Introduction to River Engineering

WOLAITASODOUNIVERSITY
DEPARTMENT OF HYDRAULIC AND WATER RESOURCES
ENGINEERING
River Engineering (HWRE-5191)
5th year HWRE (2022/23 G.C.)
By: ManamnoB.(MSc
manamnobeza@yahoo.com

1. Introduction to River Engineering
1.1 Introductions
1.2 Catchment area
1.3 River Classifications
1.4 River morphology and regime
1.5 River behaviors/characteristics
2. River Hydraulics
2.1 Introduction
2.2 Types of Flow and Water Movement in Rivers
2.3 Prandtl’s Mixing Length Theory
2.4 Fluid Shear Stress and Friction Velocity
2.5 Velocity Distribution for Rigid-Boundary Channels
2.6 Thickness of the Laminar Sublayer
2.7 Effect on Velocity-Distribution of the Movable Boundary in Alluvial Channels
3. Sediment Transport
3.1 Introduction
3.2 Sediment Transport Terminology
3.3 Properties of Sediment
3.4 Incipient Motion
3.5 Bed Forms and Bed Roughness
3.6 Bed Load, Suspended Load, Wash Load and Total Load Transport
3.7 Sediment Transport Formulae
River Engineering Course Outlines
Reading Assignment
and presentation

4. River Training and Flood Control
4.1 Introduction
4.2 Objectives of River Training
4.3 Classification of River Training Works
4.4 Methods of River Training and River Bank Protection Works
4.4.1 Transverse Structures – Groynes
4.4.2 Longitudinal Structure – Longitudinal Dikes
4.4.3 Bed Fixation by Sills
5. Preliminary Design of Bridges (Bridge Hydraulics)
5.1 Introduction
5.2 Preliminary Field and Design Procedures
5.3 Backwater Computation at Bridges
5.4 Determination of scour at bridge piers and abutments

 References:
 Principles of River Engineering, by P.Ph. Jansen, et al (1983)
 River Training Techniques, by Przedwojski, et al (1996)
 Hydraulics of Sediment Transport, by W.H. Graf (1971)
 Hydraulic Structures, 2nd ed, by P.Novak, et al (1996)
 Open Channel Hydraulics, by R.H. French (1985)
 Any literature on Sediment Transport in Rivers, River Engineering & Flood Control
 Tentative Mode Of Evaluation
Class activity /presentation = 10%
Quiz = 5%
Assignment = 15 %
Mid-term Exam = 20 %
Final Exam = 50 %

Objective of the Course:
After the completion of the course, the student should be able to
To have knowledge on river morphology and
regime
To determine quantity and type of sediment that
can be transported by river
To design river training structures
To make you familiars with bridge hydraulics

River Engineering is a branch of Civil (hydraulic)
engineering dealing with the design and construction of
various structures to improve and/or restore rivers for both
human and environmental needs.
1. Introduction to River Engineering

Introduction Cont.
Rivers have great destructive and devastating forces on
the people and environment
 Inundation or flooding
 Erosion of watershed/land erosion
 River-bed and bank scour
 Sedimentation at storage reservoirs and intake
 Hampering transport across the river, and pollution

Therefore ,River Engineering is an engineering science regulates the
relationship between natural river and human life.
The main objectives of river Engineering are to minimize the natural
adverse effects to human interests, and optimize the benefits to human
interests.
Planning and design of river training works have to be based on a
thorough understanding of the physical characteristics of the river
system by
 Field investigations, measurements and surveys
 Laboratory tests (physical model)
 Mathematical modeling
Introduction Cont.

Two types of models can be used as supporting tools, mathematical
and physical models.
 Mathematical model:
The Mathematical model gives information by computation.
This model has no restriction on the prototype areas to be modeled
or the available areas in the laboratory specified for modeling.
Also, the mathematical modeling consumes less money and time.
 Physical Model
On the other hand the physical model gives information by
measurements.
The physical models give better results in some complex cases of
study, for example the three-dimensional flows, specifically turbulence
effects, scour and deposit near structures, and determination of the
hydraulic roughness.
Introduction Cont.

