ISO 6421:2012

INTERNATIONAL ISO
STANDARD 6421
First edition
2012-08-01
Hydrometry — Methods for assessment
of reservoir sedimentation
Hydrométrie — Méthodes d’évaluation de la sédimentation dans les
réservoirs
Reference number
ISO 6421:2012(E)
©
ISO 2012

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ISO 6421:2012(E)
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ISO 6421:2012(E)
Contents Page
Foreword .iv
Introduction . v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 General . 1
4.1 Origin of the sediment deposited in the reservoir . 1
4.2 Overview of reservoir-sedimentation assessment methods . 1
5 Sediment transport balance . 2
6 Topographic survey methods . 3
6.1 General . 3
6.2 Reservoir sedimentation surveys . 3
6.3 Frequency . 4
6.4 Survey equipment . 4
6.5 Density measurements and sediment samplers . 7
7 Topographic survey using the contour method . 8
7.1 General . 8
7.2 Hydrographic survey . 9
7.3 Topographic surveys . 9
7.4 Computation of reservoir capacity .10
8 Topographic survey using a cross-sectional (range line) method .10
8.1 General .10
8.2 Reference frames/graphs . 11
8.3 Calculation of reservoir capacity .15
9 Sub-bottom mapping .19
10 Remote-sensing methods .20
10.1 General .20
10.2 Advantages .20
10.3 Limitations .20
11 Light detection and ranging .20
11.1 General .20
11.2 Aerial applications of LiDAR .21
11.3 Ground-based applications of LiDAR .21
12 Aerial imagery methods .22
12.1 General .22
12.2 Photogrammetry methods .22
12.3 Satellite imagery methods .23
13 Uncertainty analysis .23
13.1 General .23
13.2 Principles .23
13.3 Estimation of uncertainty .24
Annex A (informative) Optimization of the arrangement of ranges.28
Annex B (informative) Introduction to measurement uncertainty .32
Bibliography .40
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ISO 6421:2012(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the International
Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 6421 was prepared by Technical Committee ISO/TC 113, Hydrometry, Subcommittee SC 6, Sediment transport.
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ISO 6421:2012(E)
Introduction
Most natural river reaches are approximately balanced with respect to sediment inflow and outflow. Dam
construction dramatically alters this balance, creating a reservoir which often results in substantially reduced
velocities and relatively efficient sediment trapping. The reservoir accumulates sediment and loses storage
capacity until a balance is again achieved; this normally occurs after the reservoir fills with sediment. The rate
and extent of sediment deposition depends on factors which influence sediment yield and sediment transport,
as well as the reservoir’s trapping efficiency.
The distribution of sediment deposition in different reservoir regions is equally important. Depending upon
the shape of the reservoir, mode of reservoir operation, sediment-inflow rates and grain-size distributions,
the incoming sediment may settle in different areas of the reservoir. Declining storage reduces and eventually
eliminates the capacity for flow regulation and concomitant benefits such as water supply, flood control,
hydropower, navigation, recreation, and environmental aspects that depend on releases from storage. Water
resource professionals are concerned with the prediction of sediment deposition rates and the probable time
when the reservoir would be affected in serving its intended functions.
The estimation of sediment deposition is also important in the design and planning of storage reservoirs.
However, it is difficult to estimate the volume and rate of sediment deposition accurately from the known
criteria and available sediment transport equations. Reservoir capacity surveys indicate patterns and rates of
sedimentation, which help in improving estimation of capacity-loss rates.
This International Standard describes the following reservoir-sedimentation assessment methods:
— conventional topographic surveys (Clause 6)
— contour method (Clause 7)
— cross-sectional (range line) method (Clause 8)
— sub-bottom measurements (Clause 9)
— remote-sensing techniques (Clause 10)
— light detection and ranging (Clause 11)
— aerial applications
— ground-based applications
— aerial imagery (Clause 12)
— photogrammetry methods
— satellite imagery methods
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INTERNATIONAL STANDARD ISO 6421:2012(E)
Hydrometry — Methods for assessment of reservoir
sedimentation
1 Scope
This International Standard describes methods for the measurement of temporal and spatial changes in
reservoir capacities due to sediment deposition.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced document
(including any revisions) applies.
ISO 772, Hydrometry — Vocabulary and symbols
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 772 apply.
