Bulk properties of intertidal sediments in a muddy, macrotidal estuary

Marine Geology. 103 (1992) 445-460

445

Elsevier Science Publishers B.V., Amsterdam

Bulk properties of intertidal sediments in a muddy, macrotidal estuary J . A . S t e p h e n s a, R . J . U n c l e s a, M . L . B a r t o n a'b a n d F. F i t z p a t r i c k c

aplymouth Marine Laboratory, Prospect Place, Tlu floe, Plymouth, PLI 3DH, UK bSchool of Civil Engineering, University of Birming . .,, Birmingham, B15 2TT, UK CDeptartment of Geological Sciences, Polytechnic Southwest, Drakes Circus, Plymouth, PL4 8AA, UK (Received December 6, 1990; revision accepted May 16, 1991)

ABSTRACT Stephens, J.A., Uncles, R.J., Barton, M.L. and Fitzpatrick, F., 1992. Bulk properties of intertidal sediments in a muddy, macrotidal estuary. Mar. Geol., 103: 445-460. Measurements are presented of the bulk and mineralogical properties of intertidal sediments along the axis of the Tamar Estuary, southwest England. Bulk and dry density data indicate a significant increase in consolidation of the surface layer of the intertidal mudfiats progressing down-estuary from the turbidity maximum region to the mouth. There are very significant trends of both density and estimated critical erosion shear stress with distance. Cross-sectionally averaged, bed shear stresses due to tidal currents are comput:d using a hydrodynamicai model. These tidal stresses generally increase along the axis of the estuary from mouth to head and reach a maximum in the upper reaches. The increasing consolidation towards the mouth appears to result from the small bed shear stresses due to tidal currents in the lower reaches and the increasing proportion of coarse material there. Bed shear stresses are large in the upper reaches and regular intratidai resuspension and transport occur during spring tides, with little time available for consolidation following deposition during slack-water periods. The silt and clay fraction of the intertidal sediments increases from the mouth to the turbidity maximum region near the head (60 to > 99% dry weight). The particulate organic carbon (POC) content, assuming this to be approximated by loss on ignition, similarly increases from about 2% of dry weight near the mouth to about 8% in the turbidity maximum region. The POC content of a sample is largely dependent on the proportion of fine sediment within the sample, regardless of its position. The lower estuary is associated with intertidal sediments having relatively low silt and clay content and low associated organic material. It appears to be a fairly stable transition zone between the marine and est.uarine environments. The upper estuary, in the turbidity maximum region, appears to be almost homogeneous in both POC (8.| +0.6%) and silt content (95_+5%) during summer. The central region is an area of great variability in the size fraction, sediment type and POC content.

Introduction The complex a r r a y o f physical processes which contribute to the sediment d y n a m i c s in macrotidal estuaries a n d the ability to identify and quantify them are essential for the d e v e l o p m e n t of numerical sediment t r a n s p o r t models which can be used for pollution, ecological and engineering applications. Several studies illustrate that intertidal mudflats can periodically store or release suspended sediment ( K i r b y and Parker, 1983; Anderson, 1983) and m u s t therefore be taken into account in any sediment t r a n s p o r t model. C o n s o l i d a t i o n a n d other properties o f the inter0025-3227/92/$05.00

tidal sediment distributed along the axis o f an estuary are unlikely to be uniform. A p a r t from spatial variability due to local t o p o g r a p h i c a l features, such as meanders, one might expect systematic trends in properties progressing from the head to the mouth. T h e purpose of this p a p e r is to present m e a s u r e m e n t s o f the bulk density, d r y density, moisture content, silt fraction a n d weight loss on ignition (an estimate of particulate organic c a r b o n content) o f the surface layer o f the intertidal cohesive sediments along the axis o f a m u d d y , macrotidal estuary. Mineralogical d a t a are also presented as an aid to interpretation. The critical shear stress for erosion o f bed

© 1992 -- Elsevier Science Publishers B.V. All rights reserved

446

sediment was estimated from the dry density of the sampled sediment (Delo, 1988) as a guide to whether resuspension of intertidal sediment is likely to occur. However, we appreciate that biological and other influences may be very important to sedimem erodibility, although these are currently difficult to quantify. Estimated data on organic carbon are included because of the affinity of organic carbon compounds for both trace metals and trace organic compounds (Lamere Hennessee et al., 1986) as well as their possible significance to cohesive properties. It is known that levels of suspended sediment in some estuaries vary in response to tidal range and runoff and that in the low-salinity reaches of many partly and well-mixed estuaries a Iocalised turbidity maximum occurs (Festa and Hansen, 1978; Allen et al., 1980; Officer, 198 !). Contributions to the causes of this phenomenon have included gravitational circulation and resuspension of bed sediments (Festa and Hansen, 1978; Officer and Nichols, 1980; Officer, 1981; Uncles et al., 1985a,b,c; Uncles and Stephens, 1989). The turbidity maximum is associated with a mobile stock of bed sediment. Seasonal movements of this mobile bed sediment in the Tamar Estuary result in the formation of up-estuary shoals in summer and migration of this material to downestuary, localised depositional sites in winter (Bale et al., 1985). A significant proportion of the material involved in the relocation of these shoals is of estuarine origin and must therefore be resuspended from subtidal and intertidal mud deposits before being transported through the system. Erosion occurs from the surface of bed sediments when the critical shear stress is exceeded (Mehta, 1988). In this paper, the longitudinal distribution of estimated critical shear stress is compared with calculated, tidally-induced shear stresses derived from a cross-sectionally averaged, hydrodynamical model (Uncles and Stephens, 1989). It is possible to identify areas in the upper estuary that are likely to be subject to bed shear stresses which will result in resuspension and hence transport of material. Wave-induced shear stresses are likely to have an influence on the morphology of the mudflats in the lower reaches of the estuary,

J.A. STEPHENS ET AL.

but the magnitude of this influence is currently unknown.

Study site The Tamar Estuary in southwest England, is a macrotidal, partially-mixed system which extends 31 km from its mouth at Plymouth Sound to the limit of tidal influence at the weir near Gunnislake (Fig.l). All positions along the estuary will be given in terms of distances in kilometres from the mouth. The lower reaches of the estuary (0-16 km) exhibit extensive, intertidal mud flats which tend toward gradually increasing bank profiles and softer consistency as one progresses up-estuary. Extensive industrial development along a 6 km stretch of the east bank in the lower estuary (Devonport bank in Fig. l between 0 and 6 kin) precluded this section from being sampled. The middle part of the estuary (i 6-24 kin) exhibits less extensive intertidal mud deposits. These have a more fluid surface consistency with a much increased bank profile. The upper estuary (24-29 kin) is the turbidity maximum region and has a much more riverine appearance with reduced cross-section and steep banks. The mud banks are less prominent and the material is of a variable but fluid consistency which is deposited fairly uniformly across the whole section of the estuary during low freshwater runoff, summer conditions. The upper 2 km of the estuary (29-31 km, in the vicinity of the weir in Fig.l) gives way to steep river cliffs and there is considerably less cohesive material.

