On sediment dispersal in the Whitsand Bay Marine Conservation Zone

C H A P T E R

31 On sediment dispersal in the Whitsand Bay Marine Conservation Zone: neighbour to a closed dredgespoil disposal site R.J. Uncles, J.R. Clark, M. Bedington, R. Torres Plymouth Marine Laboratory, Plymouth, United Kingdom Abstract Some ‘thought experiment’ modelling results and interpretations of data and theory are presented to investigate the possibility that Whitsand Bay, UK, and its recently (2013) designated Marine Conservation Zone (MCZ), might have been affected in the past by intrusion of dredge-spoil sediments from the now-closed disposal site located close to the seaward boundary of the MCZ and by suspended sediment and low salinity waters from the adjacent Tamar Estuary and Plymouth Sound. The schematic modelling work (2D and 3D) is considered to provide approximate indications rather than precise predictions. The component of Tamar waters present within the MCZ is computed to be small (<10%). The location of the dredge-spoil (model tracer/particle-release) source point is crucially important to the intrusion of tracer within the MCZ. Modelled bedload sediment transport from the disposal site occurs with high waves and its magnitude and direction is dependent on near-bed tidal, wave and wind-driven currents.

Keywords: Dredge-spoil sediment disposal; Sediment transport; Bed sediments; Suspended sediments; Waves; Whitsand Bay MCZ; UK.

Introduction Whitsand Bay (the Bay) is a large coastal system in southwest England (Fig. 31.1A and B). A large shoreward part of the Bay was designated a Marine Conservation Zone (MCZ) in 2013. The MCZ protects eight habitats and their associated species and gives particular protection to four important conservation species (DEFRA, 2013). The key protected aspects are its subtidal and intertidal habitats as well as its seagrass beds and pink sea-fan and sea-fan anemone populations. The MCZ is generally shallower than 25 m and has a surface area of 52 km2. Until

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31. On sediment dispersal in the Whitsand Bay Marine Conservation Zone: neighbour to a closed dredge-spoil disposal site

FIG. 31.1 Location map and Google earth satellite image of the region: (A), Location of Plymouth near Whitsand Bay and the hindcast modelling wind-wave data site (marked by a gray square and labelled ‘N’); (B), Location of the Whitsand Bay Marine Conservation Zone, MCZ (with its seaward boundary coloured amber) showing the perimeters of the historical (yellow) and more recent (magenta) dredge-spoil disposal grounds superimposed on a Google earth image of the area. The top right and bottom left corners of the historical disposal grounds are denoted TR and BL. The current meter mooring station M (50
19.50 N, 04
15.50 W) is shown as the red-brown triangle and the wave station W (50
19.6650 N, 04
15.1620 W) as the orange inverted triangle. The village of Portwrinkle is denoted PW, Polhawn Cove as PC, the tracer input station at Devonport on the Tamar Estuary by DP, and the tracer input station in Plymouth Sound, close to the mouth of the Tamar, as TM.

March 2017 a site in the outer Bay close to the seaward boundary of the MCZ (Fig. 31.1B) was used for many years for dredge-spoil disposal. However, disposal of dredged sediment at sea can affect the water column, if only temporarily, as well as affect conditions at the seabed and the biology and ecology of the disposal area (Okada et al., 2009). In this chapter some ‘thought experiment’ tracer and particle-dispersal modelling results and interpretations of data and theory are used to indicate the conditions under which transport of fine, suspended dredge spoil sediment and coarser, deposited dredge-spoil bed sediment from the disposal site into the Bay and the future MCZ and its shoreline might have occurred over the years. The dredged sediment from Devonport Naval Base on the Tamar Estuary (DP in Fig. 31.1B), as well as that from surrounding harbour areas, including Plymouth’s marinas and docks, was disposed of for about 100 years in the outer Bay near Rame Head (located on the Rame Peninsula, southeast Cornwall, Fig. 31.1B) although initially the site was used to dispose of unwanted ordnance. Two types of dredging were, and are, undertaken: maintenance dredging, which maintains navigation depths that enable shipping to utilise the dock areas safely; and capital dredging, which is done either for a one-off project or for substantial short-term work that is repeated every several years. In recent years these Bay disposal campaigns attracted a great deal of concern from local residents; as an example from 2014, this concern resulted in the organisation of protest gatherings at Rame Head, amongst other activities (Herald, 2014; Western Morning News, 2014). Moreover, as early as 2002 a local scuba

Previous work

601

diver voiced his anxiety over what he believed was environmental damage that might have resulted from dredge-spoil disposal within the Bay (Evening Herald, 2002). In the intervening years, several newspaper articles highlighted the level of local interest in this issue (e.g. Western Morning News, 2004). Concern amongst local politicians, residents and environmentalists was understandable, especially in view of the area’s beauty, importance for tourism and, between 2011 and 2013, its designation as a Marine Conservation Zone (Guardian, 2011). These MCZs are designated under the UK’s ‘Marine and Coastal Access Act 2009’ in order to protect a range of nationally important marine habitats, their wildlife and their geology and geomorphology (JNCC, 2014). The Whitsand Bay MCZ lies immediately to the north of the dredge-spoil disposal site (amber line across the Bay in Fig. 31.1B) and parts of its seaward boundary are less than 2.5 km from any point within the disposal site. The original disposal site (yellow parallelogram in Fig. 31.1B) is denoted as PL030 and was last used for maintenance dredge spoil in 1995 (Black and Veatch, 2010); it is still marked on Admiralty Chart 199 (Admiralty, 2018) and other charts. This older site was somewhat larger than that used in later years (highlighted as the magenta parallelogram in Fig. 31.1B), which is denoted as PL031 and was first used for maintenance dredge spoil in 1994 (Black and Veatch, 2010) when disposal was undertaken toward the south and west within this area.

Previous work Physical oceanography The physical oceanography of the Bay has been described by Uncles et al. (2015), who presented measurements of waves and currents, drogue tracking, surveys of salinity, temperature and turbidity during stratified and mixed water-column conditions and bed sediment surveys. 2D and 3D hydrodynamic models were used to explore the generation of the Rame eddy (Fig. 31.2AeC) and the hydrodynamic coupling between the Bay and Plymouth Sound (the western shoreline of the Sound constitutes the eastern shoreline of the Rame Peninsula, Fig. 31.1B). Tidal currents flow around the Rame Peninsula from the Sound between site TM at the mouth of the Tamar Estuary and Rame Head from approximately 3 h before to 2 h after low water (LW) and form a transport path between them that conveys lower salinity, higher turbidity waters from the Sound to the Bay (Fig. 31.2A and B). These waters are then transported into the Bay as part of the Bay-mouth limb of the Rame eddy and subsequently conveyed to the near-shore, east-going limb and re-circulated back toward Rame Head (Fig. 31.2C). The hydrodynamic coupling between the Bay and the Sound is most graphically demonstrated using the modelled transport and dispersion of a continuous, constant-rate input of tracer material (a solute in this case) from site TM (Fig. 31.2D); higher concentrations of tracer at a location imply that a greater fraction of contaminated source waters are mixed with uncontaminated coastal and estuarine waters there. The Bay thermally stratifies during summer months and is well mixed during winter when the majority of dredge disposal activities took place. Okada et al. (2009) analyzed 5 months of current-meter data at 5 m above the bed (triangle M in Fig. 31.1B and red triangle in Fig. 31.2A and B) to show that the currents tended to be parallel to the bathymetric contours and could exceed 0.45 m s1 towards the southeast; however, these measured currents

602

31. On sediment dispersal in the Whitsand Bay Marine Conservation Zone: neighbour to a closed dredge-spoil disposal site

FIG. 31.2

Whitsand Bay drogue tracks during winter and hydrodynamic transport around Rame Head between Plymouth Sound and the Bay: (A), Drogues A, B and D (annotating the start positions) during LWþ0.5 h to LWþ4.0 h on 13 December 2004; (B), Drogues A, B, C and D (annotating the start positions) during LWþ0.5h to LWþ4.5h on 7 February 2005; (C), 3D-modelled surface (black) and bed (green) residual currents around the Rame Peninsula and in the Rame eddy at mean spring tides of winter conditions for zero winds; (D), A simulated tracer continuously released at a constant rate from location TM (Fig. 31.1B) computed using the 2D model. The concentrations apply to mid-rising water levels for average tides, average Tamar runoff and no winds. Illustrations (AeD) are reproduced with minor modifications from Uncles, R.J., Stephens, J.A., Harris, C., 2015. Physical processes in a coupled bay-estuary coastal system: Whitsand Bay and Plymouth Sound. Progress in Oceanography 137, 360e384, with permission from Elsevier.

included wind-driven and other currents over the deployment period and therefore were specific to the particular meteorological conditions during that time. They also showed that current speeds in excess of, e.g. 0.20 and 0.25 m s1, occur for less than approximately 10% and 3% of the time, respectively, demonstrating the relative slowness of currents at station M. However, large waves can occur in the Bay and these are able to initiate both suspended and bedload sediment transport. Measured data show fairly low turbidity in the Bay; nevertheless, turbidity has a strong correlation with wave height and responds rapidly to waves (Uncles et al., 2015).

