Barton On Sea Erosion Case Study

The Naish Farm section of cliffs from Chewton Bunny at Highcliffe eastward to Barton Cliff House forms the conserved stretch where erosion still continues and where there is a high degree of exposure and fossils can be collected. The cliffs here are retreating in a manner similar to that of the natural recession of the coast. A consequence of this is a low-angle, pleasant sand and gravel beach of natural appearance which is used by walkers, swimmers and surfers. It is, therefore, a useful amenity. Geologists and conservationists, in particular, consider this remaining section too important to conceal with sea-defences. At the top of the cliff there is a holiday camp rather than permanent housing, but unfortunately, as the pictures show. this much suffer some problems from the coastal retreat. Much of the remainder of the Barton and Highcliffe coast once looked natural, with eroding cliffs, like this Naish Farm area. For understanding degradation processes here consult the papers of Dr Max Barton.

The original coastline was relatively straight (examine old maps), in balance, and supplying shingle destined for Hurst Castle Spit (which contains Barton Clay fossils from here - Clavilithes). This present Naish Farm section of the coast is not natural in the sense that it represents an embayment between sea-defences. Loss of natural supply of beach material by longshore drift (from west to east) probably enhances the local erosion rate, but later when the embayment is deeper it might erode at a slower rate than would open unprotected coast. It will take some more years to find out whether this will happen. For more on the erosion rate, here, see the separate Naish Farm erosion section.

Naish Farm [Naish Estate] Section continued

- Coastal Recession of the Barton Clay

Various papers by Dr Max Barton and his co-authors have provided important information on the erosion and degradation of the Barton Clay. There has been a particular invesigation into the cliff degradation of the Barton Clay cliffs at the Naish Farm area of Highcliffe where erosion of this clay is most obvious.

First the rate at which the coast has retreated must be considered. Barton and Coles (1984) stated that the rate of recession here in the nineteen fifties was only 0.4m per annum. Although this is still quite a high rate for the south coast of England, it was not extreme because there was already an excess quantity of beach material here. Barton and Coles (1984) said that this had been discussed by Wise (1963), although the present writer has not seen this report. Certainly Burton (1931) discussed changes in the position of the Run at Mudeford and the periodic breaching of Mudeford Spit, and this resulted in a supply of sand to the western parts of the Highcliffe coast. Later when the excess of material was lost by longshore drift (eastward), the coastal recession which accelerated to 1.3m per annum.

There was a further change to a significantly rapid rate of to 1.9m per annum at Naish Farm during the nineteen seventies, probably because the Highcliffe groynes were constructed just before this decade (in 1968-1970). Beach material here, shingle and sand moves in general in an easterly direction by longshore drift. The groynes trapped much of the material from passing from west to east, as, of course, they are designed to do. This, however, led to some "starvation" of the beach at Naish Farm and some "terminal scour" as a consequence (see the Joint Local Inquiry Report of the Secretary of State, Department of the Environment, 1975). The originally fairly uniform coast now has a notable "step" in it and a new shallow bay has been created. This now, though has a sandy beach with some shingle, quite pleasant for tourists, and with "natural" fossiliferous cliffs that are educational and informative.

Naish Farm Section continued

- Collapse of Barton Clay Cliffs to Form Benches

The cliffs at Naish Farm, and in the past, all along the Barton coast section, are conspicuously terraced. These terraces or benches and their mode of formation was investigated by Barton and Coles, (1984) in their studies on the degradation of the cliffs just here (and see also other papers by M. Barton). Prior to their study the increase erosion in this area, as discussed above, steepened of the overall cliff angle from an average of 14 degrees in 1947 to a maximum recorded average of 19 degrees in 1976. The cliffs were, thus, particularly prone to landsliding and were found to collapse predominantly on a series of major shear planes (Barton, Hillier and Watson, 2006), more or less horizontal, but curved up at the back (at the landward side). The shear planes are located at certain lettered and numbered, geological horizons within the Barton Clay. These are shown in the cliff section illustration. They result in obvious benches or terraces in the cliff. These benches consist of collapsed clay debris and have top surfaces several metres or more above the specific shear planes. The benches have been named by Barton and Coles, (1984) as in the following sections:

Naish Farm Section continued

- - - The Top Scarp

The Top Scarp, shown here, is a cliff of a few metres high in brown-stained, subangular flint gravel, with some Brickearth or loam above. Both the gravel and the brickearth are of Pleistocene age. This is the Tenth River Gravel Deposit of the Geological Survey. Palaeolithic artefacts have been found in the gravels of this stretch of cliff. Slices of the gravel and loam cliff top subside on curving shear surfaces in the Barton Clay beneath.

