Hope's Nose, Torquay to Holcombe
1. INTRODUCTION - References Map
Geomorphology and Evolution
Much of this coastline is characterised by unprotected cliffs formed within distinctive reddish sandstones and breccias that back a sequence of small bays, or coves, and intervening headlands e.g. Photo 1. However, the estuary of the River Teign and a complex pattern of nearshore and offshore banks beyond its mouth introduce discontinuity (Photo 2). With the exception of the Teignmouth frontage (Photo 3), beaches are narrow and composed dominantly of sand with some fine gravels. Longshore drift south of the Teign estuary mouth, and also between Spey Point and Holcombe, is from south to north but is both weak and compartmentalised. Between northern Teignmouth and the distal end of Denn spit the net drift pathway is from north to south, creating a convergence of littoral transport at the mouth of the Teign. Thus, there are three distinct partly independent, sub-cells of beach and nearshore sediment movement as follows:
Geomorphological evolution has been largely conditioned by Holocene sea-level rise, though its basic planform is the product of earlier relative transgressive and regressive movements. An ancestral River Teign has been identified from seismic refraction studies of older buried channels (Durrance, 1971); these extend seawards, and suggest that the Teign was originally a tributary of a proto-Exe River. Early to mid-Holocene sea-level rise converted the floodplain of a river graded to at least -23m OD into a ria-like form. Subsequently, it has been infilled with sediment, a process accelerated by the southward growth of a confining spit (Denn Spit) at its entrance that has assisted sedimentation by providing shelter from wave penetration.
The mean tidal range at Teignmouth is 1.7m at neaps and 4.2m during springs. Currents at about 1km offshore are approximately 0.2 to 0.5ms-1, but increase to over 2ms-1 in the Teign entrance channel. The flood residual occurs very close inshore, but the ebb, characterised by higher velocities because of the slightly asymmetric tidal regime, is more dominant seawards of the harbour mouth. The south-west flowing flood residual moves adjacent to Denn Spit beach, whereas the ebb is close to Ness Head. Because southerly flood flow off Denn Beach is stronger than that moving northwards (at different stages of the tidal cycle), an eddy is formed within which current speeds can exceed 2.2ms-1 (HR Wallingford, 2001). A slightly less intensely developed tidal eddy also forms behind the Ness headland, caused by the reversal of tidal circulation in Lyme Bay two hours before high water. The details of this locally complex tidal range are crucial to understanding sediment dynamics offshore the Teign estuary mouth (Section 5.3).
Hydraulics Research (1970) computed a mean significant wave height of 0.85m at the estuary entrance. Whitehouse, Sutherland and Waters (2002) report on the results of a one year programme of wave measurement using four recording stations at contrasting morphodynamic locations in the nearshore/offshore zone seawards of the coastline between Teignmouth Pier and the entrance channel close to the distal point of Denn spit. Maximum energy waves approached from the east, but significant inshore wave heights exceeding 0.5m operated for only 10% of the year. A wave height greater than 2.5m occurred once, under storm conditions, whilst ten events generated heights over 1.5m and 28 events produced heights of approximately 1.0m. All of these were associated with wave approach from the east, south-east or north-east. All authorities are agreed that waves moving in from the south and south-west are much reduced in power because of refraction effects imposed further south by large headlands such as Start Point, Berry Head and Hope's Nose. However, shoaling and refraction over the system of nearshore banks reduces the energy of waves moving towards the Teignmouth coastline from all directions.
Teignmouth was one of the locations for which wave modelling excercises were undertaken as part of the DEFRA Futurecoast Project (Halcrow, 2002). An offshore wave climate was synthesised based on 1991-2000 data from the Met Office Wave Model and then transformed inshore to a prediction point in off Teignmouth at -4.65m O.D. Potential sensitivities to likely climate change scenarios were then tested by examining the extent to which the total and net longshore energy for each scenario varied with respect to the present situation. Results suggested that a one to two degree variation in wave climate direction could result in a 1-3% variation in longshore energy. Wave energy was also found to be especially sensitive to sea-level rise and Atlantic storminess. The effect is probably due to a reduction in wave refraction within the nearshore shoals and banks as water depths increase so that slightly higher haves will approach the shoreline at rather more oblique angles.
The combination of wave and tide-generated nearshore and offshore currents has major effects on sediment transport within and between the pattern of banks and shoals that constitute the complex ebb delta of the Teign (see Section 5.3). In particular, substantial increases in bedload transport due to higher shearing stresses over the shallow seabed are linked to an increase in suspended sediment loads (Hoeksta et al., 2001). Saltation may cause suspended sediment to move from crest to crest of sets of ripples, thus promoting bedform migration.
The coastline between Hope's Nose and Ness Head is relatively unaffected by interventions and remains free to behave naturally. Exceptions involve short sections of sea wall at Anstey's Cove and Oddicombe Beach as well as a section of rock revetment at the latter.
