- Email: [email protected]
Sediment transport pathways in a dredged ria system, southwest England
Estuarine, Coastal and Shelf Science 67 (2006) 491e502 www.elsevier.com/locate/ecss
Sediment transport pathways in a dredged ria system, southwest England P.L. Friend a,*, A.F. Velegrakis b, P.D. Weatherston a, M.B. Collins a a
School of Ocean and Earth Science, National Oceanography Centre Southampton, European Way, Southampton, Hants SO14 3ZH, UK b Department of Marine Science, University of the Aegean, Mytilene 81100, Greece Received 13 October 2005; accepted 2 December 2005 Available online 3 February 2006
Abstract The Fowey Ria system, southwest England, comprises the River Fowey catchment, the Fowey estuary, the cliffs and bays adjacent to the ria mouth, and part of the inner continental shelf of the English Channel. Previously, large quantities of sediment were introduced into the upper ria by ore mining activity. Today, in common with other rias, the Fowey receives a low riverine sediment input. Material from maintenance dredging in the lower ria is dumped in a spoil ground outside the ria mouth. The sediments of the system are investigated using an integrated approach to determine sediment distribution and sediment transport pathways. Surface sediments are analysed for grain size and mineralogy. Grain size trend analysis is used to examine sediment dispersal patterns away from the locus of deposition in the spoil ground. Archived data are used to investigate the seabed morphology and to determine long-term (100 year) bathymetric changes. Within the ria, mixing of sediment from several sources occurs. In the upper reaches, riverine and locally-eroded sediment is transported seawards towards the main area of commercial activity. Sand and finer-grained material moves into the ria from offshore. The bed of the inner continental shelf comprises interfluves covered by a thin veneer of sediment, with a natural composition of locally-derived lithic fragments and biogenic material. The area is mainly low/non-depositional in character, except within the partially-infilled palaeovalley and its tributaries. Sediments dumped in the spoil ground disperse in a complex pattern: coarse-grained material is moved by the action of waves and tidal currents towards the southwest and northeast; fine-grained material is transported either to the east or the west, depending upon the prevailing wave and tidal current regime. Because of its geomorphology, the lower ria acts as an efficient sediment trap, retaining (a) riverine material and sediment eroded from the upper reaches; and (b) sediment entering the ria from offshore. Despite being subjected to major anthropogenic disturbance from past mining and present-day dredging activities, the Fowey Ria conforms to the general sediment model for southwest England rias [Castaing, P., Guilcher, A., 1995. Geomorphology and sedimentology of rias. In: Perillo, G.M.E. (Ed.), Geomorphology and Sedimentology of Estuaries e Developments in Sedimentology, No. 53. Elsevier Science, Amsterdam, pp. 69e111]. A conceptual model of sediment transport pathways for the Fowey Ria system is presented as the basis for further investigations. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: ria; dredging; sediment transport; grain size trends; Fowey; southwest England
1. Introduction The term ‘ria’ is used to describe certain estuaries in Korea, parts of China and Argentina, as well as those of northwestern Spain, northwestern France, and southwestern England (Castaing and Guilcher, 1995). The original definition of a ria was a steep-sided, subaerially-eroded (non-glaciated), former
* Corresponding author. E-mail address: [email protected] (P.L. Friend). 0272-7714/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2005.12.005
river valley, drowned by Holocene sea-level rise (von Richthofen, 1886). More recently, rias have been distinguished from coastal plain estuaries by their development in areas with a high relief coast (Perillo, 1995). Elsewhere, Evans and Prego (2003) emphasise that the term ‘ria’ is useful, in that it describes all the features and deposits of an incised valley; within this, the estuarine zone can move according to climatic changes. These authors point out that, in many cases, only a small part of a ria is influenced by estuarine processes. The dominant sedimentary process in rias, in common with most modern estuaries, is infilling attributed to sea-level rise
492
P.L. Friend et al. / Estuarine, Coastal and Shelf Science 67 (2006) 491e502
during the present interglacial (Herranz and Acosta, 1984). Because the mean river discharge in rias is generally low, sedimentation is controlled mainly by the tidal regime (Berthois and Auffret, 1966). The rias of northwestern Spain (Arps and Kluyver, 1969; Vilas, 1983; Rey, 1993; Nombela et al., 1995; Garcia-Gil et al., 2000) and Brittany (Guilcher and Berthois, 1957; Guilcher et al., 1982) have been the subject of geomorphological and sedimentological investigations for a number of years; in contrast, those of southwestern England have received comparatively little attention. For the northwestern Iberian systems, recent research has focused upon: (1) sediment yield, erosion and sedimentation rates (Ria de Vigo) (Perez-Arlucea et al., 2005); (2) the distribution of sediment beyond the ria mouths (Dias et al., 2002; Jouanneau et al., 2002; Oliveira et al., 2002); and (3) heavy metal distribution within the ria sediments (Rubio et al., 2001; Evans et al., 2003; Prego and Cobelo-Garcia, 2003). Research into the rias of southwestern England (e.g. the Camel, Fal, Fowey) has focused mainly upon their geomorphology (Barton, 1964; Steers, 1964). Here, ria evolution has been compared with that of Brittany, i.e. deep incision occurring during Pleistocene low stands (Codrington, 1898), with concurrent deposition of periglacial ‘heads’ on the ria slopes. Evidence for drowning of the valleys by interglacial marine transgressions is provided by Pleistocene beaches on the ria sides, in the outer reaches. 1.1. Study area Fowey is well-known for its export of china clay, derived from the kaolinisation of local granite, whilst Fowey Harbour (Fig. 1) is an important deep-water anchorage, visited regularly by cruise ships. Dredging of the main channel in the Harbour commenced in 1904 and has been carried out continuously since then, except during World War II and during bad weather. Dredged material from the Fowey and nearby Par harbours is discharged at a spoil ground lying w2 km east of Fowey Harbour entrance, at the mouth of a small coastal embayment, Lantic Bay. At present, some 7e8 104 t y1 are removed from Fowey Harbour, whilst over a 10-year period (1985e1995), an average of 1.33 105 t y1 was discharged at the spoil ground from both of the harbours. The River Fowey catchment includes the Bodmin and St. Austell granitic intrusions, to the north and west, respectively. Tin and copper minerals, originating from inland faults, formed riverine placer deposits which were often mined (Barton, 1964). Mining and tin-streaming were responsible for severe siltation, and the eventual demise, in the Middle Ages, of the port of Lostwithiel (Fig. 1) (Gerrard, 1987, 2000; Burt, 1998). To the south and east of the granitic intrusions, Devonian sandstones, siltstones, slates, conglomerates and limestones occur, with occasional volcanic agglomerates and tuffs (Edmonds et al., 1975; Isaac et al., 1998). Head deposits and gravels overlie unconformably the Devonian sediments, in many of the catchment valleys. Within the ria itself, between Lostwithiel and part of Fowey Harbour, an eastewest trending anticlinorium exposes the Lower Devonian Dartmouth Beds, of
Fig. 1. Location map of the Fowey Ria study area, southwest England, UK (water depth contours in metres below chart datum).
red and green shales interbedded with grey sand- and siltstones (Stannier, 1990). The middle and outer Harbour reaches, including the adjacent cliffs, comprise thinly bedded slates, siltstones, sandstones, and limestones. The Bodmin granite is a partially kaolinised, coarse-grained (mean grain size 2e 3 mm) biotite granite. The St. Austell granite is predominantly a heavily kaolinised biotite granite, with lithium mica, tourmaline and topaz granites (Manning et al., 1996). The River Fowey is 43 km in length; its mean daily river discharge (1971e1991) was 4.7 m3 s1, with a maximum mean daily discharge of 97.5 m3 s1, and a Q95 of 0.75 m3 s1 (National Rivers Authority, 1994). The annual suspended sediment discharge has been estimated to be w3.3 103 t (Millward, G.E., University of Plymouth, UK e pers. comm.), using the catchment area relationship of Collins (1970). The Fowey Ria is mesotidal (mean tidal range: 3.6 m) (UKHO, 1996) with a total estuarine area of 305 ha, of which 146 ha are intertidal mud and sand deposits and 3 ha are saltmarsh. The lower reaches of the ria (Fowey Harbour, Fig. 1) and Lantic Bay are exposed to long-period Atlantic swell waves, from the southwest. Lantic Bay also receives shorter period waves from the south and southeast. Tidal currents on the inner continental shelf flow generally in an eastewest direction, parallel with the coast. A few kilometres seawards of the ria mouth, the maximum spring tide surface current velocity is w0.4 m s1 (Fennessy, 1990). A sandy bar, 150 m in length, with an elevation of about 1.5 m above the surrounding bed, crosses the Harbour entrance in the vicinity of underwater cables, where maintenance dredging is not possible. Here, fine sand occurs in the mid-channel
P.L. Friend et al. / Estuarine, Coastal and Shelf Science 67 (2006) 491e502
493
area, with gravelly sand at the channel sides. In the central channel, towards the widest part of the Harbour, sand ribbons are found on a gravelly substrate. Small sandy beaches exist on either side of the Harbour. Wave-cut platforms are found to the east of Gribben Head, and to the southwest of the Fowey Harbour entrance. Offshore, the expression of the Fowey palaeovalley is evident to a water depth of w35 m (Donovan and Stride, 1975). The offshore net sand transport direction is towards the east; however, the direction of inshore sand transport may differ, as a result of flow around headlands (Stride, 1982). It is believed that there is virtually no net sediment drift within the study area (Motyka and Brampton, 1993). Whilst cohesive sediment movement in the Tamar Ria has been the subject of detailed study (Uncles and Stephens, 1993; Uncles et al., 1994), and the geochemical signature of tin mining has been examined for intertidal sediments in the Fowey Ria (Pirrie et al., 2002), the recent sedimentology of southwest England rias has not been studied in any detail. In the absence of such data, Castaing and Guilcher (1995) suggest that the general model of ‘sandy sediments near the mouths, and mud flats/high marshes in the inner reaches’ is valid for southwest England rias. The present contribution examines the validity of the model, whilst investigating the sediments and their transport pathways in the Fowey Ria system, southwest England e a system subjected to low riverine sediment input, as well as past and present anthropogenic influences from ore mining and dredging, respectively. 2. Methods and materials 2.1. Sediment sampling Ninety offshore stations, and seven stations within the ria, were sampled by Van Veen grab (0.04 and 0.005 m3, respectively) in September 1996 (Fig. 2). A further 32 stations within the ria thalweg, and nine sites on Great Lantic beach were sampled in February 1998. All samples were subjected to grain size analysis; some samples were selected for mineralogical analysis. Grab samples were retained only if the grab jaws were fully closed. If the grab returned closed but empty, or very nearly empty on three successive occasions, the seabed was deemed to consist of bedrock. Offshore positions were fixed by DGPS (3 m) or by hand-held compass (4 m) using several sightings, where the DGPS signal was obscured by the local topography (e.g. within Lantic Bay). Within the ria, positions were fixed by navigational GPS and hand-held compass. During sampling in the upper reaches, several examples of recent channel bank collapse were observed. Intersample distance offshore was 500 m, except within the spoil ground and Lantic Bay, where it was 250 m. 2.2. Laboratory analyses Sediment samples were wet-sieved at 63 mm. The mud fraction (<63 mm) was analysed by CoulterÔcounter. The >63 mm fraction was oven-dried at 60 C, then separated into component sand (<2000 mm) and gravel fractions, by
Fig. 2. Sediment sampling sites within the Fowey Ria and on the inner continental shelf, including sampling locations on Great Lantic beach (inset).
