LABORATORY ABRASION OF GREY FLINTS, RECHARGE MATERIAL,
SHINGLE-SAND AND SHINGLE-GRAVEL MIXTURES
Compiled by U. Dornbusch. Edited by C. Moses and R. Williams.
1
2
3
4
Aim ....................................................................................................................................2
Introduction .......................................................................................................................2
Methodology......................................................................................................................3
Results ..............................................................................................................................4
4.1
Factors influencing abrasion rates .............................................................................4
4.1.1
Location ..............................................................................................................7
4.1.2
Shape .................................................................................................................8
4.1.3
Season..............................................................................................................10
4.1.4
Discussion ........................................................................................................11
4.2
Abrasion of recharge material..................................................................................11
4.3
Abrasion of shingle-sand mixtures...........................................................................12
4.4
Abrasion of shingle-gravel mixtures.........................................................................13
4.4
Discussion................................................................................................................14
5 Outlook for BAR phase II ................................................................................................14
6 References......................................................................................................................15
BAR Phase I, February 2003 – January 2005
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1 Aim
The aim of this work is two-fold:
• to assess the extent to which grey/black flint pebble abrasion rates are controlled by
flint type, pebble weight, pebble shape and surface textural characteristics.
• to measure abrasion rates of beach recharge materials, comparing abrasion rates for
sand-shingle and gravel-shingle mixtures with those of pure grey flint shingle.
2 Introduction
Pioneering laboratory abrasion experiments conducted during the Interreg II funded project
BERM (Beach Erosion in the Rives Manche), provided the first published measurements of
grey/black flint shingle pebble abrasion rates (Dornbusch et al., 2004). These baseline data,
when combined with long term field experiments, have been used to provide estimates of
beach volume losses due to in situ shingle pebble abrasion (Dornbusch et al., 2003;
Dornbusch et al., 2002).
Now that flint shingle abrasion rates are established, the BAR project is refining these early
experiments in order to investigate the key factors that control the rate of abrasion. For
example, the BERM experiments showed that the abrasion rate is controlled by the weight,
and therefore size, of individual pebbles (because changes in weight can be measured more
accurately than changes in size, pebbles are selected according to weight and changes
measured as weight changes). The experiments were conducted on pure flint shingle
pebbles with no sand or gravel present in the laboratory tumbler. During Phase I of BAR
these experimental results have been interrogated in order to identify key aspects that
require further investigation. These include:
• Abrasion of grey flint shingle pebbles recently supplied from cliffs and shore platform
in comparison to that of flint from the seabed used in beach recharge schemes.
• Abrasion of pure flint shingle pebbles from different positions on the beach and
collected at different times.
• Abrasion of pure flint shingle pebbles in comparison to that of sand-shingle and
gravel-shingle mixes in order to assess the impact of beach recharge materials on
flint shingle beach abrasion.
• Measuring of pebble shapes to elucidate large variation in the abrasion rate of
individual pebbles.
Experiments to assess the influence of the location on the beach have been carried out.
These experiments were also used to compare how the new tumbling barrels performed in
comparison with those used during the BERM experiments where average abrasion rates for
different size ranges were established. Experiments involving flints from recharge material
have so far only focussed on the size range 150-250g, which has also been used for the
experiments using mixtures of shingle with sand or gravel.
Between BERM and BAR the tumbling barrels have been changed. This was necessary,
because the hexagonal, rubber lines metal barrels used in BERM were difficult to close and
corroded so that they required a large amount of maintenance. The new barrels are made of
rubber lined plastic with one simple screw-on lid which has reduced maintenance
dramatically. A choice of different barrels sizes allowed the selection of a significantly larger
barrel than used for BERM. Because the inter-sample variation in the abrasion rate is high,
larger numbers of pebbles in one sample will provide a more stable average abrasion rate for
BAR Phase I, February 2003 – January 2005
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each sample. The increased volume of each sample also means that fewer barrels have to
be tumbled thus reducing the possible influence the barrel or the tumbling apparatus may
have on the abrasion rate. For the size range 150 to 250g the average number of pebbles in
the BERM tumblers was 24 compared to 97 in the BAR tumblers (for the size 250 to 350g,
17 compared to 64).
