Beach-dune systems near inlets - linking subtidal and subaerial morphodynamics

Filipe Galiforni Silva is a PhD student in the research group Water Engineering and Management (WEM). His supervisors are prof.dr. K.M. Wijnberg and prof.dr. S.J.M.H. Hulscher from the Faculty of Engineering Technology (ET).

Coastal dunes, tidal inlets, and beaches are among the most dynamic environments in geomorphology due to the influence of different energetic processes such as waves, wind, and currents over a relatively small area. If close to inlets, beach-dune systems may be affected by specific inlet processes that change beach characteristics and, consequently, coastal dunes. However, it is still unknown how and to what extent inlet-induced shoreline changes affect dune development. Therefore, the objective of this thesis is to understand how inlet-driven changes in adjacent coastlines affect dune development considering long-term inlet-driven changes (e.g. sand flats) and short-term inlet-driven changes (e.g. shoreline variability).

Regarding long-term changes, we found that the sand-flat geomorphological setting induces spatial variations in groundwater depths, which result in changes in sediment supply and, consequently, in coastal dune development. Moreover, results show that there is a threshold depth at which groundwater depth starts to affect dune development and that this threshold can vary spatially depending on topography. Furthermore, we found that on a sand-flat setting, storm surges may act as a depositional agent of sediment instead of an erosive agent as customarily thought. Results from both numerical modelling and field data show that supra-tidal shore-parallel deposition of sand occurs during storm surge flooding.  Moreover, the amount of sand deposited is directly proportional to storm strength, and the amount of sand deposited suggests that it may add significant potential sand for aeolian transport and, consequently, dune growth.

Regarding short-term changes, we evaluated the effect of beach width changes and shoal attachment processes on a decadal time-scale. Using both numerical modelling and field data, we found that there is a preferred cross-shore position where the foredune tends to be built which is a function of beach width and sediment supply. For narrow beaches, foredunes tend to develop at higher elevations than wide beaches due to differences in wave dissipation during storm conditions, whereas dune volume is controlled by hydrodynamic erosion and dune recovery potential by sediment supply. Furthermore, if sediment supply is limited, the effect of beach width on dune volume only appears for beach widths larger than 300 meters, suggesting that limitation in supply can dominate dune growth on regular beaches. Furthermore, we found that cumulative shoal attachment events lead to increased rates of dune volume change, even though single attachment events did not necessarily yield an increase in dune growth. Moreover, we found that beach width increase and rate of alongshore sediment spreading of the shoal after attachment is key factors determining whether the shoal will or will not influence dune growth.

Therefore, inlet-driven processes and landforms may affect beach-dune systems nearby inlets by affecting how sand supply is exchanged by subtidal and subaerial zone and how it is distributed spatially. Sand-flats may create conditions that enhance the possibility of spatial variability in dune growth and morphology, with more supply and higher dunes in the exposed part. This is relevant especially in terms of coastal management since dune heights and resilience are a crucial aspect regarding coastal protection. Shoreline changes due to inlet-processes like channel migration and shoal attachment may affect supply and space for dune development, provided that time-scale of beach width change is large enough to be translated into dune growth.