The Exeter Plymouth railway follows north margin of the Teign Estuary and runs along the cliff toe between Teignmouth and Holcombe (Photo 6). For much of this distance it has been protected by a sea-wall. The effect has been to confine the estuary and impound upper beach sediments on the open coast where parts of the line were constructed upon them. It also prevents cliff erosion sediment inputs.
The town of Teignmouth has been built upon southward trending Denn Spit at the entrance of the Teign estuary. Defences constructed to protect the town effectively impound the sediments stored in the spit and prevent free exchange with the ebb tidal delta. The Teign estuary is maintained for navigation and ports have operated at Teignmouth and Newton Abbot, although the latter has reduced in importance.
2. SEDIMENT INPUTS - F1 References Map
2.1 Marine Inputs
F1 Teign Estuary
During the early and mid Holocene, rapid sea level rise converted the lower Bovey valley into a relatively deep ria-like estuary. Following on from approximately 5,500 years before the present, the rate of sea-level transgression reduced, and the Teign estuary began to accumulate sediment from both fluvial and marine sources. This has been subsequently promoted by spit growth and weak wave action and tidal currents within the confines of the estuary. Thus, the Teign estuary is a sediment sink that has had a positive sediment budget for at least the last three to four millennia. There is a well-defined pattern of sediment sorting over the inter-tidal flats, from coarse sand and fine gravel near the entrance to silt and clay at the estuary head. At low tide sand and gravel flats, dissected by tidal channels, are exposed in the central area, whilst over 200ha of muddy flats dominate the upper estuary. A well defined mixed sand and gravel bank, known as The Salty, and other minor sandbanks, occur immediately upstream from the entrance (Photo 2), apparently trapped by the curvature of the main channel. The sediment of The Salty is probably wholly derived from marine sources (Robinson, 1975), an interpretation re-inforced by the presence of a thick mussel bed. It is therefore interpreted as a flood tide delta. Merefield (1982) reported that 8% of Teign estuary sediments have a carbonate content, i.e. shell debris mostly derived from the marine environment.
Detailed determination of sediment transport vectors immediately seawards of the entrance channel (Van Lancker, et al., 2001) indicated that wave-induced currents determined most pathways. Sediment movement was confined to the area of banks and shoals, thus representing a sustaining feed. Tidal current velocities in the main entrance channel, in excess of 2ms-1, are high enough to create some scour of the Permian breccia and sandstone bedrock into which it is incised, thus creating a small additional input. Thus, whilst ebb tidal currents are of shorter duration, but more powerful than flood currents (Halcrow, 2002), it would appear that the latter operating in conjunction with wave action at the entrance can transport sandy sediments into the estuary, creating the extensive flood tidal delta of The Salty (Photo 2). Wave action on the ebb tidal delta can also drive sediments ashore within migrating bars to supply the Denn Spit frontage in the vicinity of Teignmouth Pier (Photo 3) Due to the weaker, but longer duration flood currents is likely that there is a net input of fine sediments, although intervals of seaward flushing might be anticipated during high river flows. Further details of the complex circulation associated with the banks of the ebb tidal delta and the shoreline are discussed within Section 5.
2.2 Fluvial Inputs - FL1 References Map
FL1 River Teign
River discharges are fairly high, but strongly seasonal with a mean of 9 cubic metres per second (cumecs), but typical winter flood values of 50-100 cumecs and a maximum of 142 cumecs (Halcrow, 2002). Significant quantities of suspended sediments are discharged into the Teign estuary from the Lemon, Teign and Bovey drainage systems. A proportion is diverted into storage in upper and mid marsh mudflats, whilst a further quantity is flushed through the estuary and discharged at its mouth during high river flows. Concentrations are reported to be high (South West Water, 1989), and are probably supplemented by fine sediments released by mudflat and isolated lower salt marsh areas along the southern estuary shoreline; scour also occurs along creek margins. Little of this material makes any contribution to the marine/littoral sediment budget, although there is no reported data on total quantities of suspended sediment. The ebb tidal current may transport coarse sand, as bedload, down estuary under high river discharges. There have been a few reports of sand being trapped by banks near the estuary mouth, and other inferential evidence for ebb-current transport of possible fluvially derived sand sediments. Merefield (1978, 1982) has reported suspended concentrations of barytes of up to 3,200ppm near the estuary head, decreasing progressively towards its mouth. The only feasible origin of this mineral is via fluvial discharge. He also measured strontium concentrations, which showed a reverse pattern that was further clarified by calculating barytes: strontium ratios. Strontium is released from the breakdown of shell debris, which must therefore be introduced by the flood tide current.
