Monitoring and modelling of aeolian sand transport at the Belgian coast.

Restoration and maintenance of coastal beach and dune systems requires knowledge of aeolian sediment transport processes for the prediction of system response to wind forces over short to long-term timescales. This allows for appropriate management of storms in addition to seasonal and decadal variations. Accurate aeolian sediment transport equations are of utmost importance for modern geomorphology and coastal engineering practices. Although sand transport by wind is easily observable, reliable and accurate data sets of sand transport rates are of scarcity due to measuring difficulties. Field monitoring is essential to understand its impact in the overall sediment budgets and long-term coastal dune behavior. The main objective of this thesis is unravelling the nature of aeolian sand transport on the Belgian coast, enlarging the knowledge with the aim to improve long-term aeolian sediment transport estimates. The overall aim of the thesis is based on analyzing collected data from field experiments carried out between 2016 and 2018. This work is performed within the framework of the project CREST (Climate REsilient coaST), funded by the Strategic Basic Research (SBO) program of Flanders Innovation & Entrepreneurship, Belgium. Within the project, it is aimed to further the knowledge of coastal processes on land and under water.

To gain initial insight into the relationship between aeolian sand transport rates and wind speed, simultaneous monitoring of meteorological conditions and aeolian sand transport rates, using Modified Wilson And Cook (MWAC) sand traps, was carried out on the subaerial beach of two study sites in Belgium. The study sites comprise the natural beach-dune system of Koksijde and the managed beach-dyke system of Mariakerke. Six aeolian mathematical models, each predicting saturated transport rates, are used for objective testing. Some of the models are frequently used for long-term budget calculations. Recently, new models have been proposed in literature that require validation from qualitative field measurements. The key parameter in all these aeolian models is the shear velocity, u*. Shear velocities are calculated using vertical wind profile data from meteorological stations located on the beach. A modified Bagnold model was able to produce a strong one-to-one relation between observed and predicted transport rates. The other aeolian models produced poor results, underestimating and/or overestimating sediment transport rates.

While short-term aeolian sediment transport rates and wind speed are correlated by a modified Bagnold model, it seems of particular interest to study its relationship with annual to decadal dune behavior. To gain insight in dune behavior and the processes covering dune growth, long-term changes in dune volume at the Belgian coast are analyzed based on measured data by airborne surveys. The Belgian government has been monitoring the eastern part of the coastline since 1979, and since 1983 the entire coastline by annually or bi-annually surveying cross-shore bathymetric profiles and collecting airborne photogrammetric and, since 1999, airborne Laser Scanner (LiDAR) data. For most of the 65 km long coastal stretch, a linearly dune growth is found. It varies between 0-12.3 m³/m/year with an average linear dune growth of 6.2 m³/m/year, featuring large spatial variations in longshore directions. The dune volume is defined as the volume of sand above the dune foot level. The dune foot level along the Belgian coast is defined at +6.89 m TAW (Belgium Ordnance Datum). In this thesis, the longshore spatial and temporal variations in dune volume changes are derived and correlated with potential sediment transport. Based on a wind data set from the period between 2000 and 2017, it is found that potential aeolian sediment transport has its main drift from west to south-west direction (onshore to oblique onshore). Based on the modified Bagnold model, onshore potential aeolian sediment transport ranges maximum to 9 m³/m/year, while longshore potential aeolian sediment transport could reach up to 20 m³/m/year. An important correlation is found between observed and predicted dune development at decadal timescales when zones with dune managing activities are excluded. Most of the predicted data are within a factor 2 of the measured values. The variability in potential transport is well related to the variability in dune volume changes at the considered spatial–temporal scale, suggesting that natural dune growth is primarily caused by aeolian sediment transport from the beach. It also suggests that annual differences in forcing and transport limiting conditions (wind and moisture) only have a modest effect on the overall variability of dune volume trends.

