About the Study

SCOPAC Committee

Chairperson Councillor Mrs M Penfold MBE, West Dorset District Council.

Vice-Chair Councillor Jackie Branson, Havant Borough Council.

Technical assistance provided to Councillors by Mr Lyall Cairns (Southern Coastal Group Chair) and Dr Samantha Cope (SCOPAC Research Chair).

The STS 2012 update

The 2012 update of the SCOPAC Sediment Transport Study (STS) was funded by the Environment Agency under FDGiA, grant number LDW 41230, with additional contributions from SCOPAC.  

It is referenced as: New Forest District Council (2017). 2012 Update of Carter, D., Bray, M., & Hooke, J., 2004 SCOPAC Sediment Transport Study, www.scopac.org.uk/sts.

Sediment Transport Study 2012

Berry Head to Hope’s Nose (Tor Bay)

1. Introduction

This unit coincides with the large, well-defined coastal embayment of Tor Bay. The confining promontory headlands terminating at Hope's Nose (Photo 1), to the north, and Berry Head (Photo 2), to the south, are developed on erosionally resistant Devonian limestones. Slates, mudstones and breccia outcrops occupy much of the recessed coastline between Torquay and Brixham. Due to their varied lithological composition these latter rock materials are characterised by differential response to both marine and sub-aerial denudation. Consequently, a sequence of headlands and embayments has developed, which vary in their scales of development. With the exception of Roundham Head (Photo 3), south of Paignton, headlands within the bay form relatively minor low elevation salients. Beach development is confined to the re-entrant bays and coves, with little obvious sediment exchange between them. Sand is the dominant beach-forming sediment, with coarser material mostly adjacent to eroding headlands.

Most of this shoreline is stable, recording low rates of recession over the past 100 years (Posford Duvivier, 1998). Urban development occupies over 60% of the frontage, with seawalls and revetments built to protect residential property, infrastructure and commercial (especially recreational) facilities. At locations such as Torquay, Paignton (Photo 4), Hollicombe and Brixham (Photo 5) protection structures form esplanades and also represent an amenity resource in support of the regionally important tourism industry. Breakwaters have been constructed at Torquay, Paignton and Brixham Harbours (Photo 5), thus adding strong, artificial structures approximately at right angles to local coastline orientation.

The effect of both natural and built salients has been to compartmentalise this coastline into a sequence of discrete sub-cells of sediment movement. Whilst a weak, approximately south to north littoral transport pathway is discernible, each sub-cell appears to operate as a relatively independent coastal process system. This is especially evident from contrasts in beach morphodynamics within this unit as a whole.

A major new source of coastal data is from the Defra-funded National Network of Regional Coastal Monitoring Programmes. The Programmes consist of topographic beach surveys, nearshore bathymetry, aerial photography, lidar, coastal hydrodynamics (waves and tides) and terrestrial habitat mapping. Specifications for data collection are consistent for all regional programmes and the data and analysis reports are made freely available under the Open Government Licence from www.coastalmonitoring.org. The Southwest Regional Coastal Monitoring Programme commenced in 2006. The Lead Authority is Teignbridge District Council, with data collection, analysis and reporting led by a specialist team at Plymouth Coastal Observatory (PCO).

1.1 Coastal Evolution

The arcuate form of the Tor Bay coastline is considered to be the product of structural control created by axial planes of folding and thrusting and the effects of differential faulting. The latter has been influential in shaping the Hope's Nose (Photo 1) and Berry Head (Photo 2) promontories; in the former case, there has been intrusion of igneous dykes, thus adding further strength (Perkins, 1971). These structures are inherited from complex tectonic events that occurred not less than 250 million years ago (Lloyd, 1933).

The modern coastal planform is, however, much more recent. The presence of raised beach platforms and associated sediments at Shoalstone Point, Hope's Nose, and on Thatcher Rock (Photo 1), at between 8 and 12mOD, is evidence of erosion associated with higher sea-levels (Mottershead et al. 1987). These palaeoforms have survived because of the relative resistance of the Devonian Limestone on which they developed. An absolute date for their origin is uncertain, but they are likely to represent high sea-levels during either the ultimate or penultimate interglacial period, i.e. not more than approximately 330-360,000 years before the present. Uranium series dating of speleothems from abandoned terrestrial caves beneath Berry Head (Proctor and Smart, 1991) suggests that their formation was approximately contemporary with the shore platforms, i.e. sea-level regulated the ground water table. It is therefore reasonable to assume that the ancestral forms of both headlands, and thus Tor Bay itself, existed at least some 400-500,000 years ago. Submerged platforms, and at least one cliffline at -40mOD off Berry Head, are indicative of lower sea-levels that may be early Pleistocene in age. It is not considered that there has been any significant post-formation isostatic or tectonic displacement of the above evidence for relative sea-level change, although direct evidence is lacking.

