Beach face and berm morphodynamics fronting a coastal lagoon Felicia M. Weir a, ⁎ , Michael G. Hughes a , Tom E. Baldock b a School of Geosciences and Institute of Marine Science, The University of Sydney, NSW, 2006, Australia b Division of Civil Engineering, The University of Queensland, St. Lucia, QLD, 4072, Australia Received 1 December 2005; received in revised form 19 May 2006; accepted 19 May 2006 Available online 20 July 2006 Abstract This study documents two different modes of berm development: (1) vertical growth at spring tides or following signif icant 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 f inal 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 backf ill 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. © 2006 Elsevier B.V. All rights reserved. Keywords: Berm; Morphodynamics; Swash; Wave runup; Sediment transport shape function; Intermittently closed and open lagoon (ICOL) 1. Introduction The wave-dominated coastline of New South Wales, Australia, has approximately 130 estuaries. Many of these are coastal lakes or lagoons with entrances that naturally cycle between being briefly open to the ocean and being closed off by a wave-built berm for extended periods of time (Roy et al., 2001). The beach berms responsible for closing off coastal lagoons are created through the depo- sition of sediment at the landward extent of wave runup, resulting in the beach face profile growing both vertically and horizontally seaward. This tends to produce an in- creasing profile gradient approaching the berm crest on the seaward side and a horizontal to gently dipping back- beach profile on the landward side. Berms are ubiquitous on steep, coarse-grained beaches and several early studies proposed a simple relationship between berm height above mean sea level and wave height (Bagnold, 1940; Bascom, 1953; King, 1972). Subsequent studies have proposed relationships that include not only wave height but also wavelength (period) Geomorphology 82 (2006) 331 – 346 www.elsevier.com/locate/geomorph ⁎ Corresponding author. Tel.: +61 2 93514050; fax: +61 2 93510184. E-mail address: fweir@geosci.usyd.edu.au (F.M. Weir). 0169-555X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2006.05.015