South West Water (1989) state that upper estuary sediments are relatively poorly sorted, but that their heavy metal content - fixed by flocculation - indicates their fluvial origin. Up until the early twentieth century, the River Bovey transported significant quantities of fine clay derived from the opencast workings of Ball Clay in the Bovey Tracey basin. Clay barges navigated upstream to Newton Abbot until the late 1920s, during which time the upper estuary channel was regularly dredged. Both of these influences help to account for the poor sorting, which is also evident in some mid-estuary mudflats. Organic content declines from over 12% to less than 7% from upper to middle mudflats. Lower estuary sands and gravels are well-sorted and their very low percentage of heavy metals implies little input of terrigenous sediment.
2.3 Cliff and Shore Platform Erosion - E1 E2 References Map
E1 Hope's Nose to Ness Head
The coastline from Ness Head (Photo 2) southward to Petit Tor Point (Photo 7) is fronted by well-developed convexo-rectilinear cliffs cut into reddish Permian sandstones and breccias. At Petit Tor, the Permian Oddicombe breccia is faulted against relatively resistant crystalline Devonian Limestone (Photo 7). The outcrop pattern southwards to Hope's Nose is a complex alternation of shales, limestones, slates and mudstones, with the presence of intrusive igneous rock, e.g. at Anstey's Cove (Photo 8). The detail of coastal plan, as well as cliff morphology, is influenced by several fault and thrust planes. Collectively, these rocks make up the well-defined broad salient of the Hope's Nose peninsula cliffs, which reach a maximum elevation of nearly 90m OD. Babbacombe Bay extends north of Petit Tor comprising numerous minor salients, small coves and other coastal re-entrants e.g. Mackerel Cove (Photo 1), a few of which coincide with truncated valleys e.g. north of Oddicombe and south of Shaldon and the Ness headland (Perkins, 1971).
It has been suggested that some coves such as Anstey's on the Hope's Nose promontory were originally created as karstic solution or subsidence features (Perkins, 1971). A narrow shoreline platform, often strewn with boulders, exists seawards of most of the sandstone cliffs, with the cliff foot junction with the upper platform at or just above the level of mean high water springs (Photo 1). Platform morphology is adjusted to relative rock resistance, often with the more resistant horizons forming distinct ledges. Surfaces are characteristically rectilinear, but their restricted width is normally insufficient to inhibit wave erosion at the cliff base. Rock, mud and debris slides, falls and topples and associated debris cones occur at several sites, such as Watcombe Head, Reigate Beach, between Oddicombe and Maidencombe, south of Oddicombe Beach, Petit Tor Cove, Anstey's Cove and Ness Cove, Shaldon (Doornkamp, 1988; Sherrell, 1995, 1996). Several of these are due to either stratigraphical or faulted junctions which result in basal clays and/or mudstones underlying thicker, more competent but jointed sandstone formations. More complex slides include the failure zone of two large mudslides on either side of Watcombe Head, east Oddicombe, and the high cliffs at Labrador Bay (Photo 9) where a distinct undercliff has formed. There are several 'fossil' failure surfaces along this coastline, as at Oddicombe and Babbacombe (Doornkamp, 1988), all of which have potential for re-activation.
Beach sediment composition in the succession of small coves and bays indicates direct input from local cliff erosion. Lag deposits of boulders are frequently due to the removal of fines, by wave erosion, from slides bringing breccias to beach level, as at Maidencombe. Cliff retreat rates have not been systematically analysed, but Posford Duvivier (1998) report that cliffs between Shaldon and Petit Tor Point; and south of Oddicombe to Hope's Nose are either stable or locally receding at rates less than 0.2ma-1. A recession rate of less than 0.1ma-1 occurs in Oddicombe Bay. The highest rate approaches 1.0ma-1 over a short length of cliffs at Ness Head. With such modest rates of erosion, sediment yields to the transport system are small to moderate at present, although they would increase rapidly if recession increased due to the cliff height. For example, if the 6km section between Petit Tor Point and Ness Head eroded at 0.1ma-1, a sediment yield of 36,000m3a-1 would be produced assuming a mean cliff height of 60m. If recession increased to 0.3ma-1, a typical rate for sandstone cliffs with moderate wave exposure, the cliff sediment yield would increase to 108,000m3a-1. Bearing in mind that the breccias and sandstones of these cliffs contain a high proportion of sands and some limestone gravels, it is clear that this cliff frontage is potentially an important local source of littoral sediment.
Only very limited lengths of the cliffed coastline south of the Teign estuary have formal defences, principally short sea walls, rock revetments and gabions at Oddicombe beach and Anstey's Cove. At a few sites, small-scale cliff trimming and scaling are undertaken periodically. Devonian Limestone has been quarried in the past from Petit Tor Point, Long Quarry Point (Photo 8) and Hope's Nose (northern side).