Previous work

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A hindcast wind and wave model grid node is positioned 65 km south-southwest of the Bay’s mouth in 80 m of water (‘N’ in Fig. 31.1A; the NEXT model - a development of the NESS model, Peters et al., 1993). In winter, significant wave heights and wind speeds are predicted to occur at node N with >4.5 and > 2.5 m and >15.5 and > 9.5 m s1 for 10% and 50% of the time, respectively (HSE, 2001). Modelled wave spectral peak periods at the node typically increase with wave height in the range 5e10 s and the dominant wave directions are from the west (with 57% occurrence) and the southwest (24%). The dominant wind directions are from the west (21%) and southwest (21%) and from the northwest (12%). Due to the presence of the Rame Peninsula and coastal shoreline the Bay has a very short fetch for winds from the north and east and is not large from the west (Fig. 31.1B). However, the Bay’s fetch is considerable to the south and southwest (Fig. 31.1A). Significant wave heights were measured by Cefas (2005) at station W in the Bay (orange inverted triangle in Fig. 31.1B) during 7 December 2004 to 7 January 2005. Significant wave heights had a mean and SD of 0.65 m and 0.39 m over the deployment period and largest and smallest waves were 3.0 m, with a periodicity of 9 s, and 0.2 m. The correlation between significant wave height and wind properties maximised at a wind-velocity averaging period of 12 h prior to the measured wave height for winds that were blowing from 240
(30
south of west, Uncles et al., 2015).

Dredge-spoil disposal Okada et al. (2009) estimated the spatial distribution of dredged material disposed of at the outer Bay disposal site using measured particle-size distributions and metal concentrations. They computed that between 1999 and 2005 approximately 2 million tonnes (metric tons) of wet sediment per year was deposited at the site. They took grab samples in the area and analyzed the fine sediment fraction (<63 mm) for trace metals and all sediment <4.5 mm for size analysis. They found that a modal (most common) size of 40 mm was the most robust signature of dredge-spoil material. Their analysis of the finest fraction suggests that dredge spoil sediment influenced particle-size distributions to a distance of 6 km around the disposal site (a distance scale is shown in Fig. 31.1B). In an impact study by Cefas (2005) it was concluded that there was no evidence of longterm accumulation of dredged material either within the disposal site or in the surrounding marine area. It was thought likely that material was dispersed more widely into the English Channel. A review of the available evidence for dredge-spoil impacts was given by Elliott and Mazik (2011) and they concluded, amongst other things, that there was no evidence for longer-term increases in turbidity at the disposal site. They also thought that transport paths around Rame Head between the Tamar Estuary, Plymouth Sound and the Bay (e.g. Fig. 31.2D) should be investigated as a potential source of elevated concentrations of some contaminants associated with silt in the Polhawn Cove area (PC in Fig. 31.1B). This latter transport-path aspect is considered later in more detail. Cefas (2018) provided useful data on dredge-spoil disposal protocols. Using historical data for the Bay disposal site, they stated that there were six disposal events per 24 h per disposal campaign (based on previous licenses and disposal returns). Analysis of the disposed maintenance-dredged material determined that 62% of the sediment was cohesive fine sediment and 38% was sand and gravel, the latter of which would have had fast settling rates and

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31. On sediment dispersal in the Whitsand Bay Marine Conservation Zone: neighbour to a closed dredge-spoil disposal site

would have deposited to the bed upon disposal. They stated that 85,305 tonnes of wet sediment was disposed of at sea (60,932 m3 of wet sediment with a specific gravity of 1.4) in an average year. With a specific gravity of 2.7 for sedimentary material, this wet sediment amounted to 38,710 tonnes of dry sediment, which corresponded to 24,000 tonnes of fines (i.e. 62%) and 14,710 tonnes of sand and gravel (i.e. 38%). If the disposed fines remained in suspension during dispersal (as suspended particulate matter e SPM) then it is estimated that the average suspended sediment input rate to the sea was 121 kg s1 over the 2.3 days duration of a disposal campaign (Cefas, 2018).

Freshwater and SPM transport between the Tamar and the MCZ Modelled salinity and tracer in the MCZ The 2D (depth-averaged) model and its validation and application to tracer dispersion and salinity modelling have been described by Uncles and Torres (2013). As an example of its application, the predicted concentrations during transport from a constant-rate, continuously-released point-source of tracer solute (from site TM near the mouth of the Tamar, Fig. 31.1B) around Rame Head exhibits complex and considerable mixing due to both tidal and residual currents (Fig. 31.2D). The ‘hybrid’ salinity-simulation method outlined by Uncles and Torres (2013) is used here to investigate the transport of lower salinity waters from the Tamar Estuary around Rame Head and into the Bay. These waters are computed to move around Rame Head and hug the Whitsand Bay near-shore region, leading to a characteristic offshore gradient of increasing salinity at all states of the tide (shown for constant, high freshwater inflows at mid falling e MF, low water e LW, mid rising e MR and high water e HW tidal states in Fig. 31.3AeD, respectively). The salinity model calculation shows that having reached a final quasi-steady state after 60 tides the salinity at Polhawn Cove (PC in Fig. 31.3) is 34.32 when averaged over the final tide and the salinity at Portwrinkle (PW in Fig. 31.3) is 34.57. The maximum salinity over the modelled domain is 35.20. The section-averaged salinity across the mouth of the Tamar Estuary in final quasi-steady state, when averaged over the ebb from HW to LW (i.e. the fresher estuarine waters that are ebbed into the Sound from the Tamar) is 20.51. This ebbed water is the source of the fresher waters required to reduce modelled salinity within the local coastal waters, including the Bay and the MCZ. The estuarine-water budgets at PC and PW in terms of the fractions of estuarine waters at PC and PW, fPC and fPW, are, respectively: ð1  fPC Þ , 35:20 þ fPC ,20:51 ¼ 34:32 and ð1  fPW Þ , 35:20 þ fPW ,20:51 ¼ 34:57 Solving these equations gives fPC ¼ 0.06 and fPW ¼ 0.04; i.e., in this model simulation, 6% of the waters at PC and 4% of those at PW can be ascribed to the Tamar Estuary system outflows. There was no SPM simulation and therefore it is not possible to say what fraction of the permanently suspended fine sediment reached PC or PW from the Tamar system.

Freshwater and SPM transport between the Tamar and the MCZ

605

FIG. 31.3 2D-modelled salinity distributions in Whitsand Bay during continuously repeating mean tides and for constant high Tamar Estuary system runoff into Plymouth Sound (130 m3 s1 compared with an average value of approx. 50 m3 s1) at: (A), Mid-falling tide (MF); (B), Low water (LW); (C), Mid-rising tide (MR); (D), High water (HW).

Observed salinity and turbidity in the MCZ Measured salinity data for a 2.6 m neap-tide winter survey in the Bay when conditions were essentially vertically mixed also illustrate the transport of fresher estuarine waters around Rame Head from the Tamar system and their hugging of the coastline to the west of the peninsula (Fig. 31.4A; Uncles et al., 2015). Measured surface SPM concentrations were relatively low (<10 mg L1, Fig. 31.4B). The corresponding gridded observed surface salinity data at PC and PW were, respectively, 34.44 (with SPM concentration 5.5 mg L1) and 34.83 (with SPM concentration 4.3 mg L1) and the maximum gridded observed salinity was 35.17 (with SPM concentration 4.7 mg L1). The tracer-model results show that the delay time between starting the tracer dispersal and experiencing an effect at PC is approximately

606

31. On sediment dispersal in the Whitsand Bay Marine Conservation Zone: neighbour to a closed dredge-spoil disposal site

FIG. 31.4 Whitsand Bay hydrographic survey during essentially vertically well-mixed conditions on 16 February 2005 and the results of a bed-sediment survey on 8 March 2005: (A), Depth-averaged salinity; (B), Surface SPM concentrations (mg L1); (C), Median grain-sizes, D50, in microns (plotted as log10(D50)); (D), Percentage weight of dried sediment having grain sizes less than very fine sand (<63 mm, i.e. the silt and clay fraction). ND ¼ no data. Illustrations (AeD) are reproduced with minor modifications from Uncles, R.J., Stephens, J.A., Harris, C., 2015. Physical processes in a coupled bay-estuary coastal system: Whitsand Bay and Plymouth Sound. Progress in Oceanography 137, 360e384, with permission from Elsevier.

