ISO/TR 11651:2015

Estimation of sediment deposition in reservoirs using one dimensional simulation models

ISO/TR 11651:2015 describes a method for estimation/prediction of sediment deposition within and upstream of a reservoir using numerical simulation techniques through one-dimensional flow and sediment transport equations. Numerical simulation models for predicting sediment distribution are applicable for reservoirs, where the length of the reservoir greatly exceeds the depth and width and the reservoir has a significant through flow. ISO/TR 11651:2015 includes the theoretical basis and fundamental assumptions of the technique and provides a summary of some numerical methods used to solve the unsteady flow and sediment transport equations. Also provided are details on the application of the model, including data requirements, procedures for model calibration, validation, testing, applications and identification of uncertainties associated with the method. This Technical Report does not provide sufficient information for the development of a computer program for solving the equations, but rather is based on the assumption that an adequately documented computer program is available.

Estimation du dépôt de sédiments dans le réservoir en utilisant des modèles de simulation à une dimension

General Information

• Status: Published
• Publication Date: 13-Aug-2015
• ICS: 17.120.20 - Flow in open channels
• Technical Committee: ISO/TC 113/SC 6 - Sediment transport
• Drafting Committee: ISO/TC 113/SC 6 - Sediment transport
• Current Stage: 6060 - International Standard published
• Completion Date: 14-Aug-2015

Standards Content (Sample)

TECHNICAL ISO/TR
REPORT 11651
First edition
2015-08-15
Estimation of sediment deposition
in reservoir using one dimensional
simulation models
Estimation du dépôt de sédiments dans le réservoir en utilisant des
modèles de simulation à une dimension
Reference number
ISO/TR 11651:2015(E)
©
ISO 2015

---------------------- Page: 1 ----------------------
ISO/TR 11651:2015(E)

COPYRIGHT PROTECTED DOCUMENT
© ISO 2015, Published in Switzerland
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized otherwise in any form
or by any means, electronic or mechanical, including photocopying, or posting on the internet or an intranet, without prior
written permission. Permission can be requested from either ISO at the address below or ISO’s member body in the country of
the requester.
ISO copyright office
Ch. de Blandonnet 8 • CP 401
CH-1214 Vernier, Geneva, Switzerland
Tel. [phone]
Fax [phone]
[e-mail]
www.iso.org
ii © ISO 2015 – All rights reserved

---------------------- Page: 2 ----------------------
ISO/TR 11651:2015(E)

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Definitions . 2
4 Units of measurement . 2
5 Principles of quasi-unsteady sediment modelling . 2
6 Principles of unsteady flow models . 2
6.1 General . 2
6.2 Governing equations . 3
6.3 Numerical techniques for solution of governing equations . 6
6.3.1 Explicit finite-difference methods . 7
6.3.2 Implicit finite-difference methods . 7
6.3.3 Finite element methods . 7
6.3.4 Finite volume methods . 8
6.4 Sediment transport . 8
7 Data requirements .10
7.1 Selection of model boundaries .12
7.2 Cross-section data .12
7.2.1 General.12
7.2.2 Manning’s n values .13
7.2.3 Movable bed and dredging .13
7.3 Stage data.13
7.4 Velocity data .13
7.5 Discharge data .13
7.6 Lateral inflows and withdrawals .14
7.7 Sediment data . 14
8 Formulation, calibration, testing and validation of models .15
8.1 Formulation of numerical models .15
8.1.1 Hydrology .15
8.1.2 Geometry .16
8.1.3 Selection of transport equation .16
8.1.4 Bed mixing and armoring algorithm .16
8.2 Preliminary tests .16
8.3 Computational grid and time step.17
8.4 Convergence testing .18
8.5 Boundary and initial conditions .18
8.6 Calibration .18
8.7 Validation .19
8.8 Predictive simulation .20
8.9 Sensitivity testing .20
8.10 Specific models .20
9 Uncertainties .21
9.1 Model parameters .21
9.2 Data for model development, testing and application .21
9.3 Governing equations .22
9.4 Numerical approximations to governing equations .22
Annex A (normative) Models and case studies .24
Bibliography .25
© ISO 2015 – All rights reserved iii

---------------------- Page: 3 ----------------------
ISO/TR 11651:2015(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.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
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. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation on the meaning of ISO specific terms and expressions related to conformity
assessment, as well as information about ISO’s adherence to the WTO principles in the Technical
Barriers to Trade (TBT) see the following URL: Foreword - Supplementary information
The committee responsible for this document is ISO/TC 113, Hydrometry, Subcommittee SC 6,
Sediment transport.
iv © ISO 2015 – All rights reserved

---------------------- Page: 4 ----------------------
ISO/TR 11651:2015(E)