The detail of the modern coastline is the product of Holocene sea-level rise over the past 10,000 years. Submerged forests and interbedded marine, brackish and peat beds exposed on the foreshore at Goodrington Sands (Photo 6) and Torre Abbey, Torquay (Photo 7) provide evidence of former marsh deposits now submerged by progressive early to mid-Holocene sea-level transgression (Jukes-Browne, 1911; Perkins, 1971). Nearshore buried channels, representing the seaward extension of contemporary rivers, e.g. the Torre Stream, further confirm transgression. It is hypothesised that sandy seabed sediments occupying Tor Bay provided material for barrier beach formation from mid-Holocene times. This would have commenced when sea levels were approximately -30 to -25mOD, with shoreward barrier migration occupying several subsequent millennia of steadily rising sea-level. Massey et al. (2008) suggest relative sea-level rise of 21+/- 4m during the past 9,000 years for the barrier complex of Start Bay. It is uncertain if this can be applied to the Torbay coastline, which has not been specifically researched, but it may give an approximation of the timescale of early to mid-Holocene evolution.  As this 'super' barrier was driven towards the contemporary coastline, it encountered a series of headlands that formed the truncated interfluves between several west to east-orientated river valleys. These drain the coastal hinterland, and are long-established features that account for many of the embayments of both the pre-existing and modern coastlines. In response, the barrier segmented and sealed up the mouths of several rivers. Several former lagoons, now reclaimed and partly built over, provide evidence for this evolutionary stage. These include Torre Abbey Meadows (Photo 7), a site 1.5m below mean sea level, where there is a 5.2m thickness of estuarine and both saltmarsh and freshwater peat deposits (Perkins, 1971); the former Fleet Estuary, at the base of Abbey Road, Torquay, (now Cary Green), where a repetitive sequence of alluvial clays and silts, fluvial sands, peat and coarse beach sand is known (Doornkamp, 1988), and the low-lying areas behind Goodrington Sands (Photo 6), Broad Sands (Photo 8) and Livermead Sands. The sandstone outcrop in the centre of Goodrington Sands was surrounded by saltmarsh up until the 1860s, prior to its drainage and reclamation of landward parts as part of rapid urbanisation (Perkins, 1971).

1.2 Hydrodynamic Regime

Mean tidal range at Torquay is 3.1m, but tidal currents are weak and are not considered capable of any significant independent sediment transport. The eastward - facing orientation of this coastline results in protection, by Berry Head from waves approaching directly from between west and south. However, long period Atlantic swell waves, refracted and diffracted around Start Point and Berry Head, do enter Tor Bay, but their energy at the coastline is greatly reduced by the width and shallowness of the bay. Waves approaching from between north-east and south-east are more energetic, and can generate mean significant wave heights of between 2.5 and 3.0m (Harwood, 1983). The mean significant wave height is, however, characteristically less than 0.5m. The Southwest Regional Coastal Monitoring Programme measured nearshore waves using a Datawell Directional Waverider buoy deployed at Tor Bay in 11mCD water depth. Between 2008 and 2012 the prevailing wave direction was from the east-south-east.  Average 10% significant wave height exceedance is 0.9m (CCO, 2012).

Given that higher energy waves only occur infrequently, and that most waves approach approximately normal to shoreline orientation, the potential for littoral transport is small. Most beaches are swash-aligned and show fairly modest seasonal changes in volume and profile form, though short-term winter storm losses and longer-term summer season gains are characteristic of some.

2. Sediment Inputs

Only small streams drain into the bay and these are regulated and reach the shoreline via culverts so that there is limited potential for any significant supply of fluvial sediments. There are no reports of offshore to onshore supply of sediments to beaches.

