Hydraulic control of grain‐size distributions in a macrotidal estuary

Prévia do material em texto

Sedimentology (1980) 27,433-446 
Hydraulic control of grain-size distributions in a macrotidal estuary 
J O S E P H J. L A M B I A S E * 
Geology Department, Mchfaster University, Hamilton, Ontario, Camdo 
ABSTRACT 
The Avon River estuary of Nova Scotia was studied with the intention of analysing the relations 
between grain-size distributions and hydraulics. The Avon is macrotidal; tidal ranges up to 15.6 m 
generate tidal currents up to 1.7 m s-l. Maximum current speed increases from the mouth (seaward 
end) to the head (shoreward end) of the estuary. Mean grain size decreases from the estuary mouth to 
the head. Thus, there is an inverse relationship between mean grain size and current speed. Conse- 
quently, textural parameters do not directly reflect hydraulic conditions. Graphical dissection of 
cumulative frequency curves into their component grain populations reveals a large coarse population 
at the estuary mouth that is absent at the head. There are several relationships between hydraulics 
and cumulative curves. Shields’ criterion predicts that all sediment in the system can be transported 
so that the large coarse population at the estuary mouth is not a lag. Local maximum shear velocity 
nearly equals the settling velocity of the grain size at the boundary of the coarse (C) and intermediate 
(A) grain populations. This has been previously interpreted to signifiy a transition from traction to 
intermittent suspension transport, and implies that the C population is a function of traction and that 
the A population is related to intermittent suspension (Middleton, 1976). Each grain population is 
transported at a different rate; suspended grains travel almost an order of magnitude faster than 
grains moved by traction according to Einstein’s transport formula. Sediment transport paths in the 
estuary were determined from bedform migration directions and the computed net sediment transport 
per tidal cycle using Engelund and Hansen’s formula. The areal distribution of the transport paths, 
combined with the differential transport rates of each grain population, produces hydraulic sorting. 
Hydraulic sorting causes coarse sediment to be excluded from the estuary head and creates the inverse 
relationship between current speed and mean grain size. 
INTRODUCTION 
It has long been recognized that grain-size distribu- 
tions should reflect the hydraulic environment in 
which the grains were transported and deposited. 
Several approaches have been used t o interpret 
depositional environment from grain-size distribu- 
tions such as plotting skewness versus sorting 
(Friedman, 1961), comparing the coarsest fraction 
t o the median size (Passega, 1964) and analysing 
cumulative curve shape (Visher, 1969). Recent 
workers have examined the relationships between 
grain size distributions and hydraulics (Middleton, 
*Present address : Department of Geological Sciences, 
Virginia Polytechnic Institute and State University, 
Blacksburg, Virginia 24061, U.S.A. 
0 1980 International Association of Sedimentologists 
0037-0746/S0/0S00-0433 $02.50 
1976; Sagoe & Visher, 1977); this approach em- 
phasizes the effects of the transporting fluid on 
sediment grains, and provides a basis for interpreting 
hydraulic conditions from grain-size distributions. 
The present study examines grain-size distribu- 
tions in a macrotidal estuary, and analyses the 
observed trends in textural parameters and 
cumulative curve shape with respect t o measured 
hydraulic parameters. The aims are to define the 
characteristics of grain-size distributions that reflect 
hydraulics in a macrotidal estuary, and to use hy- 
draulics to interpret existing grain-size distributions. 
Study area 
The study was conducted in the Avon River estuary 
in central Nova Scotia, Canada. The Avon empties 
434 J. J . Lambiuse 
into the southwestern corner of Minas Basin which is 
the northeastern arm of the Bay of Fundy (Fig. 1). 
There are six major intertidal sand bodies in the 
estuary (Fig. 2); three are at the estuary mouth 
(seaward end) and the other three are near the 
estuary head (shoreward end). In addition, there is a 
narrow sand ridge near the centre of the estuary that 
is subaerially exposed only at lower low water (Fig. 2). 
