SITE DISCRIPTION - The Zwin was founded as a nature reserve in 1952 by Count Léon Loppens. Today the Zwin is a try combination of a visitor park and a nature reserve. Sustainability is a top priority for the Zwin. This key features is also visible within the implementations of SCAPE. As one of the largest pilots in SCAPE, a pumping station as well as a reservoir (pumping buffer) will be installed. Furthermore, the pilot focuses mainly on enhancing biodiversity, renovation of historical structures and reconversion of the slopes.
OVERVIEW OF THE WORKS:
An overview of the works and timeline can be found here: http://zwininverandering.eu/evolutie/
INSTALLATION OF A PUMPING STATION - The Zwin project involves installing a pumping station which must carry fresh water to the freshwater canal and divert water to the Zwin area in the event of heavy rainfall. To ensure sufficient water supply to the pumping station, modifications must be made to watercourses in the Zwin polders. On the one hand, this means constructing a reservoir at the pumping station (the pumping buffer). On the other hand, watercourses that supply the water, namely the Nieuwe Watergang and Isabellevaart, as well as several pipelines, must be profiled so enough water is supplied to the pumping station and the pumping buffer.
WATER RESERVOIR AS PUMPING BUFFER - A water reservoir will be constructed upstream from the station so the pumping station does not have to be continuously switched on and off. Such reservoirs are referred to as pumping buffers. To this end, a section of the area between the new station and Retranchement street will be excavated. The pumping buffer will have a bed surface of approx. 16 ha and be constructed with a slope gradient of 12/4, which will be sown and planted with reed fringes. This will be done by excavating approximately 70,000 m³ of soil.
Due to geographical constraints and expropriation options, only the left-hand bank (north-west side) will be widened. This means banks on the right-hand bank and connections to existing wet areas in the south will be spared, while the existing farm track will also be retained. The width of the watercourse bed will be 8 m across the whole length and the slope on the left-hand bank will be 12/4.
The new crossing under Retranchement street will be located south of the existing crossing so the downstream connection with the pumping buffer will be located entirely within the available area. This will allow the new crossing to be constructed while the old one remains in service. After diverting the stream, the remaining portion of the stream will be backfilled to ground level and compacted. The new crossing will consist of a pipeline which is 4 m wide, 2 m high and 12 m long.
HAZEGRAS FORTRESS - The current watercourse in the Hazegras fort is highly conditioned by two existing structures that enclose the area, namely the Burkeldijk overpass on the south-west side and the Hazegras lock on the north-east side (Zwin side). Bed width and bed level strongly determine the drainage capacity of the watercourse, whose flow direction will be reversed in the direction of the Zwin, even though the existing bed level slopes downwards towards the Leopold Canal.
Within Hazegras fort, the Isabellavaart will thus be deepened to a bed level of 0.8 m TAW. Bed width will remain 2 m so it is perfectly compatible with both existing structures, and the slopes will be reduced to 10/4 on both sides in order to obtain symmetry that best resembles the existing situation.
Within existing overpasses, the existing wooden floor structure will be partly removed without removing structural elements from or underneath the existing construction. The existing floor will be removed over a limited depth to allow the new floor to be constructed at 0.8 m TAW.
- Modification of the profile to restore its historical width: Before the Leopold Canal was constructed in the middle of the 19th century, water from the eastern polders on the Flemish coast drained into the Zwin area. The sources were thus located further inland and the estuary was located at the Zwin area. After the Leopold Canal was constructed, the flow direction was reversed and the source moved to the Zwin area. However, the watercourse at this site was much wider because it used to be the estuary. By constructing a pumping station, it will be possible to allow the water to resume its historical flow direction. However, the Zwin area has become silted, which means the most upward section is now much narrower. Widening this section will restore the landscape by giving it a significantly wider "estuary watercourse", which is characteristic of the historical creek area.
- Construction of gentle slopes: the flat polder landscape calls for gentle slopes, unlike hilly areas, which generally feature steep slopes. Gentle slopes make it easier for the watercourse to adapt to silting and erosion.
- The pumping reservoir will be constructed in the lowest-lying area and will be in keeping with the landscape. This will also offer a financial benefit because less soil will be excavated.
- Construction of an elongated reservoir rather than a circular one. This will be more in keeping with the typical appearance of the creek area, with its wide and expansive watercourses (see also above figure).
- Prevent 'well effect': The reservoir will be excavated deeper than the groundwater table. This means there will always be water in the reservoir to prevent a "well effect". This will also reinforce the image of a wide and expansive creek.
- Planting of reed areas and pools. Slopes will only be reinforced near the intended pipelines. All other slopes will only be planted with reed rhizomes from local reed beds. Vegetation from the landscape will thus be used to strengthen the banks. Areas for nature development have been planned on both banks of the pumping buffer. On the left-hand bank, 2 pools will be excavated and shrubbery will be planted. On the right-hand bank, the area will be slightly excavated in order to construct a reed marsh. This area will be connected to the pumping buffer via a pipe that runs under the maintenance strip. The crossing will be equipped with an adjustable valve to control water supply to the reed marsh.
- Renovation of historic structures. A polder is an area that has been reclaimed from the sea and is thus essentially man made. The project includes renovating and aesthetically reinforcing historical structures like the Burkeldijk and the Hazegras lock.