characteristics and morphological change of the river.ppt

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RIVER ENGINEERING ANDSEDIMENT TRANSPORT
COURSE CODE:WRIE-5133
PreparedBY: AMANUELB. MSC(hydraulicEngineering)
GErd

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River is any body of fresh water flowing from an upland source to a
large lake or to the sea, fed by such sources as springs and
tributary streams.
River is an important source of water for domestic, irrigation,
industrial consumption and have been useful in providing
facilities for navigation, recreation, hydropower generation, and
waste disposal.
Rivers are continuously change their forms and patterns and other
morphological characteristics in space and time due to changes in:
➢ Water discharge
➢ Sediment discharge
1. River Characteristics

1.River Characteristics
The changes in water and sediment discharge may be caused by;
1. Natural elements (natural forces)
✓ Discharge is naturally variable.
2. Anthropogenic elements
✓ Human interference in the fluvial system it includes
a. Land developments
❖ Land clearance, Infrastructure building and Land use change.
b. River developments
❖ Hydraulic structures
❖ River channelization, and Gravel and sand mining
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The Total area from which surface runoff flows to a given point of
concentration is called a catchment area
Hence, a catchment area is always connected to a certain point of
concentration, By summing the partial watershed areas of all the
tributaries, and by adding the areas draining directly into the
stream, total area of the watershed above the concentration point is
obtained.
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Example Exploitation Mechanisms , GERD

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Drainage Patterns: a watershed observed in plan form (map view) is
its drainage pattern which control the overall topography and
geologic structure of the watershed
Stream Ordering: A method of ordering, the hierarchy of natural
channels within a watershed was developed by Horton (1945). but
the modified system of Strahler (1957) is probably the most popular
today.
 Start at the headwaters of the river
 When streams/river of the same order converge the resultant
stream/river becomes a higher order stream.
 The convergence of a lower order stream with a higher order stream
does not change the status

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Strahler’s stream ordering system
The uppermost channels in a drainage network (i.e., headwater
channels with no upstream tributaries) are designated as first-
order streams down to their first confluence
Reading assignment:-Horton, Shreve, Hack and Topological

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Streams can be classified based on the balance and timing of
the storm flow and base flow components.
Ephemeral streams flow only during or immediately after
periods of precipitation. generally flow less than 30 days per year.
Intermittent streams flow only during certain times of the year.
Seasonal flow usually lasts longer than 30 days per year.
Perennial streams flow continuously during both wet and dry
times.

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Channel and Ground Water Relationships: Interactions
between ground water and the channel vary throughout the
watershed.

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The overall longitudinal profile of most streams can be
roughly divided into three zones
Zone 1, or headwaters (or upper course) often has the steepest
gradient. downstream(erosive stream characteristics)
Zone 2, the transfer zone (or Middle course) receives some of
the eroded material. It is usually characterized by wide
floodplains. This transitional reach of the stream is generally the
most stable
Zone 3, the depositional zone; Longitudinal slope flattens;
discharge and gradual deposition of sediment increases.

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1.2.1 Types of flow and Water Movement in Rivers
Laminar versus turbulent: Laminar flow occurs relatively at
low fluid velocity. The flow is visualized as layers, The shear
stress in laminar flow is given by Newton’s law of viscosity as
(1.1)
Where μ is dynamic viscosity, ρ is density of water and ν is
kinematic viscosity ( ν = 10-6 m2/s at 200C).
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2 River Hydraulics

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 The layers close to the bottom have slower velocity and therefore
there is a shear stress between the different "layers" since the
upper layer moves faster than the layer below
 This shear stress can also be called viscous shear stress since it is
caused by the viscosity of water.
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1.2 River Hydraulics

