SEATOWN

Seatown looking west to Golden Cap 1996

'Hold the Line'

Seatown is a small village located a short distance west of the River Winniford. The river roughly bisects the 2 km long shingle beach which has formed between the headlands of Golden Cap in the west and Doghouse Hill in the east. The sea cliffs decrease in height from these headlands towards the river mouth where the cliffs are probably buried by shingle. Subparallel to the seacliffs there is another significant geomorphological feature known as a "backscar". This feature is characterised by a steep gradient which reflects the exposed failure plane along which landslip movement has taken place. The area between the backscar and the seacliffs can therefore be considered to be an area of obvious historical landslides.

Anecdotal evidence suggests that the backscar has remained relatively stable since the 1960's although a cafe located between the seacliffs and backscar has apparently been destroyed by landslipping. Cracks have been observed in the Anchor Inn and other buildings in the area. Most of the buildings are painted or protected from the weather by tiles hence observation of cracks is severely limited.

In addition to erosion of the seacliffs the sea has been undermining the wall at the end of Sea Hill Lane.

A car park is located on the east bank of the river.

Shingle extraction from the beach apparently took place up to about 1987.

A well used footpath is located above the seacliffs and joins Sea Hill Lane at the location of the undermined wall.

The area is designated as a Site of Special Scientific Interest (SSSI).

Erosion is occurring along most of the coastline between Lyme Regis and Portland. At Seatown, sea erosion has produced undermining of the wall at the end of Sea Hill Lane and ongoing erosion of the seacliffs. Landslides and associated ground movements have occurred to at least the backscar. The cracks noted in various buildings may be premonitory evidence of incipient further landslips beyond the backscar.

Shingle supplied to the beach by littoral drift from the west provides some protection to the undermined wall and adjacent sea cliffs. However, it is likely that the shingle in this area is gradually depleting as natural eastward movement continues and supply is restricted by the Golden Cap headland.

Erosion is likely to continue in the vicinity of the wall unless a "protection" system is installed. The adopted method should significantly reduce maintenance in this area and minimise the risks of consequential failures (like Holbeck Hall Hotel in Scarborough). It should be noted that significant erosion and/or landslip could lead to for example the loss of the footpath, the public conveniences and the end of the highway. In these circumstances there would be a loss of amenity and access and an increased level of inconvenience to the general public. The adopted scheme must be sympathetic to this area of outstanding natural beauty and its SSSI designation.

The scheme during construction

The final scheme started in the autumn of 1997 involved the placement of rock armour from the turning circle to 100 m to the west. The armour was placed on terram mats. A number of gravel filled drains were installed across the backscar of the landslipped area immediately to the west of the buildings while the steeper backscarp was pinned with soil nails to prevent further loss.

The height and size of stone armour were reduced in size on landscape grounds

During construction, exceptionally heavy rains lead to the activation of the landslide and the consequent loss of the public toilets and part of the Anchor Inn beer garden.

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CHARMOUTH

THE OLD SEA WALL AND FACILITIES

Charmouth is a popular bathing beach. The sea front protects the Charmouth Heritage Coast Centre, beach cafe, toilet block and car park


Storms constantly threaten the old sea wall and facitilies behind it

Cliff recession to the west of the promenade at Charmouth, in Dorset, is threatening to outflank the structure unless remedial action is taken. Also, the seawall fronting the promenade is in a very poor structural condition and near the end of its useful life.

Outflanking of the promenade and/or failure of the seawall, poses a threat to a building behind, which houses a Heritage Coast Centre and Cafe. This, together with recreational facilities, are important features of this popular tourist resort.

To remedy the situation, a scheme is proposed which comprises a rock bastion, positioned at the western end of the promenade and a new stepped seawall over a 75m length frontage. The bastion will protect the vulnerable point, where the cliff and promenade meet, from wave attack and prevent outflanking of the structure. Also, with predominantly eastward drift within the coastal cell, the bastion will encourage the build-up of a storm beach, which is presently lacking to the west of the promenade. A healthy storm beach, fronting the cliffs adjacent to the bastion, will arrest, or retard, cliff recession and, hence, reduce the danger of it also being outflanked.