The major sand bodies range in length from 0.8 to 
5.6 km and in width from 0.4 to 0.9 km; they are 
elongate parallel to tidal current direction and are 
asymmetric in cross-section perpendicular to current 
direction. The sand bodies form parts of ebb and 
flood tidal deltas (Lambiase, 1977) and have mor- 
phologies that are similar to the mesotidal tidal delta 
models of Hayes (1975). 
HYDRAULIC ENVIRONMENT 
Mean tidal range in the study area is 12.0 m and 
maximum spring tidal range is 15.6 m (Canadian 
Hydrographic Service, 1976). The large tidal range 
generates strong tidal currents. Current velocities 
were measured at twenty-nine stations with an 
Endeco Type 110 direct reading current meter. 
Vertical velocity profiles were recorded every half 
hour during a complete tidal cycle at each station. 
Current speeds varied widely during a tidal cycle. 
Figure 3 depicts the maximum current speeds 
recorded at each station. Maximum flood and 
maximum ebb speeds were nearly equal at each 
location. Maximum current speed increased in the 
onshore direction. Near the estuary mouth maximum 
current speeds averaged 0.8 m s-l while at the 
estuary head they averaged 1.3 m s-l (Fig. 3). 
Maximum speeds were averaged without con- 
sidering when they were measured relative to the 
neap-spring cycle. However, twenty-three of the 
twenty-nine stations were measured at spring tide. 
Hydraulic parameters were computed from 
the vertical velocity profiles. More than 90% of the 
profiles are statistically logarithmic in shape at the 
95% level of significance. Any deviations were 
assumed to be caused by turbulence. The slope of the 
best-fit line between the log of the water depth and 
Fig. 1. Location map showing the study area (framed) and its position within the Bay of Fundy. The intertidal zone 
within Minas Basin is shown stippled. 
Hydraulic control of grain-size distribution 435 
Fig. 2. Location of the major intertidal sand bodies (stippled) and the central sand ridge (hatched), 
current speed was used to compute shear velocity 
and shear stress using standard hydraulic formulae 
(Blatt, Middleton & Murray, 1972). 
There are other, less important hydraulic processes 
operating in the study area. Waves do not noticeably 
affect sediment distribution on sand bodies in the 
Avon River estuary (Lambiase, 1977); waves tend 
to be small because fetch is limited (Fig. l), and they 
only affect intertidal sand bodies for short durations 
because water level changes rapidly. Other hydraulic 
processes are even less important than waves 
(Lambiase, 1977). 
GRAIN S l Z E DISTRIBUTIONS 
Four hundred and seventy-one samples were col- 
lected from the six major intertidal sand bodies. 
Samples were taken from as near the sediment 
surface as possible to ensure that each sample 
consisted only of grains in equilibrium with the 
present hydraulic environment. The influence of 
bedforms on grain-size distribution was minimized 
by scraping all sediment samples from bedform 
crests. All samples were sieved at $ phi intervals 
using standard techniques (Carver, 1971 ; Folk, 
1974) and the results were expressed as weight 
percentage. 
Textural parameters 
Moment measure mean, standard deviation (sorting), 
skewness and kurtosis were computed from the sieve 
data using the method of Seward-Thompson & Hails 
(1 973) for samples that were not open-ended by more 
than 5% at either the coarse or fine tail. Mean grain 
size has the areal distribution depicted in Fig. 4. 
Most samples at the estuary mouth have a mean sizecoarser than 2 phi while those near the estuary head 
tend to have a mean size finer than 2 phi. Thus 
mean grain size and maximum current velocity 
appear to be inversely related (compare Figs 3 and 4). 
Sorting also varies with position in the estuary. 
Sediments at the estuary mouth tend to be more 
poorly sorted than those near the head. Most 
samples at the mouth are moderately sorted and 
436 J. J . Lamhiase 
Fig. 3. Maximum measured current speeds. Arrows show the magnitude and direction of the maximum current speeds. 
most at the head are moderately well sorted to well 
sorted (terminology of Folk, 1974). Sediments at the 
estuary mouth are negatively skewed while those near 
the head are positively skewed. 
