HR Wallingford

M CE A

FLUIDISATION OF MUD BY I,IAVES

Laboratory and field experiments

OckendenDelo

Report SR 268Apri l 1991-

Address: Hydraulics Research Ltd, wallingford, oxfordshire oxl0 8BA, unired Kingdom.Tclephone: 0491 35381 lntemarional +4449135381 Teler 848552 HRSWALG.Facsimile: 0491 32233 Intemational + M 49132233 Regisrered in England No. 162217 4

CONTRACT

This report describes work funded by the Department of the Environnentunder Research contract PECD 7/6/L43, for which the nominated officerwas Mr P woodhead, and by HR wallingford. rr is published on behalf ofthe Department of the Envlronrnent, but any opinions expressed in chereporu are not necessarily chose of the funding Department. The workwas carried out at HR l,Iallingford in Dr E A Delo's Section of the Tida1Engineering Department, under the management of Mr M F c rhorn.

@ Crown copyright 1991

Published by perrnission of the Controller of Her Majestyts Stationeryoffice, and on beharf of the Department of the Enviionmlnt.

ABSTRACT

Previous studies at HR wallingford have investigated the effect ofmonofrequency waves on a mud bed, in a laboratory wave flume. rnreal i ty, the sea bed ls subJected to a pattern of random waves, whichgenerate much more warlable conditions in the wave boundary layer. Anextension of the previous research has been to investlgate the effeet ofa more realistic sequence of random rraves on a mud bed.

This report describes laboratory tests conducted at HR l{al1lngford onmud beds, under monofrequency waves and random rraves. It describes therecent modificati.ons to the nave flume whieh make the apparatus moreeffectlve. Laboratory tests conducted by UnLversity College of Ssanseato investigate the structure of the bed under waves are also lncluded.

The report al-so descrlbes the fleld measurement of the combined effectof tides and waves, followlng the development. of field equLpment formeasurement of near-bed cohesive sedinent processes.

A serles of tests on Fawley mud compared the critlcal shear stress forerosion and eroslon rate for equivaLent nonofrequency and random waves.The erltical shear stress for eroslon of a bed of FawLey mud of density30okgn-3 was around 0.1Nn-2. The erosion rate appeared to be higher forthe monofregue-ncy tests than for the random tests on the same nud, with1.0 1f-4kgn-2s-t- to

1.3 10-4kgn-2s-1 for monofrequency tests, and4 l-0-'J to 5 L0-)kgm-zs-l for the random tests. These erosion rates werefor wave periods of around 1.2 seconds. An estlmate for erosion ratesat different rtave perlods was nade, based on the assumption that erosionoccurs for the same proportlon of the wave cyele.

The denslty of the mud bed was not observed to change much durlng atest. Transport of mud from the paddle end of the flume towards thebeach end was observed. Transport rates per unLt width of the flumewere calculated to be 1 LO-5kgn-ls- l ro + fb-stgn-ls- l .

Laboratory measurements and in-situ rheologlcal measurements of mudunder naves were made by unlverslty college of swansea. A rheometer,whlch used the sensltlvlty of the shear save velocLty to the structureof the bed, was incorporated lnto a wave fh.rme. under the action ofwaves, fluldisatlon of the upper material oecurred, with the bedsediment loslng lts elastlc properties and changlng from a viscoelasticmaterlal to a shear-thlnnlng non-Newtonian suspension. The weakeningeffect of repeated cycllcal stressing was observed at moderateamplltudes, wlth complete loss of measurable structure occurrLng at.larger amplitudes. After a reeovery period of 2 hours (no waves), thestructure of the bed had only recovered about 70t.

A bed frame rras used to make field measurements of wave parameters atthe same time as near-bed coheslve sedlment processes. I{ave periods of1.5 to 2.5 seconds were recorded, wlth rsave herghts up to 0.2n. Thewave perlod increased as the depth increased over the Lnter.tidaLnudflats, where the frane was depl-oyed. The wave induced shear stresswas hlghest when the water depth was very shallow; however, thls was nothlgh enough to erode the consolidated nudflat.

CONTENTS

1 INTRODUCTION

TABLES

4

I^A.BORATORY TESTS

2.L Ob jec t ives2.2 Descript ion of apparatus2 . 3 D e t a i l s o f t e s t s2 .4 Resu l ts

2 .4 .L Shear s t ress on the bed2 . 4 . 2 E r o s i o n r a t e2 . 4 . 3 B e d d e n s i r y2.4.4 Mud mass transport rate2 .4 .5 Bed s t ruc tu re

FIELD MEASI'REMENTS

3.L Descript ion of apparatus3.2 Detai ls of deployments3 .3 Resu l ts

CONCLUSIONS AND RECOMMENDATIONS

4.I Conclusions4.2 Recommendations

ACKNOI{LEDGEMENTS

REFERENCES

Equatlons used in caleulation of bed shear stress from bottomorbital veloci tyI,Iave conditions during Fawley and tJentlooge laboratory testsI'Iave conditions during Poole, Hong Kong and Marsden laboratorytes tsErosion rates in laboratory wave testsSunmary of bed frame deploynents

Page

1

2

22466

101l_L21 3

L4

L4L5L6

18

1819

20

2L

1 .

2 .3 .

4 .5 .

FIGURES

1. The wave flume2. Suspended sedinent concentration along fl:me with time, and erosion

rate against peak bed shear stress, Fawley test Ftt183. Erosi.on rate against peak bed shear stress, Fawley monofrequency

and random lrave tests4. Density profiles during Fawley test Fl'1195. Bed levels along flume during Fawley test Fl'tl_96. Volume transported during FawLey nonofrequency tests7. Bed frane8. water depth, wave height and wave period at west usk field site,

24 l{'ay 1990 pm9. Water depth, wave height and wave period at West Usk field site,

25 ltlay 1990 am10. Wave i.nduced bed shear stress at l.Iest Usk site, 25 tLay 1990 arn

APPENDIX

1. Rheology of cohesive sediments under wave action - UniversiCyCol lege of Swansea repor t .

1 INTRODUCTION

The erosion of mud by waves plays a particularly

important role in the cohesive sediment transPort

along rnuddy coascs and in wide shallow estuaries

close to the sea. So far, the research has mainly

been to investigate the effect of tldal currents on

the erosion and deposition of mud. This includes

laboratory tests and, more recently, field

measurements. However, there are many shallow

areas, sueh as inter-tidal mudflats, where waves may

have a significant influence on the erosion and

deposition of cohesive sediment.

Previous studies at HR Wallingford (Ref 1-) have

investigated the effect of monofrequency rcaves on a

mud bed, in a laboratory wave flurne. In reality,

the sea bed is subjected to a pattern of random

waves, which generate much more variable condltions

in the wave boundary layer. An extension of the

previous research has been to investigate the

effect of a nore realistic sequence of random lraves

on a mud bed. This has been carried out in

laboratory wave flunes and by development of field

equiprnent for measuring cohesive sediment near-bed

processes .

This report describes laboratory tests conducted at

HR Wallingford on mud beds, under monofrequency

waves and random waves. It describes the recent

nodifications to the wave flurne whi.ch make the

apparatus more effective. Laboratory tests

conducted by Unlversity College of Swansea to

investigate the structure of the bed under waves are

also included.