4 General
4.1 Origin of the sediment deposited in the reservoir
Reservoirs are subjected to several types of sedimentation as a function of the geomorphology (geology,
slope, topography and land use, drainage density, climate, etc.) of the watershed and the biological cycles in
the reservoir or the drainage basin, in the following order of importance.
a) Erosion of the drainage basin produces dissolved substances and mineral particles with an assortment
of sizes, shapes and types that are related to the rock type and slope of the drainage basin. In addition,
landslides produce debris flows. Sediment is delivered to the reservoir both as suspended sediment load
and as bed load.
b) Sedimentation occurs due to plant debris from the drainage basin and from vascular plants and
phytoplankton in the reservoir. The debris decomposes very slowly and often forms alternating layers
with mineral deposits. The mud resulting from this type of sedimentation is very fine and extremely fluid,
often with a gelatinous texture. Accumulation of mud at a rate of several centimetres per year often causes
problems when a reservoir is drawn down or drained. It has a very high organic content resulting in heavy
consumption of dissolved oxygen.
The proportion of sedimentation caused by each type may be assessed by on-site visual observations and by
analyses of the sediment deposit.
4.2 Overview of reservoir-sedimentation assessment methods
Two basic methods for assessment of reservoir sedimentation are described.
1) Sediment transport balance:
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ISO 6421:2012(E)
The sediment load (bed load and suspended load) is measured over all the watercourses flowing into
the reservoir and then compared with the sediment load measured at the reservoir outlet. The difference
between these two quantities is assumed to represent the sediment that has been deposited in the reservoir.
The point of measurement should be sufficiently close to the reservoir periphery and particular care shall
be taken to complete outflow sampling before it meets the erodible channel downstream.
For further information, see Clause 5.
2) Capacity survey of the reservoir:
Hydrographic surveys of the reservoir are carried out at regular intervals. They reveal the geographic
distribution of sediment deposits in the reservoir and also help in determining lost storage capacity. A capacity
survey of the reservoirs is carried out using topographic survey methods or remote-sensing techniques.
— Topographic bed surveying (i.e. bathymetry) involves measuring the depth at various locations in the
reservoir, following pre-determined profiles, cross sections or using a grid for contour determination.
(See Clauses 6, 7, 8 and 9.)
— The remote-sensing technique uses images taken when the water level varies between near-empty
and near-full, to define the shoreline contours at various water levels. (See Clauses 10, 11 and 12.)
5 Sediment transport balance
In this method, the total sediment load (bed load and suspended load) is measured at suitable locations near
the mouths of all the water courses flowing into the reservoir and at all the reservoir outlets. The difference in
the incoming and outgoing total sediment load is assumed to have been deposited in the reservoir. Data on
water discharge and sediment discharge at each inflow and outflow location are required to be collected in
order to arrive at the total sediment load.
Generally, water discharge is calculated from stream gauge records (for which gauging stations should be set
up as specified in ISO 1100-1), then calibrated in compliance with the standards describing the various stream
gauging methods, e.g. ISO 748 for the velocity area method, ISO 9555 for dilution methods, etc.
A number of traditional methods are available for computing sediment transport, including an interpolation
method for estimating suspended-sediment loads when measured loads are not available. When data are
insufficient for the utilization of the interpolation method, sediment-transport curves may also be used to
compute suspended-sediment loads. However, estimates of suspended-sediment transport from transport
curves – which are also used to compute bed load, and/or total loads – may be subject to significant errors. The
equations are predicated on the presence of specific relations among hydraulic variables, sedimentological
parameters, and the rate at which bed load or bed-material load is transported. The theory supporting the
derivation of the equations tends to be incomplete, oversimplified, or non-existent.
Additionally, even the most theoretically complete equations rely on experimental data to quantify coefficients
of the equations. The availability of reliable environmental data to verify estimates from equations is often
lacking, and the equations tend to ignore or underestimate the washload component, which can comprise
a substantial fraction of the sediment depositing in a reservoir. Rainfall-runoff models based on watershed,
meteorological, and hydrological characteristics may be useful, but tend to be time-intensive and, likewise,
require reliable environmental data.
Equipment and methods for sediment load measurements are detailed in various ISO standards, such as
ISO/TS 3716, ISO 4363, ISO 4364, ISO 4365 and ISO/TR 9212.
