ESTUARINE COASTAL

A N D

SHELF SCIENCE1151-1158 -------------------------------------------

A novel shear vane used to determine the evolution of hydraulic dredge tracks

in sub-tidal marine sediments

C. H autona’*. D.M . PatersonbaUniversity M arine Biological Sta tion M illport, Isle o f Cumbrae, Scotland K A 28 OEG, U K

bG atty M arine Laboratory, University o f S t Andrews, S t Andrews, Fife, Scotland K Y 16 8L B , UK

Received 3 September 2002; accepted 27 January 2003

Available o n line at www.i

Estuarine, Coastal and Shelf Science 57

Abstract

A novel shear vane is described which can be used to record the shear strength o f discrete depth horizons o f a variety o f marine sediment types in situ. This vane, or modifications o f it, has great potential in allowing measurement o f in situ shear strength without requiring any samples to be removed from the sea bed, a process which can often destroy the fine structure o f the sediment. The vane was used to monitor the change in sediment shear strength caused by a hydraulic dredge, which was used to fish razor clams (Ensis arcuatus) in a sheltered bay within the Clyde Sea area, Scotland. D ata collected using this apparatus provided valuable and immediate information on the stratification o f the sea bed post-dredging and allowed the evolution o f the dredge track to be regularly monitored by divers over a period o f 100 d.© 2003 Elsevier Ltd. All rights reserved.

Keywords: shear vane; shear strength; hydraulic dredging; razor clams; Ensis

1 . In tr o d u c tio n

All mobile benthic fishing gears alter the physical characteristics o f sediments and m any o f these environ­m ental effects have been docum ented (Caddy, 1973; Fonteyne, 2000; Haii, Basford, & R obertson, 1990; Kaiser, Ram say, R ichardson, Spence, & Brand, 2000; L indeboom & de G roo t, 1998). In particular, shellfish dredges, such as scallop and cockle dredges, have been shown to abrade natural topographies and suspend fine sediments into the w ater colum n which later settle, blanketing the sea bed at some distance from the initial im pact site (Eleftheriou & R oberston, 1992; H all- Spencer & M oore, 2000; Thrush, Hewitt, Cummings, St. D av tnn 1995V

* Corresponding author. Present address: Gatty Marine Laboratory, University o f St Andrews, St Andrews, Fife, Scotland KY16 8LB,U K .

E-mail address: ch40®,st-andrews.ac.uk (C. Hauton).

0272-7714/03/$ - see front matter © 2003 Elsevier Ltd. A ll rights reserved, d or 10 1 n l'fi/S0272-7714('n3')00055-6

H ydraulic dredges, used to harvest razor clams (Ensis spp.) and su rf clams (Spisula spp.) in Scotland, Italy, Portugal and the USA, have the potential to cause great m odification to the physical structure o f the sea bed. They use blades or w ater jets to penetrate the sea bed to a depth o f over 30 cm and use large quantities o f sea w ater (>3000 lm in -1) to fluidise the substratum (Meyer, C ooper, & Pecci, 1981; Tuck et al., 2000). W hile it has been shown th a t the physical disturbance caused by these dredges recovers w ith time (Haii et al., 1990; Hall & H arding, 1997) concerns have been expressed regard­ing the long-term im pact o f this regular d isruption on the resident biological com m unity (H aii et al., 1990). The predicted im pacts o f sea-bed fluidisation by hy­draulic dredges range from reduced larval settlem ent and recruitm ent (Haii, 1994), causing a reduction in size o f future bivalve populations, to a situation where the sea bed becomes so fluidised tha t large burrow ing bi­valves, which require a certain degree o f sediment com ­paction to anchor themselves, can no longer burrow effectively.

1152 C. H auton. D .M . Paterson / Estuarine. Goastal and S h e lf Science 57 (2003) 1151-1158

Shallow w aters in which the hydraulic fisheries operate are am enable to the use o f scientific divers to m onitor the extent o f any physical d isruption and hab ita t m odification. To date, however, the use o f divers has been limited m ainly to hab ita t m apping and the collection o f faunal and sediment samples for ex situ analysis (Brazier, 1998; E leftheriou & R obertson, 1992; Gam ble, 1984; H aii et al., 1990; Haii, R obertson, Basford, & Heaney, 1993). W hile the recovery o f cores for laboratory analysis is a long established practice, the act o f rem oving a core from the sediment destroys the fine-scale structure and stratification o f the sample being collected (Tolhurst, Riethm üller, & Paterson, 2000). In addition, the act o f rem oving core samples from the sea bed and bringing them to the surface for analysis in­evitably causes sediment decom pression giving rise to erroneous fine-scale structural da ta (Briggs & R ichard­

son, 1996). C ore transpo rta tion can also lead to further conform ational changes (T olhurst et al., 2000).

