natural river characteristics · types of bends th f b d three types of bends can be defined, free,...

NATURAL RIVER CHARACTERISTICSCHARACTERISTICS

Karima AttiaNile Research Institute

NATURAL RIVER DEFINITION


I l f h fl i h l i hIs natural stream of water that flows in channels withmore or less defined banks.

It fl i t l kIt flows into an ocean, lake,

or other body of water

Fed along its course by tributaries.

Form a drainage basin, or watershed that collects runoffwith eroded sediments materials into the river


The sediments are typically deposited most heavilyalong the river's lower course, forming floodplains alongits banks and a delta at its mouthits banks and a delta at its mouth.

It is considered as a fundamental link in the hydrologiccycle, and they play a major role in shaping the surfacecycle, and they play a major role in shaping the surfacefeatures of the Earth.

In a few words, Natural rivers, which are self-, ,constructed, self-maintained, and seek their ownstability

Natural river classifications started since 1899 by

NATURAL RIVER CLASSIFICATION

Natural river classifications started since 1899 byDavis:

Youthful river;Mature river and;Old river

YOUTHFUL RIVERS

A river with a steep gradient that has very few tributariesand flows quickly. Its channels erode deeper rather thanwiderwider.

Brazos Ebro

Trinity

MATURE RIVERS

A river with a gradient that is less steep than those ofyouthful rivers and flows more slowly. A mature river is fedby many tributaries and has more discharge than aby many tributaries and has more discharge than ayouthful river. Its channels erode wider rather than deeper.

DanubeOhioOhio

OLD RIVERS

h l d d l ldA river with a low gradient and low erosive energy. Oldrivers are characterized by flood plains.

I dIndus

Nile

CLASSIFICATION BASED ON QUANTITATIVE

L (1957)

CLASSIFICATION BASED ON QUANTITATIVE SLOPE –DISCHARGE RELATIONSHIP

Lane (1957);SQ0.25 = K

SQ0.25 ≤ 0 0017 MeanderingSQ0.25 ≤ 0.0017 MeanderingSQ0.25 ≥ 0.010 Braided

In between the channel is considered as In between the channel is considered as intermediate sand bed stream


Leopold and Wolman (1957)


Leopold and Wolman (1957)

braided (found plotted above the relationship)meandering rivers (found plotted below the

relationship)relationship).


Ramsahoye in 1992


Ramsahoye in 1992,

for straight g

and for meandering


Khan (1971) developed classification based


Khan (1971), developed classification based on:

sinuosity, slope, and channel pattern

Schumm and Khan (1972) proposed a valley slope relation:slope relation:

• Straight: S < 0.0026• Meandering thalweg: 0 0026 < S < 0 016Meandering thalweg: 0.0026 < S < 0.016• Braided: S > 0.016

CLASSIFICATION BASED ON

S h (1963) b d h l bili

C SS C O S OCHARACTERISTICS DESCRIPTION

Schumm, (1963) based on channel stability:

stable eroding or depositingstable, eroding, or depositing

Based on mode of sediment transport:Based on mode of sediment transport:

mixed load, suspended load, and bed loadSaltation

CLASSIFICATION BASED ON

C lb l (1967) b d

C SS C O S OCHARACTERISTICS DESCRIPTION

Culbertson et al. (1967) based on:Depositional features;Vegetation;Vegetation;Braiding patterns;Sinuosity;Meander scrolls;bank heights;Levee formations andLevee formations andFloodplain types.

CLASSIFICATION BASED ON THE BASIS OF

Schumm (1977)

CLASSIFICATION BASED ON THE BASIS OF STABILITY

Schumm (1977) Sediment transportChannel stabilityMeasured channel dimensions


Croke (1992) presented a classification of flood


Croke (1992) presented a classification of flood plains that involved:

Particle size, Morphology of channels, and Bank materials


Parker (1976)


Parker (1976)Indicates that rivers with sediment transportand depth to width ratio (d/B) << 1 at formativedi h h t d t d d idischarge have a tendency toward meanderingor braiding.His classification is based on the relativemagnitude of the depth-width ratio to thechannel slope-Froude number ratio (S/F).Meandering occurs when S/F << d/B, braidingg , goccurs for S/F >> d/B and transition betweenthe two occurs when S/F ~ d/B.

LOAD CLASSIFICATIONS

Th l i i d hi h id h/d h i The low sinuosity and high width/depth ratio place the river in the bed load category

Bed load stream has 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.

CLASSIFICATION ACCORDING TO THE

Alluviam

C SS C O CCO G OSYSTEM TYPE RELATED TO ALLUVAIM

AlluviamDiluvial

Alluviam refers to loose, unconsolidated materials. Alluvium is typically made up of a variety of materials including fine particles of silt and clay materials, including fine particles of silt and clay and larger particles of sand and gravel.

DILUVIAL RIVER

There is no unique relations between There is no unique relations between discharge, sediment transport, and bed materialsMorphological changes are absent Morphologically stableFound in the upper reaches with a rock bed Found in the upper reaches with a rock bed and mountainous or torrential flow characteristics.

DILUVIAL RIVER

ALLUVIAL RIVERS

A clear relation exists between the hydraulic A clear relation exists between the hydraulic characteristics, discharge, sediment transport, and the bed material. width/depth ratio is in the order of 100Morphologically unstableFound in the downstream reaches with alluvial Found in the downstream reaches with alluvial bed

LONGITUDINAL PROFILE

UPPER PART OF THE RIVER

The upper river charterstics:High degree of confinement Confined and narrow valleyConfined and narrow valleyGeneral erosion.

