River engineering
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This document provides an outline for a course on river engineering. It discusses river characteristics, hydraulics, morphology, sediment transport mechanisms, bed load calculation methods, sediment sampling, river training works, erosion protection, discharge control, and flow forecasting. Ethiopian rivers are described as having high seasonal flow variability due to the country's highlands and marked seasonality of rainfall. Common sediment transport mechanisms include bed load, suspended load, and wash load. Methods for calculating bed load transport rates include equations developed by Nielsen and van Rijn.
River engineering
- 1. River Engineering Prepared by: Habtamu Tamiru (MSc) Email address :habtamutami@wollegauniversity.edu.et
- 2. Outlines 1. INTRODUCTION 1.1 River characteristics 1.2 River hydraulics 1.3 River morphology and regime 2. SEDIMENT TRANSPORT 2.1 Origin and properties of sediment 2.2 Limitations of particle motion 2.3 Transportation mechanisms 2.4 Bed load transport 2.5 Suspended load transport 2.6 Wash load transport 2.7 Total load transport
- 3. Outlines 3. SEDIMENT LOAD SAMPLER 3.1 Introduction 3.2 Bed load sampler 4. SCOURING 4.1 Introduction 4.2 Local scour near structures 5. RIVER TRAINING AND FLOOD CONTROL 5.1 Erosion protection 5.2 Discharge control 5.3 River flow forecasting 5.4 Hydraulics of bridge, culverts and aqueducts
- 4. Outlines Modes of Evaluation Quizzes 15% Tests 30% Assignments 25% Final exam 30% Total 100%
- 5. Wollega University CHAPTER 1: INTRODUCTION River Engineering is a branch of civil engineering (Hydraulic Engineering) dealing with the design and construction of various hydraulic structures to improve rivers for both human and environmental needs. What is River Engineering?
- 6. Wollega University Why are rivers important? River
- 7. Wollega University Characteristics of Ethiopian Rivers Owing to the highland nature of the Ethiopian landmass, surface ruggedness, the outward inclination of the highlands, and the climatic conditions, Ethiopian rivers have the following characteristics. Almost all major rivers originate from the highlands elevating more than 1500 meters above sea level, Majority of Ethiopian rivers are trans- boundary,
- 8. Wollega University Characteristics of Ethiopian Rivers Due to the marked seasonality of rainfall, Ethiopian rivers are characterized by extreme seasonal fluctuation. In the wet season, runoff is higher and rivers are full bursting their banks Due to surface ruggedness they have rapids and waterfalls along their course,
- 9. Wollega University Characteristics of Ethiopian Rivers They have cuts, steep-sided river valleys and deep gorges along their courses, Rivers in Ethiopia flow on steep slopes having steep profiles. Some of the rivers serve as boundaries, both international and domestic administrative units
- 10. Wollega University Examples Abay River Baro River Tekeze River Awash River Omo River Rift valley River
- 11. Wollega University River Hydraulics Sediment transport and water flow are interrelated and cannot be separated The natural rivers considered as an open channels and all hydraulics principles are valid River characteristics, flow types and sediment transport are the main river analysis
- 12. Wollega University River morphology and regime River morphology: is defined as the shapes of river channels and how they change in shape and direction over time. River Regime: The variability in its discharge throughout the course of a year in response to precipitation, temperature, evapotranspiration and drainage basin characteristics.
- 13. Wollega University CHAPTER 2: SEDIMENT TRANSPORT What is a sediment? How sediment is transported in a river? What is a meandering river? Rivers flowing over gently sloping ground begin to curve back and forth across the landscape. A sediment is solid materials dissolved, suspended and settled in the river
- 14. Wollega University Origin and properties of sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion sand and silt can be carried in suspension in river water and on reaching the sea bed deposited by sedimentation if buried, they may eventually become sandstone and siltstone (sedimentary rocks) through lithification.
- 15. Wollega University Properties of sediment In River hydraulics, the following physical properties of sediments are considered: Size Shape Texture
- 16. Wollega University Properties of sediment Size: This is an important particle characteristic that is readily measured. Bulk properties tend to vary with particle size in a roughly predictable manner. In fact, size alone has been found to describe sediment deposits adequately for many practical purposes.
- 17. Wollega University Properties of sediment Shape: The various shapes of sediment particles are formed in numerous ways. Shape is defined numerically by sphericity and roundness Sphericity is the ratio of the surface area of a sphere having the same volume as the particle to the surface area of the particle.