The report also describes the field measuremenc of

the combined effect of tides and araves, following

the development of f ield equipment which was

desc r i bed i n an ea r l i e r r ePo r t (Re f 2 ) .

I-ABORATORY TESTS

2 .L Ob j ecc ives

The objectives of the laboratory wave tescs were to

determine a crit ical shear stress for erosion under

random waves, and to compare the effect of random

waves with that of equivalent monofrequency waves.

The parameters of wave height, wave period and water

depth were chosen to give a range of resulting

bottom orbital velocities similar to thac found in

che field. A series of tests was run on Fawley mud

with monofrequeney vtaves. Modificati.ons were made

to the flume to generate random waves, and a further

series of tests was run on Fawley mud. In addition,

several other muds have been tested in the wave

flurne under random r{aves. These tests were not Part

of this contract, but a sunmary of the results is

included, in terms of wave conditions, shear

st resses and erosion races

2.2 Descr ipt ion of apparatus

The tests on Fawley mud were carried out in a wave,

flurne 23rn long and 0.75m wide, with a maximum water

depth of 0.55rn. There was a trough in the floor of

the flurne to hold the test bed, starting B.5m fiom

the end of the flume with the wave paddle. The

trough was 10m Long, 0.2rn wide and 0.1m deep. At

each end of the test bed section there was a raised

1ip to prevent riud escaping from the bed as bed load

transport (fig 1, with currenc flune configuration).

The flume was modified to allow for two wave

generators, which could be set for monochromatic and

random waves. For monochromatic waves, Che user

controlled the wave frequency and wave height

manually, by altering the scroke length and

frequency on the generator. The random wawe

generator nas eontfol led by a nicro-computer. For

random waves, the user input a zeto crossing period

and a signifieant wave height; the program generated

a random nave spectrum which satisfied these input

condltions. The wave spectrum generated in chis

ease lras the JONSWAP spectrum. At the other end of

the flurne to the generacors there was a shingle

spending beach to absorb the wave energy and hence

minlmise wave reflectlons back into the flume.

An ultrasonic probe mounted on a moving platforn

above the bed was used to monitor the surface level

of the nud bed at intervals along the length of the

bed.

A turbidity sensor was used to measure suspended

sediment concentrations at various depths and

positions along the flume, at time intervals

throughout. eaeh test. The instrument used was an

Analite Nepheloneter, which operates on the

principle of backscatter of infra red light. The

degree of backscatter i.s proportional to the

reflection coefficient and concentration of

suspended sediment.

At positions throughout the test section rdater

pressure transducefs were mounted below the water

surface and the signals logged onto a computer. The

wave spectrum hras determined by analysis of the

water pressure signals.

A conductivity probe was used to determine the

vertical bed density profile. The overall

conductivity of the mud i.s a function of the

conductiviuy of the pore water and the volume of

voids. It was assumed that the conductivity of the

pore water was constant and, hence, the overall

conductivity of the mud was a function of its pore

volume ie. ics density. The instrument was

calibrated using sarples of known dry density of the

mud under investigation and with saline solutions of

known density. The calibration samples had dry

densities typicall-y in the range 50kgrn-5 to

500kgn-3.

For the later tests on Marsden, Poole and Hongkong

muds, the fltrme was modified again. For these

tests, the f lune was 23m l-ong, 0.3n wide, 0.55m

maximun water depth. The test section was 8m long,

0.3n wide, 0.1n deep. Both monofrequency and random

wave generators were available. The bed leve1

readings from the ultrasonic probe and the

eoncentration readings from the turbidity were

automated. These were measured using a eomPuter

controlled robot, which travelled the length of the

test seetion at various depths. The data reeorded

was logged on to a micro-comPuter. This automatic

monitoring systbn allowed much easier collection of

data, at more points al-ong the test section.

2 .3 Deta i l s o f tes ts

After several preliminary tests to check the

operation of the equipment and instrtuaents, the

following sets of tests lrere run:

l-0 tests on Fawley nud, monofrequeney waves

6 tests on Fawley mud, random waves

3 tests on l,Ientlooge mud, nonofrequency waves

2 tests on !.Ientlooge mud, random ltaves

For each test, a uniform slurry was made up from che

bulk mud sample by di lut ion with sal ine water. This

slurry had a dry density in the range 3O0kgm-3 -

400kgn-5. This was poured into the test section of

the flune. The flurne was fil led with saline water

at 30kgn-3.

The placed nud bed was then subjected to a pattern

of waves, either mgnochromatic or random lraves. The

input wave period was fixed for the duration of all

the monofrequency tescs ac l-.2 seconds. For the

random wave tests the input wave period was fixed in

some tests at 1.2s, in others i t var ied becween l-s

and 2s. The input paraneter of wave height

(significant wave height for random waves) was

changed up to three times during a tesL; each wave

height was imposed for one hour, then it was changed

so that the resulting peak bed shear stress was

increased. Wave heights were in the range 0.0 -

0 . 1 2 n .

The vertical density profile of the bed was

determined with the conductivity probe before the

start of a test and at half-hourly intervals during

a tesc. This monitored changes in bed density

caused by consolidation or rrave action. The bed

levels at 0.5m intervals along the test section were

measured at the start of the test and at half-hourLy

intervals with the ultrasonic probe. The

concentration of suspended sediment was measured

every 30 minutes at several heights in the water

column at positions along the entire length of the

flune.

Ihe conditions of \cater depth, wave height, wave

period and resulting bottom orbi.cal velocity and

peak bed shear stress are sunmarised for chese tests

in Table 2.

2 . 4 R e s u l t s

2 .4 .L Shea r sL ress on the bed

The parameters of water depth, wave height and wave

period for these tests were chosen co give resulting

peak bed shear stresses sinilar to those in the

'fie1d. The effeet of varying these Parameters

individually has not been extensively investigated.

For monochronatic waves, the peak bed shear stress

can be calculated fron the water depth and wave

characteristics using first order linear wave theory

(Ref 3) such that:

rHU, - T siritr (2ndll,) (1)

where

Um - maximr:m botton orbital velocity (ts'l)

H : wave height (n)

T : wave period (s)

L - wave length (n)

d - rlrater depth (n)

Ihe nagnitude of the wave length is deternined

iteratively frorn:

a2 * Ek tanh (kd)

where

o : 2r /T (s-1)

B : acceleration due to gravity (*"-2)

k - 2n/L (n-11

(2>

The peak bed shear, s t ress

following relationship :

tm : 0.5 pr f rU.z

is est imated us ing the

( 3 )

where

7m : peak bed shear stress (tln'2,

pH : fluid density (kgm-3)

fH - wave friction factor

U, : maximr.m bottour orbital velocity (ns-t;

Ttre wave frlction faetor is dependent upon the wave

relmolds number and the relative roughness (Ref 4).