Methods Samples of near-surface mud were taken during 18-19 July, 1989, from just above the low-water spring tides line, at roughly l km intervals along the axis of the estuary. Where possible, samples were taken from both banks by pushing a graduated syringe (59 ml, with its end and plunger removed) a few centimetres vertically into the mud until its passage offered resistance or a 50 ml sample was obtained. A hand was then placed under the open lower end and the syringe with-

INTERTIDALSEDIMENTSIN A MUDDYMACROTIDALESTUARY

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Fig. 1. The Tamar estuary, showing its subdivision into 1 km segments, the extent of intertidal mud relative to the level of the lowest astronomical tide and the location of the sampling sites.

drawn. The volume of the sample was recordcc~ and it was then expelled into a plastic bag using the plunger, the bag was sealed and stored in the dark at 4°C. The samples were weighed to obtain the wet weight of the sample a1~.d then well mixed within the confines of the bag to obtain a homogenous medium which was then sub-sampled, weighed and dried at 70°C until no weight change occurred. From these data the dry weight of the whole sample was determined. The combustible organic material content of these samples was estimated by weight loss on ignition in a muffle furnace at 300°C. The dry sediment samples were ground up using a mortar and pestle, dessicated and weighed out into preweighed, pre-fired porcelain crucibles. The samples were fired for 2 h, removed from the furnace and allowed to cool in a desiccator before being weighed to determine the weight of the ashed sediment. The samples were then subjected to a further firing at the same temperature for an additional 12 h to see if further weight loss occurred. A temperature of 300°C was chosen to ensure that only organic material would be ignited and that

any inorganic hydrated material such as clays would be minimally affected. A similar technique was adopted by Manheim et al. (1970) for use on suspended sediment samples and, whilst not stating an ignition temperature, they acknowledge that hydrated minerals other than organic matter (if present in sufficient quantity) could contribute significantly to weight loss when subjected to very high temperatures, a conclusion also reached by Mook and Hoskin (1982). Frankel and Pearce (1973) used temperatures of 500°C for similar determinations, stating that at 550°C calcium carbonate would begin to decompose and that at higher temperatures further inorganic compounds would also be affected. All weights were accurate to 0.00001 g. The samples were then placed in the furnace for a further 12 h at a temperature of 450°C and reweighed in an eff,~rt to determine a relationship between further weight loss and possible contribution from hydrated minerals. A number of additional samples were taken adjacent to those analysed for the parameters described above and these were wet-sieved to determine the percentage of total silt and clay material (<63 ~tm particle

448

size) in the sediment sample. This was achieved by mixing 10-20 g of the sample with 25 ml of "kalgor' deflocculating agent (sodium hexametaphosphate, 6.25 g 1-~) in a 250 ml beaker and making up the resulting mixture to 200 ml with deionised water. The mixture was then placed in a sonic bath fi~r ! h and wet-sieved through a 63 lam test sieve using a mechanical shaker. The fraction retai~ed by the sieve (> 63 ~tm) was quantitatively transferred to a pre-weighed crucible and incubated at 70°C to constant weight, whilst the fraction passing the sieve was made up to 2.51 with distilled water, stirred to uniformly suspend the sediment and three accurate 10 ml aliquots were filtered onto pre-weighed, ashed (450°C for 12 h) glass-fibre filter papers for gravimetric determination of the sediment concentration. These additional samples were also used to establish a general mineralogy profile of the estuafine sediments. This was achieved by subsampling and homogenising the material with "kalgol" and drying a representative droplet onto a slide cover for 20 rain at 30°C. The sample was then covered and stored in a desiccator. The analysis was made using a Phillips 1710 diffractometer coupled with a PW ! 712 X-ray generator. The goniometer was set to scan from 4 ~' to 44 ° using standard Phillips software and both the analogue traces and the d-count were used in the interpretation. Results

Bulk density The degree of consolidation of mud sampled near the low water line of the intertidal zone is reflected in the bulk density data shown in Fig.2a. Bulk density (i.e., mass wet mud/volume wet mud) is plotted against distance from the mouth of the estuary. A least squares regression has been fitted to the data (excluding the two samples nearest the weir) to give a trend line with respect to longitudinal position. The trend line in Fig.2a explains 66% of the variance in the bulk density data and shows that the intertidal mud becomes less dense with increasing distance from the mouth of the estuary. The scatter of points about this line indicates that there

J.A. STEPHENS ET AL.

are large differences in bulk density at both the same location (on opposite banks) and between adjacent stations. In some cases (e.g. at 22 km) the large cross-estuary differences may be due to the fact that the sample station fell on a bend, where there is likely to be net erosion on the outside of the bend and net deposition on the inside of the bend. The mud deposits are clearly not homogeneously distributed and do not show a constant decrease in the degree of consolidation progressing up-estuary. However, the significant trend does imply a general, if sporadic decrease. At the head of the estuary (the weir at 31 kin), where the estuary tak¢s on riverine characteristics, the bulk density increases dramatically (the two samples nearest the head in Fig.2a). This is due to the fact that mud has been scoured from this area and the intertidal deposits are predominantly sand and fine gravel, whereas throughout the rest of the estuary the intertidal deposits comprise chiefly silt and clay-type materials (see later). DO' densiO' The dry densities of the intertidal samples (i.e., mass dry mud/volume wet mud) are shown in Fig.2b as functions of distance along the estuary. The fitted regression line (excluding the two samples nearest the weir) explains 69% o/" the variance in the dry density data and demonstrates a decreasing trend with distance from the mouth, similar to that for the bulk densities (Fig.2a). A regression of dry against bulk densities for all samples confirms a very good linear relationship. This indicates that the sample-averaged density of fundamental particles comprising the intertidal sediment does not vary appreciably throughout the estuary. Moisture content and water voids ratio Moisture content (i.e., mass of water in sample/ mass of dry mud) and water voids ratio (i.e., volume of water in sample/volume of sample) of the intertidal sediments exhibit similar trends when plotted against distance from the mouth of the estuary (Figs. 2c and d). Excluding the two samples closest to the weir, the trend lines for moisture content and voids ratio explain 70 and 40% of the