E2 Teignmouth to Holcombe
The coastline from Teignmouth northward to Holcombe (Photo 5) is fronted by well-developed steep cliffs cut into reddish Permian sandstones and breccias. North of the Teign seawall protection is continuous along the railway line between Teignmouth and Holcombe (Photo 6). The former marine sandstone cliffs behind are fully protected from marine toe erosion by the Brunel seawall and thus their potential contribution to the sediment budget has been removed since it was constructed in 1849. The beach at the base of this structure is very narrow due to reflective scour (Photo 10). Although toe erosion has halted, the cliffs remain unstable and subject to sub-aerial processes, rockfall and slides within talus accumulations. During several especially wet winters in recent decades, debris has surged across the track necessitating temporary closures of the railway line.
A pre mid-nineteenth century (pre protection) cliff retreat rate of between 0.5 and 2.0ma-1 may be assumed, which is considerably in excess of any prevailing rates elsewhere on this coastline (Posford Duvivier, 1998). If this rate is correct, then historical sediment yields from this 2km long frontage of cliffs averaging 40m in height would have been some 40,000 to 160,000m3a-1.
3. LITTORAL TRANSPORT (BEACH DRIFT) - LT1 LT2 LT3 References Map
LT1 Hope's Nose to Ness Head
The Hope's Nose promontory (Photo 4) is considered to be an absolute boundary to longshore bedload transport, though no direct proof has been offered. Between here, and the mouth of the Teign, there is a discontinuous sequence of minor headlands, coves and bays, most of which accommodate either sand-dominated or mixed sand and gravel "pocket" beaches. The most substantial beaches are at Anstey's Cove (Photo 8), Oddicombe (Photo 7), Maidencombe, and Shaldon where cliff falls have contributed well-developed backshore berms (Doornkamp, 1988). A weak, discontinuous net northwards sediment transport pathway links these beaches, although Long Quarry Point (Photo 8) may be sufficient to completely block shoreline transport. The most convincing proof of this transport direction is the northwards growth of the Ness Point spit at the most southerly point of entrance to the Teign estuary. Beach sediments within each re-entrant trap are prevented from by-passing their confining headlands except, perhaps, when incident waves are of exceptional energy. Quantities of mobile sediment are therefore likely to be relatively small, in southern parts, but probably increase northwards along the pathway due to the cumulative effect of cliff erosion yields. Material moving northwards to the vicinity of Ness Point undoubtedly makes a contribution to the complex sediment budget of offshore banks at the entrance to the Teign Estuary..
Merefield (1984) undertook an analysis of the composition of Maidencombe beach, and reported that 27% consisted of carbonate material. This was presumed to derive from sub-tidal and offshore sources that had been driven to the shoreline by waves and tidal currents. Whether this is characteristic of other beaches along this unit is unknown, although Merefield states that it is significantly higher than for most beaches along the South Devon coastline. Human interference may have influenced beach morphology in a few cases, e.g. the substantial loss of volume of Oddicombe beach between approximately 1910 and 1960. This would appear to be linked to the cessation of supply of coarse spoil from former working quarries on Hope's Nose (Perkins, 1971).
Overall, it is concluded that the sediment transport system along this unit of shoreline is weak and nearly self-contained. Beach inputs received via cliff erosion and mass movement on the one hand, and suspended transport of carbonate debris on the other, may be balanced by in situ losses via abrasion and attrition together with a likely net output northwards at Ness Point. This would help to explain apparently stable beach morphology over recent decades. Perkins (1971) infers that drift reversal takes place along some beaches, but does not suggest under what incident wave or wind conditions this might occur. Seasonal fluctuations in profile form have been observed, but volume changes have not been quantified (Posford Duvivier, 1998).
LT2 Sprey Point to Teign Entrance
All authorities are agreed that the net sediment transport pathway from north of Teignmouth to the distal point of Denn spit (Photo 2 and Photo 3) is south south-westwards. The latter feature is therefore largely a drift-fed store (Section 5.2), although there is some direct input of tidal and wave-transported sediment near the apex of this feature (Robinson, 1975). Sediment supply derives from the following sources:
Net south/south-westwards littoral drift along Teignmouth beach is reported in various studies concerned with the performance of both the seawall and groyne system. Movement in this direction takes place whenever winds and waves from the east, south-east or east-south-east are operating. Hydraulics Research (1970), report some slight drift reversal (i.e. north-eastwards) when high energy waves approach from the west-southwest. However, offshore and inshore diffraction and refraction of waves from this direction over the Teign ebb tidal delta is such that they usually have negligible capacity for littoral transport.
Southeast, east and north-east approaching waves also promote significant short-term losses of beach sediment (NRA, 1990; Lewis and Duvivier, 1974; Gundry, 1982; Teignmouth Urban District council, 1956). Loss of beach volume in front of the seawall protecting urban development on the Denn spit (Photo 3) would appear to have been a persistent feature since at least the early twentieth century. However, some losses between 1900 and 1910 (and possibly over the previous 30 years) were due to deliberate removal of sand and gravel by local building contractors. Teignmouth Urban District Council (1956) reported up to 4m of beach level fluctuation during January 1954 to October 1956, but with no clear-cut evidence, from weekly beach re-surveys, of seasonally related patterns of net accretion or depletion. As there was an approximate correlation between incident waves from the east and south-east and beach drawdown, it was concluded that variation in offshore to onshore sediment transfer was a more likely explanation of morphodynamic response than fluctuations in updrift drift rates. The surveys recorded some 70,000m3 of sand accretion along Teignmouth beach north of the pier between April and July 1956. An equivalent quantity was lost from the beach to the immediate south during the same period. The results clearly demonstrate a considerable sediment mobility, but cannot be used to infer drift because of: (i) the short study period and (ii) lack of data on cross shore exchange.