3 Methodology
The methodology employed is detailed in a work protocol (2004-07-12- Protocol lab abrasion
of shingle mixtures.doc). The main points to note are that pebble handling (weighing,
tumbling and drying) are carried out according to a strict schedule and that the selection of
samples follows clear rules. This means that the pebbles used for the tumbling experiments
are always a sub-sample of the much larger sample taken from the beach. The pebbles used
in individual tumbler barrels are selected according to the protocol shown in Figure 1.
or
Figure 1: Schematic diagram showing how tumbling samples are created.
Initially, one tumbler barrel is filled with pebbles and the interstices filled with reconstituted
seawater (for the size range 150 to 250g, this takes approximately 97 pebbles and 5.6
litres of water). The barrel contents are then evenly divided in two and placed in separate
barrels that are the same size as the original (as shown on the the two left hand
diagrams). For the purposes of these experiments, this state of being half full is referred
to as ‘100% filled with shingle’. In experiments involving shingle mixtures (shown on the
right hand), a percentage (20 or 30% of the 100%) of pebbles is removed and replaced
with sand or gravel resulting in 30/70% or 20/80% mixes.
After selection, the pebbles are dried for 24 hours at 50°C and weighed to three decimal
places (balance error ± 0.002g). They are than placed into the barrel together with the
reconstituted seawater and left for 24 hours. During the last 2.5 hours they are tumbled at an
average of 21.3 revolutions per minute. The barrel content is drained trough a sieve and the
pebbles retained are again dried for 24 hours at 50°C prior to weighing. Each sample is
tumbled three times so that three rates of abrasion are determined. For comparing samples,
the abrasion rate for the first 2.5 hour tumbling interval is used.
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4 Results
4.1 Factors influencing abrasion rates
Samples for BERM experiments were collected predominantly from the beach at Telscombe
as this beach was also used for field abrasion experiments. In anticipation of the requirement
for larger numbers of pebbles for the BAR experiments, the beach at Telscombe proved to
be too difficult to access (long distance between beach and vehicle including steps). The
beach at Newhaven provides much better access and different sources with respect to the
location on the beach. The beach features a very large backshore area from which material
could be easily obtained but the pebbles in this area are unlikely to have been affected by
waves for a few decades and therefore may abrade differently from those on the active shore
face. The BAR experiments would also necessitate that samples be collected at different
times of the year because it is impossible to collect all the material that might be used over a
two year period from one location during one day (not enough pebbles of the same size and
shape available + storage problems at the University). It was therefore necessary to assess
the influence of the position on the beach (backshore or foreshore1) and the season of
collection on abrasion rates. Theoretically, weathering or the abrasion from people walking
over the beach would have a larger impact on pebbles in the backshore area and might lead
to different rates of abrasion. Equally, different wave environments in summer and winter
may change the rate of abrasion under standardised laboratory conditions.
Because previous experiments involved abrasion rate measurements on beaches in France
(Criel and Etretat) a comprehensive set of samples was collected from Criel which had
previously shown abrasion rates comparable to those at Telscombe. However, wave climate
and geology differ due the different orientation of the French coastline and lateral variation of
the Chalk.
A total of twenty one barrels were tumbled for these experiments. Rounded grey flints were
collected from
• the foreshore at Telscombe in August 2003
• two backshore locations at Newhaven in August 2003
• foreshore locations at Criel in December 2001 and foreshore and backshore in
September 2003.
The chalk cliffs behind the beach at Telscombe consist of Newhaven Chalk (Old Nore beds
are at the bottom of the cliff) and are upper Santonian to Lower Campanian in age. The chalk
cliffs behind Newhaven west beach consist of both upper Newhaven Chalk and lower Culver
Chalk and are Campanian in age. The chalk cliffs at Criel are middle to upper Santonian in
age.
1
Foreshore in the context of this report refers to the part of the beach that is undergoing change from
waves.
BAR Phase I, February 2003 – January 2005
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Newhaven lee of old BERM 300 B
Tum bling interval (hours)
Newhaven lee of old BERM 300 A
Newhaven winterberm 300 B
Newhaven winterberm 300 A
Criel 400 foreshore 09/03 B
Criel 400 foreshore 09/03 A
Telscombe 350-450 A
Criel 300 backshore 09/03 B
Criel 300 backshore 09/03 A
Criel 400 backshore 09/03 B
Criel 400 backshore 09/03 A
Criel 300 foreshore 09/03 B
Criel 300 foreshore 09/03 A
Criel 400 foreshore 12/01
Criel 300 foreshore 12/01
Telscombe 250 - 350 B
Telscombe 250 - 350 A
Telscombe 150-250 B
Telscombe 150-250 A
Criel 200 foreshore 09/03 B
2.5
5
7.5
Criel 200 foreshore 09/03 A
Mean hourly abrasion rate (%)
0.16
0.14
0.12
0.1
0.08
0.06
0.04
0.02
0
Figure 2: Abrasion rates for all samples over three tumbling intervals of 2.5 hours each.
