Considerable interest has been expressed in the composition of the stream gravels, the movement of bed materials and the relationship of sediment composition, packing and siltation of void space to invertebrate ecology. In the autumn of 1982, freeze-samples of gravel were obtained in Dorset streams. Data were required on the depth of salmonid egg pocke and were part of a broader investigation of regional variation in the independent variables of salmonid fish length, gravel size, current velocity and the resultant dependent variable ~egg burial depth. The Dorset river gravels examined are bimodal. The grain size distribution may be resolved into two near-normal frequency distributions interpreted as representing a primary framework or lattice of gravel particles into which a secondary matrix population of sand particles has penetrated

Carling, P.A.

Aquatic Commons

FRESHWATER BIOLOGICAL ASSOCIATION 
TEESDALE UNIT 
PROJECT 73 
Report to: The Northumbrian Water Authority, 
The Department of the Environment, 
The Natural Environment Research Council, 
The Water Research Centre, 
The Welsh Office, 
The Ministry of Agriculture, Fisheries & Food. 
Date: July 1983 
Composition and bulk properties of Dorset 
river gravels. 
P.A. Carling 
CONTENTS 
Page 
1 
2 
3 
6 
7 
7 
8 
Summary 
Introduction 
Methods 
Results & Discussion 
Granulometry 
Bulk properties 
Organic content 
Diagenetic concretion 
Acknowledgement s 
References 
SUMMARY 
1. The Dorset river gravels examined are bimodal. The grain size 
distribution may be resolved into two near-normal frequency distributions 
interpreted as representing a primary framework or lattice of gravel 
particles into which a secondary matrix population of sand particles 
has penetrated. Penetration may have been pene-contemporaneous as 
suspended sand was trapped in stabilizing bedload gravel or alternatively 
may have been a later infiltration into an open-work gravel deposit. 
2. The mean grain size of the framework falls in the interval -4 to 
-5 Ø. Residual skewness results in a mode in the range -4 to -6Ø. 
3. The mean grain size of the matrix falls in the interval 0 to 2 Ø, 
whilst the mode is within the interval 0 to 1 
4. No grains larger than -7 Ø were encountered and material finer than 
4 Ø i.e. silt and clay, on average represented less than 1% of the 
total sedimentary material. 
5. A deficiency in the grain size interval -1 to -2 Ø (R. Piddle and 
Tadnoll Brook) or 0 to -1 Ø (Bere Stream) represented the saddle 
separating the two sedimentary populations of framework and matrix. 
6. The surface 20 cm of gravel had low matrix percentages, i.e. they 
were cleaner than basal gravels. 
7. Gravels are better sorted and have a higher porosity than gravels 
in upland Teesdale, only 2 % of the potential void space was filled 
with sand. 
8. The organic content of the matrix was low ~ of the order of 1% 
by weight. 
9. A tufa-like deposit commonly filled void space in basal gravels. 
INTRODUCTION 
In the autumn of 1982, freeze-samples of gravel were obtained in 
Dorset streams. Data were required on the depth of salmonid egg pockets 
and were part of a broader investigation of regional variation in the 
independent variables of salmonid fish length, gravel size, current 
velocity and the resultant dependent variable ~egg burial depth. 
The above aspects will be published at a later date when comprehensive 
data are available. Preliminary data are given by Ottaway et al. (1981). 
Considerable interest has been expressed in the composition of the 
stream gravels, the movement of bed materials and the relationship of 
sediment composition, packing and siltation of void space to invertebrate 
ecology by the F.B.A. River Laboratory staff. Consequently relevant 
- aspects of the investigation are made available here in unpublished 
form. 
Summary data on grain size composition, bulk properties and organic 
content are presented. A physical explanation for the bimodal grain-
size frequency distribution is advanced and comparisons are drawn where 
appropriate with other published information. 
In this report the use of the word siltation and similar words 
refers to the process of infilling of gravel void space by sand and 
finer particles. No specific size category such as delineated in the 
Wentworth-Lane terminology (i.e. silt< 0.063 mm) is intended. Otherwise 
grain-size nomenclature follows Wentworth-Lane (Pettijohn,1975). 
METHODS 
Twenty-five undisturbed samples of gravel were obtained in salmon 
and sea trout redds using a freeze-sampling method described elsewhere 
(Carling & Reader, 1981). Three streams were sampled; the River Piddle 
at N.G.R. SY 798941 and 806937; the Bere Stream at SY 856928 and 857927 
and the Tadnoll Brook at SY 774871. 