Introduction
Storage reservoirs built across rivers or streams lose their capacity on account of deposition of sediment.
Surveys indicate that world-wide reservoirs are losing their storage capacity, at an annual rate of about
one percent, due to accumulation of sediments. The impacts of sedimentation on the performance of the
reservoir project are manifold. Some of the important aspects are the following:
a) reduction in live storage capacity of the reservoir;
b) accumulation of sediment at or near the dam may interfere with the functioning of water intakes
and hence is an important parameter in deciding the location and level of various outlets;
c) increased inflow of sediment into the water conveyance systems and hence to be considered in the
design of water conductor systems, desilting basins, turbines, etc;
d) sediment deposition in the head reaches may cause rise in flood levels;
e) the location and quantity of sediment deposition affects the performance of the sediment sluicing
and flushing measures used to restore the storage capacity.
Hence, prediction of sediment distribution in reservoirs is essential in the following:
a) feasibility studies during planning and design of various components of new projects;
b) performance assessment of existing projects.
The most simple and earliest models to predict the sedimentation processes in reservoirs are the
empirical ones. The trap-efficiency curves derived from records of existing reservoirs are among the
most commonly used empirical methods. Recently, due to better understanding of the fundamentals
of reservoir hydraulics and morphology, along with the rapid growth of computational facilities,
development and application of mathematical models have become a normal practice.
Compared to empirical methods, the mathematical approach of the sediment distribution enables more
time and space dependent and more accurate modelling. A large number of mathematical models have
been developed during the past few decades. Flow in the reservoir can be represented by the basic
equations for conservation of momentum and mass of water and sediment.
© ISO 2015 – All rights reserved v

---------------------- Page: 5 ----------------------
TECHNICAL REPORT ISO/TR 11651:2015(E)
Estimation of sediment deposition in reservoir using one
dimensional simulation models
1 Scope
This Technical Report describes a method for estimation/prediction of sediment deposition within
and upstream of a reservoir using numerical simulation techniques through one-dimensional flow and
sediment transport equations.
Numerical simulation models for predicting sediment distribution are applicable for reservoirs, where
the length of the reservoir greatly exceeds the depth and width and the reservoir has a significant
through flow.
This Technical Report includes the theoretical basis and fundamental assumptions of the technique and
provides a summary of some numerical methods used to solve the unsteady flow and sediment transport
equations. Also provided are details on the application of the model, including data requirements,
procedures for model calibration, validation, testing, applications and identification of uncertainties
associated with the method. This Technical Report does not provide sufficient information for the
development of a computer program for solving the equations, but rather is based on the assumption
that an adequately documented computer program is available.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. For dated references, only the edition cited applies. For undated
references, the latest edition of the referenced document (including any amendments) applies.
ISO 748, Hydrometry — Measurement of liquid flow in open channels using current-meters or floats
ISO 772, Hydrometry — Vocabulary and symbols
ISO 1100-2, Hydrometry — Measurement of liquid flow in open channels — Part 2: Determination of the
stage-discharge relationship
ISO 2425, Hydrometry — Measurement of liquid flow in open channels under tidal conditions
ISO 2537, Hydrometry — Rotating-element current-meters
ISO 3454, Hydrometry — Direct depth sounding and suspension equipment
ISO 4363, Measurement of liquid flow in open channels — Methods for measurement of characteristics of
suspended sediment
ISO 4364, Measurement of liquid flow in open channels — Bed material sampling
ISO 4365, Liquid flow in open channels — Sediment in streams and canals — Determination of concentration,
particle size distribution and relative density
ISO 4373, Hydrometry — Water level measuring devices
ISO 6416, Hydrometry — Measurement of discharge by the ultrasonic (acoustic) method
ISO 18365, Hydrometry — Selection, establishment and operation of a gauging station
ISO/TS 3716, Hydrometry — Functional requirements and characteristics of suspended-sediment samplers
ISO/TR 9212, Methods of measurement of bedload discharge
© ISO 2015 – All rights reserved 1

---------------------- Page: 6 ----------------------
ISO/TR 11651:2015(E)