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Most flows in nature are turbulent. Turbulence is generated by
instability in the flow, A typical phenomenon of turbulent flow is
the fluctuation of velocity
(1.2)
Where U and W are instantaneous velocity, in x and z directions,
respectively
u and w are time-averaged velocity, U’ and w’ are instantaneous
velocity fluctuation in x and z directions respectively
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Turbulent flow is often given as the mean flow, described by u
and w. The turbulent shear stress, given by time-averaging of
the Navier – Stokes equation, is
(1.3)
both viscosity and turbulence contribute to shear stress. The total
shear stress is
(1 4)
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Steady versus unsteady: A flow is steady when the flow
properties (e.g. density, velocity, pressure etc.) at any point are
constant with respect to time.
Uniform versus non-uniform: A flow is uniform when the flow
depth and velocity does not change along the flow direction.
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Boundary layer flow:
Prandtl developed the concept of the boundary layer. It provides
an important link between ideal-fluid flow and real-fluid flow
For fluids having small viscosity, the effect of internal friction in
the flow is appreciable only in a thin layer surrounding the flow
boundaries.
However, we will demonstrate that the boundary layer fulfill the
whole flow in open channels
The boundary layer thickness (δ) is defined as the distance from
the boundary surface to the point where u = 0.995 U.
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figure1.7 Development of the boundary layer.
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Example1 Given that the flow velocity at a depth of water 10m is
1m/s. calculate x value where the boundary layer flow starts to
fulfill the whole depth and decide the type of the boundary layer
flow
Solution Based on the expression for turbulent boundary layer flow
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Prandtl’s Mixing Length Theory
Prandtl introduced the mixing length concept in order to calculate
the turbulent shear stress. He assumed that a fluid parcel travels
over a length l before its momentum is transferred.
Figure1.8 Prandtl’s mixing length theory
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Fig.1.8 shows the time-averaged velocity profile. The fluid parcel
located in layer 1 and having the velocity u1, moves to layer 2
due to eddy motion. There is no momentum transfer during
movement, i.e. the velocity of the fluid parcel is still u1 when it
just arrives at layer 2, and decreases to u2 sometime later by the
momentum exchange with other fluid in layer 2. This action will
speed up the fluid in layer 2, which can be seen as a turbulent
shear stress τt acting on layer 2 trying to accelerate layer 2
The horizontal instantaneous velocity fluctuation of the fluid
parcel in layer 2 is
(1.5)
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Assuming the vertical instantaneous velocity fluctuation having the
same magnitude
(1.6)
the turbulent shear stress now becomes
(1.7)
If we define kinematic eddy viscosity as
(1.8)
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The turbulent shear stress can be expressed in a way similar to
viscous shear stress as follows
(1.9)
1.2.2 Fluid Shear Stress and Friction Velocity
Fluid shear stress: The forces on a fluid element with unit width are
shown in Fig.1.9. Because the flow is uniform (no acceleration),
the force equilibrium in x-direction reads as
(1.10)
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For small slope we have sinβ ≈ tan β = S. Therefore
(1.11)
The bottom shear stress (z=0) in uniform flow is thus
(1.12)
Bottom shear stress: In the case of arbitrary cross section, the
shear stress acting on the boundary changes along the wetted
perimeter Fig.1.9. Then the bottom shear stress means actually
the average of the shear stress along the wetted perimeter.
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Figure 1.9: Fluid force and bottom shear stress.
Assuming uniform flow and balancing the forces acting on the darker
area in the figure above.
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(1.13)
Where P is the wetted perimeter and A the area of the cross section.
By applying the hydraulic radius (R = A/P) we get
(1.14)
In the case of wide and shallow channel, R is approximately equal to
h; eq (.14) is identical to
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Friction velocity: The bottom shear stress is often represented by
friction velocity, defined by
(1.15)
Inserting eq (1.14) into eq (1.15), we get
(1.16)
Viscous shear stress versus turbulent shear stress:
The shear stress in flow increases linearly with water depth. As the
shear stress is consisted of viscosity and turbulence, we have
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(1.17)
On the bottom surface, there is no turbulence (u = w = 0, u´ = w´ =
0), the turbulent shear stress
(1.18)
Therefore, in a very thin layer above the bottom, viscous shear stress
is dominant, and hence the flow is laminar. This thin layer is
called viscous sub-layer. Otherwise the flow is turbulent.
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Scientific Classification of flow layer: Figure 1.9 shows the
classification of flow layers. The shear stress in flow increases
linearly with water depth
Figure 1.10: Classification of flow region
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Viscous sub layer: In this layer there is almost no turbulence.
Measurement shows that the viscous shear stress in this layer is
constant. The flow is laminar. Above this layer the flow is
turbulent.
Transition layer: also called buffer layer. Viscosity and
turbulence are equally important.
Turbulent logarithmic layer: viscous shear stress can be
neglected in this layer. it is assumed that the turbulent shear stress
is constant and equal to bottom shear stress. in this layer Prandtl
introduced the mixing length concept and derived the logarithmic
velocity profile.
Turbulent outer layer: velocities are almost constant because of
the presence of large eddies which produce strong mixing of the
flow.
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Measurements show that the turbulent shear stress is constant and
equal to the bottom shear stress. By assuming the mixing length
is proportional to the distance to the bottom (l = kz),
Therefore, from engineering point of view, a turbulent layer with
the logarithmic velocity profile covers the transitional layer, the
turbulent logarithmic layer and the turbulent outer layer
At the viscous sub layer the effect of the bottom (or wall)
roughness on the velocity distribution was first investigated for
pipe flow by Nikurase, Based on experimental data, it was found
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Engineering Classification of flow layer
Hydraulically smooth flow
bed roughness is much smaller than the thickness of viscous sub
layer. Therefore, the bed roughness will not affect the velocity
distribution.
Hydraulically rough flow
bed roughness is so large that it produces eddies close to the bottom.
A viscous sub layer does not exist and the flow velocity is not
dependent on viscosity.
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Hydraulically transitional flow
velocity distribution is affected by bed roughness and viscosity
Figure 1.11: Hydraulically smooth and rough flows
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Velocity distribution
Turbulent layer
In the turbulent layer the total shear stress contains only the
turbulent shear stress. The total shear stress increases linearly
with depth
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Where the integration constant z0 is the elevation corresponding to
zero velocity (uz= z0 =0), given by Nikurase by the study of the
pipe ﬂows.
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Viscous sub layer
In the case of hydraulically smooth flow there is a viscous subl ayer.
Viscous shear stress is constant in this layer and equal to the
bottom shear stress, i.e.
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Velocity profile
The linear velocity distribution intersect with the logarithmic
velocity distribution at the elevation z =11.6ν/u∗, yielding a
theoretical viscous sub layer thickness
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Bed roughness
ks is also called the equivalent Nikurase grain roughness
The only situation where we can directly obtain the bed roughness
is a flat bed consisting of uniform spheres, where ks = diameter of
sphere. But in nature the bed is composed of grains with different
size. Moreover, the bed is not flat, various bed forms,
e.g. sand ripples or dunes, will appear depending on grain
size and current and the bed roughness can be obtained
indirectly by the velocity measurement
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Example2 Given that the flume tests with water depth h = 1m, the
measured velocities at the elevation of 0.1, 0.2, 0.4 and 0.6 m are
0.53, 0.58, 0.64 and 0.67 m/s, respectively and The fitting of the
measured velocities to the logarithmic velocity profile by the least
square method gives u∗= 0.03 m/s and z0 = 0.0000825 m. Calculate
bed roughness (ks), bottom shear stress and decide the types of flow
layer
Solution
Ks = Zo /0.033 = 0.0025m τ = ρU*2 = 0.9 N/m2 We can
confirm that it is hydraulically rough flow by u* ks /ν = 75 > 70
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Types of rivers
Rivers can be classified according to different criteria:
Classification based on variation of discharge
1. Perennial River: Perennial Rivers have adequate discharge
throughout the year.
2. Non-perennial rivers: their flow is quite high during and after
rainy seasons and reduces significantly during dry seasons.
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1.3 River morphology and regime