2.1 Cliff Erosion

E1  

The Devonian Limestones of Hope's Nose (Photo 1) and Berry Head (Photo 2) are relatively resistant to marine and subaerial denudation, although they exhibit a variety of up to 60m high rugged coastal slopes that are also adjusted to the presence of interbedded shales, intrusive dolerite sills and sandstone dykes. Wave action has exploited well defined joints and other parting planes and - as in earlier stages of the Quaternary period - is creating minor shoreline platforms. These occupy a height-range of several metres, with salt spray weathering being a probable contributory factor at higher elevations. On both Berry Head and Hope's Nose, nineteenth century quarrying has modified the natural coastal slope.

Between these two main "anchor points", sandstones, conglomerates, breccias and shales outcropping within the bay collectively offer relatively less resistance and support an intermittently cliffed coastline. Several small bays and coves are developed in weaker bedrock, defined by headlands of various degrees of prominence. Where present, harbour breakwaters and piers form strong, albeit artificial, salients. Narrow shoreline platforms are present, particularly where there are well developed near-horizontal joints or bedding planes. Several are subject to breakdown by sub-aerial weathering, particularly where poorly consolidated sandstone provides the local substrate.

Erosion is most active and rapid on sandstones, conglomerates and breccias, as evidenced by cave formation (e.g. Corbyn's, Livermead and Hollicombe Heads see Photo 9). Some cliff segments are protected by concrete walls (east Livermead and Corbyn's heads) or a rock revetment (Hollicombe Bay), but most sections remain free to erode and there is an active blowhole in the grounds of the Livermead House Hotel. Harwood (1983) estimated a current mean rate of sandstone cliff recession of 0.15m per year at Hollicombe Head (Photo 9) based on weekly re-measurement of the exposure of monumented erosion pins placed on the cliff face, between October 1981 and April 1982. This study demonstrated that recession rates are spatially variable, depending on (i) aspect-related exposure to weathering above the wave-trimmed cliff base; (ii) microscale variations in rock resistance; and (iii) cliff height. At one site, a rate of cliff face retreat of between 75 and 100mm per year appeared to be largely controlled by perennial overland flow fed by groundwater seepage.

Landslide and rock failures have been noted at several sites (Doornkamp, 1988). Occasional rockfalls have been recorded at most of the prominent sandstone/breccia headlands south of Corbyn's Head and on the cliffs south of Goodrington Sands. They are also known from some Devonian Limestone locations, e.g. Daddyhole Cove see Photo 10 (Kalaugher and Grainger, 1991), Fishcombe Point and close to Berry Head. The Meadfoot slates and grits also generate infrequent falls, one example occurring north of Saltern Cove. The slates and mudstones within Saltern Cove are also prone to this type of cliff failure, which is partially independent of wave-induced cliff erosion. Doornkamp (1988) has mapped over 80 different translational slides, both rock and debris, around the bay, though most are not currently active. Deeper-seated rectilinear slides are also known from their diagnostic morphology and associated debris. There are examples between Hollicombe Head and Elberry Cove, mostly developed in the Oddicombe Breccia. In most cases, slides also detach head deposits overlying bedrock. This includes soliflucted material, overlain by silty loess.

Some of these slope failures may be long established, even possibly recently reactivated, features. Many, however, have been inactive over the past 50-70 years. There is a good possibility that there are several relict or 'fossil' failure surfaces inherited from previous (Pleistocene) morphoclimatic conditions.

A distinctive process that adds morphological variety to the Devonian Limestone cliffline is hydraulic erosion by waves of originally solution weathered caves. There are several around the Hope's Nose peninsula and on Berry Head. Features such as the London Bridge arch, Torquay, may be partly due to solutional widening of joint planes. Several small caves between the Brixham breakwater and Shoalstone Point are regarded by Perkins (1971) as sites of original solutional pits, possibly opened up by roof collapse. There have been no systematic or site-specific studies of rates of cliff erosion for any sector of this coastline, with the apparently singular exception of that by Harwood (1983). A very broad historical rate of cliff top recession of between 0.3 and 3.0m per year for this entire length of cliffed coastline is proposed by Posford Duvivier (1998). This source suggests that long-term rates not exceeding 0.3m per year are characteristic of nearly 70% of the cliffline, with some sectors apparently stable in position over the past 100 years. No specific details of areas of higher rates are given, although they are likely to coincide with sites with a history of falls or slides, as detailed above. Potential rates of retreat are greatly reduced where there are seawalls and other protection structures, with toe stabilisation inhibiting the undermining that leads to falls and slides. These structures also reduce potential sediment input into the littoral transport system.