dry sieving. Material larger than gravel (usually one or two pieces of bedrock) was removed and discounted from subsequent analyses. The grain size distribution of the sand fraction was determined by dry sieving at half-phi intervals. Insufficient gravel was available for dry-sieve analysis. Grain size trends (McLaren and Bowles, 1985; Gao and Collins, 1992) were calculated for samples collected within the spoil ground and Lantic Bay. Offshore sand samples were amalgamated to produce five sand classes: very fine (VFS); fine (FS); medium (MS); coarse (CS); and very coarse (VCS) (Wentworth, 1922). The mineralogy of at least 100 representative grains from each size class was identified using ordinary, plane and cross-polarised light. Samples from each size class were replicated, with the results averaged. The clay mineralogy of sediments from the thalweg of the ria was identified using the general X-ray diffraction (XRD) method described by Brown and Brindley (1980). Simple techniques of measurement were used to indicate the variability between samples, rather than to provide an accurate measure
494
P.L. Friend et al. / Estuarine, Coastal and Shelf Science 67 (2006) 491e502
of abundance: the relative proportions of clay minerals were estimated on the basis of the peak areas of their 001 basal reflections on the glycolated trace. The distribution trends of kaolinite plus chlorite (K þ C) to illite (I) were used as an indicator of mixing between two sources: (1) fluvial material with a relatively low (K þ C)/I ratio; and (2) marine material with a higher (K þ C)/I ratio (Algan et al., 1994). 2.3. Archived data British Admiralty fairsheets from 1857 (D3418), 1960 (K2864/1) and 1980 (K8581/2), were used to compare the natural bathymetry near Lantic Bay before its use as a spoil ground, with the modified bathymetry after spoil disposal had been continuing for between 56 (1904e1960) and 76 (1904e1980) years, respectively. Fairsheets were scaled using the known positions of triangulation points. Data from the 1960 and 1980 surveys were combined, to provide complete spatial coverage of the area of interest. Depths were contoured, then the volumetric difference between the 1857 and the 1960/ 1980 surveys was calculated using standard engineering software, taking into account an estimated sea-level rise of 18.5 cm (average 1.5 mm y1 e Proudman Oceanographic Laboratory, UK) between the two survey periods. The average sampling density in the 1857 survey was 80 soundings km2, whilst in the 1960 and 1980 surveys, average sampling density was 120 soundings km2. For inshore work during the 1960 and 1980 surveys, and for the 1857 survey, horizontal positions were fixed by sextant. The accuracy of position fixing by sextant in areas of clearly defined coastal features (e.g. bays with headlands) was considered to be ‘good’. Decca navigation (estimated accuracy 23 m) and Trisponder positioning (3 m) were used for the 1960 and 1980 surveys, respectively. For the 1857 survey, tidally-corrected depths were measured using a standard lead line; an echo-sounder was used for the 1960/1980 surveys. For navigational safety, soundings made by line during the 1857 survey were rounded down, to ensure that recorded depths were always shallower than actual depths. For the later surveys, soundings of 20.1 m (11 fathoms) were recorded in fathoms and feet, giving a vertical accuracy of 0.15 m. Soundings >20.1 m were recorded in fathoms, giving an accuracy of 1.0 m. As it was not possible to determine whether a recorded sounding was a true sounding, or one that had been rounded down, it was assumed that no recorded depths were true depths. Therefore, the mean of the minimum and the maximum possible depths at a particular sounding location was used for the volumetric analysis (see Section3.2). The vertical precision for the 1857 survey was estimated as 0.5 m. British Admiralty sonographs from a 1979 side-scan survey (H1961/79) were used to provide information on the surface sediments and bedforms. Note: sediment grain size in the bedform areas was verified during the present study during grab sampling (see Section 2.1). Part of the inshore area was not covered during the 1979 survey, and data quality (due to the prevailing weather conditions) was intermittent. Tidal stage, tidal current speeds and wind velocities at the time of the
side-scan sonar survey were obtained from the UK Hydrographic and Meteorological Office, and Fennessy (1990). For each of the four survey days (days 231e233, 245 of the year 1979), the maximum surface current speeds in the area ranged between 0.15 and 0.20 m s1. Wind velocities were: 2e3 m s1 from the northeast (day 231; 5e6 m s1 from the southwest (days 232, 233); and 1e4 m s1 from the southsouthwest (day 245). Surveying on each day was during the spring tide ebb phase. These conditions will be discussed below within the context of the orientation of the bedforms. 3. Results 3.1. Inner continental shelf sediments On the inner continental shelf, surface sediments comprised mainly sandy gravel, with occasional muddy patches; four non-depositional bedrock areas were identified (Fig. 3). Within Lantic Bay and the northern part of the main area of anthropogenic influence (spoil ground), slightly gravelly sand was dominant. Farther south, in the main area of anthropogenic influence, gravelly muddy sand occurred, with an area of sandy gravel (on bedrock) to the southeast. Gravelly sand was found between the bedrock areas to the southwest of the mouth. Typical bivalves found were Venus striatula, Lutraria lutraria and Chlamys sp. Two species of maerl (Phymatolithon calcareum and Lithothamnium coralloides) were common in samples collected a few kilometres to the south of the ria mouth. Samples of offshore bedrock were identified as slate, comprising the clay minerals chlorite and illite. The highest quartz content (>40%) was found in the MS fraction in the main area of anthropogenic influence (Fig. 4a). From here, quartz-enriched areas extended towards the southwest, as well as northeast towards Lantic Bay. The proportion of quartz grains decreased to w5% in the south and west of the study area. Sediments within Lantic Bay were quartz-enriched (quartz content 20e30%). On Great Lantic beach, bimodal and multimodal grain size distributions were typical, with a mean grain size for the sand fraction of 620 mm. Here, angular to well-rounded quartz comprised 33e69% of the sand, with little variation in percentages between the various size fractions. Maximum feldspar content (8%) was found in the VCS fraction, with a peak value in the spoil ground (Fig. 4b). The feldspar content decreased westwards and towards the north; it was absent to the west of the study area and off the ria mouth. Feldspar levels within Lantic Bay were slightly enriched (2%). Feldspar content on Great Lantic beach (for location, see Fig. 1) was generally low (1e2%), or absent. The highest mica content (16%) occurred in the VFS fraction in the main area of anthropogenic influence and to the south of the ria mouth (Fig. 4c). Mica abundance varied inversely with grain size fraction. Mica content on Great Lantic beach averaged 2e3%. The highest lithic fragment content (60e65% in the VCS fraction) tended to occur near the bedrock areas identified previously. The lowest content (5%) was found in the main
P.L. Friend et al. / Estuarine, Coastal and Shelf Science 67 (2006) 491e502
495
Fig. 3. Distribution of surficial sediment types on the inner continental shelf (classification: Folk, 1980). Bedrock areas are shaded. The location of the sand bar, in the vicinity of buried cables, is illustrated (R: bedrock; sG: sandy gravel; gmS: gravelly muddy sand; (g)S: slightly gravelly sand; gS: gravelly sand; mG: muddy gravel; msG: muddy sandy gravel).
palaeochannel south of the ria mouth (Fig. 4d). Within the spoil ground, lithic fragment content varied between 15 and 65%. Lithic fragments (grits, and purple, grey, and green slates) comprised 13e55% of the sand fraction on Great Lantic beach. Maximum biogenic content decreased landwards from 95% (VCS fraction) in the south of the study area, to w20% near the ria mouth (Fig. 4e). The biogenic content was lowest in the spoil ground (10%). The biogenic content on Great Lantic beach varied from 0 to 53%. 3.2. Ria sediments Within the ria, grain size distributions were variable, with bimodal distributions common near the tributary mouths. The mean grain size of the sand fraction decreased significantly (r2 ¼ 0.376, p ¼ 0.003) along the thalweg from coarse sand (570 mm) at the tidal limit (see Fig. 1) to medium sand (406 mm) near the Harbour mouth. In the same direction, sediment sorting improved and the samples became more negatively skewed. In the dredged area of the Harbour, the sand fraction comprised between 3 and 34% very angular to subrounded quartz grains, with feldspar and mica present only occasionally; the lithic fragment content ranged between 18 and 51%; biogenic fragments comprised 76% of particles in VCS size fraction, and 5% in the VFS size fraction. The relative proportions of illite, kaolinite and chlorite were variable (Fig. 5). Sediments collected near the china clay terminal had the highest kaolinite:chlorite ratio (1.94); farther downstream, within the Harbour, the ratio was 0.42. Sediment from above the effective tidal limit at Restormel had a kaolinite:chlorite ratio near to unity (1.12). Close to the source of the Fowey River, the relative proportions of clay minerals in stream bed sediments were: kaolinite 52%; illite 36%; chlorite 12%. The lowest illite:chlorite ratio (1.43) occurred in sediment from the main Harbour. The proportion of kaolinite
plus chlorite to illite decreased in a landwards direction, from outside the ria mouth to inside the main Harbour. 3.3. Seabed morphology Two main backscatter types were evident in the side-scan sonar records: strong and moderate returns, with the former interpreted as exposed bedrock and the latter as surficial sediment veneer (Fig. 6). Bedform types were sand megaripples and gravel waves, with wavelengths <8e10 m. In the southeast of the study area, near the spoil ground, the bedforms were symmetrical, with crests aligned normal to the shore. Along line 233, bedforms were found with crests aligned in a southwestenortheast direction. To the south of the ria mouth, bedforms were orientated in a northesouth direction, with sediment transport directions (to the north and to the south) indicated by bedform asymmetry (Terwindt and Brouwer, 1986). A number of areas of exposed bedrock were identified in the eastern part of the study area, along with several palaeochannel features to the west. Comparison of contour plots of the Lantic Bay area, for 1857 and 1960/1980, show overall accretion between the survey periods (Fig. 7). In 1857, the bathymetric relief was more pronounced than in 1960/1980: water depths >20 m extended over nearly all the southwest quadrant; this was not the case in the 1960/1980 survey. During the 1960/1980 surveys, the contours were aligned more shore-parallel than in 1857. The 6-m contour within the embayment had moved seawards by about 100 m, suggesting a general infilling of the bay. The appearance of a distinct topographic high, at some 3 m above the surrounding seabed (around 214100E 50300N), indicates the probable main locus of sediment deposition. The volumetric increase between 1857 and 1960/1980, using the minimum and maximum possible depths derived on the basis of the survey techniques, was 3.7 (0.7) 106 m3 (see Section 2.3). As
496
P.L. Friend et al. / Estuarine, Coastal and Shelf Science 67 (2006) 491e502
Fig. 4. (a) Percentage quartz (MS fraction); (b) percentage feldspar (VCS fraction); (c) percentage mica (VFS fraction); (d) percentage lithic fragments (VCS fraction); and (e) percentage biogenic fragments (VCS fraction) on the inner continental shelf. Note: only the fractions with the highest percentage of each mineral are illustrated.
such, the bathymetric changes in the Lantic Bay area may be assumed to be real. 3.4. Grain size distributions and trends Within the main area of anthropogenic influence, the sand was poorly sorted, strongly fine-skewed and very platykurtic (Folk, 1980): the mean grain size varied between 660 (coarse sand) and 230 mm (fine sand), in Lantic Bay. In the main palaeovalley and its tributaries, the mean grain size of the seabed sediments was generally between 750 and 930 mm. In the
remaining areas, the sand was well to moderately sorted: it was generally fine-skewed to strongly fine-skewed, and platykurtic to mesokurtic. In a south to north direction, from the southern part of the main palaeovalley and the southern part of the area of anthropogenic influence, the mean grain size of the sand decreased, the sorting improved and the skewness became more negative. The mean grain size of the mud fraction was <31 mm in the region of anthropogenic influence and south of the ria mouth. The mud was generally poorly sorted, strongly fine-skewed, and very platykurtic to platykurtic. To the south of the ria
P.L. Friend et al. / Estuarine, Coastal and Shelf Science 67 (2006) 491e502
497
mouth and in the area of anthropogenic influence, the grain size distribution of the mud was mesokurtic to leptokurtic. Grain size trend vectors (Gao and Collins, 1994), for Lantic Bay and the main area of anthropogenic influence, indicate an overall north to north-easterly sand transport direction (Fig. 8a). For the mud fraction, the vectors indicated transport, in all directions, away from the site of deposition, i.e. into and out of Lantic Bay, as well as towards the east and west (Fig. 8b). Simplified trends for each fraction are illustrated in Fig. 8c, d, respectively. 4. Discussion 4.1. Ria system sedimentation
Fig. 5. Relative percentages of illite (I), kaolinite (K), and chlorite (C) within the Fowey Ria system.