Cumulative curve analysis 
Numerous authors have proposed that cumulative 
frequency curves actually are composed of two or 
more log-normally distributed grain populations, 
and that the shape of a cumulative curve is a function 
of the relative proportions of these populations 
(Tanner, 1959, 1964; Spencer, 1963; Visher, 1969). 
Several workers have advanced the idea that each 
grain population is related to a different sediment 
transport mechanism (Fuller, 1961 ; MOSS, 1962, 
1963,1972; Spencer, 1963; Visher, 1969; Middleton, 
1976; Sagoe & Visher, 1977); cumulative curve 
shape also has been attributed to the grain-size 
distribution of the source material (Shea, 1974). 
Recent work provides a theoretical basis for 
hydraulic control of cumulative curve characteristics 
(Middleton, 1976; Sagoe & Visher, 1977). 
There is divided opinion about the exact nature of 
the boundary between the log-normal grain popula- 
tions. Some workers believe that the populations 
are truncated at their boundaries (Visher, 1969; 
Sagoe & Visher, 1977); others maintain that the 
M E A N G R A I N SIZE ( P H I ) 
( 0 
[ 0 - 1 
f--J 2 - 3 
1 - 2 
> 3 
Hydraulic control of grain-size distribution 437 
0 I 2 
I I L O M L T L R S 
M E A N G R A I N SIZE ( P H I ) __ 
( 0 
0 - 1 
1 - 2 
2 . 3 a > 3 
Fig. 4. Mean grain sizes at (A) the estuary mouth, and (B) the estuary head. The central sand ridge was omitted from this 
figure because no samples were collected from it. 
populations are overlapping (Tanner, 1959, 1964; 
Spencer, 1963). Middleton (1976) presents data 
from several rivers that suggests that grain popula- 
tions are overlapping. 
Detailed studies of size distributions and sediment 
transport on several sand bodies in Minas Basin have 
demonstrated that, in this area, size distributions of 
intertidal sands are better represented by overlap- 
ping than by truncated distributions (Dalrymple, 
1977). It was also shown, using fluorescent tracer 
techniques, that although all traction fractions move 
at about the same low speed, the rate of movement 
of sand moving in intermittent suspension is a 
function of grain size (see Middleton & Southard, 
1977, Ch. 6). 
Fifty-nine cumulative frequency curves were 
dissected into their component grain populations 
using the graphical dissection method of Cassie 
(1954, 1963) assuming that the populations are log- 
normally distributed and overlapping. The accuracy 
of each dissection was tested by summing the 
individual grain populations and comparing the 
sum to the corresponding cumulative curve with a 
chi-squared statistic; all dissections were found 
to be statistically identical to the original curve. 
Graphical dissection revealed three cumulative 
curve types to which all samples can be easily 
assigned. The areal distribution of each type is 
different. Group 1 curves have a large coarse (C) 
population, a large intermediate (A) population 
and a small fine (B) population and are found at the 
estuary mouth only. Group 2 curves have a small C, 
a large A and a small B population and are found 
all over the study area. Group 3 curves have a small 
C, a large A and a large B population and are the 
dominant type of curve at the estuary head. An 
example of each of the three curve groups is illus- 
trated in Fig. 5. 
Average characteristics of the three curve groups 
are summarized in Table 1. The mean size and weight 
percentage of each grain population is listed as well 
as grain size at the boundaries between populations 
and the areal distribution of each curve type. 
Population boundaries were defined as the point of 
438 J . J . L ,ambias 
GROUP I 
GROUP 2 
, . . . . . . GROUP 3 __ - ~- 
n 
e 
a 
-3 -2 - 1 0 I 2 3 
Grain Size ( P h i ) 
Fig. 5. Typical Groups 1, 2, and 3 cumulative frequency 
curves (see text for explanation). 
equal overlap between adjacent grain populations, 
that is, the grain size at which the size-frequency 
curves of the two populations intersect. Important 
trends emerge from the data presented in Table 1. 