3 Definitions
For the purposes of this document, the terms and definitions given in ISO 772 apply.
4 Units of measurement
The units of measurement used in this Technical Report are SI units.
5 Principles of quasi-unsteady sediment modelling
Many early and contemporary sediment models simplify hydrodynamics of sediment transport models
by invoking a “quasi-unsteady” flow assumption. Instead of solving the Saint-Venant equations explicitly
or implicitly, the hydrodynamics are represented by a series of steady flow backwater computations
and associated with temporal durations. Most generalized sediment transport models still utilize
this approach. Because sediment transport and hydraulic processes respond on different time and
distance scales and because of the inherent uncertainties associated with sediment simulations, the
simplification provided by this approximation often justify the error introduced. However, because the
quasi-unsteady approach does not route water, it can be difficult to implement for reservoir modelling.
Quasi-unsteady models have been used successfully to model reservoir sedimentation but they require
external hydrologic routing computations to define reservoir stage. This process often has to be
iterative because the hydrologic routing parameters change in time as the capacity of the reservoir
changes with sediment deposition. Therefore, an unsteady approach can be advantageous.
6 Principles of unsteady flow models
6.1 General
Numerical models are used to solve sedimentation problems in river engineering, especially for long-
term simulation of long river reaches. The modelling cycle is schematically represented in Figure 1. The
prototype is the reality to be studied and is defined by data and by knowledge. The data represents
boundary conditions, such as bathymetry, water discharges, sediment particle size distributions,
vegetation types, etc. The knowledge contains the physical processes that are known to determine the
system’s behaviour, such as flow turbulence, sediment transport mechanisms and mixing processes.
Understanding the prototype and data constitute the first step of the cycle.
Mathematical
Solution
   model
Interpretation
Numerical
Prototype
 model
Interpretation
Results of
Solution
modeling
Figure 1 — Modelling cycle
In the first interpretation step, all the relevant physical processes that were identified in the prototype
are translated into governing equations that are compiled into the mathematical model.
2 © ISO 2015 – All rights reserved

---------------------- Page: 7 ----------------------
ISO/TR 11651:2015(E)

A mathematical model therefore constitutes the first approximation to the problem. It is the prerequisite
for a numerical model. At this time, many simplifying approximations are made, such as steady versus
unsteady and one- versus two- versus three-dimensional formulations, simplifying descriptions of
turbulence, etc. In water resources, one usually (but not always) arrives to the set-up of a boundary
value problem whose governing equations contain partial differential equations and nonlinear terms.
Next, a solution step is required to solve the mathematical model. The numerical model embodies the
numerical techniques used to solve the set of governing equations that forms the mathematical model.
In this step, one chooses, for example, finite difference versus finite element versus finite volume
discretization techniques and selects the approach to deal with the nonlinear terms. This is a further
approximating step because the partial differential equations are transformed into algebraic equations,
which are approximate but not equivalent to the former.
Another solution step involves the solution of the numerical model in a computer and provides the
results of modelling. This step embodies further approximations and simplifications, such as those
associated with unknown boundary conditions, imprecise bathymetry, unknown water and or sediment
discharges and friction factors.
Finally, the data needs to be interpreted and placed in the appropriate prototype context. This last
step closes the modelling cycle and ultimately provides the answer to the problem that drives the
modelling efforts.
The choice of model for each specific problem should take into account the requirements of the problem,
the knowledge of the system, and the available data. On one hand, the model must take into account all
the significant phenomena that are known to occur in the system and that will influence the aspects
that are being studied. On the other hand, model complexity is limited by the available data. There is
no universal model that can be applied to every problem. The specific requirements of each problem
should be analysed and the model chosen should reflect this analysis in its features and complexity.
6.2 Governing equations
The governing equations are the one-dimensional, cross-sectionally averaged expressions for (1)
the conservation of mass (or equation of continuity), (2) conservation of linear momentum and (3)
continuity of the bed material.
The following one-dimensional flow equations are solved to get the hydraulic parameters such as
energy slope, velocity and depth of flow at each cross-section at each time step. The sediment transport
capacities at each cross-section are then computed and compared with the sediment inflow. The scour
or deposition at each section is computed using sediment continuity equation and new cross-section
bed levels are determined accordingly. The computations then proceed to the next time step and the
cycle is repeated with the updated geometry.
© ISO 2015 – All rights reserved 3

---------------------- Page: 8 ----------------------
ISO/TR 11651:2015(E)

Conservation of mass (or equation of continuity),
∂A ∂Q
+ = q (1)
∂t ∂x
Conservation of linear momentum
 2
∂Q ∂ Q  ∂y



+ β + gA +−gA()SS = qu' (2)


 f 0

∂tx∂ A  ∂x


 
Equation of Continuity of the Bed Material
∂G ∂G
∂ ∂
bs
+ + CA +ρ Bz =0 (3)
() ()
sd∗
∂x ∂xt∂ ∂t
where
A is the cross-sectional area of the channel, and varies with x, t, and z;
t is the time;
Q is the discharge, and varies with x and t;
u’ is longitudinal component of the lateral inflow velocity and varies with x and t;
x is the longitudinal position along the channel axis;
y is the depth of flow, and varies with x and t;
g is the acceleration of gravity;
β is the momentum coefficient and varies with x, z, and t;
q is the lateral inflow per unit length of channel, and varies with x and t;
S is the bed slope, and varies with x;
0
S is the friction slope, and varies with x, t and z;
f
G is the bed load;
b
G is the suspended load;
s
C is the average spatial sediment concentration in the cross-section;
s
ρ is the density of sediment in the bed;
*
B is the deformable bed width and varies with t;
d
z is the bed elevation and varies with t.
4 © ISO 2015 – All rights reserved

