Date: 6 July 2014
 

Monitoring Team: Mark, Nick & Shawn
 

Analysis & Report: David Bills-Thompson, PFCG Technical Officer and Data Co-ordinator

 

Commentary: The Beach Monitoring report for July issued 7/07/2014 stated that "erosion of the dune toe at post 8 might have been in the order of 3 metres had the WED structure not been in place."

 

Further analysis suggests the erosion at post 8 without the protection of the WEDs would have been over 4 metres. See chart Without Weds 0714B.pdf and compare with Dune Toe 0714B.pdf

The following method was used to derive this value:

 

• A base line value of dune toe distance was established with the average of values for March and April 2013, these values being similar over consecutive months.
   

• A monthly change in toe distance was then expressed as a per unit change against the base line average for posts 7, 8 and 9 between May 2013 and May 2014. (Per Unit = 1/100 of percentage value)

 

• These values were plotted against the time scale and trend lines added to show the correlation between erosion rates at post 7 and post 8. See chart Correlation 0714B.pdf

 

There is a strong correlation between the dune toe erosion at post 7 and at post 8 as also indicated in the dune toe series of charts back to the start of data collection, so the July dune toe erosion at post 7 can be used to predict the dune toe distance at post 8 without the WEDs. Post 9 data did not correlate well with post 7 and 8 and the data was not included. (Posts 9 and 10 are in a transitional area of the beach that is significantly higher than post 7 and 8 so it is not unexpected that they show different erosion characteristics.)

A ratio of the post 8 and post 7 erosion rates was averaged between June 2013 and May 2014 and gives a value of 3.48 and then a value sourced for the post 8 dune toe distance that gave the same ratio using the July post 7 dune toe distance. The post 8 dune toe distance prediction without the WEDs was 10.538 metres or just over 4 metres more than the previous distance.

Maintaining the sand height at post 8 has had a dramatic effect in preventing dune toe erosion at the lowest part of the beach.