Less silt, more oxygen

Less silt, more oxygen

Resolving the silt problem and restoring water quality

Through changes in the river's course – straightening and deepening – as well as the disappearance of natural silt accumulation areas, the ratio of ebb and flow duration in the Ems has shifted considerably, greatly increasing silt contamination. Because the river became deeper and straighter, the incoming flow has become faster and heavier, running up to the edge of the tidal inflow in Herbrum in an ever shorter amount of time. The ebb current, on the other hand, has become slower and more consistent. Because of this asymmetry, current speeds during the flow are much greater than those during the ebb. This means that the flow current carries far more sediment into the tide than the ebb can carry back out. Silt from the harbour in Ems – where the state of Lower Saxony dredges – is no longer removed from the river system, and is thus no longer stored on land.

The Ems is much more heavily contaminated with silt than other rivers. This not only means that a lot of sediment accumulates on the river bed and along the banks, but also that there is a very high amount of dissolved silt in the river's water itself. According to measurements by the coastal research station of the Lower Saxony State Organisation for Water Management, Coast and Nature Conservation (NLWKN), the concentration of sediment in the water of the Ems is 100- to 1000-times that in the Elbe and Weser.

Primarily in the summer, when little water flows from the headwaters and the ebb is further weakened as a result, an actual layer of fluid mud forms on the river bed and only moves slightly up- and downstream with the tide. Despite the very high sediment content, this mixture remains fluid but behaves differently than water, and is not habitable for wildlife. This "system state" of the river is very rare, according to the coastal research station, and international knowledge of this topic remains "highly incomplete".

 

That is why the NLWKN and the coastal research station, among others, are taking extensive samples and sets of measurements of specific parameters of the Ems water. These measurements also serve to improve the existing calculation models for planning the measures to resolve the silt problem.

One initial measure for resolving the silt problem was commenced by the Steering Committee in January: "flexible tidal control" with the gates of the Ems barrier. Before the Federal Waterway and Shipping Administration (WSV) and the NLWKN had consolidated their very different approaches of a weir and tidal control at the Ems Barrier in a feasibility study. The effectiveness and technical feasibility of flexible tidal control with elements from both approaches was verified. It was also determined that there are currently no exclusion criteria for implementation from an environmental and shipping perspective. Because of this, the Steering Committee commissioned the NLWKN and WSV to compile the basis for a planning process. This is expected to commence in 2020.

The WSV and Lower Saxony Department of Commerce have already entered talks with harbour operators, shipping companies, and harbour-related businesses to cooperate with their representatives in finding ways to minimise shipping restrictions by blockages of the Barrier during tidal control. The basis for working out the specific operation plans for tidal control is a balance between effectiveness of silt reduction in the Ems and compatibility with maritime commerce.

In order to achieve the desired results as quickly as possible, the contract partners agreed in the Master Plan Ems 2050 to commence planning for flexible tidal control before the feasibility study for the tidal reservoirs on the Ems - the third improvement measure - is ready, presumably by late 2018. Should the tidal reservoirs prove to be feasible and effective, they can be combined with the flexible tidal control. In addition, the expected minimisation of silt in the Ems is expected to have a positive effect on the planned state of habitats in tidal polders along the Ems. They should not silt up as quickly as they do under the current conditions.

The heavy turbidity of the Ems and contamination with suspended sediment often leads to a very low oxygen content in the water. No oxygen is present is some parts of the river during the summer months. A certain increase in oxygen is always apparent in the winter. By resolving the silt problem, the oxygen content is expected to increase and the Ems should become a year-round habitat for fish once more.

Another consequence of past river enlargements is the threat of saltwater flowing further into the Ems system with the flow tide than before. The brackish water zone (the zone where freshwater and saltwater meet) has thus shifted further upriver.