As a result o f these issues, and in response to a press­ing need to quantify the physical im pact to the sea bed caused by hydraulic dredges, an alternative m eans of recording sediment characteristics in situ was sought. A review o f the literature indicated th a t sediment shear strength, a m easure o f the resistance o f the sediment to torque, could be used as an indicator o f sediment stra ti­fication and com paction and could be used to quantify the physical im pacts o f hydraulic dredges in situ (U nderw ood & Paterson, 1993). A com m on problem , however, was th a t the m ajority o f m ethods used to m easure the shear strength o f soils and sediments relied on instrum ents th a t could no t be subm erged in sea water (U nderw ood & Paterson, 1993). Consequently, an alter­native and m ore appropriate instrum ent was developed.

Q row e io guide w rd l u c u j s d Sop c o i I n r

Locking Screw

I'np C ollar

Base pini«

Sp i neili:

P io 1 O o s s -s e c tio n o f fhr1 rn m n n n i’i iÎ n a r ts o f th e sh ea r v an e : N o t to scale.

C Hauton. D .M . Paterson / Estuarine. Coastal and S h e lf Science 57 ( 2003) 1151-1158 1153

2. Material and methods

2.1. Shear vane design

The design o f the shear vane was constrained by the requirem ent for it to be used underw ater by divers wearing thick (< 5 m m ) neoprene gloves. A simple and robust design was needed to facilitate its assembly and use underw ater. Additionally, an instrum ent th a t could m easure the shear strength o f discrete horizons beneath the surface o f the sea bed was needed to record stra ti­fication with depth. The shear vane was constructed as shown in Figs. 1 and 2. The vane was m ade from m arine-grade stainless steel and the cylindrical top collar and base plate m ounting were m achined from a single bar o f steel, 6 cm in diam eter.

Two vanes were produced to accom m odate sediments o f different shear strength. The spindle length o f both vanes was 30 cm to allow m easurem ents to be taken at

different depth horizons w ithin the sediment. The vanes were held in place by a stainless steel locking screw that was tightened through the top collar against the vane spindle (Fig. 1).

The shear vane was positioned on the sediment and spring balances (Salter A bbey™ spring balances: 0-500, 0-1000, 0-3000, and 0-10000 g) were used to apply a force to the top collar via the a ttached cord, which in tu rn created an angular force on the vane. The m axi­m um force required to shear the sediment was recorded by divers using the scale on the face o f the spring balance. The depth o f the vane was adjusted by re­leasing the locking screw and pushing the vane deeper into the sediment. This allowed a series o f m easure­m ents to be recorded a t discrete horizons w ithin the sea bed. Both vanes were calibrated according to British S tandard BS 1377-7 (1990) using a variety o f natural and artificial sediments including m ud and wet and dry coarse and fine sands (Fig. 3).

Fig. 2. The in situ shear vane as it would be deployed on the sea bed showing the, cord used to apply the rotational force to the top collar.

C. H auton: D .M . Paterson / Estuarine. C oastal and S h e lf Science 57 (2003) 1151-1158

1800.0 -i

1600.0 ■

1400.0 «

1200.0 ■

a looo.o •

800.0 -

600.0 ■

400.0 ■

200.0 -

0.0

15.0 200IShear strength (kPa)

^00 0 -iV ane

3000.0 '

» 2500.0

g 2000.0 '

1500.0

1 0 0 0 .0 -

500.0

0.0 10.0 15.0 20.0 25.0 30.05.0

Shear strength (kPa)

Fig. 3. Calibration curves for the two shear vanes. Vane 1: balance reading = 4 0 .3 9 2 + (32.838 x shear strength); r — 0.791, P < 0.001. Vane 2: balance reading = 397.029 + (8 6 .3 0 9 x shear strength); r = 0 844 P <" 0.001

2.2. Field experiment design

The shear vane was used to m onitor the changes in shear strength during the developm ent o f two dredge tracks created using a hydraulic bivalve dredge (Tuck et al., 2000) as an addition to standard m easurem ents o f particle-size distribution (Buchanan, 1984) and overall dredge track dimensions.

