Beach Face and Berm Morphodynamics Fronting a Coastal Lagoon

Brief description

This study documents two different modes of berm development: (1) vertical growth at spring tides or following significant beach cut due to substantial swash overtopping, and (2) horizontal progradation at neap tides through the formation of a proto-berm located lower and further seaward of the principal berm. Concurrent high-frequency measurements of bed elevation and the associated wave runup distribution reveal the details of each of these berm growth modes. In mode 1 sediment is eroded from the inner surf and lower swash zone where swash interactions are prevalent. The net transport of this sediment is landward only, resulting in accretion onto the upper beach face and over the berm crest. The final outcome is a steepening of the beach face gradient, a change in the profile shape towards concave and rapid vertical and horizontal growth of the berm. In mode 2 sediment is eroded from the lower two-thirds of the active swash zone during the rising tide and is transported both landward and seaward. On the falling tide sediment is eroded from the inner surf and transported landward to backfill the zone eroded on the rising tide. The net result is relatively slow steepening of the beach face, a change of the profile shape towards convex, and horizontal progradation through the formation of a neap berm. The primary factor determining which mode of berm growth occurs is the presence or absence of swash overtopping at the time of sediment accumulation on the beach face. This depends on the current phase of the spring-neap tide cycle, the wave runup height (and indirectly offshore wave conditions) and the height of the pre-existing berm. A conceptual model for berm morphodynamics is presented, based on sediment transport shape functions measured during the two modes of berm growth.

Lineage

Statement: Two field campaigns were conducted over the periods 13-22 October, 2003, and 15-18 November, 2004, at Avoca Beach, New South Wales, Australia. The field measurements reported here were obtained immediately in front of Avoca Lagoon. The beach face at this site is uniformly composed of coarse sand. A surface sediment sample collected from the mid-swash zone had a mean grain diameter of 0.525 mm, and is representative of the entire profile. During the first experiment, the beach morphology was characterized by a steep beach face with a gradient that ranged from 0.097 to 0.133. During the second experiment the beach face gradient ranged from 0.067 to 0.100.

Notes

Credit
The University of Sydney

Credit
University of Queensland

Purpose
The NSW coast is a high energy, wave-dominated environment. This study looks at the entrance to the Avoca Lagoon which is situated approximately halfway along the beach and is intermittently open to the ocean during and following times of heavy rainfall. This purpose is to avoid flooding of properties along the lagoon foreshore during periods of high rainfall.