Natural River Characteristics

NATURAL RIVER CHARACTERISTICS

Karima Attia Nile Research Institute NATURAL RIVER DEFINITION NATURAL RIVER DEFINITION

‰ Isnatural stream of water that flows inchlhannelswihith more or less defined banks.

‰ It flows itinto an ocean, lklake, or other body of water

‰ Fed along its course by tributaries.

‰ Form a drainage basin, or watershed that collects runoff with eroded sediments materials into the river

NATURAL RIVER DEFINITION

‰ The sediments are typically deposited most heavily along the river's lower course, forming floodplains along its banks and a delta at its mouth.

‰ It is considered as a fundamental link in the hydrologic cycle, and they play a major role in shaping the surface features of the Earth.

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

NATURAL RIVER CLASSIFICATION

Natural river classifications started since 1899 by Davis:

‰ Youthful river; ‰ Mature river and; ‰ Old river YOUTHFUL RIVERS

A river with a steep gradient that has very few tributaries and flows quickly. Its channels erode deeper rather than wider.

Brazos Ebro

Trinity MATURE RIVERS

A river with a gradient that is less steep than those of youthful rivers and flows more slowly. A mature river is fed by many tributaries and has more discharge than a youthful river. Its channels erode wider rather than deeper. Danube Ohio OLD RIVERS

Ariverwith a low gradient and low erosive energy. Old rivers are characterized by flood plains.

IdIndus

Nile CLASSIFICATION BASED ON QUANTITATIVE SLOPE –DISCHARGE RELATIONSHIP

‰ Lane (1957); SQ0.25 = K SQ0.25 ≤ 0.0017 Meandering SQ0.25 ≥ 0.010 Braided In between the channel is considered as intermediate sand bed stream CLASSIFICATION BASED ON QUANTITATIVE SLOPE –DISCHARGE RELATIONSHIP

‰ Leopold and Wolman (1957)

braided (found plotted above the relationship) meandering rivers (found plotted below the relationship). CLASSIFICATION BASED ON QUANTITATIVE SLOPE –DISCHARGE RELATIONSHIP

‰ Ramsahoye in 1992, for straight

and for meandering

CLASSIFICATION BASED ON QUANTITATIVE SLOPE –DISCHARGE RELATIONSHIP

‰ Khan (1971), developed classification based on: sinuosity, slope, and channel pattern

† Schumm and Khan (1972) proposed a valley slope relation: • Straight: S < 0.0026 • Meandering thalweg: 0.0026 < S < 0.016 • Braided: S > 0.016

CCSSCLASSIFICATI ON BA SED ON CHARACTERISTICS DESCRIPTION

‰ ShSchumm, (1963) bdbased on chlhannel stability: stable, eroding, or depositing

Based on mode of sediment transport:

mixed load, suspended load, and bed load Saltation CCSSCLASSIFICATI ON BA SED ON CHARACTERISTICS DESCRIPTION

‰ ClbCulbertson et al. (1967) bdbased on: ƒ Depositional features; ƒ Vegetation; ƒ Braiding patterns; ƒ Sinuosity; ƒ Meander scrolls; ƒ bank heights; ƒ Levee formations and ƒ Floodplain types. CLASSIFICATION BASED ON THE BASIS OF STABILITY

‰ Schumm (1977) ƒ Sediment transport ƒ Channel stability ƒ Measured channel dimensions CLASSIFICATION BASED ON THE BASIS OF STABILITY

‰ Croke (1992) presented a classification of flood plains that involved: ƒ Particle size, ƒ Morphology of channels, and ƒ Bank materials CLASSIFICATION BASED ON THE BASIS OF STABILITY

‰ Parker (1976) ƒ Indicates that rivers with sediment transport and depth to width ratio (d/B) << 1 at formative disc harge have a tdtendency tdtoward meandidering or braiding. ƒ His classification is based on the relative magnitude of the depth-width ratio to the channel slope-Froude number ratio (S/F). Meandering occurs when S/F << d/B, braiding occurs for S/F >> d/B and transition between the two occurs when S/F ~ d/B. LOAD CLASSIFICATIONS

† The low si nuosi ty and high wid th/d epth rati o 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. CLASS IFI CATI ON A CCORDING TO THE SYSTEM TYPE RELATED TO ALLUVAIM

‰ Alluviam ‰ Diluvial

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

‰ There is no unique relations between discharge, sediment transport, and bed materials ‰ Morphological changes are absent ‰ Morphologically stable ‰ Found in the upper reaches with a rock bed and mountainous or torrential flow characteristics. DILUVIAL RIVER DILUVIAL RIVER ALLUVIAL RIVERS

‰ A clear relation exists between the hydraulic characteristics, discharge, sediment transport, and the bed material. ‰ width/depth ratio is in the order of 100 ‰ Morphologically unstable ‰ Found in the downstream reaches with alluvial bed LONGITUDINAL PROFILE UPPER PART OF THE RIVER

‰ The upper river charterstics: ƒ High degree of confinement ƒ Confined and narrow valley ƒ General erosion. MIDDLE PART OF THE RIVER

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

ƒ Relatively wide valley, ƒ Reduction in profile gradient.