For random waves, the surface elevation spectrum

generated is the JoNSIIAP spectrum:

s( f ) : 4 .732 1"0-aexp( -L .25 f ra g -a ; f -5 3 .3x (4 )

with

x : exp( - ( f f r - l

y : 0 . 0 1 5 2 i f f

- 0 . 0 0 9 8 i f f

- 1 ) 2

) f t

3 f,n

v -1 )

where

f : frequency of ordinate

f*: frequency at which spectral peak occurs

: O. 87Tz '1 o r : 0 . 2 l -7Hs 'o '5

Tz - mean zero crossing period (seconds)

H, : significant wave height (n)

S(f) : value of spectral ordinate at frequency f

I.lhere both T, and H, are input, T, is used as the

defining value and the spectrum nagnitude is defined

with an inplied gain. The near-bottom velocity

cannot now be descrlbed by a single U* and is

usually described by the standard deviation, Ur*,

of the t ime-series of instantaneous veloci t ies.

U"r" can be approximated as a function of H" and T,

( R e f 3 ) :

umlsTn/Hs : o.25/(L + Arz)3

where

A : ( 6 5 0 0 + ( 0 . 5 6 + 1 5 . 5 4 t 1 6 1 1 r t

E : T n / T .

rn : (d/s)o'5 scal ing period

d - water depthl

g - acceleracion due to gravity

(s)

and random waves with

orbital velocities

This fits che JONSWAP curve to an accuracy of better

than l-* in the range 0 < t < 0.55.

For equlvalent monochromatic

the same variance of bottom

um : "/2

u'ns ( 6 )

A peak significant shear stress for random lraves can

therefore be calculated according to equation 3,

using ./2 Ur* in'place of Ur. The equations used to

calculate a bed shear stress frorn the bottom orbital

velocity are given in Table L.

Table 2 summarises the wave conditions during each

test and the resulting botton orbital velocities and

shear stresses. For the random wave tests, the rooc

mean square bottom orbital velocity, U.*, has b'een

calculated as well as a peak significant orbital

velocicy, for comparison with the monofreguency

tests. For most of the Fawley tests, the peak shear

stress (nonofrequency equivalent) generated in the

random waves wab much lower than in the

monofrequency tests. Only test FR5 had shear

stresses sirnil-ar to those in some of the

monofrequency tests, particularly tests FU20, FM21

B

and FM25. The shear s t resses in these Cests were in

the range 0 .23 - 0 .27Nm-2 . I n t he Wen t l ooge tes rs ,

the shear stresses in tests WMl , I^n{3, i^IRl and WR2

were a l l s im i l a r a round 0 .24 - O .26Nm-2 .

Details of the wave tests on Poole, Hong Kong and

Marsden muds are given in Table 3. The shear

stresses generated in the Poole tesLs covered a

lower range than those in the HongKong tests. The

monofrequency \daves generated in the Marsden tesLs

resulted in the highest shear stresses.

A cr i t ical shear stress for erosion by waves, for a

bed of Fawley mud of density of around 300kgm-3, has

been estimated by considering test FMIB; che

nonofrequency waves rrere increased in wave height at

each step, result ing in a stepped increase in the

peak bed shear stress. The concentration of

sediment i.n suspension at O.hn above the bed, along

the length of the flume, is shown at half hour

intervals in Figure 2. Distances along the flune

have been neasured starting at the paddle end of the

test section. Some mud has been eroded after an

hour (peak shear scress 0.14Nm-2), but more mater ial

has been eroded after 1.5 hours (peak shear stress

0.24Nm-2). The mater ial in suspension also appears

to be moving down Ehe flume towards the beaeh,

particularly at the higher shear stresses, where the

eoncentration at the beach end of the flurne is much

higher than at the paddle end. Based on the point

of observed increase in coneentration in suspension,

the cr i t ical peak shear stress for.erosi-on was below

0.14Nrn-2. A better est imate for cr i t ical shear

stress can be made,by considering the erosion rate.

2 . 4 . 2 Eros ion ra te

For each test, an erosion rate for each half hour

period was calculated from the inerease in suspended

sediment eoncentrat ion. A second erosion raLe was

calculated from the depth of erosion measured with

the ultrasonic probe. The second method assumes

that there is no consolidation of the bed. The

lower part of Figure 2 shows the erosion rate

against peak bed shear stress for Eest FM18. This

shows erosion rates in the range 0.00002 -

O.00007kgrn-2"-1. I f one assumes that the erosion

rate is proport ional to the excess shear stress,

then an estimate for the critical shear stress can

be made frorn Figure 2. This suggests a critical

shear stress of around 0.1Nn-2.

Figure 3 shows the results of peak shear scress

against erosi-on rate for all the Fawley

monofrequeney ti:sts and all the Fawley random wave

tests. The erosion rates for the monofrequency

tests are, in general, higher than those for random

wave tests, wiLh erosion rates of 0.00010 -

0.00013kgrn-2s-1 (monofrequency) and 0.00004 -

0.00006kgr-2s-1 (randorn) .

These erosion r i tes are for a wave period of 1.2s,

whereas in reality the wave period is more likely to

be longer. However, if one assumes that the

critical shear stress is exceeded for the same

percentage of the time during a complete rrrave

period, then coiresponding erosion rates for a

longer period wave of period T (with the same peak

shear stress) eould be estimated by nuluiplying by

f.2/T. For a wave period of 7s, this would give

erosion rates of about 2 tO-5kgm-2"-1 for

monofrequency waves and 1 10-5kgrn-2s-1 for random

waves.

l0

The measured erosion rates f rom al l the wave tests

are sunmarised in Tab1e 4. These range from

0.00003kgm-2s-1 for the Marsden tests (which had

some of the h ighest shear s t resses, but was a h igh

densi ty bed) to 0.0004kgm-2s-1 in the Went looge

tests (a much lower densi ty bed) .

2 . 4 . 3 B e d d e n s i t l r

During the wave tests the density of the bed was

measured at half hour intervals with a eondueti.vity

meter. This instrument indicates the shape of the

density profile, and ean be used to see if the

profile changes significantly during a cest. For

these tests, each urud bed was placed at a constant

density. The effect of the rraves on the density

profile can be seen from subsequent profiles.

Figure 4 shows the density profiles recorded during

test FMI-9. These density profiles are tlrpical of

this set of wave tests. Test FM19 shows that the

constant initial density was retained Ehroughout the

test. There is no change in density at the surface

of the bed. However, the surface of the bed may be

fluidised (a changg in structure) without a change

in density.

The bed density profiles \irere measured at a point in

the centre of the test section. Frour the density

profiles, 'the bed petually appeared to increase in

depth during the test. A rise in bed levels aC one

end of the test secuion was also shown with the

readings from the ultrasonic transducer. The bed

levels lvere recorded along the length of the flume,

ats half hour intervals. Figure 5 shows the bed

levels for test FM19, indicating that material has

moved from one end of the Lesc section to che oLher.

1 1

Ac the end nearest the paddle (0m) the bed level had

dropped by approximtely lrnm afcer the first hour and

around 2mm after three hours. At the other end of

the test sect ion the bed 1evel had r isen by around

1mm. This is very typical of the wave tests

conducted in th is ser ies. I t shows the ef fect of a

f in i te length t 'est sect ion, and uni -d i rect ional

waves

2.4.4 Mud mass t ransport rate

The bed level measurements made during wave tests

nay be used to estimate mass transPort rates along

the flume (Ref 5). Typically during a test, the bed

levels went dowh at the paddle end of the flurne, and

up at the beach end. The concentrations in

suspension at the beaeh end were also higher,

indicating transport of material down the flurne.