2. The Catchment Area
• Total area from which surface runoff flows to a given
point of concentration is called a catchment area,
drainage basin, drainage area, or a watershed.
water dividing line
or water-divide
Drainage Basin features
10

The Catchment Area Cont.
11
Confluence

Watershed Forms
 Form of a watershed varies greatly, however, and is
tied to many factors including
climatic regime,
underlying geology,
morphology,
soils, and
vegetation
o Drainage Patterns: One typical feature of a
watershed when observed in plan form (map view)
is its drainage pattern
Drainage patterns are primarily controlled by the
overall topography and underlying geologic
structure of the watershed.
12

Watershed forms Cont.
• (a) Dendritic is the most common drainage pattern. Characterized by irregular branching of tributary
streams in many directions. Is more likely to be found on nearly horizontal sedimentary rocks or on
areas of massive igneous rocks
• (b) Trellised or lattice-like pattern displays a system of sub-parallel streams,
usually along the strike of the rock formations or between parallel or nearly parallel topographic
features recently deposited by wind or ice.
• (c) Radial pattern is usually found on the flanks of domes or volcanoes and
various other types of isolated conical and sub conical hills.
• (d) Parallel drainage pattern is usually found in regions of pronounced slope or
structural controls that lead to regular spacing of parallel or near parallel streams.
• (e) Rectangular drainage pattern has the main stream and its tributaries displaying right-angled
bends. This is common in areas where joints and faults intersect at right angle.
• (f) Deranged drainage pattern indicates a complete lack of structural or bed rock control. It is marked
by irregular stream courses that flow into and out of lakes.

Watershed forms Cont.
• (g) Centripetal pattern is the drainage lines converge into a
central depression.
• (h) Highly violent pattern is characteristic of areas of complex
geology.

Watershed Forms Cont.
Stream Ordering: A method of classifying, or
ordering, the hierarchy of natural channels within
a watershed was developed by Horton (1945)
Strahler (1932,1957)
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
A second-order stream is formed below the
confluence of two first-order streams and so on.
15

Stream classification
• Ephemeral streams: flow only or immediately after
periods of precipitation.
• Intermittent streams: flow only during certain times
of the year. Seasonal flow in an intermittent stream
usually lasts longer than 30 days per year.
• Perennial streams: flow continuously during both
wet and dry times. Base flow is dependably
generated from the movement of ground water into
the channel

 Channel and ground water relationships:
• The relationship is strongest in streams with gravel riverbeds in
well-developed alluvial floodplains.
17
Cont.…

Classification of watershed
Based on size
• Small < 250km2
• Medium in b/n 250 and 2500km2
• Large > 2500km2
Based on land use
• Agricultural watershed
• Forest watershed
• Mountainous watershed
• Desert watersheds
• Coastal watersheds
• Wetland watersheds
 Based on Shape
• Fan shaped watershed
• Fern shaped watershed

Classification of watershed cont.

Characteristics of watershed
• Flow length
• Stream order
• Slope (Average slope, Horton slope, relief,
hypsometric curve)
• Time of concentration
• Shape characteristics (form factor, compactness
coefficient, circularity ratio, elongation ratio,
bifurcation ratio)
• Area
• streams (location, stream density, drainage
density)

River Morphology and Regime
 The science of geomorphology studies the forces that shape the
earth's surface at the present time.
 River Morphology is defined as the shape of river channels and
how they change in shape and direction over time
 River Regime is the variability in its discharge throughout the
course of a year in response to PCP, TMP, ET and drainage basin
chxs.
 As a river group, we are interested in water , more specifically, we
are interested in the geological work done by water as it flows in a
stream channel.
 Water, when rain deposits it on upland, possesses potential
energy.
 As the water begins to flow downhill, this potential energy is
converted into kinetic energy, or the energy of motion.

