Berm formation and dynamics on a gently sloping beach; the effect of water level and swash overtopping

Abstract

Berm formation and morphological development of the beach face have been observed during a neap-neap tidal cycle on the gently sloping and accreting beach at Vejers, Denmark. During the field campaign, an intertidal bar migrated onshore and stabilized as a berm on the foreshore. A new intertidal bar occurred on the lower beach face, migrated onshore on the rising tide and finally merged with the pre-existing berm. As the tide continued to rise, the new berm translated further onshore as an intertidal bar to the uppermost part of the foreshore. The sediment transport during the berm transition was onshore directed in the upper swash and offshore directed in the lower swash. This berm development can be described through both the neap-berm, ridge-and-runnel and berm-ridge development concepts proposed by Hine (Sedimentology 1979; 26: 333-351), and all three stages were observed during only three tidal cycles. The main factors controlling this fast transformation were the gentle slope of the cross-shore profile, rapid water level translation rates, substantial swash overtopping of the berm, and low infiltration rates. Despite the onshore migration of intertidal bars and berm formation, no net foreshore accretion took place during the field campaign. This was largely due to the formation of rip channels with strong rip currents cutting through the intertidal bars and the berm, which acted as a sediment drain in the profile.

Swash Zone Bed Level Changes and Sediment Entrainment at the Surf-Swash Boundary

“…As other studies have concluded (e.g. Aagaard and Hughes, 2006;Weir et al, 2006;Baldock, 2009, Jensen et al, 2009 this indicates that the hydrodynamics vary throughout the swash zone and explain why modelling swash zone sediment transport is very difficult.…”

“…Several studies show that the hydrodynamics, and hence sediment transport, differs between the "lower" and "upper swash" (Aagaard and Hughes, 2006;Weir et al, 2006;Jensen et al, 2009). The lower swash is influenced by wave-swash interaction while the upper swash is only influenced by pure swash motion.…”

“…Sudden erosion or accretion can be due to the migration of ripples or berms (e.g. Jensen et al, 2009), but features like ripples can not be tracked using rods measurements. The method of using self-logging PTs is a very flexible method because without cables it is possible to move the transducers with the swash zone as it translates across the foreshore with the rising or falling tide.…”

Field Observations of Turbulence, Sand Suspension, and Cross‐Shore Transport Under Spilling and Plunging Breakers

“…Waves were strongly attenuated by breaking across the nearshore bar system, and the maximum significant wave height recorded at the instrument station was therefore no more than H s = 0.7 m. Over a tidal cycle, and depending on the incoming wave height, shoaling waves, breaking waves (plunging or spilling at the shorebreak; Figure 1), surf bores, and swash were all observed at the main instrument station. During the course of the experiment, net onshore transport at the instrument array caused onshore migration of the intertidal bar located between mean high and low tide levels ( Figure 2) and transformation of the bar into a berm (Jensen et al, 2009).…”

Suspended sediment transport and beach dynamics induced by monochromatic conditions, long waves and wave groups

“…The berm formation processes has been previously studied considering the effects of wave height, tidal fluctuations, water level changes induced by storms, sediment fall velocity, grain size or permeability (Bagnold 1940, Hine 1979, Weir et al 2006or Jensen et al 2009). But most of the studies dealing with berm formation, are qualitative studies, based on field experimental data highly affected by the tidal effect on gravel beaches.…”