This information has proven to be highly useful, particularly for dynamic or highly managed beaches where elevation difference plots or aerial photography are not sufficient to identify net sediment movement following replenishment or recycling activities. The ESCP have successfully applied the method in conjunction with topographic data to understand how quickly and how far replenished material moves downdrift. The results have also helped to confirm the location of drift divides; convergence zones, net sediment drift patterns as well as identifying localised drift reversals.
The images below show an example of how tracer pebbles have been used alongside traditional beach surveying methods to aid the analysis of longshore sediment transport around Portsmouth.
Historically, drift along this frontage was understood to be from west to east (left to right in the picture), as this direction of transport is seen along the rest of the beach frontage. A topographic difference plot showing the changes in beach levels between 2004 and 2016 indicates that erosion has occurred (shown in red) around the western end of the beach, suggesting material is lost from this end of the beach.
Topographic difference plot, showing the changes in beach levels
between 2004 and 2016.The red colour shows where the beach has
lowered (erosion) and the blue indicates areas that have built up (accretion).
Using this valuable information, our engineers are able to analyse trends in beach behaviour, both long term and in response to high energy wave events. This helps us to be able to estimate where material can be recycled to and from and identify areas where material is likely to be lost.
However, when results from the tracer study carried out between 2011 and 2016 are overlaid onto the same image.It becomes clear that the net sediment drift direction is in fact from west to east!