Author(s): Sara Antonio; Jebbe Van Der Werf; Erik Horstman; Ivan Caceres; Jack Puleo; Michael Larner; Joep Van Der Zanden; Suzanne Hulscher

Linked Author(s): Sara António, Jebbe van der Werf, Iván Cáceres

Keywords: Swash zone; Sediment transport processes; Hydrodynamics

Abstract: The swash zone is the highly dynamic interface between the dry and wet part of the beach. Swash processes determine whether sediment is either stored on the upper beach or transported offshore, and thus they strongly determine shoreline evolution. Previous experimental studies on sediment transport in the swash zone either focused on detailed sand transport processes at only a few cross-shore locations, or on bulk statistics of net transport rates in relation to limited wave conditions and a single initial bed slope (often 1:15 slope for large-scale laboratory experiments). Laboratory experiments with high temporal and spatial measurement resolution of swash hydrodynamics and morphodynamics for a wide range of wave conditions and beach slopes are needed to enable a more detailed quantification and understanding of sediment fluxes near the shoreline. The present research focusses on understanding the effects of beach slope on intra-swash hydrodynamics and sediment transport processes in the swash zone through a series of large-scale wave flume experiments. Laboratory experiments were conducted in the large-scale CIEM wave flume at the UPC (Barcelona, Spain). In these experiments an erosive bichromatic wave series with groups of four waves (equivalent to random time series with Hs=0.65m and Tp=3.5s) were produced in consecutive runs with each wave group repeating every two group periods (TR=2Tg=28s) in order to produce distinct types of swash interactions. The same sequence was reproduced for two different bed slopes of 1:15 and 1:25. The beach profile consisted of medium sand with D50=0.25mm with a measured mean settling velocity ws=0.034m/s. The flume was equipped with a series of instruments which continuously measured water level, bed level, suspended sediment concentrations and flow velocities, mostly focusing on the swash zone. Additionally, a combination of CCM¬+ and CCP instruments were deployed to measure sediment velocities and concentrations in the sheet flow layer over a more dispersed area of the swash. Preliminary results show that the swash zone on the 1:15 slope had shorter and more energetic swash excursions and greater water depths, while on the 1:25 slope the swash zone was wider but shallower. Furthermore, the three types of wave-swash interactions (wave capture, weak and strong wave-backwash interaction) were identified on both slopes. However, they did not occur at the same cross-shore location with respect to the shoreline, resulting in differences in where and how the sediment is picked up and mobilised by swash motions. It is hypothesised that strong wave-backwash interactions seen in the steep slope increases the tendency of offshore-directed transport, while the predominance of weak wave-backwash interactions on the gentle slope promotes onshore-directed transport. Further analysis of intra-swash sediment concentrations, both suspended load and sheet flow transport, and hydrodynamics will be done to investigate the proposed hypothesis.

DOI: https://doi.org/10.3850/IAHR-39WC2521711920221001

Year: 2022