---------------------- Page: 9 ----------------------
ISO/TR 11651:2015(E)

The momentum coefficient may be computed using Formula (4):
2
udA
β = (4)
∫
2
UA
where
u is the velocity in some elemental area dA;
U is the mean velocity in the same cross-section having a total area A.
The friction slope, S , accounts for the resistance due to external boundary stresses. The friction slope
f
is generally represented by Chezy or Manning’s equations.
For the Chezy equation, the bed resistance term in the momentum formula is described as:
gQ Q
S (5)
f
2
CAR
where
Q is the discharge;
A is the flow area;
R is the resistance or hydraulic radius.
For the Manning description, the term is:
gQ Q
S = (6)
f
24/3
MAR
The Manning number, M, is equivalent to the Strickler coefficient. Its inverse is the more conventional
Manning’s, n. The value of n is typically in the range 0,01 (smooth channel) to 0,10 (thickly vegetated
channel). The corresponding values for M are from 100 to 10.
The Chezy coefficient is related to Manning’s n by
16/
R
16/
C ==MR (7)
n
Both R and n can vary as a function of x, z, and t. Formula 6 is based on the assumption that the
Manning equation for steady, uniform flow provides a reasonable approximation for S in unsteady,
f
non uniform flow.
Formula (2) can be modified to include a term accounting for the momentum imparted to the water by
a temporally and spatially varying wind. Formulae (1) and (2) also can be written with (1) depth and
velocity, (2) stage and velocity, or (3) stage and discharge as the dependent variables.
Formulae (1) and (2) apply to the unsteady, spatially varied, turbulent free-surface flow of an
incompressible, viscous fluid in an open channel of arbitrary cross-section and alignment. The
equations are solved simultaneously for the unknowns, z (depth of flow) and Q (discharge) as a function
of time (t) and longitudinal position (x).
Formula (3) accounts for the sediment transport and thus the changes in bed levels. Various equations
are available for the calculation of sediment transport rate and alluvial roughness, e.g. the Meyer-Peter
and Muller and the DuBoys’ transport function for the calculation of bed load; the Engelund and Hansen
model, the Ackers and White model, the Yang model and the Smart and Jaeggi model for determination
of the total load and the Engelund and Fredsoe and van Rijn models for the computation of bed load
© ISO 2015 – All rights reserved 5

---------------------- Page: 10 ----------------------
ISO/TR 11651:2015(E)

and suspended load separately. All these models/equations can be applied using a single representative
grain size or using a number of grain sizes representing grain size fractions in graded material.
Formulae (1) and (2) are derived from first principles and may be obtained directly from the three
dimensional equation of mass continuity and the Navier-Stokes equations, which are general, three
dimensional statements of the conservation of momentum for any fluid flow. A number of assumptions
are required to derive Formulae (1) and (2). An unsteady flow model should generally be applied to those
conditions in which none of the major assumptions is severely violated. The assumptions are as follows:
a) flow is approximately one-dimensional, meaning that the predominant spatial variation in dynamic
conditions of hydraulic parameters (discharge, velocity and stage) is in the longitudinal direction;
b) fluid density is homogeneous throughout the modelled reach;
c) vertical accelerations are negligible, i.e. the hydrostatic pressure distribution is applicable;
d) velocity is uniformly distributed in a given cross-section. Inclusion of the momentum coefficient
in Formula (2) allows this assumption to be violated somewhat, however, there should be no flow
separation and streamlines should not be highly curvilinear;
e) neither aggradation nor degradation of the river bed occurs during computational time step;
f) turbulence and energy dissipation can be described by resistance laws formulated for steady,
uniform flow [required for Formula (4)];
g) there are no abrupt changes in channel shape or alignment;
h) velocity is zero at the channel boundary;
i) there is no super elevation of the water level at any cross-section;
j) surface tension and density of air at the free surface are negligible.
6.3 Numerical techniques for solution of governing equations
No known analytical solutions exist for Formulae (1) and (2). Consequently, numerical techniques are
used to convert Formulae (1) and (2) into algebraic equations that may be solved for z and Q at finite,
incremental values of x and t. This solution depends on the proper description of the cross-sectional
area as a function of x and t, and on the availability of accurate boundary condition data.
A variety of numerical techniques have been proposed and used to solve the unsteady flow equations.
The techniques of interest are those based on some type of gridded discretization of the problem at hand,
in which the continuous variables for which the solution is sought are solved only at specific discrete
locations of the physical domain. The algebraic equations that form the numerical
...