www.elsevier.com/locate/jmarsys

Journal of Marine Systems 51 (2004) 211–241

The stability of tidal flats in Venice Lagoon—the results of

in-situ measurements using two benthic, annular flumes

C.L. Amosa,*, A. Bergamascob, G. Umgiesserc, S. Cappuccid, D. Cloutiere,L. DeNatf, M. Flindtg, M. Bonardic, S. Cristanteh

aSouthampton Oceanography Centre, Empress Dock, Southampton, Hampshire, SO14 3ZH, UKbConsiglio Nazionale della Ricerca, Istituto Talassografico, Messina, ItalycConsiglio Nazionale della Ricerca, Istituto Grande Masse, Venice, Italy

d ICRAM, Via di Casalottizoo, 00166 Rome, ItalyeUniversite de Quebec, ISMER, Rimouski, Quebec, Canada

fTHETIS SpA, Arsenale, Venice, ItalygDepartment of Biological Sciences, University of Odense, Odense, Denmark

hDipartimento di Scienze della Terra, Universita di Parma, Parma, Italy

Received 15 December 2002; accepted 19 May 2004

Available online 27 September 2004

Abstract

Seabed properties in Venice Lagoon were examined in situ in two multidisciplinary field campaigns. The purpose of this

study was to understand the mechanisms controlling the stability of bed types. Two benthic annular flumes (Sea Carousel and

Mini Flume) were deployed simultaneously from a floating pontoon at 24 sites during summer (1998), which were considered

representative of the range in bed/habitat types. As well, bottom sampling and coring, water-column monitoring and benthic

habitat analyses were carried out. All but three sites were on cohesive sediments. Bed types included bare shelly mudflats and

regions colonised by the seagrasses Cymodocea nodosa and Zostera noltii, by filamentous cyanobacteria, and by patches of the

macrophytes Ulva rigida and Chaetomorpha sp. A subset (13) of these sites was visited during the subsequent winter to

evaluate seasonal changes. Six of the sites were intertidal, the remainder were in the sublittoral zone.

Water temperature varied between 5 and 30 jC, and salinity varied between 20 and 38 psu. In the absence of waves,

turbidity was generally low ( < 10 mg/l) and was composed of high amounts of organic matter (25–50%). This indicates that the

tidal flows were not competent to support estuarine sediment. Higher levels of turbidity were measured during wind events or

boat passage as a result of resuspension from the bed. Bed (saturated) density was, on average, 1770 kg/m3, which was

extremely high for estuarine sediments.

Sea Carousel and Mini Flume provided comparable results, despite large differences in instrument footprints. Trends from the

two instruments were similar and showed that summertime bed strength exceeded the winter by up to five times. Mean summer

erosion thresholds for Sea Carousel and Mini Flume were 1.10 and 0.82 Pa, respectively, whereas during winter, they were 0.69

and 0.74 Pa. The northern lagoon had the most resistant tidal flats due to the stabilizing effect of filamentous cyanobacteria

(Biostabilization Index: BI = 244%), microphytobenthos (BI = 153%) and Z. noltii (BI = 206%). The stabilizing effects of C.

nodosa (BI = 74%), U. rigida (BI = 115%) and shell debris (BI = 115%) were intermediate, while bare sublittoral mud beds were

the least resistant (BI = 58%). Summer erosion rates (as a function of applied stress) were lower than winter ones, probably due to

0924-7963/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.jmarsys.2004.05.013

* Corresponding author.

E-mail address: cla8@soc.soton.ac.uk (C.L. Amos).

C.L. Amos et al. / Journal of Marine Systems 51 (2004) 211–241212

water temperature changes. The algorithm E= vssb, yielded good results and indicated that erosion rates in Venice Lagoon were

high, notwithstanding the high erosion thresholds. The mean summertime friction coefficient was /= 62j and was highest in thecentral lagoon. The wintertime / = 69j showed that there was no seasonal fluctuation in bed stability. Mass settling in Venice

Lagoon was a strong function of suspended sediment concentration (S ) and a decay constant (k) of the exponential function

S(t) = Soe� kt; it was found to be in continuity with examples from other locations worldwide (and therefore normal).

D 2004 Elsevier B.V. All rights reserved.

Keywords: Benthic currents; Benthic environment; Measuring devices; Sedimentation; Intertidal flats; Microtidal lagoons

1. Introduction and background

Venice Lagoon is a complex combination of inter-

tidal marshes (barene), intertidal mudflats (paludi),

submerged mudflats (velme) and navigation channels

Fig. 1. A location diagram of sites occupied within Venice Lagoon during

1998 that were representative of the range in bottom types present in the L

March 1999 to examine seasonal changes in bed stability.

(canali) that have been subject to human activities

since 900 AD. Montanelli et al. (1970), Zampetti,

(1976) and Grillo, (1989) have documented major

losses of tidal flats that border the inner parts of Venice

Lagoon. In particular, three areas off Mestre were

this study. Twenty four sites were chosen for a survey during August

agoon. Thirteen of these sites were reoccupied during February and

C.L. Amos et al. / Journal of Marine Systems 51 (2004) 211–241 213

sequestered for industrial development to be serviced

by the artificial Malamocco canal that bisects Venice

Lagoon. Furthermore, large tracts of tidal flat in the

western and northern lagoon were taken up for ‘‘valli

da pesca’’ (embanked fish farms), such that by 1968,

more than 50% of the natural lagoon had been

reclaimed. In total, over 160 km2 of the lagoon has

been drained for commercial purposes. Further losses

of habitat have resulted from (1) controlled river input;

(2) increased boat traffic and boat size; (3) maintenance

and capital dredging; (4) relative sea level rise; and (5)

increased storminess (Carbognin and Cecconi, 1997).

Palude della Centrega (a tidal flat of the northern

lagoon) is undergoing accretion of 1.53 cm/a (Cap-

pucci, 2002), despite net losses in the central and

southern parts of the lagoon. The input of sediment to

the northern region is small and confined to a seasonal

discharge of the rivers (Zonta et al., in press), and so,

accumulation is unexpected and greater than the river

supply. Thus, the balance (budget) of sediment to the

tidal flats is governed by the throughput of the main

tidal channels that feed the flats (Treporti, Burano and

San Felice canals, see Fig. 1). Furthermore, the

trapping efficiency of the tidal flats (sensu Schubel

and Carter, 1984) must be high, or else, the palude

would suffer the same fate as their southern counter-

parts had, that is, systematic erosion. These tidal flats

are under threat of disappearing through bank erosion

within 40 years if present losses of 1 km2/a are

sustained (Consorzio Venezia Nouva, 1996).

The purpose of this paper is to present the results of

two extensive surveys carried out in Venice Lagoon

during summer of 1998 (Amos et al., 1998a,b) and the

subsequent winter (Amos et al., 2000). These surveys

were carried out through in-situ deployments of two

benthic, annular flumes at key sites in Venice Lagoon.

The objectives of the study were (1) to define seasonal

and spatial trends in tidal flat stability within the range

of habitats present in the lagoon; (2) to examine these

trends in the light of measured biophysical bed

attributes at the sediment/water interface; (3) to com-

pare the measured trends to global counterparts so that

unique aspects of the lagoon may be determined and,

as important, those aspects which are different may be

highlighted and examined further; and (4) to derive

key sedimentation algorithms, constants and coeffi-

cients that simulate the observations for purposes of

future modelling. Further objectives of this study were

to compare results from the two flumes to examine the

potential errors of field measurement resulting from

variations in instrumentation, variations in the area of

the footprint of measurement and, hence, the impor-

tance of spatial heterogeneity at the meter scale. The

overarching aim of this study was to develop a firm

understanding of the factors contributing to tidal flat

evolution within Venice Lagoon for purposes of

accurate prediction.

2. Methods and technologies

2.1. Sea carousel

Sea Carousel is a benthic, annular flume designed

for field use in subaqueous settings. The flume is 2.0

m in diameter and comprises an annulus 0.15 m wide

and 0.30 m high (H) containing a total, active volume

(Vf) of 270 l. A detailed description of the device is

presented in Amos et al. (1992). The sampling proto-

col, calibration algorithms, transforms and the detailed

format of output files are given in Amos et al. (2000).

A port in the outer wall of the annulus, at a height

of 0.2 m, was used to collected samples to calibrate

the sensors and for the collection of biological and

chemical samples (Flindt et al. 1997).

Mean flow within the Sea Carousel was deter-

mined from a relationship between azimuthal speed

and lid rotation presented in Amos, et al. (1992) and

later verified in laboratory measurements made using

a wide variety of methods, including numerical mod-

elling (Thompson et al., 2003). The advantage of in-

situ benthic flumes is that the benthic flux (WbMb )

may be defined unambiguously through a concise

definition of the mass balance equation, which is:

BM

Btþ U

BM

Bxþ V

BM

Byþ ðW �WsÞ

BM

Bz¼ 0 ð1Þ

Eq. (1) expresses the advective terms (terms 2, 3 and

4) in 3-D, which are eliminated in a closed system

under well-mixed conditions, thus reducing Eq. (1) to

input/output across two boundaries: the bed and the

mobile lid:

HBM

Btþ l

BM

Bx¼ WbMb ð2Þ

Table 1

A summary of the surface sediment properties at the sites occupied during this survey and those derived from Sea Carousel

Site Water content (%) Organic content (%) Chlorophyll (Ag/g) Carbohydr. (Ag/g) D50

(1) (2) (1) (2) (1) (2) (1) (2)(microns)

Summer survey (1998)

(1) Northern lagoon

10 – – – – – 181 – 214

20 32 47 3.7 42 24 48 26 57 28

21 31 38 3.7 7 25 46 43 54 33

22 30 38 3.2 13 – 72 – 85 33

30 43 54 5.1 23 – 199 – 235 23

31 – 32 – – – 186 – 220 –

32 28 – 2.2 – 43 236 35 279 –

33 – 39 – – 65 – 77 20

40 37 58 2.3 35 126 106 130 126 22

41 30 – 3.8 35 – 43 – 51 21

42 32 59 3.9 – – 42 – 49 22

43 – – – 23 – 29 – 34 –

44 38 40 4.1 14 – 59 – 70 32

50 36 65 5.0 35 60 46 86 72 10

51 – – – – – 54 – 64 –

52 – – – – – 49 – 59 –

53 – – – – – 40 – 104 16

34F 4 47F 11 3.7F 0.9 25F 11 55.6F 37 89F 63 64F 39 104F 77 23.6F 7

(2) Central lagoon

60 26 36 2.7 – 69 17 – 21 74

62 26 26 2.8 – 27 19 40 22 81

80 34 36 3.3 – 20 27 38 32 30

29F 4 33F 5 2.9F 0.3 – 39F 22 21F 4 39F 1 25F 5 62F 23

(3) Southern lagoon

70 26 52 2.9 – 19 34 29 41 58

90 – 20 – – – 30 – 36 –

100 – 95 – – – 30 – 36 21

110 – 154 – 49 58 23

2.6 90F 55 2.9 – 19 36F 8 29 43F 9 34F 17

Winter survey (1999)

(1) Northern lagoon

20 41 42 1.9 10 32 14 58 16

21 – – – 38 – 17 – 20

30 – 52 – 23 – 39 – 46

31 44 – 2.5 30 69 16 126 19

40 43 58 2.7 16 29 22 91 26

41 – 40 1.1 16 – 30 – 36

50 42 67 2.9 – 37 20 58 24

51 – 37 2.3 – 13 20 – 23

42F 1 49F 11 2.2F 0.6 71F 62 36F 18 22F 8 83F 28 26F 9

(2) Central lagoon

60 31 51 2.6 38 10 18 33 21

62 – – – 29 – 33 – 39

31 51 2.6 33F 4 10 25F 7 33 30F 9

(3) Southern lagoon

70 40 53 2.6 26 5 24 56 28

71 – – – 29 – 16 – 19

80 32 29 2.4 44 10 13 19 16

36F 4 41F12 2.5F 0.1 33F 8 7.8F 2.4 18F 5 37F 18 21F 5

C.L. Amos et al. / Journal of Marine Systems 51 (2004) 211–241214

C.L. Amos et al. / Journal of Marine Systems 51 (2004) 211–241 215

the first term in Eq. (2) is monitored by two internal

optical sensors (M = SVf), the second term accounts

for leakage from the flume (Amos et al., 1992); the

third term is the benthic mass flux defined as a

function of the difference between mass deposition

(D) and mass erosion (E): WbMb ¼ ðD� EÞ:Dt:A kg/

m2/s, where Dt is a time step of 10 s and A is the flume

bed area (0.874 m2). The changes in bed level

throughout the deployment are defined using Exner’s

equation:

qsð1� gÞ BhBt

¼ D� E ð3Þ

where g is the sediment porosity, qs is saturated

sediment density (kg/m3) and h is the bed level.

