SUMMARY REVIEWS 409

instrument is usable in the coastal and nearshoreenvironment giving rapid, reproducible data onsediment type and bottom dynamics that can beinterpreted correctly by concurrent field observations(Day grab) and in the context of the knowledge of thestudyarea.

The data obtained in the preliminary attempt toextend the use of the Penetrometer to obtain fieldgeotechnical data are presented in Table 3. Thestations sampled are located along the SW (Station 3)to NE (Station 1) sediment bedload transport path(Shackley & Collins, 1984). When the liquid limit(LL) is plotted versus the plasticity index (PI) on theplasticity chart (Terzaghi et al., 1968), the threestations fall in sequence (1-3) along the A line

(established by Casagrande) and above that line(clays). On this chart Stations 1 and 2 occur in thezone of high plasticity, where slight erosion isexpected (Gibbs, 1962) whilst Station 3 occurs in thezone of medium plasticity and highest resistance toerosion. When comparing these findings with the dataon bottom dynamic type obtained with the Penetro­meter (Table 3) Stations 1 and 2 are categorised asbeing in an area of transportation, and Station 3 as anarea of accumulation.

The similarity between these findings indicates thatthere is a good case for further detailed studies inorder to extend the calibration table for thePenetrometer to include geotechnical data.

References

COLLINS, M. B., C. B. PAlTIARATCHI, F. T.BANNER & G . K. FERENTINOS. 1980. The supply ofsand to Swansea Bay. In (Collins, M. B. et al.; eds.),Industrialised embayments and their enuironmentalproblems ; A case study of Swansea Bay, Pergamon Press.Oxford, 193-213.

HAKANSON, L. & M. JANSSON. 1983. Principles of lakesedimentology, Springer-Verlag, 62-65 .

SHACKLEY, S. E. 1982. The effects of dredged soil

disposal on the sublittoral non-consolidated sediments andbenthic macroinvertebrate infauna of Swansea Bay, SouthWales. Report To the Welsh Office Directorate ofEnvironmental Engineering. November 1982, 131 pp.

-- & M. B. COLLINS. 1984. Variations in sublittoralsediments and their associated macroinfauna in responseto inner shelf processes; Swansea Bay, U.K.Sedimentology , 31, 793-804.

Geophysical assessment of sediment bioturbation in some Welsh estuaries

S. E. Jones and C. F. Jago

School of Ocean Sciences, Marine Science Laboratories, Menai Bridge, Gwynedd, LL595EY

Estuaries are areas of intense human activity andconsiderable scientific interest. Their geologicalsignificance lies in their crucial participation in thesedimentary cycle (since they mark the transition fromcontinental to marine environments) and, in particu­lar, in their role as sediment traps (so that they arefrequently sites of rapid sedimentation). They aretherefore excellent environments in which to studymodem processes in order to construct models ofsedimentation that can aid interpretation of ancientsedimentary environments. As centres for commerceand recreation, estuaries are of obvious concern to thecoastal engineer who frequently has to cope with theconsequences of the same depositional processes thatinterest the sedimentologist . To the engineer , rapidsedimentation is usually a problem. Estuaries are alsobasins of mixing of fresh and salt water and as suchplay a vital part in the flux of materials, includingpollutants, from land to sea . Since pollutants are oftenscavenged by sedimentary particles, the rate ofexchange of material between the estuary and its bed(i.e . the rate of sedimentation) is again important. A

common concern in many estuarine studies istherefore the behaviour and fate of estuarinesediments and , especially, the movement of sedimentsacross the benthic boundary layer.

Organism/sediment interaction

A key contribution to boundary layer processes ismade by organisms. It is probable that such acontribution will be most felt in those environmentswhere high densities of macrobenthic organismsoccur. Estuaries are high stress environments from abiological point of view since the physico-chemicalconditions are so frequently changing ; organisms haveto tolerate large swings of salinity and temperature,and they must frequently survive drastic changes ofsedimentary regime . Rather few species have adaptedto such a fraught ecological niche. Nevertheless thefew that have so adapted often prosper, so that anestuarine fauna consists of enormous numbers ofindividuals of relatively few species. This fauna must

410 SUMMARY REVIEWS

exert a significant influence on boundary layerprocesses.