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Fig.2. Longitudinal distributions and fitted least square linear regression lines for (a) bulk density (b) dry density (c) moisture content and (d) water voids ratio. The points represented by encircled squares are not included in the regression equation.

variance in the data, respectively. These variables can be used as indices to express the degree of consolidation of sedimentary material, together with the bulk and dry density data. The moisture content increases as the bulk density decreases and the water voids ratio increases with increasing moisture content. This again indicates that the sample-averaged density of fundamental particles comprising the intertidal sediment does not vary appreciably throughout the estuary. Estimates of critical shear stress

Critical shear stresses for erosion of the intertidal sediments were estimated as functions of distance along the estuary (Fig.3a) using an empirical relationship derived by Delo, (1988): z~=0.0012p ~'2 (units Pa) where p is dry density (g mi-~). This relationship represents a great simplification of reality. The

erodibility of a cohesive bed is a function of its cohesive nature+ which in turn depends not only on density and water content but also, in a poorly understood way, on salinity, geochem;stry, clay mineralogy and microbiology. A linear trend line of estimated critical shear stress against distance is drawn in Fig.3a. The two samples nearest the weir are excluded. This trend line explains 70% of the variance in the data and implies that the critical shear stress of intertidal sediment, close to the low-water springs line, decreases fairly linearly with distance from the mouth. Minimum values occur in the turbidity maximum region of the upper estuary. These estimated critical shear stresses are compared in Fig.3b with the cross-sectionally averaged, maximum bed shear stresses which are hindcasted to occur under actual conditions of tide and runoff for the same period of time. The data are derived from a cross-sectionally averaged+ hydrodynamical model which has been used to simulate tidal condi-

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Fig.3. (a) Longitudinal distributions of estimated critical shear stress of the intertidal mud with fitted linear regression. Points represented by dots within open squares are not included in the regression equation. (b) Model-predicted maximum spring (continuous line) and minimum neap (dotted line) bed shear stresses due to tidal currents during July, 1989, superimposed on the longitudinal distribution of estimated values for critical shear stress. (c) Predicted excess shear stress for erosion along the estuary during the largest spring tide of July, 1989. Positive values imply that erosion of intertidal sediment is likely to occur. (d) Modelpredicted, bed shear stresses due to tidal currents during the largest ~,,:~:'i,g tide of July, 1989, computed using a drag coefficient of ! x 10-2 (upper line) and i x 10 -3 (lower line), superimposed on the longitudinal distribution of estimated values for critical shear stress.

tions during July, 1989. Bed shear stresses due to surface gravity waves have been ignored. Simulated data in Fig.3b show longitudinal distributions of the maximum, tidally-induced bed shear stresses for the largest spring and smallest neap tide during July, 1989. The modelled bed shear stresses were calculated within the hydrodynamical model (Uncles and Stephens, 1989) using a drag coefficient multiplied by the water density and the square of the calcu-

lated, cross-sectionally averaged current speeds. The modelled equations of cross-sectionally averaged continuity and momentum are given in Uncles and Stephens (1990). A constant drag coefficient, Ca, of 1.6 x l0 -3 was used. This value led to good agreement between modelled and observed elevations and currents along the estuary at both neap and spring tides (Uncles and Stephens, 1990). The choice of CD was made by comparing modelled M2 and M4 tidal constituents

INTERTIDAL SEDIMENTS IN A MUDDY MACROTIDAL ESTUARY

of water level with tide gauge data obtained independently from those data presented in Uncles and Stephens (I 990). Because the bed shear stresses are derived from cross-sectionally averaged currents, it follows that stresses exerted in the deep channel and on the higher margins of the tidal mud banks will be higher and lower, respectively, than those shown in Fig.3b. Maximum shear stresses during a tidal cycle are attained at approximately the mid-tide water level for much of the estuary. At mid tide the sampled sites are covered by a depth of water equal to about half the spring tidal range. If the estuarine cross-sectional shape is approximated by a "V", with the deepest water in the centre, then it is straightforward to estimate the maximum shear stress at the site from the simulated, cross-sectionally averaged, maximum shear stress. In the upper half of the estuary the tidallyinduced bed shear stresses at the sample sites will be similar to simulated data shown in Fig.3b. In the lower half of the estuary, !he maximum tidallyinduced shear stresses at the sample sites wdl be lower than the simulated, cross-sectionally averaged, maximum shear stresses. Comparing the simulated, maximum bed shear stresses exerted by the small neap tide with estimated, critical erosion shear stresses (Fig.3b) shows that neap tides probably cause little resuspension of material along most of the estuary. Regular periods of resuspension during neap tides, and thus all tides, would be likely to occur ir~ the upper 3 km of the estuary. Here, the maximum bed shear stress always exceeds the critical shear stress for erosion of intertidal mud and resu~pen sion and transport are likely to occur. Under spring-tide conditions the potential for resuspension and transport is greatly increased in the upper half (15-31 km) of the estuary. The modelled, maximum bed shear stresses due to tidal flows increase progressing into the estuary due to funnelling of water. A maximum is reached at about 20 km, which is accentuated by ~ local narrowing of width in this region, and the stresses decrease further up-estuary due to frictional damping of the tide resulting from the shallowness of the estuary. Stresses increase again near the weir

451

due to freshwater runoff significantly complementing the ebb tidal flow. A plot of the excess shear stress (the difference between modelled shear stress and critical erosion shear stress) during the largest spring tide of July, 1989, is shown in Fig.3c. The simulated, bed shear stress during the spring tide is greater than the estimated, critical shear stress for erosion of the intertidal sediment near the head and at around 17 km and 20 km from the mouth and is likely to cause resuspension and transport of sediment there. Generally, the simulated bed shear stresses in the upper half (15-31 km) of the estuary are not a great deal lower than the estimated critical shear stress of the intertidal deposits. This suggests that the areas of intertidal mudflats near the lowwater springs line of the upper estuary are potential sources of mobile sediment, which may be available for resuspension and transport. The simulated, tidally-induced bed shear stresses in the lower half of the estuary are much smaller than the estimated, critical bed shear stresses for erosion. This iron,lies that the area is one of net deposition in the absence of significant wave activity. Enhanced deposition of mud may occur in the lower reaches during high runoff events which are associated with large suspended sediment levels from fluvial inputs and from erosion of intertidal mud in the upper estuary. Unfortunately. the drag coefficient, Ca, used in cross-sectionally averaged hydrodynamical models of estuaries is not a well-defined, fundamental physical property and can vary widely between estuaries. Values of CD in the literature range from 0.8 x 10 -3 (Giese and Jay, 1989) to over 30 x 10 - 3 (Swift and Brown, 1983). As an illustration of the effects of varying Co for the Tamar, Fig.3d shows the modelled bed shear stresses due to tidal currents derived by running the cross-sectionally averaged model with an unrealistically low Ca of i x 10 - 3 (the lower line) and an unrealistically high value of I x 10 -2 (the upper line). With a very large CD of I x 10- 2, tidally-induced resuspension occurs in the upper and central estuary but still does not appear to occu~ in thc lower 8 km of estuary. With the very low drag coefficient, it appears that resuspension does not occur in the lower and central estuary, but is still likely to

