Tidal control in the Lower Ems

Abstract

The lower Ems river suffers from hyper-turbid conditions. These conditions are caused by channel deepening over the last 50 years, resulting in an asymmetrical tidal wave. This asymmetry has resulted in an increased
import of sediment into the Ems river. Recently some effort has gone into determining whether or not the local tidal barrier (Emssperrwerk) can be used to alter the asymmetry of the tidal waves propagating through the Ems estuary. Previous modelling studies, using state-of-the-art models, have been able to reproduce prototype tests involving the same tidal barrier but very little effort has thus far gone into determining what the optimal barrier control regime is and what the restriction imposed on the barrier operation mean for the overall
effectiveness of the barrier. The operation of the tidal barrier was schematised and described using two factors: closure percentage and closure duration. The effectiveness of a broad spectrum of combinations determining the tidal barrier operation is modelled using a simple one-dimensional model which takes only into account the (simplified) hydrodynamics in the lower Ems river. The results of the model indicate that only in part of the Lower Ems the tide can be adjusted to become symmetrical or slightly ebb-dominant. This requires a closure duration longer than approximately 240 minutes. The length of this stretch of river is increase further by increasing the closure percentage. Similarly, increasing the closure percentage also leads to a decrease in the tidal volume and the amplitude of the M4 component. For this reason, the closure percentage should be kept as low as possible, as long as the barrier operation is able to reduce the sediment import into the Lower Ems. The required length of the stretch of the Lower Ems with a symmetric tide is estimated to be around 12 km. Applying restrictions to the barrier operations showed that shorter barrier
operations should be preferred, since short operations allow a larger number of
tidal waves to be altered, increasing the effectiveness in a realistic setting. Based on these considerations an optimal barrier operation consists of a closure duration of approximately 300 minutes and a closure percentage of approximately 60%. The results showed that the method used for evaluating the barrier operation is unable to distinguish between water levels signals consisting of altered and unaltered waves. The effectiveness of the barrier operation is
still estimated by considering the whole water level signal. The impact of the barrier operation in the model interfered with the seaward model boundary. This error increases in magnitude with increasing closure percentages. Making the results for those combinations unreliable. In order to make more reliable conclusions about the effect of the tidal barrier operation, subsequent models should encompass a larger domain and include a more detailed schematisation of the channel geometry, river discharge and the tidal barrier itself.