Estimation of Total Sediment Quantity in
Streams
(Seminar)
Prepared By
Luay Kadhum Hameed
2010
Chapter 1
Introduction
Sediments are small particles, like sand ,gravel, clay and silt. The
water in a river has a natural capacity of transporting sediment, given the
velocity ,depth, sediment characteristics etc. Man made structure in a river
may change the sediment transport capacity over a longer part of the river,
or locally. Erosion may take place in connection with structure, such as
bridges, flood protection works etc. The hydraulic engineer has to be able to
assess potential scour problem. During a flood, the risk for erosion damages
is at its highest.[Olsen,2002]
Sediment is a phenomenon of soil erosion process, which generally starts
with soil first being detached by wind or water or other forces than further
transported and finally either becomes suspended particles in water or wind
and finally settled on land surface. Sediment represents the soil quantity
suspended or deposited.
Sediment can be classified as deposited or suspended. Deposited sediment is
that found on the bed of river or lake. Suspended sediment is that found in
the water column where it is being transported by water movements.
Sediment information is used for measuring effects of changing agricultural
practices, for engineering design of the facilities, such as bridges ,locks,
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dams and hydropower structures, for reservoir study to help reservoir
maintenance.
Sediment causes many problems when constructing hydropower plants and
irrigation projects in tropical countries .Deposition and filling of reservoirs is
one problem ,and the water intake has to be designed for handling the
Sediments .The Sediments reaching the water turbine may cause wear on the
components. Various instruments has been developed to measure the solidmaterial discharge .Such measurements are necessary to determine directly
the amount of sediment load and/or to establish or check analytical or
empirical relations which permit direct calculation of the sediment load.
Very few of the developed instruments are universally accepted. It becomes
necessary to use the instruments with extreme care, and with in the range of
the hydraulic and sediment parameters as specified by the manufactures.
[Graf,1971]
Chapter 2
Bed load measuring devices
Bed load: The part of total load which has more or less continuous
contact with the bed. Thus the bed load must be determined in relation to the
effective shear stress which acts directly on the grain surface.
Description of bed load samplers:
The basic principle of mechanical trap-type bed-load samplers is the
interception of the sediment particles which are in transport close to the bed
over a small incremental width of the channel bed. Most of the particles
close to the bed are transported as bed load but the sampler will inherently
collect a small part of the suspended load (related to vertical size of intake
mouth).
Popular instruments to sample bed load transport are: Bed load
transportmeter Arnhem (BTMA), Helley-Smith sampler (HS) and Delft Nile
bed load and suspended load sampler (DNS).
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The bed-load transport measured by a mechanical sampler is dependent on
its efficiency (instrumental errors), on its location with respect to the bed
form geometry (spatial variability) and on the near-bed turbulence structure
(temporal variability).
The efficiency of the bed-load sampler depends on the hydraulic coefficient,
the percentage of width of the sampler nozzle in contact with the bed during
sampling and on sampling disturbances generated at the beginning and the
end of the sampling period.
Typical instrumental problems of a (bag-type) bed-load sampler are:




the initial effect; sand particles of the bed may be stirred up and
trapped when the instrument is placed on the bed (over sampling),
the gap effect; a gap between the bed and the sampler mouth may be
present initially or generated at a later stage under the mouth of the
sampler due to migrating ripples or erosion processes (under
sampling),
the blocking effect; blocking of the bag material by sand, silt, clay
particles and organic materials will reduce the hydraulic coefficient
and thus the sampling efficiency (under sampling),
the scooping effect; the instrument may drift downstream from the
survey boat during lowering to the bed and it may be pulled forward
(scoop) over the bed when it is raised again so that it acts as a grab
sampler (over sampling).
According transport in a river is often determined by measurements.
A water bottle is lowered into the river, and water with sediment is
extracted. The sediment concentration is determined in a laboratory. The
water bottle is not able to reach all the way down to the bed, so there will be
an unmeasured zone 2-10 cm from the bed . often the measured sediment
will be denoted suspended load , and the unmeasured load denoted bed load.
