Table Of ContentG.W. ANNANDALE
Rand Afrikaans University, Auckland Park Johannesburg,
P.O. Box 524, 2000 Johannesburg, South Africa.
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To my wife Nicolene
for her encouragement,
support and hard work.
VII
PREFACE
Books on the subject of reservoir sedimentation are generally lacking. There
is however a need for collecting, documenting and evaluating the knowledge
available in this field of engineering. Research on reservoir sedimentation
in recent years was aimed mainly at water resources projects in developing
countries. These countries, especially in Africa, often have to cope with
long droughts, flash floods and severe erosion problems. Large reservoir
capacities are required to capture water provided by flash floods so as to
ensure the supply of water in periods of drought. The problem arising
however is that these floods, due to their tremendous stream power, carry
enormous volumes of sediment which, due to the size of reservoirs, are
virtually deposited in toto in the reservoir basin, leading to fast deterio-
ration of a costly investment. Accurate forecasting of reservoir behaviour
is therefore of the utmost importance.
The point of view taken in this book is that of the water resources engineer
who wishes to predict the effect of sediment deposition on a reservoir and
its immediate environment. Practical procedures to estimate sediment yield,
calculate sediment profiles and assess the influence sediment retention has
on the river downstream of a reservoir are presented. Preventative measures
are also discussed.
The practical approach adopted in this book by illustrating theory with
examples should appeal to the design engineer. However, new theory presented
regarding processes involved in the deposition of sediment in reservoirs
should also stimulate the more academically minded research engineer to some
deeper thought.
T
Chapter 1
PROBLEMS RELATED TO RESERVOIR SEDIMENTATION
The role of the water resources engineer in a counrry's economy often goes
unnoticed but is of vital importance, especially in the case of developing
and semi-arid countries. Supply of sufficient water for urban, industrial
and farming use in semi-arid countries requires large reservoirs to compen-
sate for the irregular flow of water. In capturing the excess water, often
supplied by flash floods, large volumes of sediment are usually deposited in
the reservoir basin. It is these deposits of sediment that are of great
concern to the water resources engineer. Effective evaluation of the effects
of sediment deposits requires a quantitative analysis of sediment processes,
the subject matter of this book.
In order to appreciate the influence of reservoir sedimentation on the
environment it is important to view the reservoir as part of a river system.
Before the existence of a reservoir, the river in which it is built endeav-
ours to attain a condition of stability. Construction of a reservoir dis-
turbs the normal flow pattern however and sediment is deposited in the
reservoir basin, in an attempt to restore the original progress that has been
made towards stability. During this process the river geometry is changed
both upstream of the dam wall, due to deposition of sediment, and downstream,
not by an excess of sediment, but by a lack thereof in the water. The
"sediment hungry" water entrains sediment downstream of a dam wall, leading
to degradation. The influences of these processes, as far as the water
resources engineer is concerned, can be subdivided into implications for the
environment, economic life and the design of a reservoir.
Large reservoirs invariably have tremendous influence on the environment, and
engineers have in the past been accused, often justifiably, of being insensi-
tive to these issues. However, the attitude of engineers has changed in
recent years and their contribution in quantifying certain impacts of pro-
posed projects on the environment plays an important role in many studies.
The intention is not to deal with environmental issues in great detail, but
merely to present a few examples of the influence disturbed sediment dis-
charge in rivers has on the environment and to highlight the importance of
quantitative analyses. In viewing processes downstream of reservoirs it is
not only the scour processes that are of importance but also the effect
reduced water and sediment\ discharge has on the livelihood of marshes and
floodplains. It is of course not within the field of expertise of the
engineer to investigate the influence of different flow patterns (both of
water and sediment) on plant and animal life, and a life science specialist
will have to be co-opted for this task. The engineer however will be respon-
sible for quantifying sediment and water discharge patterns downstream of a
reservoir, information which is required to reliably determine the potential
influence of a reservoir on the original river system.
The effect of degradation as regards loss of fertile soil and changes in
aquatic environment due to changes in river geometry, also require rigid
calculation procedures. In the region upstream of the reservoir sediment
deposition can, due to drastic changes in the cross-sectional geometry of
rivers, have considerable influence on flood lines and also result in other
environmental damage.
Anticipating the effect reservoir sedimentation will have on flood lines
upstream of reservoirs is very important, as inaccurate assessment can result
in unplanned inundation, which could lead to considerable losses of both crop
and wild plant life. Deposited sediment can also, in cases where reservoir
levels are often low, result in damage to the surrounding areas. This occurs
due to wind-blown sediment, which is entrained from the beaches surrounding
the water perimeter. These sediments can cause considerable damage to plant
life in the vicinity.
The vulnerability of the economic life of a water resources project to
reservoir sedimentation stems from the nonlinear relationship between yield
of water and reservoir capacity. Severe reductions in yield of water from a
reservoir can result from relatively small changes in reservoir volume. A
conflict therefore arises in areas where reservoir sedimentation is a prob-
lem, and the water resources engineer must be able to accurately forecast the
balance between decreasing yield and increasing demand for water as time
passes. Misjudgement during the planning stages of a project can turn out to
be a considerable financial burden on a community or country.
