Location and probability of shoal margin collapses in a sandy estuary

Abstract

Channel bank failure, collapses of shoal margins and beaches due to flow slides have been recorded in Dutch estuaries for the past 200 years but have hardly been recognized elsewhere. Current predictions lack forecasting capabilities, because they were validated and calibrated for historic data of cross‐sections in specific systems, allowing local hindcast rather than location and probability forecasting. The objectives of this study are to investigate where on shoal margins collapses typically occur and what shoal margin collapse geometries and volumes are, such that we can predict their occurrence. We identified shoal margin collapses, generally completely submerged, from bathymetry data by analyzing DEMs of Difference (DoD) of the Western Scheldt for the period 1959‐2015. We used the bathymetry data to determine the conditions for occurrence, specifically to obtain slope height and angle and applied these variables in a shoal margin collapse predictor. We found 299 collapses along 300 km of shoal margin boundaries over 56 years, meaning more than 5 collapses occur on average per year. The average shoal margin collapse body is well approximated by a 1/3 ellipsoid shape, covers on average an area of 34,000 m2 and has an average volume of 100,000 m3. Shoal margin collapses occur mainly at locations where shoals take up a proportionally larger area than average in the cross‐section of the entire estuary, and occur most frequently where lateral shoal margin displacement is low. A receiver operating characteristic curve shows that the forecasting method predicts the shoal margin collapse location well. We conclude that the locations of the shoal margin collapses are well predicted by the variation in conditions of the relative slope height and angle within the Western Scheldt, and likely locations are at laterally relatively stable shoal margins. This provides hypotheses aiding the recognition of these features in sandy estuaries worldwide.