It is therefore recommended to continue the monitoring programme, perhaps in a more concise form.
- Detailed bathymetric grid Selenapolder 1994, more