2.3. Field measurements

F our sites were m arked with buoys on a razor clam Ensis arcuatus bed in Lam lash H arb o u r on the Isle o f A rran, Scotland (55.50°N, 05.10°W). The razor-clam bed was located at the northern end o f Lam lash H arbour at a depth o f between 4 and 6 m below chart datum (CD). Two sites were designated as control sites (cl and c2) and two as im pacted sites (il and i2) and all four sites were sufficiently far apart to avoid pseudo­

replication (H urlbert, 1984). Before dredging the two im pacted sites, all four sites were surveyed on four occa­sions during the period O ctober 2000 to M arch 2001. The two im pacted sites were dredged using a single tow o f the U niversity M arine Biological Station (UM BSM ) hydraulic dredge on the 30 April 2001. All four sites were re-visited on four occasions during the period M ay-A ugust 2001 to quantify the extent o f any recovery o f the im pacted sites, relative to the control sites.

O n each occasion, one core sample (0.1 m 2, core depth 20 cm) was collected from each site and stored at - 2 0 °C for particle-size analysis using a M alvern™ M astersizer E laser particle counter (undertaken by Fisheries Research Services, M arine L aboratory A ber­deen). O n each o f the post-dredge surveys divers also used a tape m easure to record the m ean w idth and depth o f the track (five replicate readings for each track).

2.4. Shear vane measurement

The shear vane was used to record the shear strength a t tw o horizons: ‘to p ’ ( - 4 cm) and ‘bo ttom ’ ( - 2 0 cm) for each site. Five replicate m easurem ents o f shear strength were m ade for each horizon a t each site on every visit. Because o f the' time th a t was taken to develop and calibrate the shear vane, it was no t available for use th roughout the study. Shear vane m easurem ents were recorded once before dredging (11 M arch 2001), on the day o f dredging (30 A pril 2001), and then on every post­dredge survey during the period M ay-A ugust 2001.

3. Results

3.1. Sediment granulometry

Sand fractions (2 -+0.063 mm, —1.0—+ + 3 .5 0 ; Bu­chanan, 1984) dom inated the core samples collected at the experim ental site in Lam lash H arbour before and after dredging, constituting approxim ately between 89 and 97% o f the to tal dry weight in each core. There was, however, a significant increase in the proportion o f silt (particles <0.063 mm in diam eter; Buchanan, 1984) in the core samples collected from the im pacted sites after dredging (Table 1, Fig. 4). The dredge suspended a large am ount o f fine sediment into the water colum n, which was no t dispersed by any appreciable current and therefore settled back dow n onto the dredge track, where it rem ained for the rest o f the experim ent (100 d).

Particle-size analysis and classification according to the Phi scale (B uchanan, 1984) allowed cum ulative curves to be plo tted for each o f the 32 core samples (not shown), from which the inclusive graphic standard deviation (ct,) was determ ined according to Folk (1974). The hydraulic dredge reduced the sediment from ‘m oderately sorted’ to ‘poorly sorted’ within the dredge tracks (Table 2).

C. H auton, D .M . Paterson / Estuariae, Coastal and S h e lf Science 57 (2003 J 1151-1158 m s

3.2. Track dimeris inns

The U M BSM dredge left 13.9 cm deep tracks in the sea bed a t the im pacted sites (il and i2). W ave action and b io turbation reduced the depth o f these two tracks to 2.9 cm after nearly 100 d (Fig. 5). By this time, the tracks appeared as shallow furrows in the sea bed and were usually discernible from an accum ulation o f shells with adherent seaweed lying along the axis o f the furrow. The dredge created straight tracks tha t were initially dem arcated by parallel em bankm ents caused by the ‘snow -plough’ effect o f the base o f the dredge and stabilising runners. The m ean w idth o f the dredge tracks between the parallel em bankm ents increased from approxim ately 100 cm im m ediately after dredging, to 115 cm after 100 d (Fig. 6). This small increase was attribu ted to a gradual erosion o f the em bankm ents by wave action, which caused the apices to be flattened and broadened.

