| dc.description.abstract | During the mid-Holocene, when sea-level rose rapidly, many estuaries formed. When
sea-level rise slowed down during the late Holocene, some of these estuaries filled with
sediment, where others remained largely unfilled. Vegetation has an important ecoengineering
effect and contributes to estuary infilling. It is unclear however, how net
importing estuaries will respond to the current and projected sea-level rise. To this end
we conducted scaled laboratory experiments in the 20 x 3 m tilting flume the Metronome,
which can simulate tidal currents. By asymmetric tilting, a flood dominated, net importing
estuary was created, and sediment was supplied to the seaward and landward
boundaries. A mud simulant was used and seeds of plant species with eco-engineering behaviour
were supplied into the ow periodically. The channel network evolution through
time was mapped using a recently developed network extraction tool. This tool can extract
fluvial networks from elevation data in an objective way.
It was found that sea-level rise enhances sediment mobility which, in combination with
flood dominance, leads to higher sediment fluxes to the upstream part of the experiment
than in experiments without sea-level rise. Sea-level rise also allows tidal currents to propagate
further upstream which promotes channel formation upstream in the experiment.
Most importantly, sea-level rise causes the morphological units in estuaries to shift landward
and the upstream part of the system to silt up, given enough sediment is available to
fill the created accommodation. In the case of low sediment availability, which is common
in systems in which dams and dikes are built, coastal squeeze may prevent transgression
and estuarine environment will drown. Two examples, the Scheldt and the Wadden sea,
are considered to illustrate what the fate of estuaries could be, considering the current
and predicted sea-level rise. It is found that if sea-level rise stays on the low end of the
predictions (40 cm/100 yrs), the Scheldt and the Wadden sea area may aggrade to maintain
a stable position. In case sea-level rises faster, these systems are likely to transgress
but are prone to coastal squeeze because of man-made hard boundaries directly landward
of them. It is therefore expected that many of these systems will drown in the centuries
to come, unless we change our coastal management strategy and create more space for
systems to silt-up and keep up with sea-level. The findings of this study can contribute
to managing estuaries sustainably in terms of sediment budgets, navigability and ecology. | |