The macrobenthos can affect the bed in many ways.Their activities influence the transport of bothdissolved and particulate material across thesediment/water interface and can significantly alterthe microbiology and geochemistry of the sedimentsdown to depths of 0.5 m. This bioturbation may takethe form of ventilation of the pore waters by directpumping of the overlying water, and/or sedimentmixing, and/or biodeposition of faecal pellets.Biological effects therefore occur both at the surfaceof, and within, the sediment. Grain size, sorting,packing, permeability, pore pressure and bedroughness can be modified in important, though atpresent poorly understood, ways (Rhoads & Boyer,1982). By altering its fabric, organisms affect thegeotechnical and dynamic properties of the sediment,so that erosion or deposition may be initiated orenhanced. The type and magnitude of these effectsvary with the nature and activities of the organisms.

An additional level of complexity is provided byfeedback from sediments to organisms; the organismsare influenced by the physico-chemical condition ofthe sediments (Aller, 1980). Community structuresduring prolonged quiescent spells may not be thesame as those that become established in theimmediate aftermath of storms (Rhoads & Boyer,1982). Biological effects are frequently seasonal(Grant , Boyer & Sanford 1982); effects in the springand summer (when recruitment and primary produc­tivity are increasing) differ from those in winter (whenmetabolic rates decline) .

Aims and techniques

The primary aims of this work are to establishquantitative criteria for evaluating the effects ofselected infauna on the strength and stability ofestuarine sediments, and to monitor possible peri­odicity in animal/sediment interaction relating toseasonal variations. Three organisms have beenselected because they occur in large numbers in manyestuaries. These are Lanice conchilega (the 'sandmason'), a polychaete worm whose dwelling tubes,consisting of cemented sediment particles, candevelop into an extensive framework within the hostsediment; Arenicola marina (the 'lugworrn'), anotherpolychaete, which by building regular, large If-shapedburrows , causes considerable reworking of thesediment; and Corophium uolutator, an amphipodcrustacean , which also builds Uvshaped burrows buton a smaller scale than those of Arenicola.

The major problem in any study of sediment fabricis that it is a difficult parameter to evaluate in anunconsolidated sediment owing to the impossibility ofobtaining undisturbed samples. However , the re­quired measurements can be made using remote

sensing geophysical techniques (Davis & TaylorSmith, 1980; Richardson & Young, 1981; Cratchley,Davis & Taylor Smith, 1982). In particular, studies ofacoustic shear wave velocity and propagation charac­teristics can yield information relating to the sedimentfabric, including its anisotropy.

G = V;xp

where G is the rigidity modulus, V. is shear wavevelocity and p is bulk density. And electricalresistivity can be related to sediment porosity (TaylorSmith, 1987):

FF = R,edimen.lRpore fluid

FF=n-m

where FF is formation factor, R is resistivity, n isporosity and m is dependent on the sediment fabric.

These measurements can be made in situ withminimal disturbance of the sediment. The shear waveprobe is constructed from piezoelectric ceramicbimorph plates, placed unclamped in a protectivesheet aluminium frame, then potted and sealed inresin (Shirley & Hampton, 1978). The probe transmitsan acoustic pulse through the upper few centimetersof the bed ; the pulse is intercepted by an identicalprobe inserted 5-15 cm from the transmitter, and thereceived information is displayed on an oscilloscope.The resistivity probe consists of four equispacedelectrodes mounted on a perspex frame ; current ispassed between the outer electrodes and the potenialdrop between the inner two is measured.

Measurements have been made in sands in severalWelsh estuaries (Cefni, Conwy, Mawddach, Dwyryd)with emphasis on the Taf estuary in southwest Wales ,where it is known that the sedimentation rate is high(Jago, 1980).

