Rip current dynamics at a nourished sandy beach, Egmond aan Zee (the Netherlands)

Abstract

Recession of the coastline is nowadays combatted with large deposits of sand. The nourished sand alters the nearshore morphology and changes the cross-shore and alongshore currents that force rip currents. While it has been documented that nourishments contribute to the onshore movement and growth of the sandbars, little is known about the long term (<years) effect of repeated nourishments to the rip system. This study focuses on the effect of nourishments on the natural dynamics of rips. Rips at the sandy straight coast of Egmond were analysed at two adjacent beaches, one natural and one frequently nourished over the years. Alongshore rip locations were automatically detected out of the video images, spanning 16.4 years for the natural site and 7.7 years for the nourished site. The resulting dataset is used for the determination of the rip distribution and the rip spacing. To quantify the rip migration and rip lifetimes, the rip locations were manually corrected for successive years. The rip system appears to be regular spaced in summer where the number of rips gradually increases. In winter, the number of rips rapidly decreases and the rip system becomes irregular spaced. The NOM of the bar system is related to the gradual decrease of the number of rips in the system. The placement of beach and shoreface nourishments halted the NOM of the bar system and thereby the decreasing trend of the number of rips. The beach nourishments substantially increased the rips in the system until the next winter. Overall the nourishment combinations reduced the mean rip migration rate with 4 m/day and increased the mean rip spacing values with 25 m. Attempts to relate the sudden changes in the rips system to the wave forcing were unconvincing. Reorganization of the rip locations corresponded to high-energetic wave event. However, similar conditions occurred for the stable periods in between. More research on short term periods is needed to distinguish the forcing mechanisms behind modifications in the rip system.