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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)
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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
!
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c.9oo
LI
o.ooo14
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o.ooooT
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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
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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
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@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
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' 150
U . J
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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