There is a Iarge C population at the estuary mouth 
that is absent near the estuary head; this population 
has a mean size of 0.6 phi and equally overlaps the A 
IJopulation at an average of 1.6 phi. Another 
important trend is that the C-A and A-B population 
boundaries occur at coarser grain sizes near the 
estuary head than at the mouth. The hydraulic 
significance of this trend will be discussed later. 
Thus, there is an essential difference in the grain-size 
distributions at the estuary head and mouth. 
S E D I M E N T T R A N S P O R T P A T H S 
Sediment transport paths were examined to deter- 
mine their effect on grain-size distributions. Trans- 
port paths were delineated from bedform migration 
directions and current velocity data using the 
Engelund & Hansen (1967) transport equation. 
Bedform migration directions were measured by 
planting a stake at a bedform crest during low tide 
and measuring displacement of the bedform crest 
relative to the stake during the next low tide. Net 
sediment transport direction in channels was 
computed by calculating instantaneous transport 
rates for each vertical velocity profile using Engelund 
& Hansen's formula. The instantaneous rates were 
summed. vectorially for a complete tidal cycle 
yielding the net transport direction and magnitude. 
It should be noted that the Engelund & Hansen 
formula was the most reliable of five that were tested 
against transport rates calculated from bedform 
migration rates in Minas Basin (Dalrymple, 1977). 
Generally, the formula yielded accurate predictions 
of the dominant sediment transport direction and 
areal trends in the magnitude of net transport, but 
individual predictions of transport rate often 
difered from transport rates calculated from bed- 
form migration rates. 
There are both ebb and flood transport zones with- 
in the study area (Fig. 6). Numerous studies in tide- 
dominated areas have reported distinct flood and ebb 
transport zones (Coastal Research Group, 1969; 
Ludwick, 1974; Dalrymple, 1977; Knight, 1977). 
Ebb and flood transport zones tend to form elliptical 
cells centred on sand body crests; similar patterns 
have been noted in other tidally influenced areas 
(Houbolt, 1968; Klein, 1970; Dalrymple, Knight & 
Middleton, 1975). 
The transport paths illustrated in Fig. 6 do not 
appear to cause the observed inverse relationship 
between mean grain size and current vefocity as the 
paths indicate that a sediment exchange between 
the estuary head and mouth is possible. However, 
transport paths do influence grain-size distributions; 
the effects will be discussed later. 
I N T E R P R ETA TI O N OF G R A IN- S I 2 E 
D I S T R I B U T I O N 
Relationships between cumulative curves and 
hydraulics 
Sand bodies and bedforms in the Avon River 
estuary are in equilibrium with maximum, rather 
than average, flow conditions (Lambiase, 1977). It 
will be shown that grain-size distribution also is 
related to maximum flowconditions. 
If grain-size distributions reflect local hydraulic 
conditions, then the coarsest grain size at each 
location should be a function of maximum local 
flow strength. For each current station, the com- 
petence of the maximum measured speed was cal- 
culated using Shields' criterion (Shields, 1936), and 
the predicted competence was plotted against the 
coarsest sieve size containing sediment (Fig. 7). 
Essentially all sediment in the system can be trans- 
ported by the local currents; this includes most 
stations at the estuary mouth as well as all those at 
the head. Thus the Iarge C population at the estuary 
mouth is not a lag deposit except possibly at a few 
stations (Fig. 7). However, current speeds were not 
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440 J . J . Lambiase 
Fig. 6. Sediment transport paths. Arrows indicate the direction of net sediment transport, arrow length is not related 
to transport rate. 
0 0 
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t, ! L G L I L L L i L L i . L L u . l I I I I I I I I 
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Observed Maximum G r a i n S i z e (Ph i ) 
Fig. 7. ,Predicted maximum grain size versus observed 
maximum grain size. Predicted size is based on Shields' 
parameter and observed size is the coarsest sieve fraction. 
All predicted values are from stations monitored at 
spring tide except those points that are circled. 
monitored at spring tide at any of these stations; 
because current speeds are greatest a t spring tide, it 
is likely that all sediment can be transported by the 
existing hydraulic regime. 