452

occur near the head and at around 20 km from the mouth. In view of the high critical shear stresses estimated from measured dry densities of intertidal mud in the area opposite Devonport, between 1-2 km from the mouth (Fig. 1 and Fig.3b), erosion of consolidated mud due to tidal currents is unlikely to occur in the absence of significant wave activity. This is consistent with one measurement of mud deposition rate in the area (Clifton and Hamilton, 1979). Except for winds from due south (see Fig.I), the Tamar is sheltered to some extent by flanking valley slopes. Quantitative data on surface waves are lacking, but anecdotal evidence suggests maximum significant wave heights of about ! m in the deeper, more exposed reaches of the lower estuary near the mouth. Such waves can be expected to influence the morphology of the fringing mudflats there, but currently we have no information on the magnitude of that influence. Wave heights are smaller in the central and upper reaches.

Weight loss on ignition Weight loss on ignition from the sediment samples when fired at 300C for 2 h is represented as a percentage of the total dry weight and plotted against distance from the mouth in Fig.4a. The data show a scattered distribution, with large differences between opposite intertidal areas at some longitudinal positions. Although largely subjective, the application of cluster (Milligan, 1980) and correlation analysis to the weight-loss data suggest that the estuary may be roughly subdivided into three main regions: Region (A) (0-15 kin), which shows a strong, linear increase in the percentage weight loss on ignition with distance up-estuary (80% of the variance is explained by the trend with distance). Mean and standard deviation for the region are 4.7_+ 1.6%. Region (B) (16-23km). a region of spatially fluctuating weight loss on ignition (6.7 + !.6%) and with no significant spatial trend. Region (C) (24-3u km), a region having fairly uniform weight loss on ignition (8.1 +_0.6%). The two samples at the head of the estuary lie outside

J.A..~TEPHENS ET AL.

of the turbidity maximum region and have very low weight loss on ignition. Further weight loss occurred from the sediments when fired at 300°C for an additional 12 h. However, the additional percentage weight loss was, on average, only 0.3% and is therefore negligible. This suggests that a period of 2 h is sufficient for complete combustion at 300°C and that further reduction in weight is due to loss from another source. On firing the samples at 450°C for an additional 12 h they yielded a further mean weight loss of 4%. The uniformity of this weight loss suggested that it was probably not due to the loss of additional organic material, but could possibly be attributed to the loss of structural water from hydrated minerals and inorganic material at the higher temperatures. Mook and Hoskin (1982) demonstrated that an organic free sediment sample (comprising 87% clay and silt) exhibited a certain stability between 300 and 400°C and that a significant and pronounced weight loss occurred at temperatures above 400°C, which was possibly attributable to water loss from hydrated minerals and other inorganic material. The total weight loss at 450°C is consistent with values obtained by Bale (1987) for intertidal sediments in the Tamar. If the total weight loss on ignition at 45(J~'C is divided by the weight loss at 300'~C (2 h) a mean ratio of 1.6 is obtained, which is the same as that derived by Meade et al. (1975) for a relationship between loss on ignition and the concentration of particulate organic carbon (POC). Thus, it would appear that the weight loss on ignition at 300°C for 2 h is directly attributable to POC, rather than water driven out of the sediment matrix.

Size fraction analysis The silt and clay (<63 ttm particle diameter) fraction of the intertidal sediments is shown in Fig.4b. The size fraction ranges from about 60% of dry weight near the mouth of the estuary to over 99% at the penultimate station near the head, with a large scatter of values. At longitudinal positions 8.9 and 11 km, where values were obtained for both banks, the size fraction varied by

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Loss on ignition I% by weight)

Fig.4. (a) Percentage weight loss on ignition along the estuary showing three suggested subdivisions for the changes in physical characteristics. (b) Silt content by weight along the estuary showing three suggested subdivisions for changes in the characteristics of composition, in section A, for both diagram (a) and diagram tb), a trend line (linear regression) has been fitted, in section C the fitted line represents the mean of the data (points represented by encircled squares are not included). (c) Relationship between silt content and bulk density with fitted linear regression. (d) Relationship between silt content and loss on ignition (2 h) with fitted linear regression.

as much as 20% on opposite sides of the estuary. We have no data on size spectra within the silt and clay size fraction, but some data are presented in Bale (1987). As with weight loss on ignition, cluster and correlation analysis of the silt and day size fraction indicate that the estuary can be very roughly subdivided into three main regions: Region (A) (0-15 km), displaying a linear trend between size fraction and distance along the estu-

ary. This trend explains 80% of the variance in the size fraction data for the region. Mean and standard deviation are 78_+ 13%. Region (B) (16-23 kin), a region of spatially fluctuating silt and clay content (up to 25% difference in size fraction between adjacent stations) with mean and standard deviation of 83-+ 10%. Region (C) (24-30 kin), a region of fairly uniform silt and clay size fraction with mean and standard deviation of 95_+5%. The size fraction

454

of the sample at 31 km (weir) is about 65%, which is similar to that at the mouth. The silt and clay size fraction is plotted against loss on ignition (which is considered to be an estimate of particulate organic carbon (POC) content) in Fig.4d. There is a very significant correlation with 66% of the variance explained. As the percentage of silt increases the percentage weight loss on ignition increases. Therefore, the weight loss on ignition (estimated POC content) of the intertidal sediment samples is largely related to their silt and clay content which, in turn, is largely dependent on the long-term, fluvial input of particulate matter and its subsequent transport and accumulation within the estuary. The strength of this correlation (Fig.4d) is an indication of the terrigenous nature of the organic carbon, which may be largely derived from plant detritus. The comparison of intertidal silt content with bulk density in Fig.4c indicates a strong relationship, with 54% of the variance being explained. Therefore, the more consolidated intertidal sediments with higher bulk densities (usually occurring towards the mouth of the estuary) have a lower silt content than the intertidal sediments with lower bulk densities (us~lal!y occurring towards the head). This implies both that deposited mud in the lower reaches is associated with a larger sand component, as would be expected closer to the coastal sea, and that consolidation anywhere within the estuary is enhanced by the inclusion of coarser, sandy material in the sediment matrix. This is consistent with the finding in Delo (1988) that the addition of small amounts of sand to a muddy bed increases the density of the bed.