An average bed level was calculated at each time

interval. The part of the profi le above this

average level was then ealculated to be the volume

transported. A 'mass t ransPort rate was ealculated

by urultiplying the volume of material by the density

of the mud surfbce. Volumes from successive

profi les were subtracted to give the volume

transported during a tine interval. Volumes were

calculated per unit width of the flume. The total

volume transported during each test is shown in

Figure 6. Aftei: an init ial sharp rise during the

first 30 minutes, the vdlurne transporLed in each

test remains roughly constant. Test FM2lr shows the

largest volume transported at around 0.08m3 per

metre width of the flurne, which is two to three

times larger than the volume transPorted in the

other tests. This g ives a mass t ransport rate of

around 4 10-5kgrn-1s-1 for che first 30 rninutes of

t es t FM24 .

t 2

2 . 4 . 5 Bed structure

The structure of a mud bed under wave action has

been studied by the University College of Swansea

(U C Swansea). Laboratory measurements and in-situ

rheometrical measurements of the mechanical response

of cohesive sediment under waves were made prior to

and during bed deforuration and loss of structure. A

full account of the work is given in Appendix 1-, and

summarised here.

An in-situ rheoneter was developed, which utilised

the sensitivity of shear wave velocity to the

structure of the bed (through nechanical rigidiry).

The dimensions of the rheometer allowed it to be

incorporated into flune experiments with minimal

disturbance.

Shear wave Cransducers were mounted in the test

secti.on part of a wave flurne. The test section was

100mrn deep, with the transducers mounted at 23nn,

53mn and 80mn above the base. The transducers

respond to shear wave arrival and dlmarnic pressure

changes within the bed. Shear wave velocities and

attenuation coeffici.ents rtrere measured at the three

vertical locations. The dynarnical bed response was

investigated for tlro Cest procedures: waves of

consCant frequency and constant anplitude, and

constant frequency and increasing amplitude

A pronounced non-homogeneous distribution of

increasing bed rigidity with deprh was found.

was due to the non-linear dependence of the

rigidity modulus, G, on the fractional volume

solids for cohesive sediments.

This

o f

13

Flu id isat ion of the upper mater ia l occurred, wi th

the bed sediment los ing i ts e last ic propert ies and

changing f rom a v iscoelast ic mater ia l to a shear-

thinning non-Newtonian suspension (the flocs being

fu l ly supported by the f lu id) . This f lu id isat ion of

the upper material also corresponded with weakening

of the lower layers. The weakening effect of

repeated cyclical stressing was observed at moderate

arnplitudes, with complete loss of measurable

structure occurring at larger amplitudes. After a

recovery period of 2 hours (no waves), the

structure of the bed had not completely reeovered

(recovery only -70t) .

These flume tests indicated that in-situ rheometry,

based on shear wave propagation techniques, can be

used to determine the dynarnical behaviour of a

visco-elastic sediment under uni-directional wave

loading.

F IELD I,IEASUREI,IENTS

3.1 Descr ipt ion of apparatus

A bed frame was designed to enable simultaneous

field measurements of tide and. wave induced flow and

near-bed cohesive sediment processes (Ref 2). The

frame was triangular in shape with an open

structure to offer as little resistance to the flow

as possible. I t had three legs which could be

pushed into the bed to prevent movement (Fig 7).

The following instruments were mounted on the frame:

- 3 Braystoke current meters

- 4 electromagnetic current meters (EMCM)

- 3 Partech curbidity sensors

- I pressure sensor

- 3 ul trasonic probes

L4

The Braystoke current meters were mounted at 0.lm,

0.5m and 1.0m above the bed and in such a way that

they could swivel freely through 360 degrees to

align themselves with the current. These were used

to measure tidal flows and to give a field

calibration of the EMCMs.

The EMCMs measured 2 components of veloeities.

These were mounted at 0. I-rn and 0. 5n above the bed

and set to measure in the x,y and x,z planes at eaeh

height. The output from these was logged at 5Hz to

enable high frequency wave and turbulence

fluctuations to be analysed.

The Partech turbidity sensors were mounted at 0.1m,

0.5rn and 1.0m abo'qe the bed in positions that would

not interfere with flow around uhe EMCMs.

Ihe pressure sensor rras mounted at 0.8m above the

bed. The output was logged at 5Hz and enabled water

depth and wave conditions to be found.

The ultrasonic probes were mounted at approximately

0.Ln above the bed, ei-ther attached to a spur on

eaeh leg of the bedfrane or about 3 metres away from

the nain bedfrane. These enabled the bed level

change during the tide to be monitored.

The output from each of the instruments was logged

onto a computer over the monitoring period. Full

details of the bed frame, instrumentation and

analysis of data are given in Reference 5.

3.2 Detai ls of deployments

The frame was deployed on seven occasions between

Novenber l-989 and Decenber 1990. The first five

15

3 . 3 R e s u l t s

deployments were on spr ing t ides at a s i te on

inter- t ida l mudf lats at the mouth of the River Usk,

near Newport , Gwent . The Usk is a t r ibutary of the

River Severn, and therefore has a spring tidal range

of around 12m. The bed frame was submerged for

about 3 hours on each t ide. Deta i ls of the

deployments and'a surmary of results are given in

Table 3. During some of the deplo)rments

diff iculties were experienced with the pressure

transducer and no information about wave conditions

was recorded. Some wave activity was recorded

during the Apritr and May 1990 deplo)rments and on one

tide during the October deployment.

Two further deploynents were made at Eastham Docks

in the Mersey estuary, on spring and neap tides.

This was a sheltered site with very l itt le wave

activity during these deployments. Details of these

deployments are given in Table 4.

Wave hei-ghts of up co 0.2m were recorded during the

April and May deploynents ac West Usk. l,Iave periods

of 1.5s to 2.5 s 'eeonds were recorded over the same

period. Figures 8 and 9 show the wave height, wave

period and water depth for two of the recorded

tides. As expected, the wave period increased as

the water depth increased, and then decreased again

as the wacer depth decreased. There was l i t t le-

change in the wave height over the same period.

The wave induced'shear stress was calculated fron

the spectral analysis of the high frequency velocity

fluctuations (r"r) and from the pressure wariations

("r ) (se.e Ref 6, Appendix A) . Very h igh values of

re, rdere obtained in a large proportion of the

measurements. These were thoughc to be unrealiscic

16

and were most l ikely due to measurement errors

caused by either flexing of the instrument mountings

or disturbance of the flow around the current

meters. Nevertheless, i t was c lear that even for

qui te snal l waves, . the wave induced shear s t ress can

be h igher than the, t ide induced shear s t ress. This

means that waves are an important factor in

determin ing whether mater ia l is eroded or deposi ted.

The mud uray then be transported away from the site

by the t ida l ve loc ic ies.

Figure 10 shows the wave induced shear stress for 25

May 1990 am, which gave reasonable values of the

wave induced shear stress, cal_culated frorn both

methods. The values of shear stress deereased as

the water depth increased at the start of the

monitoring period and then increased as the water

depth deereased towards the end of the monitoring

pe r i od .