The new stepped seawall, whilst restoring protection to property and recreational facilities, will reduce swash heights and wave reflections, compared to the present vertical seawall and provide greater amenity value. Some setback in the beach is expected on the eastern side of the bastion, due to easterly drift. It is thus proposed that shingle, which tends to accumulate on the beach to the east of the promenade, be used as a replenishment source. Significant accretion of this beach is presently causing the mouth of the River Char to be deflected eastward, which has necessitated continuing maintenance. Using shingle from the beach for replenishment will reduce the throughput of material to some extent and this may have a beneficial effect on the river mouth.

The estimated cost for the scheme is £386,000, which includes contingency, lead-in costs and design and supervision allowances. Based on predicted erosion contours for the immediate area, in a 'do nothing' situation, possible scenarios (providing for loss of the seawall and promenade in 5 and 10 years) were investigated in the financial scheme appraisal. Benefit to cost ratios of 2.11 and 1.17 were obtained for loss of protection in year 5 and 10 respectively. These figures indicate that the scheme is economically viable, even under the lowest assumed erosion rate. However, what was clear from the benefit assessment analysis was that benefits from tourism far outweighed the benefits to be gained from protecting property and other structural assets. This, it is concluded, is not an unexpected finding for a scheme of the nature of that proposed, for a unique resort such as Charmouth.

The small resort of Charmouth, 4.5 km to the east of Lyme Regis in Dorset, is situated just upstream of the mouth of the River Char. Despite its small size, it is a very popular resort for day visitors and holidaymakers, there being a number of caravan parks within the resort and the immediate area. The seafront is approached down Lower Sea Lane. Here, on the western side of the river which flows across a shingle beach to the sea, is a small promenade, car parking area and building. The building houses a cafe and Heritage Coast Centre.

The river mouth is flanked to the west and east by eroding cliffs; the coastal geology comprising Liassic strata, mainly shales, marls and mudstones, which weather to soft clays. To the west of the River Char, the cliffs form the eastern flank of Black Ven. one of the largest and most active coastal landslides in Europe. Instability results from a combination of geological and hydrogeological factors, coupled with continuous basal removal of material by marine erosion. The calculated long term mean retreat rate of the cliffs is 0.71m pa (Bray et al, 1991). On the eastern side of the River Char, the coastal topography rises to Stonebarrow Hill; the basal Liassic strata being capped here by poorly consolidated Lower Cretaceous sediments. Landslips are also a prominent feature of Stonebarrow, the mechanism for failure being similar to that of Black Ven. The mean rate of cliff retreat has been estimated to be between 0.2 and 0.4m pa (Bray et al, 1991).

The foreshore consists of a shingle ridge with sand at low water; the lower foreshore to the west being sheltered by extensive limestone reefs. The River Char is partly trained by a concrete wall and drains seaward through the ridge. Its discharge is however restricted by banks of gravel, or shingle, deposited by marine processes. This causes the river to 'pond up' for some distance behind the shingle ridge. This is a feature which has amenity benefits (freshwater bathing lagoon) and is consequently encouraged.

An important coastal process along this stretch of foreshore, and for that matter Lyme Bay, is the movement of marine sediments through littoral drift. The tendency for the mouth of the River Char to deflect eastward and cause subsequent sediment accumulation against groynes, indicates a net eastward drift at Charmouth; this having been estimated to be between 3000 and 5000 m3pa (Bray et al. 1991). Movement further to the east of Charmouth is however blocked by landslide debris beneath Golden Cap. The pattern within this coastal cell is thus for beach accretion towards the east, with major landslide sites to the west supplying beach material. The west is consequently characterised by high sediment throughput and relatively low volumes of shingle. No material is sufficiently stable on the western side to provide any long-term protection to the base of cliffs.