At the estuary mouth, the coarsest sieve fraction 
nearly equals the predicted competence of the 
measured currents (Fig. 7). At the estuary head, 
however, currents are more competent than is 
required to transport the coarsest grain size (Fig. 7). 
As previously stated, several authors believe that 
each grain population contributing to a cumulative 
curve is related to a different depositional mech- 
anism. Visher (1969) associated the C population 
with surface creep o r traction, the A population with 
saltation and the B population with suspension. 
Middleton (1976) argued that, because saltation is 
unimportant in water (Kalinske, 1943), the A popula- 
tion actually is transported by intermittent sus- 
pension rather than saltation. 
If grain populations reflect transport mechanisms, 
then each boundary between populations should 
Hydraulic control ojgrnin-size distribution 44 1 
represent a transition from one mechanism to 
another. Middleton (1976) examined the boundary 
between populations C and A, and related the 
boundary to a transition from traction to inter- 
mittent suspension for a number of modern rivers. 
Hydraulic theory predicts that the coarsest 
sediment that can be suspended will have a settling 
velocity, w , that is approximately equal in magnitude 
to the root mean square of the vertical velocity 
fluctuations, d?’; the criterion for suspension then is 
w = 4 s ‘ (1) 
Because turbulent velocity fluctuations are difficult 
to measure, some workers have related the magnitude 
of 4 V ’ to the magnitude of the shear velocity, U*. 
Bagnold (1973) found the relationship 
McQuivey (1973) found ratios up to 1.74 for small 
flume data and Bowden (I 962) measured turbulence 
in tidal currents and found d?’ to be 1.2 times as 
large as U*. Middleton (1976) combined Eqns 1 and 
2 to form a criterion for suspension of 
w / u * m I (3) 
This criterion was confirmed experimentally for 
shallow, flat bed conditions by Francis (1973). 
Dalrymple (1977) showed by hydraulic measure- 
ment and the use of fluorescent tracers that the 
criterion for suspension worked well in the intertidal 
environment, provided that comparison was made 
between the settling velocity of the sediment and the 
maximum shear velocity (averaged over approxi- 
mately 1 h). He also showed that the criterion 
worked better when the boundary between the 
traction and intermittent suspension populations was 
established from the size distribution by using 
graphical dissection and the point of equal overlap 
than it did when the size ‘break’ was obtained by 
fitting straight line segments to the log probability 
The relationship between C-A boundary and the 
initiation of suspension was tested further in the 
Avon River estuary. For each current station, the 
settling velocity of the grain size at the C-A popula- 
tion boundary was obtained from data given by Graf 
and Acaroglu (1966) for natural grains. Maximum 
U* values for each tidal cycle, computed following 
the criterion of Dalrymple (1977), were compared to 
plot. 
H E A D 
S e t t l i n g V e l o c i t y A t The C - A Boundary (m/s) 
Fig. 8. Maximum measured shear velocity ( U * ) versus 
settling velocity (w) of the grain size at the boundary of 
the C and A grain populations. The average ratio of 
w/U* -0.9 is shown with a solid line, and the condition 
of w = U* is plotted as a dashed line. 
the appropriate w , and w / U * ratios fit the suspension 
criterion of Equation 3 (Fig. 8). w/U* ratios range 
from 0.38 to 1.70 with a n average of 0.90 which is 
close to the criterion of w/U*=0+30 established by 
Engelund (1973). 
The observed shift towards coarser grain sizes 
at the C-A and A-B boundaries from the estuary 
mouth to the head (Table 1) is a response to the 
increased flow strength in that direction. This 
supports the contention that cumulative curves do 
reflect hydraulic conditions and that the shape of the 
curve depends on flow strength. 
Sediment transport rates calculated with Ein- 
stein’s (1 950) transport equations also support the 
theory that the boundary between the C and A 
populations represents a transition from traction to 
intermittent suspension transport. Measured and 
computed values of the appropriate hydraulic 
parameters were input into Einstein’s formula;the coarsest grain size at which the suspended load 
discharge was above zero agreed closely with the 
grain size at the C-A boundary in the cumulative 
curve. Thus, the boundary between cumulative curve 
populations C and A probably reflects a transition 
from traction to intermittent suspension. 
