Field attrition experiments
The aim of this part of the BERM project is to estimate the volume of beach lost through shingle abrasion and closely relates to the tumbling experiments.
To estimate the susceptibility of pebble abrasion tumbling experiments are run in the laboratory. Under controlled conditions we can for example determine the susceptibility different types of flint or other rocks.
To calibrate the tumbling experiments with the real world, attrition experiments have to be carried out on the beach. The process of measuring attrition in the field involves: weighing pebbles, putting them on the beach face so that they are moved over the beach by the waves, recovering the pebbles from the beach and weighing them again. The weight difference can then be related to the time the pebbles have spent on the beach allowing an assessment of the abrasion rate.
The main problem is recovering the pebbles from the beach because they are likely to have been
a) moved in a longshore direction
b) moved in a cross shore direction
c) buried
Overcoming these three problems is part of the project and initial trials involve
1) attaching a pebble to a fixed location (e.g. with a thread to a pole driven into the beach)
2) observing a pebble during its time on the beach
3) putting out pebbles of distinct colours
All trials carried out so far involved only a very small number of pebbles so that the results may not be statistically significant but measurable abrasion takes place on flint, quartzite and limestone during only one tide!
Figure 1: Limestone (F) and two quartzites (D, M) after spending two tides on a pebble beach during which time the limestone lost 0.12% and the quartzites lost ~0.078% of their weights.
On an annual basis this may result in a significant reduction of size and weight of those pebbles that are moved on the shore face but further experiments involving a greater number of pebbles and carried out over a longer time span are need to support the initial trial results.
U.Dornbusch 29-01-2001