Clear-water bed shear stress (so) in the flume varies

with lid rotation (Ur) as a power function that approx-

imates the quadratic stress law: so = 0.43Ur1.57 Pa,

r2 = 0.99, n = 14.

The effect of suspended sediment concentration

(S ) on the suppression of the bed shear stress may be

large, and consequently, a stress reduction algorithm

has been applied, following Amos et al. (1992), Li

and Gust (2000) and Cloutier et al. (in review). The

calculation of Ws comes from the transform:

Ws=(yM/yt)/St m/s, where St is the suspended sedi-

ment concentration at time t. Fluid density is also

corrected for solids content: q=(qo(1�Vs) + qsVs) kg/

m3, where qo is the clear water density (f1026 kg/

m3), Vs is the suspended sediment volume and qs is

the assumed particle density (2650 kg/m3). The

impact of S on seawater density is small compared

with changes in temperature and salinity. For exam-

ple, for S = 500 mg/l and qo = 1026 kg/m3, q = 1026.3

kg/m3. By using Gibbs et al. (1977), Ws is trans-

formed into an equivalent sedimentation diameter.

The suspended particle size spectrum is derived by

binning the diameters evaluated each 20 s into a

series of 22 size classes ranging from 10 to 300 A,under still-water settling Ws =Wb. The effects of

natural variations in roughness and turbidity on the

Notes to Table 1:

The water content was calculated from CT scanner analyses. Water content (

from CT scanner analyses of gravity cores; organic content (1)—surface se

suspension samples collected from Sea Carousel; chlorophyll (1)—surface s

measured in suspension in Sea Carousel, where CHLORO=0.318F A(Cappucci, 2002); carbohydrate (2)—fluoresence (F) measured in suspensi

estimates of bed shear stress are presented by

Thompson et al. (2004a) and may be large. Never-

theless, a constant drag coefficient (Cd = 4�10� 2)

was used. The errors in estimation of Cd for naturally

rough muddy beds under fully turbulent flows in

annular flumes is evaluated by Thompson et al.

(2003).

2.2. Mini Flume

The Mini Flume is an annular benthic flume that

is 0.30 m in diameter and stands 0.30 m high. It

offers advantages over Sea Carousel in that it is

stand-alone, is easier to operate, suffers no disper-

sion to the ambient water and monitors erosion over

a much smaller area of bed (0.0324 m2). The

descriptions and calibrations of Mini Flume are

found in Amos et al. (2000) and Thompson et al.

(2004b).

The flow structure in the flume, the thickness of the

boundary layer and the wall effects have been exam-

ined in LDV measures carried out under various bed

roughnesses (Fung, 1997; O’Brien, 1998; Thompson

et al., 2004b). The gradient of flow in this boundary

layer was used to define the friction velocity (U*),

which is linearly related to U.08 (the flow speed at a

height of 80 mm): U*=0.141(U.08), and so = 1.50�10� 3qU.08

2 Pa.

Flume operation was held constant throughout the

study and comprised a period of eight stepwise

increases in current speed (the erosion period), fol-

lowed by four stepwise decreases in current speed (the

deposition period). The erosion threshold and erosion

rates were determined in a similar manner to those for

Sea Carousel; the deposition threshold and deposition

rates were defined for cases of subthreshold flow, as

well as in still water. In this fashion, the degree of

retention of the flow (sensu Partheniades, 1971) could

be examined.

The methodology followed for chlorophyll and

phaeopigment analysis was an adaptation of the

1)—surface sediment analyses (Cappucci, 2002); water content (2)—

diment analyses (Cappucci, 2002); organic content (2)—from filtered

ediment analyses (Cappucci, 2002); chlorophyll (2)—fluoresence (F)

g/g, r2 = 0.09, n= 9; carbohydrate (1)—surface sediment analyses

on in Sea Carousel, where CARBO= 0.375F Ag/g, r2 = 0.83, n= 9.

C.L. Amos et al. / Journal of Marine Systems 51 (2004) 211–241216

fluorometric procedure described by Parsons et al.

(1984). Carbohydrate content was quantified using the

saline extraction procedure of Underwood et al.

(1995).

2.3. CT scanner analyses

Bulk density was evaluated using X-ray computed

tomography, which offers advantages over standard

methods of analysis by being digital (yielding spectra

of the Hounsfield Unit) and three-dimensional, being

able to resolve to a voxel volume of 0.06 mm3. Orsi

(1994) transformed HU into a computed tomographic

number CT with the expression: CT= 1+(HU/1000),

so that air is CTc 0, water is CTc 1 and natural,

fine-grained sediment is 1 < CT < 3. The transform

from CT to salt water (36 psu) wet bulk density is

qb = 390 + 670(CT) kg/m3, r2 = 0.992, n = 11 (Amos et

al., 1996a). Solid volume (Vs) per unit volume of

sediment was determined as Vs=(qb� qx)/(2650� qx)

m3, where x = 7 and 27 jC (q7 and q27). Once Vs was

known, porosity (g) was found: g=(1�Vs), from

which the dry bulk density (qbs) was determined:

qbs = qs(1� g) and the water content (Wc) of the

sediment was found from Wc = 100gqx/qs(1� g)%.

Results were used in Eq. (3) to solve for eroded depth.

Cores were analyzed at 1.5-mm intervals from the

core top to a depth of 15 mm. Thereafter, analyses

were made each 5 mm of depth. The mean and

standard deviation of density were plotted against

depth to produce core profiles used in the estimation

of eroded depth (h) and in the determination of eroded

aggregate density used in the transform of settling rate

to sedimentation diameter (Cristante, 2000).

2.4. Site descriptions

Venice Lagoon has been subdivided into northern,

central and southern regions for the purposes of this

study (Fig. 1). The northern region, situated north of

the city of Venice, was occupied by sites 20, 30, 40

and 50 (additional sites within the immediate area of

each site were annotated incrementally within each

decade); the central region, between the inlets of

Lido and Malamocco, was occupied by sites 61, 70

and 80; and the southern region, south of the

Malamocco inlet, was occupied by sites 60, 62, 90,

100 and 110. A summary of the characteristics of

bottom sediments at each site is given in Table 1.

The sites were chosen to be representative of the full

range of subenvironments of the lagoon (Bergamasco

et al., in review) and were distinct in terms of water

mass character (Umgiesser, 2000). Twenty-four sites

were occupied during August 1998. Of these, 13

sites were reoccupied during February 1999. the two

surveys were chosen to represent summer and winter

conditions, respectively. The mean water depth of the

lagoon was 0.5 m below chart datum (lowest low

water) and was thus defined as either intertidal or

sublittoral. Sites 33, 40, 53 and 80 were on intertidal

flats and were occupied over the high tide; the

remainder were in a sublittoral setting and thus

permanently submerged. All sites were on cohesive

clayey silt, with the exception of sites 10, 60 and 62

(Table 1), which were sandy and colonised by the

seagrass Cymodocea nodosa. Sites 20, 21, 22 and 70

were on bare mudflats; sites 30, 31, 32 and 42 were

dominated by microphytobenthos; sites 50, 51, 52,

53, 61, 100 and 110 were occupied by the seagrass

Zostera noltii in patches that covered 20–60% of the

bed; sites 60 and 62 were composed of shelly sand

colonised by patches of C. nodosa and Ulva rigida;

site 80 was covered entirely by U. rigida; and site 90

was covered in a dense mat of C. nodosa. Site 70

was on a bare muddy bed adjacent to the Margera

shipping canal and hence subject to the largest ship

wake effects; site 51 was a replicate of site 50, and

site 32 was a night-time replicate of site 30. All sites

other than 32 were occupied during daylight and

over high tide. The influence of seagrasses on

turbulence and the resulting bed shear stresses within

Sea Carousel have been measured by Thompson et

al. (2004a). A constant drag coefficient, equivalent to

a rough muddy bed, has been used throughout this

study, which, in the presence of dense seagrasses,

may be in error by up to an order of magnitude. An

evaluation of these errors is provided by Thompson

et al. (2003).

3. Results

Two field campaigns were mounted in Venice

Lagoon: a summer and winter programme. A sum-

mary of site characteristics is given in Table 1. Sea

Carousel and Mini Flume were deployed simulta-

C.L. Amos et al. / Journal of Marine Systems 51 (2004) 211–241 217

neously adjacent to each other. Conditions of water

depth, water temperature, turbidity and salinity, and

physical setting were recorded on-site (Table 2).

Gravity cores up to 1.5 m long and push-syringe

cores up to 0.15 m long were also collected for CT

scanning and for the evaluation of sediment properties

and composition. A parallel survey of seabed stability

was undertaken by Hydraulics Research using the

instrument Sederode at sites similar to those described

herein, but on different days (Feates and Mitchener,

1999). The results of this work are reported in Wall-

ingford (1999). As well, a series of self-logging

current meters and OBS sensors were deployed

throughout the lagoon for periods up to 14 days.

The results of these deployments are reported in

Amos et al. (2000).

The summer mean water temperature was f 27

jC, and the salinity was f 32 psu. Temperature was

highest in the northern lagoon probably because of the

greater expanse of intertidal flats; however, salinity

was lowest (28 psu, site 30) due to freshwater inflow

from the Dese river. Near-bed summer turbidity (S0)

varied considerably but was highest in the north

(76F 63 mg/l) due to prevailing strong Scirocco

winds blowing from the south. S0 was considerably

less (although more consistent) in the relatively deep

central and southern regions (10–14F 16 mg/l).

There was no significant differences in water

temperature between sites occupied during the winter

survey (f 7 jC). (Gularte et al, 1980 have demon-

strated that the erosion rate of cohesive material

increases with water temperature; thus, we might

expect to see higher erosion rates during the summer.)

The pattern and range of wintertime salinity were

similar to those observed during the summer survey,

with the exception of site 30 (close to the Dese river);

thus, variations in salinity were not considered in this

study. The highest levels of turbidity were found in

the southern lagoon due to the presence of winds from

the north. Results of the analyses of Sea Carousel

(together with the water column measurements) are

presented in Table 2; results of the Mini Flume

analyses are given in Table 3.

