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A number of basic requirements for human life, such as water supply, means of transport by navigation, hydropower, drainage and discharge of waste water, are provided by rivers. However, also some negative aspects of river can be indicated in relation to human living conditions, such as floods, if they are above certain limits. They have great destructive and devastating forces on the river, people and environment.  Inundation or flooding (Sudan, Ethiopia), are among the negative impacts of floods of Blue Nile. Erosion, watershed land erosion (Ethiopia), river-bed and bank scour (Egypt and Sudan), sedimentation at storage reservoirs and intake structures (Ethiopia, Sudan and Egypt) hampering transport across the river, and pollution (all Nilotic countries) etc.

Ø  River Engineering is an engineering science regulates the relationship between Natural River and human life.

Ø  The regulation can be done by Human interference with Natural River by Planning, Design and Implementation of Engineering Works.

Ø  The main objectives are to minimize the natural adverse effects to human interests, and optimize the benefits to human interests.

Ø  Planning and design of river training works have to be based on a thorough understanding of the physical characteristics of the river system.

Ø  Such understanding will be obtained by applying the following methods of investigation:

o  Field investigations, measurements and surveys

o  Laboratory tests (physical model)

o  Mathematical modeling

Ø  River planning and regulations need careful field investigation and measurements because the results of human activities in a river may not be easy to predict, especially in a quantitative way.

Ø  To decide in such planning the collected field data should be evaluated by supporting tools.

Ø  Two types of models can be used as supporting tools, mathematical and physical models.

Ø  The two types of model have their own field of applications and sometimes they are used in conjunction.

River Morphology

Types of rivers

Ø  Different types of rivers can be distinguished.

Ø   The major distinction is between diluvial rivers and alluvial rivers.

(i) Diluvial rivers

Ø  This type of river is characterized by the fact that no unique relations exist between the discharge of water, the sediment transport and the bed material.

Ø  Morphological changes due to the interaction of the hydraulic and sedimentological characteristics of the river are absent and the river systems can be considered as morphologically stable.

Ø  In general, diluvial rivers are found in the upper reaches with a rock bed and mountainous or torrential flow characteristics.

(ii) Alluvial rivers

Ø  Alluvial streams may be defined as an open conduit, with geometric dimensions – cross section, longitudinal profile, and slope – changing with time, in dependence of discharge, material of stream bed and banks, and quantity and quality of the sediment carried by the water.

Ø  Alluvial channels are virtually free to adjust their dimensions and shape in response to changing hydraulic conditions and shape in place;

Ø  It is evident, therefore, that most parts of the stream bed and its banks are composed of the material transported by the stream under recent flow conditions (discharges), or at least during the latest geological age.

Ø  In these rivers a clear relation exists between the hydraulic characteristics, the bed material, the sediment transport and the morphological bed and plan form development.

Ø  The alluvial rivers are therefore morphologically unstable.

    Longitudinal profile

Ø  The longitudinal profile can generally be subdivided into three parts:

Ø  The upper river, where erosion takes place; the middle river where erosion and deposition are more or less in equilibrium; and the lower river, where sedimentation takes place.

Ø  In theory the middle river is only a very short stretch (in the limit of a point), but for practical purposes the longest part of a river is often regarded as middle river.

Ø   As stated before, the upper reach is normally diluvial, the middle and lower reaches are classified as alluvial reaches.

Ø  In Figure below a longitudinal profile of a river is given.

Stream Form and Classifications

  • Rivers can be broadly classified in terms of channel patterns based upon configuration as viewed on a map or from the air.
  • Channel pattern describes the plan form of a channel (see Fig. below).
  • In many cases, a stream will change pattern within its length.
  • The plan form deformation is controlled in nature by lithology and its variation along the river length, slope, discharge and sediment load variation during the flood, outside controls and human intervention.
  • Three different types can be distinguished:
  (i) The straight river, (ii) The meandering river and (iii) The braiding river. 

(i)                 The straight river

Ø  A straight channel can be defined as one that does not follow a sinuous course.

Ø  The straight channel can be defined as the stretch of the river which has sinuosity less than 1.5  (sinuosity is the ratio of channel length to valley length.)

Ø  Sinuosity varies from a value of unity for a straight reach to a value of three or more.

Ø  A sinuosity of 1.5 is usually taken as the dividing line between meandering and straight channels.

