4- Why has the pebble and shingle ridge at Porlock Bay become progressively more vulnerable to breaching?

During the past 4000 years the coastal system at Porlock Bay has become what Orford (2004 p 1) describes as ‘sediment constrained and storm wave dominated’ and ‘a barrier subject to over washing, landward migration, breaching and tidal lagoon development’. A report by DEFRA and the Environment Agency in 2001 stated that ‘studies reveal a history of transitions between freshwater and saline conditions indicative of a barrier that was periodically breached and resealed as it migrated inland’.  Ballance (2001) identifies that ‘in some periods heavy rainfall and occasional breaches caused an extension of the flooded area behind the ridge.  This happened from 1946-1951 and from 1959 to 1962’. In the recent past major breaches and inundations of the hinterland of the pebble ridge occurred in 1910, 1981 and 1990 for example and Posford Duviuier (1992) reported that the ridge was overtopped on every high tide between 1939 and 1960.  

As a consequence the pebble ridge has become progressively weakened and unstable over time and increasingly susceptible to breaching as ‘major changes in the ridge morphodynamics have taken place’ (Pethick 1996 p 6) as a result of several inter-related factors:

  • Head sediment supply of gravel, sands and pebbles from the cliffs between Foreland Point and Gore Point to the west of Porlock Bay has ‘virtually ceased’ (Orford 2004 p 4) as a result of decreasing rates of relative sea level rise particularly over the past 2000 years (Jennings et al 1998).  As a consequence the current sediment supply entering the system and estimated to be in the range of 250 – 2000 cubic metres per year (Orford 2004 p 4), is failing to compensate for the volume of material lost, particularly in the central Porlock and Bossington beach portions of the ridge.  The volume of sediment here has been reduced significantly as a result of highly destructive storm wave and overtopping events (where sediment is carried over the ridge by storm wash over) and the steady accumulation of material to the east at the Hurlestone Point end of the barrier as a result of ongoing longshore drift.  The ridge has become therefore progressively unstable as a result of coastal processes ‘forcing the barrier into a more swash – aligned structure’ (Pethick p 8);
  • The pebble ridge is becoming increasingly longer and thinner making it more vulnerable to breaching and overtopping.  A study by Posford Duvivier (1992) using measurements from historic maps revealed that the ridge increased in length by 6% as it was pushed landward by the sea from 1809 -1888.  Whilst it was pushed landward the two ends of the pebble ridge nevertheless remained anchored at Porlock Weir at the western end and Hurlstone Point to the east.  Subsequently the ridge had to bend or bow in the middle in order to keep itself anchored causing this 6% increase in length and corresponding reductions in its thickness;
  • The construction of groynes on the beach face and sea walls at the back of the beach face at Porlock Beach to protect the main road from Porlock Weir to Porlock and at Porlock Weir itself adjacent to the quay and dock at the western end of the ridge in the early 19th century, has interrupted and reduced the replenishing sediment supply entering the system from the west.   The image in Resource 27 shows wooden groynes constructed on the pebble ridge east of New Works.

The vulnerability of the pebble ridge was summarised in a Porlock Ridge and Saltmarsh SSSI report by Natural England in 2008 (p 1):

“This major source of coarse sediment has long since disappeared, leaving only a relatively insignificant input of sediment from occasional cliff falls.  The inputs of sediment to the beach ridge from this modern source are too small to sustain the earlier beach profile and the increase in the length of the ridge as it continues to rollback, in a lengthening curve into Porlock Bay.  This means that the ridge has been growing steadily thinner ever since it was formed, a condition exacerbated by the further reduction in shingle inputs caused by the construction of groynes at Gore Point at the western extremity of the ridge”.  

Until 1996 the response to breaching and overtopping events along the ridge was to undertake compensatory replenishment of the lost sediment.  This commonly involved dredging material (mostly gravel, sand and mud) from the entrance to Porlock Weir harbour and transporting it along the ridge by digger where it was used to strengthen points of identified weakness and to fill breaches.  Bulldozers were also used to push pebbles and shingle that had washed over the ridge during storms back onto the ridge again to reinforce it – see for example the ridge restoration work of 1991 shown in the photograph in Resource 28.   The period 1967 – 1971 saw a prolonged period of such ridge replenishment (Ballance 2001 p 4).  The photographs in Resource 29 and Resource 30 show the ridge being replenished and strengthened following storm damage in 1952.

Consolidate Your thinking

The construction of groynes, sea walls and beach replenishment through dredging and deposition of sediment, are often referred to as techno-centric and interventionist coastal management practices.  Explain what you understand techno-centric practices to be and how they differ from eco-centric or non-interventionist coastal management policies.  See pages 37-38 of the following document to begin your research

Go to question 5 - Why did the storm of 1996 cause a catastrophic breach in Porlock Ridge?