Abrasion rates are given as the percentage weight loss per hour (Figure 2). In actual weight
loss this is equivalent to values from ~0.1 g to almost 2 g per pebble, depending upon pebble
size, during a 2.5 hour tumbling interval.
All samples show the decrease of the abrasion rate with time that was found in previous
experiments. There is also a difference in the abrasion rate for sample pairs (A and B) that is
consistent throughout the 3 tumbling intervals. Nevertheless, since the revolution rate of the
tumblers is consistently different by ~1.2 revolution/minute (at a mean rate of the two
tumblers of 21.3 revolutions/minute) barrels were swapped over 3 times during the 2.5 hour
tumble for the last 6 samples in Figure 1. This does not seem to have changed the difference
between the sample pairs (Figure 3). The difference between the paired samples appears to
be randomly distributed so that factors associated with the barrels or tumblers do not seem
to have any influence.
Figure 4 shows that the weight/abrasion rate relationship found in previous studies holds only
partially if all samples are considered. The samples with a mean weight of 200g abrade at a
consistently lower rate than the heavier ones but there does not seem to be an obvious
difference in the abrasion rate between the 300 and 400g samples. However, if only samples
from the same beach are analysed (Figure 4b) the relationship between size and abrasion
rate shows up clearly again. This indicates that factors other than size have a significant
impact on the abrasion rate.
BAR Phase I, February 2003 – January 2005
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Te
ls
Te com
C
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Le rie l s co be
2
C 0 m b 150
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00 ep 0 0 B
Te B s tem -25
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N New Cr m b ER 20
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C o erb ks h A
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D ore re
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00
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Le
Mean hourly abrasion rate for the first 2.5
hours (%)
0.1
criel 200
criel 300
B
0.12
weight
0
Stdev in % of A
0.14
14
0.12
12
0.1
10
0.08
8
0.06
6
0.04
4
0.02
2
0
0
0.18
0.16
0.14
0.12
0.1
0.08
0.06
0.04
0.02
0
400
abrasion rate
350
0.08
300
0.06
250
0.04
200
0.02
150
BAR Phase I, February 2003 – January 2005
Science Report: Report on waves and tides
Standard deviation (%)
A
450
400
350
300
250
200
150
100
50
0
Mean weight of sample (g)
Criel 200
September
03
Telscomber
200 August
03
Telscomber
300 August
03
Criel 300
December
01
Criel 400
December
01
Criel 300
foreshore
Septem 03
Criel 300
backshore
Septem 03
Criel 400
foreshore
Septem 03
Criel 400
backshore
Septem 03
Newhaven
300
winterberm
Newhaven
300 lee of
old berm
Telscombe
400 August
03
Mean hourly abrasion (%)
0.16
16
Figure 3: Abrasion rates for all samples during the first 2.5 hour interval together with the
standard deviation between the paired samples in %.
Figure 4: Comparison of the mean
hourly abrasion rate for the first 2.5
hour interval (columns) and the
average weight of the sample
(squares). A) (above) all samples.
B) (below) only samples from Criel
collected September 2003
(foreshore and backshore).
criel 400
100
6
4.1.1 Location
The location on the beach from which samples were collected for use in the laboratory
experiments may have influenced results if the pebbles were to exhibit different surface
textural characteristics. For example, pebbles on the storm berm or at the back of the beach
may exhibit more brittle surfaces, or a greater development of surface chattermarks, because
of the high wave energies that they have experienced in comparison to pebbles further down
the beach. Alternatively, they may experience lower abrasion rates because they are less
chattermarked or have lost some of the brittle surface due to the fact that they are not
constantly battered, unlike pebbles in the surf zone for example. Similarly, their removal from
the active surf zone may mean that they are affected by weathering processes which may
also alter the surface texture possibly making it smoother. Trampling by people may also
provide a gentle form of abrasion that could reduce the thickness of the brittle surface,
equivalent to some time spent in the tumbler.
