Impact of sea-level rise on the morphological equilibrium state of tidal inlets

This study addresses the question whether the geomorphology of a tidal inlet (i.e. the coastal inlet and associated tidal basin) can maintain equilibrium under a rising relative sea level. When a tidal inlet system is exposed to a constant rate of sea-level rise (SLR), the system will be permanently in a state that deviates from the Equilibrium State corresponding to zero SLR. This is a physical requirement to create a permanent incentive to import sediment into the system. In case the rate of sediment import matches the rate of sea-level rise, a new state of dynamic morphological equilibrium is reached. If the actual import rate is less than this, the system’s morphological state will deviate increasingly from its equilibrium and finally degenerate. In this case SLR has exceeded an upper boundary (named ‘state limit’, denoted as SLRlimit), and the system will not be able to reach a state of dynamic morphological equilibrium any longer and drown. Because the SLRlimit predictions are sensitive to a variation of internal sediment exchange rate and availability of sediment at the seaward boundary of the system, we assess the SLRlimit predictions quasi-probabilistically to gain quantitative insight into the inherent uncertainties. This technique leads to an estimate of the probability distribution for the SLRlimit of two selected tidal inlet systems in the Dutch Wadden Sea. The calculated probabilities comply with the available fragmentary geological data, which suggest that the former tidal inlets in the western region of the Netherlands were drowned under the influence of a sea-level rise of 80 cm to a few metres per century.