Development within the coastal zone at Charmouth is restricted. The only residential properties presently at risk from coastal erosion are on the western side of the River
Char, fronting Higher Sea Lane

The seafront at Charmouth is exposed to the full force of the sea

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There are principally four, separately identifiable, problems currently being faced along Charmouth sea frontage, from the River Char for a distance of approximately 0.5km to the west. These problems, which are attributable to the geology of the area and marine transport processes, are as follows:

a) a threat to residential properties along Higher Sea Lane posed by cliff recession; the area being subject to ground movement, mainly in the form of mud flows;

b) erosion of the cliff at the western extremity of the promenade, which is threatening to outflank the structure;

c) structural integrity of the seawall which, has suffered damage in recent storms;

d) deflection of the River Char eastward, due to shingle accretion on the eastern side of the promenade (west bank of the river).

A key, and common, factor in all four problems is eastward littoral drift which, from the promenade westward, is not enabling sufficient material to build up to form a storm beach to protect the cliff base, or the promenade. To the east, however, this results in considerable shingle accumulation.

These problems are considered below in more detail:

Approximately 0.5km west of the mouth of the River Char, is an area of mudflow, 2.2 ha in extent. This is roughly triangular in shape, with its base fronting the beach for a distance of 250m, the apex being 120m inland. To the north of the mudflow area is Higher Sea Lane, which services eleven properties. Nine of these are on the northern side of the roadway, with the remaining two on the seaward side; the former being of the order of 55m to 80m from the cliff top and the latter between 25m and 40m.

A study of the area by the Institute of Geological Sciences (IGS, 1971), identified it as a fossil mudflow. Quoting the report:

'The direction of flow had been west to east, downhill towards the River Char. The recent movement is then seen to be the activation of the wettest part of the fossil mudflow by erosion of the coast perpendicular to the original direction of movement.'

Dobbie & Partners (1974), in preparing a feasibility report on remedial measures recommended for stabilising the upper cliff, agreed with the conclusion of the IGS report that the problem was principally one of drainage in the upper cliff. However, Dobbie's further concluded that, in their view, erosion at the foot of the cliff significantly affected the retreat of the upper cliff, despite the apparent disparities in the rates of erosion and retreat.

With no adequate storm beach at the foot of the mudflow and with little likelihood of one building up under present hydraulic conditions, the process of erosion and retreat will continue. At what rate, however, cannot be reliably predicted and will depend on climatic conditions; re-activation of mudflows being more likely during, or immediately following, periods of heavy rainfall.

The absence of an adequate storm beach to the base of the cliffs, immediately to the west of the promenade, means that this section of cliff is also gradually retreating. The stratigraphic division for the Liassic rocks forming these cliffs is 'shales with beef'. A particularly vulnerable point is at the junction with the promenade wall, where continuing retreat of the cliff poses a threat to the structure through outflanking.

To protect this weak point, gabion baskets were installed in circa 1981. External abrasion usually precludes their use in the inter-tidal zone on gravel beaches, or sites which are exposed to severe wave attack. As a consequence, they have deteriorated due to the ravages of the marine environment and require regular maintenance (in terms of repairing wire cages and replacing lost packing stone). Even with the protective gabions, overtopping by storm waves and sea spray is continuing to erode the cliff, albeit at a slower rate. In the course of time, the promenade structure will thus be outflanked and destroyed, unless a more permanent long-term engineering solution is established. In addition to the engineering unsuitability of the gabions in this application, they are aesthetically very unsightly on the amenity beach.

