Chapter 1: Introduction to River Hydraulics

Hydraulic Structures II

1. Chapter one Introduction to River Hydraulics Gemedo G. (MSc.)

2. General Rivers often adjusts their roughness, velocity, slope, depth, width, and plan form in response to human activities and natural conditions. When a river channel is modified locally, it may initiate changes in the channel and flow characteristics. The study of river engineering traditionally deals with all hydraulic engineering works on river River hydraulics includes the evaluation of flow characteristics of river and its changes over time. Results from analyses of river hydraulics are critical for project formulation, design, construction, and operation. Eg. , designing of dams, spillways, levees, and guide banks requires both hydrological and hydraulic computations.

3. Introduction A river is the general term for a channel and the water in it. Water collected together and flowing down slopes form a stream flow . The space where a stream flow run is a channel. The area supplying water into a channel is a drainage basin. The boundary between drainage basins is a watershed divide/ridge. A river system is composed of the main stream and many tributaries. A drainage pattern is plan of river system.

4. The primary function of a river is the conveyance of water and sediment and in addition, rivers serve for; Social/economic/cultural functions: for water supply, irrigation, hydropower, transportation and recreation; and means of waste disposal. Ecological functions: habitats for aquatic plants and animals. Rivers are complex dynamic systems due to; They continuously change their forms and patterns (shape, size, position) and morphological characteristics (spatial and temporal). Interaction between hydrodynamics, morpho dynamics and ecological processes.

5. The changes in water and sediment discharge may be caused by: Natural forces: variation of discharge Anthropogenic elements: Human interference in the fluvial system River developments: Hydraulic structures, river channelization, gravel and sand mining; Land developments: land use change, land clearance and infrastructure building. River morphology (fluvial morphology): is the study of the forms and patterns of rivers and processes developed these forms by the action of running water. It is time dependent and varies with discharge, water surface slope, velocity, depth and width of channel, sediment input and characteristics. It can be substantially influenced by engineering works

6. River forms and patterns: it includes river channel geometry, shape and size of channel cross section. River channel pattern/ plan form: Straight channel, meandering channel and braided channel Classifications of rivers Rivers can be classified according to various criteria: i . Based on variation of discharge in river Perennial Rivers: have adequate discharge throughout the year. They obtain supply from melting snow and precipitation. Non-perennial rivers: The flow is quite high during and after rainy season but they carry little flow in non-rainy season. Flashy rivers: The flood rises and fall in a river is sudden and therefore, the flood flows occur suddenly, and rise and fall of water level is very quick. Virgin rivers: In arid zones a river water may completely dry before it joins other water body due to percolation or excessive evaporation.

7. ii. Based on stability of river Aggrading/accreting rivers : It is a river collecting sediment and building up its bed and it is a silting river. Such river increases its bed slope, which is called building up of slope. The silting may be due to heavy sediment load, construction of obstruction across the river. Figure 1-1. Aggrading and degrading Rivers

8. Degrading rivers: here, the river’s bed is constantly getting scoured (eroded) to produce and dissipate available excess land slope,. Stable River: A river which does not change its alignment, slope and its regime significantly. iii. Based on the location of river: Mountainous rivers: The rivers that flow in hilly and mountainous regions. Rivers in flood plains: After the boulder stage, a river enters the flood plains having alluvial soil. The bed and banks of the are made up of sand and silt. Delta Rivers: When River enters a deltaic plain, it splits into a number of small branches due to very flat slopes. Figure 1-2. Delta River formations

9. d. Tidal rivers: Just before joining a sea or an ocean, the river becomes a tidal river. In a tidal river, there are periodic changes in water levels due to tides. The river receives the sea water during flood tides, but during outgoing tides, it delivers water back to the sea. iv. Based on the plan-form of river: Straight rivers: They are straight in plan and have cross-sectional shape of a trough. The maximum velocity of flow usually occurs in the middle of the section. They may exist in the mountainous region and rare in flood plains. Figure 1-3. Straight channels

10. b. Meandering rivers: follow a winding, crooked course. They consist of a series of bends of alternative curvature in the plan. The successive curves are connected by small straight reaches of the river, called cross-overs or crossings Figure 1-4. Meandering patterns c. Braided rivers: they flows into two or more channels around alluvial islands developed due to deposition of silt. Figure 1-5. Braided river patterns

11. Sediment and Morphological processes A river develops various landforms through channel (river morphological) processes. The main channel processes or fluvial processes are erosion, transportation and sedimentation. Erosion predominates in the upper reach area of a drainage basin, and valleys composed of channels and slopes are formed. If the transport capacity of the river is affected by water management structures such as diversion or storing, sediment deposition may occur.

12. The good understanding of sediment transport process is important for: Morphological boundary conditions for design of hydraulics structures and river training works: This are discharge, water levels, velocities, bed levels and the future changes in river morphological characteristics near the structure. Reservoirs Sedimentation: Reservoirs are suffering from excessive sedimentation due to either the upstream sediment supply was never considered or underestimated. In addition, changes in sediment yield due to changed land use in the upstream catchment can cause detrimental sedimentation. To remedy, it requires changes in the operation of reservoir, sometimes with drastic consequences

13. Sediment problems at Intakes: Many canals of irrigation projects suffer from excessive sedimentation, which is entering through the head works because of: The sediment transport in the river was not properly assessed and appears to be much higher than anticipated; The morphological changes have taken place after construction of the intake structures. Environmental Impact Assessment (EIA): To assess the potential impacts of hydraulic structures and river training works, possible morphological changes should be identified.