Assuming that the findings of Middleton (1976) 
and Francis (1973) are valid, and that each grain 
population is a function of a different sediment 
transport mechanism, each population will have a 
different transport rate. The A population will be 
transported at a faster rate than the C population 
because the A population moves by intermittent 
suspension while the C moves by traction. Einstein’s 
transport formula predicts that grains travelling 
442 J . J . Lambiase 
mainly by suspension are transported almost an 
order of magnitude faster than grains that move 
mainly by traction. 
Hydraulic sorting 
The relationship between transport mechanisms and 
cumulative curves reveals the process that produces 
the observed areal trends in textural parameters and 
cumulative curve shape; that process is hydraulic 
sorting. Coarse sediment is excluded from the 
estuary head because it is transported by traction, 
and, therefore, moves more slowly than finer 
sediment that is intermittently suspended. However, 
differential transport rates alone will not produce 
hydraulic sorting because all size fractions would 
eventually reach the estuary head. It is the sediment 
transport paths (Fig. 6) that combine with different 
transport rates for each grain population to cause 
hydraulic sorting. 
Mechanism 
Near the centre of the estuary (Fig. 9), a bedrock 
ledge projects into the estuary at the southern end 
of the central sand ridge. The ledge acts as an ebb 
shield by deflecting ebb currents to the east (Fig. 9). 
The ridge was observed to be composed of coarse 
sand, but the next sand body towards the estuary 
head (i.e. Hantsport Bar, Fig. 9), has very little 
coarse sediment. Thus, the ebb channel near 
Hantsport (Fig. 9) is the limit of penetration of 
coarse sediment. 
The arrows in Fig. 9 illustrate how the process of 
hydraulic sorting occurs. Coarse and fine sediment 
are transported in the flood direction along the 
western side of the estuary. Both size fractions cross 
the bedrock ledge and are dumped into the ebb 
channel. Because of its faster transport rate, the 
intermittent suspension population can cross the 
ebb channel before it i s entrained into the ebb 
system on the eastern side of the river. The slower 
moving traction population cannot reach Hantsport 
Bar because it is entrained by the ebb channel. 
The length of the arrows in Fig. 9 is proportional 
to the travel distances of each grain population 
during a tidal cycle. The distances were converted 
from sediment transport rates calculated with 
Engelund and Hansen’s transport formula for a 
station north of the bedrock ledge. Transport 
distances in the ebb channel were estimated from 
transport rates for five stations near Hantsport Bar. 
Fig. 9. Schematic diagram of the hydraulic sorting 
process. The insert illustrates the area where hydraulic 
sorting occurs. See the text for an explanation of the 
process. 
Relative transport rates of the traction and inter- 
mittent suspension populations are based on 
Einstein’s transport formula. 
Integrated net sediment transport rates at several 
cross-sections of the estuary suggest that there is 
little, if any, net sand transport into or out of the 
estuary head (Lambiase, 1977). However, these cross- 
sections are based on only a few stations each, and 
Amos (1976) presents data that implies a post-1970 
net shoreward transport of sand. The problem is 
unresolved, but the important factor for hydraulic 
sorting is that net shoreward transport is much 
slower than the circulation rate. 
Thus, almost as much fine sand must exit from the 
estuary head as is introduced so that the overall 
transport paths of the traction and intermittent sus- 
pension populations are as illustrated (Fig. lo). The 
traction load circulates at the estuary mouth; some 
of it penetrates as far as Hantsport where its slow 
transport rate causes it to be returned towards the 
estuary mouth. The intermittent suspension popula- 
tion also circulates at the mouth, but reaches the 
estuary head as well, due to its relatively rapid 
transport rate. Intermittently suspended sediment 
Hydraulic control of grain-size distribution 443 
Fig. 10. Transport paths of the traction and intermittently suspended grain populations. Arrows indicate direction of 
net transport; arrow length is not related to transport rate. 
circulates at the head and is mixed with the traction 
load near Hantsport (Fig. 10). 
