Natural sediment sinks have been removed in many estuaries and tidal basins, and the resulting loss in accommodation space likely led to an increase in the suspended sediment concentration (SSC). The effect of land reclamations and changes in other sediment sinks on SSC has been investigated using the Ems Estuary as a well-documented example. Coastline reconstructions and stratigraphic observations in the Ems Estuary, on the Dutch–German border, have been used to quantify the long-term (centennial) changes in sediment sinks. These data indicate that approximately 2–3 · 106 dry weight tons of fine-grained sediments annually accumulated in the estuary from the end of the 16th century up to the beginning of the 20th century. In the 20th century and especially from 1960 to 1994, large amounts of sediment were dredged and removed from the Ems Estuary. Since the 1990s, sediment removal rates through dredging activities sharply decreased, while available monitoring data show that on average SSC in the estuary increased during this period. The potential impact of sediment sinks on SSC has been investigated using a simple mass balance and a complex numerical model. Both methodologies indicate that the reduction in sediment sinks lead to an increase in suspended sediment concentration of several 10s of mg/l averaged over the estuary. The numerical model also resolves the spatial variation in changing suspended sediment concentrations, suggesting that locally the increase may exceed 100 mg/l. This work reveals that a decrease in accommodation space for fine-grained sediments by land reclamations or by reducing fine-grained sediment extraction will lead to an increase in suspended sediment concentrations, and provides methodologies to estimate the impact of interventions on SSC of estuaries and tidal basins.@en

Subsidence caused by extraction of hydrocarbons and solution salt mining is a sensitive issue in the Netherlands. An extensive legal, technical and organisational framework is in place to ensure a high probability that such subsidence will stay within predefined limits. The key question is: how much subsidence is acceptable and at which rate? And: how can it be reliably assured that (future) subsidence will stay within these limits? To address the issue for the Wadden Sea area, the concept of ‘effective subsidence capacity’ is used. To determine the ‘effective subsidence capacity’, the maximum volumetric rate of relative sea-level rise, that can be accommodated in the long term, without environmental harm, is established first. The volume of sediment that can be transported and deposited by nature into the tidal basin where the subsidence is expected, ultimately determines this ‘limit of acceptable average subsidence rate’. The capability of the tidal basins to ‘capture’ sediment over the lunar cycle period of 18.6 years is the overall rate-determining step. Effective subsidence capacity is then the maximum average subsidence rate available for planning of human activities. It is obtained by subtracting the subsidence volume rate ‘consumed’ by natural relative subsidence in the area (sealevel rise plus natural shallow compaction) from the total long-term acceptable subsidence volume rate limit. In the operational procedure for mining companies, six-years-average expectation values of subsidence rates are used to calculate the maximum allowable production rates. This is done under the provision that production will be reduced or halted if the expected or actual subsidence rate (natural + man induced) is likely to exceed the limit of acceptable subsidence. Monitoring and management schemes ensure that predicted (6-year average) and actual (18.6-year average) subsidence rates stay within the limit of acceptable subsidence rate and that no damage is caused to the protected nature. A GPS based early warning system is used for early detection of unexpected behaviour. In support of SSM (State Supervision of Mines, the government regulator), TNO-AGE (an independent government advisory group) applies an independent Bayesian statistical analysis of all data, as they become available, to calculate the probability of scenario's under which future subsidence will exceed the defined limits. It is external to the operator's annual measurement and control loop and ensures that preventive actions can be taken in time in case such scenarios emerge. Regular communication keeps the authorities and the general public informed on the use of the effective subsidence capacity to demonstrate that the actual average subsidence rate stays strictly within the defined bounds and that, from a scientific point of view, there is no reasonable doubt that damage to the tidal system will not occur now or in the future.@en