3.1. Summer survey

Mean surface bed density was highest in the

central lagoon (qb = 1907F 60 kg/m3), intermediate

in the north (qb = 1783F 108 kg/m3) and lowest in

the south (qb = 1667F 282 kg/m3; Table 2). The

scatter between sites within a given region was

greater than the overall trends, although the maxima

in density showed systematic increases from north to

south (Fig. 2B). The mean bed density was much

higher than that of newly deposited material (Sills and

Elder, 1984) or of typical estuarine muds

(1250 < qb < 1350 kg/m3; Dyer, 1984) and indicates

that they have suffered compaction through loading

(Jones, 1944; Whitehouse et al., 1999). Van Rijn

(1993) defines material of qb>1400 kg/m3 as hard

mud (solid), which takes longer than 100 years to

form in estuaries.

The mean summer and winter surface sediment

densities were similar, indicating no seasonal changes.

There was no clear difference in density between

intertidal and sublittoral sites.

The results from Sea Carousel and Mini Flume

showed that water depth was not a significant

control on bed stability. However, the intertidal flats

exhibited more scatter than the sublittoral region

did, hosting the highest and lowest erosion thresh-

olds of the survey. Bed stability fell into two broad

regions; higher bed stability to the north of the city

of Venice [sc(0) = 1.26F 0.75 Pa] and lower bed

stability to the south [sc(0) = 0.68F 0.20 Pa]. The

north showed higher erosion thresholds, higher

settling rates (Ws = 0.00125F 0.00069 m/s), lower

friction coefficients (/ = 26F 22j) and higher fluo-

rescence (chloro a) within the water column, as well

as in eroded material, than did the south (Ws =

0.00041F 0.00032 m/s; / = 38F 24j). The overall

summertime bed strength in the lagoon was high

[sc(0) = 1.10F 0.67 Pa]. An example of a time

series from Sea Carousel is shown in Fig. 3. The

figure illustrates eleven 5-min increments of lid

rotation and current speed applied within the flume,

which was sitting in 1.2 m of water on a rising tide

(Fig. 3A). S increased in step with current velocity.

The net effect of dispersal from the flume is evident

by the divergence in measured and corrected S (Fig.

3B). Ambient S was consistently high (163 mg/l),

largely due to boat traffic (principally the Burano

Vaporetto) close to this site. Peaks in erosion rate

(Ep) are also evident and coincide with the onset of

each successively higher current velocity. This ero-

sion is Type I in form (asymptotically decaying with

Table 2

A summary of site conditions and the results from Sea Carousel deployments

Site Temp (jC) Sal (psu) Depth (m) S0 (mg/l) sc(0) (Pa) U (degrees) qb (kg/m3) Ws2 (m/s)

Summer survey (1998)

(1) Northern lagoon

10 30 36 1.4 – – – 1975 –

20 29 36 1.4 64 0.35 5,34 1763 –

21 27 31 1.0 19 0.22 10,33 1843 0.00085

22 27 30 1.2 163 0.44 6,26 1840 0.00133

30 27 28 1.8 51 0.80 6,16,0,11 1705 0.0011

31 27 28 1.2 74 1.90 44,63 1909 –

32 27 28 1.4 108 0.83 8 – 0.0012

33 29 32 0.8 75 0.79 61 1833 0.0029

40 29 33 0.7 9 3.06 82 1672 0.0012

41 30 33 1.2 74 1.51 49 – 0.0012

42 30 33 1.0 113 0.83 11,23 – 0.0031

43 27 36 3.4 118 1.37 21 – 0.00039

44 26 36 1.3 253 0.86 16,36 1820 0.0022

50 28 30 1.5 7 1.42 17 1630 0.00072

51 27 30 2.5 67 1.51 50,0,28 – 0.00091

52 27 30 2.2 14 2.12 21,0,9 – 0.00088

53 27 30 0.8 15 2.23 73 1630 0.00095

27.9F 1.3 31.8F 2.8 1.5F 0.7 76F 63 1.26F 0.75 26F 22 1783F 108 0.00125F 0.00069

(2) Central lagoon

60 27 36 2.0 10 0.36 82,88 1865 –

61 25 34 2.1 7 1.13 32 1992 0.00039

80 28 34 0.5 14 0.85 46 1865 0.00073

26.7F 1.2 34.7F 0.9 1.5F 0.7 10F 3 0.78F 0.32 62F 24 1907F 60 0.00056F 0.00017

(3) Southern lagoon

70 26 36 1.3 42 0.59 21,10 1720 0.00073

90 28 36 1.6 2 0.86 48 2100 0.00009

100 25 35 2.0 9 0.39 78 1495 –

110 28 36 1.2 4 0.88 18,56 1352 –

26.7F 1.3 35.7F 0.4 1.5F 0.3 14F 16 0.68F 0.20 38F 24 1667F 282 0.00041F 0.00032

Winter survey (1999)

(1) Northern lagoon

20 6 35 2.0 32 1.08 70,19 1803 0.00019

21 6 35 1.6 15 1.13 67,13 – 0.00083

30 6 20 1.6 103 0.30 63,25 – 0.00042

31 8 20 1.4 38 0.76 27,29 1718 0.00016

40 9 33 1.2 21 0.84 51,2 1672 0.00026

41 9 32 1.2 15 1.16 49,15 1824 0.00052

50 5 36 1.4 20 0.99 71,42 1691 0.00005

51 5 36 1.2 12 0.48 52 1857 0.00026

6.7F 1.6 30.8F 6.4 1.4F 0.3 32F 28 0.84F 0.29 40F 22 1761F 70 0.00034F 0.00023

(2) Central lagoon

60 7 36 2.5 64 0.53 68 1728 0.00022

61 7 38 2.7 63 0.62 70 – 0.00027

7 37F 1 2.6F 0.1 63F 0.5 0.57F 0.04 69F 1 1728 0.00024F 0.00002

(3) Southern lagoon

70 8 37 2.5 94 0.61 73 1710 0.00017

71 7 37 2.9 72 0.71 57 – 0.00029

80 7 36 1.7 89 0.38 39 1960 0.00020

7.3F 0.5 36.7F 0.5 2.4F 0.5 85F 9 0.57F 0.14 56F 14 1835F 125 0.00022F 0.00005

S0 is the ambient near-bed turbidity at the start of deployments, sc(0) is the surface erosion threshold stress, / is the internal friction coefficient,

qb is the wet sediment bulk density within the topmost 10 mm and Ws2 is the mean aggregate settling velocity.

C.L. Amos et al. / Journal of Marine Systems 51 (2004) 211–241218

Table 3

A summary of results derived from Mini Flume during this study

Site S0 (mg/l) sc(0) (Pa) sd (Pa) Ws2 (m/s) D50 (microns) Smax (mg/l)

Summer survey

20 – 0.35 – – – –

21 80 0.31 >0.17 – – 400

22 – 0.43 [0.13 – – –

30 115 0.59 0.56 3.46� 10� 5 39 1018

31 105 0.23 0.46 – – –

32 156 – – 1.38� 10� 4 59 123

40 62 2.36 – 3.44� 10� 5 31 293

41 172 0.34 – 4.23� 10� 4 85 346

42 290 0.31 – 8.86� 10� 5 42 1900

44 140 0.58 >0.95 – – –

52 0 1.98 – – – –

53 – 1.75 [0.14 – – –

60 0 0.38 >0.47 – – –

62 0 0.65 >0.41 2.78� 10� 4 88 40

70 40 0.56 >0.32 – – –

80 44 0.66 >0.36 3.07� 10� 4 75 612

100 – 1.61 – – – –

Mean 93F 80 0.82F 0.67 >0.40 1.86� 10� 4F 1.39� 10� 4 60F 21 591F 570

Winter survey

20 22 0.73 > 0.45 7.19� 10� 5 71 39

21 27 1.12 > 0.40 8.33� 10� 5 45 350

30 30 – – 4.34� 10� 4 91 682

31 22 0.69 0.56 1.56� 10� 4 – –

40 21 1.16 0.90 2.45� 10� 4 65 907

51 55 0.58 1.39 1.34� 10� 4 47 1251

60 37 0.60 – 6.73� 10� 5 42 140

61 30 0.74 1.00 1.38� 10� 4 52 1233

70 22 0.37 > 0.78 5.80� 10� 4 123 138

71 15 – > 1.21 2.26� 10� 4 64 78

80 26 0.70 1.00 1.94� 10� 4 59 438

Mean 28F 10 0.74F 0.24 >0.85 2.12� 10� 4F 1.53� 10� 4 66F 23 526F 444

S0 is the ambient near-bed suspended sediment concentration, sc(0) is the surface erosion threshold, Ef is the erosion rate constant in Eq. (5), sdis the depositional threshold stress, Ws2 is the aggregate mass settling rate, D50 is the median sedimentation diameter and Smax is the peak

turbidity at the onset of still-water settling.

C.L. Amos et al. / Journal of Marine Systems 51 (2004) 211–241 219

time and hence termed ‘‘benign’’; Fig. 3C), although

it was Type I/II at the highest flows, indicating the

onset of ‘‘chronic’’ erosion. Note that the erosion

rates fall below zero at the end of each increment,

which indicates numerical error in defining the

corrected suspended sediment mass when S is rap-

idly changing. The assumption of equilibrium (i.e.,

no bed erosion) at the end of each velocity incre-

ment is important in the development of synthetic

core plots and their interpretation, as we assume that

ss = sc(h) = sb(h), where sc and sb are the critical

shear stress and bed strength at depth h, respective-

ly. The magnitude of peak erosion rate varied with

eroded depth and was not a function of the applied

flow velocity (bed shear stress). As a result, the

mean erosion rate (Em averaged over each increment

of flow) was used. Fluorescence was initially high

in the example shown and decreased after bed

erosion (Fig. 3D) diagnostic of resuspension of a

‘‘fluff’’ layer as the flume landed. Fluorescence

increased also during the initial period of erosion,

indicating the erosion of a surface biofilm that

typifies summer conditions in the northern lagoon

(Sitran et al., 2000).

A typical time series taken from Mini Flume is

shown in Fig. 4. The four phases of flow for each test

Fig. 2. (A) Ambient (near-bed) turbidity (S0) measured in Venice Lagoon during summer (1998) and winter (1999). During the summer,

turbidity was greatest around the city of Venice and least in the southern lagoon, whereas during winter, the highest turbidity was in the south.

Peaks in turbidity (sites 31 and 41) were due to sustained wind events. Winter-time turbidity was lower due to prevailing calm conditions. (B)

Surface sediment bulk density measured at each site during summer and winter surveys. In general, density was highest in the south, where

habitat loss through erosion has been greatest. No significant difference between summer and winter bed density was found. The cross-hatching

denotes the region of fluid mud (qb < 1200 kg/m3). All sediments were ‘‘solid soils’’ by definition (qb > 1200 kg/m3; Terzaghi and Peck, 1969;

van Rijn 1993) and most were highly consolidated compared with other estuarine counterparts.

C.L. Amos et al. / Journal of Marine Systems 51 (2004) 211–241220

are indicated in Fig. 4A, i.e., (1) an equilibrium phase;

(2) an erosion phase; (3) a settling phase under flow;

and (4) a still-water settling phase. Maximum azi-

muthal flow velocity (U.08) was 0.4 m/s. Erosion (in

phase 2) of the bed is evident in the trend in S in Fig.

