Temporal and spatial behaviour of rip channels in a multiple‐barred coastal system

Abstract

A 15-month data set of daily time-averaged video images (Argus) has been analyzed to describe the spatial and temporal variability of the rip channels on a multiple-barred coast at Noordwijk aan Zee, The Netherlands. The landward boundary of the intertidal bars and a proxy of the subtidal bar crest, defined as the intertidal and subtidal bar lines respectively, were derived from the Argus images. Local seaward-directed deviations of the bar lines represent the cross-shore and alongshore locations of the rip channels. The average intertidal rip spacing (l) was 243 m, but the rips were not spaced regularly (σ λ /l = 0.47). Some intertidal rips were observed to fill up during falling tide, but the majority remained open. The filled intertidal rip channels had more landward positions and migrated more slowly (2.4 versus 4.6 m/day) in the alongshore direction than the open intertidal rip channels. The number and the alongshore migration rate of open intertidal rip channels increased with the preceding wave heights (r = 0.26, p < 0.01) and alongshore component of the offshore wave power (r = 0.25, p < 0.01), respectively. The shape of the intertidal bar lines was similar to the subtidal bar line shape, suggesting that the intertidal morphology is coupled to the subtidal alongshore variability. The phase of two bar lines could vary from in phase (0°) to out of phase (180°). The phase changes gradually, due to different alongshore migration rates of the intertidal and subtidal bar lines. Figure 4. Time series of the hydrodynamic parameters H rms (black) and P l (gray) at the offshore location MPN (a), location X 1 (b) and location X 2 (c). Vertical dash-dot bars mark storm events, which induced a morphologic reset on the intertidal beach. Note the different vertical scales of P l .Figure 5. Rectified and merged Argus image taken around low tide on 27 May 2002 (a) showing the dunes, the beach (region of interest of intertidal zone marked by dashed line) and a part of the subtidal zone with the inner bar line (white solid line). The region of interest regarding the intertidal zone was produced with a higher resolution (b) and consequently classified based on the surface composition (c) with the intertidal bar line (black solid line). with the alongshore position versus time (a) including their smoothed positions (solid lines). Spatial variability of the number of days on which rip channels appeared (b) and the time-averaged cross-shore position (c), and temporal variability of the number of rips N (d) and alongshore-averaged cross-shore position (the solid line shows the filtered data with a window of 14 days) (e). Except for (a), the distinction of IRC type was made: fIRCs are represented by black-filled bars or dots, and oIRCs by unfilled, white bars or dots. The extreme storm events are numbered and marked by horizontal dash-dot lines. with the alongshore position versus time (a) including their smoothed positions (solid lines). Spatial variability of the number of days on which rip channels appeared (b) and the time-av...

Coupling mechanisms in double sandbar systems. Part 1: patterns and physical explanation

“…Clear coupling patterns in double sandbar systems, such as in Figures 1, 8 and 9, are however rarely observed in the fi eld (Van Enckevort and Wijnberg, 1999;Castelle et al, 2007;Quartel, 2009). More often, inner-bar rip channels possess remarkably smaller and more variable alongshore scales than the outer bar (e.g.…”

“…cally face outer-bar horns, a situation reminiscent of the commonly observed 180° out-of-phase relationship of inner-bar patterns and shoreline rhythms (Sonu, 1973). Quartel (2009) observed a coupling of the intertidal morphology to the subtidal alongshore variability, with the phase between the two bar variabilities varying from in-phase (0°) to out-of-phase (180°), with gradual phase changes due to different alongshore migration rates of the bars. In addition, Castelle et al (2007) noted the systematic occurrence of two inner-bar rip channels within one outer-bar crescent, revealing the existence of a possible relationship at half of the outer-bar wavelength.…”

Coupling mechanisms in double sandbar systems. Part 2: impact on alongshore variability of inner‐bar rip channels

“…Clear coupling patterns in double sandbar systems, such as those presented in Van Enckevort and Wijnberg, 1999;Castelle et al, 2007;Quartel, 2009 and Part 1), are however rarely observed in the fi eld (Hom-ma and Sonu, 1962;Goldsmith et al, 1982;Bowman and Goldsmith, 1983). More often, inner-bar rip channels possess remarkably smaller and more variable alongshore scales than the outer bar (Barusseau et al, 1994;Van Enckevort et al, 2004;Lafon et al, 2004Lafon et al, , 2005.…”

Synthetic Imagery for the Automated Detection of Rip Currents

“…These techniques were used to extract datasets of up to 4 years (e.g., Holman et al, 2006); timescales that would be unfeasible to measure in-situ. A major limiting factor on the length of rip datasets that can be generated is that all attempts thus far required various degrees of manual input (Bogle et al, 2000;Gallop et al, 2009;Holman et al, 2006;Quartel, 2009;Ranasinghe et al, 1999;Turner et al, 2007;Whyte et al, 2005). The majority of research using video images uses the raw image products (snapshot, timex, daytime, etc.)…”