The top-left photograph was taken in 2001 about midway between Barton and Highcliffe, and just west of the western end of the Barton section of sea-defences. There is a greensward on the top of the cliff here. Notice the block with brickearth which is out of position on the right, still with its soil and grass capping. A semicircular area has collapsed on the left. Incidently, you can see sand lenses in the periglacial, braided-river gravels.

The top-right photographs and the bottom photographs, all taken in 2003, shows collapse of the gravel at the Top Scarp at Naish Farm, just east of Chewton Bunny. Here coast erosion is a constant problem. Clearly some of the falls are quite recent.

NAISH FARM SECTION continued

- - - F2 Bench

This bench is near the top, in front of the top scarp. It is a narrow bench based in a shear plane at the base of bed F2, and with Plateau Gravel forming much of the back-scarp cliff. You can see it in the photograph above and in the central photograph below. The bench contains collapsed gravel debris and is broken up by a number of fissures parallel to the cliff. The flint gravel is progressively disrupted and becomes mixed with clay and turves of soil and grass, and slides gradually down the cliff.

LOCATION:

Naish Farm Section continued

- - - D Bench

As shown in the left and central photographs above, the middle bench, forms the extensive central part of the cliffs. There is the F scarp at the landward (northern) side of it with exposure of the upper part of the Middle Barton Clay. Shearing curves down from the scarp and continues nearly horizontally near the base of bed D, and beneath the terrace shown. See Barton, Hillier and Watson (2006 ) for information on the initial origin of the D shear surface by a lateral rebound response to coastal recession.

This middle bench is complex in pattern with much colluvium (debris) and also ponds developed as a result of rotational movement (notice the bullrushes in the pond in the central photograph, although they are have largely died off in this winter scene). These ponds, shown in the photographs, are mostly present near the F scarp. Small alluvial fans extend into the ponds and miniature deltas develop, as shown in the right-hand photograph (looking southward with reflection from the sun; note the little distributaries on the delta; note also how the reeds have grown on the shallower southern slope, away from the clay scarp). Not shown in these pictures are amphitheatre-like features which develop in places as a result of specific landslides.

LOCATION:

NAISH FARM SECTION continued

D Scarp and D Shear Surface

The D Scarp contains the D Shear Surface, shown in the photographs here. The name has been given by Barton and Coles (1984) because of this shear plane. Do not confuse this reference to D with the strata present in this scarp. It is mostly in Bed C, the Voluta suspensa Bed, the shear surface being almost at the base of Bed D. Lower down and in front of it is the A3 Bench of colluvium. Debris is falling onto this from the shear surface, which is very slowly moving forward. If you stand on the beach in this area, you may sometimes see small pieces of clay falling from the shear plane. However, this only happens when sliding is very active. This fresh, damp debris is browner in colour than the older material. It is dominantly weathered sandy clay from the Barton Formation, but with some yellowish-brown, flint gravel intermixed. This gravel has come from the Pleistocene, Plateau Gravel high in the cliff and has fallen into the other colluvium on a higher bench. Apart from this there is some plant material, and pieces of septarian nodules.

LOCATION:

NAISH FARM SECTION continued

- - - A3 Bench

This is the low bench extending upto about 3m above the beach, and obvious from the shore at Naish Farm. The shearing is at the base of A3. Further east, near the old gun emplacement, this disappears eastward under the beach, and the lowest sea cliff is then formed by bed C, beneath the D Bench.

Thus, the presence of the three shear surfaces at various elevations within the cliff produce a benched type of cliff profile, comparable with that seen in the Gault and Lias Clay controlled cliffs of Fairy Dell, near Charmouth, Dorset and can be seen in the London Clay of the Isle of Sheppey. Barton and Coles (1984) pointed out that excavating the toes of such landslides has reactivated slides of this type in the USA.