These differences were tested on flint pebbles collected from the beaches at Criel and
Newhaven. At Newhaven samples of the same size were collected in the summer of 2003
from the position of the previous winter’s storm berm and from a position landward of the last
storm berm (Figure 5), i.e. from a position that has been out of the abrasion influence of the
sea for several decades but which is heavily subjected to passage of people.
Mean hourly abrasion (%)
0.18
0.16
0.14
0.12
0.1
0.08
0.06
0.04
0.02
0
New haven 300
w interberm Summer 03
A)
New haven 300 lee of old
berm Summer 03
B)
Figure 5:
A) Approximate location of samples.
B) Comparison of the abrasion rate for the first 2.5 hour tumbling interval of the A and B
sample (columns) and the average of the two barrels (green bars) for samples from
Newhaven.
The abrasion rate for the samples from further back on the beach is 20% higher.
BAR Phase I, February 2003 – January 2005
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At Criel samples of all three weight groups were collected from the active foreshore and the
flat backshore area just to the west of the eastern terminal groyne. Public access here is
likely to be limited but there is no information as to whether the flat back beach area has
been smoothed by bulldozers.
0.16
Mean hourly abrasion (%)
0.14
0.12
0.1
0.08
0.06
0.04
0.02
0
Criel 200
foreshore
Criel 200
backshore
Criel 300
foreshore
Criel 300
backshore
Criel 400
foreshore
Criel 400
backshore
Figure 6: Comparison of the abrasion rate for the first 2.5 hour tumbling interval of the A
and B sample (columns) and the average of the two barrels (green bars) for samples from
Criel collected in September 2003.
The results are not consistent in that for the size classes 300 and 400g the averaged
abrasion of the backshore samples is higher while for the size class 200g it is smaller (Figure
6). It would appear that location on the beach does have some influence on the abrasion
rate. One explanation for this may be a link between the brittleness of the surface and the
energy conditions to which they have been subject. However, this surface must have a
considerable thickness as the difference in the abrasion rate persists even after the third
tumbling interval (Figure 2). Also this effect seems to survive several years of subaerial
exposure as is the case at Newhaven.
As a consequence of these results all experiments involving shingle mixes use shingle
pebbles collected from the same source and involve two barrels filled with 100% shingle as
controls.
4.1.2 Shape
Though the above comparisons show that abrasion rates can vary significantly between the
average of whole samples (inter-sample variability), previous experiments have drawn
attention to the fact that the abrasion of individual pebbles in one sample can vary
considerably (Figure 7). The standard deviation of the individual abrasion rates (intra-sample
variation) averaged over all 21 samples (see 4.1.1, Figure 2) is 24%, ranging from 16% to
55%; the maximum difference is considerably higher.
BAR Phase I, February 2003 – January 2005
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Telscombe 300 A
Le Criel 400
Telscombe 400
Hourly abrasion rate (%)
0.25
Telscombe 300B
Telsc 200 A
Criel 200 A
Le Criel 300
Telsc 200 B
Criel 200 B
0.2
0.15
0.1
0.05
0
150
200
250
300
350
400
450
Pebble weight (g)
Figure 7: Abrasion rates of individual pebbles from the same sample plotted against the
individual pebble weights.
To assess what might be causing this variation all three pebble axis have been measured for
227 pebbles from the paired samples of the Criel backshore samples in the size range 250 to
350 (2x64 pebbles) and 350 to 450 (49 and 50 pebbles). Correlation analysis shows the
following results (Figure 8).
Pearson
ABRA- WEIGHT
SION
Correla- ABRASION 1.000
.267
tion
WEIGHT
.267
1.000
A
.022
.363
B
.253
.654
C
.133
.472
b/a
.162
.213
c/b -.053
-.050
c/a
.075
.133
KRUMBEIN
.126
.188
.060
-.029
zing
number
A
B
C
b/a
c/b
.022
.253
.133
.162
-.053
.363 .654 .472
1.000 -.069 -.341
-.069 1.000 .198
-.341 .198 1.000
-.699 .751 .360
-.247 -.529 .719
-.744 .165 .872
-.809 .468 .722
.297 .238 -.599
.213
-.699
.751
.360
1.000
-.218
.611
.868
-.026
c/a KRUMBEIN
.075
.126
zing
number
.060
-.050 .133
-.247 -.744
-.529 .165
.719 .872
-.218 .611
1.000 .632
.632 1.000
.291 .922
-.655 -.570
.188
-.809
.468
.722
.868
.291
.922
1.000
-.380
-.029
.297
.238
-.599
-.026
-.655
-.570
-.380
1.000
Figure 8: Correlation analysis for 227 pebbles of two different size classes.