4 d for a source positioned at the mouth of the Tamar (TM in Fig. 31.1B) and approximately 7 d at PW (see later). Therefore, the appropriate input rate of freshwater (for an interpretation of Fig. 31.4A and B) is the very high freshwater flow rate of 144 m3 s1 from the Tamar system that occurred 4 d earlier (12 February 2005) during a time of spring tides. Based on regression analysis of large quantities of Tamar salinity and SPM concentration data measured during 1982e85 (Uncles and Stephens, 1989, 1990) it was possible to estimate the salinity and turbidity of waters ebbing into the Sound from the Tamar system during 12 February 2005; estimated ebbed salinity was 25.26 and estimated ebbed SPM concentration was 16.8 mg L1. Therefore, the fractions of Tamar-system waters present at PC and PW, respectively, during the 16 February survey are given by: ð1  fPC Þ , 35:17 þ fPC ,25:26 ¼ 34:44 and ð1  fPW Þ , 35:17 þ fPW ,25:26 ¼ 34:83

Using modelled tracer to represent SPM

607

Solving these equations gives: fPC ¼ 0.07 and fPW ¼ 0.03; i.e., 7% of the waters at PC and 3% of those at PW can be ascribed to the Tamar Estuary system outflows in these measured data, which is consistent with simulated salinity data at high freshwater inflows (6% and 4% for PC and PW, respectively, Fig. 31.3). The SPM concentrations that are predicted to occur at PC and PW, assuming no deposition or resuspension of sediment during transport is given by: ð1  0:07Þ  4:7 mg L1 þ 0:07  16:8 mg L1 ¼ 5:5 mg L1 and ð1  0:03Þ  4:7 mg L1 þ 0:03  16:8 mg L1 ¼ 5:1 mg L1 The prediction for PC is in accordance with measurement (5.5 mg L1) whereas that for PW overestimates the SPM (5.1 mg L1 as opposed to the gridded observed value of 4.3 mg L1) although the difference is insignificant for the present purpose. Therefore, approximately 1 mg L1 of SPM or less (i.e. between 0.03  16.8 and 0.07  16.8 mg L1) can be ascribed to the Tamar system at these MCZ sites, which is substantially smaller than typical in-situ concentrations (Fig. 31.4B). Data from spring-tide and neap-tide tidal-cycle anchor stations located in the mouth of the Tamar provide an alternative indication of the magnitude of SPM concentrations in waters ebbing from the Tamar Estuary system. A small neap tide on 8 October 1981 with a freshwater flow of 90 m3 s1 (for the whole Tamar system) had an ebb-tide SPM concentration of 2 mg L1, and a large spring tide on 13 October 1981 with a freshwater flow of 52 m3 s1 (for the whole Tamar system) had an ebb-tide SPM concentration of 8 mg L1 (Uncles et al., 1985; Uncles and Lewis, 2001). These concentrations are small and the small fractions of Tamar system waters estimated to occur at PC and PW (<10%) render them insignificant. Nevertheless, strong wave activity in Plymouth Sound waters could suspend fine-grained seabed sediments at greater concentrations that might impact MCZ waters, although there are no measurements to demonstrate this. Similarly, strong wave activity in the MCZ could suspend deposited bed sediments, including dredge-spoil sediments, although grab-sample measurements of median grain sizes of the surficial bed sediments within the Bay and MCZ (Fig. 31.4C and D) show that the majority of bed sediment is sand sized rather than silt and clay sized and therefore more likely to be moved as bedload transport and less readily suspended and transported in the water column by tidal currents and waves in the Bay (Uncles et al., 2015).

Using modelled tracer to represent SPM Two types of ‘thought experiment’ are considered for various winds and tides. The first uses a constant and continuous input rate of tracer corresponding to 1000 Units s1 of solute, which attempts to represent very fine, continually suspended SPM. This is equivalent to the average input rate of fine dredge-spoil sediment, which amounts to 121 kg s1 over a 2.3 d disposal campaign for an average year (estimated from data in Cefas, 2018), so that 1 Unit

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31. On sediment dispersal in the Whitsand Bay Marine Conservation Zone: neighbour to a closed dredge-spoil disposal site

is equivalent to 121 g of fine sediment. In the first case considered this input rate is applied continuously until quasi steady-state concentrations are reached. It therefore represents an extreme case of potential spoil-site impact with the MCZ and its shoreline. In the second case, a 2.3-d tracer calculation is undertaken that simulates schematically a dredge-spoil disposal campaign, again with a fine-sediment input rate of 121 kg s1, noting the maximum transient concentrations at PC and PW as an indication of MCZ impact. All of the dredgespoil sediment is released over the same arbitrarily sized 100 m  100 m area of sea surface and the fine sediment (tracer) is mixed throughout the water column. Of interest are the equivalent local concentrations of SPM at the tracer release source point and within the MCZ.

Neaps, springs and mean tides without winds Dredge-spoil disposal was undertaken in the southwest reaches of the reduced spoil site in later years (within the magenta parallelogram in Fig. 31.1B). Continuous release and dispersion of a tracer from a point in the southwest corner of the spoil site (the 37 m deepest point, BL, Fig. 31.1B) shows little or negligible interaction with the eastern MCZ or coastline for mean neap, mean or mean spring tides (Fig. 31.5AeC). This is quantified as negligible (‘N’) concentrations at PC and PW for mean neap and mean tides, whereas concentrations of order 0.01 Units m3 occur at the mean spring tide, which compares with 1.7 Units m3 at the tracer source location, BL (upper three data rows in Table 31.1). The delay times incurred for an effect to be registered at PC from start-up of the tracer release decrease with increasing tidal range, between approximately 3 de1 d, and between 8 d and 2 d at PW (Table 31.1). Delay times of 4e8 d are similarly incurred for tracer transport between sites DP and TM (Fig. 31.1B) and the MCZ (bottom row in Table 31.1). Similar results for modelled peak transient tracer concentrations due to a source at BL are derived for the schematic 2.3d dredge-disposal simulation (third data column under headings of ‘Polhawn Cove’ and ‘Portwrinkle’ for BL and TR source inputs in Table 31.1). Continuous release and dispersion of a tracer from a point in the north-eastern corner of the older, larger spoil site (the 23 m shallowest point, TR, labelled on the yellow parallelogram in Fig. 31.1B) generally shows strong interaction with the MCZ and coastline for mean neap, mean and mean spring tides (Fig. 31.6AeC). Although concentrations are negligible (denoted by ‘N’ in Table 31.1) at PW during the neap tide, they are substantial for the other tides and at PC for all tides, with concentrations <1 Units m3 that decrease with increasing tidal range (Table 31.1) due to enhanced mixing and advection by tidal currents. The delay times from start-up of the release and dispersion for an effect to be registered at PC and PW are similar to those when the source is located at BL (approximately 1e9 d, Table 31.1). Peak concentrations for the 2.3-d numerical experiments with the same input rates (third data column under headings of ‘Polhawn Cove’ and ‘Portwrinkle’, Table 31.1) tend to be smaller than the final, tidally-averaged values for the long-term tracer inputs at TR, but the reduction is less than a factor of two or three. At the tracer source points BL and TR the tracer-equivalent SPM concentrations are between approximately 2 and 5 Units m3 or w200e600 mg L1. These concentrations compare with predicted depth-averaged concentrations ranging between 400 and 500 mg L1 just after the release of disposed sediment at a

Using modelled tracer to represent SPM

609

FIG. 31.5

Concentrations of a simulated tracer continuously released at a constant rate from location BL at the southwest corner of the closed dredge-spoil site (Fig. 31.1B) and a photograph of Rame Head taken from the Whitsand Bay coastline. Computed concentrations are shown at HW using the 2D model running for continuous mean neap, mean and mean spring tides: (A), Tracer concentrations at HW for mean neap tides; (B), Tracer concentrations at HW for mean tides; (C), Tracer concentrations at HW for mean spring tides; (D), Rame Head.

more off-shore release site (Cefas, 2018) that subsequently reduce to <20 mg L1. At PC and PW the final concentrations are between w40 and 100 mg L1 for a continuous source at TR, which is greater than the peak concentrations computed from the 2.3-d simulations and negligible or of order 1 mg L1 for a continuous source at BL (Table 31.1). For the long-term, continuous inputs, approximate quasi-steady conditions at PC and PW occur 10 d after tracer release for both release points BL and TR (240 h in Fig. 31.7). There is a delay between the start of tracer dispersal and its impact at PC and PW, after which there is a tidally fluctuating but slowly increasing concentration approaching near-quasi-steady conditions after 10 d. The concentrations and effects due to a tracer source at BL (Fig. 31.7A and B)

