Determination of grain size distribution

 

(1) Volume-based sample

(2) Surface-based sample - line count sample

 
Common procedure for the determination of the grain size distribution of bed-load in rivers; sampling of the (coarse) grains of the surface layer; sample analysis with consideration of fines and conversion to charactersistics of subsurface layer;

 Sample protocol
 Analysis sheet

 

  Sampling procedure according [2], [6] - determination of the frequency distribution of the (coarse) grains of the surface layer

(1)
Define a representative sample location (either at dry or wet conditions)

(2)
Document the meta data of location and sample in the sample protocol

(3)
Tighten a string at the sample location along the surface layer of the river bed (or along the river banks, gravel banks, areas of sediment depositon, etc.)

(4)
Take pictures of the sample location (close view) and the hydraulic situation in the relevant river reach (close view and overview)

(5)
Measure the grain diameter of the b-axis of every grain situated below the tightened string; grains with diameters <1 cm are not considered

line_count_sample_string

     Fig. 1: Line count sample - relevant grains
grain_axes     Fig. 2: Grain axes

(6)
Count and classifiy all detected grains according the grain size by use of the sample protocol

(7)
Document the maximum grain size at the sample location (below the string)

(8)
Record the lengths of the sections of fines (<1 cm) along the string
Note:
This sampling step does not refer to the common procedure according [2] and [6]; however, in case of large proportion of fines (<1 cm) along the string, this information may be alternatively used to better assess the weight-fraction of fines (<1 cm) during data analysis (see step 3 of analysis procedure)

 

  Analysis procedure according [2], [6] - conversion to weight fractions, to conditions of the subsurface layer and consideration of fines

(1)
Transfer the data of the sample protocol to the analysis sheet

(2)
Convert the requency distribution of the (coarse) grains to weight fractions according:
 
    

(3)
Convert from conditions in the surface layer to conditions in the subsurface layer by assuming the fraction of fines (<1 cm) according:
 
    

  • To account for subsurface conditions typically observed in gravel bed rivers fFu can be assumed according [2]:
    fFu=0.25
    (1-fFu)=0.75
     
  • To account for surface (active) layer conditions in gravel bed rivers and thereby considering also fines (<1 cm) fFu can be assumed according [3]:
    fFu=0.11
    (1-fFu)=0.89
     
  • If above discussed options are not representative for the conditions at the sample location, the fraction of fines (<1 cm) fFu can be assessed by use of the documented lenghts of fines on the sample protocol:
    (i)
    Assume a representative mean diameter of fines and roughly estimate the number of fine grains along the captured sections of fines
    (ii)
    By knowing the number of fines and the total number of grains of the sample (in this case case the total of the fine grain and the captured (coarse) grains) determine fFu

(4)
Complete the grain size distribution by assuming a Fuller-distribution for the fraction of fines (<1 cm) according:
 
    

  

  What to look out during line count sampling and data analysis [2], [6]

a.
Detect of at least 150 grains in total and of at least 30 grains of the mean grain classes for every sample

b.
Define an appropriate location referring to the respective work (modelling) task and the hydraulic conditions
(e.g. specific areas of expected bed-load deposition during floods, upper or lower areas of gravel banks, channel banks, etc.)

c.
To account for spatial variabilities of the grain size distribution and uncertainties of the sampling and analysis procedure perform several samples at one location

   

(1) Volume-based sample

Common procedure for the determination of the grain size distribution of bed-load in rivers; sampling of all sediment fractions of the subsurface layer;

 Analysis sheet

 

  Sampling and analysis procedure [4], [5], [6]

(1)
Define a representative sample location (either at dry or wet conditions); depending on river morphology and sediment characteristics, define the required sample volume according the expected maximum grain size, if possible according  [1], [2], [5]:

   (i)  Required sample volume [m³]:   2.5 x dmax
   (ii) Required sample volume [kg]:    0.1 x 10b x ρs x dmax3

   with:
   dmax   ... maximum grain diameter [m]
   ρs        ... grain density [kg/m³]
   b          ... level of accuracy, high: 5, medium: 4, low: 3 [-]

(2)
Plan and prepare the accessibility to the location as well as transportation of the excavated sediments considering the required sample volume

(3)
Document the meta data of location and sample in the analysis sheet

(4)
Take pictures of the sample location (close view) and the hydraulic situation in the relevant river reach (close view and overview)

(5)
Perform the sampling and transport the sample to the laboratory facility

(6)
Dry the sediment sample according the procedure discussed in [4]

(7)
Sieve the dry sediment sample by use of a sieve tower and the procedure discussed in [5]

(8)
Eventually, in case sediment fractions are as well relevant for suspended-load transport, perform a hydrometer analysis of the fine sediment fractions; this procedure is discussed in [5]

(9)
Summarize results of sieving (oversize weights of every considered sieve element) in the analysis sheet by means of diameter-specific total oversize weights and deliver the cumulative oversize weights

 

  What to look out during volume sampling and data analysis [6]

a.
If the volume sample is related to the subsurface layer of the river bed the surface (active) layer has to be removed at the sample spot before sampling

b.
For practical purposes it is sometimes beneficial to select and weight (very) coarse sediments in the field (weighting machine, water displacement); these fractions then do not have to be transferred to the laboratory facility; according [6] this procedure is reasonable for grains with diameters larger than 0.032 m

c.
The analysis with the sieve tower is limited to a certain maximum sample volume which is smaller than the total sample volume. Sieve analysis is therfore accomplished in portions and the total of the oversize sediment related to the different grain sizes is delivered in the analysis sheet.

d.
Define an appropriate location referring to the respective work (modelling) task and the hydraulic conditions
(e.g. specific areas of expected bed-load deposition during floods, upper or lower areas of gravel banks, channel banks, etc.)

e.
To account for spatial variabilities of the grain size distribution and uncertainties of the sampling and analysis procedure perform several samples at one location; the sample does not account for any stratification in the subsurface layer of the river bed

 

  References

[1]
Bunte, K., Abt, S.R. (2001): Sampling surface and subsurface particle size distributions in wadable gravel- and cobble-bed streams for analysis in sediment transport, hydraulics, and streambed monitoring, U.S. Dep. of Agric., For. Serv., Rocky Mt. Res. Stn., Fort Collins, Colorado, USA.Gen. Tech. Rep. RMRS-GTR-74, DOI
[2]
Fehr, R. (1987): Geschiebeanalysen in Gebirgsflüssen – Umrechnung und Vergleich von verschiedenen Analyseverfahren, Annual Communications of the Laboratory of Hydraulics, Hydrology and Glaciology at ETH Zurich, Nr. 92 (German), WEBLINK
[3]
Recking, A. (2013): An analysis of nonlinearity effects on bed load transport prediction, Journal of Geophysical Research – Earth Surface, 118, 1264–1281, DOI
[4]
ÖNORM EN ISO 17892-1 (2015): Geotechnical investigation and testing - Laboratory testing of soil - Part 1: Determination of water content, 15.06.2015, WEBLINK
[5]

ÖNORM EN ISO 17892-4 (2015): Geotechnical investigation and testing - Laboratory testing of soil - Part 4: Determination of particle size distribution, 01.05.2017, WEBLINK
[6]
Rickenmann, D. (2014): Methoden zur quantitativen Beurteilung von Gerinneprozessen in Wildbächen, WSL Report 9 (German), WEBLINK


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