The highest correlation exists between the abrasion rate and the pebbles’ weight. However,
plotting both against each other does not show a convincing correlation (Figure 9). None of
the pebble shape factors has any significant correlation with the abrasion rate. This holds
also true when the analysis is carried out for individual barrels. It is probable that factors
other than shape, such as flint geological and surface textural properties, have significant
influences on the rate of abrasion.
BAR Phase I, February 2003 – January 2005
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500
pebble weight (g)
450
400
350
300
250
200
150
100
50
0
0
0.05
0.1
0.15
0.2
0.25
Mean hourly abrasion (%)
Figure 9: Scatter plot of individual pebble weights and abrasion rates together with a
linear regression curve.
4.1.3 Season
rie
l3
00
D
C
ec
rie
em
l4
be
00
r0
D
1
ec
em
C
rie
be
l3
r0
00
1
f
C
or
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es
l3
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rie
sh
l4
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fo
C
re
rie
sh
l4
or
00
e
ba
ck
sh
or
e
0.16
0.14
0.12
0.1
0.08
0.06
0.04
0.02
0
C
Mean hourly abrasion (%)
Seasonal changes in wave climate may change the abrasion rate for samples collected from
the same position on the beach. Alternatively, longshore transport can lead to a different
population of beach material being in the same position at different sample times. The
comparison between samples collected from the foreshore at Criel in December 2001 and
September 2003 seem to indicate that the earlier samples abrade at a higher rate (Figure
10).
Figure 10: Comparison of the abrasion rate for the first 2.5 hour tumbling interval of the A
and B samples (columns, only one sample each for December 2001) and the average of
the two barrels (green bars) for samples from Criel collected in December 2001 and
September 2003.
BAR Phase I, February 2003 – January 2005
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4.1.4 Discussion
The present data show that the abrasion rate of whole samples depends on the average
weight of the sample (thus confirming previous results) and the location from which it was
collected, i.e. which beach + location on that beach. Samples taken from the same position
on a beach but at different times also seem to differ in their abrasion rates possibly due to
differences in the wave energy experienced by the pebbles prior to collection or by the fact
that a different population was sampled (longshore transport may replace one type of shingle
with another).
It is therefore essential, that the experiments to compare pure shingle with shingle and
sand/gravel mixes use shingle collected from one population.
The large variations in the abrasion rate of individual pebbles within one sample still eludes
an explanation as factors like weight or shape show only weak correlations with the abrasion
rate and in combination do not increase the predictability of the abrasion rate.
4.2 Abrasion of recharge material
Recharge material was collected in July 2004 from the Sovereign Harbour end of Pevensey
Bay. It was originally dredged from the Hastings Bank. From the mix of shingle, gravel and
sand in the recharge material, only pebbles in the size range of 150-250g were selected to
match the size and shape of the grey flint used in previous experiments. Altogether four
samples (barrels) of the recharge material have been tumbled.
Comparing the abrasion rates with those of the same sized material from both Criel and
Telscombe (Figure 11) shows that the abrasion rate for the first 2.5 hours is very similar
(averages for Criel = 0.072, Telscombe = 0.071, recharge = 0.075). However, the rate at 7.5
hours appears to be slightly lower than that for Telscombe. Although the highest rate for one
sample comes from the recharge material, so does the lowest. It therefore appears, from this
preliminary experiment, that the difference in abrasion rate of similar sized pebbles of
geologically recent dark grey flints and those extracted from the sea bed for the use of beach
recharge is very small. More detailed experiments will be conducted to further investigate this
preliminary finding.