610 TABLE 31.1

State

31. On sediment dispersal in the Whitsand Bay Marine Conservation Zone: neighbour to a closed dredge-spoil disposal site

Simulated tracer concentrations and start-up delays (i.e. times required for the tracer to first affect the sites) for various constantly repeating mean neap-tide, mean-tide and mean spring-tide tidal states, constant fresh breezes and constant near gales (Beaufort wind-scale speeds of 10 and 15 ms-1, respectively) that are tabulated for the constant-rate input source sites BL and TR (denoted as ‘source’) and for Polhawn Cove and Portwrinkle coastal sites. Concentration (BL input, Units m-3) & start-up delay (days) Polhawn Cove

Portwrinkle

Concentration (TR input, Units m-3) & start-up delay (days)

Tide

Source

Neap Tide

4.4

N

2.6d

N

N

8.1d

N

5.3

0.94

3.1d

0.37

N

8.8d

N

Mean Tide

2.3

N

2.1d

N

N

4.7d

N

3.2

0.72

1.5d

0.38

0.63

5.2d

0.43

Spring Tide

1.7

0.01

1.0d

0.01

0.02

1.6d

0.02

2.6

0.35

0.7d

0.27

0.29

1.6d

0.19

10 ms-1 Wind

Source

N

2.3

0.43

2.1d

0.43

0.51

2.1d

0.65

3.8

0.37

1.9d

0.21

0.38

1.9d

0.23

S

2.4

0

e

0

0

e

0

3.3

0.06

0.5d

0.36

0.13

1.0d

0.29

E

1.8

0

e

0

0

e

0

3.3

0.05

0.5d

0.13

0.05

1.0d

0.07

W

1.8

0

e

0

0

e

0

4.2

0

e

0

0

e

0

15 ms-1 Wind

Source

N

2.5

0.30

1.6d

0.15

0.30

1.6d

0.22

4.0

0.18

1.3d

0.11

0.19

1.3d

0.13

S

2.3

0

e

0

N

1.3d

N

3.4

N

0.5d

0.07

0.02

0.5d

0.11

E

0.7

0

e

0

0

e

0

1.7

0.07

0.5d

0.15

0.10

0.5d

0.14

W

0.6

0

e

0

0

e

0

1.4

0

e

0

0

e

0

NE

0.7

N

1.3d

N

N

1.3d

N

2.0

0.03

0.7d

0.10

0.03

0.7d

0.09

NW

1.1

0

e

0

0

e

0

3.3

0

e

0

0

e

0

SE

1.4

0

e

0

0

e

0

1.6

N

0.7d

N

N

0.7d

N

SW

1.2

0

e

0

0

e

0

2.8

0.04

1.6d

0.27

0

e

0

Polhawn Cove

Portwrinkle

Polhawn Cove

Portwrinkle

Concentration (DP input, Units m ) & start-up delay (days) -3

State Tide

Source

Mean Tide

6.8#

Polhawn Cove 0.52#

4.1d

e

Portwrinkle 0.51#

7.8d

e

Source

Polhawn Cove

Source

Portwrinkle

Polhawn Cove

Source

Portwrinkle

Polhawn Cove

Portwrinkle

Concentration (TM input, Units m ) & start-up delay (days) -3

Source

3.8*

Polhawn Cove

0.51*

3.6d

e

Portwrinkle

0.48*

7.2d

e

All continuous inputs concentration data are averaged over the final simulated tide. The three columns under each heading of ‘Polhawn Cove’ and ‘Portwrinkle’ correspond, respectively, to concentrations during final quasi-steady conditions for continuous, constant-rate inputs, the associated delay time in days before the site is affected by inputs, and the maximum concentrations for 2.3 d constant-rate inputs. Input rates are 1000 Units s-1 and corresponding concentrations are Units m-3. ‘N’ denotes ‘negligible - i.e. the concentration is extremely small. Also tabulated are concentrations corresponding to constant, continuous tracer inputs of 1000 Units s-1 at site DP (Devonport on the Tamar Estuary, Fig. 31.1B) and at site TM in Plymouth Sound near the mouth of the Tamar Estuary (Fig. 31.1B); 2.3 d simulations were not undertaken for DP and TM. Data are presented after 20 days for TM (*) point inputs and after 30 days for DP (#) point inputs.

Using modelled tracer to represent SPM

611

FIG. 31.6 Concentrations of a simulated tracer continuously released at a constant rate from location TR at the northeast corner of the closed dredge-spoil site (Fig. 31.1B) computed at HW using the 2D model running for continuous mean neap, mean and mean spring tides, and a photograph looking west along the Whitsand Bay coastline: (A), Tracer concentrations at HW for mean neap tides; (B), Tracer concentrations at HW for mean tides; (C), Tracer concentrations at HW for mean spring tides; (D), Whitsand Bay, looking west.

are much less pronounced than those for a source at TR (Fig. 31.7C and D). The decreasing concentrations at PC from neaps to springs due to a tracer source at TR illustrate the extra dispersion that occurs during the stronger tides.

Mean tides with winds Considering first some fresh-breeze simulations (Beaufort wind-scale speed of 10 m s1): except for northerly (south-going) fresh breezes, the continuous release and dispersion of a tracer from the southwest corner of the spoil site (BL) during continuously repeating

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31. On sediment dispersal in the Whitsand Bay Marine Conservation Zone: neighbour to a closed dredge-spoil disposal site

FIG. 31.7 Time-dependence of simulated tracer concentrations at Polhawn Cove, PC, and Portwrinkle, PW (Fig. 31.1B) due to continuous and constant input rates of tracer at the southwest and northeast corners of the closed dredge-spoil disposal site (BL and TR), starting at zero time; (A), Polhawn Cove concentration due to a tracer source at BL; (B), Portwrinkle concentration due to a tracer source at BL; (C), Polhawn Cove concentration due to a tracer source at TR; (D), Portwrinkle concentration due to a tracer source at TR. Only spring tides affect these coastal sites for inputs at BL, although concentrations are very small, whereas all tides affect concentrations at Polhawn Cove for inputs at TR and all except neap tides at Portwrinkle.

3.45-m tidal range mean tides and constant 10 m s1 winds shows almost no interaction with the MCZ or coastline in the eastern Bay. For both a continuous source and a 2.3-d source at BL there are zero concentrations at PC and PW except for northerly winds (Table 31.1), when the final quasi-steady mean and peak 2.3-d transient concentrations there are approximately 0.5 Units m3 (60 mg L1) at the coast and approximately 2 Units m3 at BL (w200 mg L1). For a continuous source at TR the final quasi-steady mean concentrations at PC and PW are less than approximately 0.13 Units m3 (<16 mg L1) except for a northerly wind, when the concentrations are approximately 0.4 Units m3 (50 mg L1, Table 31.1). For a 2.3-d source at TR the peak transient concentrations at PC and PW maximise at less than 0.4 Units m3 for a southerly breeze (50 mg L1, Table 31.1). For illustration, 97% of hourly averaged wind speeds were less than 10 m s1 during 2006. These indicative SPM concentrations in mg L1 are upper bounds because they assume that all the disposed fine sediment remains in suspension. Similar results apply to continuous and 2.3-d source calculations for 15 m s1 near-gales, in particular the generally dominating importance of northerly winds for source inputs at both BL and TR; northerly winds enhance the Rame eddy (Fig. 31.2AeC) and lead to final quasisteady mean or peak 2.3-d transient concentrations at PC and PW of 0.3 Units m3 (40 mg L1) or less and concentrations that are zero or of the order of (or less than) 0.1 Units m3 (12 mg L1) for all other near-gale winds (Table 31.1 and Figs. 31.8AeD and 31.9AeD). An exception is the effect at PC of a southwesterly near-gale for a 2.3-d source at TR, where the peak transient concentration reaches 0.27 Units m3 (30 mg L1; Table 31.1).

3D particle dispersal in the bay

613

FIG. 31.8 Concentrations of a simulated tracer continuously released at a constant rate from location BL in the southwest corner of the closed dredge-spoil disposal site (Fig. 31.1B) for near-gale constant winds of 15 m s1 and mean tides, computed using the 2D model and shown for HW conditions: (A), Tracer concentrations at HW for a northerly (south-going) near gale; (B), Tracer concentrations at HW for a westerly (east-going) near gale; (C), Tracer concentrations at HW for an easterly (west-going) near gale; (D), Tracer concentrations at HW for a southerly (northgoing) near gale.