- 18. Wollega University Properties of sediment Sieve Separation: Coarse grains (larger than 0.062 mm) are ordinarily separated by sieves having mesh openings corresponding to the grain sizes measured. The U.S. standard sieve series is based on a 200-mesh screen with a diameter of 0.074 mm. Sets of sieves with openings larger than this diameter include 0.125-mm, 0.25-mm, 0.50-mm, 2-mm, and 4-mm sizes
- 19. Wollega University Motion of sediment particles Sediment transport is the movement of solid particles (sediment), typically due to a combination of gravity acting on the sediment, and/or the movement of the fluid in which the sediment is dragged. Gravity force is a major cause for movement of sediment particle in a fluid.
- 20. Wollega University Motion of sediment particles Initiation of motion: For a fluid to begin transporting sediment that is currently at rest on a surface, the boundary (or bed) shear stress exerted by the fluid must exceed the critical shear stress. 𝜏𝑠𝑢𝑟𝑓𝑎𝑐𝑒= 𝜏𝑤𝑎𝑡𝑒𝑟
- 21. Wollega University Sediment transport Mechanisms The Three common sediment transporting mechanisms are: 1. Bed load transport 2. Suspended load transport 3. Wash load transport Wash Load
- 22. Wollega University Sediment transport Mechanisms The bed load is the part of the total load that is more or less in contact with the bed during the transport. It primarily includes grains that roll, slide or bounce along the bed. Thus the bed load movement is governed by the shear velocity at the bed and effective resistance of the sediment particle. Bed Load Transport
- 23. Wollega University Sediment transport Mechanisms The suspended load is the part of the total load that is moving in suspension without continuous contact with the bed as a result of agitation of fluid turbulence. Many sediment deposits contain large proportion of fine sediments, which are readily set in motion by tidal currents. The determination of the rate of suspended load transport is straight forward by comparison with measurement of the rate of bedload transport. Suspended load transport
- 24. Wollega University Methods of computing bed load transport When the bed shear stress exceeds a critical value, sediments are transported in the form of bed-load and suspended load. For bed-load transport, the basic modes of particle motion are rolling motion, sliding motion and siltation motion Background Bed load transport Shields parameter Critical Shields parameter Bed shear stress
- 25. Wollega University Methods of computing bed load transport Methods
- 26. Wollega University Methods of computing bed load transport Physically the transport rate is related to the characteristics of the bed-load layer: Sediment concentration (Cs) Thickness ( ) Sediment velocity along the bed ( ) Bed load transport rate (qs) Bed load transport rate
- 27. Wollega University Methods of bed load transport rate calculation He is one the scientists conducted his researches in field of ecology and genetics. Lars Peter Nielson was quantified the rate of sediment transport by developing the following equations: 1. Lars Peter Nielsen (1992)
- 28. Wollega University Methods of bed load transport rate calculation He was hydraulic Engineer Leo van Rijn was quantified the rate of sediment transport by developing the following equations: 2. Leo van Rijn (1984, 1993)
- 29. Wollega University Methods of bed load transport rate calculation He was hydraulic Engineer Leo van Rijn was quantified the rate of sediment transport by developing the following equations: 2. Leo van Rijn (1984, 1993)
- 30. Wollega University 3. SEDIMENT LOAD SAMPLER What is sediment sampling? Where possible, sediment should be collected from a depth of 60 cm below the water surface. The sampler should wade into the pond until the desired depth is reached, and then measure sediments directly into sampling jars, or by using a stainless steel pan. Suspended sediment load is usually measured with both manual and automatic samplers. Continuous measurements are made possible by using turbidity as a replacement measure of suspended sediment concentrations
- 31. Wollega University 3. SEDIMENT LOAD SAMPLER What is sediment sampling? The collection and analysis of the silt, sand, mud, clay in a stream or river bed, either dry or wet, is a common gold exploration method
- 32. Wollega University RIVER TRAINING AND FLOOD CONTROL What is river training work? Flood? 'River training' refers to the structural measures which are taken to improve a river and its banks. River training is an important component in the prevention and mitigation of flash floods and general flood control, as well as in. other activities such as ensuring safe passage of a flood.
- 33. Wollega University RIVER TRAINING AND FLOOD CONTROL Erosion protection Erosion is the geological process in which earthen materials are washed away and transported by natural forces such as wind or water. Sediment is entered into a river or water sources during soil erosion and they are interrelated terms.
- 34. Wollega University RIVER TRAINING AND FLOOD CONTROL Discharge control The amount of sediment moved by a stream in a given time, measured by dry weight or by volume. If very huge amount of sediment yield is discharging into the reservoir or sources of water river training works should be provided.
- 35. Wollega University RIVER TRAINING AND FLOOD CONTROL River flow forecasting River flow forecasting is required to provide basic information on a wide range of problems related to the design and operation of river systems. The availability of extended records of rainfall and other climatic data, which could be used to obtain stream flow data, initiated the practice of rainfall-runoff modelling.