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

Straight Point to Otterton Ledge

1. Introduction

This relatively short length of coastline is characterised by differential marine and sub-aerial denudation of Triassic rocks of varying lithologies. The confining headlands of Otterton Ledge (Photo 1) and Straight Point (Photo 2) are developed in relatively resistant sandstones. Mudstones, overlain by sandstones and conglomerates, occupy the western and central sectors and their degradation has promoted landsliding and mudflows (Photo 3). The coastal frontage of the town of Budleigh Salterton consists of near-vertical cliffs (Photo 4), with the easternmost sector protected by a short length of seawall (Photo 5). Periodic landslips and more frequent flows and slides provide input of a range of sediment types to the beach. Confined by headlands the "pocket beach" consists of a steep coarse pebble or cobble backshore ridge and a coarse to fine sandy foreshore. Average beach crest height is some 4m (maximum 5m O.D.), becoming progressively higher and wider, and less coarse in composition, from west to east; and maximum inter-tidal width approximately 600m.

Budleigh Spit at the mouth of the River Otter, composed predominantly of large, spherical to discoidal clasts derived from updrift cliff erosion, has grown progressively eastwards (Photo 6). This has enclosed a formerly open estuary and diverted the mouth of the river to a position adjacent to Otterton Ledge (Photo 1). This feature may have a partial barrier origin, and thus be related to mid to late Holocene sea-level rise. The latter factor has caused basal wave erosion to activate landslipping, though it is probable that the larger failure surfaces are the outcome of the removal of ancient landslide debris - i.e. they are re-activated features. Sea-level rise and accelerated erosion has also been critical to the present definition of the two terminal headlands. There is no direct evidence that either is now by-passed by longshore bedload transport, thus creating an effectively closed coarse sediment transport sub-cell between them.

The distinctive metaquartzite clasts, uniquely derived from the Pebble Beds outcropping on this shoreline, are known to occur also in beaches to the east - notably the Chesil barrier beach, but also as far east as Langley Point in East Sussex. This implies that at earlier stages of Holocene sea-level transgression, there was unimpeded west to east littoral transport. This carries further implications of the possible existence of an early Holocene prototype 'super' barrier, occupying Lyme Bay, which was subject to segmentation, disruption and partial submergence as it moved shorewards under the influence of storm waves. It can be postulated that segmentation became complete as sea-level re-occupied the ancient cliffs of this coast and differential erosion resulted in the emergence of headlands such a Straight Point and Otterton Ledge and many others to the east.

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). In 2009, the Southwest Coastal Monitoring Programme completed a 100% coverage swath bathymetry survey, extending offshore 1km from MLW between Petit Tor Point and Portland Bill.

Given the orientation of this coastline, it is exposed to relatively infrequent, but high energy, waves that approach from the east and south-east. Swell waves from the west and south-west suffer substantial refraction and energy loss before breaking; there is very limited fetch to generate local waves from the south-west because of the sequence of headlands south to Start Point.

An unpublished set of offshore wave height data for Budleigh Salterton (thought to have been collected by the Institute of Oceanographic Sciences between 1973 and 1977) gives a maximum significant wave height of 4m for south-easterly approaching waves. The equivalent value for waves moving from due east is 3.95m. The Programme measured nearshore waves using a Datawell Directional Waverider buoy deployed at Dawlish in 11mCD water depth, from 2011 to 2012.  Prevailing wave direction is south-by-east.  Average 10% significant wave height exceedance is 1.05m (CCO, 2012). The mean tidal range at the mouth of the River Otter is 4.1m, but there are no reported measurements of tidal current velocities.

2. Sediment Inputs

2.1 Cliff Erosion

E1 Littleham to Budleigh Salterton

Between Littleham Cove and the eastern boundary of Budleigh Salterton, cliffs of up to 130m in height are developed in a sequence of relatively weakly resistant soft rock lithologies with an overall 5° easterly dip. Contrasts in cliff morphology, and geomorphological processes currently active have been described in some detail by Grainger and Kalaugher (1987a and b, 1988, 1991); Kalaugher and Grainger (1981, 1990); Kalaugher, Grainger and Hodgson (1987) and David Roche Geoconsulting (2000). Rockfalls and mudslides are of frequent occurrence and there is clear evidence of former larger-scale landslide events. Cobble and gravel beaches provide varying degrees of cliff-toe protection against marine erosion, but debris created by weathering and mass movement temporarily conceals beach sediments and is pushed seawards into the zone of wave attack. Shore platform development is limited to the more resistant formations, but platforms are of restricted width and often concealed by debris derived from previous cliff falls (Posford Duvivier, 1997).

