Predicted ripple dimensions in relation to the precision of in situ measurements in the southern North Sea

Abstract

Ripples are common morphological features in sandy marine environments. Their shapes and dimensions are closely related to local sediment properties and forcing by waves and currents. Numerous predictors for the geometry and hydraulic roughness of ripples exist but, due to their empirical nature, they may fail to properly reflect conditions in the field. Here, measurements of tide and wave generated ripples in a shallow shelf sea are reported. Discrete and continuous methods for the extraction of ripple dimensions from digital elevation models (DEMs) are inter-compared. The range of measured ripple dimensions is quantified and compared to the results of empirical predictors. The repeatability of a measurement for inactive conditions is taken as the precision of measurements of bedform dimensions. The accuracy of measurement is assessed via comparison to predicted dimensions. Results from field data show that the precision of measurements is limited to 10 % of the absolute ripple dimensions. The application of different methods for the detection of ripple heights may result in form roughness heights differing by a factor of up to 2 between the traditional statistical estimate and a full evaluation of the spatial bathymetry.