Analysis of Coastal Monitoring Programme 2007 and 2012 aerial photography and lidar datasets indicates that cliff input is minimal, with volumes of cliff input yielding shingle or sand grade beach material sediment input estimated at less than 1,000m³ per year. The available data has resulted in a change from the 2004 arrows when no quantitative data was available. No significant landslides or cliff erosion events have been measured during the interval 2007-2012. (See PCO Annual Survey Reports for further details)

3. Littoral Transport

LT1 Northward Transport within Pocket Beaches

Because of low wave energy along much of this shoreline and a prevailing wave approach normal to shoreline orientation, rates of littoral drift are weak. There are several sectors where it is difficult to identify any net movement direction, but slight northwards directed pathways are apparent within most of the pocket beaches from sediment accumulations against the infrequent structures and harbour breakwaters that interrupt them. The weak nature of longshore transport is indicated by only very limited sediment accumulations against the updrift, southern sides of headlands. Analysis of Coastal Monitoring Programme topographic baseline and lidar datasets in 2007 and 2012 data indicated there is some minor cross-shore sediment movement between the nearshore sub-tidal and inter-tidal beach, which could predominate over longshore transport. There is no discernible net transport between these independent pocket beaches. Within each pocket beach there was a slight net northward transport of less than 1,000m³ per year of beach grade material, although there is no trend within Eldberry Cove.  (See PCO Annual Survey Reports for further details). The available data has resulted in a change from the 2004 arrows when no quantitative data was available. Both Hope's Nose and Berry Head are considered to be absolute boundaries to bedload longshore transport.

4. Beach Characteristics

Beaches are composed predominantly of reddish pink sand, with some coarser material adjacent to headlands, and are confined to small bays and "pocket" coves contained by coastal salients. Input is presumed to derive from local cliff weathering and erosion, and from updrift when headlands are occasionally by-passed. The possibility of some net input from offshore is implied by Merefield (1984), who stated that 7% of the volume of Goodrington Sands was composed of carbonate material. This was all broken shell debris, and could only have derived from the nearshore or offshore seabed. However, abrasion and solution ensure this source has a short residence time, and its resupply is seasonal.

Several of the larger beaches have a postulated barrier origin (see Section 1.1), and have now been built over. Many are backed by seawalls and revetments, which prevent landward migration. Most beaches, such as Goodrington Sands, are trapped by enclosing headlands and appear to be effectively closed transport sub-cells. Cross-shore transport is a function of incident waves; beach volumes deplete and profiles flatten in response to higher energy waves, from the north-east, east or south-east (Posford Duvivier, 1998). They subsequently recover, usually at a slower rate, when modified swell waves from the south and south-west operate. These fluctuations are seasonal, with little apparent long-term change. In the absence of data from beach monitoring, it is currently difficult to add any further observations on beach morphodynamics. Visual evidence, such as the presence of some gravel in coves between Brixham and Broad Sands, at Livermead and immediately east of Corbyn's Head, but with finer more silty sediments at Paignton, Broad Sands and Goodrington Sands (Doornkamp, 1988) indicates scope for research into the sorting and provenance of beach material. Transfer of local accumulations of boulders, gravel and cobbles in coves and beaches (e.g. Meadfoot, Torquay) along the Devonian Limestone cliffs to the beaches of central Tor Bay seems inherently unlikely. Thus, the presence of isolated gravel beach berms is more probably related to nearshore outcrops of Permian conglomerate and breccia at these sites.

Significant foreshore depletion of Torre Abbey Sands occurred between about 1920 and 1960 (Perkins, 1971), although it has been stable since then. The result has been the complete loss of any remaining beach at high tide at this amenity location. The reason for this behaviour is uncertain, as it post-dates the cessation of beach mining known to have occurred at several Tor Bay sites during the second part of the nineteenth century.

Some of the land behind the Broad Sands, Goodrington Sands, Paignton and Torre Abbey beaches has been reclaimed from marshland and a lagoon still remains behind Goodrington Sands (Photo 6). As a consequence, these frontages may be susceptible to subsidence due to (i) uneven partially compacted fill materials used, and (ii) compression of soft estuarine and freshwater marsh sediments. Problems of subsidence have been clearly identified affecting Princess Gardens immediately behind Torquay Harbour and a need for remedial actions has been highlighted (Bolitho, 2003). Due to their low-lying nature hard defences that prevent beach profile adaptation by landward migration protect these reclaimed areas. This could result in some narrowing and steepening of the beaches by 'coastal squeeze.'