In theoryyyy 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. LOWER PART OF THE RIVER

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

Plan form is th e ri ver confi gurati on as vi ewed on maps or from aerial photographs or other spectral images

PARAMETERS RESPONSIBLE ABOUT FORMING RIVER PLAN FORM

† There is a range of flows responsibl e for creating channel plan form, rather than one singggle flow magnitude

† Effective” discharge refers to the range of flow magnitudes that transports the majority of a river’s annual sediment load over the long- term PARAMETERS RESPONSIBLE ABOUT FORMING RIVER PLAN FORM

† The “domi nant disch arge” ref ers to th e flow magnitude that determines channel shape, or cross section width and depth THE CALCULATION OF THE RANGE OF DISCHARGES RESPONSIBLE FOR FORMING AND MAINTAINING CHANNEL FORM

Dominant or effective discharge PARAMETERS RESPONSIBLE ABOUT FORMING RIVER PLAN FORM

† Bankf ull” disc harge is t he flow magni tud e th at is contained within a channel without overtoppin g its banks. † This 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 formation and flood plain formation. Bank full Discharge PLAN FORM DEFORMATION

† The plan form ddfeformati on is controll llded in nature by:

ƒ Lithology and its variation along the river length; ƒ Discharge; ƒ Sediment load variation during the flood, ƒ Outside controls and ƒ Human intervention. PLAN FORM CLASSIFICATION

† Rivers can be cl assifi ed in terms of ch annel pattern (plan form) that three different types can be distinguished:

ƒ Straight river, ƒ Meandering river and ƒ Braiding river. Braided

Meandering

Straight STRAIGHT RIVER CHARACTERISTICS

† The straiihght channel can be ddfidefined 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

† CiConsists of a numb er of consecutive bends. † Hilicoidal flow resulting in scouring near the outer bend and sedimentation near the inner bend. † More predictable, than braiding rivers † They normally have one relatively deep channel MEANDER GEOMETRICAL RELATIONS

crest

tough MEANDER GEOMETRICAL RELATIONS

† RdiRadius of curvature (()r): 2.3 to 2.7 times the bankfull width MEANDER GEOMETRICAL RELATIONS

† MdMeander WWlavelength (λ): A full meand er wavelength is the distance between two similar ppgoints along the channel between which waveform is complete. It was found to occur between 6 and 15 times the bankfull width. MEANDER GEOMETRICAL RELATIONS

Sinuosi ty: Is a measure used to quantif y th e difference between meandering and straight channels. It is defined as the channel lengg()th (L) measured along the center of the channel divided by the valley length (Lv) measured along the valley axis s = L/Lv Sinuosity = 1 ((gstraight channels ) Sinuosity = 3.0 (highly sinuous, or “tortuous” meanders). MEANDER GEOMETRICAL RELATIONS

† Arc angle (θ): the angl e 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 ppperpendicular to the valle y or straight line axis. MEANDER GEOMETRICAL RELATIONS CROSS SECTION IN MEANDER CHANNEL TYPES OF BENDS

† Three types of bbdends can be defined, free, limited, and forced bends. These types are defined according to the different external constrains and degree of fdfreedom to attttiain lateral formation. FREE BEND TYPE

† The free bend is usuall y associ ated wi th 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. LIMITED BEND TYPE

† In this type, the bend cut into solid rock or hard strata in deep gorges and exhibit meanderinggp pattern similar to that of rivers in flood plains. In this type, 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 iifnformati on about the origi n of incised bends is found. FORCED BEND TYPE

† In this type the channel is highl y restri cted from external movements. The bank line movements are mainlyyy controlled by either natural or man made 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. THE BRAIDED RIVER

† Channel divid ed around is lan ds. † Can have several more or less parallel 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

† The characteri sti cs of bbiddraided channel can be presented as follows: ƒ Wide ƒ Banks are poorly defined and unstable ƒ Two or more channels ƒ Sand bars are found between sub-channels ƒ Sand bars and sub-channels change their pospostoition ve ry rap apdyidly ƒ Often steep slopes with large suspensions ƒ Sediment overload CROSS SECTION OF BRAIDED CHANNEL

† In a braided river each branch seppyarately 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. † The flood plain generally fills rapidly when natural levees are overt opped ddiuring extreme hihihgh level s, but drains only slowly via small channels through the natural levees. Back swamps may then be fddhldformed, due to the slow drainage. THE BRAIDED RIVER