BAR Phase I, February 2003 – January 2005
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11
2.5
5
7.5
Cr
ie
l2
00
Cr
b
ie
l 2 ack
sh
00
Ba or e
Cr
ie
09
l 2 cks
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3
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A
f
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09
es
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/
03
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00
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B
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09
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/0
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3
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09
m
Te
be
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/0
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3
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15
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B
ns
0
m
-2
be
Pe e y
5
R
ve
15 0 A
ns ech
0ar
25
Pe e y
g
Re
0
e
ve
B
A
ch
ns
1
ar
50
Pe e y
-2
Re ge
ve
50
B
ch
ns
1
ar
50
ey
-2
Re ge
5
C
ch
15 0
ar
0
ge
-2
50
D
15
025
0
Mean hourly abrasion rate (%)
0.09
0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0
Figure 11: Comparison of mean hourly abrasion rates of grey and recharge flints of the
same size.
4.3 Abrasion of shingle-sand mixtures
To assess the influence of sand on the abrasion of grey flint shingle six samples (barrels)
were prepared. The sand and gravel used in the experiments were sieved from the recharge
material collected from Pevensey Bay. The grey flint shingle pebbles used were from
Newhaven. In all barrels the water content remained the same but the amount of shingle and
the admixture was changed. Two samples (100%a and 100%b in Figure 12) were control
samples where the barrel was filled in the same way as the standard procedure, i.e. half the
barrel was filled with shingle and the interstitial space filled with water. Sample 80%c
contained only 80% of the shingle of the control samples and 60%f only 60 %. These
reduced fillings were used to establish the effect a reduced shingle load on its own has on
the abrasion rate and as such to confirm or contrast preliminary results from BERM
experiments. If for example a reduction in the shingle load only would lead to a reduction in
the abrasion rate a reduction in the abrasion rate by replacing shingle with sand would be
more difficult to be attributed to the influence of the sand. Samples D and E contained 80%
flint shingle as sample C but also 20% of sand (0.5 to 2mm). The amount of 20% sand was
selected as this a common amount of sand within beaches along the BAR coast, as has
been found by the project’s analysis of beach sediment.
Results are shown in Figure 12. The main findings are that a reduction of the shingle load
(comparison of C and F with A and B) is associated with an increase in the abrasion rate
(supporting preliminary results obtained during the BERM project), and that the addition of
sand reduces the abrasion rate significantly (>50%). The presence of sand seems also to
halt abrasion rates at a low level much earlier (after 5 hours) than for samples with shingle
only. In the case of the latter, the decline in abrasion rate continues for many more hours
whilst for the former it seems to remain relatively constant from the second tumbling interval
onwards.
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210
0.09
0.08
205
0.07
0.06
200
0.05
0.04
195
0.03
0.02
190
0.01
0.00
Mean pebble weight (g)
Mean hourly abrasion (%)
0.10
2.5 hours
5 hours
7.5 hours
mean w eight
185
A-100%
B-100%
C-80%
D-20/80% E-20/80%
F-60%
Figure 12: Mean hourly abrasion rates and average sample weights for grey flints from
Newhaven. Labels on the x-axis reflect the composition of the sample with 100%
indicating that all the sample was flint, 80% and 60% that 80 and 60% of the sample were
flint and 20/80 that 80% was flint shingle and 20% was sand (0.5 – 2 mm).
4.4 Abrasion of shingle-gravel mixtures
The influence of gravel was investigated using six samples (barrels) of flint shingle from
Newhaven with a weight range of 150 to 250g. Barrels A and B are the control with 100%
shingle, barrels C and D contain 30% gravel (4 – 8 mm) and 70% shingle and barrels E and
F 20% gravel and 80% shingle (Figure 13).
The addition 20 and 30% gravel reflects the range of gravel found within beaches along the
BAR coast, as has been found by the project’s analysis of beach sediment.
210
0.10
205
0.08
200
0.07
195
0.06
0.05
190
0.04
185
0.03
180
0.02
Mean pebble weight (g)
Mean hourly abrasion rate (%)
0.09
2.5 hours
5 hours
7.5 hours
mean w eight
175
0.01
0.00
170
A-100%
B-100%
C-30/70% D-30/70% E-20/80% F-20/80%
Figure 13: Mean hourly abrasion rates and average sample weights for shingle and
shingle-gravel mixes.
The results are very similar to those from the shingle-sand mixture experiment. The addition
of gravel reduces the abrasion rate of flint shingle significantly. The addition of 20% gravel
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leads to a 30% reduction in the abrasion rate and the addition of 30% to a reduction of 50%.
Similar to the results for sand, the difference in the abrasion rate for the mixtures between
the second an third tumbling interval seems to be much smaller than for the pure shingle
samples.