The preceding discussion assumes that grain size 
distributions of samples collected from intertidal 
sand bodies reflect processes that occur in adjacent 
channels. However, several studies in Minas Basin 
indicate that sediment moves in sediment transport 
zones, and that each zone includes a channel and 
part of an intertidal sand body (Knight, 1977; 
Dalrymple, 1977; Lambiase, 1977). Each sediment 
transport zone appears to act as a unit through 
which all available sediment is transported so that 
grain-size distribution at a point within the zone 
probably is indicative of processes operating within 
that zone. 
Efects on grain-size distribution 
The hydraulic sorting mechanism described above 
strongly influences grain-size distribution in the 
Avon River estuary. Exclusion of coarse sediment 
from the estuary head causes mean grain sizes to be 
finer at the head than at the mouth (Fig. 4). This 
produces the observed inverse relationship between 
mean grain size and current velocity. Areal trends 
in sediment sorting and skewness are also caused by 
hydraulic sorting. Sediment sorting is better at the 
estuary head than at the mouth because the range of 
available grain sizes is reduced at the head by 
hydraulic sorting. Sediments are negatively skewed 
at the mouth because of the presence of a large 
coarse grain population, and positively skewed 
sediments dominate the estuary head because the 
coarse population is absent. 
Hydraulic sorting explains the observed areal 
distribution of cumulative curve shape. Because the 
large C population is excluded from the estuary 
head, Group 1 curves do not occur there. Cumulative 
curves with a large B population are common at the 
estuary head not only because increased current 
strength allows suspension at a coarser grain size, 
but absence of the coarse tail makes the amount of 
sediment in suspension more important relative 
to the total available sediment. The failure of 
Shields’ criterion to predict coarsest grain size at the 
estuary head (Fig. 7) is a function of hydraulic 
444 J . J . Lambiase 
sorting. The coarse grain sizes that the strong 
currents are capable of transporting are excluded 
from the estuary head. 
DISCUSSION 
Grain size parameters as indicators of hydraulics 
The relationships between grain-size distributions 
and hydraulics in the Avon River estuary indicate 
that the hydraulic environment does control 
sediment distribution, and that the grain-size distri- 
bution reflects this control. However, hydraulic 
sorting affects the reliability of some grain size 
parameters with the result that cumulative frequency 
curve characteristics, as determined by graphical 
dissection into normal components, are better 
indicators of hydraulic environment than are 
textural parameters such as moment measures. 
Several aspects of cumulative curves reflect 
hydraulic environment. The relative proportion of 
the different grain populations reveals the importance 
of each transport mechanism as well as the avail- 
ability of each size fraction.Grain size at the 
boundary of the C and A grain popuIations varies 
directly with flow strength because the C-A boundary 
directly reflects maximum U* values. Coarsest grain 
size corresponds to the theoretical competence of the 
maximum flow. Areas affected by hydraulic sorting 
do not fit this last relationship, but a disagreement 
between predicted and observed maximum grain 
size allows recognition of hydraulically sorted areas. 
Graphical textural parameters or moment meas- 
ures are not good indicators of hydraulic conditions 
in the Avon River estuary because of hydraulic 
sorting. The largest anomaly is that mean grain size 
is inversely related to current speed. The use of 
textural parameters alone would yield incorrect 
patterns of flow strength. As might be expected, the 
best sorted sediments are those with mean grain 
sizes in the fine sand size range. However, this is 
caused by the hydraulic sorting process in this 
estuary which is a different sorting process from the 
one identified by Inman (1949) that can produce the 
same relationship. Textural parameters cannot 
provide information about the relative importance 
of transport mechanisms, or accurate U* values. 
The hydraulic sorting process in the Avon River 
estuary would not have been detected if just textural 
parameters had been used. 
