The intertidal zone plays an important role in the protection of the coast. It is a very dynamic area subject to waves, tide, and wind and topographic changes can be large over a short period of time. For macro-tidal coasts (tidal range > 4 m) like the Belgian one, tide is an important factor in the intertidal beach morphodynamics but it remains unclear what specific hydrodynamic conditions lead to topographic changes. This is mainly due to a scarcity of reliable field data of sediment transport and beach topography. This study investigates the intertidal beach morphodynamics based on extensive measurements of hydrodynamics, sediment transport, and beach topography resulting in a conceptual model of hydrodynamic forcing and topographic response. Two study sites along the Belgian coast are examined: a natural, multi-barred beach (Groenendijk) and a managed beach with a featureless intertidal zone (Mariakerke). The monthly to seasonal dynamics in beach topography is investigated based on multiannual monthly cross-shore beach profiles. It is found that topographic changes on this scale are mainly event-driven with, in general, erosion during energetic events and beach recovery in between. The ridges and runnels at Groenendijk move onshore and become more pronounced during energetic conditions, while the intertidal beach topography is smoothened during calm conditions. Monthly variations in intertidal beach volume are on average 2% of the total beach volume and they can be up to 7% for energetic (non-storm) events. There is a large alongshore variability in topographic response to hydrodynamic forcing and this response can even be opposite over a distance of tens of meters. In comparison to the nearshore hydrodynamics and sediment dynamics it is found that the intertidal beach grows when waves are small (wave steepness < 0.010), whereas it erodes when waves are large (> 0.025). For medium wave steepness (0.010-0.025) this is opposite, which is attributed to a sudden rise in sediment supply. This rise is likely related to waves breaking over the sandbanks in front of the coast and at beaches southwest of the study sites. On a daily scale, the relationship between wave steepness and intertidal beach volume is heavily distorted by spring-neap variations in tidal current direction. Strong alongshore currents transport sediment away from the beach during spring tide which enhances erosion. In contrast, currents are cross-shore and wave-dominated during neap tide. The impact of variations in tidal current direction on the intertidal beach topography is in the same order of magnitude as wave impact. The effect of variations in sediment supply is subordinate to the impact of waves and tide. The main mode of sediment transport in the intertidal zone is in suspension. Currents peak at approximately 1.5 m above the bed, resulting in a peak in sediment transport at 1/3 of the high tide level. The mean sediment transport is onshore by tidal currents during calm conditions, while it is offshore by undertow during energetic wave conditions (wave height > 0.6 m or wave steepness > 0.025). Oscillatory transport is always onshore because of wave asymmetry but is subordinate to the mean transport. A good qualitative and quantitative relationship is found between measured sediment transport rates and observed topographic changes. This is ascribed to the use of acoustic backscatter intensity from a high resolution current sensor, allowing the estimation of suspended sediment concentrations over the full water column. Furthermore, this good relationship is attributed to the use of a static laser scanner to obtain accurate beach volume changes. In general, it can be concluded that waves are not necessarily the dominant forcing factor on macro-tidal beaches. Tidal currents can be equally important and they can contribute significantly to the transport of sediment. Besides waves and tidal currents, variations in sediment supply influence the changes in the intertidal beach topography. A conceptual model is provided to illustrate the relationship between hydrodynamic forcing and topographic response. |