3.3. In situ shear strength

The sea bed in Lam lash H arbour was vertically stratified before dredging such that there was a signifi­cant difference (t test, P < 0.01) between the in situ shear strength recorded for the top ( - 4 cm) and the bottom horizon (—20 cm). A t the start o f the study the mean shear strength o f the top horizon was 15.7 kPa while tha t o f the bo ttom horizon was 72.6 kPa. The hydraulic dredge completely hom ogenised this vertical structure, reducing the shear strength to approxim ately 5 k P a to a depth o f a t least - 2 0 cm (Fig. 7). D uring the

Table ISummary o f the two-factor orthogonal A NO VA comparing the percentage o f silt in cores before and after dredging between control and impacted sites

s uurce o f F (versusvariation SS df MS residual) / > = 0.01

Sampling interval (Ti) 0.0063 1 0.0063Sand type (Dr) 0.0282 1 0.0282Interaction (Ti x Dr) 0.0215 1 0.0215Residual 0.0205 28 0.0007

Total n vaM, t i

Balanced A N O V A performed using G M AV5 statistical software (Underwood, 1997). Significantly different groups isolated using an a posteriori Student-N ewm an Keuls analysis (SN K ). Percentage data were arcsine square root transformed before analysis. SS, sum o f squares; MS, mean square; df, degrees o f freedom; s, significant.

post-dredge survey, the shear strength o f the bottom horizon began to increase, although even after 100 d there was no significant difference (i test, P > 0.05) between the shear strength o f the top and bottom horizons a t sites il and i2 (Fig. 7).

4. Discussion

This article has described the design and in situ use of a simple shear vane, which was developed to allow scientific divers to record additional da ta on the m echanical properties o f the sediment w ithin the tracks created by a hydraulic dredge. This shear vane has great potential for m onitoring sedim ent changes in situ w ithout requiring samples to be removed for later

1? n

£

Bet o re

Control Im pacted Control Im pacted

Site

Fig. 4. Increase in percentage silt in the core samples collected from the impacted sites after dredging, showing the mean + SD (n = 8). Lower case letters denote significantly different groups (two factor orthogonal A N O V A and a posteriori Student-N ew m an-K euls multiple comparison ((Underwood, 1997), P < 0.01, arcsine square root transformed data).

1156 C. H auton. D .M Paterson / Estuarine. Coastal and S h e lf Science 57 (2003) 1151—115ft

Table 2Summary o f the inclusive graphic standard deviations (ui) calculated according to Folk (1974) for the sediment cores collected at the experimental site

S a m p l i n g

¡n im a i Site M can' tt \ {^ i SD (®) n ClasniGes Lion

H d b r ï ( l u n tro l DAI IHJki Ü M o d e ra te ly s.Ortfid!m p a u u ! b .Hü Li.114 S M o d era te ly sotildC o n tro l [1.Ï7 0 ,04 S M o d e ra te ly so rted(l r r 1 i 1 J l l i ' | 1 ft, V D n j - i r l i r e i i r l w l

analysis in a laboratory. Indeed, prelim inary studies (data no t shown) dem onstrated th a t the fluidised structure o f the sediment w ithin a dredge track was totally changed by the process o f rem oving a core sample. This ruled out any ex situ analysis o f the sediment from a dredge track using instrum ents such as the G eonor Fall Cone appara tus (M eadow s & M ead­ows, 1991; M eadows, Reichelt, • M eadow s, & W ater- w orth, 1994; M eadow s & T ait, 1985, 1989) or load resistance penetrom eters (M eadows, M urray, M eadows, M uir W ood, & W est, 1998; M urray, M eadows, M ead­ows, W est, & M uir W ood, 2000). M edium-scale changes in the vertical stratification o f the sediment were m easured using the in situ shear vane, providing valuable additional da ta to augm ent routine m easure­m ents o f particle-size analysis. Shear vanes o f similar design have been described previously (Briggs & Richardson, 1996; U nderw ood & Paterson, 1993). The instrum ent described herein, however, represents two advances on earlier designs because it can be used underw ater and it does no t require the opera to r to hold the vane spindle norm al to the sediment surface as the integral base plate supports the spindle. This second feature means th a t the instrum ent is m ore stable and is not as prone to operator error as are previously described submersible torque vane m eters (Briggs & Richardson 1996)

Days post-dredge

0 0 20 0 600 80.0 100 0 120 0400

-4iH I rack il

I rack Í2

20 a ■

Fig. 5. Reduction in depth o f the two dredge tracks created in Lamlash Harbour on 30 April 2001 (mean ± SD. n = 5).

t i n n

g M 0.0

I% 100.0

OO0

70.0, : 0.0 20.0 40.0 60.0 80.0 100.0 120.0

Days post-dredge

Fig. 6. Increase in linear distance between the embankments o f both dredge tracks created in Lamlash Harbour (mean ± SD , n = 5).