In the subsequent 45 years, Teignmouth beach has not been subject to a comparable monitoring exercise, though Lewis and Duvivier (1974) and Gundry (1982) observe the effectiveness of the groyne system in trapping inputs of sediment that probably derive from offshore, and which have helped to maintain beach levels. Small-scale, occasional replenishments have also assisted. Lewis and Duvivier (1974) report fluctuations of the shoreline of up to 14m between 1958 and 1971 (thought to be derived from local authority profile data).
HR Wallingford (2001) state that the low gradient sandy foreshore (median grain size diameter of 0.2mm) of Teignmouth beach is a dissipative form, though it often features a shallow bar. In contrast, the backshore element has a mean slope of 8o, is composed of coarse sand and some gravel (median grain size of 0.4mm) and is more reflective. This section of the beach is subject to especially rapid drawdown during winter storms from the south-east and east (Posford Duvivier, 1998), assisted by wave reflection from the backing seawall. The latter, first constructed in 1908, is built over a substantial store of backshore sediment, which has a further negative effect on contemporary beach stability. COAST 3D project researchers (HR Wallingford, 2001) report that the direction of longshore transport consistently correlated with offshore wave approach, with the magnitude occurring being a function of wave height. Short-term reversal of the net drift pathway, associated with waves approaching from angles greater than 140o tended to move material northwards. Data obtained from the swash zone indicated that it was morphodynamically very active, showing rapid alternation between accretion and erosion as the tide rose and fell. The key controlling variable was the interaction of prevailing type and height of shoaling and breaking waves and antecedent beach slope. Grain size characteristics and water table fluctuation also exerted influences but they were not individually researched. Suspended sediment transport increased shorewards through the surf zone, with net transport related to a balance between wave asymmetry, moving sediment onshore, and the backwash (undertow) current, taking it offshore. Further research has demonstrated transfer of sand from the distal area of Denn Spit into the channels at the estuary entrance (see Section 5.2 and Section 5.3). Whether this quantity is in balance with the longshore flux of sediment south of Teignmouth pier has yet to be determined.
LT3 Sprey Point to Holcombe
North of Teignmouth pier, to approximately Sprey Point, (a blunt salient fronted by a seawall with a small breakwater on its northern side), frequent but brief reversals of net southwards longshore drift direction are directly related to changes in incident waves. Suspended transport direction, however, is controlled by reversal of tidal currents during each tidal cycle (HR Wallingford, 2001). No net pathway can be readily determined. North of Sprey Point, littoral movement is northwards in response to wave climate, but there is no clear evidence of any by-passing of Holcombe (Parson and Clerk) headland. Posford Duvivier (1998) estimated a net northward drift potential of 68,000m3a-1 at Holcombe by modelling transport using a hindcast wave climate. It probably overestimates actual transport for it was assumed that all material was sand and that abundant material was always available for transport. A weakly defined drift divergence must therefore exist in the vicinity of Sprey Point.
The railway line and defending wall have been built over a pre-existing backshore sediment store, a fact that helps to account for Mean Low Water retreat, inter-tidal narrowing and back beach lowering since at least the 1880s (Posford Duvivier, 1998). Considerable wave reflection occurs from the sea wall at high tide (Photo 6 and Photo 10) and this is probably effective in entraining sediment and promoting beach scour.
In summary, the longshore transport system for the open coast is a set of closed or partially closed sub-cells, though onshore and offshore exchanges are probable, especially in the vicinity of the Teign estuary entrance. Drift convergence occurs at the entrance to the Teign estuary, with complex linkage to the largely self-contained circulation of sediment in the near- and offshore areas of banks and shoals, where much of the available littoral sediment is presently stored.
4. OUTPUTS - O1 References Map
4.1 Offshore Transport
O1 Offshore Transport at Mouth of Teign
There is evidence for some onshore-offshore transport at the mouth of the Teign, as part of a complex circulation associated with the banks there. See Section 5 for a full discussion.