MIDDLE PART OF THE RIVER

The middle river where erosion and deposition The middle river where erosion and deposition are more or less in equilibrium. The dominant characteristic of the river’s central reaches is:

Relatively wide valley, Reduction in profile gradient Reduction in profile gradient.

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

LOWER PART OF THE RIVER

The lower river where sedimentation takes The lower river, where sedimentation takes place. The sediment input is larger than output.

PLAN FORM

Pl f i h i fi i i d Plan form is the river configuration as viewed on maps or from aerial photographs or other spectral imagesg

PARAMETERS RESPONSIBLE ABOUT FORMING RIVER PLAN FORM

Th i f fl ibl f There is a range of flows responsible for creating channel plan form, rather than one single flow magnitudeg g

Effective” discharge refers to the range of flow magnitudes that transports the majority of a river’s annual sediment load over the long-term


Th “d i di h ” f h fl The “dominant discharge” refers to the flow magnitude that determines channel shape, or cross section width and depthp

THE CALCULATION OF THE RANGE OF DISCHARGES RESPONSIBLE FOR FORMING AND MAINTAINING CHANNEL FORM

Dominant or effective dischargeg


B kf ll” di h i h fl i d h Bankfull” discharge is the flow magnitude that is contained within a channel without overtopping its banks.pp gThis flow is significant in creating the shape and size of alluvial channels. It is the discharge magnitude when the channel depth to width ratio in its minimum value. It is the break point between channel It is the break point between channel formation and flood plain formation.

Bank full Discharge

PLAN FORM DEFORMATION

Th l f d f i i ll d i The plan form deformation is controlled in nature by:

Lithology and its variation along the river length;

hDischarge;Sediment load variation during the flood, Outside controls and Outside controls and Human intervention.

PLAN FORM CLASSIFICATION

Ri b l ifi d i f h l Rivers can be classified in terms of channel pattern (plan form) that three different types can be distinguished:g

Straight river, Meandering river and Braiding river.

Braided

Meandering

Straight

STRAIGHT RIVER CHARACTERISTICS

Th i h h l b d fi d h The straight channel can be defined as the stretch of the river which has sinuosity less than 1.5 It is the transition stage between meandering and braiding Attributed to outside controls

MEANDERING RIVER CHARACTERISTICS

C i f b f Consists of a number of consecutive bends.Hilicoidal flow resulting in Hilicoidal flow resulting in scouring near the outer bend and sedimentation near the inner bendnear the inner bend.More predictable, than braiding riversgThey normally have one relatively deep channel

MEANDER GEOMETRICAL RELATIONS

tcrest

tough


R di f ( ) 2 3 2 7 i h Radius of curvature (r): 2.3 to 2.7 times the bankfull width


M d W l h (λ) A f ll d Meander Wavelength (λ): A full meander wavelength is the distance between two similar points along the channel between which p gwaveform is complete. It was found to occur between 6 and 15 times the bankfull width.


Si i I d if h Sinuosity: Is a measure used to quantify the difference between meandering and straight channels. It is defined as the channel length (L) g ( )measured along the center of the channel divided by the valley length (Lv) measured along the valley axisaxis

s = L/LvSinuosity = 1 (straight channels) y ( g )Sinuosity = 3.0 (highly sinuous, or “tortuous” meanders).


A l (θ) h l b h Arc angle (θ): the angle swept out by the radius of curvature between adjacent inflexion points.pMeander arc length (Z): the distance measured along the meander path between repeating (inflexion) points(inflexion) points.Amplitude (a): width of meander belt measured perpendicular to the valley or p p ystraight line axis.

CROSS SECTION IN MEANDER CHANNEL

TYPES OF BENDS

Th f b d Three types of bends can be defined, free, limited, and forced ,bends. These types are defined according to the different to the different external constrains and degree of f d t tt i freedom to attain lateral formation.

FREE BEND TYPE

Th f b d i ll i d i h b d The free bend is usually associated with broad flood plains consisting of relatively erodible material. In this type, the river bends follow yp ,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 is found that this type is not disturbed by the external factors and experienced the highest degree of freedom to form the bend shape.

LIMITED BEND TYPE

I hi h b d i lid k 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 g pflood plains. In this type, the banks of the channel are composed of consolidated parent material that limits the lateral erosion Such material that limits the lateral erosion. Such rivers are called incised rivers and these bends are called incised bends or entrenched bends. H h i f ti b t th i i However, no much information about the origin of incised bends is found.

FORCED BEND TYPE

I hi h h l i hi hl i d In this type the channel is highly restricted from external movements. The bank line movements are mainly controlled by either y ynatural or man made activities. Sometimes in this type the river impinges onto an almost straight parent bank at large angle (600 to 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.

THE BRAIDED RIVER

Ch l di id d d i l d Channel divided around islands. Can have several more or less parallel branches which are not fixed but tend to branches which are not fixed but tend to change alignment continually. The braiding river has many disadvantages, being less stable and less predictable than meandering rivers.

THE BRAIDED RIVER

Th h i i f b id d h l b The characteristics of braided channel can be presented as follows:

WideWideBanks are poorly defined and unstableTwo or more channelsSand bars are found between sub-channelsSand bars and sub-channels change their position very rapidlypos t o e y ap d yOften steep slopes with large suspensions Sediment overload

CROSS SECTION OF BRAIDED CHANNEL

In a braided river each branch separately tends to p yform sections similar to those in a single meandering channel. As the discharge and therefore the water level of 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 plains separated by natural levees from the main flow channel. The flood plain generally fills rapidly when natural l t d d i t hi h l l levees are overtopped during extreme high levels, but drains only slowly via small channels through the natural levees. Back swamps may then be f d d h l dformed, due to the slow drainage.

THE BRAIDED RIVER

natural river characteristics · types of bends th f b d three types of bends can be defined, free,...

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