Where distinct stratigraphic differences were evident in the cores, 
samples were split accordingly. Alternatively, where pockets of fish 
eggs occurred, cores were also split into sections representing gravels 
above the eggs and gravels below the eggs. Several cores displayed no 
stratification and were analysed in bulk. A total of 37 samples and 
sub-samples was examined. 
Samples were dried to room temperature and sieved mechanically on 
a Fritz Analysette. Sieving time was 10-15 minutes using an intermittent 
mode of amplitude sufficient to mobilise the coarsest particle. 
As grain-size distributions are logarithmically distributed, samples 
were sieved at 1 phi Ø intervals in the range 4.0 Ø (0.063 mm) to -7.0 Ø 
(129 mm). The use of the phi (log2) conversion is common practice and 
effectively normalises grain-size distributions prior to statistical 
analysis. The phi scale is shown in relation to the grain size in 
millimetres in Table 1. A fuller description is given by Sumner (1978). 
Samples were bimodal. The coarse mode (<0 Ø), here termed the 
framework, contained no organic detritus. The fraction containing the 
fine mode (>0 Ø), here termed the matrix, was ashed at 550°C for 2 hrs 
to ascertain the loss on ignition. The percentage weight loss was taken 
to represent the approximate combustible organic content. 
Seven high quality cores from the R. Piddle at Affpuddle were used 
to derive estimates of particle density, void ratio and similar bulk 
properties. 
Some basal gravel contained a diagenetic concretion. A sample was 
analysed by X-ray diffraction to determine mineral composition. 
RESULTS AND DISCUSSION 
Granulometry 
As will be indicated below most samples were bimodal and gravel 
void space was partially silted. There were no open-work gravels (sensuo 
- Cary, 1951). 
Identification of bimodality is dependent on the magnitude of a 
secondary peak in the sand range that can be accepted as a genuine 
secondary population. Minor fluctuations may occur in the cumulative 
frequency curve resulting from sieving errors and the inevitability of 
treating continuous series data as group frequency data. Experience 
had shown that these should only be expected to be of the order of 1 
or 2%. A cut-off point was adopted such that a secondary mode would 
only be identified if it contained > 5% more by weight than in the 
adjoining interval. 
Using the above procedure, thirty-three sub-samples were bimodal. 
The remaining four samples were unimodal. Three of these had the mode 
in the coarse gravel whilst the remaining sample, with a mode in the 
sand size sediment, represented a sandy organic deposit; either a flaser 
or a buried vegetal mat (p. 7). 
The coarse mode in the bimodal sediments commonly fell in the -4 
to -6 range, whilst the fine mode fell in the 0 to +1 Ø range. The 
saddle between the two populations was in the 0 to -1 Ø range or occasionally 
in the -1 to -2 Ø range. There was little variability from this rule. 
Taking the mid-point in each modal class (in millimetres) as a 
representative grain-size diameter for the matrix (dm) and the framework 
(df), the ratio dm/df varied little, variation being mainly associated 
with changes in the df value (see also Potter, 1955). For 33 analyses 
52% of ratio values = 0.0313, 30% = 0.0156 with 18% constituting other 
values. Extreme values were 0.0078 and 0.0625. The significance of the 
ratio is discussed below (p. 6). 
Although graphical statistical measures have been used to describe 
bimodal sediments (Dyer, 1972), these are of little value if the bimodal 
population may be disassociated rationally into component parts. For 
example, the mean or median grain size may be poorly represented in a 
bimodal population, whilst skewness is largely a measure of the magnitude 
of the population modes. Consequently the populations were disassociated 
and statistical parameters calculated for each modal population (Table 2). 
Although the resolution of mixed frequency distributions into near-normal 
components may be based on numerical or semi-graphical methods (e.g. Mundry, 
1972; Harding, 1949), the common deficiency of fluvial sedimentary grains 
in the size-range 0 to -1 Ø (Pettijohn, 1975) p. 42)as noted here suggests 
the juxta-position of hydraulically "independent" populations, broadly 
representing a traction population 0 and a suspension population 
>0 Ø. Plotting two contiguous normal distributions as a cumulative 
frequency curve on probability paper results in a distinctive sigmoidal 
curve (Harding, 1949). The bimodal sediments reflected this model (Fig. 1). 
Fig. 1 Cumulative frequency distributions of Dorset river gravels. 