Results

Preliminary measurements, including some in con­trolled laboratory experiments, show that all threespecies produce burrows that significantly alter thegeotechnical properties of the host sediments (to date,measurements have been made in sands) . Burrowingleads to grain bioturbation and hence texturalhomogeneity. But burrows are frequently patchy intheir distribution through the bed, so burrowingdiminishes structural homogeneity. Furthermore,different kinds of burrows can product quite differenteffects . Thus , in general, open burrows decreaseresistivity (and hence increase porosity) and decreaseshear wave velocity (and hence decrease the rigiditymodulus). By contrast, in the case of armouredburrows, while burrowing still decreases resistivity(increasing porosity), it increases shear wave velocity(and hence increases the rigidity modulus). So, forexample , over the range of burrow densities that have

SUMMARY REVIEWS 411

SEDIMENTOLOGICAL- CHARACTERISTICS

I Icomposition texture

tMICROSTRUCTURE

f-- I I -primary biogenicstructures structures

tGEOPHYSICAL AND

GEOTECHNICAL PROPERTIESI I

electrical shear wave f--

resistivity velocity

t .t tporosi y shear strength

tHYDRODYNAMIC

- PROPERTIESI

scour

Fig. 1. Interrelationships between the geophysical , geotech­nical, and hydraulic properties of a sediment bed .

been encountered in the field , the open burrows ofCorophium decrease (by 33%) and the armouredburrows of Lanice increase (by 40%) the strength ofsands. Given the vast numbers of individuals thatcolonise many estuaries, these results are verysignificant.

However, there are complex interactions between

several important variables which prevent simpleconclusions. Thus, for example, for a given porosity,V. (and hence G) is notably different for burrowedand un burrowed sands, but v,. is also dependent onthe carbonate content of the sands. So while burrowedsands can be identified by their acoustic properties,their acoustic signature is dependent on theinteraction of porosity, carbonate content, and burrowdensity. Comparable interactions influence the electri­cal formation factor .

The results also show that the shear wave velocity insands is dependent on the degree of water saturationof the sand. Thus a saturated sand, as it drains on anintertidal bank, can experience an increase of 80% inits rigidity modulus during its transition from fullysaturated to fully drained . So the strength of the sandis sensitive to the position of the water table, and itwill vary during the tidal cycle. Accordingly, the samesand would be capable of supporting varying stressesat different stages of the tide.

Having shown that the biological effect isimportant, the next stage will be to monitor this effectduring a tidal cycle . Burrows alter the porosity andpermeability of a sand and so may control its drainagebehaviour; this could be significant in the light of theabove discussion. Also by altering permeability,burrows modify seepage rates and pore pressures insaturated sands, and may therefore affect the bedresponse to a current-induced boundary shear stress(i.e. the resistance of the bed to scour). Experimentsto investigate such phenomena are underway at thetime of writing. In situ geophysical measurements andnear-bed hydraulic measurements are being madesimultaneously while the bed itself is observed withunderwater television.

Conclusions

Geophysical techniques allow in situ measurement ofgeotechnical properties. These properties are alteredsignificantly by biological activity (such as burrow typeand burrow density) and by water saturation. As aresult, there is a pronounced spatial variability in suchproperties that depends on the nature of theorganisms and the height of the water table. Theremay also be a temporal variability that is both tidaland seasonal. However, there are complex inter­relationships between the geophysical, geotechnical,and hydraulic properties of the bed (see Fig. 1) so thatassessment of biological effects is not simple.

References

ALLER, R. C. 1980. Relationships of tube-dwelling benthoswith sediment and overlying water chemistry . In (Tenore,K. R. & B. C. Coull; eds.), Marine benthic dynamics,Univ. South Carolina Press, Columbia, 285-308.

CRATCHLEY, R. c., A. M. DAVIS & D. TAYLOR

SMITH. 1982. Enhancement of the role of geophysics inmarine geotechnical investigations. Oceanology Interna­tional '112 Conf. Papers. Brighton, 01R24.1.

DAVIS, A. M. & D. TAYLOR SMITH. 1980. Dynamicelastic moduli logging of foundation materials . In (Ardus,

412 SUMMARY REVIEWS

D.; ed.), Offshore site investigations, Graham andTrotman, London.

GRANT, W. D., L. F. BOYER & L. P. SANFORD. 1982.The effect of bioturbation on the initiation of motion ofintertidal sands. J. Mar. Res., 40, 659-77.

JAGO, C. F. 1980. Contemporary accumulation of marinesand in a macrotidal estuary, Southwest Wales. Sedim.Geol., 26, 21-49.