3D particle dispersal in the bay Three dimensional (3D) hydrodynamic models provide a more realistic representation of the currents within the Bay than can be achieved with a 2D (depth-averaged) model. In order to illustrate the possible dispersion of fine sediment discharged to the Bay at locations within the closed dredge-disposal site, a 3D hydrodynamic model (FVCOM) has been run for the winds and tides corresponding to November and December 2006 and these outputs used to drive a particle dispersal model. FVCOM is a finite-volume unstructured grid model. It

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31. On sediment dispersal in the Whitsand Bay Marine Conservation Zone: neighbour to a closed dredge-spoil disposal site

FIG. 31.9 Concentrations of a simulated tracer continuously released at a constant rate from location TR in the north-east corner of the closed dredge-spoil disposal site (Fig. 31.1B) for near-gale constant winds of 15 m s1 and mean tides, computed using the 2D model and shown for HW conditions: (A), Tracer concentrations at HW for a northerly (south-going) near gale; (B), Tracer concentrations at HW for a westerly (east-going) near gale; (C), Tracer concentrations at HW for an easterly (west-going) near gale; (D), Tracer concentrations at HW for a southerly (northgoing) near gale.

is particularly valuable for applications to coastal seas and estuaries (e.g. Torres and Uncles, 2011). It uses a finite-volume method to discretise the equations for continuity, momentum, salinity, temperature and other variables. The methods used ensure that the conservation laws are obeyed, both within individual control volumes and over the whole modelled domain (e.g. Chen et al., 2003, 2006). The output water levels, 3D currents and turbulence coefficients derived from the application of FVCOM to the Bay during November and December 2006 have been utilised to drive the movements of neutrally buoyant particles introduced to the Bay at sites BL and TR (Fig. 31.1B). Although particle-tracking computer

3D particle dispersal in the bay

615

models have been used for many years to study both the dispersion of neutrally-buoyant materials and particles of suspended sediment (e.g. Kelsey et al., 1994), their application has become increasingly useful with the rapid advances in computer technology and now efficiently allows simultaneous simulation of many thousands of particles.

November 2006 simulations e light winds, large tides Winds speeds were low during 4e11 November 2006 and generally substantially less than 5 m s1. It was a time of large spring tides (5.1 m tidal range on 6 November). Particles were released into the water surface at location BL (Fig. 31.1B) at 02:00 (HW) on 4 November and thereafter at every subsequent LW and HW until 10:00 (LW) on 6 November, simulating schematically 2.3 days of particle inputs. Particle distributions at 14:00 of 4 November (HW), 04:00 of 6 November (HW), and 17:00 of 7 November (HW) illustrate the dispersion of particles into the Bay and its MCZ, their drift to the west, and their interaction with the coastline (red particles in the uppermost panel of Fig. 31.10). Large numbers of released

FIG. 31.10 Particle positions at selected high water times for the November release scenario. Particles were released from disposal site locations (Fig. 31.1B) BL (top two rows) and TR (bottom two rows). Particles released at the surface are shown in red; particles released near the bed (2 m above the bed) are shown in yellow.

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31. On sediment dispersal in the Whitsand Bay Marine Conservation Zone: neighbour to a closed dredge-spoil disposal site

particles were transported westward, close to the Bay’s mouth at its boundary with the English Channel (corresponding to the majority of tracer material illustrated in Fig. 31.5C). A similar dispersion pattern occurred for the near-bed release of particles (yellow particles in the second panel down of Fig. 31.10), except that the interaction of particles with the coastline and the MCZ within the eastern Bay was much more pronounced. Repeating these simulations for particle inputs at TR (bottom two panels in Fig. 31.10) demonstrated that the MCZ and coastline of the eastern Bay are greatly impacted in the short term by dispersing particles from both surface and near-bed releases at TR, which is also a feature of the continuous tracer release at TR during zero-wind spring tides (Fig. 31.6C). It is noted that similar dispersal patterns result from a single HW or LW release of surface and near-bed particles at BL or TR (not shown). Under these environmental conditions of strong tides and light winds a release of neutrally buoyant particles within the closed dredge-spoil disposal site can be expected to lead to the dispersal of particles into the MCZ and to their interaction with the coast. In the eastern Bay the effect is much stronger for particle releases at TR, owing to its closer proximity to the coast and shallower waters.

December 2006 simulations e strong winds, large tides Winds were generally strong and typically from the south or southwest during 2e6 December 2006, usually greater than 10 m s1 (fresh to strong breeze) and reaching more than 19 m s1 (gale force) on 3 December. It was again a time of spring tides (4.4 m tidal range on 5 December). Particles were released into the water surface at location BL (Fig. 31.1B) at 14:00 (HW) on 2 December and thereafter every subsequent LW and HW until 22:00 (LW) on 4 December, simulating schematically 2.3 days of particle inputs. Particle distributions at 02:00 of 3 December (HW), 16:00 of 4 December (HW), and 05:00 of 6 December (HW) illustrate the dispersion of particles (red particles in the uppermost panel of Fig. 31.11) to the southeast and their overall drift to the east (corresponding to the majority of dispersing tracer material illustrated in Fig. 31.8B). Particles had largely left the region by 05:00 of 6 December (Fig. 31.11). A similar dispersion pattern occurred for the near-bed release of particles (yellow particles in the second panel down of Fig. 31.11), except that the southerly (north-going) component of gale-force winds at 02:00 of 3 December had transported the majority of near-bed particles to the south into deeper English Channel waters (a wind-driven, nearbed mechanism described for this area by Uncles et al., 2015). Repeating these simulations for particle inputs at TR (bottom two panels in Fig. 31.11) demonstrated very similar behaviour to those for BL, showing that the Bay, its coastline and MCZ, are largely protected from particle intrusion by strong south and south-westerly winds. Again, similar dispersal patterns resulted from a single HW or LW release of surface and near-bed particles. Under these conditions of spring tides and strong south and southwesterly winds a release of neutrally buoyant particles can be expected to lead to seaward flushing of the particles to the east and southeast. If the near-bed particle releases in the disposal area during these conditions were interpreted as resuspended sediment due to wind waves and tidal currents, then offshore sediment transport would be anticipated.

3D particle dispersal in the bay

617

FIG. 31.11 Particle positions at selected high water times for the December release scenario. Particles were released from disposal site locations (Fig. 31.1B) BL (top two rows) and TR (bottom two rows). Particles released at the surface are shown in red; particles released near the bed (2 m above the bed) are shown in yellow.

November 2006 LW-release simulations e light winds, large tides The disposal of dredge-spoil sediment to the Bay was most likely to have occurred close to LW when tidal currents were directed away from Rame Head and the neighbouring coastline. A 2.3-d simulation of disposal at BL every LW during strong tides shows little interaction with the coast in the eastern Bay for surface particle releases (top panel, Fig. 31.12). Interaction with the coast and the MCZ is much more pronounced for near-bed particle releases (second-down panel, Fig. 31.12). For particle releases at TR there is considerable particle intrusion into the MCZ, both for surface and near-bed releases (bottom two panels, Fig. 31.12). The LW particle-release distributions during the December 2006 large tide, strong wind conditions show essentially the same behaviour as the combined HW-LW particlerelease distributions for the same conditions (Fig. 31.11) due to the dominating, Bayflushing effects of these strong winds.

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31. On sediment dispersal in the Whitsand Bay Marine Conservation Zone: neighbour to a closed dredge-spoil disposal site

FIG. 31.12 Particle positions at selected high-water times for the November low water release scenario. Particles were released from the BL (top two rows) and TR (bottom two rows) sites. Particles released at the surface are shown in red and particles released near the bed (2 m above the bed) are shown in yellow.

Deposited dredge-spoil sediment and bedload transport Approximately 38% of the disposed dredge-spoil sediment comprised sand and gravel (Cefas, 2018) and this material would have been deposited to the seabed and initially overlain the in-situ sediment. Sediment sizes measured from a grab-sample survey (Uncles et al., 2015) demonstrate that the in-situ sediment grain diameters within the MCZ generally are >100 mm (Fig. 31.4C). However, only two grab-sample sites were located within the dredge-spoil site so that the spatial variability of bed sediment within the site cannot be determined (Fig. 31.4C). Smaller sediment sizes occur to the south of the disposal site (Fig. 31.4C and D) where depths exceed 40 m and a large but well-defined zone possessing substantial amounts of silt and clay exists (Fig. 31.4D). The placement of this zone cannot readily be ascribed directly to the physical accumulation of sediment leaving the Tamar system in its freshwater discharges and, although speculative, may be enhanced by off-shore transport

Deposited dredge-spoil sediment and bedload transport

619

of fine suspended sediment away from the spoil site (evident for near-bed released neutrally buoyant particles during strong southwesterly winds and spring tides in Fig. 31.11) and its subsequent deposition. Tidal currents and waves produce shearing stresses on the seabed that have an important influence on sediment transport and sediment grain size distributions. Several mechanisms for transport are considered here.