4B and began at U.08 = 0.15 m/s. Erosion was Type I

and returned to zero within each increment of flow,

demonstrating the absence of dispersal from the

flume. Settling (phase 3) took place at all flow

velocities, indicating that the depositional threshold

was above U.08 = 0.17 m/s (a value higher than the

initial erosion threshold). The 10-min increment was

not enough to define an asymptote to the log-linear

settling trend, and thus, the degree of retention could

not be determined. However, a decay constant (k; after

Einstein and Krone, 1962) was found in each case

(Table 4).

The erosion threshold is clear from the trends in

S (Fig. 5). S (above ambient, solid dots) shows an

exponential trend with applied bed shear stress that

is valid for the range of applied stresses. In the case

shown, the surface critical bed shear stress

sc(0) = 1.90 Pa and is found by solving the regres-

sion of S versus ss at ambient turbidity (S0 = 74 mg/

l). The relationship between erosion rate and bed

shear stress shows much greater scatter and, hence,

Fig. 3. A time series from Sea Carousel at site 22 (northern lagoon) undertaken during August 1998. A time series of step-wise increases in current

velocity was applied, which was constant between sites. Bed response was evident in the levels of turbidity (B) and the peaks erosion rate at the

onset of each successive increase in flow rate (C). Erosion rate dropped rapidly to zero within the 5-min increment of each flow rate. The signal of

fluorescence (D) showed a systematic decrease in organic matter with time indicative of a peak in organic matter at the sediment surface.

C.L. Amos et al. / Journal of Marine Systems 51 (2004) 211–241 221

has not been used to derive sc(0). It is expressed as

a function of the square root of the excess bed

shear stress (so(t)� sc(0))0.5 following Parchure and

Mehta (1979), where so(t) is the applied stress at

time t and sc(0) is the surface bed shear strength,

yielding:

lnðEm=Ef Þ ¼ aððsoðtÞ � scð0ÞÞ0:5 ð5Þ

Fig. 4. (A) A time series plot current speed from Mini Flume at site 21 (northern lagoon). The time series is subdivided into several phases: (1)

an initial phase to allow conditions to equilibrate to ambient levels; (2) an erosion phase, wherein the flow is accelerated step-wise at 5- and 10-

min intervals; (3) a settling phase, in which the flow is decreased in a series of 10-min steps and stirred vigorously between each step; and (4) a

phase of still water settling. This scheme was chosen to derive the various sedimentation parameters defining erosion and deposition within a

single deployment. Increment 3 was not undertaken in Sea Carousel due to errors resulting from leakage. (B) The time series of suspended

sediment concentration (S) within Mini Flume showed clearly bed erosion and subsequent settling. (C) Erosion and settling rates defined from

the rate of change in the levels of S.

C.L. Amos et al. / Journal of Marine Systems 51 (2004) 211–241222

The coefficients Ef (termed ‘‘floc erosion rate’’; kg/

m2/s) and a (m/N0.5) are empirically derived. The

equation prescribed by Delo and Ockenden (1992)

has also been evaluated:

Em ¼ vsb kg=m2=s ð6Þ

where v and b are empirical coefficients.

A selection of synthetic cores derived from the

Sea Carousel time series is presented in Fig. 6.

The northern sites generally exhibited a surface

biofilm 0.7–1.5 mm thick, which was evident by

a low friction coefficient (/ = 6j). Beneath the

biofilm was found a soil characterised by a higher

friction coefficient (/ = 26j; Fig. 6A). The surface

biofilm in Fig. 6A is equated with the high

fluorescence evident in the time series from Sea

Table 4

A summary of settling characteristics derived from Sea Carousel

during this study

Site Smax

(mg/l)

k (1/s) Ds

(microns)

Fluorescence

(mV)

BI %

Summer survey

20 – – – 152 51

21 834 � 237 48 300 30

22 1059 � 735 56 227 61

30 724 � 616 118 628 123

31 – – – 588 250

32 1480 � 1141 110 743 109

33 244 � 149 76 206 110

40 – – – 335 486

41 – – – 135 239

42 823 � 594 183 131 131

43 – – – 92 217

44 1200 � 999 45 186 121

50 2084 � 1209 80 193 234

51 454 � 216 41 171 249

52 1467 � 962 65 156 349

53 160 � 63 48 277 367

60 – – – 55 49

62 229 � 82 24 60 140

70 1864 � 1578 37 108 90

80 556 � 157 28 85 115

90 – – – 95 100

100 228 – 17 95 73

110 – – – 155 193

Winter survey

20 1032 � 249 53 44 154

21 730 � 528 47 53 70

30 1500 � 1018 30 124 46

31 787 � 369 23 50 116

40 632 � 357 40 69 133

41 1002 � 555 43 95 163

50 566 � 145 19 63 164

51 228 � 75 28 62 66

60 334 � 123 21 57 80

62 313 � 143 37 103 94

70 248 � 76 22 75 94

71 284 � 190 28 52 109

80 521 � 197 20 42 48

Smax is the maximum suspended sediment concentration at the onset

of settling, k is the decay constant in equation S(t) = Smaxe� kt (after

Einstein and Krone, 1962), Ds is the median sedimentation diameter

(microns) evaluated using Gibbs et al. (1977).

The value of fluorescence is taken at the start of each experiment

and is a measure of the organic content of the suspended material

(Table 1), and BI is the Biostabilization Index (after Heinzelmann

and Wallisch, 1991).

C.L. Amos et al. / Journal of Marine Systems 51 (2004) 211–241 223

Carousel shown in Fig. 3D, and hence, is high in

organic matter. Strength increases linearly with

depth beneath the biofilm as the inorganic sediment

is encountered in the erosion process. The higher /value (linear increase with depth) is diagnostic of

self-weight consolidation. Some profiles increase in

strength monotonically and exhibited extremely

high values (Fig. 6C, site 53), while others (Fig.

6B, site 30) show changes in / diagnostic of

stratification at the scale of 1–2 mm. In Fig. 6B,

a layer of / = 0 was found 2.3 < h < 3.5 mm, which

is diagnostic of no consolidation with depth. Such

layers have been found elsewhere and may reflect

bioturbation.

3.2. Winter survey

Spatial trends in bed density during the winter

were within the scatter of results, which yielded a

mean value of 1773F 89 kg/m3. This bed density is

diagnostic of ‘‘hard mud’’ that is not typical of

estuaries (van Rijn, 1993). The winter survey

showed no relationship between bed strength and

water depth, and surface erosion thresholds were

uniformly lower than during the summer survey.

The pattern of erosion during the Sea Carousel

deployments were similar to that in the summer

survey. That is, the highest erosion thresholds were

again found in the northern lagoon [sc(0) = 0.84F0.29 Pa] and were lower in the central and southern

lagoons [sc(0) = 0.57F 0.14 Pa]. The overall bed

strength was only 50% [sc(0) = 0.69F 0.20 Pa] of

the summertime value, despite no significant differ-

ences in surface sediment bulk density. Erosion rates

and net eroded mass (E) were lower than during

summer, yielding higher friction coefficients. In the

central and southern lagoons, / was uniformly high

(up to 69j), diagnostic of a stable substrate (Alga-

mor, 1967). Synthetic core plots from the winter

survey are shown in Fig. 6 (D, E and F). The plots

show profiles of decreasing friction coefficient with

depth, which is more typical of examples from the

literature (Parchure and Mehta, 1979; Mehta, 1989).

Sites 20 and 30 show clearly the loss of the surface

biofilm, which has been replaced by a layer of rapid

strength increase (sensu the collapse zone of Droppo

and Amos, 2001).

Mass settling rates were also lower during the

winter than during the summer by a factor of 2–3

(Ws = 0.00029F 0.00019 m/s), although there were

no significant differences in settling rate across the

Fig. 5. Bed shear stress plotted against suspended sediment concentration (S ) to derive the surface erosion threshold. The solid dots were used in

the regression and were selected by eye; the open dots reflect ambient turbidity levels prior to bed erosion.

C.L. Amos et al. / Journal of Marine Systems 51 (2004) 211–241224

lagoon. The decrease in settling rate is likely to be

water temperature related, either through the inhibi-

tion of organic floc formation or through increases in

water density.

4. Interpretation and discussion

4.1. Intercomparison of instrumentation

A comparison was made of erosion thresholds for

Sea Carousel, Mini Flume and Sederode during the

summer (Fig. 7A) and winter (Fig. 7B). The summer

survey showed the highest overall bed strengths and

the highest variability (scrit = 1.10F 0.69 Pa). The

erosion threshold was highest and showed the great-

est spatial heterogeneity in the northern region

(1.26F 0.75 Pa); both strength and variability de-

creased southwards (0.68F 0.20 Pa). Mini Flume

worked intermittently during the summer due to

leakage and power failure. As a consequence, mean

erosion threshold appeared to be much lower than

for the Sea Carousel (0.82F 0.67 Pa). Sederode

appeared to give consistently low values of erosion

threshold at all sites, yielding a mean value of

0.24F 0.11 Pa. There is no relationship between

the results of Sederode and Sea Carousel. The

results from Sederode were carried out on box cores

that were collected in batch and then analyzed

aboard the pontoon after initial tests with a shear

vane. It is likely that desiccation and disturbance

during sampling and storage is the cause of the

mismatch in results. Cappucci (2002) found that

Mini Flume yielded erosion thresholds of laborato-

ry-deposited sediment from site 40 that were twice

of those derived from the CSM of Paterson (1989).

Thus, the absolute accuracy of field instruments is

questionable.

4.2. Spatial and seasonal trends in bed stability

The winter survey showed a stronger convergence

in results between Sea Carousel and Mini Flume

(Fig. 7B). Nevertheless, the mean values of sc(0) foreach survey differed by only 0.05 Pa, which is

within the error margins of the instruments. The

trends in results between Sea Carousel and Mini

Flume were similar during the winter survey, al-

though the correlation in threshold values was poor

(r2 = 0.23; n = 8). The mean erosion threshold from

Sea Carousel was 0.69F 0.21 Pa. As in the summer

survey, the highest threshold was found at site 40.

The mean erosion threshold from Mini Flume was

0.73F 0.27 Pa. It too recorded the highest erosion

threshold at site 40. Sederode, by contrast, yielded

lower overall values (0.34F 0.11 Pa). The general

Fig. 6. Synthetic cores constructed from a Sea Carousel time series of bed erosion undertaken during the summer at sites 22 (A), 30 (B) and 53

(C), and during the winter at sites 20 (D), 30 (E) and 80 (F). The examples demonstrate the range in near-surface sediment macrofabric evident

by changing slopes (/) in the failure envelopes. In general, / was greater in the winter than in the summer, which signifies more stable material.

This contrasts with the lower erosion thresholds measured during winter, which suggests that / was largely influenced by physical properties,

whereas erosion threshold was largely controlled by biological activity.

C.L. Amos et al. / Journal of Marine Systems 51 (2004) 211–241 225

similarities between Sea Carousel and Mini Flume

during the winter survey suggest that the trends are

valid. Differences in results cannot be due to sys-

tematic instrument errors, as these would lead to a

constant over- or underprediction, which was not the

case. Differences in threshold estimates of 0.19F

Fig. 7. (A) A comparison of three devices for measuring surface sediment erosion threshold—Sea Carousel, Mini Flume and Sederode (from

HRWallingford, 1999)—made during summer 1998. There is a large difference in results between devices. Mini Flume behaved intermittently,

thus the low values at sites 31, 41, 42 and 43. Trends show high thresholds in the north and lower values in the central and southern lagoons.