From west to east, the principal lithostratigraphic divisions of the outcropping Triassic Rocks are:

1. Relatively resistant Straight Point Sandstone, creating the prominent Straight Point headland that forms the western boundary of this unit. It forms near vertical cliffs and moderately wide shore platforms (Photo 2);  
2. Littleham Mudstone, a calcareous mudstone that is closely-fissured but with well defined master joints that control the form of exposed faces of the cliff that it supports. It degrades readily by mudsliding;
3. Sandy Conglomerate and the Budleigh Salterton Pebble Beds. The latter unit forms an inland escarpment that meets the coast at West Down Beacon. There is significant seepage at the interface between this permeable unit and the underling relatively impermeable mudstones;  
4. Otterton Sandstone, a poorly cemented, permeable formation, occupying the cliffed frontage of Budleigh Salterton;

Cliff height increases westwards from the Otter estuary, with progressively more exposure to wave erosion from north-east to south-west. Kalaugher, Grainger and Hodgson (1987) have proposed the following "geomorphological process units" as a summary of the principal contrasts of cliff morphology and behaviour; these occur in a west to east sequence, viz:

1. Unit 1: Cliff development in moderately weak, fissured Littleham Mudstone, with well-defined cliff top embayments resulting from several previous sliding failures (e.g. The Floors). These occur as large-scale but infrequent, 5-25 year, events (e.g. some 75m of cliff top retreat between 1933 and 1937 at Littleham Cove, and the collapse of a central block between 1963 and 1969). The mean rate of recession between 1920 and 1990 was approximately 1.5m per year. Grainger and Kalaugher (1987) have described an example of another characteristic of this unit, which is persistent mudsliding over the lower cliff. Between 1981 and 1985, a large monitored mudslide near West Down Beacon created approximately 100m of displacement, as its lobate toe pushed forward across the beach in eleven discrete and rapid "surges" of movement (Photo 3). Each surge accounted for 5-15m of movement in as many hours, with the mudslide toe moving by planar sliding induced by undrained loading over one or more shear surfaces. It was observed that at least one surge event was apparently triggered by sea-level fluctuation during a single tidal cycle, although the controlling factor is likely to have been previous large debris falls from the rearwards cliff face. The latter accumulates temporarily on ledges and can independently generate small mudslides; however, their principal role is to feed the larger scale events. Although mudslides and flows create basal debris accumulations, they have a relatively short residence time; in consequence, there is a generally positive correlation between rates of basal cliff erosion/recession and the magnitudes of major sliding events.  
2. Unit 2: An upper cliff free face, developed in Conglomerate and Pebble Beds above an undercliff of Littleham Mudstones i.e. within the highest cliffs Unit 1 (lower cliff-face) and Unit 2 (upper cliff-face) occur together to create a compound cliff profile (Photo 3). The overlying strata retreat via brittle fracture falls, to give a distinct backscar, whilst the mudstones generate smaller-scale mudflows subject to intermittent surging. Seepage at the junction between argillaceous and arenaceous strata contributes to overland flow and gulleying as additional erosional processes. Both may reduce debris loading at the cliff base, and thereby play a secondary role in inducing sliding and flowage.  
3. Unit 3: Budleigh Salterton Pebble Beds overlie Otterton Sandstone, supporting steep, near-vertical cliff profiles subject to weathering by processes such as spalling, and falls (Photo 3). These produce impersistant basal debris stores.  
4. Unit 4: Steep Otterton Sandstone cliffs that become progressively lower in an eastwards direction, with free faces deeply etched by aeolian-induced 'honeycomb' weathering related to the presence of thin concretionary and marl bands. Basal undercutting, promoting occasional slab failures, operates along the westernmost sector; a vegetated debris store protects the cliff base further east with a narrow platform to seawards. This stability condition is reinforced by a seawall with gabions fronting the eastern developed frontage of Budleigh Salterton. Uniform lithology excludes seepage erosion, but various weathering processes and superficial mass movements continue (Roche, 2000).