Rias have been classified into three categories from a sedimentological perspective: (1) marine, (2) estuarine, and (3) riverine (Castaing and Guilcher, 1995). Within the marine sub-environment, sand is transferred from the inner shelf to the ria, either by longshore currents induced by wave activity or by tidal currents, especially in the rias bordering the English Channel. In the estuarine domain, the marine signature diminishes, the biogenic content of the sediments declines rapidly, whilst the riverine influence increases. Within the riverine section, current velocities are controlled by river discharge; hence, riverine sedimentation dominates: a characteristic common to all such settings is substantial siltation, caused by trapped riverine deposits. In the case of the Fowey Ria, a decrease in the biogenic content in a landwards direction within the marine sub-environment, together with a fining of the sediment, improved sorting and more negative skewness, suggests landwards transport of sediment. Here, trends in the grain size parameters are similar to the Case 1 scenario described by McLaren and Bowles (1985). These findings are supported by the clay mineral analysis, in which the (K þ C)/I ratio decreases from offshore into Fowey Harbour; this suggests that fine
Fig. 6. Side-scan sonar interpretation of the seabed morphology and surficial sediments of the study area (adapted from Admiralty Chart, K8581/6).
498
P.L. Friend et al. / Estuarine, Coastal and Shelf Science 67 (2006) 491e502
Fig. 7. Bathymetry of the Lantic Bay area in (a) 1857, and (b) 1960/1980. Depths converted to metres below Chart Datum.
sediment moves in a northerly direction into the ria, from outside the mouth (Weaver, 1989; Algan et al., 1994). Evidence for the transport of sediment into the mouths of rias (e.g. the Goyen Ria, Brittany; Ria de Muros y Noya, Spain) is provided by asymmetrical sand waves with their lee side facing landwards, as well as decreasing biogenic sediment content and mean grain size in a landward direction (Guilcher et al., 1982; Junoy and Vieitez, 1989; Somoza and Rey, 1991; Rey, 1993). In terms of its size, morphology, and sediment characteristics, the Fowey Ria is remarkably similar to the Goyen Ria, Brittany: a typical sandy ria, some 6 km in length, in which the sediment distribution in the outer ria is influenced mainly by swell waves from the southwest (Guilcher et al., 1982). Sediments in Fowey Harbour have a mean grain size similar to the lower Goyen Ria sediments (range 200e400 mm), but
are less well-sorted; this indicates that wave action is less influential in the Fowey Ria. In both rias, the biogenic content decreases landwards, from w80% in the outer reaches. In the Goyen Ria, erosion of periglacial deposits on the channel sides supplies sediments to the inner ria. In comparison, the inner ria sediments in the Fowey are likely to derive from riverine input, as well as erosion of the mudflats, saltmarshes and channels in the upper reaches. Evidence from the geochemical analysis of sediments suggests that reworking of waste derived originally from past mining activities farther upstream, occurs in the northern section of the ria (Pirrie et al., 2002). The lower reaches of the northern Spanish coast rias are characterised by infilling with sand, the siliceous fraction of which is believed to derive from a terrigenous source, carried offshore during the Pleistocene marine regressions, then
Fig. 8. Lantic Bay and the main region of anthropogenic influence: residual grain size trends for (a) sand-size fraction, and (b) mud-size fraction. Simplified trends for each fraction are illustrated in (c) and (d), respectively.
P.L. Friend et al. / Estuarine, Coastal and Shelf Science 67 (2006) 491e502
remobilised during the Holocene transgression in response to strong wave action. The origin of the mud within the rias is also attributed to terrigenous sources, as it includes kaolinite from the alteration of inland rocks and clay minerals (Castaing and Guilcher, 1995). In the case of the Fowey, changes in grain size parameters within the riverine and upper estuarine section of the ria thalweg conform to the Case 1 scenario (see above), indicating a sediment transport direction from the tidal limit near Restormel (see Fig. 1) towards Fowey Harbour. 4.2. Sediment transport pathways The present study uses a multi-disciplinary approach for the identification of sediment transport pathways. Changes in the surface sediment clay mineral content, the mineral, biogenic and lithic fragment content, the seabed morphology and bathymetry, as well as the distribution of grain size parameters may all be used as indicators of sediment transport directions. In the case of the Fowey, sediments entering the lower reaches may be derived from (1) riverine input; (2) erosion of sediments in the upper reaches; (3) spillage of kaolin in the centre of commercial activity; and (4) offshore, via the ria mouth. On the inner continental shelf, dredged sediment is used as a tracer for sediment transport pathways in an area where the natural sedimentary regime is low/non-depositional. Here, the distribution of bedrock areas to the southwest of the Fowey Ria mouth (Fig. 3) correlates well with the positions of known shore platforms (Everard et al., 1964); it delineates interfluves of the incised river valley and a what appears to be a palaeo-tributary channel entering the main palaeochannel from the west. The sandy gravel belt, aligned and extending in a northesouth direction between the interfluves south of the ria mouth, correlates with the bathymetric contours (Fig. 1), indicating the position of the main palaeochannel. A second palaeo-tributary channel appears to extend eastwards towards the Lantic Bay region. The spoil ground sediments have mineral compositions that are similar to the sediments found within the dredged area of Fowey Harbour (see Section 3.2). They differ considerably from the natural sediments, which comprise mainly lithic and biogenic fragments. Evidence for the transport of sand away from the area of anthropogenic deposition, into Lantic Bay, is provided by (1) the enrichment of quartz, feldspar and mica in sediments to the northeast; (2) the residual grain size trend vectors; (3) purple and green lithic fragments from the Dartmouth Beds (which outcrop from the mid-Harbour to the upper estuary), found in the beach sands; (4) bathymetric changes; and (5) the predominant wave direction. Residual trend vectors, together with the small percentage of mud found in the Lantic Bay samples (<1.5%), indicate that fine sediments move away from the locus of deposition, being transported ultimately to the east or west depending, presumably, upon wave intensity and the direction of the prevailing tidal flow. The general bedform orientation in the south-eastern quadrant of the study area correlates with the eastewest (floode ebb) direction of the surface tidal currents (see Section 1.1 and Fig. 6). From the bedform asymmetry to the south and southwest of the mouth, there is some evidence for the
499
existence of preferred flood and ebb flow directions, with the ebb jet aligned along the main axis of the palaeovalley (see Fig. 6). This pattern supports the existence of a clockwise rotating gyre to the east of Gribben Head, as reported by local seafarers (M. Sutherland, Fowey Harbour Commissioners e pers. comm.); this has been suggested by Stride (1982), as a mechanism near headlands for causing sediment transport in the opposite direction to the general (residual) trend. Bedrock areas identified to the southwest of the Harbour entrance are seaward extensions of the shore platforms described by Everard et al. (1964). In the present study, the existence of a shore (wave-cut) platform to the east of the Harbour entrance is demonstrated, together with a large bedrock interfluve farther to the south (see Fig. 3). Bathymetric changes that have occurred in the area of anthropogenic deposition, over a period of 120 years (see Fig. 7) indicate an increase in the volume of the sedimentary cover, of w3.7 106 m3. Since 1904, when dredging commenced, it is interesting to note that some 6 106 t of material has been removed from the Fowey Ria to the spoil ground since dredging commenced. If all the material had remained in place on the seabed, it would have a volume of 3.3e3.8 106 m3, assuming a packing density of 1800e 1600 kg m3 (Rieke and Chilingarian, 1974). Although the exact amount of material removed from Par Harbour is not known, it is believed to be similar in volume to that removed from the Fowey Ria. This suggests that some 3e4 106 m3 of sediment has been transported away from the spoil ground, since dredging commenced. Combining the evidence provided by the various different research approaches, a conceptual model of suggested sediment transport pathways for the study area has been developed (Fig. 9). 5. Concluding remarks Although subjected to severe anthropogenic disturbance from mining and dredging activities, the Fowey Ria conforms to the general ria sediment model of Castaing and Guilcher (1995). Large offshore areas of exposed bedrock, to the southwest and southeast of the study area, show that the inner continental shelf area is a low/non-depositional environment. Sediments originating from offshore, and most probably the spoil ground, enter the mouth of the ria under infilling conditions experienced by modern ria systems. An undredged area across the ria mouth forms a sill which, together with the ‘U’ shaped profile caused by dredging in the main Harbour, acts as an effective trap for sediments entering the lower ria, either from offshore or from the riverine section. Several preferred sediment transport directions in the area of anthropogenic influence are linked to the prevailing hydrodynamic conditions. Long-period swell waves, together with waves produced under local storm conditions, resuspend fine-grained material; this makes it available for transport by tidal currents.
500
P.L. Friend et al. / Estuarine, Coastal and Shelf Science 67 (2006) 491e502
Fig. 9. General model of suggested sediment transport pathways for the lower Fowey Ria and inner continental shelf area.
The dredged sediment acts as a useful tool to trace sediment transport pathways, in environments which are naturally low/non-depositional. Future studies are recommended, as outlined below: (1) The use of numerical wave and surface current modelling for the inner continental shelf area, to provide estimates of critical shear stress at the seabed, for the assessment of erosion, transport and deposition; (2) Long-term monitoring of the intertidal flats, marsh areas and channel migration patterns, to understand the rates of erosion and accretion within the ria; (3) Geochemical analysis of sediments for Sn and Cu signatures in the southern part of the ria, and in the paleovalley (and its tributaries) on the inner continental shelf, for use as a tracer of sediment movement in these areas; (4) The effect of dredging, on changes in the estuarine geometry and tidal characteristics within the ria (e.g.
Speer and Aubrey, 1985; Friedrichs and Aubrey, 1988), should be studied; (5) A sediment budget should be established for the ria system, by quantifying the input from rivers/tributaries, field runoff and offshore sources.