Tidal currents The fastest tidal current constituents generate higher frequency components of the tide through various hydrodynamic mechanisms; e.g. the dominant tide in the English Channel is M2 (with period 12.4 h) and this will generate an M4 tide (with period 6.2 h) and possibly higher overtides and a residual (tidally averaged) current (Sinha and Pingree, 1997). The M4 tide and residual current are frequently much smaller than the M2 current and therefore have little effect on the magnitude of the bed shear stress. However, the direction of the peak tidal stresses is effectively determined by the M4 and residual currents because these introduce a difference between the peak flood and peak ebb current speeds and stresses. There is a remarkable similarity between the direction of peak tidal stresses and the direction of sediment transport paths for sand-sized bottom sediments over the northwest European shelf (Stride, 1973) and over the English Channel region in particular (illustrated in Pingree and Griffiths, 1979). The maximum stress has an important influence on bed sediment types because it determines the sizes of sediment grains that are moved and therefore the sizes of those that remain at a location. At the deepest (37 m) disposal site corner, BL, the 2D-modelled depth-averaged peak tidal current speeds close to LW (modelled currents at LW are shown in Fig. 31.2A) vary from 0.12, 0.20 and 0.31 m s1 for mean neaps, mean tides and mean springs, respectively. At peak tidal current speeds close to HW the currents are directed approximately oppositely to those at LW and are somewhat slower. Depending on wave influences, which alter the effective seabed frictional drag coefficient for tidal currents, the peak bed shear stresses corresponding solely to the peak, approximately west-directed LW tidal current (ignoring wave-induced currents that directly shear the bed) vary from 0.02 Pa at neaps with no waves to 0.24 Pa at springs with 3.0-m high, 9 s waves (0.31 Pa with 4.5-m high, 10 s waves). If these tidal currents were by themselves capable of moving bed-deposited dredge-spoil sediment as bedload, then this sediment would be transported in a direction slightly north of west and would not intrude into the MCZ (Fig. 31.2A). At the shallowest (23 m) disposal site corner, TR, the northwest-directed (into the Bay) peak tidal current speeds close to LW (Fig. 31.2A) range from 0.17, 0.29 and 0.37 m s1 for mean neaps, mean tides and mean springs, respectively. At peak tidal current speeds close to HW the corresponding southeast-directed (out of the Bay) currents are somewhat slower. Ignoring the direct effects of wave-induced currents on the bed, the peak bed shear stresses corresponding solely to the peak LW tidal current vary from 0.04 Pa at neaps with no waves to 0.48 Pa at springs with 3.0-m high, 9-s waves (0.62 Pa at springs with 4.5-m high, 10-s waves). If these tidal currents were by themselves capable of moving bed-deposited dredge-spoil sediment as bedload, then this sediment would be transported in a northwesterly direction and would intrude into the MCZ (Fig. 31.2A).

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31. On sediment dispersal in the Whitsand Bay Marine Conservation Zone: neighbour to a closed dredge-spoil disposal site

The estimated tidal stresses at BL and TR, if sufficiently strong to cause bedload sediment transport of deposited dredge-spoil sediment, are such that preferential sediment transport toward approximately the west at BL and northwest at TR would occur close to LW, which would dominate opposing transport close to HW toward approximately the east at BL and southeast at TR. With wave-affected tidal currents, again ignoring the direct effects of wave-induced currents at the sea bed, 1 mm diameter grains (very coarse sand) and finer would be preferentially moved toward the MCZ at TR during mean spring tides with 3 m waves (Shields’ method in SandCalc software; Soulsby, 1997) and 0.4 mm grains (medium sand and finer) at BL. The local, in-situ sediment is coarser and would not be transported (Fig. 31.4C).

Wave-induced currents The maximum significant wave height measured by Cefas (2005) at station W in the MCZ region (Fig. 31.1B) during 7 December 2004 to 7 January 2005 was 3.0 m, with a periodicity of 9 s. The mean depth at the measuring site was 23 m, so that a 3 m wave is anticipated to be somewhat smaller at BL in 37 m depth and somewhat higher in shallower waters closer to the shore, because one effect of wave shoaling is to increase wave height, particularly just before breaking (Masselink and Hughes, 2003). Nevertheless, the 3.0 m wave will be used here as the observed ‘standard’ high wave, although larger significant wave heights occur. Dhoop and Mason (2018) show from several years of recent measurements in the western-most part of Whitsand Bay (Looe Bay, within the MCZ) that 5 m and 6 m waves have a 1-y and 10-y return period, respectively; they also show that the median storm duration for the area during the observations was 13 h. There are three types of wave-induced current that are relevant to sediment transport as bedload: the oscillatory currents that accompany surface gravity wave oscillations in water level; the steady, long-term (wave averaged) currents that are generated by large amplitude (nonlinear) waves; and the steady, long-term (wave averaged) currents that result from wave breaking, which form the near-shore longshore drift. Oscillatory wave currents At site BL the peak wave currents at the bed vary from 0.03 m s1 for a 1.5-m high, 6-s wave to 0.19 m s1 for a 2.5-m high, 8-s wave to 0.59 m s1 for a 4.5-m high, 10-s wave. The maximum bed shear stresses associated with these peak wave currents during fastest mean-spring tidal currents vary from 0.18 to 0.76e3.5 Pa, respectively. Therefore, stresses on the seabed due to the combined wave currents and tidal currents are considerable during the occurrence of high waves. At BL, with 3-m high, 9-s waves and tidal currents, the bed shear stresses range from 1.2 Pa at mean neap tides to 1.5 Pa at mean spring tides, both of which are capable of moving sediment grains greater than 2-mm diameter (gravel). At site TR, the neap to spring bed shear stresses reach 3.5e4 Pa for 3-m high, 9-s waves, which are capable of moving sediment grains of diameter greater than 4 mm (gravel). In conjunction with LW-dominant tidal currents, and in absence of near-bed, wind-driven currents that could alter the dominant current and sediment transport directions, these stresses are able to preferentially transport sediment approximately west at BL (not toward

Deposited dredge-spoil sediment and bedload transport

621

the MCZ) and approximately northwest at TR (toward the MCZ). However, strong winds are necessary to drive high waves and the associated wind-driven currents near the seabed could modify the directions of preferential transport at TR and BL (as shown in Fig. 31.11) to be either away from, or toward, the MCZ. Stokes drift currents Due to the Stokes drift velocity, nonlinear waves may not only move bed sediment but also preferentially transport it in the direction of wave advance. The nonlinear Stokes drift theory solutions are valid for waters deeper than approximately 0.01 gT2 (Soulsby, 1997), where T is wave period and g acceleration due to gravity. The minimum depths of validity range between approximately 2e10 m for 4e10 s waves, so that the nonlinear Stokes theory is applicable over the dredge spoil disposal site and, in particular, at sites TR and BL. Solutions for the Stokes wave celerity and Stokes drift velocity through the water column are given by Komar (1998). An onshore current exists at the bed that varies with wave period and wave height, together with a compensating offshore flow in the central part of the water column. At the 23-m-deep TR site, a 3-m high wave with 9-s periodicity would drive a 0.04 m s1 current (a 4.5-m high wave with 10-s periodicity would drive a near-bed current of 0.1 m s1), compared with 0.01 m s1 at the 37-m deep BL site (0.03 m s1 for a 4.5 m wave). Therefore, a unidirectional Stokes drift current of 0.01 m s1 at BL would convey near-bed suspended sediment w0.5 km onshore (w1.5 km for a current of 0.03 m s1) over a 13-h storm duration, which is less than that required to reach the MCZ (2.3 km). At TR, a unidirectional Stokes drift current of 0.04 m s1 would convey near-bed suspended sediment w2 km onshore (w4.5 km for a near-bed current of 0.1 m s1) over a 13-h storm duration, which is greater than that required to reach the MCZ (w0.5 km) or the coast at PC (1.5 km). The other disposal site corners lie between these extremes for BL and TR. The Stokes drift current speeds with 3-m high waves are generally much less than peak tidal current speeds, although they are comparable at TR during neap tides. They are also generally less than tidal residual current speeds, although again they are comparable at TR. Longshore drift currents Waves breaking at the shoreline may also preferentially transport sediment due to longshore drift (Komar, 1998; Masselink and Hughes, 2003). Because the dominant wave directions are from the west (270
, 57% occurrence) and the southwest (225
, 24% occurrence) and the shoreline normal (the line perpendicular to the coastline) at the coast is oriented between approximately 175
at PW increasing to approximately 215
at PC, wave refraction and wave breaking at a non-normal angle to the coastline is likely to occur, together with the formation of a longshore drift (Komar, 1998; Masselink and Hughes, 2003). The occurrence of a longshore drift in Whitsand Bay has been noted by Vincent and Osborne (1993), who estimated a drift speed of w0.1 m s1 during their observations. The speed of the longshore drift depends on the square root of the root-mean-square breaker height and the wave angle at its break point (Komar, 1998). As an example (given in Masselink and Hughes, 2003), a breaker height of only 1 m and a wave-direction to shoreline-normal angle of 10
produces a current of 0.63 m s1 at the mid-surf zone position, which in the MCZ would convey sediments and other materials along the coast from PW toward PC and Rame Head (Fig. 31.1B).