With the exception of the cliffline behind the seawall, which has been stable since at least 1890, all sectors of this coastline are retreating (Roche, 2000), as confirmed from analysis of Coastal Monitoring Programme aerial photography (2012) and lidar (2007 and 2012) surveys. The available data has resulted in a change from the 2004 arrows when no quantitative data was available to reflect a cliff input of material of less than 1,000m³ per year. Posford Duvivier (1997, 1998, 1999) and Posford Duvivier/British Geological Survey (1999) estimated a mean rate of recession of 0.1 to 0.4 m per year for its entire length during the last century. This figure conceals considerable spatial and temporal variability, and may be an under-estimate, because recession would appear to have been more rapid in recent years at some locations. For example, rates of retreat along the western frontage of Budleigh Salterton are currently no more than 0.4m per year, but may be up to 5.0m per year between The Floors and Straight Point. Posford Duvivier (1997) calculated sediment yield from cliff recession to be in the order of 44,000m³ per year of silt and clay; 7,500m³ per year of fine sand; 3,500m³ per year of medium to coarse sand, and 1,600m³ per year of fine to medium gravel and metaquartzite cobbles from the Budleigh Salterton Pebble Bed.

Shoreface erosion, based on an actively eroding shoreface zone 1,000m in width, was calculated to result in between 0.8 and 4.4mm per year of vertical downwearing. This produced a sediment yield of between 2,000 and 6,000m³ per year for the shoreline between Otterton Ledge and western Budleigh Salterton, and 9,400m³ per year for the sector in front of Budleigh Salterton. Most of this is clay, silt and fine sand, and is removed from the local sediment budget as suspended load. Of the products from cliff erosion, only the coarser fractions are stable on the beach, particularly the discoidal metaquartzite pebbles released from the Budleigh Salterton Pebble Bed. Medium and coarse sands contribute to the lower foreshore and nearshore subtidal zone.

2.2 Fluvial Inputs

FL1 River Otter

The estuary of the River Otter has been considerably reduced in area as a result of:
(i) sedimentation and (ii) land-claim behind the barrier spit that has grown eastwards across its mouth. The river now discharges, via an incised 5m wide channel, adjacent to Otterton Ledge. Rendel Geotechnics and the University of Portsmouth (1996) estimate that coarse bedload discharge is a maximum of 320m³ per year, although this material could become deposited within the estuary rather than being supplied directly to the coast. River discharge in combination with tidal flows maintain the inlet and are sufficiently strong to entrain beach gravels and transport them a short distance seaward where they accumulate within an ebb tidal delta (Photo 6). Ebb current velocities at the mouth of the Otter are significantly higher than on the flood so that the gravels are preferentially transported seaward at the inlet. Wave action tends to drive material back shoreward from the delta to the barrier such that a circulation of sediment occurs. This is primarily a circulation of existing beach sediments and not of newly derived materials.

3. Littoral Sediment Transport and Beach Characteristics

» LT1  

Sediments released by cliff erosion and landsliding, and shoreface abrasion, provide a range of materials, from large boulders to fine sand and silt. The coarser fraction of cobbles and gravel is retained on the intertidal beach, particularly the discoidal, rounded quartzite grit and vein quartz clasts of the Budleigh Salterton Pebble Beds, characteristically with long axes between 19 and 90mm. Taking the beach system of this unit as a whole, it is morphodynamically least stable over the western sector. It is in this area that slides and flows across the beach temporarily subdivide it into several isolated 'pocket' beaches. Progressive retreat of the position of mean low water in the immediate lee of Straight Point, since circa 1890, may be due to the release from storage of beach sediments confined by earlier, but subsequently eroded, cross-beach slumps, slides and flows. Carr and Blackley (1975) specifically examined the evidence for longshore grading based on a one year programme of systematic sampling of the coarse (upper beach) sediments. Using axial measurements of samples of individual pebbles (200 pebbles per sample site), mean roundness; skewness and kurtosis indices were measured for a sequence of cross-profiles between Littleham Cove and central Budleigh Salterton. Regression analysis failed to reveal any positive correlation between mean particle size ('b' axis) and mean roundness index. However, it was apparent that clasts were more discoidal in shape in the western sector, where wave energy is higher than it is eastwards. This might suggest that discs are more mobile in comparison with spheres. Grading patterns were not consistent throughout the period of sampling, with the smallest particles sometimes concentrated at the eastern end of the beach but more often clustering towards the central section. This could be the product of 'pulses' of supply of large clasts from cliff landslip/landslide events, thus demonstrating the importance of punctuated input from this source. This was particularly evident from clast dimensions taken from the winter stormbeach. The fact that samples were more normally distributed to either side of the central sector of the beach may be evidence of short-distance sorting and, possibly, abrasion wear. Comparisons of pebbles sampled from the cliffs with those from the beach demonstrated that they were similar (with allowance for marine sorting on the beach), suggesting strongly that the beach material is derived from the eroding cliffs in the immediate vicinity. This feature gives a specific distinction to the beach of this unit in that most cobble-dominated beaches are associated with wave energy reduction due to submarine topography; in this case, however, it would appear that it is well adjusted to wave refraction (May, 2003).  