4.4 Discussion
The results of the laboratory abrasion experiments carried out in BAR phase I appear to
provide good news for the coastal manager. Firstly, flint pebbles from recharge material
appear to abrade at similar rates to those of grey flint pebbles and secondly, tumbling grey
flint pebbles in more ‘beach-like’ mixes with sand and gravel records lower abrasion rates. It
is therefore likely that field rates of abrasion are lower than those measured only flint shingle
in the laboratory.
However, little knowledge exists of the material mix on the beach foreshore that is regularly
moved by wave action. Although the bulk beach usually contains ~20-30% sand and similar
amounts of gravel, the surface layer is often highly sorted so that sand is absent and it is
composed of pebbles of a narrow size range. Whether this sorting takes place and is
maintained whilst the waves move over the beach or whether it is only produced by the last
waves during the receding tide is largely unknown. It is possible, therefore, that the field
abrasion rates might be somewhere between that of pure shingle and sand-shingle mixes in
the laboratory.
The mechanism that leads to the reduction of the abrasion rate by adding smaller particles is
at the moment unknown. It could be due to either a ‘ball-bearing’ effect of the smaller
particles or by actually restricting the movement of the pebbles by forming a matrix between
them. This could only be clarified by observing pebble movement inside the barrels during
tumbling.
5 Outlook for BAR phase II
Experiments using smaller amounts of sand or gravel need to be carried out to find out at
which point (at which percentage of sand / gravel) abrasion rate slows down, eg samples
could be run with 5 and 10% sand /gravel content.
These experiments should be combined with observations of the processes inside the
barrels that will help in understating the mechanism through which the reduction in the
abrasion rate is achieved.
To measure the abrasion rate under even more realistic conditions shingle needs to be
tumbled with a combined mixture of sand and grave to assess the combined influence of a
more complex mixture.
The preliminary experiments conducted during BAR Phase I have identified two areas that
require further examination:
Investigating the influence of sediment mixtures over a wider range mixing ratios
Experiments using lower amounts of sand or gravel need to be carried out to investigate the
point at which (i.e. the percentage of sand / gravel) abrasion rate slows down, e.g. samples
could be run with 5 and 10% sand /gravel content. Equally, some experiments should be run
with higher amounts to assess the full range of environmental conditions. These experiments
should be combined with observations of the processes inside the barrels that will help in
understating the mechanism through which the reduction in the abrasion rate is achieved.
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To measure the abrasion rate under even more realistic conditions shingle needs to be
tumbled with a combined mixture of both sand and gravel to assess the combined influence
of a more complex mixture.
Investigating the factors that influence the abrasion rate of individual pebbles
The influence of flint shingle pebble surface textural characteristics on abrasion rates
requires further investigation. For instance, it is unclear at the moment whether the abrasion
rates measured are due to a gradual wearing away of the chattermarked surface or whether,
under high energy surf zone conditions, the chattermarked surface is constantly re-forming.
The geochemistry of flint shingle pebbles requires further investigation, particularly in relation
to how it affects pebble hardness and the development of surface textural characteristics
such as chattermarks.
6 References
Dornbusch, U., Moses, C.A., Robinson, D.A. and Williams, R.B.G., 2004. Laboratory
abrasion tests on beach flint shingle. In: R.N. Mortimore and A. Duperret (Editors),
Coastal Chalk Cliff stability. Engineering Geology Special Publications. The
Geological Society, London, pp. 131-138.
Dornbusch, U., Robinson, D.A. and Williams, R.B.G.and Moses C.A., 2003. Estimation of
abrasion on flint shingle beaches in East Sussex, UK. Proceedings of the
International Conference on Coastal Sediments 2003. CD-ROM Published by World
Scientific Publishing Corp. and East Meets West Productions, Corpus Christi, Texas,
USA. ISBN 981-238-422-7, pp. 13 pages.
Dornbusch, U., Williams, R.B.G., Moses, C.A. and Robinson D.A., 2002. Life expectancy of
shingle beaches: measuring in situ abrasion. Journal of Coastal Research, Special
Issue 36: 249-255.
This report is partly based on the following reviews, protocols and reports:
2003-11-14-Report_Laboratory abrasion 01.doc
2004-01-06-Report_Laboratory abrasion 02- shape.doc
2004-07-12-Protocol_Laboratory abrasion of shingle mixtures.doc
2004-09-21-Report_Laboratory abrasion 03-sand mixtures.doc
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