The wave induced shear stress was not high enough co

erode any of the bed in this case. This is probably

because at the l^Iest Usk site, the nudflats consist

of a base of very consolidated mud, on top of which

there is sometimes a soft unconsolidated deposit.

This sof t deposi t is easi ly eroded, and on the

occasions when it has been seen at the l,Iest Usk site

it has been eroded very quickly when the mudflats

are first submerged, and before monitoring begins

(as th is requi res at least hn depth of water) . I t

is thought that erosion of the consolidated rnudflats

would only take place during storms. So far

monitoring has not taken place during a stormy

per iod .

T7

CONCLUSTONS AND RECOI.IMENDATIONS

4.L Conc lus ions

1 . The wave flume was modified to generaLe either

monofrequency or random waves. A series of

cests on Fawley mud compared the criuical

shear stress for erosi-on and erosion rate for

equivalent nonofrequency and random traves. The

cri t ical shear stress for erosion of a bed of

Fawley mud of density 300kgn'3 was around

0. 1Nm-2.

The erosion rate appeared to be higher for the

monofrequency tests than for the random tests

on the same mud, r^rith 1-.0 10-akgm'2s-1 to

1.3 10-4kgp'2s-1 for monofrequency tests, and

4 10-5 to 6 L0'5kgn'2s'1 for che random tests.

These erosion races were for wave periods of

around 1.2 seconds. If one assumes that

erosion takes place for the srme proportion of

the wave cycle even for a longer period rrave,

then erosion rates of around 1 19-51*-2r'1

were estimated for a random wave of period

around 7s (a more realistic wave peri-od for che

f ield) .

The density of the mud bed did not change much

during a test. However, the denslty profiles

rcere measured half way along the test section

and some changes in bed thickness were

observed. Transport of rnud from the paddle end

of che flune towards the beach end was

observed. Transport rates per unlt width of

Lhe flurne were calculated to be 1 19-5p*'1"-1

Co 4 l_0-5kgm-1"-1.

2 .

3 .

18

5 .

Laboratory measurements and in-si tu rheological

measurements of mud under waves were made by

Universi ty Col lege of Swansea (Appendix 1). A

rheometer, which used the sensitivity of the

shear wave velocity to the structure of the

bed, was incorporated into a wave flume.

Under the action of waves, fluidisation of the

upper material occurred, with the bed sediment

losing i.ts elastic properties and changing from

a viscoelastic material to a shear-thinning

non-Nerctonian suspension. The weakening effect

of repeated cyclical stressing was observed at

moderate arnplitudes, with complete loss of

measurable structure occurring at larger

anplitudes. After a recovery period of 2 hours

(no waves), the structure of the bed had only

recovered about 70*.

A bed frame was used to make field measurements

of wave parameters at the same time as near-bed

cohesive sediment processes. Wave periods of

l - .5 to 2.5 seconds were recorded, with wave

heights up to 0.2n. The wave period increased

as the depth increased over the inter-tidal

nudflats, where the frame was deployed. The

wave induced shear stress was highest when the

water depth r{as very shallow; however, this was

not high enough to erode the consolidated

nudflat although it probably eroded all- soft

uneonsolidated deposits on the surface befbre

the bed frame nonitoring started.

In-situ rheometry, based on shear \rave propagation

techniques has been successfully used to determine

the dlmamical behaviour of mud under uni-directional

waves. It is recourlrended that this technique is

4.2 Reeommendations

19

used more extensively to cover wider ranges of the

wave parameters and the resulting effect. The

structure of the mud appears to be one of the most

important factors in deterrnining the behaviour of

the mud. Other parameters' such as density, are not

so useful as tt has been shown that the density can

remain unchanged even though the structure has

changed.

The bed frame has been used on a number of occasions

to measure \rave Parameters at inter-tidal sites.

During the deplo1rments the wave induced shear

stresses have not been high enough to erode the

consolidated urudflats, al-though thin layers of soft

unconsolidated deposits have been eroded very

quickly in the shallow water as the tide rises (and

before the noniLoring equipment can be started). It

is recommended that the bed frane is deployed during

a period of high activity when the consoli-dated

nudflat night erode. In addition it would be useful

to deploy the bed frame sub-tidally, so that the

effects of the high wave shear stresses in very

shallow water rdere not observed.

5 ACKNOSLEDGEI.IENTS

HR ilallingford grauefully acknowledges the

rheological measurements of Dr D J A Wil-liams and

Dr P R t{illiarns of the Department of Chemieal

Engineering, University College of Swansea.

HR !ilallingford also acknowledges the work of

Ing. Ismael Piedra Cueva, who continued the research

on wave fluidisation at HR with the aid of grants

fron the Uruguayan Government and the EEC (DG XII).

20

REFERENCES

1 .

2 .

3 .

4 .

5 .

6 .

Hydraulics Research. "Laboratory experiments

on a near-bed turbid layer". HR Wall ingford,

England. Report SR 88, June L987.

Delo E A, Diserens A P. "Erosion of nud by

waves: Development of a field measurement

system". Hydraulics Research, Wallingford,

England. Report SR 225, January 1-990.

Soulsby R L and Snalhnan J V. "A direct method

of calculating bottom orbital velocity under

waves", Hydraulies Research, I.Ial-lingford,.

England. Report No SR 75, February 1985.

Delo E A. "Estuarine muds manual" Hydraulics

Research, l{allingford, England. Report No SR

L64, February l -988.

Piedra Cueva I. "Erosion of nud by waves:

Analysis of Fawley wave fh.rme tests'r,

Hydraulics Research, l,Iallingford, England.

Report No IT 358, October 1990.

Diserens A P, Delo E A, Ockenden M C.

"Estuarine sediments - near bed processes:

Field measurement of near bed cohesive sediment

processes", HR l la l l ingford, England. Report SR

262, Apri l l -991-

2L

Tables

TABLE I Equations used in calculation of bed shear stress from bottomorbi ta l ve loc i ty

Input parameters

U.r" Root-mean-square bed orbital velocity for random r,raves (r"-1)Tz Mean zero crossing period (s)v Viscosity of f lu id (rnzs- l ;p Density of fluid (kgn-3)ks Nikuradse equivalent grain roughness (n)

Calculated parameters

Um Orbital velocity for equivalent monochromatic wave (r"-1)A Semi-orbital excursion length (n)R" I.Iave Relmolds numberr Relative roughness (AA)Fs Smooth friction factorFr Rough friction factorFH Wave friction factorr Representative bed shear stress for random wave spectrum(tirn-2)

Equations used

um : -/2 ums

r : 0 . 5 p F r U r 2

A : U r T . / 2 r

\ : u r A / v

F" - 2 R"-o'5 if Rw < 5 105 Laminar

Fs : 0.0521 q-0'187 if Rw > 5 105 Snoorh rurbulenr

Fr : 0.3 i f r < 1.57 Rough turbulent

Fr : 0.00251- exp(5 .2L r-oJ\ i f r > 1.57 Rough rurbulenr

F, - max(Fs,Fr)

O O N O O o N o : q 9 1 4 9m h N N - e F r N N e N

r o o o o o o o o o o o o

O \ O o e 6 F l \ O 1 4 \ O t a a g ' !i i e F O N N F N N N

o o o o o o o ( 3 0 0 0

n \ oG t N

o o

c c t @ 6 0 h( ] O C T F N

r o o o o o

h c ( l o t \ l a o $ F mN o r r \ l N a \ l n I ( \ t o F a 6 - 6 e ( \ l ( \ r ( \ r