R. Piddle, 0 Bere Stream, X Tadnoll Brook. The sigmoidal 
curve reflects the mixing of two normally distributed populations 
of sediments. 
Because of the little variation between samples, samples were averaged 
for each stream to produce one grain size frequency curve. The resultant 
curves were disassociated into a matrix population and a framework 
population by fitting normal distributions around each population mean 
(Fig. 2). Distributions were truncated at the saddle frequency, this 
interval not being used in any calculations. The method is described 
by Bliss (1967; p. 156). Observed frequencies were compared with expected 
values using the Kolomogorov-Smirnov test (Siegel, 1956). None of the 
component distributions was significantly different from normal at the 
0.01% level. Descriptive statistics for each observed population are given 
in Table 2. Slight residual log normality in the distributions, despite 
phi-transformation, is evident in the probit plots and skewness values, 
although kurtosis approaches normality. The gravel framework in the 
Fiddle is closest to a normal distribution whilst the Bere Stream gravels 
are skewed with the mean and mode in separate class intervals. The 
Tadnoll Brook data are also skewed but are only based on two cores (four 
sub-samples) which may not be a representative sample. 
Assuming the deposits are a two component system of spheres of 
equal density, i.e. gravel and sand, the sand can only represent up to 
22% of the total deposit weight if the gravel particles are contiguous 
and tightly packed or 32% if the gravel, still in contact, is loosely 
packed (Fraser, 1935). Any additional percentage weight of sand would 
indicate that the sand and gravel were deposited contemporaneously and 
many gravel particles would be isolated in a sand mass. Smaller percentages 
would indicate either contemporary deposition or later infiltration of 
sand into a stable gravel bed. The matrix represented between 4 and 
61% by weight. The majority of values were low yielding a mean value 
of 19%( s.d. 13%). Basal sections of compacted sediment had high values 
25% (s.d. 17%) whilst surface gravels were cleaner ~13% (s.d.8%). 
In a similar vein the ratio dm/df for a compact gravel cannot exceed 
0.154 (Fraser, 1935) if the matrix was a later infiltration population, 
otherwise deposition was contemporaneous. 
These data are interpreted as representing, contemporaneous deposition 
of coarse gravel from traction with some suspended sand being trapped in 
the stabilising framework (e.g. Smith, 1974) and later, partial infilling 
of the void space by fine sand infiltrating the stable bed. The few high 
percentage values represent sand deposits with additional gravel mixed in. 
The cleaner surface gravels may be low in matrix component owing to the 
digging activity of fish, hydraulic winnowing by scour and a propensity for 
find sand to settle and fill gravels from the base upwards (Beschta & 
Jackson, 1979). 
Bulk properties 
The density of particles of bed material was 2.45 g cm-3 (s.d. 0.0447), 
bulk density was 1.60 tonnes m-3 (s.d. 0.19). 
The gravels are better sorted than gravels in Teesdale where bimodality 
is not ubiquitous (Carling & Reader, 1982). The degree of sorting and low 
sand content result in low bulk densities and consequently in high void 
ratios (E) and porosities (λ) (Table 3). 
The few data for Dorset are plotted in Figs. 3 + 4 in relation to the 
Teesdale data presented in Carling & Reader (1982). Values of λ are 
better described by Komura's relationship than by the Teesdale curve in 
terms of median diameter. A discrepancy between the Teesdale and Dorset 
data also exists in terms of the percentage of silt and clay in the 
deposits. The high porosity is clearly reflected in the proportion by 
volume of the potential void space filled by sand (i.e. the ratio between 
the sand volume and the sand and void volume combined) where on average 
only sane 29% of the potential void space is filled. 
Organic content 
Organic detritus was finely comminuted (<1 mm). In only one sample 
were any large wood or reed fragments found. Two samples, which were 
very black and apparently organically rich, were dominated by sand; 
the organic percentage only being 3-4%. These deposits may represent 
decayed vegetal mats which entrapped sand and were subsequently buried 
by gravels, or alternatively, flasers of sand deposited with fine organic 
detritus in hollows in the gravel bed. 
The matrix infilling the gravel framework had organic percentages 
in the range 0.42 to 3.46%. The mean value for all deposits was 1.14% 
(s.d. 0.79%). 
Diagenetic concretion 
The X-ray diffraction analysis showed the concretion to be calcite, 
with small amounts of quartz. The latter probably represent small sand 
grains in the calcareous deposit. The material resembles tufa which is 
formed by evaporation (Pettijohn, 1975, p. 357). The possible mechanism 
of formation for these sub-aqueous deposits is precipitation from a super-
saturated solution. 