4.2 Estuarine Output - EO1 References Map
EO1 Teign Estuary
Several researchers (e.g. Hydraulics Research Station, 1965, 1970; Riddle and Murray-Smith, 1990; Robinson, 1975; HR Wallingford, 2001) have demonstrated net seawards transport of sand and fine gravel in the Teign estuary entrance channel. This takes place via the ebb tidal channel, and occurs as both suspended and bedload transport. Much of this material (there are no quantitative estimates) is likely to be sediment introduced into the lower estuary on the flood tidal stream, i.e. from marine sources, especially material drifting southward down Denn spit. However, a proportion will be mostly suspended organogenic particulate matter introduced upstream via the River Teign. The sediments are flushed out of the estuary inlet seaward until the ebb tidal current disperses and wave action tends to drive material back landward. Deposition occurs where the two opposing forces are evenly balanced forming the ebb tidal delta. Detailed determination of sediment transport vectors immediately seawards of the entrance channel (Van Lancker, et al., 2001) indicated that wave-induced currents determined most pathways. Sediment movement was confined to the area of banks and shoals, thus representing a sustaining feed. Tidal current velocities in the main entrance channel, in excess of 2ms-1, are high enough to create some scour of the Permian breccia and sandstone bedrock into which it is incised, thus creating a small additional input. Daily drag-dredging of the channel, to maintain navigation access to Teignmouth Dock, stimulates sediment suspension and may provide some increase of net seawards transport.
4.3 Dredging - References MapDredging of the mid to upper channel of the Teign was undertakenin the past when Newton Abbot was a port for the export of china clay. The entrance of the Teign is drag dredged freyuently to maintain a navigation channel to Teignmouth docks.
4.4 Beach Mining - References MapSome sand and gravel has historically been removed from Denn spit by local building contractors. It is thought that this practice ceased in the early twentieth century.
5. SEDIMENT STORES AND SINKS - References Map
5. 1 Teign Estuary
The tidal Teign estuary is approximately 9km in length, less than 1km wide (at its widest point) and occupies a surface water area of between 1.2km2 (mean low water springs) and 3.5km2 (mean high water springs). There is progressive up-estuary distortion of the tidal wave, giving an ebb current of longer duration than the flood current in the upper estuary, however, this situation is reversed in the mid and lower estuary, so that ebb current velocities at the entrance are faster than on the incoming flood. The latter starts to penetrate before the ebb has ceased, resulting in each current flows following separate channels at the entrance. This mutually evasive pattern disappears rapidly upstream, with residual tidal currents, in either direction, following the main axial channel (South-West Water, 1989).
The narrow entrance is the product of the southward growth of Denn Spit. This, coupled with a sharp bend in the main channel immediately up-stream, ensures that the entire estuary is virtually removed from the influence of external wave action. Internally generated waves are relatively insignificant because of limited fetch.
Freshwater discharge via the River Bovey and its tributaries is gauged at Preston, and normally varies between a summer mean of 5m3s-1 and a winter mean of 10 to 20m3s-1. Flood peak discharges are characteristically 50m3s-1, with exceptionally high peaks of 150 - 200m3s-1 recorded every 6 to 7 years (during the past 35 years). Variations in river discharge affect both the rate of rise of the flood tide in the upper estuary and the overall sea-level slope (Sea Sediments Ltd, 1979; Whitehouse et al., 2001). River flow velocity is normally less than 0.1ms-1.
In terms of salinity structure two distinct classifications have been identified as follows (Halcrow, 2002):
There is only limited development of saltmarsh (approximately 13ha) on both the northern and southern margins of the estuary. It occurs mostly as isolated patches of mid and upper marsh communities that have been invaded by Spartina anglica since the late 1950s. Vegetation has not, therefore, played a major role in trapping fine sediment. Before its development as a gravel spit, the site of the Denn was an area of upper-middle saltmarsh. There is likely to be a small contemporary input of fine sediment from erosion of mudflats along the south shore, removed as suspended load. Walls and bunds protect the north shore, where there has been some piecemeal land claim.
Teign Estuary Partnership has developed to promote the integrated management of the estuary. They have prepared an Estuary Management Plan (Teign Estuary Partnership, 2000) that presents a series of guiding principles and strategic objectives with supporting information covering a wide range of topics. They have also sponsored research into a variety of topics. Further details are provided at the project website at: http://www.teignestuary.org/
5.2 Denn Spit and Teign Entrance Channel
The growth of the sand and gravel-dominated, roughly triangular, form of Denn Spit certainly predates the growth of the town of Teignmouth. Urban development now impounds much of this store, with its upper seaward face being fixed by the presence of a substantial seawall for nearly a century. The form of this feature is clearly the result of progressive southwards extension and expansion fed by littoral drift over several centuries. The lengthening of the distal point has deflected the entrance channel of the lower Teign estuary. Further migration southwards has been inhibited by Shaldon Cliffs, thus tidal velocities have been intensified by the constricted cross-sectional area now partly fixed by a training wall. Sediment arriving at, or close to, the distal end would appear to have a short residence time before removal by tidal and wave-generated currents (Hydraulics Research, 1965). The flood-generated eddy promotes southerly flow off Denn beach, and maintains a permanent flood channel that intervenes between the beach foreshore and Spratt Sand (Robinson, 1975; HR Wallingford, 2001). Flow velocities here are up to 2.2ms-1, sufficient to transport particles of up to medium sand size into the Lower Teign estuary. Whitehouse (2001) describes observations of migratory megaripples and sandwaves within this channel, thus confirming the significance of sand transport by tidal currents. Waves approaching from the easterly quarter are needed to rework coarser sediment incorporated into the beach, which are then more likely to enter the offshore anticlockwise circulation system. Phases of accretion of Denn beach foreshore are apparently related to phases of onshore bank migration (Robinson, 1975).