F F F F E F F . ! F F E F E

tft r.l

c) (fr n r A r n l ' | r n r n r n r n r | h

E o o o e 0 0 0 0 0 c ,

o o N t \tt lo N (\l

o o o c )

(\l ^t nt (\t N (\t N a\l .\l

\ t nN N

o o

\o ri .o

o o o

o o. \ l i l N O e

e F e

o o o < t oc t O O O r Om ].t r?t nt l.l

EE8ee ??ee8 es

e N l r t F G . I

E== *r

o@

t oo cN . =

i O F l - t N N N o o l \r 4 1 4 N ( \ l m G J t 4 t 4 N

O O O O O < > r C f O C )

I \ - t \ \ t h l \ t \ o N N( \ l ( \ . l e e ( \ l ( \ l l O n t i l

O O O O O O T O O O

oN ! )e c

( ] E\o

o o< ) cO E

vo0,o-b

o c ' \ O \ O e | O \ O € | \ . f . \ l o q q 1 4 f - r O e ! !t r 3 :E t rF= =88888 =8= ==O ( > < ) ( > T O O O O O O O O O O O O O O

$$3pPp BBP 8s$Sb8 t rh t r 8E- i e ( f o o e o o e o F

O C t C t O C t O I C r C t O O ( : C t C t ( > O O C t O < t O

N I \Cf, CI

o o

Rt ro oc t o

c r o a o c o c r o q , N l m c t h @ Q t 4 r Q 1 4 s ! \ l==88 E=B 888888 889 5Eo o o o ( t o c , o o o o o o o 0 0 0 ( ]

c r oqqggBg :39 EssEBg 6EE sso c ,- - c r c ) c r o

< ) c t o c t o c r c t o 0 0 0 0 0 0

o@

( t oN C

E

onte a A

r Cer .F\ O E

eo PP f ,8PP EPP 39 t s98 Ph t r 586 e o F e 6 6 e e F e e a i O e c l F e C r F

5 6 'E

nt rnN t t( t o( t o

F ( ! 1 4 N FN N . \ t n t I \c ) < t o o c ,

T O O O O C )

a^o,

@ O o \ O G ( r O i e | O . O J n r t N C o ( \ l n € E r ! n ! ^sgSBgg '=gg ;888s5 888 E5d d d d d d S ; ; ; E d d d d d d d d d d

O E!g @

I * ^ r ^ r N ^ r ^ r i . v . v , . r r B p 8 8 B ^ r ^ r ^ r 8 :o . . o . o( , F e e C O e F e F e

o t u o r o o888 88

bb lbba 'bbda ' db lbab 33 : : :ia i i= 'aa: 'aa '=a"z-raA EEE EESEEESEEEEE E.EC. f .T .E Eg$ EE

. O O t \ 6 F gN r.t i.1 l.l a\l Nc t ( > o o c r c t

o o c t o o ( t

N O \ ( t O r O o\O (\l I'l ti .\r <,o c r c r o o oo o c t o c t o

ocor a h

c t c6l '=

o|\tF ( tr C

Cl 'F

\ o E

ocOr o

o c(\l 'F

N e E

ovo c )c ( \ t g 'v F C

e < t E. c \ oo(,

c oc, q,

O r o . h L) r C P- A o ' = 9 l' E E

aE

eL

D

o o\al Co E

qo

( , @{ , 3

9EE Eo o

! I Lc( E LL O

Lo . t

F -

N 9F S g)rO 'Fo d ,

r F S

Lo oo . >@

E D 3c

o co @o o( ) 9

o ' EL tt)q, 'FN U ,

att 0o o ,o o3 3

P8o o

FSg L

889 9E E

L Lo o

F -

9 vc co op v6 0

56o ( . !

' t ' Po t.- O)o c ,e !o oo o= =

o ov vo o= =

O G O O T O | \ I Fr O F a ! e c t F

6 t't rn l.t rt tt '.l an r4

o ( t o c t c > 0 0 c r e

o€t O

c t cGl '=F E

oo oc ( \ r

v F g )r C-O er 'F

O \ O E

Loo-o o 0 )> r O C6 r . -= c r E

L <O se O -o d ,I !

o > >I D L 9o ! ' -L 6o t t ci 88

.lt

t ,(1,P

LoPog

!o

oot8

6=?€o

toro,cLl

io

thcoI

?oooo=

t\.l

uJ6

F

P

-e ggBtsSeg&sg s$snf i f i

: : P g R E N m N R e N F r . r r ^ e- - - - - - - - - - Q C e E e d et a a ! r L r 4 4 6 r r 4 r r r B r

E58EE688888888e8 ???Er rceeEee8ceS ee3e3e

P6 .0 r oF C

E=

> t q 1 e l ! 6 - c o . oN N N t l N t o t 6 {o o o o o o o o

f r - t . - N 9 . o F - N @N N N F I I ' I

o o o o o o o o

F O O i r . \ l ( \ l n t t \ l

O r O \ 6 O l . \ l ( \ J

o o o

rfr.c rf tt trt t.lc t ( t o c r o

o

t ao c- E

o^ r oF C

o E\o

N 1 . l \ t \ O o \ - O . 6 \ OE e T F T N F N N\= o o o o o o o o

o o\ O c

F t . -O E

o@

ct ttN C

E

onlF ( ,

r COt 'F

€ E

I \ - O r O n 6 g FO . O \ @ $ ^ l ! n r d eO O O F n J

o o o o o o o o

.+ t l l . l F m \O F \$

. o \ o \ o F F F . * \ oo o o

o c ) o c r o c t o o

9 F c , O F ! 4 C t \ l . t

. " 888 Bgg P=E d d d d d d d c ;

€ tlt t\ o. co .c888 889< ) o o o o c ,

n \ t \ t c ( l C t c t\ f \ t $ I \ N ( l to o c > c ) c t oo o o ( t o o

m \ O ] . t \ O . $ No llt l.l .t h \O tne .

o o o o ( 5 0

q r o o . € I . | € c r ? . tt 88 ==8 8Po o o o ( t o o ( t

838 858 $ tc r O O O F O O O

0 0 ( > o o o o o

E o ^ o t \ N . c r . r 6E 888 888 88E c ; d d d d d d d

- BE5 6 ' E

ctqt

. 1 hc t col '=

ct.\JF O

. CCl .Fr O E

cta(,a o

c r cs 'Eot\l ('e C

O EI

E ?.C= 0 E

(t{,( . a aO T

gEE Io o

E LcO LL O

$Lo o

9 -

N PF T

ot

8 ' 6. F ELc r oo - >

o8 r t. a co @o o( ) b

o cL EI'e , . -N 6

t r oa r oO Gt t

PPq q

t tL L

e8ecL Lo o

F -

P Pc co ov v6 0

55C ' ( ,

3:.- tlt

8Co oo 6= 3

F . \ I

o oo o= - ,

c)feO . =

E

.\l

C'Poc

v.coo

3

o=

at!