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31. On sediment dispersal in the Whitsand Bay Marine Conservation Zone: neighbour to a closed dredge-spoil disposal site

Is the Whitsand Bay MCZ a ‘natural’ habitat? It is of interest to consider whether the MCZ was, at its political creation, a natural system or a system modified to some extent by historical anthropogenic influences. Currently, there is much debate about what a legitimate baseline would be for a Marine Protected Area (MPA) site against which to measure its future condition. It is also of interest to consider how the MCZ may evolve following the relatively recent cessation of near-by dredge-spoil disposal.

Potential anthropogenic influences In 2001 a large ‘spike’ occurred in the sediment volumes extracted from the Tamar Estuary due to capital dredging (Widdows et al., 2007), together with a consequent ‘spike’ in dredgespoil disposal within the Whitsand Bay disposal site. An image of a silt-laden seabed at a site in Whitsand Bay was published in 2002 (Evening Herald, 2002). The same article stated that, according to a local diver with more than 40 years of diving experience in the area, parts of the Bay had been affected by silt deposits where, in earlier times, only sand and gravel had been visually evident. Local experience therefore indicated potential anthropogenic influences, possibly due to spoil disposal, although influences due to the Tamar Estuary discharge into Plymouth Sound or land runoff into Whitsand Bay could not be discounted at that time. Modelling and observational work presented here indicates that the Tamar influence on sediment concentrations is likely to be small. Local land runoff and its associated sediment loads into Whitsand Bay are also likely to be small, unlike those, for example, during rare spate conditions in mountainous, semi-arid coastal regions (Uncles, 2013). An additional anthropogenic influence could have resulted from sewage outfalls that are located within Whitsand Bay (Cefas, 2005), but their effects are again likely to be small away from the immediate vicinity of the outfalls. The modelling work and analysis described here indicates that the location of the dredgespoil (model tracer/particle-release) dispersal source point is critical to the extent of tracer/ particle-release intrusion into the (recently defined) MCZ and its interaction with the coast. Inputs of suspended fine sediment from a source in the southwest corner of the disposal site (BL) have little computed effect on the MCZ, except potentially during northerly winds when an enhanced Rame eddy could transport some of the suspended sediment inshore, especially during larger tides. Modelled tracer and particle-release (proxies for suspended fine-sediment) intrusion into the MCZ region becomes much more pronounced when starting within the disposal site but closer to the shore. Therefore, it is very likely that dredge spoil disposal has influenced the MCZ region over the approximately 100 years of disposal, particularly if, and when, it was undertaken within the near-shore, eastern areas of the enlarged disposal site of earlier times (Fig. 31.1B). The Okada et al. (2009) analysis of seabed sediments confirms the anthropogenic influence, inasmuch as dredge-spoil sediment was found to affect particle-size distributions to a distance of 6 km around the disposal site, which is well inside the MCZ region (Fig. 31.1B). Similarly, at site TR it is predicted that tidal currents will transport some of the smallergrained sediment within the deposited dredge spoil toward the MCZ (as bedload) especially at spring tides and especially in the presence of large waves, which cause sediment

Is the Whitsand Bay MCZ a ‘natural’ habitat?

623

movement and resuspension as well as onshore transport due to the Stokes drift. However, wind-driven currents close to the bed could greatly affect the direction of both bedload transport and near-bed, suspended-load transport for the finer sediments suspended by wave activity (Fig. 31.11). For example, a southerly wind blowing into the Bay, or a wind with a strong southerly component, will lead to a near-bed residual flow of water out of the Bay and into deeper English Channel waters (Uncles et al., 2015 and illustrated in Fig. 31.11). Therefore, a potential near-bed transport of suspended sediments and bedload transport out of the Bay could occur when wave-induced bed stresses dominate tidal stresses, such that the threshold of sediment motion is exceeded and sediment resuspension occurs throughout a tide over a median 13-h storm duration period (Dhoop and Mason, 2018). At site BL it is predicted that bedload and suspended sediment transport will occur in the presence of waves, but bedload transport is unlikely to intrude into the MCZ and significant suspended sediment intrusion is only likely at a time of northerly winds during the disposal operation itself, due to the small fetch and reduced wave activity and sediment resuspension associated with northerly winds. It is very likely that over approximately 100 years of disposal, suitable combinations of large tides, waves and wind-driven currents have led to the intrusion of suspended and deposited dredge spoil sediments into the MCZ region, as indicated by the measurements of Okada et al. (2009), as well as offshore dredge-spoil sediment transport into deeper waters (due to near-bed wind-driven currents) where longer-term deposition is more likely. If intrusion occurred, it is very unlikely that the relatively small-grained sediment associated with the dredge spoil remained permanently within the MCZ region. The importance of waves and wave-induced sediment resuspension in shallow MCZ waters likely led to the continuing transport of the finer sediment until it was flushed out of the Bay and into deeper English Channel waters. Looking forward in time, it is probable that both the disposal-site sediment composition and that of the MCZ will change to some small extent due to potential bedload transport of non-cohesive dredge-spoil sediment or resuspension of compacted cohesive sediment from the disused site into the MCZ and the wider English Channel area. In the long term this bed supply will either be exhausted or immovable (the site was originally used for ordnance and it is possible that other non-sedimentary material was inadvertently deposited in earlier times). Silt and clay-sized sediment currently forms <5% of the MCZ surficial bed sediments (Fig. 31.4D) but it is not known what proportion of this comprises dredge-spoil fine sediment; clearly some does, as demonstrated by Okada et al. (2009). The potential evolution of this sediment grain-size distribution is perhaps one item of relevance to future monitoring and management of the Whitsand Bay MCZ.

An indication of dredge-spoil movement from sediment grab samples According to Okada et al. (2009), a modal grain size of w40 mm provides a robust signature of dredge-spoil sediment. This modal size has been searched-for in a laser-diffraction study to determine the granulometry of the fine-sediment component (<63 mm, e.g. Spencer, 2015) of the grab samples collected during 8 March 2005 (Fig. 31.4C and D). Okada et al. (2009) did not specify upper or lower bounds on their w40 mm estimate of dredge-spoil

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31. On sediment dispersal in the Whitsand Bay Marine Conservation Zone: neighbour to a closed dredge-spoil disposal site

modal grain size; therefore, we arbitrarily take an observed modal size range for our laserdiffraction data of between 35 and 45 mm to represent a strong indication of the presence of dredge-spoil sediment (in bold in Table 31.2 and as heavy green tick marks next to stations in Fig. 31.13A and B) and an observed modal size range for our data of between 30 and 35 mm and 45 and 50 mm to represent a weaker indication of the presence of dredge-spoil sediment (in bold in Table 31.2 and as light green tick marks next to stations in Fig. 31.13A and B). Near-shore stations had negligible quantities of fine sediments in the 8 March 2005 grab samples (highlighted by circles around the stations in Fig. 31.13B). Strong indications of dredge-spoil sediment presence occurred within and to the west of the dredge-spoil site, and strong and weaker indications of dredge-spoil sediment presence occurred in a long east-west swathe to the south of the dredge-spoil site (Fig. 31.13A and B). Weaker indications of dredge-spoil sediment presence occurred northwest of the disposal site within the MCZ (Fig. 31.13A and B). These data imply that the majority of dredge-spoil sediment deposited within the disposal site has eventually been transported offshore. The most likely cause for this implied transport can be attributed to strong southerly and southwesterly winds, or winds with a strong southerly component, during times of spring tides. Strong southerly winds generate high waves and cause an offshore-directed, near-bed residual flow of water out of the Bay that is particularly marked at spring tides (modelled and depicted in Fig. 6(k) of Uncles et al., 2015 and illustrated for modelled near-bed particles in Fig. 31.11). These mechanisms appear to have resulted in the long-term, near-bed suspended and bedload transport of dredge-spoil sediment out of the disposal site and into deeper areas.

Conclusions This article has presented some ‘thought experiment’ modelling results and interpretations of data and theory in order to investigate the possibility that Whitsand Bay, and its recently (2013) designated MCZ, might have been affected in the past, and may be affected in the future, by the intrusion of dredge-spoil sediments from the now-closed disposal site located close to the seaward boundary of the MCZ and by outflows of suspended sediment and low salinity waters from the adjacent Tamar Estuary and Plymouth Sound. The schematic modelling work has utilised both depth-averaged and three-dimensional hydrodynamics and has not attempted to emulate actual dredge-spoil disposal protocols; as such, these calculations are considered to provide indicators and not predictors of actual events. Observations and modelling show that an important transport path exists between the Tamar system and Plymouth Sound and the MCZ, although the component of Tamar waters present within the MCZ is computed to be small (<10%). The corresponding computed SPM levels due to Tamar waters are of order 1 mg L1, which is substantially less than the local, insitu SPM concentrations and, depending on the magnitude of any inherent contamination of the imported particles, is unlikely to be influential. Considering the dispersion of suspended fine-grained dredge-spoil sediment during its disposal, or due to subsequent resuspension of fine sediment from the spoil-site seabed, model calculations show that the location of the dredge-spoil (model tracer/particlerelease) source point is crucially important to the intrusion of suspended sediment (modelled tracer/particles) within the MCZ and its interaction with the coast.