[Olsen,2002]
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Chapter 3
Suspended load measuring devices
Suspended load: The part of total load which is moving without
continuous contact with the bed as the result of the agitation of the fluid
turbulence. The appearance of ripples will increase the bed shear stress(flow
resistance. On the other hand , more grains will be suspended due to flow
separation on the lee side of the ripples . Thus the suspended load is related
to the total bed shear stress.[Zhou Lin,2001].
The main purpose of suspended sediment sampling is to determine the
instantaneous mean discharge –weighted suspended sediment concentration
over stream cross section . The measured suspended sediment discharge is
computed by these concentrations combined with water discharge.[Edwards
and Glyssom,1999].
Characteristics of ideal sediment sampler:
The requirements of an ideal time integrating suspended sediment sampler
were summarized by Nelson and Benedict (1951).
1. the velocity at the entrance of the intake tube should be equal to the
local stream velocity.
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2. the intake should be pointed into the approaching flow and should
protrude upstream from the zone of disturbance caused by the
presence of the sampler.
3. the sample container should be removable and suitable for
transportation to the laboratory without loss or spoilage of the
contents.
Description of suspended load samplers:
This section describes different samplers to measure the
suspended load.. Suspended load samplers can be classified as a direct
method (Delft Bottle sampler and acoustic samplers) or and indirect
method.
Indirect methods may be point-integrating (trap/bottle samplers, pump
samplers, optical samplers, impact samplers) or depth-integrating (USD-49
and collapsible bag sampler). The most important characteristics of the
point-integrating samplers (sampling period, minimum cycle period and
overall accuracy). Point integration method obtains a sample at a given point
, where it remains for a certain time. Depth integrating method obtains a
sample while it is moved vertically through a given distance. .[Edwards and
Glyssom,1999].
(1)Bottle and Trap samplers:
The basic principle of all mechanical bottle and trap samplers is the
collection of a water-sediment sample to determine the local sediment
concentration, transport and/or particle size by physical laboratory analysis.
Optimal sampling of a water-sediment volume by means of a mechanical
instrument requires an intake velocity equal to the local flow velocity (isokinetic sampling) or a hydraulic coefficient, defined as the ratio of the intake
velocity and local flow velocity, equal to unity. Differences between the
intake velocity and local flow velocity result in sampling errors.
(2)USP-61point-integratingsampler:
The USP-61 suspended load sampler consists of a streamlined bronze
casting (= 50 kg), which encloses a small bottle (= 500 ml). The sampler
head is hinged to provide access to the bottle. The intake nozzle, which can
be opened or closed by means of an electrically operated valve, points
directly into the approaching flow.
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(3) Delft Bottle sampler: The Delft Bottle suspended load sampler is
based on the flow-through principle, which means that the water entering the
intake nozzle leaves the bottle at the backside.
(4) USD-49depth-integrating samplers
The USD-49 depth-integrating sampler is a depth integrating sampler.
The sampler is lowered at a uniform rate from the water surface to the
streambed, instantly reversed, and then raised again to the water surface. The
sampler continues to take its sample throughout the time of submergence. At
least one sample should be taken at each vertical selected in the crosssection of the stream.
(5) Pump sampler
Chapter 4
Sediment transport formulas:
There exist a large number of sediment transport formulas . Some of
the formulas are developed for bed load , and some for total load . All
formulas contain empirical constants , so the quality of the formula depend
on the data set used to calibrate the constant. In other words, some formulas
work well for steep rivers ,and some for rivers with smaller slopes, finer
sediments etc. The formulas give very different result for the same case , and
there is often an order of magnitude between lowest and highest value. It is
therefore difficult to know which formula to use. Different researchers also
have varying opinions and preferences as to what formula to use.
The formulas can be divided in two groups: Bed load formulas and total
load formulas. The bed load formulas are developed for data sets where only
bed load occur. When used in situations where the sediment transport is
mainly suspended load , the formulas may give very inaccurate results. The
total load formulas should work for both modes of transport.[Olsen,2002].
