Beach cusp morphodynamics

Abstract

Detailed measurements of three-dimensional beach cusp morphology were made on a steep gradient, low energy, microtidal beach in Perth, Western Australia. During the field campaign a variety of wave conditions and tidal ranges were experienced, and these differing hydrodynamic conditions were reflected in a consistent pattern of morphological changes to the beach cusp system. A useful parameter to delineate between trends of cusp destruction and re-formation appeared to be the surf similarity parameter ξ = tan β⁄√H o ⁄L o , where H o is offshore wave height, L o is deep water wave length and tan β is beach gradient. For ξ<1·2 the beach cusps were planed off, whereas cusp morphology was enhanced when ξ>1·2.A small storm was experienced at the start of the field campaign period and resulted in considerable erosion of the beach face. The cusp morphology across the lower beachface was destroyed, but a subtle remnant of the pre-storm cusp morphology was preserved on the upper beachface. When cusps reformed after the storm, under the influence of declining wave conditions, they appeared at the same location and with the same dimensions as the pre-storm cusp morphology. Hence, it is considered that the cusp re-formation was controlled more by the antecedent morphology than the hydrodynamic conditions. This indicates that positive feedback between swash hydrodynamics and beachface morphology, necessary to form beach cusps, does not require a large variation in relief.

Relationship between high-frequency sediment-level oscillations in the swash zone and inner surf zone wave characteristics under calm wave conditions

“…that wave heights (H m ) and water levels have a strong linear correlation as observed by Thorn and Guza [34], Wright et al [42], Horn [43], and Masselink and Hegge [18]. Therefore, the third canonical variate pair indicated the beach cusp moved on to the beachface with the water level variations.…”

“…The sediment-level oscillations gave a window to investigate the beach cusp morphodynamics. Masselink et al [18,19] proposed a threshold to describe the beach cusp development with the surf similarity parameter. When ξ b <1.2, that beach cusp is planed o , whereas cusp morphology is enhanced when ξ b >1•2.…”

“…Because the tide range was small, signi cant swashes reached the highest pegs during the entire experiment. Although the diurnal tide range for the area is 1.6 m, the range from low to high tide on the day of our experiment was 0.6 m. We also made an accuracy test by taking repeated measurements as in Sallenger [16], Masselink et al [18]. The accuracy of the technique was ±2 mm in the upper swash zone (A2, A3, B2, B3).…”

“…Both transects consisted of 3 steel pegs (10 mm diameter and 1.2 m in length) which were inserted approximately 0.8 m into the sediment (cf. [9,[17][18][19]). …”

Technical note: Optimizing the utility of combined GPR, OSL, and LiDAR (GOaL) to extract paleoenvironmental records and decipher shoreline evolution

“…Three-dimensional grid modelling can be used to visualise how good gain control can distinguish barrier facies boundaries (Figure 4a) and isolate beachfaces by interpolating the highest amplitude reflections between a series of shore perpendicular transects (Figure 4b). The use of 3-D models is not necessary for extracting sea level 30 and storm records, but could be useful in studying shoreline rotation (Harley et al, 2011;Short and Trembanis, 2004) or smaller-scale and more irregular features such as beach cusps (Coco et al, 1999;Masselink et al, 1997). …”