ACKNOWLEDGEMENTS 
The F.B.A. River Laboratory staff are thanked for their assistance 
and hospitality during the visit to Dorset. Dr. P. Fox carried out liaison 
work with local land owners who kindly allowed access to their properties. 
Dr. D.T. Crisp, Messrs. J. Blackburn, B. Martin, S. Welton, A. Strevens and 
S.C. Shinn helped with the freeze-sampling and Mr. J. Morgan arranged an 
ample supply of CO2. Mr. P. Boole carried out the laboratory analysis and 
Dr. J. Hilton arranged for the X-ray diffraction test. Mrs. D. Jones typed 
the manuscript. 
REFERENCES 
Beschta, R.L. & Jackson, W.L. (1979) The intrusion of fine sediments 
into a stable gravel bed. J. Fish.Res. Bd. Can. 36 (2) 204-210. 
Bliss, C.I. (1967) Statistics in Biology, Vol. 1. McGraw-Hill, New 
York, 558 pp. 
Carling, P.A. & Reader, N.A. (1981) A freeze-sampling technique suitable 
for coarse river bed-material. Sedimentary Geology, 29, 233-239. 
Carling, P.A. & Reader, N.A. (1982) Structure, composition and bulk 
properties of upland stream gravels. Earth Surf. Processes and 
Landforms, 7, 349-365. 
Cary, A.S. (1951) Origin and significance of open-work gravel. Trans. 
Am. Soc. Civ. Engr. 116, 1296-1308. 
Dyer, K.R. (1970) Grain-size parameters for sandy gravels. J. Sedim. 
Petrol. 40 (2), 616-620. 
Fraser, H.J. (1935) Experimental study of the porosity and permeability 
of clastic sediments. J. Geol. 43; 910-1010. 
Harding, J.P. (1949) The use of probability paper for graphical analysis 
of polymodal frequency distributions. J. Mar. Biol. Assoc., 28, 
141-153. 
Mundry, E. (1972) On the resolution of mixed frequency distributions 
into normal components. Mathematical Geology, 1, (4), 55-60. 
Ottaway, E.M.; Carling, P.A.; Clarke, A. & Reader, N.A. (1981) Observations 
on the structure of brown trout, Salmo trutta Linnaeus, redds. 
J. Fish Biol. 19, 593-607. 
TABLE 1. Grain size distribution of Dorset stream gravels. 
* saddle interval (see text) 
d s.d. skewness kurtosis 
Piddle framework 
Piddle matrix 
Bere framework 
Bere matrix 
Tadnoll framework 
Tadnoll matrix 
-4.50 
1.10 
-4.54 
1.35 
-5.06 
0.71 
1.13 
1.31 
1.31 
0.92 
1.14 
1.08 
0.07 
0.90 
0.66 
1.19 
0.43 
0.76 
2.31 
3.39 
2.79 
4.47 
2.47 
3.24 
TABLE 2. Summary statistics for Dorset stream gravels. 
TABLE 3. Bulk properties of R. Piddle gravel. 
Sample No. 
Bulk density 
(tm-3) 
Void ratio Porosity Matrix vol. Matrix + void vol. 
1 
2 
3 
4 
5 
6 
7 
1.48 
1.45 
1.70 
1.74 
1.72 
1.83 
1.31 
0.64 
0.64 
0.44 
0.44 
0.46 
0.35 
0.84 
0.39 
0.39 
0.31 
0.31 
0.32 
0.26 
0.46 
0.36 
0.26 
0.14 
0.34 
0.32 
0.34 
0.24 
Fig. 2 Frequency distributions of Dorset river gravels, showing the 
bimodal distributions. Calculated normal distributions for 
the sub-populations of framework and matrix are shown as continuous 
curves. 
Fig. 3 Porosity λ of R. Piddle gravels, , plotted in relation to 
the median grain size of the deposits. Other symbols and curves 
are from Carling & Reader (1982) see text. 
Fig. 4 Porosity λ of R. Piddle gravels, , plotted in relation to 
the percentage of material less than 0.063 mm in the deposits 
(P0.063). Circles and curve are from Carling & Reader (1982) 
see text. 


Composition and bulk properties of Dorset river gravels

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Composition and bulk properties of Dorset river gravels

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