Acknowledgements Funding for this study was provided by the Maritime Division of English Nature and the EU project: EUROSTRATAFORM (EVK3-CT-2002-00079). The following are thanked for logistical support: Capt. M. Sutherland (Fowey Harbour Commissioners); the captain and crew of the dredger, Lantic Bay; J. Davis; E. Burfoot; J. Oake. Dr. W. Farnham (University of Portsmouth) is thanked for identification of the maerl. The Hydrographic Office, Taunton kindly provided the Admiralty
P.L. Friend et al. / Estuarine, Coastal and Shelf Science 67 (2006) 491e502
survey data. A. Bale (Plymouth Marine Laboratory) is thanked for providing the sea-level rise data. K. Davis (National Oceanography Centre Southampton) is thanked for producing the graphics. Two anonymous reviewers are thanked for their useful comments and suggestions. References Algan, O., Clayton, T., Tranter, M., Collins, M.B., 1994. Estuarine mixing of clay minerals in the Solent region, southern England. Sedimentary Geology 92, 241e255. Arps, C.E.S., Kluyver, H.M., 1969. Sedimentology of the Northwestern shores of the Ria de Arosa, NW Spain. Leidsche Geologische Mededelingen 37, 135e145. Barton, R.M., 1964. An Introduction to the Geology of Cornwall. Pitman Press, Bath, 168 pp. Berthois, L., Auffret, G., 1966. Dynamique de la se´dimentation dans les rias et les estuaires des petits cours d’eau tributaires de la Manche. Cahiers Oce´anographiques 18, 761e764. Brown, G., Brindley, G.W., 1980. X-ray diffraction procedures for clay mineral identification. In: Brindley, G.W., Brown, G. (Eds.), Crystal Structures of Clay Minerals and Their X-ray Identification. Mineralogical Society, London, pp. 305e361 (Monograph 5). Burt, R., 1998. History of metalliferous mining. In: Selwood, E.B., Durrance, E.M., Bristow, C.M. (Eds.), The Geology of Cornwall. University of Exeter Press, Exeter, pp. 211e225. Castaing, P., Guilcher, A., 1995. Geomorphology and sedimentology of Rias. In: Perillo, G.M.E. (Ed.), Geomorphology and Sedimentology of Estuaries e Developments in Sedimentology, No. 53. Elsevier Science, Amsterdam, pp. 69e111. Codrington, T., 1898. On some submerged valleys in South Wales, Devon and Cornwall. Quarterly Journal of the Geological Society, London 54, 251e278. Collins, M.B., 1970. Discussion of design curves for suspended sediment load estimation. Proceedings of the Institute of Civil Engineers 46, 85e88. Dias, J.M.A., Jouanneau, J.M., Gonzalez, R., Araujo, M.F., Drago, T., Garcia, C., Oliveira, A., Rodrigues, A., Vitorino, J., Weber, O., 2002. Present day sedimentary processes on the northern Iberian shelf. Progress in Oceanography 52, 249e259. Donovan, D.T., Stride, A.H., 1975. Three drowned coastlines of probable Late Tertiary Age around Devon and Cornwall. Marine Geology 19, M35eM40. Edmonds, E.A., McKeoun, M.C., Williams, M., 1975. British Regional Geology: SW England. HMSO, London, 136 pp. Evans, G., Howarth, R.J., Nombela, M.A., 2003. Metals in the sediments of Ensenada de San Simon (inner Ria de Vigo), Galicia, NW Spain. Applied Geochemistry 18, 973e996. Evans, G., Prego, R., 2003. Rias, estuaries and incised valleys: is a ria an estuary? Marine Geology 196, 171e175. Everard, C.E., Lawrence, R.H., Witherick, M.E., Wright, L.W., 1964. Raised beaches and marine geomorphology. In: Hosking, K.F.G., Shrimpton, G.J. (Eds.), Present Views of Some Aspects of the Geology of Devon and Cornwall. Royal Geological Society of Cornwall, Truro, pp. 283e310. Fennessy, M.J., 1990. Tidal Stream Atlas of the South Cornwall Coast. The author (publisher), Plymouth, Devon, 15 pp. Folk, R.L., 1980. Petrology of Sedimentary Rocks. Hemphill, Texas, 185 pp. Friedrichs, C.T., Aubrey, D.G., 1988. Non-linear tidal distortion in shallow well-mixed estuaries: a synthesis. Estuarine, Coastal and Shelf Science 27, 521e545. Gao, S., Collins, M.B., 1992. Net sediment transport patterns inferred from grain size trends, based upon definition of ‘‘transport vectors’’. Sedimentary Geology 81, 47e60. Gao, S., Collins, M.B., 1994. Analysis of grain size trends, for defining sediment transport pathways in marine environments. Journal of Coastal Research 10, 70e78. Garcia-Gil, S., Duran, R., Vilas, F., 2000. Side scan sonar image and geologic interpretation of the Ria de Pontevedra seafloor (Galicia, NW Spain). Sciencia Marina 64, 393e402.
501
Gerrard, S., 1987. Streamworking in medieval Cornwall. Journal of the Trevithick Society 14, 7e31. Gerrard, S., 2000. The Early British Tin Industry. Tempus Publishing, Stroud, 176 pp. Guilcher, A., Berthois, L., 1957. Cinq anne´es d’observations se´dimentologiques dans quatre estuaires te´moins de l’Ouest de la Bretagne. Revue de Ge´omorphologie Dynamique 8, 66e86. Guilcher, A., Andrade, B., Dantec, M.H., 1982. Diversite´ morpho-se´dimentologique des estuaires du Finiste`re. Norois 29, 205e228. Herranz, P., Acosta, J., 1984. Estudio geofisico de la Ria de Muros y Noya. Boletin del Instituto Espagnol Oceanografia 1, 48e78. Isaac, K.P., Selwood, E.B., Shail, R.K., 1998. Devonian. In: Selwood, E.B., Durrance, E.M., Bristow, C.M. (Eds.), The Geology of Cornwall. University of Exeter Press, Exeter, pp. 31e64. Jouanneau, J.M., Weber, O., Drago, T., Rodrigues, A., Oliveira, A., Dias, J.M.A., Garcia, C., Schmidt, S., Reyss, J.L., 2002. Recent sedimentation and sedimentary budgets on the western Iberian shelf. Progress in Oceanography 52, 261e275. Junoy, J., Vieitez, J.M., 1989. Cartografia de los sedimentos superficiales de la Ria de Foz, Lugo. Thalassas 7, 9e19. Manning, D.A.C., Hill, P.I., Howe, J.H., 1996. Primary lithological variation in the kaolinised St. Austell granite, Cornwall, England. Journal of the Geological Society, London 153, 827e838. McLaren, P., Bowles, D., 1985. The effects of sediment transport on grain-size distributions. Journal of Sedimentary Petrology 55, 457e470. Motyka, J.M., Brampton, A.H., 1993. Coastal Management Mapping of Littoral Cells. HR Wallingford, Report SR328, 102 pp. National Rivers Authority, South-West Region, 1994. Seaton, Looe and Fowey Catchment Management Plan Consultation Report. National Rivers Authority, Bodmin, Cornwall, 117 pp. Nombela, M.A., Vilas, F.V., Evans, G., 1995. Sedimentation in the mesotidal Rias Bajas of Galicia (north-western Spain): Ensenada de San Simon, Inner Ria de Vigo. In: Flemming, B.W., Bartholoma, A. (Eds.), Tidal Signatures in Modern and Ancient Sediments. International Association of Sedimentologists, pp. 133e149 (Special Publication 24). Oliveira, A., Rocha, F., Rodrigues, A., Jouanneau, J., Dias, A., Weber, O., Gomes, C., 2002. Clay minerals from the sedimentary cover from the Northwest Iberian shelf. Progress in Oceanography 52, 233e247. Perez-Arlucea, M., Mendez, G., Clemente, F., Nombela, M., Rubio, B., Filgueira, M., 2005. Hydrology, sediment yield, erosion and sedimentation rates in the estuarine environment of the Ria de Vigo, Galicia, Spain. Journal of Marine Systems 54, 209e226. Perillo, G.M.E., 1995. Definitions and geomorphic classifications of estuaries. In: Perillo, G.M.E. (Ed.), Geomorphology and Sedimentology of Estuaries, Developments in Sedimentology, No. 53. Elsevier, Amsterdam, pp. 17e47. Pirrie, D., Power, M.R., Wheeler, P.D., Cundy, A., Bridges, C., Davey, G., 2002. Geochemical signature of historical mining: Fowey Estuary, Cornwall, UK. Journal of Geochemical Exploration 76, 31e43. Prego, R., Cobelo-Garcia, A., 2003. Twentieth century overview of heavy metals in the Galician Rias (NW Iberian Peninsula). Environmental Pollution 121, 425e452. von Richthofen, F., 1886. Fu¨hrer fu¨r Forschungsreisende. Ja¨necke, Hannover, 734 pp. Rey, J., 1993. Relacio´n morpho-sedimentaria entre la plataforma continental de Galicia y las rias bajas y su evolucio´n durante el Cuaterario. Instituto Espanol de Oceanografia, Madrid, Special Publication 17, 233 pp. Rieke, H.H., Chilingarian, G.V., 1974. Compaction of Argillaceous Sediments e Developments in Sedimentology, No. 16. Elsevier, Amsterdam, 424 pp. Rubio, B., Pye, K., Rae, J.E., Rey, D., 2001. Sedimentological characteristics, heavy metal distribution and magnetic properties in subtidal sediments, Ria de Pontevedra, NW Spain. Sedimentology 48, 1277e1296. Somoza, L., Rey, J., 1991. Holocene fan deltas in a ‘‘ria’’ morphology. Prograding clinoform types and sea-level control. Cuadernos de Geologia Ibe´rica Madrid 15, 37e48. Speer, P.E., Aubrey, D.G., 1985. A study of non-linear propagation in shallow inlet/estuarine systems. Part II: theory. Estuarine, Coastal and Shelf Science 21, 207e224.
502
P.L. Friend et al. / Estuarine, Coastal and Shelf Science 67 (2006) 491e502
Stannier, P., 1990. Cornwall’s Geological Heritage. Twelveheads Press, Truro, 47 pp. Steers, J.A., 1964. The Coastline of England and Wales. Cambridge University Press, Cambridge, 750 pp. Stride, A.H., 1982. Offshore Tidal Sands e Processes and Deposits. Chapman and Hall, London, 222 pp. Terwindt, J.H.J., Brouwer, M.J.N., 1986. The behaviour of intertidal sand waves during neapespring tidal cycles and the relevance of palaeoflow reconstruction. Sedimentology 33, 1e31. UKHO, 1996. Admiralty Tide Tables. The United Kingdom Hydrographic Office, Taunton, 442 pp.
Uncles, R.J., Barton, M.L., Stephens, J.A., 1994. Seasonal variability of finesediment concentrations in the turbidity maximum region of the Tamar Estuary. Estuarine, Coastal and Shelf Science 38, 19e39. Uncles, R.J., Stephens, J.A., 1993. Nature of the turbidity maximum in the Tamar Estuary, UK. Estuarine, Coastal and Shelf Science 36, 413e431. Vilas, F.V., 1983. Medios sedimentarios de transicion en la Ria de Vigo: secuencias progradantes. Thalassas 1, 49e55. Weaver, C.E., 1989. Clays, muds, and shales. In: Developments in Sedimentology, vol. 44. Elsevier, Amsterdam, 819 pp. Wentworth, C.K., 1922. A scale of grade and class terms for clastic sediments. Journal of Geology 30, 377e392.