5.3 Nearshore and Offshore Banks of the Outer Teign Estuary
The complexities of the morphology and migration of the banks, bars and shoals of the outer Teign estuary were first systematically studied in 1848-1850 (Sprat, 1856). He concluded that there was a 3 to 7 year cycle of "circular" sediment movement, which occurred in three phases, as follows:
Throughout these changes, recorded cartographically in Robinson (1975), Spratt Sand shows comparative stability, though with some changes in length and breadth. This is also true of the channel that defines its shore-facing margin, which acts as the main route for the entry of the flood tide into the Teign estuary. Tracer results have proved that tidally transported sand can move up to 100m south-westwards (towards Denn Point) in 48 hours (HR Wallingford, 2001).
At this last location, tidal stream velocities are capable of moving the coarsest grades of sediment available, i.e. fine gravel. Experimentation with marked pebbles did demonstrate that they were moved upstream to The Salty, thus confirming the operation of a flood current very close inshore. However, most sediment that enters the inner estuary is rapidly removed seaward by the higher velocity ebb tidal current, which therefore functions as the main process of feeding and sustaining the offshore/nearshore banks. The ebb stream initially moves past Denn Point, and then swings slightly north-east to join the anticlockwise movement of the tide circuit early in each cycle. During its third quarter, it moves eastwards, somewhat closer to the Shaldon-Ness Point shoreline (Photo 2), and then southwards during the final quarter. Current velocities slacken during the last two quarters.
The incoming flood stream takes a more circuitous route, but with a dominant south-westerly directed flow (which commences half an hour before the ebb has ceased) that is parallel to the shoreline. This creates the flood channel between Spratt Sand and Denn Beach foreshore, as previously noted.
Thus, the overall effect of tidal motion is to create an anticlockwise water circulation pattern. As this resembles the direction of sand bank movement over 3 to 3½ year periods, it is tempting to conclude that tidal transport is a dominant process. It is, however, more likely that both waves and tidal currents act together in a complex inter-relationship to create this circuit of bank movement. Robinson (1975) concluded that the Inner Pole is mostly the product of wave action, as tidal currents here are relatively weak; however, they may transport sand that has been entrained by shoaling waves at the peak of the flood tide. Ness Pole is also the result of wave and tidal interaction, thus denying that it is generated entirely by wave driven by spit growth starting at the base of Ness Cliff (Sprat, 1856; Hydraulics Research, 1965; 1970). This conclusion is largely based on the evidence that the composition of Ness Beach is of medium sand, whilst particle size analysis of sediment samples from Ness Pole reveal a bimodal distribution, with peaks of coarse sand and fine gravel. Robinson (1975) did concede that the transient presence of a "tenuous" ridge linking Ness Pole and the northern tip of Ness Beach could indicate a slight tendency towards net offshore movement. However, there was no convincing evidence identified of any net northwards-directed longshore transport across the boulder-strewn beach at the foot of Ness Head Cliff. This ridge feature is perhaps more likely to be built from sediment supplied by the ebb current flowing westwards from the harbour mouth. Once established, it might feed the growth phases of Ness Pole, as well as temporarily trapping sediment that might otherwise have been removed further offshore. Its subsequent breakdown and erosion is likely to be initiated by wave action. The precise location of the initial stage of development of the Ness Pole must be linked to where tidal current velocities start to diminish. All later cyclic bank movements, especially between the Outer and Inner Poles, are driven by both tidal currents and waves, possibly with the latter acting in a subsidiary role.
Overall, Robinson (1975) is able to conclude that the sediment transport circulation pattern covering the entire outer Teign estuary is virtually self-contained and in a condition of dynamic equilibrium. He argued that inputs of sediment from further offshore would seem to be precluded by significant differences in grain size, as the inshore banks are made up of medium to coarse sand and fine gravel, whereas very fine sands and biogenic debris are predominant offshore. Other authors, however, have presented evidence of a significant eed from the erding cliffs to the south (Craig-Smith, 1970; Nunny, 1980)
Several further insights into sedimentary processes occurring in this complex near/offshore area have come from the various research teams contributing to the COAST 3D project (Whitehouse et al, 2001). These include the resolution of several tidal vortices created during both the flood and ebb stages of the tidal cycle; and the more general - if still provisional - conclusion that nearshore patterns of water movement are dominated by wave-generated currents, even when ebb flow is at its maximum (Sutherland, 2001). However, high resolution ripple profile data from nearshore showed rapid migration of these superficial bedforms with tidal current movement, and reversal of cross-section asymmetry with change in flow direction (Hoekstra et al., 2001). Bedload transport rates on Spratt Sand were calculated from bedform migration rate and sedimentology, and were as high as 3mhr-1 during the flood tide. These forms are confined to a shallow layer of very fine sand on top of the coarser sands and fine gravels of the bank, thus absence of sufficient supply of sediment prevents their growth. There was a positive correlation between mean bedform height and the rate of bedload transport (Hoekstra et al., 2001).
