NATURAL RIVER. Karima Attia Nile Research Institute

Size: px

Start display at page:

Download "NATURAL RIVER. Karima Attia Nile Research Institute"

Marilyn Willis
5 years ago
Views:

1 NATURAL RIVER CHARACTERISTICS Karima Attia Nile Research Institute

2 NATURAL RIVER DEFINITION

3 NATURAL RIVER DEFINITION Is natural stream of water that flows in channels with ih more or less defined banks. It flows into an ocean, lake, 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

5 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 selfconstructed, self-maintained, and seek their own stability

7 NATURAL RIVER CLASSIFICATION Natural river classifications started Davis: since 1899 by Youthful river; Mature river and; Old river

8 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

A mature river is fed by many tributaries and has more

9 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

10 OLD RIVERS A river with a low gradient and low erosive energy. Old rivers are characterized by flood plains. Indus Nile

11 CLASSIFICATION BASED ON QUANTITATIVE SLOPE DISCHARGE RELATIONSHIP Lane (1957); SQ 0.25 = K SQ SQ Meandering Braided In between the channel is considered as In between the channel is considered as intermediate sand bed stream

12 CLASSIFICATION BASED ON QUANTITATIVE SLOPE DISCHARGE RELATIONSHIP Leopold and Wolman (1957) braided (found plotted above the relationship) meandering rivers (found plotted below the relationship).

13 CLASSIFICATION BASED ON QUANTITATIVE SLOPE DISCHARGE RELATIONSHIP Ramsahoye in 1992, for straight and for meandering

15 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 < Meandering thalweg: < S < Braided: S > 0.016

17 CLASSIFICATION C BASED ON CHARACTERISTICS DESCRIPTION Schumm, (1963) based on channel stability: stable, eroding, or depositing Based on mode of sediment transport: mixed load, suspended load, and bed load Saltation

18 CLASSIFICATION C BASED ON CHARACTERISTICS DESCRIPTION Culbertson et al. (1967) based on: Depositional features; Vegetation; Braiding patterns; Sinuosity; Meander scrolls; bank heights; Levee formations and Floodplain types.

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

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

21 CLASSIFICATION BASED ON THE BASIS OF STABILITY Parker (1976) Indicates that rivers with sediment transport and depth to width ratio (d/b) << 1 at formative discharge have a tendency toward meandering or braiding. 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.

22 LOAD CLASSIFICATIONS The low sinuosity i and high h width/depth h 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.

23 CLASSIFICATION C ACCORDING 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.

24 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.

25 DILUVIAL RIVER

26 DILUVIAL RIVER

27 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

28 LONGITUDINAL PROFILE

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

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

31 LOWER PART OF THE RIVER The lower river, where sedimentation takes place. The sediment input is larger than output.

32 PLAN FORM Plan form is the river configuration i as viewed on maps or from aerial photographs or other spectral images

34 PARAMETERS RESPONSIBLE ABOUT FORMING RIVER PLAN FORM There is a range of flows responsible for creating channel plan form, rather than one single flow magnitude Effective discharge refers to the range of flow magnitudes that transports the majority of a river s annual sediment load over the longterm

35 PARAMETERS RESPONSIBLE ABOUT FORMING RIVER PLAN FORM The dominant discharge refers to the flow magnitude that determines channel shape, or cross section width and depth

36 THE CALCULATION OF THE RANGE OF DISCHARGES RESPONSIBLE FOR FORMING AND MAINTAINING CHANNEL FORM Dominant or effective discharge

37 PARAMETERS RESPONSIBLE ABOUT FORMING RIVER PLAN FORM Bankfull discharge is the flow magnitude that is contained within a channel without overtopping 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.

38 Bank full Discharge

39 PLAN FORM DEFORMATION The plan form deformation is controlled in nature by: Lithology and its variation along the river length; Discharge; Sediment load variation during the flood, Outside controls and Human intervention.

40 PLAN FORM CLASSIFICATION Rivers can be classified in terms of channel pattern (plan form) that three different types can be distinguished: Straight river, Meandering river and Braiding river.

41 Braided Meandering Straight

42 STRAIGHT RIVER CHARACTERISTICS The straight channel can be defined d 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

43 MEANDERING RIVER CHARACTERISTICS Consists of a number 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

44 MEANDER GEOMETRICAL RELATIONS crest tough

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

46 MEANDER GEOMETRICAL RELATIONS 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 bankfull width.

47 MEANDER GEOMETRICAL RELATIONS Sinuosity: i Is a measure used to quantify the difference between meandering and straight channels. It is defined as the channel length (L) measured along the center of the channel divided by the valley length (Lv) measured along the valley axis s = L/Lv Sinuosity = 1 (straight channels) Sinuosity = 3.0 (highly sinuous, or tortuous meanders).

48 MEANDER GEOMETRICAL RELATIONS 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 p to the valley or straight line axis.

49 MEANDER GEOMETRICAL RELATIONS

50 CROSS SECTION IN MEANDER CHANNEL

51 TYPES OF BENDS 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.

52 FREE BEND 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.

53 LIMITED BEND TYPE In this type, the bend cut into solid rock or hard strata in deep gorges and exhibit meandering gpattern 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 information about the origin i of incised bends is found.

54 FORCED BEND TYPE In this type the channel is highly hl restricted from external movements. The bank line movements are mainly controlled by either natural or man made activities. Sometimes in this type the river impinges onto an almost straight parent bank at large angle (60 0 to 90 0 ). 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.

55 THE BRAIDED RIVER Channel divided id d around islands. 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.

56 THE BRAIDED RIVER The characteristics i of braided d channel can be presented as follows: Wide Banks are poorly defined and unstable Two or more channels Sand bars are found between sub-channels Sand bars and sub-channels change their position posto very rapidly apdy Often steep slopes with large suspensions Sediment overload

57 CROSS SECTION OF BRAIDED CHANNEL 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. The flood plain generally fills rapidly when natural levees are overtopped during extreme high h levels, l but drains only slowly via small channels through the natural levees. Back swamps may then be formed, due to the slow drainage.

58 THE BRAIDED RIVER

Surface Water and Stream Development

Surface Water and Stream Development Surface Water The moment a raindrop falls to earth it begins its return to the sea. Once water reaches Earth s surface it may evaporate back into the atmosphere, soak