Introduction to River Engineering

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1. WOLAITA SODO UNIVERSITY DEPARTMENT OF HYDRAULIC AND WATER RESOURCES ENGINEERING River Engineering ( HWRE-5191 ) 5 th year HWRE (2022/23 G.C.) By: Manamno B. (MSc [email protected]

2. 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

3. 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 River Engineering Course Outlines

4. 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, 2 nd 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 %   River Engineering Course Outlines

5. 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

6. 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

7. 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

8. 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.

9. 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.

10. 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

11. The Catchment Area Cont. 11 Confluence

12. Watershed Forms Form of a watershed varies greatly, however, and is tied to many factors including climatic regime, underlying geology, morphology, soils, and vegetation 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

13. Watershed forms Cont. ( a) D endritic is the most common drainage pattern. Characterized by irregular branching of tributary streams in many directions. I s 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.

14. 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.

15. 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

16. Stream classification Ephemeral streams: f low 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

17. Channel and ground w ater r elationships: The relationship is strongest in streams with gravel riverbeds in well-developed alluvial floodplains. 17 Cont.…

18. 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

19. Classification of watershed cont.

20. 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)

21. 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.

22. River Morphology and Regime This kinetic energy can do various works as it moves downhill Turbulence and friction within the moving water uses up kinetic energy. 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. 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

23. 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

24. Rivers classification Classification based on variation of discharge Perennial River: Perennial rivers have adequate discharge throughout the year. Non-perennial rivers: their flow is quite high during and after rainy seasons and reduces significantly during dry seasons. Flashy rivers: in these rivers, there is a sudden increases in discharge. The river stage rises and falls in a very short period. Virgin rivers: these are those rivers which get completely dried up before joining another river and sea. 24

25. Rivers classification Cont. Classification based on the location of reach: Mountainous rivers: they flow in hilly and mountainous regions. These rivers are further divided into rocky rivers and Boulder Rivers . 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. 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. 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

26. Rivers classification Cont. Based on stability of the river:- Aggrading Degrading Stable Deltaic 26

27. Rivers classification Cont. 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

28. Rivers classification Cont. 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

29. Rivers classification Cont. Classification based on plan-from: 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. 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. Braided rivers: flow in two or more channels around alluvial islands developed due to deposition of silt. 29

30. Rivers classification Cont. 30 Straight river Meandering river Braided river

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32. 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

33. Stages of rivers Cont. 33

34. Stages of rivers Cont. 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. 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

35. Stages of rivers Cont. 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

36. Stages of rivers Cont. 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

37. Stages of rivers Cont. 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

38. 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

39. Cont… An alluvial river usually has the following three stages: Flow in a straight reach Flow at bends 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

40. Cont… 40

41. 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.

42. Cont.… 42

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

44. 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

45. 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

46. Meander Cont. so that sediments are eroded from the outer bank and redeposit on the inner ban k 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.

47. Cont.…

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

49. Cont.…

50. 50 Cont.…

51. 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.…

52. 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 ).

53. Meander parameter Meander parameters

54. Meander Parameters Meander parameter Cont.….

55. Meander length, (M L ) 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 (M B ) 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. Meander parameter Cont.….

56. 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 Meander parameter Cont.….

57. Sinuosity (C) : is the ratio of channel length along the centerline of the channel (S) to the meander length (M L ) 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 . Meander parameter Cont.…. C = S/M L

58. 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.

59. Cont.…. 59

60. Cont.…. 60

61. 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

62. 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

63. 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

64. Cont.…. 64

65. Cont.…. 65 The three longitudinal profile zones

66. Cont.…. 66 Generalized longitudinal section of a river

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

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73. END !!!