Mineralogy The mineralogy of the estuary can be defined primarily by the occurrence of combinations of its clay mineral types, as identified by the X-ray diffraction analysis. Division of the estuary into regions has been attempted on this basis. In some instances, these regions can be further sub-divided by the occurrence of accessory minerals [minerals which are present in such small amounts that their presence or absence is not significant in determining the mineralogical classification of the parent

J.A. STEPHENS ET AL.

rock, Battey (1981)]. All the analysed samples contained quartz and halite. The sub-divisions of the estuary, as defined from its mineralogy, are not separated by distinct boundaries. There are three main regions: Region (A) (lower estuary, 0-13 km), which is characterised by mixed-layer, chlorite-montmorillonite and kaolinite-montmorillonite, with a very diverse population of accessory minerals, represented in most cases by the oxide form. Locations where samples were taken from intertidal areas on opposite sides of the estuary show differences in the species of accessory minerals present. Region (B) (middle estuary, 14-22 kin), a region characterised by samples which either contain pure chlorite or chlorite is absent. There do not appear to be any mixed-layer clay minerals and the number of accessory species is considerably reduced, although calcite is present at positions 15, 16 and !7 km (Fig. I) in the form of fibres, suggesting Mytilus as a possible source of origin and indicating deposition of marine sediments. Region (C) ( upper estuary, 23-29 km), a region which can be further subdivided into two zones: (i) (lower, 23-26 km) characterised by mixed layer chlorite-montmorillonite-halloysite and kaolinite-montmorillonite clays with calcite as the predominant accessory mineral, and (ii) (upper, 27-29 kin) characterised by pure chlorite and kaolinite clays with the dominant accessory minerals being goethite, grossular garnet and rhodochrosite. The two samples taken at the head of the estuary (30-31 km) are intrinsically different from samples at any of the other locations. Both samples contain mixed-layer chlorite and montmorillonite clays. The accessory minerals are characteristic of the catchment-area metamorphic aureole mineralogies and are of varying sizes. These accessory minerals are characterised by oxides indicating exposure and frequent resuspension and winnowing. The samples appear to have a bimodal grain size distribution. Discussion

A turbidity maximum occurs in the upper reaches of the Tamar Estuary. A tidal resuspension morel which ignores density effects, but which has

INTERTIDAL SEDIMENTS IN A MUDDY MACROTIDAL ESTUARY

a spatially independent, runoff-dependent erodibiiity constant provides a reasonable description of the magnitude and location of the turbidity maximum at ili~h water during both spring and neap tides (Uncles and Stephens, 1989). Suspended sediment is removeJ from the supply of bed sediment and incorporated i.~to the turbidity maximum during spring tides and is largely deposited and returned during neap tides. Both the model (Uncles et al., 1988) and observations (Bale et al., 1985) show that the mobile bed s~'diment which is associated with the turbidity maxi~aum migrates seasonally and is located in the upper few kilometres of estuary during summer and dis~}ersed throughout the central estuary during winter. Limited, intratidal profiling data on current speeds and suspended sediment concentrations imply that the critical shear stress for erosion of bed sediments increases progressing down-estuary from the turbidity maximum region to the mouth. Similar implications follow from a longitudinal survey of the shear strength of the intertidal mudflats, in which shear strength was measured using a hand-held vane device (Uncles and Stephens, 1989). The bulk and dry density data in Fig.2, and the estimated critical shear stresses for erosion in Fig.3, support these previo us indications that a significant increase in consolidation of the surface layer of the intertidal mudflats occurs progressing downestuary from the turbidity maximum region to the mouth. Although there is considerable spatial variability along the estuary due to the non-homogeneity of the intertidal sedirrents, there are very significant trends of both density and estimated critical shear stress for erosion with distance. The generally increasing consolidation towards the mouth appears to depend on at least two factors. First, the much lower bed shear stresses due to slower tidal currents in the lower reaches (Fig.3b) allow long-term deposition of suspended sediment. This sediment can then consolidate during the course of time (Delo, 1988; Mehta, 1988). Resuspension due to wave activity, or a combination of tide and wave-induced currents may occur, and may affect the morphology of the mudflats, but regular and periodic intratidal resuspension due to tidal currents is likely to be small. Second,

455

the silt and clay fraction of the intertidal sediments decreases in the lower reaches, implying a larger sand content and consequent strengthening of the bed (Delo, 1988). The bed shear stresses due to tidal currents are large in the upper reaches (Fig.3b). The silt and clay content of the intertidal sediments ,~salso high (Fig.4b). Therefore, regular intratidal resuspension and transport are likely to occur, with little time available for consolidation following deposition during slack-water periods. Erosion, resuspension, transport and deposition c)cles in the upper estuary lead to the accumulation of a mobile stock of fine sediment which "feeds" the turbidity maximum and shifts longitudinally in response to seasonal variations in freshwater runoff and tidal range (Uncles et al., 1988; Uncles and Stephens, 1989). The data presented here are concerned with properties of those intertidal sediments deposited close to the low-water springs line. Amos et al. (1988) have shown that the vane shear strength and other bulk properties of the sediments can vary across the width of an intertidal mudflat, from low to highwater line. The bed shear strength tended to increase moving up the mudflat. The shear strength increased with time when high evaporation was taking place. The subtidai deposits were subjected to higher bed shear stresses and comprised mainly coarser, non-cohesive sediments. In the lower and central reaches of the Tamar, the deep channel bed comprises mainly coarse sediments or highly consolidated mud, so that the sampled, intertidal sediment probably represents the most readily available supply of sediment for potential resuspension. In the turbidity maximum region of the upper estuary, the mobile stock of fine sediment extends across the width of the estuary. Therefore, it is subjected to stronger bed shear stresses than the sampled sediment and does not experience evaporative water losses. The potential for resuspension of these subtidal sediments is therefore likely to be even higher than that for the sampled sediments. The cross-sectionally averaged, bed shear stresses computed by the model for the sampling period represent the extremes of tidal conditions that could be expected to occur then. These stresses

J.A. STEPHENS ET AL.