14. Erosion Running water carries sediment by erosion and corrosion. Erosion: Gravel being brought by running water scours the channel and removes sediment from the river bed. It makes a channel broader and deeper. Corrosion: is the process by which stream water reacts chemically with rocks and dissolves them. Transportation There are three different processes in transporting sediment load. Corrosion: as defined above. Suspensions: is the process by which fine materials are transported in or on water surface without contact with the river bed and it is the suspended load (materials transported by suspension) that create turbidity of stream water. Traction: Gravel of larger diameter slides or rolls, and sand hops or bounds on a river bed. Sediment load carried by traction is known as bed load .

15. Sedimentation A flood carries a huge volume of bed load from mountains to the river plain. Particles of bed load are deposited in order of size, and alluvial plain is formed. An alluvial fan composed of gravel is formed in the uppermost reaches of an alluvial plain. The surface of an alluvial fan is like a segment of a cone. A delta being developed near a river mouth consists of fine materials and sand. A flood plain consisting of natural levees and back swamps occupies the transitional area between an alluvial fan and a delta.

16. Channels in alluvial plains Alluvial streams are those, which flow through sandy material, shape their channel through it and carry water and sediment. In dealing with alluvial streams the material in the bed and banks of the channel is generally assumed to be non-cohesive, though some of the fine sediment in transport may settle on the banks and make the bank material cohesive.

17. Channels running in a mountainous area are characterized as follow: River bed materials are rock or gravel; The channel patterns is straight or braided; Debris flow and traction transport heavy sediment loads; and The gradient of river beds is steep. Characteristics of channels in alluvial plains differ on an alluvial fan, flood plain and delta. River bed materials are gravelly on an alluvial fan, sandy on a flood plain and muddy on a delta. Gravelly sediments show a braided channel pattern and are moved by traction. A sand bed channel usually meanders in flood plains but straight in some case. Channels bifurcate on a delta are usually straight, cut channels meander in the upper reaches. Muddy sediment loads are carried by suspension.

18. Behaviors of Rivers The main factors altering the behavior of rivers are silt and sediment that flows in the river. Sediment carried by river poses numerous problems like increasing of flood levels, silting of reservoirs and etc. Straight reaches In a straight reach of a river, the river cross section is in the shape of a trough, with high velocity flow in the middle of the section. The water surface level will be lower in the middle and higher at the edges. Figure 1-6. Cross-section of straight reaches

19. Alluvial Channel Bends usually develop in alluvial rivers and they are characterized by scouring on the concave side and silting on the convex side. Because of curved flow, it is subjected to centrifugal forces and there is a transverse slope of water surface due to the super elevation of water surface at the concave bank. As a result, the bottom water moves from the concave bank to the convex bank and also carries with it the bed material and deposits it near the convex bank. To replace the bottom water, water dives in from the top at the concave bank and flows along the bottom carrying sand and silt to the convex bank where it is deposited. This secondary motion is primarily responsible for the erosion of the sediment on the concave bank and the deposition of the sediment on the convex bank. The depth of flow in a river at the bend becomes deeper at the concave bank.

20. Figure 1-7. The movement of water in a river bends

21. The continued action of flow developed around river bends causes further erosion and deposition respectively, on concave and convex banks of river. Meander patterns are usually associated with wide flood plains comprising easily erodible material. When consecutive curves connected with short straight reaches, the river is said to be a meandering river. The four variables which govern the meandering process are: valley slope, silt grade and charge, discharge, and bed and side materials. All these factors considerably affect the meandering patterns, and all of them are interdependent.

22. During floods, the river carries tremendous amount of silt charge. When the silt charge is in excess of quantity required for stability, the river starts building up its slope by depositing the silt on the bed. The river reach becomes an aggrading or of accreting type which is the primary process consequently leads to meandering. Figure 1-8. Meandering Rivers and their Parameters

23. The various meandering parameters are defined as follow: Meander Length ( 𝑴 𝑳 ): It is the tangential distance between the corresponding points of a meander. Meander belt or width ( 𝑴 𝑩 ): It is the distance between the outer edges of clockwise and anti-clockwise loops of the meander. Meander Ratio: It is the ratio of meander belt to meander length. Tortuosity: It is the ratio of the length along the channel to the direct length of the river reach. Crossings or Cross-overs : It is the short straight reaches of the river, connecting two consecutive clockwise and anti-clockwise loops.

24. Cut-Off It is a process by which an alluvial river flowing along bends abandons a particular bend and establishes its main flow along a comparatively straighter and shorter channel. During the development of meanders, there is always a lateral movement of the meanders due to their gradual lengthening. Increased frictional losses and bank resistance tend to stop lateral movement. When the bend and the bank resistance become too large, the flow finds it easier to cut across the neck than to flow along the loop and results in a cutoff. Thus, cutoff is a natural way of counter balancing the effect of the ever-increasing length of a river course due to the development of meander.

25. Figure 1-9. Cutoff development