Test of self‐organization in beach cusp formation

Abstract

[1] Field observations of swash flow patterns and morphology change are consistent with the hypothesis that beach cusps form by self-organization, wherein positive feedback between swash flow and developing morphology causes initial development of the pattern and negative feedback owing to circulation of flow within beach cusp bays causes pattern stabilization. The self-organization hypothesis is tested using measurements from three experiments on a barrier island beach in North Carolina. Beach cusps developed after the beach was smoothed by a storm and after existing beach cusps were smoothed by a bulldozer. Swash front motions were recorded on video during daylight hours, and morphology was measured by surveying at 3-4 hour intervals. Three signatures of selforganization were observed in all experiments. First, time lags between swash front motions in beach cusp bays and horns increase with increasing relief, representing the effect of morphology on flow. Second, differential erosion between bays and horns initially increases with increasing time lag, representing the effect of flow on morphology change because positive feedback causes growth of beach cusps. Third, after initial growth, differential erosion decreases with increasing time lag, representing the onset of negative feedback that stabilizes beach cusps. A numerical model based on self-organization, initialized with measured morphology and alongshore-uniform distributions of initial velocities and positions of the swash front at the beginning of a swash cycle, reproduces the measurements, except for parts of one experiment, where limited surveys and a significant low-frequency component to swash motions might have caused errors in model initialization.

Field observations of swash zone infragravity motions and beach cusp evolution

“…Differences between observations and numerical predictions also could originate from the use of the IMLE technique with run-up observations. The IMLE might be affected by the presence of rhythmic cuspate features causing time lags between swash motions in cusp horns and bays [Coco et al, 2003] that could increase concentration of energy at wave numbers related to the beach cusp spacing.…”

“…[6] Two other experiments (already reported by Burnet [1998] and Coco et al [2003Coco et al [ , 2004) in which pre-existing beach cusps were flattened by a bulldozer on a limited control section also were conducted. Beach cusps present at the edge of the control section were left undisturbed, and the extent to which they might have affected swash flow circulation patterns is unknown.…”

“…Swash fluid flow was measured by continuously recording 2-hour videotapes during daylight hours. Digitized (2 Hz) images were analyzed to derive swash front elevation time series along cross-shore profiles spaced every 5 m alongshore [Burnet, 1998;Coco et al, 2003]. Beach morphology was surveyed over a region encompassing the upper beach and the swash zone that included the area covered by beach cusps.…”

“…In contrast, field observations of infragravity motions in the swash zone [Guza and Thornton, 1982;Holman and Sallenger, 1985;Holland et al, 1995;Raubenheimer and Guza, 1996;Ruessink et al, 1998;Holland and Holman, 1999;Raubenheimer, 2002] have not been used to show how the edge wave dispersion relationship might be modified by the presence of swash morphology. Here the interactions between swash zone infragravity motions and evolving beach cusps are investigated with field observations [Burnet, 1998;Coco et al, 2003Coco et al, , 2004 obtained, before, during, and after beach cusp formation.…”

Detecting nonlinearity in run-up on a natural beach

“…1a) used here were digitized from sub-aerial video imagery collected at the US Army Corps of Engineers Field Research Facility at Duck North Carolina, on 6 September 1994 (these timeseries have been used previously in Burnet (1998) and Ciriano et al (2005). In addition, Coco et al (2003) provide details on the methods used to extract run-up timeseries from video). Timeseries were collected at 2 Hz and converted to variations in water level elevation with the help of surveyed measurements of beachface morphology and standard camera rectification techniques.…”