Sediment transport pathways in a dredged ria system, southwest England P.L. Friend a,*, A.F. Velegrakis b, P.D. Weatherston a, M.B. Collins a a
School of Ocean and Earth Science, National Oceanography Centre Southampton, European Way, Southampton, Hants SO14 3ZH, UK b Department of Marine Science, University of the Aegean, Mytilene 81100, Greece Received 13 October 2005; accepted 2 December 2005 Available online 3 February 2006
Abstract The Fowey Ria system, southwest England, comprises the River Fowey catchment, the Fowey estuary, the cliffs and bays adjacent to the ria mouth, and part of the inner continental shelf of the English Channel. Previously, large quantities of sediment were introduced into the upper ria by ore mining activity. Today, in common with other rias, the Fowey receives a low riverine sediment input. Material from maintenance dredging in the lower ria is dumped in a spoil ground outside the ria mouth. The sediments of the system are investigated using an integrated approach to determine sediment distribution and sediment transport pathways. Surface sediments are analysed for grain size and mineralogy. Grain size trend analysis is used to examine sediment dispersal patterns away from the locus of deposition in the spoil ground. Archived data are used to investigate the seabed morphology and to determine long-term (100 year) bathymetric changes. Within the ria, mixing of sediment from several sources occurs. In the upper reaches, riverine and locally-eroded sediment is transported seawards towards the main area of commercial activity. Sand and finer-grained material moves into the ria from offshore. The bed of the inner continental shelf comprises interfluves covered by a thin veneer of sediment, with a natural composition of locally-derived lithic fragments and biogenic material. The area is mainly low/non-depositional in character, except within the partially-infilled palaeovalley and its tributaries. Sediments dumped in the spoil ground disperse in a complex pattern: coarse-grained material is moved by the action of waves and tidal currents towards the southwest and northeast; fine-grained material is transported either to the east or the west, depending upon the prevailing wave and tidal current regime. Because of its geomorphology, the lower ria acts as an efficient sediment trap, retaining (a) riverine material and sediment eroded from the upper reaches; and (b) sediment entering the ria from offshore. Despite being subjected to major anthropogenic disturbance from past mining and present-day dredging activities, the Fowey Ria conforms to the general sediment model for southwest England rias [Castaing, P., Guilcher, A., 1995. Geomorphology and sedimentology of rias. In: Perillo, G.M.E. (Ed.), Geomorphology and Sedimentology of Estuaries e Developments in Sedimentology, No. 53. Elsevier Science, Amsterdam, pp. 69e111]. A conceptual model of sediment transport pathways for the Fowey Ria system is presented as the basis for further investigations. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: ria; dredging; sediment transport; grain size trends; Fowey; southwest England
1. Introduction The term ‘ria’ is used to describe certain estuaries in Korea, parts of China and Argentina, as well as those of northwestern Spain, northwestern France, and southwestern England (Castaing and Guilcher, 1995). The original definition of a ria was a steep-sided, subaerially-eroded (non-glaciated), former
* Corresponding author. E-mail address: [email protected] (P.L. Friend). 0272-7714/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2005.12.005
river valley, drowned by Holocene sea-level rise (von Richthofen, 1886). More recently, rias have been distinguished from coastal plain estuaries by their development in areas with a high relief coast (Perillo, 1995). Elsewhere, Evans and Prego (2003) emphasise that the term ‘ria’ is useful, in that it describes all the features and deposits of an incised valley; within this, the estuarine zone can move according to climatic changes. These authors point out that, in many cases, only a small part of a ria is influenced by estuarine processes. The dominant sedimentary process in rias, in common with most modern estuaries, is infilling attributed to sea-level rise
492
P.L. Friend et al. / Estuarine, Coastal and Shelf Science 67 (2006) 491e502
during the present interglacial (Herranz and Acosta, 1984). Because the mean river discharge in rias is generally low, sedimentation is controlled mainly by the tidal regime (Berthois and Auffret, 1966). The rias of northwestern Spain (Arps and Kluyver, 1969; Vilas, 1983; Rey, 1993; Nombela et al., 1995; Garcia-Gil et al., 2000) and Brittany (Guilcher and Berthois, 1957; Guilcher et al., 1982) have been the subject of geomorphological and sedimentological investigations for a number of years; in contrast, those of southwestern England have received comparatively little attention. For the northwestern Iberian systems, recent research has focused upon: (1) sediment yield, erosion and sedimentation rates (Ria de Vigo) (Perez-Arlucea et al., 2005); (2) the distribution of sediment beyond the ria mouths (Dias et al., 2002; Jouanneau et al., 2002; Oliveira et al., 2002); and (3) heavy metal distribution within the ria sediments (Rubio et al., 2001; Evans et al., 2003; Prego and Cobelo-Garcia, 2003). Research into the rias of southwestern England (e.g. the Camel, Fal, Fowey) has focused mainly upon their geomorphology (Barton, 1964; Steers, 1964). Here, ria evolution has been compared with that of Brittany, i.e. deep incision occurring during Pleistocene low stands (Codrington, 1898), with concurrent deposition of periglacial ‘heads’ on the ria slopes. Evidence for drowning of the valleys by interglacial marine transgressions is provided by Pleistocene beaches on the ria sides, in the outer reaches. 1.1. Study area Fowey is well-known for its export of china clay, derived from the kaolinisation of local granite, whilst Fowey Harbour (Fig. 1) is an important deep-water anchorage, visited regularly by cruise ships. Dredging of the main channel in the Harbour commenced in 1904 and has been carried out continuously since then, except during World War II and during bad weather. Dredged material from the Fowey and nearby Par harbours is discharged at a spoil ground lying w2 km east of Fowey Harbour entrance, at the mouth of a small coastal embayment, Lantic Bay. At present, some 7e8 104 t y1 are removed from Fowey Harbour, whilst over a 10-year period (1985e1995), an average of 1.33 105 t y1 was discharged at the spoil ground from both of the harbours. The River Fowey catchment includes the Bodmin and St. Austell granitic intrusions, to the north and west, respectively. Tin and copper minerals, originating from inland faults, formed riverine placer deposits which were often mined (Barton, 1964). Mining and tin-streaming were responsible for severe siltation, and the eventual demise, in the Middle Ages, of the port of Lostwithiel (Fig. 1) (Gerrard, 1987, 2000; Burt, 1998). To the south and east of the granitic intrusions, Devonian sandstones, siltstones, slates, conglomerates and limestones occur, with occasional volcanic agglomerates and tuffs (Edmonds et al., 1975; Isaac et al., 1998). Head deposits and gravels overlie unconformably the Devonian sediments, in many of the catchment valleys. Within the ria itself, between Lostwithiel and part of Fowey Harbour, an eastewest trending anticlinorium exposes the Lower Devonian Dartmouth Beds, of
Fig. 1. Location map of the Fowey Ria study area, southwest England, UK (water depth contours in metres below chart datum).
red and green shales interbedded with grey sand- and siltstones (Stannier, 1990). The middle and outer Harbour reaches, including the adjacent cliffs, comprise thinly bedded slates, siltstones, sandstones, and limestones. The Bodmin granite is a partially kaolinised, coarse-grained (mean grain size 2e 3 mm) biotite granite. The St. Austell granite is predominantly a heavily kaolinised biotite granite, with lithium mica, tourmaline and topaz granites (Manning et al., 1996). The River Fowey is 43 km in length; its mean daily river discharge (1971e1991) was 4.7 m3 s1, with a maximum mean daily discharge of 97.5 m3 s1, and a Q95 of 0.75 m3 s1 (National Rivers Authority, 1994). The annual suspended sediment discharge has been estimated to be w3.3 103 t (Millward, G.E., University of Plymouth, UK e pers. comm.), using the catchment area relationship of Collins (1970). The Fowey Ria is mesotidal (mean tidal range: 3.6 m) (UKHO, 1996) with a total estuarine area of 305 ha, of which 146 ha are intertidal mud and sand deposits and 3 ha are saltmarsh. The lower reaches of the ria (Fowey Harbour, Fig. 1) and Lantic Bay are exposed to long-period Atlantic swell waves, from the southwest. Lantic Bay also receives shorter period waves from the south and southeast. Tidal currents on the inner continental shelf flow generally in an eastewest direction, parallel with the coast. A few kilometres seawards of the ria mouth, the maximum spring tide surface current velocity is w0.4 m s1 (Fennessy, 1990). A sandy bar, 150 m in length, with an elevation of about 1.5 m above the surrounding bed, crosses the Harbour entrance in the vicinity of underwater cables, where maintenance dredging is not possible. Here, fine sand occurs in the mid-channel
P.L. Friend et al. / Estuarine, Coastal and Shelf Science 67 (2006) 491e502
493
area, with gravelly sand at the channel sides. In the central channel, towards the widest part of the Harbour, sand ribbons are found on a gravelly substrate. Small sandy beaches exist on either side of the Harbour. Wave-cut platforms are found to the east of Gribben Head, and to the southwest of the Fowey Harbour entrance. Offshore, the expression of the Fowey palaeovalley is evident to a water depth of w35 m (Donovan and Stride, 1975). The offshore net sand transport direction is towards the east; however, the direction of inshore sand transport may differ, as a result of flow around headlands (Stride, 1982). It is believed that there is virtually no net sediment drift within the study area (Motyka and Brampton, 1993). Whilst cohesive sediment movement in the Tamar Ria has been the subject of detailed study (Uncles and Stephens, 1993; Uncles et al., 1994), and the geochemical signature of tin mining has been examined for intertidal sediments in the Fowey Ria (Pirrie et al., 2002), the recent sedimentology of southwest England rias has not been studied in any detail. In the absence of such data, Castaing and Guilcher (1995) suggest that the general model of ‘sandy sediments near the mouths, and mud flats/high marshes in the inner reaches’ is valid for southwest England rias. The present contribution examines the validity of the model, whilst investigating the sediments and their transport pathways in the Fowey Ria system, southwest England e a system subjected to low riverine sediment input, as well as past and present anthropogenic influences from ore mining and dredging, respectively. 2. Methods and materials 2.1. Sediment sampling Ninety offshore stations, and seven stations within the ria, were sampled by Van Veen grab (0.04 and 0.005 m3, respectively) in September 1996 (Fig. 2). A further 32 stations within the ria thalweg, and nine sites on Great Lantic beach were sampled in February 1998. All samples were subjected to grain size analysis; some samples were selected for mineralogical analysis. Grab samples were retained only if the grab jaws were fully closed. If the grab returned closed but empty, or very nearly empty on three successive occasions, the seabed was deemed to consist of bedrock. Offshore positions were fixed by DGPS (3 m) or by hand-held compass (4 m) using several sightings, where the DGPS signal was obscured by the local topography (e.g. within Lantic Bay). Within the ria, positions were fixed by navigational GPS and hand-held compass. During sampling in the upper reaches, several examples of recent channel bank collapse were observed. Intersample distance offshore was 500 m, except within the spoil ground and Lantic Bay, where it was 250 m. 2.2. Laboratory analyses Sediment samples were wet-sieved at 63 mm. The mud fraction (<63 mm) was analysed by CoulterÔcounter. The >63 mm fraction was oven-dried at 60 C, then separated into component sand (<2000 mm) and gravel fractions, by
Fig. 2. Sediment sampling sites within the Fowey Ria and on the inner continental shelf, including sampling locations on Great Lantic beach (inset).