Coastal Monitoring Programme data provides no quantifiable evidence for a weak net westward transport along this frontage, and therefore the 2004 westward LT2 arrows have been removed from the updated map. In the western portion of the bay the sheltering influence of Otterton Ledge is diminished and south-east approaching waves generate a stronger potential for westward drift, although the extent to which there is any consistent net westward drift is uncertain. Limited cross-shore sediment movement is likely between the nearshore sub-tidal and inter-tidal beach.

On the basis of present knowledge, it is considered probable that the coarse sediment transport system between the two confining headlands of this shoreline unit is closed. Although Coastal Monitoring Programme data is inconclusive inputs from cliff erosion may be balanced by outputs related to some net offshore removal, together with abrasion losses, or they may result in slow accumulation against Otterton Ledge and within the Otter tidal delta. Substantial outputs of fine sediment take place via shoreface erosion, supplemented by discharge from the Otter. This is removed as suspended load, with unknown final destination(s). Coarse bedload sediments delivered by the River Otter are likely to be only a very small component; some of this would appear to be retained, as storage, in the offshore tidal delta. During the nineteenth and early twentieth centuries, informal "pebble picking" and pebble crushing for local road metal represented an output, but this practice was apparently stopped in the 1920s. In summary, the contemporary sediment budget is best described as dynamically stable.

LT1 Littleham Cove to Otterton Ledge (see introduction to littoral transport)

The beach pebble gradings outlined above provide some indirect evidence that longshore beach drift occurs in both eastwards and westwards directions. This is supported by the relatively symmetrical planshape of this shoreline. Reversals probably occur frequently in response to changes in incident waves, with those approaching from the east and south-east responsible for net westward drift and those from the south and south-south-west generating eastward drift. However, shelter against south-east approaching waves provided by Otterton Ledge and refractive effects set up by both this headland and the nearshore tidal delta of the River Otter tend to reduce the power available for westward drift. Furthermore, east and south-east approaching waves operate for only a short period each year, with refracted south and south-westerly waves therefore the dominant feature of the local wave climate. Consideration of the net outcome of these opposing forces leads to the tentative conclusion that there is a weak eastwards net longshore transport pathway with support to this view provided by (a) the growth of the barrier spit across the entrance to the Otter Estuary; and (b) a slight eastwards expansion in beach width towards Budleigh Salterton with significant gravel and sand storage against Otterton Ledge (Photo 1). The revised transport arrows reflect the low reliability and confidence in this marginal net eastward trend of less than 1,000m³ per year.

4. Sediment Outputs

Offshore Losses at Otterton Ledge

In 2009, the Coastal Monitoring Programme completed a 100% coverage swath bathymetry survey, extending offshore 1km from MLW between Petit Tor Point and Portland Bill. The seabed is generally gently sloping comprising coarse or mixed sediment. The subtidal extensions of the rock headland at Straight Point exposed in parallel ridge features, extend approximately 700m south-southeastwards, and southwards from Otterton ledge approximately 250m offshore. The seabed sediments appear to be of greater thickness at the western end of this section, with few rock outcrops discernible offshore of the Floors. Further east, between Budleigh and Otterton Ledge, the thickness of seabed sediment is not sufficient to mask the underlying bedrock, with extensive areas of bedrock exposed. The 2004 sediment transport study included an arrow indicating offshore loss at Otterton Ledge. Due to this new data the lack of sediment in this area provided the confidence to remove this arrow. Posford Duvivier (1998) suggested that long-term coast recession and erosion of the Budleigh Salterton Pebble Beds might result in the accumulation of a reserve of well-sorted cobbles and pebbles on the nearshore bed.

