Influence of retention basins on tidal dynamics in estuaries: application to the Ems estuary
We present a quick assessment method using an idealized one-dimensional linear model to explore the influence of multiple retention basins, whose construction is proposed as a measure to reduce tidal amplitudes in estuaries/tidal channels. To this end, we have developed a process-based network model for the cross-sectionally averaged water motion, including width and depth convergence (thus extending earlier studies), bottom friction, radiation damping and allowing for multiple basins at arbitrary locations.

For frictionally dominated tidal channels, model results show that construction of a retention basin generally leads to a reduction of the tidal amplitude at the channel head. This reduction is stronger for larger basins and for basins closer to the landward side. Strikingly, for weak to moderate friction, a basin situated sufficiently close to the seaward side may trigger the opposite and undesired effect (tidal amplification) and nonlinear interaction among basins may occur.

The model is then applied to the Ems estuary (Germany), where the construction of nine designated retention basins is currently under consideration. For parameter values representing the present-day Ems situation, constructing all nine proposed basins is estimated to result in a tidal amplitude reduction of 0.87 m (from 1.51 m to 0.64 m). A systematic model analysis of all 29 = 512 combinations shows that 86% of this reduction can be achieved by selecting only four of these basins. Importantly, shifts in the frictional regime, as experienced by the Ems in the past, may drastically change the effect of retention basins.

Even though the efficiency and flexibility of this exploratory model allow for extensive sensitivity studies, we recommend to combine it with a detailed model, for the purpose of both efficiency and accuracy.