Relationships between grain-size parameters and 
hydraulics in the Avon River estuary suggest that 
some cumulative frequency curve characteristics are 
better than textural parameters for palaeohydraulic 
interpretation of sedimentary deposits. Because 
textural parameters are unreliable in the Avon, their 
general applicability to the interpretation of ancient 
sediments is suspect. 
Likelihood of hydraulic sorting occurring in other areas 
A discussion pointing out the inadequacy of tex- 
tural parameters because of hydraulic sorting must 
consider whether hydraulic sorting is unique to the 
Avon River estuary or is a process that might occur 
commonly. To evaluate its likelihood of occurrence, 
it is necessary to examine the causes of hydraulic 
sorting. 
In the Avon River estuary hydraulic sorting is 
produced by a combination of sediment transport 
paths and differential transport rates of each grain 
population. Differential transport rates of the 
traction and intermittent suspension grain popula- 
tions should occur everywhere so that the specific 
feature of the Avon that causes hydraulic sorting is 
the nature of the sediment transport paths. The 
crucial factor is that the correct transport path 
geometry is established. In the Avon, a bedrock ledge 
controls transport paths (Fig. 9); it is not the presence 
of this ledge that causes hydraulic sorting, but the 
geometry that the ledge establishes. The important 
features are that sediment being transported in the 
flood direction must cross an ebb channel that is 
oriented transverse to flood current direction 
(Fig. 9), and that the ebb channel has a width that 
will allow intermittently suspended sediment to 
cross while the traction load cannot. 
The geometry that causes hydraulic sorting in the 
Avon River estuary is common. Numerous examples 
of estuaries that have shallow, flood transport areas 
separated by deep, meandering ebb channels that 
often become oriented transverse to flood currents 
are contained in the literature. These include 
Yaquina Bay, Oregon (Kulm & Byrne, 1967) and 
estuaries in the Danish Moraine Archipelago 
(Schou, 1967) and on the New England Coast 
(Coastal Research Group, 1969). Thus it is likely 
that hydraulic sorting occurs in other estuaries and 
may be responsible for some of the reported ex- 
amples of a shoreward decrease in mean grain size. 
Also, the same process may operate on a smaller 
scale and cause the local grain size changes ass- 
ociated with ebb and flood shields in some estuaries 
(J. Boothroyd, 1977 personal communication). 
Hydraulic control of grain-size distribution 445 
CONCLUSIONS 
In the macrotidal Avon River estuary, grain-size 
distribution is controlled by local current speed. 
This control is not reflected by textural parameters, 
but it is exposed by cumulative frequency curve 
analysis. Dissection of cumulative curves into their 
component grain populations reveals that the 
settling velocity a t the C-A population boundary 
approximates the maximum value of the local shear 
velocity. Each grain population is transported by a 
different mechanism. The C population reflects 
traction and the A population is a function of inter- 
mittent suspension. Consequently, each grain 
population is transported at a different rate. 
Hydraulic sorting is produced by the geometry 
of the sediment transport paths in the estuary in 
conjunction with differential transport rates for each 
grain population. Fast moving intermittently sus- 
pended grains can bypass a n ebb channel that ‘traps’ 
slow moving grains transported by traction. This 
excludes coarse sediment from the estuary head, 
creating a n inverse relationship between mean grain 
size and current speed. This, in turn, causes textural 
parameters t o be unreliable. Hydraulic sorting may 
not be limited to the Avon River estuary because 
numerous estuaries have similar transport path 
geometries, and differential transport rates should 
occur everywhere. 
ACKNOWLEDGMENTS 
This study was part of a Ph.D. programme super- 
vised by D r G. V. Middleton. I wish to thank Dr 
Middleton for his discussion and criticism of the 
project and this manuscript. Drs R. W. Dalrymple 
and R. J. Knight made many helpful suggestions. 
Field assistance was provided by Neal Schoen and 
Bob Mepham. Typing of the manuscript and 
drafting were done by the staff of Beak Consultants 
Limited, Calgary, Alberta. The research was 
supported by the Canada Department of Energy, 
Mines and Resources, and by Imperial Oil Limited. 
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