The average summertime erosion thresholds were 0.24 (Sederode), 0.82 (Mini Flume) and 1.10 Pa (Sea Carousel). (B) A comparison of the

three erosion devices made during winter 1999. Sea Carousel and Mini Flume yielded similar mean values and similar spatial trends, whereas

Sederode predicted lower threshold values. In addition, erosion thresholds were uniformly lower in the winter by about 50%. Seasonal

differences were greatest in the northern lagoon.

C.L. Amos et al. / Journal of Marine Systems 51 (2004) 211–241226

0.13 Pa (21F13% of the maximum value) are

probably due to differences in sizes of the footprints

of the two flumes and spatial heterogeneity. The

majority of the signal is thus due to changing bed

types across the lagoon. The mean summertime

erosion threshold determined by Sea Carousel was

almost twice that determined during the winter

survey (Fig. 8). The greatest differences were found

in the northern lagoon, while the southern lagoon

showed no seasonal trends due to the paucity of

biostabilizers.

4.3. Seasonal changes in turbidity and bed density

Near-bed turbidity (S0) in the Venice Lagoon was

influenced by wave activity either due to boat traffic

or wind events. In the absence of waves, levels of

turbidity fell below 10 mg/l, and a large part of this

material was organic matter (25F 11%), thus, tidal

patterns in Venice Lagoon favour sediment deposi-

tion. The organic content during winter was higher

than during the summer (71F 62%) and may have

been due to the development of an organic-rich

Fig. 8. A comparison of summer and winter surface sediment

erosion thresholds derived from Sea Carousel throughout Venice

Lagoon. Summer thresholds were about twice the winter-time

values; differences were most apparent in the northern lagoon,

where the best developed intertidal flats were found.

C.L. Amos et al. / Journal of Marine Systems 51 (2004) 211–241 227

nepheloid layer, as described by Droppo and Stone

(1994). Peaks in turbidity were correlated with strong

wind events and so were due to resuspension from the

bed rather than from river runoff or from the open sea

(Cappucci, 2002). The mean near-bed ambient turbid-

ity of the Venice Lagoon was 54F 52 mg/l. There was

no significant difference in turbidity between the

winter (49F 32 mg/l) and summer (57F 62 mg/l)

surveys (Fig. 2A). The spatial trends in turbidity did,

however, vary; in the winter, the highest turbidity was

in the southern/central region, while in the summer,

highest turbidities were in the north.

The bulk density of surface (topmost 1 cm) sedi-

ment was extremely variable (1352 < qs < 2100 kg/m3)

and was considerably more dense than typical freshly

deposited fines (1200 kg/m3 according to Sills and

Elder, 1984). This material has therefore been subject

to significant consolidation and was not recently

deposited (van Rijn, 1993).

4.4. The factors controlling erosion threshold

A considerable amount of information regarding

the factors controlling the surface erosion threshold

[sc(0)] has been published recently. Sutherland et al.

(1998b), Tolhurst et al. (2000) and Riethmuller et al.

(2000) provide reviews of the subject. Considering

first abiotic sediments, sc(0) has been shown to be

strongly linked to the sediment bulk density (qb).

Miniot (1968) was perhaps the first to demonstrate

this relationship using crushed limestone and natural

muds. Einsele et al. (1974) found a strong relationship

between sc(0) and void ratio (i.e., the ratio of volume

of voids to volume of sediment) or porosity [g = 100e/(1 + e)] for laboratory clays, which can be equated

with bulk density if the sediment aggregate density is

considered constant. They also found that strength

increased with depth and that the rate of increase was

greatest at the surface (/ = 26j) and decreased with

depth (/ = 6j). Lambermont and Lebon (1977) found

bed strength to increase most rapidly near the surface

and then more slowly with depth. This was substan-

tiated by Parchure and Mehta (1979) and Mehta

(1989), who defined three zones of decreasing / with

depth below the sediment/water interface. Williamson

and Ockenden (1996) derived a linear relationship of

dry sediment weight to sc(0), as have Villaret and

Paulic (1986), Berlamont et al. (1993), Mitchener and

Torfs (1996) and Cappucci (2002). These various

relationships are compared with the data of this and

previous surveys of the Sea Carousel (Amos et al.,

1998a,b) in Fig. 9. The winter data from this study

cluster within the results from an Arctic Sound (Amos

et al., 1996b), suggesting low biostabilization. The

best fit regression of previous Sea Carousel data and

the winter survey data herein is:

scpð0Þ ¼ 5:44� 10�4ðqbÞ � 0:28 Pa;

r2 ¼ 0:46; n ¼ 73 ð7Þ

where scp(0) is the predicted surface strength due to

bed density. As a first approximation, this (solid) line

Fig. 9. A scattergram of surface sediment erosion threshold stress and wet sediment bulk density for the summer and winter field campaigns

using Sea Carousel. Winter results fall along the regression line (solid) developed for other sites over a wide range of bulk densities. The

summer survey shows a large scatter in results; thresholds largely fall above the regression line, especially the sites in the northern lagoon (30,

40 and 50). The surface strength is attributed to the biostabilization of the intertidal flats during summer. Site 40 exhibited the highest strength

detected by Sea Carousel and has been the focus of a study by Cappucci (2002). The significance of sediment buoyancy is discussed in

Sutherland et al. (1998a), and although not discovered in this study, the presence of floating aggregates near site 30 during summer 1998 may be

diagnostic of this phenomenon in Venice Lagoon. Key relationships taken from the literature are also plotted for comparison: Cappucci (2002),

Williamson and Ockenden (1996), Mitchener and Torfs (1996), Villaret and Paulic (1986) and Berlamont et al. (1993).

C.L. Amos et al. / Journal of Marine Systems 51 (2004) 211–241228

is interpreted as neutral stability, as it is expressed

only in terms of bed density. Biostabilization should

add to the strength; bioturbation should reduce it. The

northern summer sites (30, 40 and 50) show dramatic

departures from the best-fit line, evident as strength

increases (biostabilization). These sites are high in

chlorophyll a and dissolved carbohydrates (Table 1),

which are diagnostic of algal activity (Sutherland et

al., 1998c) and are high in organic matter. A Bio-

stabilization Index (BI, sensu Heinzelmann and Wall-

isch, 1991) has been calculated for all sites as follows:

BI=[sc(0)/scp(0)]� 100% (Table 4). The mean sum-

mer and winter BI for this study is 174F% and

103F%, respectively. Thus, during summer, biosta-

bilization strengthens the bed on average to twice the

level due solely to cohesion. The northern lagoon

shows winter BI values around 160%, while the

southern and central lagoons fall below 50%. The

most notable example of biostabilization during the

summer is site 40, where a Biostabilization Index of

over 486% was found. This site has been examined by

Cappucci et al. (2004), who made extensive measure-

ments of bed stability using the Cohesive Sediment

Meter (Paterson, 1989). His results, largely collected

C.L. Amos et al. / Journal of Marine Systems 51 (2004) 211–241 229

during the summer season, show a trend that fits

closely with that found herein (Fig. 9). The data of

Villaret and Paulic (1986), who undertook experi-

ments on an artificially deposited bed, fit closely the

neutral regression line. Their data were derived, in

part, under oscillatory flow, which suggests that the

relationship applies to wave motion as well as to mean

tidal flows. The results of Berlamont et al. (1993) pass

through the biostabilized sites (40 and 50) in the

northern lagoon. The results of Williamson and Ock-

enden (1996) and Mitchener and Torfs (1996) show

trends well above those found herein. Site 70

exhibited a sc(0) = 0.5 Pa in both seasons, as it was

susceptible to resuspension by boat wakes.

Fig. 10. A detailed scattergram of surface sediment erosion threshold stres

using Sea Carousel. The best-fit regression line of all data is shown as the

beds; (2) mobile fluid muds (Bingham behaviour); (3) dense fluid muds (B

mud (solid, 100 years old) following van Rijn (1993). The solid line (Eq. (

at the liquid limit (qbi1100 kg/m3) and the assumption that the erosion th

curve was fitted to data from Canadian Arctic (Amos et al., 1996b), h

biostabilization may be defined.

Fig. 10 illustrates the data derived from Sea Car-

ousel over the lifetime of its use. Note that some gas-

rich beds exhibit densities below 1000 kg/m3 within

the topmost 2 mm (see Sutherland et al., 1998a),

resulting in a buoyant lift force (1). Sediments be-

tween 1000 and 1100 kg/m3 (2) are usually described

as fluid muds (Bingham response) possessing no yield

strength (van Rijn, 1993), yet, clearly, the examples

from Hamilton Harbour, Ontario, indicate this not to

be the case. Also plotted in the figure are (3) the

region of ‘‘dense fluid muds’’ (Bingham response),

(4) the region of ‘‘fluid solids’’ (qb>1250 kg/m3) and

(5) the ‘‘solid’’ region (qb>1400 kg/m3). Note that the

data represented by Eq. (5) predicts yield strength

s and wet sediment bulk density for a wide range of sites monitored

dotted line (Eq. 7). Five regions are illustrated: (1) bouyant (gassy)

ingham behaviour); (4) stiff mud (solid, 10 years old); and (5) hard

8)) attempts to satisfy the laboratory analyses of sediment behaviour

reshold of silica (q= 2650 kg/m3) may be considered as infinite. The

ence neutral. On the basis of this fit, regions of disturbance and

Fig. 11. The Biostabilization Index from Sea Carousel measured

during summer (August) 1998. The sites have been clustered and

ranked into principal bed types according to their ability to stabilize

their substrate: (1) filamentous cyanobacteria [BI = 244%;

sc(0) = 2.28F 0.77 Pa]; (2) Z. noltii [BI = 206%; sc(0) =1.68F 0.42 Pa]; (3) surficial microphytobenthos [BI = 153%;

sc(0) = 1.03F 0.43 Pa]; (4) shelly lag [BI = 115%; sc(0) =1.11F 0.25 Pa]; (5) U. rigida [BI = 115%; sc(0) = 0.85 Pa]; (6) C.

nodosa [BI = 74%; sc(0) = 0.70F 0.24 Pa]; and (7) sublittoral bare

mud [BI = 58%; sc(0) = 0.40F 0.12 Pa]. The plot is subdivided into

biostabilized (BI>100%) and destabilized regions (BI < 100%) and

shows that the lagoon bed is more stabilized during the summer, and

perhaps even destabilized during the winter.

C.L. Amos et al. / Journal of Marine Systems 51 (2004) 211–241230

where arguably none should exist; this suggests that

the premise of neutral conditions may not be true. All

the examples from Venice Lagoon fall within region

(5), i.e., a ‘‘solid bed’’, and thus, this distinction may

be less relevant. The distinction becomes more im-

portant when comparing the data from Venice with the

other data.

The trends in Fig. 10 may be examined more

rigorously by the following arguments: (1) Neutral

bed strength should go to zero at 1100 kg/m3 bed

density by definition of the onset of stationary fluid

muds (Sills and Elder, 1984); (2) bed strength should

approach infinity as qb! 2600 kg/m3, as this reflects

solid rock; and (3) neutral material in the plot comes

from the Arctic (D, Amos et al., 1996b). A neutral

relationship has been fitted by iteration to fit the two

density limits and pass through the Arctic data. This

relationship has the form:

scð0Þ ¼ 5� 10�12ðqb � 1100Þ4 Pa ð8Þ

where all units are SI. This power function is shown

as a solid curve in Fig. 10. It is similar in form to that

proposed by Whitehouse et al. (1999). Data falling

towards the left of the line are interpreted as biosta-

bilized, and those falling to the right are considered

disturbed (perhaps bioturbated). The results falling on

the line defined by Eq. (7) for qb < 1400 kg/m3 appear

to be strongly biostabilized and, using the definition

of Heinzelmann and Wallisch (1991), would, in most

cases, result in BI!l. For high densities subject to

biostabilization, Eq. (8), with the addition of an

appropriate BI, is recommended. The appropriate BI

is likely dependent on bed type and vegetation cover.