  • It can also be defined as the transition between meandering and braiding and have normally a meandering channel constricted between the relatively straight banks of the high water channel.
  • In this type, although a river may have a relatively straight alignment, its thalweg (or flow path of greatest depth along the channel) usually wanders back and forth from one bank to the other.
  • This type of the plan form is mainly attributed to outside controls (examples of the outside controls are valley wall, protection work, development on the top of islands, infrastructures, towns, limestone banks and mountains) which sometimes results in internal movements grab the thalweg line to sweep from side to side inside the channel.

(ii)               The meandering river

Ø  The most common channel pattern is the meandering stream.   A meandering channel is one that is formed by a series of alternating changes in direction, or bends.

  • The meandering river consists of a number of consecutive bends and giving an S-shaped appearance to the plan view of the river although sometimes these bends are stable, there is generally a tendency, due to hilicoidal flow for scour to occur at the outer bend and sedimentation at the inner bend.
  • Thus the bends tend to develop to such an extent that eventually a short cut occurs.
  • In aerial photographs the old meanders and short cuts are often clearly visible. (see figure).
  • From an engineering point of view, meandering rivers are more predictable, than braiding rivers and they normally have one relatively deep channel.
  • A meandering river can be described by a number of characteristics through geometrical relations to express its features.
  • The meandering river consists of a series of deep pools in the bends and shallow crossings in the short straight reach connecting the bends.
  • The thalweg flows from a pool through a crossing to the next pool forming the typical S-curve of a single meander loop (see Fig. below)
  • A meandering channel is one that consists of alternate bends.
  • As the thalweg, changes from side to side within the channel, the momentum of the flow affects the cross-sectional geometry of the stream.
  • In bends, there is a concentration of flow due to centrifugal forces.
  • This causes the depth to increase at the outside of the bend, and this area is known as a pool.
  • As the thalweg again changes sides below a bend, it crosses the centerline of the channel.
  • This area is known as the riffle or crossing.




  • One of the main causes of the meandering process is the slight departures from symmetry of flow and bank erosion which tends to deviate the bulk of the flow to one side or the other of the channel.
  • There also appears to be some connection between the longitudinal slope of the stream and the intensity of the meandering; field measurements have shown that slopes of meandering streams generally are very mild.
  • A general characteristic of all meandering watercourses is the migration of the bends downstream and under certain circumstances even laterally.
  •  Figure below shows the plan view and cross section of a typical meandering stream

The following meandering geometrical characteristics can be defined as:

i.      Radius of curvature (r): generally the river forms a series of regular sinusoidal curves with an average radius of 2.3 to 2.7 times the bank full width (Newbury and Gaboury 1993).

ii.      Meander Wavelength (λ): A full meander wavelength is the distance between two similar points along the channel between which waveform is complete.

  • It was found to occur between 6 and 15 times the bank full width.
  • The bank full width is the width of the channel at water level during an average 1 to 2 year peak discharge event.
  • The bank full discharge is the dominant channel forming discharge.
  • The bank full width can be calculated by either using theoretical relationships or by on the ground measurements using field indicators.

iii.      Sinuosity (Р): is the ratio of channel length along the center line of the channel to the length of the valley measured along the center of the meander belt or center of the valley. Sinuosity generates resistance to flow and alters the hydraulic slope of the channel (Rosgen, 1996).

iv.      Arc angle (θ): the angle swept out by the radius of curvature between adjacent inflexion points.

v.      Meander arc length (Z): the distance measured along the meander path between repeating (inflexion) points.

vi.      Amplitude (a): width of meander belt measured perpendicular to the valley or straight line axis.

  • Three types of bends can be defined, free, limited, and forced bends.
  • These types are defined according to the different external constrains and degree of freedom to attain lateral formation.
  • The first type, the free bend is usually associated with broad flood plains consisting of relatively erodible material.
  • In this type, the river bends follow the curves of the valley so that each river bend includes a promontory of the parent plateau.
  • It is found that this type is not disturbed by the external factors and experienced the highest degree of freedom to form the bend shape.