RHOADS, D. C. & L. F. BOYER, 1982. Effects of marinebenthos on physical properties of sediments. A succes­sional perspective. In (McCall, P. L. & M. J. S. Tevesz;

eds.), Animal sediment relations, Plenum PublishingCorporation, 3-52.

RICHARDSON, M. D. & D. K. YOUNG, 1981.Geoacoustic models and bioturbation. Mar. Geol., 38,205-18.

SHIRLEY, D. J. & L. D. HAMPTON. 1978. Shear-wavemeasurements in laboratory sediments. J. Acoust. Soc.Am. 63,607-13.

TAYLOR SMITH, D. 1987. Geotechnical studies inTremadog Bay. Proc. Geol. Ass., 98,385-96.

The application of remote-sensing to theidentification of coastal water and sediment

movement

J. C. Rimmer

Department of Oceanography, University College of Swansea, Singleton Park, Swansea, SA28PP

Remote-sensing, using visible light, has been usedextensively and with considerable success in recentyears by oceanographers and geologists as asupplementary research technique. Many of theoceanographic investigations have concentrated onbiological measurements, using a satellite designedspecifically for phytoplankton chlorophyll identifica­tion. However, a series of highly successful satellitesdesigned for terrestrial measurements, Landsat 1 to 5,have also shown potential for use of monitoringcoastal water and sediment movement.

The space-borne and airborne remote-sensingsystems used in this research application utilize visibleand infrared light from the sun. Light reaching the seais scattered by inorganic particles in suspension (thusincreasing radiance recorded at the sensor), or isabsorbed at specific wavelengths by phytoplankton ororganic matter (reducing the radiance at thesewavelengths). Radiance upwelling from the sea isrecorded in various wavelength bands in digital form.The data can be displayed and analysed on a digitalimage processing system.

An airborne Thematic Mapper (ATM) has beenused in investigations in Swansea Bay, northernBristol Channel. The ATM has wavebands equivalentto those on Landsat 5 (see Williams, 1984) andmethods of data analysis developed for Landsatsatellite data can be applied to airborne data to studysmaller scale features. Data from the ATM have beenused to investigate water quality in the area (Collins &Pattiaratchi, 1984; Rimmer, Collins & Pattiaratchi,1987) and water and sediment movement in thevicinity of an offshore linear sandbank (Pattiaratchi,Hammond & Collins, 1986). Present research isconcentrating on the use of remotely sensed images toidentify transport paths and to estimate currentvelocities, mass water flux, and sediment concentra­tions in the bay. Circulation in Swansea Bay is

complex. An offshore rectilinear current systemseparates into 2 branches to the east of MumblesHead (Ferentinos & Collins, 1980). Water travels intothe Bay south of the outer headlands and forms ananticlockwise semi-rotatory current system (Borthwick& Collins, 1978). There is a dominant south-westerlymovement along the western coast for 9 hours of thetidal cycle when water leaves the Bay between theheadlands. The circulation pattern has been investig­ated using remote-sensing techniques to give a,synoptic view over different tidal states.

Fig. 1 shows water transport paths identified fromATM images over Mumbles Head. The flow featuresare seen on the image as streamlines between watermasses of differing sediment concentrations and hencedifferent spectral signatures. The images reveal theanticlockwise rotatory system developing at themid-flood tidal state (Fig. la). When fully developed(Fig. Ib) the water leaves the Bay between theheadlands. An interesting feature identified from theimages was the formation of an eddy to the south ofthe headland at the start of the ebb phase (Fig. lc),produced when water flowing out from the Bayaround the headland develops vorticity as it flows intodeeper water. The eddy persists for about an houruntil the offshore current reaches too great a velocityfor its maintenance, and the eddy is then broken down(Fig. Id).

It is possible to estimate the velocity of thenear-shore current using a simple algebraic modelpresented by Bohm, Salusti & Travaglioni, (1986).The model was developed for large scale features onsatellite images, where it was found that streamlinestend to follow the isobaths. The model is, however,applicable to small scale investigations with airbornedata if the Rossby number is less than 1. Currentvelocities have been calculated for a series of images,hourly from high water to low water at positions V