BI has been plotted against the various bed types

occupied in this study in Fig. 11. The highest BI

(244%) was found in those regions dominated by

filamentous cyanobacteria, such as those described

by Grant and Gust (1987) and Dade et al. (1990).

Mudflats occupied by patches of Z. noltii were also

strongly biostabilized (BI = 206%) but were largely

neutral during the winter. Mudflats without plants but

dominated by a biofilm of microphytobenthos were

less biostabilized (BI = 153%). Remaining bed types

showed little evidence of biostabilization: U. rigida

(BI = 115%), shell lags (BI = 115%), C. nodosa

(BI = 74%) and bare mudflats (BI = 58%). Note that

in almost all cases, winter strengths show BI < 100%,

as would be expected of bioturbated material.

The significance of this estimate of BI has been

tested for the summer survey against measures of

surface chlorophyll a (CHLORO) and dissolved car-

bohydrate (DCHO) shown in Table 1. Relationships

for the variables was found in the forms: BI = 2.87(D-

CHO)%, r2 = 0.81, n = 17 and BI = 3.83(CHLORO)%.

There appears to be a general positive trend in both

C.L. Amos et al. / Journal of Marine Systems 51 (2004) 211–241 231

parameters that suggests that DCHO and CHLORO

may be valuable proxies of BI. Riethmuller et al.

(2000) also found a positive linear trend with chloro a

based on observations made on the tidal flats of the

German Wadden Sea. A plot similar to Fig. 12 was

attempted for the winter survey, and, as expected, the

data were flat (diagnostic of no trends between the

variables).

4.5. The factors controlling erosion rates

The mean summer and winter erosion rates have

been estimated using Eqs. (5) and (6) and are

shown in Fig. 13A and B, respectively. The data

herein are compared with predictions derived from

Parchure and Mehta (1979), Partheniades, (1971),

Lee et al. (1981), Thorn and Parsons (1980), Dixit

(1982) and Villaret and Paulic (1986). The range in

results spans five orders of magnitude; the results

from Venice Lagoon appear to cut across these

Fig. 12. A scattergram of Biostabilization Index (BI, sensu Heinzelmann an

1998 against surface analyses of chlorophyll a and dissolved carbohydrate

[BI = 2.87(DCHO)%; r2 = 0.81, n= 17]. The plot suggests that the majo

biological activity at the sediment surface. A similar plot was made for th

predictions being relatively insensitive to applied

excess bed shear stress. The power relationship of

Em to ss derived from this study is compared in Fig.

13B with the results of Anderson (2001), McCave

(1984), Lavelle et al. (1984), Fukuda and Lick

(1980), Sheng and Lick (1979), Gularte et al.

(1977, 1980) and Lee et al. (1981). The results

herein suggest higher erosion rates than those pub-

lished in the literature, with the exception of Ander-

son (2001). This latter study indicates erosion rates

three orders of magnitude higher than any others, a

response explained as due to pellet erosion (solid-

transmitted shear stresses). Curve 8 (Fig. 13B)

represents the lagoon-average trends during summer

from this study and demonstrates higher erosion at

lower stresses and lower erosion rates at higher

stresses than during winter (curve 9). The former

trend appears related to the low surface bulk density

of material. Gularte et al. (1980) proposed that

warmer water results in lower erosion rates than

d Wallisch, 1991) for the Sea Carousel data collected during summer

. There is a significant correlation between the variables in the form

rity of the summertime stabilization in Venice Lagoon is due to

e winter survey, but no correlation was found.

Fig. 13. (A) Bed erosion rate as a function of excess bed shear stress (so(0)� sc) for seven algorithms presented in the literature that use Eq. (5)

(curves 1–7) largely derived from laboratory studies. These are compared with the mean estimates of Ef and a for the summer (1998) and winter

(1999) surveys (curves 8 and 9). The data from Venice indicate very high critical values and resulting floc erosion rates resulting in low erosion

rates with excess stress. (B) Bed erosion rate as a power function of applied stress from seven data sets presented in the literature (curves 1–7)

that use Eq. (6). The data from Venice Lagoon, using mean estimates of v and b, are shown from the summer (1998) and winter (1999) surveys

(curves 8 and 9). These plots suggest that Eq. (6) provides the best fit to previous data and that erosion rates are high in comparison with

laboratory equivalents.

C.L. Amos et al. / Journal of Marine Systems 51 (2004) 211–241232

C.L. Amos et al. / Journal of Marine Systems 51 (2004) 211–241 233

do colder ones. Temperature has been accounted for

in the estimation of fluid density and hence is

accounted for in the estimation of bed shear stress.

Perhaps, the effects of bioturbation were lower

during wintertime.

4.6. The factors controlling settling rates

Still-water settling has been monitored for most

sites at the end of each erosion experiment. The

change in concentration of S with time was fitted to

an exponential function that resulted in values of

r2>0.85 in all cases. The equation of the trend in S

is that proposed by Einstein and Krone (1962):

S(t) = Smaxe� kt mg/l, where � k is the decay constant

that has been plotted against Smax in Fig. 14. The plot

Fig. 14. The relationship between still-water settling decay constant (k) an

Sea Carousel. A strong correlation is found between data collected in Ve

surveys in UK and Canada. These results illustrate that settling rate is not s

lagoon. At high concentrations, winter settling in this study appears to

temperature f 30 jC), which supports the field measurements of Cloutie

shows a positive relationship between Smax and k in

the form � k = 134� 0.70(Smax) 1/s, r2 = 0.57, n = 57.

The relationship has been derived in association with

data from five other sites in Canada and UK and is

valid for both summer and winter surveys. This trend

shows that there is nothing unusual about mass

settling rate in Venice Lagoon and that it appears to

behave as other estuaries around the world at all times

of the year.

The mean mass settling rate of sediment has been

considered by Dyer (1984), Whitehouse et al. (1999)

and others to be strongly related to suspended sed-

iment concentration (S ). At values of S encountered

in this study, floc settling would be expected where

Ws increases as some power function of S. Recent

data on organic-rich sediments from Hamilton Har-

d maximum suspended sediment concentration (Smax) derived from

nice during summer and winter campaigns and other Sea Carousel

ite or season dependent and that a single trend is valid for the entire

be faster than the summer settling in Hamilton Harbour (water

r and Hequette (in review).

C.L. Amos et al. / Journal of Marine Systems 51 (2004) 211–241234

bour, Canada, showed data on settling rates that were

independent of S (Amos et al., 2003), which was also

the case in this study. A similar finding was

made by Milligan and Hill (1998) based on con-

trolled laboratory experiments within the range

S < 250 mg/l, and Partheniades et al. (1968) based

on experiments undertaken in a laboratory annular

flume for S < 1000 mg/l.

Fig. 15. (A) mass settling rate (Ws2) vs. suspended sediment concentration

bed erosion at site 31 (winter, 1999). (B) A comparison of mean mass settli

during the summer and winter campaigns. No trends with S were evident

during the summer than during winter.

There appear to be two types of material in Venice

Lagoon: a surface organic-rich fluff and an inorganic

silt below. Estimates of settling rate have been made on

both of these populations by (1) monitoring settling of

the fluff suspended on lowering of the Sea Carousel

(Ws1) and (2) monitoring settling of S after the erosion

process (and hence dominated by sediment aggregates,

Ws2). The results from site 31, presented in Fig. 15A,

monitored during still-water settling in Sea Carousel before and after

ng velocity and suspended sediment concentration for sites surveyed

in either survey, although settling rates were higher by a factor of 2

C.L. Amos et al. / Journal of Marine Systems 51 (2004) 211–241 235

show no significant difference in settling rate and no

trend with S. Given the limited range in S represented

in Fig. 15A, results from other sites in both the summer

and winter surveys were evaluated (Fig. 15B) over a

range in S < 2000 mg/l. Again, no relationship between

Ws2 and S was found. The mean summertime

Ws2=1.07�10�3F6.6�10�4 m/s and the mean winter-

timeWs2=2.95�10�4F 1.91�10� 4 m/s, showing that

settling in cold water was less than in warm water. This

finding is in keeping with laboratory studies of Lau

and Krishnappan (1992) and Lau (1994) and field

measurements of Cloutier and Hequette (in review).

Fig. 16. Still-water mass settling rates determined from Sea Carousel (Ws1

The preerosion settling (Ws1) refers to organic fluff suspended during the lo

sediment aggregates held in suspension by the flow. No difference in settlin

was evident. The highest settling rates were found in the northern lagoon an

the high levels of S ). Trends were similar in both devices. No differences

The aggregate settling rates determined by Sea

Carousel and Mini Flume were similar in magnitude

despite differences in scale of the settling chamber

(Fig. 16). No clear spatial trends in the settling rate

were found. Maxima during the summer survey were

found at northern sites, represented by 30, 40 and 50

(Fig. 16). These sites exhibit the highest chloro a and

carbohydrate contents, suggesting that organic bond-

ing of flocs is the dominant mechanism in the aggre-

gation of suspended material.

The deposition threshold (sd) could be evaluated

only in Mini Flume. Fig. 17 shows an example of

and Ws2) and Mini Flume (Ws2) during the summer field campaign.

wering of the instrument, whereas post-erosion (Ws2) refers to eroded

g rate was evident between the two populations, and no trend with S

d the lowest occurred around the city of Venice (which may explain

in trends in pre- and post-erosion settling were evident.

Fig. 17. (A) A time series of sediment settling under applied (subcritical) flows monitored in situ within Mini Flume. The plot shows that

settling followed an exponential decay law in which the decay constant (k) depends on the applied stress. (B) The decay constant (k) is plotted

against applied shear stress. The deposition threshold (sd) is found by regressing k against sd and solving for k = 0. In this example, sd = 0.56 Pa,which is below the surface erosion threshold [sc(0) = 0.76 Pa]. In many cases, k showed an inverse relationship to so or was not related, and thus,a deposition threshold could not be defined.

C.L. Amos et al. / Journal of Marine Systems 51 (2004) 211–241236

phases 3 and 4 (Fig. 4A). The time series of S is

shown as a function of time for each of four levels of

applied stress and still water. The decay constant (k)

for each stress level is plotted in Fig. 17B. The

evaluation of the regression equation of k versus ssat k = 0 yields sd = 0.56 Pa. The surface erosion

threshold for the same deployment was sc(0) = 0.59Pa. The depositional threshold is thus close to the

surface erosion threshold. This is explained by Par-

theniades et al. (1968), who showed that aggregates in

suspension form a spectrum of sizes and settling rates

which are controlled largely by the stress levels that

eroded the aggregates. As the flow is reduced, so a

part of the size spectrum will settle, leaving behind

the finer aggregates (Seq, the degree of retention).

Estimates of sd from other sites are summarised in

C.L. Amos et al. / Journal of Marine Systems 51 (2004) 211–241 237

Table 3. In some cases, sd>sc(0), although no clear

trend was evident. A time-dependent deposition

threshold is probably more realistic than to assume

a constant value.

5. Conclusions

This paper provides a description of two surveys

undertaken in Venice Lagoon during the summer of

1998 and the subsequent winter using two benthic

annular flumes: Sea Carousel and Mini Flume. The

major conclusions of this study are listed below.