The influence of sediment deposition on reservoir design affects decisions
regarding the placement of sluices in dam walls and estimation of excess
pressure due to deposited sediment on structures such as dam walls and intake
towers. It is therefore very important to accurately predict sediment
distribution in a reservoir. Large volumes of deposited sediment will not
necessarily mean that pressures due to sediment on a dam wall will be
3
considerable. The shape of the reservoir basin and sediment transport
characteristics may be such that most of the sediment is distributed in the
upstream reaches, perhaps not even reaching the dam wall. In other cases
large volumes of sediment may deposit close to the wall, leading to forces on
the structure that are in excess of the normal hydrostatic forces. The
influence of sediment distribution on the placement of sluices follows a
similar line of reasoning and further emphasizes the need for accurate
calculation procedures to forecast distribution of deposited sediment.
The material in the rest of the book has been organized to enable the water
resources engineer to quantify most of the phenomena associated with reser-
voir sedimentation. Chapter 2 reviews basic hydraulic principles and can be
omitted by those familiar with the details. Chapter 3 deals with morpho-
logical principles of river mechanics, and the most popular sediment trans-
port theories are reviewed, compared and evaluated in Chapter 4. Practical
techniques for estimating yield of sediment from catchments and retention
thereof in reservoirs are presented in Chapter 5. Chapter 6 deals with
measures that can be taken to prevent reservoir sedimentation, and existing
techniques for forecasting the distribution of deposited sediment are pre-
sented and evaluated in Chapter 7. Chapter 8 presents modern concepts of
processes involved in reservoir sedimentation together with four new calcula-
tion procedures, illustrated by example. Techniques for forecasting scour
downstream of reservoirs are reviewed in Chapter 9.
4
Chapter 2
OPEN CHANNEL HYDRAULICS
2.1 INTRODUCTION
A study of reservoir sedimentation being part of the study of open channel
flow in movable bed channels, requires knowledge of the basic principles of
open channel hydraulics as well as of river behaviour. Basic principles of
open channel hydraulics will be dealt with in this chapter, while basic
principles of river behaviour will be dealt with in Chapter 3.
Relationships describing the behaviour of fluid flowing in open channels will
be derived by using the Reynolds transport theorem as point of departure.
This theorem allows an understanding of the relationship between a system and
a control volume; the latter approach being the most popular amongst civil
engineers. Subsequent to the derivation of conservation of mass, linear
momentum and energy equations, the concept of specific energy is reviewed.
Under the heading of uniform flow, means of estimating channel roughness and
calculation of uniform flow phenomena are discussed, whereafter laws gover-
ning the behaviour of gradually varied flow are dealt with.
2.2 REYNOLDS TRANSPORT THEOREM
Of the three approaches used to solve problems in fluid engineering, viz.
control volume, differential analysis and dimensional analysis, the one
method probably used more often than the other two, especially in open
channel hydraulics, is control volume analysis. Control volume analysis is a
large-scale, integral type analysis which requires assumptions regarding
energy losses, etc. within the control volume. This requirement idof import
as the relationships used to solve control volume problems only relate
changes in fluid properties at the boundaries of the control volume.
Dimensional analysis usually requires costly laboratory equipment and is
often used for solving problems in large engineering projects where the
expense can be justified.
Differential analysis on the other hand often demands numerical solution of
differential equations resulting from the application of the conservation
laws to an infinitesimally small control volume. Such analysis requires a
5
computer for the successful application thereof and is a field currently
being developed under the name of computational hydraulics.
Although the field of computational hydraulics is becoming more popular, the
analysis is involved and requires a lot of time. Control volume analysis on
the other hand may sometimes give crude answers but is much quicker to apply
and therefore popular in general engineering practice. The latter will
consequently be the main subject of this chapter.
In the application of control volume analysis it is important to relate the
properties of a control volume to the properties of the system of which the
control volume forms a part. A system, being defined as an arbitrary quan-
tity of mass of fixed density, is separated from its surroundings by its
boundaries and the interaction between the system and its surroundings is
described by the laws of mechanics. The conversion of the mathematics of
system behaviour to that describing behaviour of fluid flowing through a
control volume is carried out by the so-called Reynolds transport theorem.
pstem 1 System 2
Figure 2.1 Flow of three systems through control volume ab.
6
The Reynolds transport theorem can be derived by first considering flow in an
one-dimensional system, whereafter the equation can be generalized to repre-
sent three-dimensional flow.
Consider the one-dimensional flow of three systems flowing in the x-direction
as indicated in Figure 2.1. At time t (Figure 2.l(a)) system 2 completely
fills the control volume ab, whereas systems 1 and 3 are situated on either
side. As the systems flow from left to right the situation changes over a
time interval dt so that at time t+dt a small portion of system 1 has moved
into the control volume and a portion of system 2 has moved out of the
control volume. At time t+dt the control volume is therefore filled with
parts of systems 1 and 2.
Each one of these systems has a generalized property B, which can be either
mass, energy or momentum. If an intensive value 6 is defined as
then the total amount of the property B present in the control volume can be
expressed as
Bc = JcGpdV (2.2)
where dm = pdV = differential mass of the fluid; p = mass density of the
fluid; dV = infinitesimal volume of fluid; and Bc = property B present in
control volume c.
The rate of change of the generalized property B within system 2 at time t
can then be written as
d (B~)= - 1 B (t + dt) - -1 €3 (t)
dt dt c dt c
where t = time; A = cross-sectional area of flow; and v = flow velocity.
