The field investigation, using tracers, of meso-scale shingle beach behaviour

Abstract

Meso-scale (tidal time-scale) shingle beach processes are examined, using a combination of direct (trapping, tracing and core) and indirect (survey) data on an open shingle beach at Shoreham, W. Sussex.

Traditionally, studies of the coastal zone have been restricted to macro-scale investigations; these utilise morphometric comparisons, which are indirect and record the superimposition of many processes occurring over long periods. In order to gain an insight into the processes themselves, there is a need to study the appropriate processes at meso- and micro-scale (Horikowa, 1981). The technology to study micro-scale processes is still under development. Similarly, although approaches to the understanding of meso-scale processes have been available for the past two decades, there has been a reluctance to use such technology; this is due to the highly variable results produced, especially during high energy conditions. Against this background the present investigation seeks to develop more definitive measurement techniques, to study processes which influence shingle beach behaviour.

The literature review undertaken identified that results of transport experiments are sensitive to the various methods used. In particular, a number of fundamental requirements are identified; these have resulted in the development of the 'electronic' pebble.

Shingle transport was monitored during a (5 week, 2 Phase) field measurement programme, during a range of wave energy conditions. The first phase involved the coordinated deployment of trapping and tracer studies, using the 'electronic' and aluminium pebbles. The deployments were supported by daily measurements of beach form, together with the automatic recording of breaking waves. The results demonstrate that reliable transport measurements were possible during storms, using tracers (especially the 'electronic' types where recoveries of 78% were achieved). In contrast, the traps were susceptible to damage and, similarly, interacted with the processes being measured.

The second phase of the field programme involved a high intensity (1 to 2 tide resolution) morphometric study, incorporating transport layer thickness measurements and the monitoring of shingle beach behaviour. Wave observations were obtained at the same time. The morphometric study allowed Powell's (1990) SHINGLE beach profile model to be validated. The transport layer experiments reveal a direct relationship between breaker wave height and disturbance depths. Furthermore, shingle beaches display transport layer efficiencies which are comparable to morphometrically-similar sand beaches. At the same time, tracers record reliably the mobile layer thicknesses. Using novel (Grid and Column) tracer injection methods, shingle transport is shown to vary within the beach system. Differential across-shore transport is most pronounced during storm conditions (where an association with breaker zone transport, rather than swash, was found). Longshore transport is found also to decay with depth. The assumption that tracer material which moves vertically also undergoes horizontal advection was validated. Transport rate calculations indicate that shingle transport efficiency (K) increases with increased wave power. The reliability of the measurements obtained were confirmed by use of morphometric data.

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