Many shallow lakes suffer from accumulation of fine sediment. This sediment build-up is caused by different factors, like shoreline erosion, peat degradation, internal organic production and external inflow or run-off. In order to create lee area’s and improve water quality and clarity, Tauw has developed the ‘Sediment Storer’ and ‘Sediment Settler’. If installed properly, both innovations have proven to be useful in restoring the ecological balance of shallow lakes.
In shallow lakes fine sediment is easily kept in suspension, thereby inhibiting growth of higher order aquatic plants and natural water quality improvement. In the largest protected peaty grassland of Western Europe, the Wormer- and Jisperveld area (The Netherlands), innovative and sustainable measures have been undertaken to reduce hydrodynamics, sediment resuspension and turbidity in order to remediate contaminated sediment, enhance biodiversity and improve water quality.
Tauw has developed (and built) two types of light weight geotextile constructions, to steer fines and store sediment: the Sediment Settler and Sediment Storer. Both constructions were easy to handle and to install. Despite their low weight, they have proven to be both resilient and strong.
First, a pilot was conducted to store 500 cubic meters of sediment behind a Sediment Storer of 50 meters in length. Several weeks after installation of the Sediment Storer in March (2013) vegetation started to develop. By the end of the growing season the sediment top layer was totally covered by a pioneer vegetation of reedmace (Typha), thereby isolating the contaminated sediment underneath and forming a natural cap over the years.
Schematisation of Sediment SettlerIn January 2014 four Sediment Settlers (25 meters each) were placed perpendicular to water currents in order to reduce those and induce settlement of sediment on the lee side. Due to the Sediment Settler, fines transport decreased by 20-40 %. One year after installing the Sediment Settler a few decimeters thick sediment layer had already been formed at its lee side. Even plant species that have disappeared 30 years ago returned on top of the constructions.
After conducting two successful pilots in 2013, last winter over 2,5 kilometers of geotextile constructions were installed. About 15.000 cubic meters of slightly contaminated sediment was hydraulically dredged and transported to Sediment Stores in order to alter hydrodynamics and induce nature development. By doing so, maintenance costs were minimized and sediment had been beneficially re-used. The project was finalized in April 2015 and the monitoring for the coming years has already started.
This project has shown that it is possible to control sediment settling and transport, improving recreational and ecological quality of this wetland area. Nautical bottlenecks were reduced and in lee areas vegetation developed. Dredged material can easily be kept and stored in submerged basins. So, synergies were realized between sediment remediation, water quality improvement, nature restoration and shore line protection.