Beyond the im m ediate issue o f hydraulic dredge im pacts, the scope for using divers to m ake in situ m easurem ents is lim ited by the depth to which they can safely operate, essentially less than 30 m, as well as by underw ater visibility. W ith m odification, this vane could be used to record shear strength remotely, using the m anipulator arm o f a rem otely operated vehicle (ROV) to provide a calibrated ro tational force on the top collar. Furtherm ore, the length o f the vane could be reduced to allow for stratified sam pling o f different depth horizons to be m ade a t a higher resolution than reported in this study. Clearly, w ith some m odifications, this device could have a wide application as a cheap and robust instrum ent for use in deep waters.

It is clear th a t the U M BSM hydraulic blade dredge significantly altered the physical nature o f the sea bed, creating linear tracks o f fluidised sand as well as suspending a large am ount o f silt in to the w ater colum n which subsequently settled ou t onto the surface o f the sea bed. The dredge rem oved all m easurable sediment stratification and created a vertically hom ogenous bed to a depth o f a t least 20 cm. The tracks created in Lam lash H arbour existed for over twice as long as the 40-d period recorded in a previous study o f hydraulic dredging (Tuck et al., 2000). The persistence o f any dredge track depends upon how exposed a particular site is to wave action, currents ánd storm events. The site used for this w ork was sheltered from the prevailing south­westerly wind direction by hills rising to over 500 m. In addition, the tidal currents across this bed never exceeded 0 .5 m s -1 (A dm iralty C hart # 2126). In earlier studies o f hydraulic dredging, the experim ental sites were located in the m ore exposed sound o f R onay near the island o f G rim say in Scotland (57.45°N, 7.25°W), subject to current speeds in excess o f 1.5m s -1 (Tuck et al., 2000). As a result the dredge track features would have been eroded m ore quickly in the higher energy environm ent around G rim say than in the sheltered bay used in this study. F requent re tu rn visits by dredge fishers to dredge

Track ilTrarV i?

C. H auton , D .M . Paterson / Estuarine , Coastal and S h e lf Science 57 (2003) 1151-1158 1 157

120 0

100 0

800

60.0 -

40.0 -

20.0

00

Site cl

Top horizon Bottom horizon

3/01 4/01 5/01 6/01 a/oi

120 0 Site ilIm pacted sites dredged100.0

800

600

400

20 0'ri'2a 0 0

7/013/01 401 5/01 8/01 9/01

S ite c2160.0

1 20 .0 ■

40.0 -

0.0

20.0 Site i2

80.0 -

60.0 -

40.0

20.0

8/01

D ate

Fig. 7. Changes in the in situ sediment shear strength caused by a single pass o f the UM BSM dredge (mean ± SD , n = 5). Asterisks indicate significant differences between the shear strength o f the top ( - 4 cm) and bottom (—20 cm) horizons (/ tests, P < 0.01).

sheltered sites may have a significant im pact on biological recruitm ent, productivity and ultim ately benthic com ­m unity structure. The persistence and ultim ate conse­quence o f these im pacts in sheltered environm ents, especially in areas regularly fished by hydraulic gear, should be carefully considered by fisheries m anagers and conservationists alike, as an integral p a rt o f any long­term ecosystem m anagem ent strategy.

In sum m ary, the shear vane described here represents a valuable new tool for the analysis o f sediment structure in situ, which will allow detailed quantification o f the im pact o f fishing gears on soft sediments, an area o f research which to date has often been over-looked (R iem ann & Hoffm an, 1991; Palanques, Guillén, & Puig, 2001).

Acknowledgements

This w ork was conducted M iile in receipt o f funding from the E uropean Com m ission D G X IV under contract C F P 99/078 fo r a Study Project entitled ‘Assessments o f the Im pact and Efficiency o f H ydraulic D redging in Scottish and Italian W aters’. We gratefully acknowledge the assistance o f the skipper and crew o f R V Aora and the U M B SM dive team . The assistance o f T. Tolhurst during the design, and M . Parker and W. Sloss during the construction, o f the shear vane apparatus is gratefully acknowledged. The involvement o f P.G. M oore, in subm itting the original project proposal and securing funding for the work, is gratefully acknowl­edged.,

1158 C H n u t on D M Patprsnn I Kxtunrinp C.nnstnl a n d S h p l f Srip.nrp 5 7 (2 0 0 3 ) 1151—1158

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