456

generally increase along the axis of the estuary from mouth to head aod reach a maximum in the upper reaches when magnification of the tide by funnelling is just balanced by frictional dissipation due to shallowing water. There are local variations in this trend at sites with pronounced topographic influences. It appears that under low neap-tide conditions, tidally-induced resuspension from the intertidal sediments is likely to occur only in the upper 3 km of estuary (Fig.3b). Under spring-tide conditions, however, there appear to bc two main intertidal regions which, if bed sediment is available, are certain to experience regular periods of resuspension: the upper 3 km of the estuary and the region between 17 and 21 kin, where the already high bed shear stresses are enforced by local shallowing and constriction of the main channel. The regior, between 17 and 21 km is included in the area labelled (B) in Fig.4a and b and the large variability in the data for this region may be due, in part, to the ability of the tidal currents there to regularly resuspend and transport coarser, noncohesive sediment from the bed of the deep channel. This coarser material may subsequently be deposited on the intertidal areas, together with fine, cohesive sediment. An additional source of variability in this area may arise from the existence of channel meanders and associated rapid variations in currents and shear stresses which would not be apparent from a cross-sectionally averaged hydrodynamical model. The intertidal samples were taken from the upper few centimetres of bed. Because the density of a mud bed tends to increase with depth (Delo, 1988; Mehta, 1988), it is probable that the nearsurface sediment involved in the most recent deposition events have lower bulk and dry densities and, therefore, lower critical erosion shear stresses for erosion than we have estimated. Moreover, in the turbidity maximum region of the upper 10 km of estuary, the fine sediment extends across the width of the estuary and is not subjected to evaporative water losses and subsequent strengthening (Amos et al., 1988). Therefore, it is likely that the near-surface, intertidal (close to the lowwater line) and subtidal sediments in the whole of the upper region from 17-30 km is subject to

resuspension during spring tides, especially during spring and fall when spring tides are significantly stronger than during the summer period simulated in Fig.3b. The sediment samples in the immediate vicinity of the weir are up-estuary of the turbidity maximum and its associated stock of mobile fine sediment. The intertidal area here is generally subjected to strong ebb flows, deposition of coarser, noncohesive, terrigenous sediment and considerable shear-strengthening during winter spate conditions. The silt and clay fraction of the intertidal sediments increases from the mouth to the turbidity maximum region near the head (60 to > 99% dry weight), but with a large amount of local variability. The POC content, assuming this to be approximated by loss on ignition, similarly increases from about 2% of dry weight near the mouth to about 8% in the turbidity maximum region. The scatter of data is again large with trend lines for POC and size fraction against distance explaining ordy about 30% of the variance in both cases. The regression of POC versus silt and clay size fraction explains 66% of the variance in the data. This implies that the POC content of a sample is largely dependent on the proportion of fi:le sediment within the sample, regardless o f its position. Claster analysis (Milligan, 1980) and correlation analysis of the POC and size-fraction data with distance along the estuary defines three main regions. These regions are also apparent from the mineralogical data. The lower estuary is associated with imertidal sediments having relatively low silt a~ad clay content and low associated organic material. The mio.eralogy of this region can be charac~erised by mixed-layer clays and a diverse number of accessory minerals, represented in most cases by the oxide form. The occurrence of oxide forms of the accessory minerals suggests exposure and winnowing. A fairly linear, up-estuary increase in POC and silt and clay size fraction occurs with distance. The lower estuary therefore appears to be a fairly stable transition zone between the marine and estuarine environments. Fine sediment eroded from the upper estuary during freshwater spates will be carried down-estuary in the surface layers

457

INTERTIDAL SEDIMENTS IN A MUDDY MACROTIDAL ESTUARY

of the water column during the ebb flows. A fraction of this suspended sediment will be deposited on the intertidal areas, where bed shear stresses due to tidal flows are small (Fig.3b), particularly when spates coincide with neap tides. Sediment resuspended by wave activity in the exposed coastal area will be transported into the estuary during the flood tide and deposited on the intertidal areas, particularly at high-water slack. The central region is an area of great variability in the size fraction, sediment type and POC content. Current speeds are fast and the estuary meanders as it becomes increasingly shallow and narrow towards the head. Samples contain either pure chlorite or chlorite is absent. There do not appear to be any mixed-layer clay minerals and the ,~umber of accessory minerals is reduced it,, relation to the rest of the estuary. This region appears to be the winter "storage" area for the mobile stock of fine sediment. • During high runoff winter periods, it appears that the mobile stock of bed sediment associated with the turbidity maximum is displaced downestuary into the central reaches (Bale et al., 1985; Uncles et al., 1988). This sediment is mobilised and moved up-estuary during low runoff, spring and summer periods. Initial movement and transport is enhanced by strong spring tides during spring. However, some of the fine sediment will remain in the central region during summer and will be associated with coarser sediment in localised, low shear-stress areas, which experience transport only under extreme conditions of runoff and tides. During summer, fine sediment may be eroded and flushed from the turbidity maximium region during brief, high runoff periods and may be temporarily deposited in the central region until sufficiently strong spring tidal currents, coupled with low runoff, are able to move the fine sediment back into the turbidity maximum area. Fine suspended sediment which reaches the lower estuary tends to remain there. Therefore, the intertidal areas in the central region experience very large seasonal changes and these are reflected in the variability observed in these summer data. The upper estuary, in the turbidity maximum region, appears to be almost homogeneous in both

POC (8. I +0.6%) and silt content (95 + 5%), which implies a regular resuspension, transport and deposition cycle of fine-sediment mixing there. This intertidal sediment is part of the mobile stock of material associated with the turbidity maximum. The region has a mineralogy which is distinct from that further down-estuary, but which is not homogeneous and can be subdivided into upper (27-29 km) and lower (23-26 km) reaches. The lower is characterised by mixed layer chloritemontmorillonite-hailoysite and kaolinite-montmorillonite clays with calcite as the predominant accessory mineral. The upper zone is characterised by pure chlorite and kaolinite clays with the dominant accessory minerals being goethite, grossular garnet and rhodochrosite. The reason for this inhomogeneity in the mineralogy of the intertidal sediments within the turbidity maximum region is not known, but it may indicate a sorting mechanism in which certain mineral types are deposited preferentially according to density. Conclusions

(1) The bulk and dry density data and the estimated critical shear stresses for erosion support previous indications that a significant increase in consolidation of the surface layer of the intertidal mudflats occurs progressing down-estuary from the turbidity maximum region to the mouth. Although there is considerable spatial variability along tim estuary due to the non-homogeneity of the intertidal sediments, there are very significant trends of both density and estimated critical shear stress for erosion with distance. (2) The generally increasing consolidation towards the mouth appears to depend on at least two factors. First, the much lower bed shear stresses due to slower tidal currents in the lower reaches allow long-term deposition of suspended sediment. This sediment can then consolidate during the cours~ of time. Resuspension due to wave activity, or a combination of tide and wave-induced currents may occur, but regular and periodic intratidal resuspension due to tidal flows is likely to be small. Second, the silt and clay fraction of the intertidal sediments decreases in the lower