The old sea wall was vulnerable to wave attack and lacked adequate beach protection

As with the beaches to the west, eastward littoral drift, coupled with wave action, also prevents the build up of an adequate storm beach fronting the western end of the promenade; the seaward face of which is a vertical seawall. The danger associated with a depleted storm beach was highlighted during storms on 30 August 1992. Severe wave conditions removed coping stones and facings on the front of the seawall. Also, water penetrating behind and under the promenade decking, caused fill material to be washed out. In addition, the gabion baskets at the western end of the promenade were severely damaged, necessitating emergency works to restore protection. This event has exposed the structural inadequacy of the present seawall, with no adequate storm beach to dissipate wave energy.

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On the eastern side of the promenade, significant accretion of the shingle ridge is presently occurring. The predominant eastward drift of material is causing the mouth of the River Char, which drains through this ridge, to be deflected eastward. This is by no means a recent phenomenon; historically the mouth of the river has cyclically moved eastward and back again dependent on the predominant drift direction during any given period in time. However, recent eastward movement of the river mouth has been further than experienced previously, according to local knowledge. The river mouth had moved so far east that it was beginning to cut into the flanks of the Blue Lias cliffs. It was also seriously affecting the amenity value of the beach. To remedy this situation a straight channel was mechanically excavated through the shingle ridge, in May 1992. The tendency during storms is for this channel to block with shingle, necessitating continuing maintenance. The predominantly eastward drift being experienced at Charmouth, correlates with the finding from a study by Hydraulics Research (HR) of recent beach changes east of West Bay Harbour (HR, 1991). Marked changes in wave climate have been found to have taken place some time between 1980 and 1984. Since 1984, the wave 'records' at West Bay indicate that there have been fewer big storms from the south-east sector.

THE SCHEME UNDER CONSTRUCTION IN 1994

the scheme involved a new, stepped sea wall and a rock groyne on the western end to protect the flank and build up beach material in order to slow erosion of the western cliffs.


Rock armour was placed seawards of the workings before being located in the groyne to the west of the sea wall


Excavation in front of the old sea wall


Piling for the foundations of the new sea wall



The rock armour arrived by road from the Mendips. The stone chosen was Carboniferous Limestone


Nearing completion with the final placement of the rock armour at the western end of the sea wall

Initial consideration was given to a scheme aimed at combating all of the four problems discussed in the preceding section. This comprised a new stepped sea wall, fronting the promenade and a rock revetment extending from the existing promenade westwards some 130m to the eastern flank of the mudflow, there, terminating in a rock bastion. To maintain an adequate beach over this frontage, shingle replenishment from material accumulations east of the promenade was envisaged. However, when estimating cliff top erosion rates for a financial scheme appraisal, it became clear that only three properties in Higher Sea Lane were likely to be threatened, 40 to 50 years hence and, possibly, only six within a 100 year timescale. The discounted benefits for protecting these properties made the initially conceived scheme untenable, even if erosion rates 50% higher were assumed.

A scheme of reduced extent is thus proposed, which excludes measures to protect properties in Higher Sea Lane. This is aimed at alleviating the problems of outflanking of the promenade, the structural integrity of the seawall and deflection of the river mouth. The proposal is for a new stepped seawall over a 75m long frontage, seaward of the existing promenade structure, with a rock bastion positioned at its western end. The rock bastion at the western end of the promenade will protect this vulnerable point from wave action and, thus, the problem of outflanking of the structure presently being experienced. It is proposed that the bastion be taken seaward, 30m from the existing cliff line to approximately the Om OD contour. With predominantly eastward drift along this section of coastline, the immediate effect of the bastion will be to impede this drift, thereby encouraging the build-up of a shingle storm beach on the western side fronting the cliffs. The design crest level of the bastion has been set at the level and slope required for this generated beach. With the build-up of a spending beach at the foot of the cliffs, basal erosion will be retarded, or arrested, thereby reducing the possibility of the bastion being outflanked. When accretion tops the crest level of the bastion, eastward drift of material, over and around the seaward end of the bastion, will be re-established.