River Morphology and Regime
This kinetic energy can do various works as it moves downhill
1. Turbulence and friction within the moving water uses up kinetic
energy.
2. Kinetic energy is used when the water erodes away the surface
of the stream channel it is passing through or moves sediments
along the streambed.
3. By the time the river ends in a standing body of water, for
example when it enters a lake, all of this kinetic energy has been
converted into thermal energy, or heat energy.
• This heat energy, uniformly spread through the environment, is
the most degraded form of energy and cannot accomplish
anything.
Potential Energy -> Kinetic Energy -> Heat Energy

Rivers and their Characteristics
Rivers are the natural canals which carry a huge quantity
of water drained by the catchments as runoff. They take
off From Mountains, flow through plains and finally join
the sea or an ocean.
Rivers are important arrangements of the hydrological
cycle.
Rivers carry a large amount of silt or sediment which is
washed down from the catchments area and also eroded
form the bed and banks of river.
23

Rivers classification
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.
3. Flashy rivers: in these rivers, 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.
24

Rivers classification Cont.
 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.
3. Delta rivers: when a rivers 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.
25

 Based on stability of the river:-
1. Aggrading
2. Degrading
3. Stable
4. Deltaic
26

1. Aggrading (accreting) type: For a river collecting sediment and building
up its bed called an aggrading/Accreting/ type.
 Is a silting river.
 It builds up its slope.
 The silting is mainly due to various reasons, such as: heavy sediment
load, construction of an obstruction across a river, sudden intrusion
of sediment from a tributary, etc.
2. Degrading type: If the bed is getting scoured year to year, it is called
degrading.
 If the river bed is constantly getting scoured, to reduce and dissipate
available excess land slope.
27

3. Stable river: If there is no silting or scouring, it is
called a stable river.
 A river that does not change its alignment, slope and its
regime significant.
28

Classification based on plan-from:
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.
29

30
Straight river
Meandering river
Braided river

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:
Rocky stages
Boulder stage
Alluvial stage
Deltaic stage
32

Stages of rivers Cont.
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.
2. Boulder stage: the bed and banks are usually composed of
large boulders, gravels and shingles.
The bed slope 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.
34

3. 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.
35

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.
36

4. 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.
37

Behaviors Of Rivers In Alluvial Stages
 The behaviors of alluvial rivers depend to a large extent on the
sediment carried by it.
 The sediment carried by the river poses numerous problems,
such as:
 Increasing of flood levels
 Silting of reservoirs
 Silting of irrigation and navigation channels
 Splitting of a river into a number of interacted channels
 Meandering of rivers
 Specially the meandering causes the river to leave its original
course and adopt a new course.
38

Cont…
 An alluvial river usually has the following three stages:
1. Flow in a straight reach
2. Flow at bends
3. Development of meanders
A. Flow in a straight reach:
 The river cross section is in the shape of a ditch/channel, with
high velocity flow in the middle of the section.
 Since the velocity is higher in the middle
 Due to the existence of this transverse gradient from sides
towards the center, transverse rotary currents get developed.
 However, straight reaches are very few in alluvial channels.
39

Cont.…
41
B. Bends:
 Every alluvial river tends to develop bends, which are
characterized by scouring on the concave side and silting on
the convex side.
 The silting and scouring in bends may continue due to the
action of centrifugal force.
 These rotary currents cause the erosion of concave edge and
deposition on the convex edge forming shoal on this edge.
 When once the bend forms, it tends to make the curvature
large and larger.

Deep, fast water and erosion on the outside of the bend
Slow, shallow water and deposition on the inside of the bend
Cont.…

Cont.…
Development of Meanders:
Once a bend in the river has been developed,
either due to its own characteristics or due to the
impressed external forces, the process continues
furthest downstream.
The successive bends of the reveres order are
formed. It ultimately leads to the development of
a complete S-curve called a meander.
44

Meander Cont.
• A meander is one of a series of regular sinuous
curves in the channel of a river or other watercourse.
• Meanders are a result of the interaction of water
flowing through a curved channel with the
underlying river bed.
• This produces helicoidal flow, in which water moves
from the outer to the inner bank along the river bed,
then flows back to the outer bank near the surface of
the river.
• This in turn increases carrying capacity for
sediments on the outer bank and reduces it on the
inner bank

Meander Cont.
• so that sediments are eroded from the outer bank and
redeposit on the inner bank of the next downstream
meander.
• It is produced as a watercourse erodes the sediments of
an outer, concave bank (cut bank) and deposits
sediments on an inner, convex bank
• The result of this coupled erosion and sedimentation is
the formation of a sinuous course as the channel
migrates back and forth across the axis of a flood plain.