  • The second type is the limited bend.
  • In this type, the bend cut into solid rock or hard strata in deep gorges and exhibit meandering pattern similar to that of rivers in flood plains.
  •  In this type of bends, the banks of the channel are composed of consolidated parent material that limits the lateral erosion.
  •  Such rivers are called incised rivers and these bends are called incised bends or entrenched bends.
  • However, no much information about the origin of incised bends is found.
  • The third type of bends is the forced bend.
  • In this type the channel is highly restricted from external movements.
  • The bank line movements are mainly controlled by either natural or manmade activities.
  • Sometimes in this type the river impinges onto an almost straight parent bank at large angle (600 to 900).
  • The free bend has the smallest sinuosity and arc angle.
  • Next in values is the limited bend followed by the forced bend to some extent.
(iii) The braiding river
  • A braided river is generally wide with poorly defined and unstable banks, and is characterized by a steep, shallow water course with multiple channel divisions around alluvial islands.
  • The braiding river can have several more or less parallel branches which are not fixed but tend to change alignment continually.
  • The division of the discharge between the different branches also tends to change.
  • The braiding river has many disadvantages, being less stable and less predictable than meandering rivers.
  • The characteristics of braided channel can be presented as follows:

o   wide

o   banks are poorly defined and unstable

o   two or more channels

o   sand bars are found between sub-channels

o   sand bars and sub-channels change their position very rapidly

o   often steep slopes with large suspensions

o   sediment overload

  • The two primary causes believed to be responsible for the braided condition are:(Overloading, that is, the stream may be supplied with more sediment than it can carry resulting in deposition of part of the load, and
  •  (2) Steep slopes, which produce a wide shallow channel where bars and islands form readily.
  • If the channel is overloaded with sediment, deposition occurs, the bed aggrades, and the slope of the channel increases in an effort to maintain a graded condition.
  • As the channel steepness, the velocity increases; multiple channels develop and cause the overall channel to widen.
  • The multiple channels, which form when bars of sediment accumulate within the main channel, are generally unstable and change positions with both time and stage.
  • Another cause of braiding is easily eroding banks.
  • If the banks are easily eroded, the stream widens at high flow and at low flow bars form which become stabilized, forming islands.
  • In general, a braided channel has a steep slope, a large bed material load in comparison to its suspended load, and relatively small amounts of silts and clays in the bed and banks.
  • The braided stream is difficult to work with in that it is unstable, changes its alignment rapidly, carries large quantities of sediment, is very wide and shallow even at flood flow and is, in general, unpredictable.
  • Leopold and Wolman (1957) investigated many rivers and found empirically that some rivers are braided and some others are meanders is attributed to a large extent the channel slope and the discharge.
  • He related the channel slope Si and bank full discharge Qb to classify the pattern

Si = (0.0116)/Qb0.44

  •  It would seem that the relationship separates rivers that were braided (found plotted above the relationship) from meandering rivers (found plotted below the relationship).
  • Ramsahoye in 1992 gave a rise to the following relationship

Ø  For straight channel

Si = (0.038)/Qb0.387

Ø  For meandering channel

Si = (0.048)/Qb0.802

Cross section

  • The thalweg, defined as the line connecting the deepest points in the channel, is indicated in Figure below.
  • At the crossing between two bends the thalweg is not the deepest point of the cross section, but the point that occurs between the deepest points in the upstream and downstream bends.
  • In a braided river each branch separately tends to form sections similar to those in a single meandering channel.
  • As the discharge and, therefore, the water level of a river varies, one can distinguish between a low water channel and a high water channel, with flood plains separated by natural levees from the main flow channel (Figure below).
  • The flood plain generally fills rapidly when natural levees are overtopped during extreme high levels, but drains only slowly via small channels through the natural levees.
  • Back swamps may then be formed, due to the slow drainage. Note: In alluvial rivers the width/depth ratio is in the order of 100.
  • The low sinuosity and high width/depth ratio place the river in the bed load category (schumm 1977).
  • Bed load stream have width/depth ratios greater than 40, sinuosity is less than about 1.3, and bed load (sand and gravel) is greater than about 10% of the total sediment load.
  • On a sketch of a cross section with a natural scale very little can be seen and distorted scales are therefore used.
  • It should always be remembered that the cross sections and longitudinal profiles of rivers are in reality very flat.

Dependent and independent variables

Independent variables

  • When describing the river system in terms of dependent and independent variables, the natural river system has to be considered.
  • The natural functions of the river system are to provide drainage of rainfall water and to evacuate the erosion products from the river basin.
  • When considering the river system as the system of channels along which the water and sediment will be evacuated the independent variables of the system are:
  • The discharge to be evacuated Q (m3/s)
  • The sediment eroded and to be evacuated S (m3/day)
  • The characteristics of the sediment D (m), p (kg/m3), ε (porosity)