:
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Where
4-1 Total sediment load
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4-2 Bed load transport formulas:
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4-3 Suspended load transport
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Fig. 1 Illustration of vertical distribution of c and u
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The calculations below show the different in results for some equations:
Calculate qB by formuls:
formuls
Kalinske-Frijlink
Meyer-Peter
Einstein-Brown
qB (m3/m.sec)
0.0000121
0.0000215
0.0000167
Comment: The total bed load transport in the river depends on the width of
the river . When the accuracy of sediment transport formulas is concerned,
experts say that if the formula gives the correct order of magnitude , it is a
good formula .
It is not surprising that the formulas give more or less the same result ,
because all formulas include parameters to be determined by the fitting of
experimental results.
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Rating curve
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Chapter 5
Summary
The methods and equipments used for sampling suspended
sediment are different from those used for deposited sediment .Also
sampling method for measurement of quantity sediment in transport are
different than for measurement of sediment quality. The reason for these
differences reflects the fact that sediment quantity must include the sand –
size fractions which are unequally distributed in depth . For bed sediments it
may be necessary to collect deposited sediments with minimum disturbance
in order not to lose the fine material on the sediment surface , or because the
vertical distribution of the sediment components is important .
Various instruments for measuring the sediment transport rate are described.
Usually the sediment transport is represented as the summation of the bed
load and suspended load transport.
To measure the bed load transport, two measuring methods are available:
simple mechanical trap-type samplers (collecting the sediment particles
transported close to the bed) and the recording of the bed profile as a
function of time (bed form tracking).
To measure the suspended load transport, a wide range of instruments is
available from simple mechanical samplers to sophisticated optical and
acoustical (electronic) sensors. Most instruments are used as pointintegrating instruments which means the measurement of the relevant
parameters in a specific point above the bed as a function of time. Some
instruments are used as depth-integrating samplers, which means continuous
sampling over the water depth by lowering and raising the instrument at a
constant transit rate.
Estimating sediment movement is problematic for several reasons . The
measurements are a time consuming and expensive ; the accuracy of
measurements is likely to be poor, and even if there are a good data, where
the soil came from and when is not known .But it is sometimes necessary to
collect samples in order to develop sediment rating curves for detailed
analysis of long- term trends in system stability, to assess water quality ,or to
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determine conditions under which large volume of sediment are delivered to
the stream system.
All instruments are described in terms of their measuring principle, practical
operation, inaccuracy and technical specifications
Guidelines for the selection of the most appropriate sampling technique for a
certain environment are given, based on the following criteria:
1.
2.
3.
4.
5.
type of process/parameters to be measured,
type of sampling environment,
type of sampling,
type of project and required accuracy,
available instruments and available budget.
Measured load is a portion of the total sediment load that is obtained by the
sampler in the sampling zone, unmeasured load is a portion of the total
sediment load that passes beneath the sampler ,both in suspension and on the
bed. The mathematics methods for calculating the sediment transport are
been used ,this method is used to find quantity of bed load and total load in
canals or rivers .The factors which effect on the measurement of sediment
are size of particle ,location ,velocity of flow ,quantity of sediment with
discharge which flowing from catchment and accuracy the man tests for
taking sample. It is not separate limits between bed load and suspended load
because the type of devices used which needed to spacing for taking
samplers .The purpose from estimations by equations to a known ability of
canal to carry the sedimentations and compared with actual measuring of
sediments in field.
The question remains on which formula to use . Three approaches exist:
1. Some formulas work better in particular situations, for example steep
rivers etc. The problem with this approach is the difficult and
inaccurate classification of the formulas
2. Do a measurement in the river ,and use the formula that best fits the
result (julien,1989). The problem is the difficulty of obtaining a good
measurement.
3. Use several formulas ,and choose an estimate close to the average
value.
The final approach depend on the information available and the
experience and knowledge of the engineer.
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