Using sediment sampling and digital side-scan sonar, Van Lancker, et al. (2001) determined a well-sorted pattern of seabed sediment, with coarse sands and fine gravel close to the estuary entrance and fine sand in the adjacent nearshore area. This was considered to be a product of the seawards reduction in the effective shearing stresses applied by tidal currents. An additional pattern of three distinct "lobes" of deposition was evident, each with a pattern of sorting that indicated (moving seawards) the deposition of bedload; fall-out of suspended sediment and the reworking of very fine sand. There was no apparent sorting in areas between these lobes. Residual transport vectors were determined primarily from the spatial pattern of indices of sorting and skewness of superficial sediments, confirmed in part by the profile asymmetry and orientation of megaripples. This work suggested that net movement was essentially on- to offshore, with wave-induced currents becoming progressively more important in maintaining the confined pattern of circulation. However, there was some indication that there could be an input of sand from the offshore zone beyond the area of the main banks. It was also noted that tube worms on the shallow near and offshore seabed have a high capacity to trap suspended sediment; in so doing, they create irregular microtopography and increase surface roughness. This could be a previously underestimated mechanism promoting net sedimentation.
Much of the research carried out for the COAST 3D investigation was concerned with fundamental, rather than site-specific hydrodynamic and sedimentological parameters. However, it has added some valuable insights into local process mechanisms and transport pathways. None of this new knowledge represents a challenge to the main deductions of Robinson (1975), but it would appear to give greater emphasis to the contribution of tidally induced transport. The concept that the sediment budget in the Teign estuary approaches is closed from external sources may now need some revision.
6. COASTAL DEFENCE AND HABITAT INTERFACE ISSUES - References Map
The main habitats present include vegetated sea cliffs, vegetated shingle, some small lagoons and intertidal mudflats and saltmarshes within the Teign Estuary.
The sea cliffs are varied in nature comprising of hard limestones and softer breccias and sandstones that offer a range of ecological niches and important exposures of Permian stratigraphy e.g. Oddicombe and Petit Tor Point (Photo 7). The majority of cliffs retreat relatively slowly and are undefended so there are few immediate issues.
Some 13 ha of saltmarsh and 220 ha of intertidal flats are recorded within the Teign estuary. Studies have not been undertaken to formally quantify the detailed distributions of estuarine habitats, review their "health," or determine whether they could be affected by maintenance of existing defences. In particular, the Exeter - Plymouth railway runs along the northern shore and its embankment could contribute to "squeezing" of intertidal habitats as sea level rises. The Teign Estuary Partnership has developed to promote the integrated management of the estuary. They have prepared an Estuary Management Plan (Teign Estuary Partnership, 2000) that presents a series of guiding principles and strategic objectives with supporting information covering a wide range of topics. They have also sponsored research into a variety of topics, including a study of the small lagoons found along the northern estuary margin. Further details are provided at the project website at: http://www.teignestuary.org
7. OPPORTUNITIES FOR CALCULATION AND TESTING OF LITTORAL DRIFT VOLUMES - References Map
The discontinuous nature of the shoreline of this unit with its numerous headlands, boulder aprons, pocket beaches, Teign tidal inlet and nearshore banks means that it is unsuited for definitive studies of drift. There are, however, opportunities to study drift occurring on the beach immediately south of Ness Head, between Sprey Point and Holcombe and along Denn Spit. An initial approach would be to model the littoral drift potential at these sites based on an analysis of a long-term hindcast wave climate. Uncertainties encountered in applying numerical model studies would include:
A potentially useful approach might be to undertake detailed sediment budget analysis of each of the three main beach sub-cells (i) Petit Tor Point to Ness Head (ii) teign inlet to Sprey Point and (iii) Sprey Point to Holcombe.
8. KNOWLEDGE LIMITATIONS AND MONITORING REQUIREMENTS - References MapThe SMP has summarised existing knowledge (Posford Duvivier, 1998) and the recently completed COAST 3D Project Teignmouth experimental programme has added various original insights into the complexities of wave and tidal current-driven sediment transport in the near- and offshore zones. Several of the observations and provisional conclusions deserve further investigation and there are several uncertainties, requiring focus concentrated on the following issues:
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MMIV © SCOPAC Sediment Transport Study - Hope's Nose, Torquay to Holcombe (inc. Exe Estuary")