458

reaches, implying a larger sand content and consequent strengthening of the bed. (3) The cross-sectionally averaged, bed shear stresses computed by the hydrodynamicai model for the sampling period represent the extremes of tidal conditions that could be expected to occur then. These stresses generally increase along the axis of the estuary from mouth to head and reach a maximum in the upper reaches when magnification of the tide by funnelling is just balanced by frictional dissipation due to shallowing water. There are local variations in this trend at sites with pronounced topographic influences. (4) The bed shear stresses due to tidal currents are large in the upper reaches. The silt and clay content of the intertidal sediments is also high. Therefore, regular intratidal resuspension and transport are likely to occur, with little time available for consolidation following deposition during slack-water periods. Erosion, resuspension, transport and deposition cycles in the upper estuary lead to the accumulation of a mobile stock of fine sediment which "feeds" the turbidity maximum and shifts longitudinally in response to seasonal variations in freshwater runoff and tidal range. (5) The silt and clay fraction of the intertidal sediments increases from the mouth to the turbidity maximum region near the head (60 to > 99% dry weight), but with a large amount of local variability. The POC content, assuming this to be approximated by loss on ignition, similarly increases from about 2% of dry weight near ~he mouth to about 8% in the turbidity maximum region. The scatter of data is again large with trend lines for POC and size fraction against distance explaining only about 30% of the variance in both cases. The regression of POC versus silt and clay size fraction explains 66% of the variance in the data. This implies that the POC content of a sample is largely dependent on the proportion of fine sediment within the sample, regardless of its position. (6) The lower estuary is associated with intertidal sediments having relatively low silt and clay

J.A. STEPHENS ET AL.

content and low associated organic material. The mineralogy of this region can be characterised by mixed-layer clays and a diverse number of accessory minerals, represented in most cases by the oxide form. The occurrence of oxide forms of the accessory minerals suggests exposure and winnowing. A fairly linear, up-estuary increase in POC and silt and clay size fraction occurs with distance. (7) The lower estuary therefore appears to be a fairly stable transition zone between the marine and estuarine environments. Fine sediment eroded from the upper estuary during freshwater spates will be carried down-estuary in the surface layers of the water column during the ebb flows. A fraction of this suspended sediment will be deposited on the intertidal areas, where bed shear stresses due to tidal flows are small, particularly when spates coincide with neap tides. Sediment resuspended by wave activity in the coastal area will be transported into the estuary during the flood tide and deposited on the intertidal areas, particularly at highwater slack. (8) The upper estuary, in the turbidity maximum region, appears to be almost homogeneous in both POC (8. I _+0.6%) and silt content (95 +_5%) during summer, which implies a regular resuspension, transport and deposition cycle of fine-sediment mixing there. This intertidal sediment is part of the mobile stock of material associated with the turbidity maximum. The region has a mineralogy which is distinct from that further down-estuary. (9) Th~ central region is an area of great variability in the size fraction, sediment type and POC content. Samples contain either pure chlorite or chlorite is absent. There do not appear to be any mixed-layer clay minerals and the number of accessory minerals is reduced in relation to the rest of the estuary. This region appears to be the winter "storage" area for the mobile stock of fine sediment associated with the turbidity maximum. It also temporarily receives fine sediment durin,, brief, spate conditions in summer. This sediment is subsequently moved back up-estuary.

459

INTERTIDAL SEDIMENTS IN A MUDDY MACROTIDAL ESTUARY

Appendix

1

Table of primary and accessory minerals present in the Tamar sediment samples and their general chemical composition. //mixedlayer chlorites; *pure chlorites Distance (kin)

Primary minerals

31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 !1 i0 9 8.1 6.7 4.2 3.4 2.2 1.7 1.1

Accesssory minerals

!

2

3

4

5

6

7

8

#

*

*

m

*

!

*

*

#

*

m

*

•

*

•

*

•

*

#

*

* p

* *

* *

*

m/h m/h m/h

* * *

p p p

* * *

m/h

*

p

*

*

*

Go Go

*

p •

* p

* *

* •

Go •

p

*

•

,~

*

* • • , • •

*

Gr Gr Gr Gr

Sp

p

#

Sp Sp

Go * * *

*

*

•

* * m/h

*

•

Go Go

*

*

#

#/*

9

Go Sp Sp

Gr Gr Gr

Di Di Di Di Di Di Di Di Di Di Di

Wi

Rd

Th

Rd

Z

Wi

Rd Ce Ce

Wi An An An Hu

Ce Ce Ce

Az Ca Ca Ca Ca Ca

An Ca Ca Ca

Rd Gy Gy Gy

Hu Hu Hu

An An An

Rd

Hu

Ce Ce Ce

Hu Hu Wi Hu Hu Hu Hu Di

He

T

S

T

S

St

Z

St

T

p • •

p

*

,

•

p

*

*

*

p

*

*

*

Gr

Di Di

Hu

Sp

Az Az

P #/* #/* # # # #/* #/*

* *

*

p

*

*

*

p

*

*

* * *

* *

* * *

m m m

* * *

p p p

*

m

*

p

*

*

m

*

p

*

•

#

*

*

m

•

m

*

p

m

p

p

*

minerals:

(Mg, Fe)(AI, Si) Ow(OH)s (Ca,Na)o.os(AI,Mg,Fe)4_6(Si,Al)sOw (OH).H20 AI2Si2Os(OH)t

3 = kaolinite 4 = others: h = halloysite AI2Si2Os(OH),.H20 m = mixed montmorillonite

Micas: 5 = muscovite 6 = biotite 1= lepidolite p = phlogopite

KAI2(Si3AI)O w(OH)2 K2(Mg,Fe 2 +)6_sAIo-. l(Sio_ sAl2-3)O2o (OH,F),, K2(Li,_2AI2_3)Sia_6Ai,,_ 202o(OH,F),t K 2Mg6(Si6AI6)O2o(OH,F)4

Feldspars: 7 = plagioclase (Na,Ca)2AISiaOs 8 = microcline (Na,K)AISi3Os 9 = barium plagioclase BaAI2Si2Os Accessory

minerals:

An = Anthophyllite

(Mg,Fe 2 +)7SiaO22(OH)2

Di Gr Gr

Go Go Go Go Go

Sp Sp Sp Sp Sp

Gr Gr

Hu

Az = Azurite Ba = Baryte Ce = Celestine Ca = Calcite Di = Diopside G r = Garnet G o = Goethite Gy = Gypsum He = Heamatite Hu = Humite Rd = Rhodochrosite Sp = Spinel St = Staurolite Th = Thorite T = Tourmaline Wi = Wilkietite W = Witherite Z = Zircon

Az

Az Az

Gr

*

Clay minera!s: 1 =chlorite 2 = montmorillonite

Gr

Sp

Di

p

#

Primary

Go

*

Ce

Az Ba Ba Ba Z Ba Zo O~a/W Ba/W Ba/W Ba/W Ba/W Ba/W

Cua(COa)2(OH)2 BaSO4 SrSO4 CaCO3 CaMg(Si206) (Ca,Mg) aAl2(Si04)3 Fe2Oa.H20 CaSO,.22H20 Fe203 Mg2SiO4.MgxTi~(OH,F)202 MnCO3 MgAI204 (MgFe 2 ÷ ),(AIFe a + )906[SIO414(O.OH)2 ThSiO4 (Na,Ca)a(Mg, Fe 2 ÷ ,AI,Li)a(AI,Fe a ÷ )6 OzT(OH,F)4 Ca4(SiO4,PO4,SO,)a(O,OH,F) BaCO3 ZrSiO4

460

References Allen, G.P., Salomon, J.C., Bassoulet, P., Du Penhoat, Y. and De Grandpre, C., 1980. Effects of tides on mixing and suspended sediment transport in macrotidal estuaries. Sediment. Geol., 26: 69-90. Amos, C.L., Van Waggoner, N.A. and Daborn, G.R., 1988. The influence of subaerial exposure on the bulk properties of fine-grained intertidal sediment from Minas Basin, Bay of Fundy. Estuarine Coastal Shelf Sci., 27: ! - ! 3. Anderson, F.E., 1983. The northern muddy intertidal: a seasonally changing source of suspended s~iments to estuarine waters - - a review. Can, J. Fish. Aquat. Sci., 40: 143-159. Bale, A.J., 1987. The characteristics, behaviour and heterogeneous chemical reactiviq of estuarine suspended particles. Ph.D. Thesis, Plymouth Polytech., Plymouth, UK. Bale, AJ., Morris, A.W. and Howland, RJ.M., 1985. Seasonal sediment movement in the Tamar estuary. Oceanol. Acta, 8(I): I-6. Battey, M.H., 1981. Mineralogy for Students. Longman, New York, 355 pp. Clifton, R.J. and Hamilton, E.I., 1979. Lead-210 chronology in relation to levels of elements in dated sediment core profiles. Estuarine Coastal Mar. Sei., 8: 259-269. Delo, E,A., 1988. Estuarine muds manual. Report SR 164. Hydraul. Res., Wallingford, UK. Festa, J.F. and Hansen, D.V., 1978. Turbidity maxima in partially mixed estuaries: A two dimensional numerical model. Estuarine Coastal Mar. Sci., 7: 347-359. Frankel, S.L, and Pearce, B.R., 1973. Determination of water quality parameters in the Massachusetts Bay (1970-1973). Mass. Inst. Technoi. Rep. No. MITSG 74-8. Giese, B.S. and Jay, D.A., 1989. Modelling tidal energetics of the Columbia River Estuary. Estuarine Coastal Shelf Sci., 29:549-571. Kirby, R. and Parker, W.R., 1983. Distribution and behaviour of fine sediment in the Severn Estuary and inner Bristol Channel, U.K. Can. J. Fish. Aquat. Sci., 40: 83-95. Lamere Hennessee E., Blakeslee, P.J. and Hill, J.M., 1986. The distributions of organic carbon and sulfur in surficial sediments of the Maryland portion of Chesapeake Bay, Sediment. Petrol,, 56: 674-683. Manheim, F.T., Meade, R.H. and Bond, G.C., 1970. Suspended matter in surface waters of the Atlantic Continental Margin from Cape Cod to the Florida Keys. Science, 167: 371-376.

J.A. STEPHENS ET AL.

Meade, R.H., Sachs, P.L., Manheim, F.T., Hathaway, J.C. and Spencer, D.W., 1975. Sources of suspended matter in waters of the Middle Atlantic Bight. J. Sediraent. Petrol., 45: 17i-188. Mehta, A.J., 1988. Laboratory studies on cohesive sediment deposition and erosion. In: J. Dronkers and W. van Leussen (Editors), Physical Processes in Estuaries. Springer, New York, pp.427-445. Milligan, G.W., 1980. An examination of the effects of six types of error perturbation on fifteen clustering algorithms. Psychometrika, 45: 325-342. Mook, D.H. and Hoskin, C.M., 1982. Organic determination by ignition: Caution advised. Estuarine Coastal Shelf Sci., 15: 697-699. Officer, C.B., 1981. Physical dynamics of estuarine suspended sediments. Mar. Geol., 40: 1-14. Officer, C.B. and Nichols, M.N., 1980. Box model applications to a study of suspended sediment distributions and fluxes in partially mixed estuaries. In: Estuarine Perspectives. Academic Press, San Diego, Calif., pp.329-340. Swirl, M.R. and Brown, W. S., 1983. Distribution of bottorr: stress and tidal energy dissipation in a well-mixed estuar.~. Estuarine Coastal Shelf Sci., i 7:297-317. Uncles, R.J. and Stephens, J.A., 1989. Distribution of st~spended sediment at high water in a macrotidal estuary J. Geophys. Res., 94(C!0): 14395-14405. Uncles, R.J. and Stephens, J.A., 1990. Computed and observed currents, elevations, and salinity in a branching est,,~ary. Estuaries, 13: 133-144. Uncles, R.J., EIliott, R.C.A. and Weston, S.A., 1985a. Observed fluxes of water, salt and suspended sedilaent in a partly m-'.xed estuary. Estuarine Coastal Shelf Sci., 20:147- ! 67. Uncles, R.J., Eiliott, R.C.A. and Weston, S.A., 1985b Dispersion of salt and suspended sediment in a partly mixed estuary. Estuaries, 8: 256-269. Uncles, R.J., Elliott, R.C.A. and Weston, S.A., 1985c. Lateral distributions of water, salt and sediment transpor~ in a partly mixed estuary. In: B.L. Edge (Editor), Proceedings of the 19th International Conference on Coastal Engineering, ASCE, pp.3067-30/7. Uncles, R.J., Stephens, J.A. and Woodrow, T.Y., 1988. Seasonai cycling of estuarine sediment and contaminant transport. Estuaries, 11: 108-116.