(1) The wet bulk densities of surface sediments were

extremely high and required conditions of conso-

lidation in excess of 100 years to form (van Rijn,

1993). While this may be reasonable for the

majority of the lagoon, where erosion is taking

place, the high values at site 40 are unexplained,

given the site is accreting at a rate of 1.53 cm/a.

(2) Sea Carousel and Mini Flume results showed

similar overall trends in mean erosion thresholds.

The lagoon-averaged summer values of sc(0)were 1.10 and 0.82 Pa, respectively; winter

values were 0.60 and 0.74 Pa. Neither instrument

compared well with Sederode, which predicted

values lower than those found herein.

(3) Winter values of sc(0) corresponded well with

previous Sea Carousel values at similar bed

densities. In particular, the Artic sediments of

Manitounuk Sound, Quebec, overlap those

herein. This suggests that, in general, biostabili-

zation in winter was low. The neutral relationship

recommended for Venice Lagoon is sc(0) =5�10�12(qb� 1100)4 Pa.

(4) The Biostabilization Index (BI) for the summer

survey was 174%, indicating that bed strength

was almost double that due solely to cohesion.

During winter, BI = 103%, indicating that bed

strength was due only to cohesion. Values above

150% in the northern lagoon (site 40) indicate

biostabilization even during winter; values below

100% in the southern lagoon reflect disturbance.

(5) The highest BI (486%) was found in northern

Venice Lagoon (site 40) at a site dominated by

filamentous cyanobacteria (BI = 244%). Interme-

diate biostabilization was evident at sites occu-

pied by Z. nolti (BI = 206%) and on mudflats

dominated by surface microphytobenthos

(BI = 153%). Low biostabilization was found at

sites occupied by U. rigida (BI = 115%), shell

debris (BI = 111%) and C. nodosa (BI = 74%).

Finally, bare sublittoral muds appeared to be

destabilized (BI = 58%) and of low strength.

(6) Mean erosion rates (Em) at a given stress were

higher than reported in other studies and were

best simulated by the expression Em= vssb kg/

m2/s, where the mean summer and winter values

of v and b were 0.0029 and 0.99, and 0.0012

and 2.13, respectively. This expression lacks a

critical value, and thus, it should be used only in

cases where the flow exceeds the appropriate

erosion threshold.

(7) Still-water settling showed an exponential

decrease in S, with time defined by the

expression S(t) = Smaxe� kt. The decay constant

k was strongly related to Smax and fell in line

with previous studies. This suggests that depo-

sition rates in Venice Lagoon are normal.

(8) The deposition threshold of suspended material sdwas often higher than the surface erosion

threshold sc(0), and no relationship between thesethresholds was found. The data herein support the

concept of a time-dependent sd, in which, after

bed erosion, deposition of the material eroded at

the highest antecedent stress level will normally

take place to a level of Seq, known as the ‘‘degree

of retention’’ (Partheniades et al., 1968).

(9) The still-water settling velocity (Ws) was

evaluated for the suspended fluff layer and for

eroded aggregates. No significant difference in

settling between the two types of material was

found. Ws appeared to be independent of S, in

agreement with the work of Partheniades et al.

(1968) and Milligan and Hill (1998).

(10) The internal friction coefficient (/) was derivedfrom synthetic cores constructed from data from

each site. A complex bed structure was found,

which showed differences between the two

seasons. During the summer, a surface organic-

rich biofilm of low / (6j) was apparent,

overlying an inorganic bed of higher / (>26j)./ appears to be an important factor in determin-

ing the erosion of a bed, yet, is complex in its

vertical structure and thus difficult to define.

Marine Systems 51 (2004) 211–241

Notation

D Deposited mass within the flumes (kg/m2/s)

D50 Median sedimentation diameter (m)

E Eroded mass within the flumes (kg/m2/s)

Ef Floc erosion rate in Eq. (5) (kg/m2/s)

h Bed level (m)

H Flume duct height (m)

k Decay constant (1/s)

M Total suspended mass within the flumes (kg)

Mb Benthic mass either eroded or deposited in

unit time (kg)

Mo Digital motor input

S(t) Suspended sediment concentration at time t

(kg/m3)

S0 Ambient suspended sediment concentration

(kg/m3)

U Mean azimuthal current velocity (m/s)

Ur Lid rotational speed (m/s)

Uz Mean azimuthal current velocity at height z

above bed (m/s)

V Mean radial current velocity (m/s)

Vs Suspended sediment volume (m3)

Vt Sea Carousel flume volume (0.270 m3)

W Mean vertical current velocity (m/s)

Wc Water content

Wb Benthic flux (m/s)

Ws1 Organic fluff mass settling velocity (m/s)

Ws2 Eroded aggregate mass settling velocity (m/

s)

a Empirical coefficient in erosion Eq. (5) (m/

N0.5)

b Empirical coefficient in erosion Eq. (6) (m/

N0.5)

v Empirical coefficient in erosion Eq. (6) (kg/

m2/s)

/ Internal friction coefficient

g Sediment porosity

l Fluid viscosity (poises)

ls Linear X-ray attenuance coefficient in sedi-

ment

lw Linear X-ray attenuance coefficient in salt

water

q Clear water fluid density (kg/m3)

qb Sediment (wet) bulk density (kg/m3)

qbs Sediment (dry) bulk density (kg/m3)

qs Turbidity-corrected fluid density (kg/m3)

qx Seawater density at x jC (kg/m3)

so Fluid-transmitted bed shear stress (Pa)

C.L. Amos et al. / Journal of238

sb(z) Bed shear strength at depth z (Pa)

sc(z) Critical bed shear stress for erosion at depth z

(Pa)

sd Critical shear stress for onset of deposition

from suspension (Pa)

ss Turbidity-corrected bed shear stress (Pa)

Acknowledgements

This study was funded under an EU contract to

THETIS (MAS3-CT97-0145) and through funding

under the CORILA 3.2-Hydrodynamics and Morphol-

ogy of the Venice Lagoon. The field work was carried

out with the support of a wide variety of experts

without whose efforts, the project would not have

succeeded. In particular, we wish to thank Robert

Murphy and Bruce Hart (Geological Survey of

Canada-Atlantic) for their technical help during field

operations. The work has been heavily supported by

THETIS, Venice. We are indebted to Roberto

Marescalchi and Carlo Piovesan for technical support,

Roberto Cattelan for laboratory support, and Sergio

Bomben for Pontoon maintenance, safety and design.

We also thank Paolo Ciavola (University of Ferrarra),

F. Corbani and Anna (University of Parma) for their

support. This paper is dedicated to the memory of Dr.

Adrian Cramp (University of Wales, Cardiff), a great

friend to us all. We will miss his dry humour, good

sense and companionship.

References

Algamor, G., 1967. Interpretation of strength and consolidation data

from some bottom cores off Tel-Aviv-Palmakhim coast, Israel.

In: Richards, A.F. (Ed.), Marine Geotechnique, Univ. of Chi-

cago Press, Urbana, IL, pp. 131–153.

Amos, C.L., Grant, J., Daborn, G.R., Black, K., 1992. Sea carou-

sel—a benthic annular flume. Estuarine, Coastal and Shelf Sci-

ence 34, 557–577.

Amos, C.L., Sutherland, T.F., Radzijeski, B., Doucette, M., 1996a.

A rapid technique to determine bulk density of fine-grained

sediments by X-ray computed tomography. Journal of Sedimen-

tary Research 66, 1023–1039.

Amos, C.L., Sutherland, T.F., Zevenhuizen, J., 1996b. The stability

of sublittoral, fine-grained sediments in a subarctic estuary.

Sedimentology 43, 1–19.

C.L. Amos et al. / Journal of Marine Systems 51 (2004) 211–241 239

Amos, C.L., Brylinsky, M., Sutherland, T.F., O’Brien, D., Lee, S.,

Cramp, A., 1998a. The stability of a mudflat in the Humber

estuary, South Yorkshire, UK. In: Black, K.S., Paterson, D.M.,

Cramp, A. (Eds.), Sedimentary Processes in the Intertidal Zone-

Geological Society of London Special Publication, vol. 139,

pp. 25–43.

Amos, C.L., Cloutier, D., Cristante, S., Cappucci, S., Levy, A.,

1998b. The Venice Lagoon study (F-ECTS) field results—Au-

gust, 1998. Geological Survey of Canada Open File Report

3711, 31 pp. + appendixes.

Amos, C.L., Cloutier, D., Cristante, S., Cappucci, S., 2000. The

Venice lagoon study (F-ECTS)—February, 1999. Geological

Survey of Canada Open File Report 3904, 47 pp. + appendixes.

Amos, C.L., Droppo, I.G., Gomez, A.E., Murphy, T.P., 2003. The

stability of a remediated bed in Hamilton Harbour, Lake

Ontario, Canada. Sedimentology 50, 149–168.

Anderson, T.J., 2001. The role of fecal pellets in sediment settling at

an intertidal mudflat, the Danish Wadden Sea. In: McAnally,

W.H., Mehta, A.J., (Eds.), Coastal and Estuarine Fine Sediment

Processes. Elsevier, Amsterdam, pp. 387–401.

Bergamasco, A., Denat, L., Flindt, M., Amos, C.L. in review. Feed-

back effects between biotic factors and physical processes in

estuarine environments from science to management. Coastal

and Nearshore Oceanography.

Berlamont, J., Ockenden, M., Toorman, E., Winterwerp, J., 1993.

The characterisation of cohesive sediment properties. Coastal

Engineering 21, 105–128.

Cappucci, S., 2002. The Stability and Evolution of an Intertidal Flat

in Venice Lagoon, Italy. PhD Thesis, Southampton University,

unpublished, 132 pp.

Cappucci, S., Amos, C.T., Hosoe, T., Umgiesser, G., 2004. SLIM: a

numerical model to evaluate the factors controlling the evolution

of intertidal mudflats in Venice Lagoon, Italy. Journal of Marine

Research 251, 257–280 (this issue).

Carbognin, L., Cecconi, G., 1997. The Lagoon of Venice, environ-

ment, problems, remedial measures. Field guide of IAS Envi-

ronmental Sedimentology Conference. Venice Publ. Consiglio

Nationale della Ricerca, Venice. 71 pp.

Cloutier, D., Hequette, A. in review. The effect of high water

viscosity on sediment. Resuspension along a subarctic beach,

Kuujjuaraapik, Northern Quebec. Journal of Coastal

Research.

Cloutier, D., LeCouturier, M.N., Amos, C.L., Hill, P.R., in review.

The effects of suspended sediments on turbulence in an annular

flume. Journal of Geophysical Research.

Consorzio Venezia Nouva, 1996. The Morphological Restoration of

the Venice Lagoon Quadri Trimestrali of Consorzio Venezia

Nuova, Venice. 24 pp.

Cristante, S. 2000. Sedimentologia Analitica di Sedimenti della

Laguna di Venezia. B.Sc. Thesis, University of Parma, Italy,

unpublished, 75 pp.

Dade, W.B., Davis, J.D., Nichols, P.D., Nowell, A.R.M., Thistle,

D., Trexler, M.B., White, D.C., 1990. Effects of bacterial exo-

polymer adhesion on the entrainment of sand. Geomicrobiology

Journal 8, 1–16.

Delo, E.A., Ockenden, M.C., 1992. Estuarine Muds Manual. HR

Wallingford Report SR309, 23 pp.

Dixit, J.G. 1982. Resuspension potential of deposited kaolinite beds.

M.Sc. Thesis, unpublished, University of Florida, Gainesville.