At the Belgian coast, the beach profile is also regularly altered by human intervention to limit aeolian sand towards the hinterland as it often results in large depositions of sand on neighboring roads and tram tracks. Each year, the municipalities do large investments in the maintenance of their streets and sewer systems. A field experiment was designed to carry out simultaneous measurements of wind and sediment transport across a human-constructed high berm with a steep seaward cliff that is backed by a dyke. In front of the dyke, a trench is excavated to prevent aeolian sand being blown to the hinterland. Two sets of measurements were carried out, one with oblique onshore and one with winds directly onshore. Over-steepened velocity profiles and thus large shear velocities were measured at the steep cliff during the onshore wind event compared to the back beach due to flow compression and acceleration. The fetch effect has been measured across the flat berm where maximum transport was achieved at a distance of 20 to 35 m of the berm lip. The fetch effect is characterized with an overshoot during the oblique onshore wind event. Sand flux rapidly increased towards a maximum value followed by a decrease to a lower equilibrium value which was approximately half of the maximum mass flux obtained at the critical fetch distance. The evolution of the vertical mass flux profiles downwind caused the grain distribution above the surface (decay rate) to increase almost linear with increasing fetch length further away from the berm lip, until an equilibrium is achieved. This means that the distribution of particle trajectories changed similar until it was stable for different transport events on a flat dry beach surface. Based on this study, the steep cliff in front of the human-constructed coastal berm is very sensitive to erosion due to aeolian sand transport. Sand being eroded from the berm lip is deposited in front of the dyke and in the trench.

When studying aeolian sediment transport in coastal zones, often a location is chosen where the number of supply-limiting factors is minimal (e.g. moisture, shells, vegetation) to ensure better comparison between predicted and observed values. However, as is often the case in a natural coastal environment, the beach contains bed irregularities caused by wind action, patches of pebbles, beach wrack, shells and shell-fragments, vegetation and beach litter. The effect of these small-scale bed features is frequently disregarded when conducting field experiments, even sometimes called insignificant. Therefore, the effect of largely scattered shell pavement on aeolian sand transport on the upper beach of a natural beach-dune system was studied during a short-term field experiment in the winter of 2016 in Belgium. The coverage of shell pavement on the upper beach increased towards the dunes and was highest just in front of the dune foot. Continuous sand transport occurred during strong highly oblique onshore wind and was measured during two experiments. During the two experiments, spatial variations in aeolian sand transport indicate that there was a consistent decrease in transport rate with distance downwind. Within 162 m, aeolian sand transport decreased by factor of 10 from the high waterline in the direction of the dunes. The negative gradient in transport caused local deposition of sand on the upper beach in the form of mobile rippled sand strips. This accumulation of sand acted as a new source area for aeolian transport to the dunes when the intertidal beach was inundated. However, as this region is also very sensitive to wave run-up, the accumulated sand may be removed again from the upper beach. The vertical distribution and median grain size of airborne sand particles across the shell-fragmented beach remained constant.

The main conclusions of this research are that on a short-time scale (hours to days), aeolian sediment transport rate is cubic related with wind speed by a modified Bagnold model. On decadal timescales, an important correlation between observed and predicted dune development is also found. This indicates that dune growth is primarily caused by aeolian sediment transport from the beach and that annual differences in forcing and transport limiting conditions only have a slight effect on the overall variability of dune volume trends. It also suggests that the modified Bagnold model proves to perform strong on longer timescales. During moderate onshore and oblique onshore wind, measurements on a high flat berm with a steep seaward cliff indicate the presence of the fetch and overshoot effect. It was observed that the evolution of the vertical mass flux profiles downwind causes the exponential decay rate to increase almost linear with increasing fetch length until an equilibrium decay rate is achieved. The effect of shell pavement and moisture on a beach is significant and cannot be disregarded. Aeolian sand transport can be reduced by a factor 10 within a short distance downwind, causing local accumulation of sand which is not entering the dunes directly. Though, shells do not have an influence on the vertical distribution and grain size of airborne sand particles downwind. Further research should focus on better quantifying aeolian sediment transport rates by more innovative monitoring techniques, especially when long-term monitoring is required. Further research should also focus more on the influence of bed roughness and its feedback to the wind shear velocity on aeolian sediment transport. Additionally, a change in decadal dune behavior due to climate change is also very relevant to study.