High frequency in-situ field measurements of morphological response on a fine gravel beach during energetic wave conditions

Abstract

This paper presents novel data collected over four weeks at Loe Bar, a fine gravel barrier (D50 = 3.5 mm), exposed to energetic wave conditions (Hs10% = 2.4 m). Combined remote and in-situ measurements were used to identify 3D morphological response, profile change and runup behaviour during successive energetic periods (Hs = 2.5 m). Small intertidal beach volume losses occurred under high steepness waves (H0/L0 > 0.01), but, in general, the overall beach volume was remarkably stable. In fact, the greatest morphological response was evident not in cross-shore sediment exchanges, but due to the alongshore redistribution related to the three-dimensional beach cusp system. Specifically, during energetic wave conditions the cusp morphology underwent horn growth and embayment deepening, leading to increased cusp definition. An additional explanation for the stability of the cross-shore beach profile is related to the development of the beach step. During energetic conditions and long period waves, significant deposition (c. 1 m) occurred at the top of the beach step above the still water line. The enhanced step provided a focal point for wave breaking, considerably curtailing the swash motion and reducing the runup limit, thereby providing protection to the upper beach. This study provides much needed insight into complex morphological response and runup characteristics on gravel beaches, fundamental for improved model development, and leading towards better predictive tools.