Fast flowing water
has more energy
River cliff
forms
Slip-off
slope

 As the outer banks of a meander continue to be
eroded through processes such as hydraulic action
the neck of the meander becomes narrow and
narrower.
Cont.…

Cont.….
• The higher velocities at the outside bend lead to
higher shear stresses and therefore result in
erosion.
• Similarly, lower velocities at the inside bend
cause lower sheer stresses and deposition occurs.
• Thus meander bends erode at the outside bend,
causing the river to becoming increasingly
sinuous (until cutoff events occur).

Meander parameter
Meander parameters

Meander Parameters
Meander parameter Cont.….

Meander length, (ML)
 Is the lowest distance between two meander loops or
 It is the axial length of one meander, i.e the tangential distance
between the corresponding points of a meander.
Meander width/belt (MB)
 It is the distance between the outer edges of clockwise and anti-
clockwise loops of meander.
 Meander Ratio: It is the ratio of meander belt to meander length.
 Is the ratio of the meander width to meander length, i.e. = MB/ML.
Crossovers
 the short straight reaches of the river connecting two consecutive
loops.

 Radius of curvature (R): is the radius of the maximum interior
circle in a meander loop
 Has an average radius of 2.3 to 2.7 times the bank full width
 Meander Wavelength (λ): is the distance between two similar
points along the channel between which waveform is complete.
 Tortuosity: it is the ratio of length along the channel (i.e arcual
length) to the direct axial length of the river reach

 Sinuosity (C): is the ratio of channel length along the centerline of
the channel (S) to the meander length (ML)
 Arc angle (θ): the angle swept out by the radius of curvature
between adjacent inflexion points.
 Meander arc length (Z): the distance measured along the meander
path between repeating (inflexion) points.
 Amplitude (a): width of meander belt measured perpendicular to
the valley or straight-line axis.
C = S/ML

Longitudinal Profile of Rivers
The longitudinal profile can generally be
subdivided into three parts:
The upper river, where erosion takes place
The middle river where erosion and deposition are
more or less in equilibrium
The lower river, where sedimentation takes place.

Cont.….
Zone 1, or headwaters (or upper course)
 often has the steepest gradient. Sediment erodes from slopes of
the watershed and moves downstream. Typically erosive
stream characteristics
 Deep narrow valley (V shaped)
 Interlocking spurs
 Fast flowing river (erosion predominant)
 Pot holes in the river bed
 Rapids and Waterfalls
61

Cont.….
Zone 2, the transfer zone (or Middle course),.
Receives some of the eroded material
It is usually characterized by wide floodplains
and meandering channel patterns. Longitudinal
slope of the stream gradually eases;
Tributaries join the main stream, and therefore
often sudden changes of flow regime.
Erosion and deposition are more or less in
equilibrium;
Stream characteristics obtained from the
middle course are frequently used as basis for
design of stream training projects.
62

Cont.….
Zone 3, the depositional zone (or Lower course):
Longitudinal slope flattens;
Discharge increases,
The primary depositional zone
Gradual deposition of sediment eroded upstream,
hence relatively short-period shifting and changing of
the main stream channel.
It is important to note that erosion, transfer, and
deposition occur in all zones, but the zone concept
focuses on the most dominant process.
63

Cont.….
65
The three longitudinal profile zones

Cont.….
66
Generalized longitudinal section of a river

69
Flow, channel size, and sediment characteristics change throughout the
longitudinal profile

Introduction to River Engineering

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