dry sieving. Material larger than gravel (usually one or two pieces of bedrock) was removed and discounted from subsequent analyses. The grain size distribution of the sand fraction was determined by dry sieving at half-phi intervals. Insufficient gravel was available for dry-sieve analysis. Grain size trends (McLaren and Bowles, 1985; Gao and Collins, 1992) were calculated for samples collected within the spoil ground and Lantic Bay. Offshore sand samples were amalgamated to produce five sand classes: very fine (VFS); fine (FS); medium (MS); coarse (CS); and very coarse (VCS) (Wentworth, 1922). The mineralogy of at least 100 representative grains from each size class was identified using ordinary, plane and cross-polarised light. Samples from each size class were replicated, with the results averaged. The clay mineralogy of sediments from the thalweg of the ria was identified using the general X-ray diffraction (XRD) method described by Brown and Brindley (1980). Simple techniques of measurement were used to indicate the variability between samples, rather than to provide an accurate measure
494
P.L. Friend et al. / Estuarine, Coastal and Shelf Science 67 (2006) 491e502
of abundance: the relative proportions of clay minerals were estimated on the basis of the peak areas of their 001 basal reflections on the glycolated trace. The distribution trends of kaolinite plus chlorite (K þ C) to illite (I) were used as an indicator of mixing between two sources: (1) fluvial material with a relatively low (K þ C)/I ratio; and (2) marine material with a higher (K þ C)/I ratio (Algan et al., 1994). 2.3. Archived data British Admiralty fairsheets from 1857 (D3418), 1960 (K2864/1) and 1980 (K8581/2), were used to compare the natural bathymetry near Lantic Bay before its use as a spoil ground, with the modified bathymetry after spoil disposal had been continuing for between 56 (1904e1960) and 76 (1904e1980) years, respectively. Fairsheets were scaled using the known positions of triangulation points. Data from the 1960 and 1980 surveys were combined, to provide complete spatial coverage of the area of interest. Depths were contoured, then the volumetric difference between the 1857 and the 1960/ 1980 surveys was calculated using standard engineering software, taking into account an estimated sea-level rise of 18.5 cm (average 1.5 mm y1 e Proudman Oceanographic Laboratory, UK) between the two survey periods. The average sampling density in the 1857 survey was 80 soundings km2, whilst in the 1960 and 1980 surveys, average sampling density was 120 soundings km2. For inshore work during the 1960 and 1980 surveys, and for the 1857 survey, horizontal positions were fixed by sextant. The accuracy of position fixing by sextant in areas of clearly defined coastal features (e.g. bays with headlands) was considered to be ‘good’. Decca navigation (estimated accuracy 23 m) and Trisponder positioning (3 m) were used for the 1960 and 1980 surveys, respectively. For the 1857 survey, tidally-corrected depths were measured using a standard lead line; an echo-sounder was used for the 1960/1980 surveys. For navigational safety, soundings made by line during the 1857 survey were rounded down, to ensure that recorded depths were always shallower than actual depths. For the later surveys, soundings of 20.1 m (11 fathoms) were recorded in fathoms and feet, giving a vertical accuracy of 0.15 m. Soundings >20.1 m were recorded in fathoms, giving an accuracy of 1.0 m. As it was not possible to determine whether a recorded sounding was a true sounding, or one that had been rounded down, it was assumed that no recorded depths were true depths. Therefore, the mean of the minimum and the maximum possible depths at a particular sounding location was used for the volumetric analysis (see Section3.2). The vertical precision for the 1857 survey was estimated as 0.5 m. British Admiralty sonographs from a 1979 side-scan survey (H1961/79) were used to provide information on the surface sediments and bedforms. Note: sediment grain size in the bedform areas was verified during the present study during grab sampling (see Section 2.1). Part of the inshore area was not covered during the 1979 survey, and data quality (due to the prevailing weather conditions) was intermittent. Tidal stage, tidal current speeds and wind velocities at the time of the
side-scan sonar survey were obtained from the UK Hydrographic and Meteorological Office, and Fennessy (1990). For each of the four survey days (days 231e233, 245 of the year 1979), the maximum surface current speeds in the area ranged between 0.15 and 0.20 m s1. Wind velocities were: 2e3 m s1 from the northeast (day 231; 5e6 m s1 from the southwest (days 232, 233); and 1e4 m s1 from the southsouthwest (day 245). Surveying on each day was during the spring tide ebb phase. These conditions will be discussed below within the context of the orientation of the bedforms. 3. Results 3.1. Inner continental shelf sediments On the inner continental shelf, surface sediments comprised mainly sandy gravel, with occasional muddy patches; four non-depositional bedrock areas were identified (Fig. 3). Within Lantic Bay and the northern part of the main area of anthropogenic influence (spoil ground), slightly gravelly sand was dominant. Farther south, in the main area of anthropogenic influence, gravelly muddy sand occurred, with an area of sandy gravel (on bedrock) to the southeast. Gravelly sand was found between the bedrock areas to the southwest of the mouth. Typical bivalves found were Venus striatula, Lutraria lutraria and Chlamys sp. Two species of maerl (Phymatolithon calcareum and Lithothamnium coralloides) were common in samples collected a few kilometres to the south of the ria mouth. Samples of offshore bedrock were identified as slate, comprising the clay minerals chlorite and illite. The highest quartz content (>40%) was found in the MS fraction in the main area of anthropogenic influence (Fig. 4a). From here, quartz-enriched areas extended towards the southwest, as well as northeast towards Lantic Bay. The proportion of quartz grains decreased to w5% in the south and west of the study area. Sediments within Lantic Bay were quartz-enriched (quartz content 20e30%). On Great Lantic beach, bimodal and multimodal grain size distributions were typical, with a mean grain size for the sand fraction of 620 mm. Here, angular to well-rounded quartz comprised 33e69% of the sand, with little variation in percentages between the various size fractions. Maximum feldspar content (8%) was found in the VCS fraction, with a peak value in the spoil ground (Fig. 4b). The feldspar content decreased westwards and towards the north; it was absent to the west of the study area and off the ria mouth. Feldspar levels within Lantic Bay were slightly enriched (2%). Feldspar content on Great Lantic beach (for location, see Fig. 1) was generally low (1e2%), or absent. The highest mica content (16%) occurred in the VFS fraction in the main area of anthropogenic influence and to the south of the ria mouth (Fig. 4c). Mica abundance varied inversely with grain size fraction. Mica content on Great Lantic beach averaged 2e3%. The highest lithic fragment content (60e65% in the VCS fraction) tended to occur near the bedrock areas identified previously. The lowest content (5%) was found in the main
P.L. Friend et al. / Estuarine, Coastal and Shelf Science 67 (2006) 491e502
495
Fig. 3. Distribution of surficial sediment types on the inner continental shelf (classification: Folk, 1980). Bedrock areas are shaded. The location of the sand bar, in the vicinity of buried cables, is illustrated (R: bedrock; sG: sandy gravel; gmS: gravelly muddy sand; (g)S: slightly gravelly sand; gS: gravelly sand; mG: muddy gravel; msG: muddy sandy gravel).
palaeochannel south of the ria mouth (Fig. 4d). Within the spoil ground, lithic fragment content varied between 15 and 65%. Lithic fragments (grits, and purple, grey, and green slates) comprised 13e55% of the sand fraction on Great Lantic beach. Maximum biogenic content decreased landwards from 95% (VCS fraction) in the south of the study area, to w20% near the ria mouth (Fig. 4e). The biogenic content was lowest in the spoil ground (10%). The biogenic content on Great Lantic beach varied from 0 to 53%. 3.2. Ria sediments Within the ria, grain size distributions were variable, with bimodal distributions common near the tributary mouths. The mean grain size of the sand fraction decreased significantly (r2 ¼ 0.376, p ¼ 0.003) along the thalweg from coarse sand (570 mm) at the tidal limit (see Fig. 1) to medium sand (406 mm) near the Harbour mouth. In the same direction, sediment sorting improved and the samples became more negatively skewed. In the dredged area of the Harbour, the sand fraction comprised between 3 and 34% very angular to subrounded quartz grains, with feldspar and mica present only occasionally; the lithic fragment content ranged between 18 and 51%; biogenic fragments comprised 76% of particles in VCS size fraction, and 5% in the VFS size fraction. The relative proportions of illite, kaolinite and chlorite were variable (Fig. 5). Sediments collected near the china clay terminal had the highest kaolinite:chlorite ratio (1.94); farther downstream, within the Harbour, the ratio was 0.42. Sediment from above the effective tidal limit at Restormel had a kaolinite:chlorite ratio near to unity (1.12). Close to the source of the Fowey River, the relative proportions of clay minerals in stream bed sediments were: kaolinite 52%; illite 36%; chlorite 12%. The lowest illite:chlorite ratio (1.43) occurred in sediment from the main Harbour. The proportion of kaolinite
plus chlorite to illite decreased in a landwards direction, from outside the ria mouth to inside the main Harbour. 3.3. Seabed morphology Two main backscatter types were evident in the side-scan sonar records: strong and moderate returns, with the former interpreted as exposed bedrock and the latter as surficial sediment veneer (Fig. 6). Bedform types were sand megaripples and gravel waves, with wavelengths <8e10 m. In the southeast of the study area, near the spoil ground, the bedforms were symmetrical, with crests aligned normal to the shore. Along line 233, bedforms were found with crests aligned in a southwestenortheast direction. To the south of the ria mouth, bedforms were orientated in a northesouth direction, with sediment transport directions (to the north and to the south) indicated by bedform asymmetry (Terwindt and Brouwer, 1986). A number of areas of exposed bedrock were identified in the eastern part of the study area, along with several palaeochannel features to the west. Comparison of contour plots of the Lantic Bay area, for 1857 and 1960/1980, show overall accretion between the survey periods (Fig. 7). In 1857, the bathymetric relief was more pronounced than in 1960/1980: water depths >20 m extended over nearly all the southwest quadrant; this was not the case in the 1960/1980 survey. During the 1960/1980 surveys, the contours were aligned more shore-parallel than in 1857. The 6-m contour within the embayment had moved seawards by about 100 m, suggesting a general infilling of the bay. The appearance of a distinct topographic high, at some 3 m above the surrounding seabed (around 214100E 50300N), indicates the probable main locus of sediment deposition. The volumetric increase between 1857 and 1960/1980, using the minimum and maximum possible depths derived on the basis of the survey techniques, was 3.7 (0.7) 106 m3 (see Section 2.3). As
496
P.L. Friend et al. / Estuarine, Coastal and Shelf Science 67 (2006) 491e502
Fig. 4. (a) Percentage quartz (MS fraction); (b) percentage feldspar (VCS fraction); (c) percentage mica (VFS fraction); (d) percentage lithic fragments (VCS fraction); and (e) percentage biogenic fragments (VCS fraction) on the inner continental shelf. Note: only the fractions with the highest percentage of each mineral are illustrated.
such, the bathymetric changes in the Lantic Bay area may be assumed to be real. 3.4. Grain size distributions and trends Within the main area of anthropogenic influence, the sand was poorly sorted, strongly fine-skewed and very platykurtic (Folk, 1980): the mean grain size varied between 660 (coarse sand) and 230 mm (fine sand), in Lantic Bay. In the main palaeovalley and its tributaries, the mean grain size of the seabed sediments was generally between 750 and 930 mm. In the
remaining areas, the sand was well to moderately sorted: it was generally fine-skewed to strongly fine-skewed, and platykurtic to mesokurtic. In a south to north direction, from the southern part of the main palaeovalley and the southern part of the area of anthropogenic influence, the mean grain size of the sand decreased, the sorting improved and the skewness became more negative. The mean grain size of the mud fraction was <31 mm in the region of anthropogenic influence and south of the ria mouth. The mud was generally poorly sorted, strongly fine-skewed, and very platykurtic to platykurtic. To the south of the ria
P.L. Friend et al. / Estuarine, Coastal and Shelf Science 67 (2006) 491e502
497
mouth and in the area of anthropogenic influence, the grain size distribution of the mud was mesokurtic to leptokurtic. Grain size trend vectors (Gao and Collins, 1994), for Lantic Bay and the main area of anthropogenic influence, indicate an overall north to north-easterly sand transport direction (Fig. 8a). For the mud fraction, the vectors indicated transport, in all directions, away from the site of deposition, i.e. into and out of Lantic Bay, as well as towards the east and west (Fig. 8b). Simplified trends for each fraction are illustrated in Fig. 8c, d, respectively. 4. Discussion 4.1. Ria system sedimentation
Fig. 5. Relative percentages of illite (I), kaolinite (K), and chlorite (C) within the Fowey Ria system.