5. Summary

This appears to be a self-contained unit characterised by relatively slow rates of sediment flux and shoreline change. The following issues are significant:

1. Except for slow cliff erosion there are few inputs of sediment into the shoreline system so that the beaches are essentially a finite resource.
2. The beaches are subject to seasonal changes and Torre Sands has suffered significant depletion. Many beaches have very little dry width at high tide. This affects their amenity value and means that storm waves can strike defences during storms.
3. Although cliff erosion is slow, studies have revealed a wide distribution of relic or inactive coastal landslides. Some of these areas of ancient landslides could be susceptible to reactivation following increased toe erosion, or changes in groundwater levels that might be anticipated with future climate change and sea-level rise (Halcrow et al. 2001; Halcrow, 2002).
4. Building upon landward parts of barrier beaches and reclamation of marshes within the main embayments has impounded some potentially valuable sediments and prevents beach profile adaptation by landward migration, potentially resulting in some future narrowing and steepening of the beaches.
5. There is very limited quantitative understanding of processes and rates of change due to a lack of monitoring and detailed previous studies on this frontage.

6. Opportunities for Calculation and Testing of Littoral Drift Volumes

Data collected by the Defra-funded National Network of Regional Coastal Monitoring Programmes is pivotal for future improvement in estimating beach change.  The Programmes consist of topographic beach surveys, nearshore bathymetry, aerial photography, lidar, coastal hydrodynamics (waves and tides) and terrestrial habitat mapping.  Specifications for data collection are consistent for all regional programmes and the data and analysis reports are made freely available under the Open Government Licence from www.coastalmonitoring.org.

The Southwest Regional Coastal Monitoring Programme commenced in 2006. The Lead Authority is Teignbridge District Council, with data collection, analysis and reporting led by a specialist team at Plymouth Coastal Observatory (PCO). Although a relatively short duration of quality data collection, analysis of Coastal Monitoring Programme data between 2006 and 2012/13, combined with other datasets, academic research and historical studies may in the future enable sediment budgets, transport rates and directions to be identified or verified for the major pocket beaches e.g. Broad Sands, Goodrington Sands, Paignton and Torre Sands where each bay would appear to operate as a relatively closed system for sand and gravel. Potentially, a beach plan shape model could be set up to simulate the beach responses to south-west and south-east approaching waves that would tend to cause significant beach re-orientations within the confined bays.

7. Knowledge Limitations and Monitoring Requirements

Notwithstanding results from the Southwest Regional Coastal Monitoring Programme, and the summarised information collated in the Durlston Head to Rame Head SMP2 (SDDCAG, 2011), and given the combination of future sea-level rise, climate change (including wave climate) and the apparent absence of more than minimal fresh sediment inputs, it would be advisable to consider:  

1. Quantitative assessment of the wave climate for each of the major beach embayments. It ideally requires a representative long-term hindcast offshore wave climate to be compiled for an offshore point within the bay based on some 20-30 years of wind data. A shoaling and refraction analysis should then be undertaken to determine the specific climates for the selected beaches. Ideally, there should also be field wave measurement to validate the hindcasting and refraction analyses. A magnitude-frequency analysis should also be linked to a quantitative study of the recurrence probabilities of extreme water levels. This is considered important for it is storm waves and storm tidal surges in combination that will define sea-wall overtopping and beach erosion criteria.
2. Sample beach sediments and investigate the reasons for spatial variation in sediment sorting on the larger beaches, with a view to determining possible nearshore or offshore sources of supply.
3. Derive estimates of historical cliff recession and shoreline change from comparisons of Ordnance Survey maps, sequent air photo and other remotely sensed imagery. This should perhaps involve some more detailed assessment of the depletion of Torre Sands with investigation into the likely causal factors. Use should be made of cliff recession data to estimate the likely magnitude of cliff erosion sediment inputs to local beaches.
4. Determine the potential for reactivation of currently inactive, relict, coastal landslides and cliff failure surfaces. It would be valuable to produce cliff behaviour assessments for each main section of cliff line. The Futurecoast study (Halcrow, 2002) provides a preliminary assessment, although greater detail based on methods outlined by Rendel Geotechnics, (1998) and Lee and Clark (2002) is required.
5. Attempt to access hydraulic studies that are believed to have been undertaken for Brixham and Torquay harbours.