Droppo, I.G., Amos, C.L., 2001. Structure, stability, and transfor-

mation of contaminated lacustrine surface fine-grained laminae.

Journal of Sedimentary Research 71, 717–726.

Droppo, I.G., Stone, M., 1994. In-channel surficial fine-grained

sediment laminae: part I. Physical characteristics and forma-

tional process. Hydrological Processes 8, 101–111.

Dyer, K.R., 1984. Coastal and Estuarine Sediment Dynamics. John

Wiley & Sons, Chichester. 342 pp.

Einsele, G., Overbeck, R., Schwarz, H.U., Unsold, G., 1974. Mass

physical properties, sliding and erodibility of experimentally

deposited and differently consolidated clayey muds. Sedimen-

tology 21, 339–372.

Einstein, H.A., Krone, R.B., 1962. Experiments to determine modes

of cohesive sediment transport in salt water. Journal of Geo-

physical Research 67, 1451–1461.

Feates, N.G., Mitchener, H.J.,1999. Sederode measurements in

Venice Lagoon. Hydraulics Research Report TR90, 14

pp. + appendixes.

Flindt, M., Salomonsen, J., Carrer, M., Bocci, M., Kamp-Nielsen,

L., 1997. Loss, growth and transport dynamics of Chaetomor-

pha area and Ulva rigida in the Lagoon of Venice during an

early summer field campaign. Ecological Modelling 102,

133–141.

Fukuda, M.K., Lick, W., 1980. The entrainment of cohesive sedi-

ments in freshwater. Journal of Geophysical Research 85,

2813–2824.

Fung, A., 1997. Calibration of flow field in Mini Flume. Contract

Report to Geological Survey of Canada, Atlantic. unpublished.

Gibbs, R.J., Matthews, M.D., Link, D.A., 1977. The relationship

between sphere size and settling velocity. In: deVries Klein, G.

(Ed.), Sedimentary Processes, Processes of Detrital Sedimenta-

tion. SEPM Reprint Series, Tulsa, OK, vol. 4, pp. 4–25.

Grant, J., Gust, G., 1987. Prediction of coastal sediment stability

from photopigment content of mats of purple sulphur bacteria.

Nature 330, 244–246.

Grillo, S., 1989. Venezia, Le Difese a Mare Publ. Arsinale Editrice,

Venice. 253 pp.

Gularte, R.C., Kelley, W.E., Nacci, V.A., 1980. Erosion of cohesive

sediments as a rate process. Ocean Engineering 7, 539–551.

Gularte, R.C., Kelley, W.E., Nacci, V.A., 1977. Threshold erosion

velocities and rates of erosion for redeposited dredges material.

Second International Symposium on Dredging Technology,

UK.

Heinzelmann, C., Wallisch, S., 1991. Benthic settlement and bed

erosion, a review. Journal of Hydraulic Research 29, 355–371.

Wallingford, H.R., 1999. Sederode measurements in the Venice

Lagoon. Internal Project Report Code WP-1b-T035.0.

Jones, O.T., 1944. The compaction of muddy sediments. Quarterly

Journal of the Geological Society of London, 137–160.

Lambermont, J., Lebon, G., 1977. Erosion of cohesive soils.

Journal of Hydraulic Research 16, 27–44.

Lavelle, J.W., Mofjeld, H.O., Baker, E.T., 1984. An in situ erosion

rate for a fine-grained marine sediment. Journal of Geophysical

Research 89, 6543–6552.

Lau, Y.L., 1994. Temperature effects on settling velocity and depo-

C.L. Amos et al. / Journal of Marine Systems 51 (2004) 211–241240

sition of cohesive sediments. Journal of Hydraulic Research 32,

41–51.

Lau, Y.L., Krishnappan, B.G., 1992. Size distribution and settling

velocity of cohesive sediments during settling. Journal of Hy-

draulic Research 30, 673–684.

Lee, D.Y., Lick, W., Kang, S.W., 1981. The entrainment and depo-

sition of fine grained sediments in Lake Erie. Journal of Great

Lakes Research 7, 224–233.

Li, M.Z., Gust, G., 2000. Boundary layer dynamics and drag re-

duction in flows of high cohesive sediment suspensions. Sedi-

mentology 47, 71–86.

McCave, I.N., 1984. Erosion, transport and deposition if fine-

grained marine sediments. In: Stow, D.A.V., Piper, D.J.W.

(Eds.), Fine-Grained Sediments: Deep Water Processes and Fa-

cies. Blackwell Scientific, Oxford, pp. 35–69.

Mehta, A.J., 1989. On estuarine cohesive sediment suspension be-

haviour. Journal of Geophysical Research 94, 14303–14314.

Milligan, T.G., Hill, P.S., 1998. A laboratory assessment of the

relative importance of turbulence, particle composition, and

concentration in limiting maximal floc size and settling behav-

iour. Journal of Sea Research 39, 227–241.

Miniot, C., 1968. Etude des proprietes physiques de differents sedi-

ments tres fins et de leur comportment sous des actions hydro-

dynamiques. La Houille Blanche 7, 591–619.

Mitchener, H., Torfs, H., 1996. Erosion of mud/sand mixtures.

Coastal Engineering 29, 1–25.

Montanelli, I., Samona, G., Valconover, F., 1970. Venezia, Caduta e

Salvezza. Sansoni Nuovi. 28 pp.

O’Brien, D.J., 1998. The Sediment Dynamics of a Macrotidal Mud-

flat on Varying Timescales. PhD Thesis, University of Wales,

Cardiff, unpublished, 132 pp.

Orsi, T.H., 1994. Computed tomography of macrostructure and

physical property variability of seafloor sediments. PhD Thesis,

Texas A&M University, unpublished, 181 pp.

Parsons, T.R., Maita, Y., Lalli, C.M., 1984. Determination of chlor-

ophylls and total carotenoids. A Manual of Chemical and Bio-

logical Methods for Seawater Analysis. Pergamon Press,

Oxford, pp. 101–112.

Parchure, T.M., Mehta, A.J., 1979. Erosion of soft cohesive sediment

deposits. Journal of Hydraulic Engineering 111, 1308–1325.

Partheniades, E., 1971. Erosion, deposition of cohesive materials.

In: Shen, H.W. (Ed.), River Mechanics V. II. Water Resources

Publ., Fort Collins, CO, pp. 25–91.

Partheniades, E., Cross, R.H., Ayora, A., 1968. Further results on

the deposition of cohesive sediments. Proceedings of the Coastal

Engineering Conference, London, 723–742.

Paterson, D.M., 1989. Short-term changes in the erodibility of in-

tertidal cohesive sediments related to the migratory behaviour of

epipelic diatoms. Limnology and Oceanography 34, 223–234.

Riethmuller, R., Heineke, M., Kuhl, H., Keuker-Rudiger, R., 2000.

Chlorophyll a concentration as an index of sediment surface

stabilization by microphytobenthos? Continental Shelf Research

20, 1351–1372.

Sills, G.C., Elder, D.M., 1984. The transition from sediment suspen-

sion to settling bed. In: Mehta, A.J. (Ed.), Estuarine Cohesive

Sediment Dynamics. Springer-Verlag, New York, pp. 192–205.

Sitran, R., Cappucci, S., Bergamasco, A., 2000. Diatoms, chloro-

phyll and carbohydrates in the surface sediments of the Venice

Lagoon: effects on the erosion threshold. Proceedings of

ELOISE Conference, Rende, Italy. 2 pp.

Schubel, J.R., Carter, H.H., 1984. The estuary as a filter for

fine-grained suspended sediment. In: Kennedy, V.S. (Ed.),

The Estuary as a Filter. Publ. Academic Press, Orlando,

FL, pp. 81–105.

Sheng, Y.P., Lick, W., 1979. The transport and resuspension of

sediments in a shallow lake. Journal of Geophysical Research

84, 1089–1826.

Sutherland, T.F., Amos, C.L., Grant, J., 1998a. The effect of buoy-

ant biofilms on the erodibility of sublittoral sediments of a tem-

perate microtidal estuary. Limnology and Oceanography 43,

225–235.

Sutherland, T.F., Amos, C.L., Grant, J., 1998b. The erosion of

biotic sediments: a comparison of methods. In: Black, K.S.,

Paterson, D.M., Cramp, A. (Eds.), Sedimentary Processes in

the Intertidal Zone. Geological Society of London, London,

pp. 295–307.

Sutherland, T.F., Grant, J., Amos, C.L., 1998c. The effect of

carbohydrate production by the diatom Nitzchia curvilineata

on the erodibility of sediment. Limnology and Oceanography

43, 65–72.

Terzaghi, K., Peck, R.B., 1969. Soil Mechanics in Engineering

Practise Publ. John Wiley & Sons, New York. 729 pp.

Thompson, C.E.L., Amos, C.L., Jones, T.E.R., Chaplin, J., 2003.

The manifestation of fluid-transmitted bed shear stress in a

smooth annular flume—a comparison of methods. Journal of

Coastal Research 19 (4), 1094–1103.

Thompson, C.E.L., Amos, C.L., Umgiesser, G., 2004a. A compar-

ison between fluid shear stress reduction by halophytic plants in

Venice Lagoon, Italy and Rustico Bay, Canada—analyses of in

situ measurements. Journal of Marine Systems.

Thompson, C.E.L., Amos, C.L., LeCouturier, M., Jones, T.E.R.,

2004b. Flow deceleration as a method of determining drag co-

efficient over roughened flat beds. Journal of Geophysical Re-

search 109, CO3001.

Thorn, M.F.C., Parsons, J.G., 1980. Erosion of cohesive sediments

in estuaries: an engineering guide. Proceedings of Third Inter-

national Symposium on Dredging Technology, 349–358.

Tolhurst, T.J., Riethmuller, R., Paterson, S., 2000. In situ versus

laboratory analysis of sediment stability from intertidal mud-

flats. Continental Shelf Research 20, 1317–1334.

Umgiesser, G., 2000. Modelling residual currents in the Venice

Lagoon. In: Yanagy, T. (Ed.), Interactions between Esuaries,

Coastal Seas and Shelf Seas. Terra Scientific Publishing, Tokyo,

pp. 107–124.

Underwood, G.J.C., Paterson, D.M., Parkes, R.J., 1995. The

measurement of microbial carbohydrate exopolymers from in-

tertidal sediments. Limnology and Oceanography 40 (7),

1243–1253.

Van Rijn, L.C., 1993. Principles of Sediment Transport in Rivers,

Estuaries and Coastal Seas. Aqua Publications, Amsterdam.

Villaret, C., Paulic, M., 1986. Experiments on the erosion of de-

posited and placed cohesive sediments in an annular flume.

Report, Coastal and Oceanographic Engineering Department.

University of Florida, Gainesville.

C.L. Amos et al. / Journal of Marine Systems 51 (2004) 211–241 241

Whitehouse, R.J.S., Soulsby, R.L., Roberts, W., Mitchener, H.J.,

1999 Dynamics of Estuarine Muds. HR Wallingford Report

SR 527, 81 pp.

Williamson, H.J., Ockenden, M.C., 1996. ISIS, and instrument for

measuring erosion bed shear stress. Estuarine, Coastal and Shelf

Science 42, 1–18.

Zampetti, P., 1976. Il Problema di Venezia. Sansoni Nuova SpA.

157 pp.

Zonta, R., Costa, F., Collavini, F., Zaggia, L., in press. Objectives

and structure of the DRAIN project: an extensive study of the

delivery from the drainage basin of the Venice Lagoon (Italy).

Environment International.