LOCATION:

NAISH FARM SECTION CONTINUED

- - - Degradation Processes

The degradation processes, according to Barton and Coles (1984), include scarp slumping, spalling (including toppling and soil falls), bench sliding (involving movement of colluvium over a preferred bedding plane), debris sliding (including movement of screes over clay scarps), mud sliding (i.e. traditional lobate 'mudflows'), mud runs (true flows), stream (or gully) erosion and man-related processes. The diagram alongside, after Barton and Coles, shows the main processes. Regular surveying and monitoring using a variety of techniques and having to surmount difficult field conditions, has begun to elucidate the characteristics, rates and inter-relationships of these processes. A flow chart representing the systematic transfer of soil from the in situ state via various colluvial modes (including the three bench levels) en route to the sea and an accompanying colluvial soil budget for the year July 1981 to July 1982 has been drawn up. It is shown that bench sliding is by far the most significant process in terms of volume of colluvium moved through the undercliff, accounting for 93 percent of the total volume of colluvium contained in the area. Mud and debris sliding, although important processes, are of relatively minor importance.

NAISH FARM SECTION CONTINUED

- Exposure Ratio

Barton (1998) has put forward the theory of Exposure Ratio. This, ER = (sum of exposed strata height/total cliff height) x 100. In other words it is just the relative height of exposed strata.

The table below, from Barton (1998), gives approximate values of exposure ratio in the Naish Farm area, east of Highcliffe. Accumulations of colluvium now cover the basal scarp at locations 1 and 2 because of the construction of a strong point at Chewton Bunny. Prior to this the exposure ratio at both of these locations would have been larger.

No.
 
Easting Range
(N.G.R.)
Recession Rate (m. /annum)
(1982-1993)
Exposure Ratio (%)
(maximum value)
1 2182-2183 0.74 34
2 2186-2190 1.14 39
3 2215-2225 1.14 54
4 2230-2235 1.71 55
5 2245-2250 0.94 47
6 2260-2270 0.60 23
Mean  
1.045 42

An important point made by Barton (1998) is that the type of cliff degradation that takes place results in much of the cliff being covered by colluvium (see diagram above). This means that only certain limited stratigraphical horizons are well-seen at any one time. Before the sea-defences were constructed various parts of the Barton succession were seen at different places along the coast. For instance, the Earthy Bed (E) is actually present at Naish Farm but is now rarely seen in good condition. The main exposures of this were further east and I used to collect from it in the area now covered by the Barton-on-Sea sea-defences. The old geological literature on the coast shows that various parts of the dipping succession have been seen at various places and at various different times. The cliffs have never been static and neither have the positions of the exposures. Dr Barton explains that certain specific stratigraphical levels, of around 0.5 to 1.0m above the preferred plane shear surfaces, have never received the stratigraphical and palaeontological study such has been devoted to the rest of the sequence. From a stratigraphical point of view, the colluvium never makes up the deficiency since the solid strata are completely remoulded within the interior of the benches by the intense shearing suffered by the softened material as it accomodates to the markedly non-circular slip surfaces.

Dr Barton states that the prediction of the likely exposure ratio of benched-form clay cliffs following partial toe protection will need to encompass a series of factors which he lists. The main objective is to avoid both the high rates of recession associated with rapid toe erosion and the eventual slope coverage associated with zero toe erosion. The aim therefore is for the state of moderate toe erosion (perhaps aiming at a value of about 0.3 to 0.6 metres per annum, although this would be subject to negotiation).

Such moderate toe erosion might be achieved at Naish Farm by an offshore breakwater or by partial beach replenishment. With the Chewton Bunny Stongpoint already reducing the exposure ratio at the western end of Naish Farm it is unlikely to please geologists and conservationists; with the cliff still retreating it may not please local objectors. It is, however, one possible way to resolve the problem of these cliffs and the original ideas in the Barton (1998) paper should be studied. If this scheme had originally been applied to the whole Christchurch Bay coastline and the blanketing sea-defences never constructed, it might have been acceptable to all. If and when the sea-defences ever collapse an offshore bar right round Christchurch Bay might be a solution but is likely to be unacceptable or impracticable or both (see section on the "Oceanus" scheme mentioned below).

LOCATION:

Between Naish Farm and the Barton Sea Defences

(see also the note on the D shear surface given above in the Naish Farm section)

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