Rias have been classified into three categories from a sedimentological perspective: (1) marine, (2) estuarine, and (3) riverine (Castaing and Guilcher, 1995). Within the marine sub-environment, sand is transferred from the inner shelf to the ria, either by longshore currents induced by wave activity or by tidal currents, especially in the rias bordering the English Channel. In the estuarine domain, the marine signature diminishes, the biogenic content of the sediments declines rapidly, whilst the riverine influence increases. Within the riverine section, current velocities are controlled by river discharge; hence, riverine sedimentation dominates: a characteristic common to all such settings is substantial siltation, caused by trapped riverine deposits. In the case of the Fowey Ria, a decrease in the biogenic content in a landwards direction within the marine sub-environment, together with a fining of the sediment, improved sorting and more negative skewness, suggests landwards transport of sediment. Here, trends in the grain size parameters are similar to the Case 1 scenario described by McLaren and Bowles (1985). These findings are supported by the clay mineral analysis, in which the (K þ C)/I ratio decreases from offshore into Fowey Harbour; this suggests that fine
Fig. 6. Side-scan sonar interpretation of the seabed morphology and surficial sediments of the study area (adapted from Admiralty Chart, K8581/6).
498
P.L. Friend et al. / Estuarine, Coastal and Shelf Science 67 (2006) 491e502
Fig. 7. Bathymetry of the Lantic Bay area in (a) 1857, and (b) 1960/1980. Depths converted to metres below Chart Datum.
sediment moves in a northerly direction into the ria, from outside the mouth (Weaver, 1989; Algan et al., 1994). Evidence for the transport of sediment into the mouths of rias (e.g. the Goyen Ria, Brittany; Ria de Muros y Noya, Spain) is provided by asymmetrical sand waves with their lee side facing landwards, as well as decreasing biogenic sediment content and mean grain size in a landward direction (Guilcher et al., 1982; Junoy and Vieitez, 1989; Somoza and Rey, 1991; Rey, 1993). In terms of its size, morphology, and sediment characteristics, the Fowey Ria is remarkably similar to the Goyen Ria, Brittany: a typical sandy ria, some 6 km in length, in which the sediment distribution in the outer ria is influenced mainly by swell waves from the southwest (Guilcher et al., 1982). Sediments in Fowey Harbour have a mean grain size similar to the lower Goyen Ria sediments (range 200e400 mm), but
are less well-sorted; this indicates that wave action is less influential in the Fowey Ria. In both rias, the biogenic content decreases landwards, from w80% in the outer reaches. In the Goyen Ria, erosion of periglacial deposits on the channel sides supplies sediments to the inner ria. In comparison, the inner ria sediments in the Fowey are likely to derive from riverine input, as well as erosion of the mudflats, saltmarshes and channels in the upper reaches. Evidence from the geochemical analysis of sediments suggests that reworking of waste derived originally from past mining activities farther upstream, occurs in the northern section of the ria (Pirrie et al., 2002). The lower reaches of the northern Spanish coast rias are characterised by infilling with sand, the siliceous fraction of which is believed to derive from a terrigenous source, carried offshore during the Pleistocene marine regressions, then
Fig. 8. Lantic Bay and the main region of anthropogenic influence: residual grain size trends for (a) sand-size fraction, and (b) mud-size fraction. Simplified trends for each fraction are illustrated in (c) and (d), respectively.
P.L. Friend et al. / Estuarine, Coastal and Shelf Science 67 (2006) 491e502
remobilised during the Holocene transgression in response to strong wave action. The origin of the mud within the rias is also attributed to terrigenous sources, as it includes kaolinite from the alteration of inland rocks and clay minerals (Castaing and Guilcher, 1995). In the case of the Fowey, changes in grain size parameters within the riverine and upper estuarine section of the ria thalweg conform to the Case 1 scenario (see above), indicating a sediment transport direction from the tidal limit near Restormel (see Fig. 1) towards Fowey Harbour. 4.2. Sediment transport pathways The present study uses a multi-disciplinary approach for the identification of sediment transport pathways. Changes in the surface sediment clay mineral content, the mineral, biogenic and lithic fragment content, the seabed morphology and bathymetry, as well as the distribution of grain size parameters may all be used as indicators of sediment transport directions. In the case of the Fowey, sediments entering the lower reaches may be derived from (1) riverine input; (2) erosion of sediments in the upper reaches; (3) spillage of kaolin in the centre of commercial activity; and (4) offshore, via the ria mouth. On the inner continental shelf, dredged sediment is used as a tracer for sediment transport pathways in an area where the natural sedimentary regime is low/non-depositional. Here, the distribution of bedrock areas to the southwest of the Fowey Ria mouth (Fig. 3) correlates well with the positions of known shore platforms (Everard et al., 1964); it delineates interfluves of the incised river valley and a what appears to be a palaeo-tributary channel entering the main palaeochannel from the west. The sandy gravel belt, aligned and extending in a northesouth direction between the interfluves south of the ria mouth, correlates with the bathymetric contours (Fig. 1), indicating the position of the main palaeochannel. A second palaeo-tributary channel appears to extend eastwards towards the Lantic Bay region. The spoil ground sediments have mineral compositions that are similar to the sediments found within the dredged area of Fowey Harbour (see Section 3.2). They differ considerably from the natural sediments, which comprise mainly lithic and biogenic fragments. Evidence for the transport of sand away from the area of anthropogenic deposition, into Lantic Bay, is provided by (1) the enrichment of quartz, feldspar and mica in sediments to the northeast; (2) the residual grain size trend vectors; (3) purple and green lithic fragments from the Dartmouth Beds (which outcrop from the mid-Harbour to the upper estuary), found in the beach sands; (4) bathymetric changes; and (5) the predominant wave direction. Residual trend vectors, together with the small percentage of mud found in the Lantic Bay samples (<1.5%), indicate that fine sediments move away from the locus of deposition, being transported ultimately to the east or west depending, presumably, upon wave intensity and the direction of the prevailing tidal flow. The general bedform orientation in the south-eastern quadrant of the study area correlates with the eastewest (floode ebb) direction of the surface tidal currents (see Section 1.1 and Fig. 6). From the bedform asymmetry to the south and southwest of the mouth, there is some evidence for the
499
existence of preferred flood and ebb flow directions, with the ebb jet aligned along the main axis of the palaeovalley (see Fig. 6). This pattern supports the existence of a clockwise rotating gyre to the east of Gribben Head, as reported by local seafarers (M. Sutherland, Fowey Harbour Commissioners e pers. comm.); this has been suggested by Stride (1982), as a mechanism near headlands for causing sediment transport in the opposite direction to the general (residual) trend. Bedrock areas identified to the southwest of the Harbour entrance are seaward extensions of the shore platforms described by Everard et al. (1964). In the present study, the existence of a shore (wave-cut) platform to the east of the Harbour entrance is demonstrated, together with a large bedrock interfluve farther to the south (see Fig. 3). Bathymetric changes that have occurred in the area of anthropogenic deposition, over a period of 120 years (see Fig. 7) indicate an increase in the volume of the sedimentary cover, of w3.7 106 m3. Since 1904, when dredging commenced, it is interesting to note that some 6 106 t of material has been removed from the Fowey Ria to the spoil ground since dredging commenced. If all the material had remained in place on the seabed, it would have a volume of 3.3e3.8 106 m3, assuming a packing density of 1800e 1600 kg m3 (Rieke and Chilingarian, 1974). Although the exact amount of material removed from Par Harbour is not known, it is believed to be similar in volume to that removed from the Fowey Ria. This suggests that some 3e4 106 m3 of sediment has been transported away from the spoil ground, since dredging commenced. Combining the evidence provided by the various different research approaches, a conceptual model of suggested sediment transport pathways for the study area has been developed (Fig. 9). 5. Concluding remarks Although subjected to severe anthropogenic disturbance from mining and dredging activities, the Fowey Ria conforms to the general ria sediment model of Castaing and Guilcher (1995). Large offshore areas of exposed bedrock, to the southwest and southeast of the study area, show that the inner continental shelf area is a low/non-depositional environment. Sediments originating from offshore, and most probably the spoil ground, enter the mouth of the ria under infilling conditions experienced by modern ria systems. An undredged area across the ria mouth forms a sill which, together with the ‘U’ shaped profile caused by dredging in the main Harbour, acts as an effective trap for sediments entering the lower ria, either from offshore or from the riverine section. Several preferred sediment transport directions in the area of anthropogenic influence are linked to the prevailing hydrodynamic conditions. Long-period swell waves, together with waves produced under local storm conditions, resuspend fine-grained material; this makes it available for transport by tidal currents.
500
P.L. Friend et al. / Estuarine, Coastal and Shelf Science 67 (2006) 491e502
Fig. 9. General model of suggested sediment transport pathways for the lower Fowey Ria and inner continental shelf area.
The dredged sediment acts as a useful tool to trace sediment transport pathways, in environments which are naturally low/non-depositional. Future studies are recommended, as outlined below: (1) The use of numerical wave and surface current modelling for the inner continental shelf area, to provide estimates of critical shear stress at the seabed, for the assessment of erosion, transport and deposition; (2) Long-term monitoring of the intertidal flats, marsh areas and channel migration patterns, to understand the rates of erosion and accretion within the ria; (3) Geochemical analysis of sediments for Sn and Cu signatures in the southern part of the ria, and in the paleovalley (and its tributaries) on the inner continental shelf, for use as a tracer of sediment movement in these areas; (4) The effect of dredging, on changes in the estuarine geometry and tidal characteristics within the ria (e.g.
Speer and Aubrey, 1985; Friedrichs and Aubrey, 1988), should be studied; (5) A sediment budget should be established for the ria system, by quantifying the input from rivers/tributaries, field runoff and offshore sources.