Presently, this method is not commonly used for assessment of reservoir sedimentation, because of the availability
of improved techniques and because of a number of practical difficulties and limitations. These include:
1) substantial costs and human resources involved for continuous, long-term measurements at
several locations;
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ISO 6421:2012(E)
2) inadequacy of spatial and temporal representativeness of limited observations due to typically large
variations of sediment load with time and discharge, and also in the cross section;
3) change in masses, and in proportions of fine and coarse fractions of the transported sediment with time;
4) limited accuracy of sediment measurements due to issues associated with
i) sampler efficiencies and sampling techniques, and
ii) potential disturbances induced due to measuring equipment and procedures;
5) large variations in estimates of the bed-load transport rates (in the absence of actual measurements),
made using different sediment transport relations or calculated as a fraction of a measured
suspended load.
NOTE New surrogate technologies for monitoring sediment transport are being developed that may provide cost-
effective and quantifiably accurate sediment-discharge data at gauging stations. ISO 11657 (under development) describes
a number of sediment-surrogate monitoring technologies, including the use of continuous turbidity and stream flow
measurements to estimate suspended-sediment transport. Bulk-optic, laser-optic, digital-optic, pressure-difference, and
acoustic techniques for metering suspended-sediment transport are being investigated. All of these techniques require
in-stream calibrations to accepted standard monitoring instruments and techniques.
6 Topographic survey methods
6.1 General
In topographic surveying, in order to assess the volume of sediment deposit along with its location in the
reservoirs, direct measurements of the depths or elevations of the reservoir bed and the coordinates of the
measurement points are periodically carried out. The main survey methods are the cross-sectional (or range
line) method and the contour method. The selection of a method depends on the quantity and distribution of
sediment indicated by field inspections, shape of the reservoir, purpose of the survey, and desired accuracy.
While the contour survey method is generally applicable for all types of reservoir shapes, the use of the range
method should be limited to relatively straight reaches. A suitable combination can also be used.
For smaller reservoirs, a reconnaissance sedimentation survey may be carried out. This survey has been
designed to determine the approximate rate of loss of storage capacity; the thickness of the deposited sediment
is measured in 15 to 20 or more well distributed locations in a reservoir by means of a simple measuring device
known as a spud (see 6.4.5).
6.2 Reservoir sedimentation surveys
6.2.1 Advantages
a) A reservoir survey can be less costly than taking continuous sediment measurements at several locations
in the catchment.
b) The accuracy of these surveys is usually high, particularly if advanced equipment is used.
c) The survey can be carried out at any convenient time to get the total sedimentation after the last survey.
d) The time required for a survey can be considerably shortened with the use of advanced equipment.
6.2.2 Limitations
a) Topographic surveys do not provide any information about the variation of sediment yield with time, and
give only the total sediments accumulated since the last survey. The above information can only be
obtained by gauging.
b) The unit weight of sediment deposits is required for estimating sediment yield. The temporal and spatial
variation in the unit weight may introduce errors in the results.
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ISO 6421:2012(E)
c) This method does not provide sub-catchment-wise sediment yield; this can only be obtained by sediment
sampling of different streams.
d) This approach is not very effective where sedimentation is small, as the error of measurement may mask
the true sedimentation rates.
e) Sediment outflow data are also required to estimate the total sediment inflow.
6.3 Frequency
The frequency at which reservoir surveys are taken depends on individual site characteristics. Generally,
reservoirs are surveyed every 3 to 10 years. The survey frequency depends on the sediment accumulation
rate; reservoirs that have high accumulation rates are surveyed more often than those with lower rates. For
reservoirs which are losing capacity very slowly, a survey interval in the order of 20 years of even longer may
be adequate. For reservoirs which are losing capacity rapidly, or where the impact of sediment management is
being evaluated, a survey interval as short as 2 to 3 years may be used.
The cost of running a survey also plays a critical part in deciding the survey frequency. Special circumstances
may necessitate a change in the established schedule. For example, a reservoir might be surveyed after a
major flood that has carried a heavy sediment load into the reservoir.
A survey may also be run following the closure of a major dam upstream in the same catchment, since the
reduction in the free drainage area leads to a reduction in the sediment accumulation rate of the downstream
reservoir. The volume of the sediment that has accumulated in a reservoir is computed by subtracting the
revised capacity from the original capacity at a reference reservoir elevation (usually the full reservoir level).
Since this is the difference of two large numbers, an error, even by a few percentages in either of the two
numbers will significantly influence the results.
The minimum survey interval depends on the precision of the survey technique and the rate and pattern of
storage loss. For instance, if a survey technique incorporates an error in the order of 2 % of the total reservoir
volume, and if the reservoir is losing capacity at 0,25 % per year, a 4-year survey interval may be too short to
produce reliable information unless most sediment inflow is focused into a small portion of the impoundment.