5. Sediment Stores and Sinks

5.1 River Otter Estuary

The alluvial flats of the lower River Otter occupy a formerly much larger estuary inlet now largely infilled. Today, it is narrow, bounded by cliffs to the east and embankments to the west, and has a total area of less than 35ha. The estuary tidal prism has been substantially reduced by land claim such that its inlet is of marginal stability and is deflected significantly eastward by Budleigh Spit. Its creek-dissected intertidal flats are of small extent, but support about 20ha of saltmarsh to either side of the main channel above the estuary mouth. It is an unusually species-rich, well-zoned vegetation community, with only small clumps of invasive Spartina. This area of low to mid saltmarsh has been stable in area since the late 1950s. The estuary has acted a local sink for fine sediments delivered by the river and small inputs of suspended marine sediments delivered from the exchange of tidal waters.

5.2 Budleigh Spit

The shingle barrier spit that encloses the Otter estuary is dominantly composed of highly rounded discoidal metaquartzite clasts derived from the Budleigh Salterton Pebble Beds. Whilst longshore transport from the west has provided sediment feed that has sustained its progressive eastwards growth over a distance of nearly 500m, this feature has several diagnostic characteristics of a barrier structure. It is therefore likely that it has developed as a result of shoreward movement of an originally detached, offshore beach supplied by a reserve of clastic sediments released by earlier coastal erosion. Its presence can be traced back to at least the mid sixteenth century. Only later in its development would it have become dependent upon gravel supplies from the local eroding cliffs. During the previous 100-130 years, it appears to have been stable in both planform and elevation, although the crest is now reinforced by rock-filled gabions. Without any specific historical or contemporary evidence for overwashing, overtopping, crest cutback or lowering, it is difficult to confirm barrier morphogenesis. The beach face exhibits multiple berms, indicating probable short-term fluctuations in cross profile form. Sediment exchange between the beach and nearshore zones is therefore likely, but not proven. The stability of Budleigh Spit is due in some part to protection from waves from the east and south-east afforded by Otterton Ledge.

5.3 River Otter Tidal Delta

A small ebb tidal delta, composed dominantly of gravel with some sand, has accumulated seawards of the mouth of the Otter adjacent to Otterton Ledge. It is evident from the visual pattern of wave refraction at low water, and from aerial photography. In other respects, little is known of the size or evolution of this feature. Its materials are derived from gravels entering the Otter inlet that become entrained and transported seawards by ebb tidal currents in combination with river flows. It may also partly be the product of occasional high magnitude river flood and bedload discharge events from the Otter that would strip gravels off of the spit and deliver them seawards. Wave action tends to drive material back shoreward from the delta to the barrier such that a circulation of sediment occurs. Otterton Ledge may be important to the stability of the delta for it acts as: (i) a barrier preventing eastward drift of gravels away from the delta and (ii) shelter against south-east approaching waves that would otherwise tend to drive the deltas shoreward and westward.

Due to its relatively stable, natural beach sustained by local cliff erosion this unit is not currently the subject of immediate shoreline management problems, thus the incentive for investment in monitoring systems is relatively low. Even the short stretch of seawall does not appear to have promoted problems of beach drawdown. Nonetheless, with the exception of research on cliff degradation, knowledge is qualitative and there are several uncertainties. Without improved understanding, effective proactive management or future responses to problems may be constrained. These uncertainties are primarily:

1. The directions and quantities (rates) of net longshore transport.
2. The overall sediment budget, especially on/offshore exchanges.
3. Nearshore wave climate.
4. Spatial and temporal variation in beach profile form and elevation, especially its storm response.
5. The bathymetric form and sediment storage volume of the Otter tidal delta.

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 at present a relatively short-term time series of data has been collected (~ 6 years), longer term Coastal Monitoring Programme data, when combined with other data sets, academic research and historical studies may enable sediment budgets, transport rates and directions to be identified and/or validated in the future. Baseline habitat mapping has been carried out by the Southwest Regional Coastal Monitoring Programme.

Notwithstanding results from the Southwest Regional Coastal Monitoring Programme, the Durlston Head to Rame Head SMP2 (SDDCAG, 2011), has also summarised existing knowledge.