Northern Sea Wall, Kent The North Kent Shoreline Management Plan (SMP) defines management units along the North Kent coast ranging from Management Unit 4a - 1A at the Isle of Grain, to Management Unit 4b - 9D at Dover. The Northern Sea Wall is bounded entirely by the whole of Unit 5E, which covers a distance of 4.5km from west of Reculver Towers through to Plumpudding Island. Management unit 5E is split into 11 subunits and includes five relatively short sections near the western end of Reculver and six longer lengths along the Northern Sea Wall (Figure 1). Figure 1: Northern Sea Wall and Associated Sub-Units A study of the area in 1993 compiled by Robert West and Partners, and a scheme strategy plan produced by Canterbury City Council in 1996, lead to works being carried out along the entire length. The beach was recharged with 110,000m 3 of shingle interspersed between rock groynes and covered sub-units 6-11, or collectively known as the Northern Sea Wall. However, a major shortcoming to the scheme was due to the absence of reliable historic data. This resulted in some uncertainty with respect to the sediment budget for the area, beach response to storm attack, expected losses and future recharge requirements.
In order to monitor the newly placed beach, a programme of monitoring commenced to record changes. The monitoring programme ran for a five-year period from September 1997 to November 2002. The aim of this longer term study was to obtain a better understanding of the performance of the new beach and to establish needs for future beach replenishment and recycling. Two problems immediately became apparent; firstly, the outfalls had a tendency to block on a regular basis due to shingle accumulating at the mouth, which reduced the flow of water out to sea. To compensate for this, regular recycling operations are necessary which remove the shingle and stockpile it at the crest of the beach (Figure 2). Secondly, a shingle bank called the Wantsum Delta, was losing material on a regular basis and required frequent recycling operations to maintain it (Figure 3). The maintenance of the bank is particularly important as it protects the saline lagoons that are designated SSSI sites and are of significant importance to bird life. Figure 3 : Recycling Operations to unblock the mouth of an outfall Figure 2 : Recycling Operations to maintain the Wantsum Delta The monitoring consisted of 19 beach level stations (BLS) numbered 50 through to 68, at regular intervals along the Northern Sea Wall. Profiles were taken from these points perpendicular to the wall down to the toe of the beach (i.e. the shingle/mud foreshore transition). The level of the beach surface was measured with an electronic theodolite and an electronic distance measurer (EDM) over the course of each profile line. Profiles were taken biannually thus enabling the calculation of cross-sectional beach areas and the monitoring of relative changes in beach volumes (Figure 4). By
assigning stretches of beach that were representative of beach profiles, a sediment budget was compiled for the Northern Sea Wall for the five years subsequent to the replenishment (Figure 5). Figure 4: Changes in the beach profile for BLS 65 Figure 5: Sediment Budget for Annual changes in Beach Volume by beach section.
The confidence levels for some of these sections are poor; for example sub-unit 5E.8 is eroding by 1 908m 3 /year ( + / - 3 728m 3 ). This is due to the uncertainty brought about by the recycling operations that were not quantified by monitoring. In contrast, high confidence levels were attained for areas that were subject to no recycling. Figure 6 shows a section of the western sea wall that is eroding by -6 809m 3 /year ( + / - 737m 3 ). Figure 6: Erosion at the Western End of the Northern Sea Wall The five years of monitoring along the Northern Sea Wall produced a good dataset, but it also raised new questions and highlighted areas where the monitoring strategy could be improved. This monitoring scheme has been superseded by the Strategic Regional Coastal Monitoring Project. By working in conjunction with the Environment Agency (EA), information on recycling operations will be monitored, quantified and fed back into the sediment budget. The project will continue to take bi-annual profiles at the existing BLS sites, but in addition to these, new profile lines will be set up at volatile areas highlighted in the original monitoring project. All the bench marks have been tied into the Ordinance Survey (OS) national network and monitoring will be conducted with GPS surveying equipment. Profiles will also be taken within 24 hours of a storm event providing vital information on the beach response to extreme events.
To supplement this, an Aerial Beach Management Survey (ABMS) will be flown each summer, providing profile lines through photogrametric analysis for existing survey lines and additional intermittent locations. Most significantly, an annual beach plan survey will be conducted with GPS surveying equipment to map the entire beach and foreshore. This will also cover the stockpiles of recycled material and enable the entire beach volume, accretion and erosion trends to be established. The benefits of monitoring data are already becoming apparent at this site. Extra groynes proposed at the eastern end are no longer required as the monitoring has indicated that the beach is now accreting, with the short term trend in erosion being attributed to an unusually high occurrence of Easterly winds. Sections of beach that are stable or accreting, have been earmarked as potential sources of recycling material for the future and can be used should their be a reoccurrence of the problem. The need for continuous recycling operations at the outfalls puts a good case forward for works to extend them, hopefully negating the need for any further recycling at these locations. English Nature are concerned with the stability of Wantsum Delta and the maintenance of the Brackish lagoons, but such detailed monitoring data allows them to plan for the future, measure trends and assess the necessity of any works. Performance of the beach will aid future scheme design and help predict the longevity of the initial works and the requirement for future replenishments. Information from the monitoring programme therefore provides a significant management tool in helping to maintain an important coastal defence and its adjoining environmentally sensitive areas. In addition, data from the monitoring of this coastline and the lessons learnt, can be fed into subsequent scheme designs in other areas. The Northern Sea Wall For further Information Contact: Strategic.Monitoring@Canterbury.gov.uk