L

oI € 6 6 r \ r ^ r N N ( \ rO .U ! o c r o

(\' (\I

? Q t o l . l

o o ( : '

o t n t n o o c t c ) o| , t F F n l n h . c r . lS {t { lq ftt t l h h

e888

F nl r.l

5s9 cga E$o . o . o - - - - - -

c,co. t h

c r c(\l .=F E

oo c tC a\l

v ? o, C

Tt Ct .Fo ( ) Egoo-

9 EEO . . F= o E

L E ,o 99 0 -6 0 ,= t t

o > >g ' L P@ t l . -L 6o t ce88

oQ'

o=

Pg r .o ot s c

arPrAe,v

iovoL

Io

csu,Lo-L

oot6

Jo,6

|l,

ooo-olcLJ

!

eov

?oooot

1.1

UJao

F

?ol

J

6EE EEEEE? T?Eo o 6 0 6 0L L L L L L

ot ol o,c c c c c

o o o 9 9 9 8 €i i 6 P8P ? .? .o o o o o o 6 do - o - o - - - - - =

3-

14 \O rno t t \ f o

o o o o o 0o o c ) c t c ) oo o o o ( t oo o c r o c r o

, . ro . C @ r l

6 C t O C 'o c ) o o o < ) 9o o o c ) c t o eo o o o o o oo o o o o o o

EEx@ev >0 ,- .llP ! 96 . - Cs 4 l l

c 5 6 oo v L \' 6 e d ?o o o \L L O ' C ' )u r s E r z

rt

c,I

o

axt\f

o

hN

ct

!4

o

oa.t6

tft |/rN N

. b $ B o oe ( ! r I I g c

a 'a ' b 3 I o I €e v O ) o

r t S c c o c Lee , ! E E I e E3-

6

(t

t lr.to

t

6

c)

lft.tt

oaoo

otnt t

rf

o

r O OAI .\I

o @

t a

\ f r . |

o o

r+

<)ct

r C )

C'( > v

r o o r nro ro (\lo o c t

a

ts tt r.t \O| \ t m G t n lo o ( t ( t

ry=

NE

2

ooP

Lot-

r t r r | . l t t l . l | . |E o o o o o o

v(0

t-

L - C( u e9 O -6 t ut !

o > . >( t r L 9o a t . -L a to r a , c> o o< . o t t

opai6'P

G'

@!

Lo

PoL

!o

c

oovoL

coaoLUJ

rs

u,JcoF

Ip\c

€6L

o o o o ot l t t ! a

IEg€

€ot

J

o o(:' (>t l r +

o oc co oE E

' r nm

h n 6 6

N \ O t - N

^ t ^ J N ^ '

\ O F 6

N r

O\ rat t N F l

1 4 ( ( r or o r o e

o o ( )

h l \ \ Or O r F F

o < ) o

C a

.g ' e3

' l l * ^g se(,oL

9 c ^t ' E go@ r a= i x ^

q EE V

r | \ *nl

o

r @ltl

o

h t n

e |\.1

oo

r l U- P U o - E

d c E- o

o c r o o o o c t o o c t o c ,c t c r o h o c t n c r 6 0 0 66 F n e 6 F C t F F F e C )

a .o ( r o < t o c r o ( t o o o o

o o c ) ( t o o o o o o o oo t t t o o h o o o c ) h o t n

F n t F ( \ l t l m n | \

( ) C t O ( f O O c t ( t O O o e

o c t o o c ) o o o o o or n r n n R l c t c r o o h o u lF F F O n l . l \ t €

c r o o o o 0 0 0 c ) o o

c t o o o o c t c r o c t oO C r C r e O O O 9 o h h. \ l ( \ l h e . \ l I . \ t a ! r n \ O n C O

O ( t C ) O o < t ( t c t c t e F

( f o o o o o o o o c r c r oc o e o c ) o o o e o 9 0 < )t'l \t h € .t \o I \o ln N h l.to ( t o ( t o c r o o o o o o

o c ) o c ) o c r c r < ) o o c ) c ,O F C T O O O O O O | f T ! n o6 h N e t ! a O € c O e . l N G ( ,( t O o e O o c r ( ) e F F e

$ o\ f {

o o

€ \ oa 4 t l

o o

F \ f { ( \ 1 O \ 6 h \ O i O I \ ( \ l lN N N N T A l ' l t . | l . l F F N N

o ( t o o o o o ( t o o o o

( r a \ | F ( > . \ l C O r O \ O l n N O Fa.t tt ta (v t't (\r .\l nr .\, |\l lo ?tl

o o ( t o o o o o o o 0 0

h \ t O nO O F O

< t o o o

c ) o h c t@ l s e e

o o o o

Itt |tt

n l F r . l

h \ o r . t c t < tt l t t l . l t ! . t

o o o o o

€ € c O c ( r . l }r.t lfl Ft r.t .t

o o ( > c r o

C f O c l O r Ott m l.t nt l.l0 0 c t ( 5 0

'iO N .\l

hh O . F

C'

l.l h. n j n l

\ O n

o ( ]

o o or n r n N| . . r n h

( \ l h r n.+ .S .{t

o c t o

h ( \ r \ $rf! .t' .f

cr <t ct

l \ O . r r N o \

G O C O € N F

a O n r O \ O . lF l . | t n e NO N ( ) N c ( 'O F O

o o r n< t O q )o c r o

F N \O ltl |'r \O \O N l.t rtr cO t.lN m m m { r F r . t e ( \ I N N N

o o o o o c r o o o o c r < t

b f x ^Eg"EE

r C ) . 1

trt(\l

oaoL

! ! c" = ( \ rh . = o e: X t - z; g

o o < t 6 0O \ n l \ \ t no F ( 3 O O

c r o o ( f o

o o o ( > oro \o .t \o \ol n h 6o o o o o

oct' , I

o

oo

t t t t ^ l

o

o oe o o

. O r a . a r F F

. o oo

< ) 8 8. Q r r . . . r ^ . { lt !

! c r o(t

o oO c r o. e | . r t . 3 t n ( \ lN . o oo

o 8 8r O . . r t , C ( l ( ttn. o oo

c'er o E -r - o

O l eJ l <v q

- E6

o c

E e E{ r n( ) .c oo xo g

E!o

!

8_ .53 e e

O e

o xgE

c t o o 6 h@ @ @ \ O t sF F e O C f

( 9 0 0 0 0

c t c ) o o oN N e O @@ c o c ( t n t o

o o o o o

o o o ( f , oo o o N @c\t $,t ^J o o

o o o o o

o o o < > r ntt! rn nl rn N€ o 6 c o N No o o o o

o o@ of.l ..t

oFI

t af.l

8688S 8838t t t t f . t t l t t ( \ t t t t o I . l

( ) N ! 4 G r F o | . t F o F \ O u \

a.t tt t4 tt a.t l. l t l r.t Ft rl t l l .!