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3. Flashy rivers: in these rives, there is a sudden increases in
discharge. The river stage rises and falls in a very short period.
4. Virgin rivers: these are those rivers which get completely dried
up before joining another river and sea
Classification based on the location of reach:
1. Mountainous rivers: they flow in hilly and mountainous
regions. These rivers
are further divided into rocky rivers and Boulder Rivers.
2. Rivers in flood plains: after the boulder stages, a river enters the
flood plains having alluvial soil. The bed and banks of river are
made up of sand and silt.
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3. Delta Rivers: when a river enters a deltaic plain, it splits into a
number of small branches due to very flat slopes. There is shoal
formation and braiding of the channels in the delta rivers.
4. Tidal rivers: just before joining a sea or ocean, the river becomes
a tidal river. In a tidal river, there are periodic changes in water
level due to tides.
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Classification based on plan-form:
1. Straight rivers: these rivers are straight in plain and have cross-
sectional shape of a trough. The maximum velocity of flow usually
occurs in the middle of the section.
2. Meandering Rivers: follow a winding course. They consist of a
series of bends of alternate curvature in the plain. The successive
curves are connected by small straight reach of the river called
crossovers or crossings.
3. Braided rivers: flow in two or more channels around alluvial
islands developed due to deposition of silt.
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Figure 1.13: Straight, meandering, and braided rivers
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Stages of rivers
As the river flows from its origin in a mountain to a sea, it passes
through various stages. A river generally has the following 4
stages:
1. Rocky stages
2. boulder stage
3. alluvial stage
4. deltaic rivers
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1. Rocky stage: it is also called the hilly or mountainous stage or the
incised stage.
 The flow channel is formed on the rock by degradation and
cutting.
 The cross section of the river is usually made up of rock.
 The river has a very steep slope and velocity of water is quite
high.
 As the beds and banks are rocky, erosion hazard is less.
 It is ideal for the construction of dam.
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2. Boulder stage: the bed and banks are usually composed of large
boulders, gravels and shingles.
 The bed slop is quite steep
 The river first flows through wide shallow and interlaced
channels and then develop a straight course.
 Most of the diversion head works are constructed in this stage.
3. Deltaic stage: is the last stage of the river just before it discharge
into the sea. The river is unable to carry its sediment load. As a
result, It drops its sediments and gets divided into channels on
either side of the deposited sediment and form the delta.
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4. Alluvial stage: the river in this stage flows in a zig- zag manner
known as meandering.
 The cross section of the river is made up of alluvial sand and silt.
 The materials get eroded form the concave side (the outer side) of
the bend and get
 deposited on the convex side (inner side) of the bend.
 The bed slope is flat and consequently the velocity is small.
 The behavior of the river in this stage depends up on the silt
charge and the flood discharge.
 River training works are required in the alluvial stage.
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Types of alluvial rivers in flood plains (alluvial stage) is future sub
divided in to
A. Aggrading or accreting type: is a silting river. such as: heavy
sediment load, construction of an obstruction across a river, sudden
intrusion of sediment from tributary.
B. Degrading type; if the river bed is constantly getting scoured it
is known as degrading.
C. Stable type: a river that does not change its alignment, slope and
its regime is called Stable River.
D. Deltaic River: it splits into a number of small branches due to
very flat slopes.
River Engineering

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