Offshore to onshore transport across distinct landforms at Church Norton Spit

Minor funds contribution (2015-2016) \ University of Sussex (lead): £4,000

1. Background and motivation

Church Norton Spit at Pagham Harbour, West Sussex has undergone cycles of accretion and breaching since the harbour’s formation in 1910.

In the last 14 years, the spit has experienced excessive growth, although the source of this material is unknown. The material for this growth must therefore come from the sub-tidal as there is no other input from land or alongshore.

Some of the material will move as part of bedforms (green circled in Figure 1) but some may move using bedforms as a transport path (the orthogonal bar in the red circle). This latter process had been postulated by Julian Orford in a few papers in the 1990s, but remained a general untested hypothesis.

The aim of the project was to establish whether the orthogonal bars act as transport pathways for pebbles across the intertidal.

Figure 1: Aerial photography showing location of the more or less stationary orthogonal bar (red) and other intertidal shingle features (green)

Figure 1: Aerial photography showing location of the more or less stationary orthogonal bar (red) and other intertidal shingle features (green)

2. Project

SCOPAC contributed £4,000 to an undergraduate student project to investigate whether the orthogonal bars are acting as sediment transport corridors. The project used painted and tagged pebbles (using the RFID technology perfected in previous Coastal Partners* research www.coastalpartners.org.uk/services/tracer-pebbles) during 3 deployments (17/18 February, 4/5 March and 18/19 April 2016).

Pebbles previously collected from the feature included two sizes (approximately 100g and 300g) and were placed in rows parallel to the shore across the feature and left for two high tides after which all pebbles found were collected again for the next test. Placement and collection positions were collected with dGPS and an example of the layout and movements of pebbles during the first deployment can be seen in Figure 2.

* the group formerly known as Eastern Solent Coastal Partnership, rebranded in 2020 as Coastal Partners

Figure 2: Example of pebble placement (yellow) and collection (red) positions from the first deployment. Note that the aerial photography in the left hand panel is from June 2013 (CCO) illustrating some of the shore parallel movement of the orthogonal bar.

3. Results

Wave conditions were different for all three events (Figure 3) with the first deployment being the most energetic followed by deployment 2 and 3.

  • Those pebbles that moved, were transported almost exclusively landwards and obliquely towards the north. The oblique movement is consistent with slow advance parallel to the beach observed from survey data over the last few years and is a residual of the general obliqueness of waves measured at Rustington to have come from ~250°during the deployment
  • The average distance travelled for all pebbles in deployment 1 was 7.77 m while the average distance travelled during deployments 2 and 3 was 1.57 m and 0.96 m respectively. This is consistent with wave energy over each of the deployments.
  • While smaller pebbles seemed to have travelled further than larger ones, there was no statistical correlation between size and distance
  • The full report contains attempts to further analyse the data by position but the sample seems to be too small to allow for any inferences.

Figure 3: Wave and tide conditions during deployments taken from Rustington wave buoy and Littlehampton tide gauge.

4. Summary

The measurements on the orthogonal bar at Church Norton Spit show that pebbles move along the bar even under comparatively low energy conditions and support the assumption that these features act as transport corridors.

It is recommended that a future study deploy the tracers for more than one tidal cycle to identify longer term fluctuations in transport. In addition, further work on quantifying the volume of shingle moving along the orthogonal bars would be of benefit to understand their significance as a transport mechanism in relation to the wider sediment cell.

Uwe Dornbusch, 21-6-2016

This project was instigated by Dr Uwe Dornbusch of the Environment Agency, Arun DC and Chichester DC and was led by Dr Cherith Moses from the University of Sussex with input from Professor David Sear from the University of Southampton. Coastal Partners provided the tracer study retrieval equipment.

 

The project was carried out as part of an undergraduate thesis by Lucy White at the University of Sussex.

Download the thesis (PDF, 3Mb)