8888 8888 8888\ \ \ \ \ \ \ \ \ \ \ \h h h 6 c t ( o @ ( ( t o o o c ,c r o o o o o o ( )\ \ \ \ \ \ \ \ \ \ \ \nNxR ts83P 8838

o o o c f , ( fO. O. O\ O\ O.\ \ \ \ \\ \ \ \ \O O e T O FA J N N N N

ovtrtltoo.oato!(,octE€

!

oou

r \ l O r ! O O < t O \ N u l c O F ( ) F

588R ESPS 6$8Ne O F C T . \ I O | \ t O F O F O

\t I\ t\ O\ t\t t r t F t m m

c o o t l N n ltsRSNEF C T F C t C I

88888\ \ \ \ \\ C \ f r t r i \ to o o o o\ \ \ \ \\ C 6 6 \ O l \N N N N N

( , l ( 'P Ee , - r nI D X E . A NF g h

O C t v

O - PA I . F A

l A E ( , m9 F \

g gd e oL

T P N ( \ r c l o0 , !-O ot! = E nl t\t a\t t\t

! F O - v

9 Q t 6O - 9

L E( U > l \ . $ o i t \

E 8== I dB5 tsf ' - O - o - e O t \ l Ov r F o

!

J 6 ( > ( ' \ O \h g l a o c o c o c t = \ \ \ \U F e F NJ o s( a u ( , \ \ \ \

F C t = ( \ t l . t t o c t

Figures

$.gH

Eg

.P=PF86F3

9 'eaE*EEE E

E EO g ] J

F o o !

Efr{Fg "r$

6o(It=E.ottcEo

baE5o fC go oO+,G

o 9tb>->

EooEo=

s'tr€

Fig 1 The wave flume

Concentrotion 6long f lume, test FM 1 8

g O mins

1- 60 mins

o 90 mins

A 1 2 O m i n s

X 1 5 O m i n s

V 1 8 O m i n s

nIEoJ

co

.:"co

coo

o . 1 3

o . 1 2

o . 1 1

o . 1

o.09

o.oa

a.o7

o.o6

o.05

o.o4

o.o3

a.o2

o.o1

o1 3 5

Distonce olong flume (m)

Id

N

l ^F ( )O IY Uv o

o -

9 6F

c :O Foo

u

o.2

Peok bed sheor stress (Nm-Z;

Erosion rote, test FM18

Suspended sediment concentrct ion c longf lume with t ime, ond erosion rate ogoinstpeak bed shecr stress, Fcwley test FMlB

Fig 2

Fowley monofrequency tests

c cs e

* v *z v

z o

6E ea9 c

ztt

qN

It

!

o

c.9oo

LI

o.ooo14

o.ooo13

o.ooo12

o . o o o 1 1

o.oool

o.oooo9

o.ooooa

o.ooooT

o.oooo6

o-oooo5

o.oooo4

o.oooo3

o.oooo2

o.ooool

o.1 0.2

Peok bed shedr stress (ttm-Z;

o.3

Io

NIEo

oo

c.9oo

LI

o.ooo14

o.ooo13

o.ooo12

o.ooo11

o.ooo1

o.oooo9

o.ooooa

o.ooooT

o.oooo6

o.oooo5

o.oooo4

o.oooo3

o.oooo2

o.ooool

o .1 0 .2

Peok bed sheor stress (mono equiv.)

o.5

Fowley rondom tests

ErosionS I T C S S ,

random

rate ogcinst peok bedFcwley monofrequencywove tests

sheorcnc

Fig 3

Test Fl/19

o 0 mins

+ J0 mins

o 60 mins

A 90 mins

X i20 mins

V 180 mins

oa

-o

c'l

o

o

oo

ol' 6

-

0 . 1

0.09

0.08

0.07

0.06

0.05

0.04

0.0J

0.02

0.01

0

Fig 4 Densi ty prof i les dur ing Fcwley testFM19

Test FM19

tr 30 mins

+ 6 0

o 9 0

A 120

x 150

v 180

Eo

oo-

o.:. go

Q)

o,

o

0.02

0.015

0.01

0.005

0

-0.005

-0.01

-0.015

-0.02

-0.025

Fig 5 Bed levels c long f lume dur ing Fcwleytest F-M 1 9

Fowley monofrequency iests

FMz5

FM'I6

FMl7

c

' ;o

oEoo

r,-

o

o

@c

q)

5

80Time ofter stort of test (mins)

f iq 6 Volume trcnsported dur ing Fcwleymonofrequency tests

f ig 7 The bed frcme

3.5

3

2.5

2

1 . 5

1

Woter depth 24 Moy 1990 Pm

o 24/5/9opm

Esoo

!

do3

o.5

o- 1 5 0 -5O 50

Time relotive to high woter (mins)

Wove chorocteristics 24 Moy 199O pm

o

co

oo

v.9oo

o

o3

1 . 5

F

co'q,

c

o . 1 5 I]c

.vu ' ' | i

.9a

o Hs 24 Moy 1990 PM

a rz 24 Moy 1990 PM

o.o5

o- 1 50 -50 50

Time relotive to high woter (mins)

Fig B Wcter depth, wove height ond wcveper iod at West Usk f ie ld s i te,24 Mcy 1 990 pm

Water depth 25 Moy 1990 om

^ 25/5/90om

3 . 5

E

o -v

6- o t <9 " '

o- l c u

2ovcoq)o

o 1 ' 5

.9ooo

o 1;

o.5

o- 1 5 0

' 150

U . J

4.25

!

.9o

^ - - oU . I D >

'co

Gu . r ' =

.9a

o.o5

o

-50 50Time relotive to high woter (mins)

-50 50Time relotive to high woter (mins)

150

Wove chorocteristics 25 Mov 199O om

A HS 25 MOY 1990 AM

+ -fz 25 Moy l99O AM

Wcter depth, woveper iod at West Usk25 Mcy 1 990 cm

height and wcvef ie ld s i te,

Fig 9

25 Moy 1990 om

A T w

B Tew(1)

+ Tew(2)

o Tew(J)

c!IE

z

a@

aa

a

0.7

u.o

0.5

0.4

0.3

0.2

0 .1

0-40 0

Time relotive to high woter (mins)

Wcve induc ed bedWest Usk si te. 25

sheor stress aiMcy 1 990 cm

Fig 10

Appendix

IRI.IEOIJC)GY' c'F Co)IIESIVE SEDrME TTIS' IJINI)EIR

I^IAVE ACTIOhit

R.EPOR.T

b5z

D. J . A- WILLIAMSA A} i rD P- R. . h ' I I -L IAMS

Department of Chernical Engineering

University College of Ssansea

Singleton Park

Swansea, SA2 8PP

FEBRUARY 1990

lTo rhot correspondence should be addressed.

FIGURE ( I }

Water surface-

3.55m

Testsignals

FIGURE (2 }

(!Ivl

c{E

(\l

>,

2000

Time (sec.)

3000

RUNS

FIGURE ( -: }

GI' ,a 100

GI

5(!

1000Time (sec.l

RUNS8-13

2000

Fr'b *

80

70f , = 0.83 Hz

o to= 0 sec.ot=600sec.

60

50

f401n

(\

5i30

Disptacement - Stroke

FIGURE ( 5 }

(tlo

Initiat butkdensity of slurry

(q)

oooo-

ooooo

oooooaoo

o

o

o

o

' oo()aO

ao

<- #1

co

o

Oa

oa

oo_.<:-Oa

OC

o

ETJ qC,

.9rD(U+

l r I

#3

1300 , 1400

Butk density (kglm3)1200 1500