625

Conclusions

TABLE 31.2

Grain-size distributions of Whitsand Bay surficial sediments derived from laser diffraction measurements of the fine-sediment (<63 mm) fraction obtained in grab samples on 8 March 2005 at stations illustrated in Fig. 31.13.

Station

Lat. 50 N decimal minutes

Long. 4W decimal minutes

% Fines (<63 mm)

Mode (mm)

Mean (mm)

D10 (mm)

D50 (mm)

X1

17.00

20.00

5

7

29

1

10

X2

17.00

19.00

13

9

18

1

9

X3

17.00

18.00

20

7

20

1

8

X4

17.00

17.00

12

5

18

1

7

X5

17.00

16.00

13

14

23

1

10

X6

17.00

15.00

17

6

18

1

9

X7

17.00

14.00

15

6

16

1

8

X8

17.00

13.00

22

6

14

1

7

X9

17.00

12.00

11

7

16

2

9

3

18.00

13.00

9

50

29

3

22

4

18.00

14.00

15

35

21

2

13

5

18.00

15.00

64

42

23

2

16

6

18.00

16.00

78

42

27

3

17

7

18.00

17.00

28

42

20

2

12

8

18.00

18.00

11

46

24

2

15

9

18.00

19.00

8

8

20

2

11

10

18.00

20.00

5

8

16

2

9

2

18.03

12.07

19

42

21

2

13

17

18.84

14.19

0.3

8

23

2

12

11

19.00

20.00

2

9

18

2

10

12

19.00

19.00

13

24

17

2

10

13

19.00

18.00

16

38

26

2

19

14

19.00

17.00

10

7

21

2

8

15

19.00

16.00

5

42

22

2

12

16

19.00

15.00

0.2

29

39

2

17

18

19.50

14.30

0.1

NFS

NFS

NFS

NFS

19

20.02

15.00

0.1

NFS

NFS

NFS

NFS

20

20.02

16.00

0.1

NFS

NFS

NFS

NFS (Continued)

626

31. On sediment dispersal in the Whitsand Bay Marine Conservation Zone: neighbour to a closed dredge-spoil disposal site

TABLE 31.2

Grain-size distributions of Whitsand Bay surficial sediments derived from laser diffraction measurements of the fine-sediment (<63 mm) fraction obtained in grab samples on 8 March 2005 at stations illustrated in Fig. 31.13.dcont'd

Station

Lat. 50 N decimal minutes

Long. 4W decimal minutes

% Fines (<63 mm)

Mode (mm)

Mean (mm)

D10 (mm)

D50 (mm)

21

20.02

17.00

10

50

21

2

11

22

20.02

18.00

1

8

19

2

10

23

20.02

19.00

4

24

18

2

10

24

20.02

20.00

1

26

21

2

13

25

21.03

20.00

1

50

25

2

15

26

21.03

19.00

1

12

32

3

15

27

21.03

18.00

0.4

NFS

NFS

NFS

NFS

29

20.56

16.00

0.1

NFS

NFS

NFS

NFS

28

20.57

17.00

0.1

NFS

NFS

NFS

NFS

Station names (not used elsewhere in this article) and their locations (indicated in Fig. 31.13) are tabulated, together with the percentage weight of fine sediment obtained per grab sample (% Fines) and, in microns, the modal (Mode), mean (Mean), D10 (diameter for which 10% of the fine sediment by volume comprises smaller particles) and D50 (diameter for which 50% of the fine sediment by volume comprises smaller particles; i.e. the median size). Mode sizes between 30 and 50 mm are shown in bold. The occurrence of negligible fine sediment in a grab sample is denoted by ‘NFS’.

FIG. 31.13 Median grain sizes of the fine-sediment component of surficial sediment samples and the percentage mass of fine sediment in a sample: (A), Median grain-sizes, D50, in microns (plotted as log10(D50)); stations are highlighted that have a fine-sediment component modal grain size in the range 30e35 and 45e50 mm, plotted as light green tick marks, and 35e45 mm, plotted as heavy green tick marks; (B), Percentage weight of dried sediment having grain sizes less than very fine sand (<63 mm, i.e. the silt and clay fine-sediment fraction). ND ¼ no data. Illustrations (A)e(B) are equivalent to Fig. 31.4C and D with additional information on mode grain sizes (green tick marks) and the location of stations with negligible fine sediment in their samples (circled stations in (B)). Illustrations (AeB) are reproduced with minor modifications and additional data from Uncles, R.J., Stephens, J.A., Harris, C., 2015. Physical processes in a coupled bay-estuary coastal system: Whitsand Bay and Plymouth Sound. Progress in Oceanography 137, 360e384, with permission from Elsevier.

Conclusions

627

With the exception of dispersion during northerly winds, which enhance the Rame eddy, the inputs from a source in the southwest corner of the disposal site (BL) have little computed effect on the MCZ. However, these northerly winds have a short fetch over Whitsand Bay and are unlikely to generate waves capable of causing resuspension of deposited dredgespoil sediment at BL, so that a combination of large tides (for resuspension of fine-grained sediment - unless during the disposal operation itself) and northerly winds would be necessary to cause intrusion of suspended sediment into the MCZ. Disposal at the northeast corner site (TR) which might, but is not known, to have occurred historically when the disposal area was larger and extended closer to shore, has a more generalised effect on the MCZ and coastline. Northerly winds remain an important wind influence for dispersion from TR into the MCZ, but other winds can also drive suspended finesediment transport into the MCZ. Both modelled tracer and particle-release simulations show that the tidal dispersion of suspended fine sediment from TR is particularly important, even in the absence of winds, and strongly influences the MCZ and its coastline. Modelled peak tidal currents within the dredge-spoil site occur close to LW and are directed approximately westward at BL and approximately northwestward toward the MCZ at TR. Modelled peak currents close to HW are somewhat slower and directed oppositely. Ignoring the direct effects of wave-induced currents at the sea bed (i.e. taking into account only the effect of waves on the seabed tidal-current frictional drag), wave-influenced peak tidal currents during high (3 m) waves and at spring tides are computed to be capable of moving 1 mm grain-sized bed sediment at TR (0.2 mm sediment without wave-affected drag) and 0.4 mm sediment at BL (no transport without wave-affected drag). Bed shear stresses are much higher when the direct effects of wave currents at the seabed are taken into account and are computed to be capable of moving grains larger than 2 mm at BL and 4 mm at TR. In the absence of wind-driven currents, preferential transport toward the MCZ from TR is likely to occur due to modelled shoreward-dominant tidal currents. However, wind-driven near-bed currents will be associated with fast winds and high waves, especially from the south and southwest, and these currents will tend to transport dredge-spoil sediment seaward as bedload and near-bed suspended load (as shown in Fig. 31.11). Nonlinear currents due to the Stokes drift associated with waves will tend to drive nearbed sediment shoreward and toward the MCZ at TR, where they are comparable with tidal residual and wind-driven currents for the highest waves, although they are much slower at BL and unlikely to contribute significantly to onshore transport there. If sediment and other materials are moved inshore to the wave breaker zone, then potentially they will be transported by longshore currents and conveyed west to east along the coast, in the direction of PW to PC and Rame Head. Because dredge-disposal sediment has a small modal grain size of w40 mm it is easily moved by spring tidal currents at TR, even without waves, and by high waves at BL, even without tidal currents. High waves at BL will generally be associated with southerly or southwesterly winds, which drive an offshore flow of near-bed water and, potentially, an offshore transport of near-bed suspended sediment. This scenario is apparent in the offshore presence of dredge-spoil sediment, based on its w40-mm grain-size signature, in deeper English Channel waters. Finally, the schematic nature of these model calculations is again emphasized; accurate calculations of transport and concentrations for realistic conditions would require the application of actual dredge-spoil disposal protocols.

628

31. On sediment dispersal in the Whitsand Bay Marine Conservation Zone: neighbour to a closed dredge-spoil disposal site

Acknowledgements James Clark acknowledges support from the Natural Environment Research Council, UK (NERC grant NE/L007010 and the NERC LTSS National Capability Program that underpins numerical modelling work at PML). Ricardo Torres and Michael Bedington acknowledge support from the NERC National Capability LOCATE project and the EU Interreg Atlantic Area Program project MYCOAST (EAPA_285/2016). We thank Pierre Cazanave for preparing the meteorological data used in the 3D hydrodynamic and particle-dispersal models. Reg Uncles thanks Professor Steve de Mora, Chief Executive of the Plymouth Marine Laboratory, for the award of a Senior Research Fellowship; he also thanks Mr Dave Peake for discussions on the dredge-spoil issues affecting Whitsand Bay.

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