Acknowledgements Funding for this study was provided by the Maritime Division of English Nature and the EU project: EUROSTRATAFORM (EVK3-CT-2002-00079). The following are thanked for logistical support: Capt. M. Sutherland (Fowey Harbour Commissioners); the captain and crew of the dredger, Lantic Bay; J. Davis; E. Burfoot; J. Oake. Dr. W. Farnham (University of Portsmouth) is thanked for identification of the maerl. The Hydrographic Office, Taunton kindly provided the Admiralty
P.L. Friend et al. / Estuarine, Coastal and Shelf Science 67 (2006) 491e502
survey data. A. Bale (Plymouth Marine Laboratory) is thanked for providing the sea-level rise data. K. Davis (National Oceanography Centre Southampton) is thanked for producing the graphics. Two anonymous reviewers are thanked for their useful comments and suggestions. References Algan, O., Clayton, T., Tranter, M., Collins, M.B., 1994. Estuarine mixing of clay minerals in the Solent region, southern England. Sedimentary Geology 92, 241e255. Arps, C.E.S., Kluyver, H.M., 1969. Sedimentology of the Northwestern shores of the Ria de Arosa, NW Spain. Leidsche Geologische Mededelingen 37, 135e145. Barton, R.M., 1964. An Introduction to the Geology of Cornwall. Pitman Press, Bath, 168 pp. Berthois, L., Auffret, G., 1966. Dynamique de la se´dimentation dans les rias et les estuaires des petits cours d’eau tributaires de la Manche. Cahiers Oce´anographiques 18, 761e764. Brown, G., Brindley, G.W., 1980. X-ray diffraction procedures for clay mineral identification. In: Brindley, G.W., Brown, G. (Eds.), Crystal Structures of Clay Minerals and Their X-ray Identification. Mineralogical Society, London, pp. 305e361 (Monograph 5). Burt, R., 1998. History of metalliferous mining. In: Selwood, E.B., Durrance, E.M., Bristow, C.M. (Eds.), The Geology of Cornwall. University of Exeter Press, Exeter, pp. 211e225. Castaing, P., Guilcher, A., 1995. Geomorphology and sedimentology of Rias. In: Perillo, G.M.E. (Ed.), Geomorphology and Sedimentology of Estuaries e Developments in Sedimentology, No. 53. Elsevier Science, Amsterdam, pp. 69e111. Codrington, T., 1898. On some submerged valleys in South Wales, Devon and Cornwall. Quarterly Journal of the Geological Society, London 54, 251e278. Collins, M.B., 1970. Discussion of design curves for suspended sediment load estimation. Proceedings of the Institute of Civil Engineers 46, 85e88. Dias, J.M.A., Jouanneau, J.M., Gonzalez, R., Araujo, M.F., Drago, T., Garcia, C., Oliveira, A., Rodrigues, A., Vitorino, J., Weber, O., 2002. Present day sedimentary processes on the northern Iberian shelf. Progress in Oceanography 52, 249e259. Donovan, D.T., Stride, A.H., 1975. Three drowned coastlines of probable Late Tertiary Age around Devon and Cornwall. Marine Geology 19, M35eM40. Edmonds, E.A., McKeoun, M.C., Williams, M., 1975. British Regional Geology: SW England. HMSO, London, 136 pp. Evans, G., Howarth, R.J., Nombela, M.A., 2003. Metals in the sediments of Ensenada de San Simon (inner Ria de Vigo), Galicia, NW Spain. Applied Geochemistry 18, 973e996. Evans, G., Prego, R., 2003. Rias, estuaries and incised valleys: is a ria an estuary? Marine Geology 196, 171e175. Everard, C.E., Lawrence, R.H., Witherick, M.E., Wright, L.W., 1964. Raised beaches and marine geomorphology. In: Hosking, K.F.G., Shrimpton, G.J. (Eds.), Present Views of Some Aspects of the Geology of Devon and Cornwall. Royal Geological Society of Cornwall, Truro, pp. 283e310. Fennessy, M.J., 1990. Tidal Stream Atlas of the South Cornwall Coast. The author (publisher), Plymouth, Devon, 15 pp. Folk, R.L., 1980. Petrology of Sedimentary Rocks. Hemphill, Texas, 185 pp. Friedrichs, C.T., Aubrey, D.G., 1988. Non-linear tidal distortion in shallow well-mixed estuaries: a synthesis. Estuarine, Coastal and Shelf Science 27, 521e545. Gao, S., Collins, M.B., 1992. Net sediment transport patterns inferred from grain size trends, based upon definition of ‘‘transport vectors’’. Sedimentary Geology 81, 47e60. Gao, S., Collins, M.B., 1994. Analysis of grain size trends, for defining sediment transport pathways in marine environments. Journal of Coastal Research 10, 70e78. Garcia-Gil, S., Duran, R., Vilas, F., 2000. Side scan sonar image and geologic interpretation of the Ria de Pontevedra seafloor (Galicia, NW Spain). Sciencia Marina 64, 393e402.
501
Gerrard, S., 1987. Streamworking in medieval Cornwall. Journal of the Trevithick Society 14, 7e31. Gerrard, S., 2000. The Early British Tin Industry. Tempus Publishing, Stroud, 176 pp. Guilcher, A., Berthois, L., 1957. Cinq anne´es d’observations se´dimentologiques dans quatre estuaires te´moins de l’Ouest de la Bretagne. Revue de Ge´omorphologie Dynamique 8, 66e86. Guilcher, A., Andrade, B., Dantec, M.H., 1982. Diversite´ morpho-se´dimentologique des estuaires du Finiste`re. Norois 29, 205e228. Herranz, P., Acosta, J., 1984. Estudio geofisico de la Ria de Muros y Noya. Boletin del Instituto Espagnol Oceanografia 1, 48e78. Isaac, K.P., Selwood, E.B., Shail, R.K., 1998. Devonian. In: Selwood, E.B., Durrance, E.M., Bristow, C.M. (Eds.), The Geology of Cornwall. University of Exeter Press, Exeter, pp. 31e64. Jouanneau, J.M., Weber, O., Drago, T., Rodrigues, A., Oliveira, A., Dias, J.M.A., Garcia, C., Schmidt, S., Reyss, J.L., 2002. Recent sedimentation and sedimentary budgets on the western Iberian shelf. Progress in Oceanography 52, 261e275. Junoy, J., Vieitez, J.M., 1989. Cartografia de los sedimentos superficiales de la Ria de Foz, Lugo. Thalassas 7, 9e19. Manning, D.A.C., Hill, P.I., Howe, J.H., 1996. Primary lithological variation in the kaolinised St. Austell granite, Cornwall, England. Journal of the Geological Society, London 153, 827e838. McLaren, P., Bowles, D., 1985. The effects of sediment transport on grain-size distributions. Journal of Sedimentary Petrology 55, 457e470. Motyka, J.M., Brampton, A.H., 1993. Coastal Management Mapping of Littoral Cells. HR Wallingford, Report SR328, 102 pp. National Rivers Authority, South-West Region, 1994. Seaton, Looe and Fowey Catchment Management Plan Consultation Report. National Rivers Authority, Bodmin, Cornwall, 117 pp. Nombela, M.A., Vilas, F.V., Evans, G., 1995. Sedimentation in the mesotidal Rias Bajas of Galicia (north-western Spain): Ensenada de San Simon, Inner Ria de Vigo. In: Flemming, B.W., Bartholoma, A. (Eds.), Tidal Signatures in Modern and Ancient Sediments. International Association of Sedimentologists, pp. 133e149 (Special Publication 24). Oliveira, A., Rocha, F., Rodrigues, A., Jouanneau, J., Dias, A., Weber, O., Gomes, C., 2002. Clay minerals from the sedimentary cover from the Northwest Iberian shelf. Progress in Oceanography 52, 233e247. Perez-Arlucea, M., Mendez, G., Clemente, F., Nombela, M., Rubio, B., Filgueira, M., 2005. Hydrology, sediment yield, erosion and sedimentation rates in the estuarine environment of the Ria de Vigo, Galicia, Spain. Journal of Marine Systems 54, 209e226. Perillo, G.M.E., 1995. Definitions and geomorphic classifications of estuaries. In: Perillo, G.M.E. (Ed.), Geomorphology and Sedimentology of Estuaries, Developments in Sedimentology, No. 53. Elsevier, Amsterdam, pp. 17e47. Pirrie, D., Power, M.R., Wheeler, P.D., Cundy, A., Bridges, C., Davey, G., 2002. Geochemical signature of historical mining: Fowey Estuary, Cornwall, UK. Journal of Geochemical Exploration 76, 31e43. Prego, R., Cobelo-Garcia, A., 2003. Twentieth century overview of heavy metals in the Galician Rias (NW Iberian Peninsula). Environmental Pollution 121, 425e452. von Richthofen, F., 1886. Fu¨hrer fu¨r Forschungsreisende. Ja¨necke, Hannover, 734 pp. Rey, J., 1993. Relacio´n morpho-sedimentaria entre la plataforma continental de Galicia y las rias bajas y su evolucio´n durante el Cuaterario. Instituto Espanol de Oceanografia, Madrid, Special Publication 17, 233 pp. Rieke, H.H., Chilingarian, G.V., 1974. Compaction of Argillaceous Sediments e Developments in Sedimentology, No. 16. Elsevier, Amsterdam, 424 pp. Rubio, B., Pye, K., Rae, J.E., Rey, D., 2001. Sedimentological characteristics, heavy metal distribution and magnetic properties in subtidal sediments, Ria de Pontevedra, NW Spain. Sedimentology 48, 1277e1296. Somoza, L., Rey, J., 1991. Holocene fan deltas in a ‘‘ria’’ morphology. Prograding clinoform types and sea-level control. Cuadernos de Geologia Ibe´rica Madrid 15, 37e48. Speer, P.E., Aubrey, D.G., 1985. A study of non-linear propagation in shallow inlet/estuarine systems. Part II: theory. Estuarine, Coastal and Shelf Science 21, 207e224.
502
P.L. Friend et al. / Estuarine, Coastal and Shelf Science 67 (2006) 491e502
Stannier, P., 1990. Cornwall’s Geological Heritage. Twelveheads Press, Truro, 47 pp. Steers, J.A., 1964. The Coastline of England and Wales. Cambridge University Press, Cambridge, 750 pp. Stride, A.H., 1982. Offshore Tidal Sands e Processes and Deposits. Chapman and Hall, London, 222 pp. Terwindt, J.H.J., Brouwer, M.J.N., 1986. The behaviour of intertidal sand waves during neapespring tidal cycles and the relevance of palaeoflow reconstruction. Sedimentology 33, 1e31. UKHO, 1996. Admiralty Tide Tables. The United Kingdom Hydrographic Office, Taunton, 442 pp.
Uncles, R.J., Barton, M.L., Stephens, J.A., 1994. Seasonal variability of finesediment concentrations in the turbidity maximum region of the Tamar Estuary. Estuarine, Coastal and Shelf Science 38, 19e39. Uncles, R.J., Stephens, J.A., 1993. Nature of the turbidity maximum in the Tamar Estuary, UK. Estuarine, Coastal and Shelf Science 36, 413e431. Vilas, F.V., 1983. Medios sedimentarios de transicion en la Ria de Vigo: secuencias progradantes. Thalassas 1, 49e55. Weaver, C.E., 1989. Clays, muds, and shales. In: Developments in Sedimentology, vol. 44. Elsevier, Amsterdam, 819 pp. Wentworth, C.K., 1922. A scale of grade and class terms for clastic sediments. Journal of Geology 30, 377e392.