The promenade seawall, in its present form, is not amenable to retaining beach material; over a ca 75 m length its vertical face having a high wave reflection coefficient. It is also in poor structural condition with cracks and exposed reinforcing on the front face. Without extensive repairs it is close to the end of its useful life. In view of these factors, the proposal is to construct a new stepped wall, seaward of the existing structure. Such a stepped profile will reduce swash heights and wave reflections, which are a primary cause of toe scour, and also enhance its amenity value.

The promenade being on the downdrift side of the bastion, initial erosion of the shingle beach fronting it is anticipated until a sediment transport path is re-established. Even after this has occurred, some set-back in the beach can be expected in the lee of the bastion. To maintain an adequate beach fronting the promenade, it is proposed that shingle which tends to accumulate east of the promenade be used as replenishment, or re-cycling, source. To ensure that sufficient supply material is available for recycling without jeopardising the level of sea defence afforded by the beach to the east, regular monitoring will be required. Periodic maintenance will thus be involved in placing beach material where it is needed. Nevertheless, using the shingle ridge at the mouth of the River Char as a supply source will be advantageous in that it will limit the build-up of material here. Whilst eastward deflection of the river mouth, under present drift conditions, cannot be fully prevented without further training works to the outlet channel, a reduction in the throughput of shingle will, to some extent, diminish the tendency for eastward deflection.

The site lies within the Dorset Area of Outstanding Natural Beauty (AONB) with the cliffs to the west and east of the River Char being designated Sites of Special Scientific Interest (SSSI). These are noted for the interest in fresh geological exposures by continuing erosion.

The proposed rock bastion, in impeding longshore transport of material and promoting the build up of a storm beach, will have an impact on the local geomorphology. In the case of longshore drift, the effect will be short-term, since a transport path will be re-established once material accumulation overtops, or passes around, the seaward end of the bastion. With an estimated net easterly drift rate of 3000 to 5000M3 pa, this would occur naturally within the first year or two of the bastion being installed. However, it is proposed that this beach be artificially renourished from the shingle bank to the east of the promenade immediately following the construction of the bastion so that disruption in longshore transport is minimised. With the significant accumulations of shingle presently to the east of the promenade, around the mouth of the River Char, it is not anticipated that any short-term disruption in longshore transport will have a significant effect on beaches further east.

With the build up of a spending beach on the western side of the bastion, erosion of the cliff over a limited length will be arrested or retarded. For SSSI"s, where interest lies in geological exposure, as a general rule English Nature have an interest in preserving the same, or an adequate, rate of coastal erosion. The westerly influence of the bastion will be limited and any beach build-up will front cliffs where geological interest is not as important as much further west i.e. Black Ven.

During the consultation process, some initial concerns were however expressed by English Nature and the Countryside Commission (note now Countryside Agency) over the possible affects of a bastion on coastal processes. As the result of a site meeting and discussions held between Officer of WDDC and English Nature, the proposed length of the bastion was reduced from 40m to 30m. This, it was mutually agreed, would go some way to ameliorating any possible adverse impacts on coastal processes. On the basis of this design revision, objections to a planning application for the scheme were withdrawn by English Nature. (Supplementary documentation in support of MAFF grant aid, submitted with the Engineer's Report, refers). Other responses from statutory consultee's were favourable to the scheme.

Another area of concern, which has been given consideration, is access. The foreshore between Charmouth and Lyme Regis is a publicised route for walkers, with Black Ven being of particular interest to fossil hunters. Although there are already dangers along this stretch of coast, of being cut-off by rising tides, the proposed bastion should not increase this risk. So that the bastion should not restrict, or unduly limit, public access, steps are proposed across the root of the bastion.

The scheme has been discussed with members of Charmouth Parish Council, the initial response to its principles and scope being favourable.

Prior to the storm events of 30 August 1992, it had been the practice of Charmouth Parish Council to place beach chalets along the rear of the promenade for the summer months. Many of these were destroyed or severely damaged by this storm and the Parish Council has agreed to a permanent resiting of the replacement chalets in a location where they are less likely to suffer damage.

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