6.4 Survey equipment
6.4.1 General
The basic survey items are
a) horizontal or distance measurement, and
b) vertical or depth measurement.
The principal equipment and instruments required for the hydrographic and topographic coverage in relation to
the measurements are detailed in the subsequent subclauses.
6.4.2 Positioning equipment
6.4.2.1 General
The global positioning system (GPS) is a space-based global navigation satellite system that provides reliable
location and time information, in all weather conditions and at all times, anywhere on or near the Earth when
and where there is an unobstructed line of sight to four or more GPS satellites. It is maintained by the United
States government and is freely accessible by anyone with a GPS receiver.
There are two general operating methods by which GPS-derived positions can be obtained:
1) absolute point positioning;
2) relative (differential) positioning (DGPS).
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ISO 6421:2012(E)
6.4.2.2 Absolute point positioning
A GPS satellite continuously transmits microwave radio signals composed of two carriers, two codes and a
navigation message. The GPS receiver picks up the GPS signal through receiver antenna and processes it using
built-in software. GPS receivers on the ground calculate their positions by making distance measurements to
four or more satellites. The satellites function as known reference points that broadcast (free) satellite identity,
position and time information via codes on two carrier frequencies. Measurements of the distance to each
individual satellite are made by analysing the time it takes for a signal to travel from a satellite to a GPS
receiver. Trilateration is then used to establish a GPS receiver’s position. The absolute point positioning is
highly dependent on the accuracy of the known coordinates of each satellite, accuracy of modelled atmospheric
delay and the accuracy of the resolution of the actual time measurement process performed in a GPS receiver
(clock synchronization, signal processing, signal noise, etc.). For many applications, absolute point positioning
does not provide sufficient accuracy.
6.4.2.3 Differential GPS (DGPS)
6.4.2.3.1 General
Differential positioning is the technique or method used to position one point relative to another. DGPS requires
two or more GPS receivers to be recording measurements simultaneously. Differential positioning is more
concerned with the relative difference in position between two users, who are simultaneously observing the
same satellite, than with the absolute position of the individual user. Since errors in the satellite position and
atmospheric delay estimates are effectively the same at both receiving stations, they cancel each other to a
large extent. Differential positioning can be performed by using code- or carrier-phase measurements and can
provide results in real-time or be post-processed. A DGPS utilizing code-phase measurements can provide
a relative accuracy of a few metres. A DGPS utilizing carrier-phase measurements can provide a relative
accuracy of a few centimetres.
6.4.2.3.2 DGPS (code-phase)
A code-phase DGPS consist of two GPS receivers, one set up over a known point and one moving from point
to point or placed on a moving platform, measuring pseudo ranges to at least four common satellites. Since the
satellite positions are known and one of the receivers is over a known point, a “known range” can be computed
for each satellite observed. This “known range” can then be subtracted from the “measured range” to obtain
a range correction or pseudo-range correction (PRC). This PRC is computed for each satellite being tracked
at the known point. The PRC can then be applied to the moving or remote receiver to correct its measured
range. Code-phase DGPS has primary applications to real-time positioning systems where the accuracies at
the meter level are tolerable.
A real-time dynamic DGPS includes reference station, communications link and user equipment. If the results are
not required in real time, the communication link could be eliminated and positional information post-processed.
a) Reference station: The reference station measures timing and ranging information broadcast by the
satellites and computes and formats range corrections for “broadcast to user” equipment. The reference
receiver consists of a GPS receiver, antenna and processor. Using the technology of differential pseudo-
ranging, the position of the survey vessel can be found relative to that of the reference station. The pseudo-
ranges are collected by a GPS receiver and transferred to a processor where pseudo-range corrections
are computed and formatted for data transmission. The reference station is placed on a known survey
measurement in the area having an unobstructed view of the sky. The antenna should not be located near
objects that will cause multipath or interference.
b) Communication links: The communication link is used as a transfer media for the differential corrections.
c) User equipment: The remote receiver should be a multichannel single frequency GPS receiver. The
receiver shall be able to accept the differential corrections from the communications link and then apply
those corrections to measured pseudo range.
d) Separation distances: The maximum station separation between the reference and the remote station in
order to meet hydrographic surveying standard of 2 m, can be maintained up to a distance of 300 km.
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