Operation and Maintenance
- Results related to morphology
- Effects of rising water level on morphology
- Results related to ecology
- Effects of rising water level on ecology
Recent images show that the two westernmost banks have merged with the shore and only one remains offshore. In the meantime the first (peninsular) and second bank still emerge. At these locations vegetation succession takes place by lack of morphodynamics and a relative quiet environment. In the lee of the banks sedimentation takes place and shoreline vegetation and marshland have developed. The initial expectation of the project was that the sand bars at Mirnserklif would not have to be defended by any artificial structure. This is only partly true: the Mirnserklif project positively contributes to Natura2000 and EU WFD goals for the area, but this effect is gradually decaying and maintenance nourishments seem to be needed in order to restore the optimal situation.
1.Results related to morphology
The created sandbanks are influenced by erosion and sedimentation processes as a result of the natural dynamics in the area. They slowly move towards the coast and lose height. Most of the sand is deposited in the shallow zone landward of the banks. The first bank has merged with the coast, thus has become a peninsula. As the sheltering effect of the banks decreases, regional morphological processes become stronger and the system gradually returns to a stable state without banks, but with a sediment-rich foreshore and a more diverse habitat.
This shows that in the present situation (small-scale undefended sand nourishments) local dynamics and sediment transport lead to temporary sedimentation and a more diverse habitat. Mirnserklif is an example of how sacrificial shoreline nourishments can be successful in low-dynamic conditions in terms of waves and currents.
2. Effects of rising water level on morphology
As the mean water level of the IJsselmeer has been stable since the creation of the islands at Mirnserklif, the project provides no insight into the morphological effects of a slowly rising water level. It seems reasonable, however, to expect that – by lack of significant bank-building mechanisms – the nourished banks will get submerged, wave action will increase and the vulnerable shores and habitats will be less protected and bound to get submerged.
If the rate of lake level rise remains small, the rising water level it will prevent vegetation succession on the banks, which is actually desirable. When combined with a more natural water level variation, this can lead to the development of desired vegetation (Natura 2000 regulations define exactly which species must be protected). In that case, a more flexible management of the water level may well enable the Mirnserklif to grow along with a gradually rising water level in the IJsselmeer.
3. Results related to ecology
In the first year after construction, the sand banks contributed greatly to the natural values and flora and fauna diversity at Mirnserklif. Vegetation mapping carried out in 1996 at the Mirnserklif showed that there was a special vegetation composition, with amongst others Rumex maritimus and Chenopodium rubru . On the peninsula dry and wet pioneer vegetation, helophytes and different types of sedge vegetation developed. As the 3rd and 4th island got submerged, terrestrial vegetation disappeared here. In 1998, some very rare breeding pairs of the little gull (Larus minutus) were observed. Also a significant increase in numbers of black terns, common terns and gulls was observed. In relation to the EU Water Framework Directive there is a positive effect on macrophytes and fish species.
The current natural value in the area, however, is lower than the initially intended. Some banks are permanently submerged, some are attached to the mainland and possibly influenced by succession. The peninsula is less suitable for breeding birds as the whole area is now accessible by beasts of prey. In order to keep the Natura 2000 objectives (namely island with pioneer vegetation) maintenance nourishments would be needed.
4. Effects of rising water level on ecology
More water level variation leading to temporary flooding of the bank reduce succession. As a result, artificial maintenance of the vegetation is not necessary. needed as a result. Erosion and other morphodynamic processes, however, will influence and displace the banks. In case of permanent submergence the terrestrial habitat will disappear, like in the case of the third and the fourth bank. Higher mean water levels will reduce the occurrence of shore plants. An increase in wave activity onshore behind the isles may lead to erosion and regional leveling. Higher mean water levels are likely to shift the movement of the shore zone inland.