Chapter one. River morphology.pptx

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1. Love and peace for our country , Ethiopia!! CHAPTER ONE: RIVER MORPHOLOGY By inst. Amanuel B. MSC (HYDRAULIC ENGINEERING)

2. 1.1.1 River River is any body of fresh water flowing from an upland source to a large lake or to the sea, fed by such sources as springs and tributary streams. Rivers may be classified as; Perennial:- continuous flow (have adequate discharge though out the year, there is dependable base flow contribution) Ephemeral:- flows only for short period during or following precipitation (less than 30 days per year) Intermittent:- flow only during certain times of the year (seasonal flow, lasts more than 30 days per year). Rivers also have multitude of functions, which includes the following: Social/ economic/ cultural functions Ecological functions 1.1 Introduction

3. 1.1 Introduction Rivers are complex dynamic systems interaction between: Hydrodynamics , Morphodynamics Ecological processes. Rivers are continuously change their forms and patterns and other morphological characteristics in space and time due changes in: Water discharge Sediment discharge The changes in water and sediment discharge may be caused by Natural elements (natural forces) Discharge is naturally variable. 2. Anthropogenic elements Human interference in the fluvial system it includes:

4. Cont’d 1/22/2023 a. Land developments Land clearance Infrastructure building Land use change b. River developments Hydraulic structures River channelization Gravel and sand mining River morphology is the study of the forms and patterns of rivers and the processes that developed those forms. It is time dependent and varies particularly with discharge, sediment input and characteristics, and with bank material

5. Cont’d River forms and patterns: River channel geometry River channel pattern/ planform River channel planform geometry River bed grade/ slope River morphological processes: processes due to flowing water Erossion Transportation Deposition River forms and patterns and morphological processes interact in a feedback mechanism.

6. Cont’d why good understanding of sediment transport process is important? Reasons:- Morphological boundary conditions for design of hydraulic structures and river training works. Sedimentation in Reservoirs

7. Cont’d Sediment problems at Intakes Environmental Impact Assessment Environmental Impact Assessment 1.2 Watershed/ Catchment The concept of watershed is important in the river system Because rivers do not exist in isolation. They are part of a larger system – fluvial system   Total area from which surface runoff flows to a given point of concentration is called a watershed, a catchment area, a drainage basin, or a drainage area . Hence a watershed is always connected to a certain point of concentration, the lowest point of the respective basin.

8. Cont’d Watershed boundaries The imaginary line delimiting various watersheds is known as water dividing line or water-divide. Watershed geomorphology/ characteristics Drainage area – A It is the single most important watershed characteristics for hydrologic design It reflects the volume of water that can be generated from precipitation V = Rainfall depth x Area

9. Cont’d Watershed Length - L It is the second most important watershed characteristics in hydrologic design. It is useful for the computation of the concentration time - T c ; It is the distance measured along the main channel from the watershed outlet to the basin divide;   While the length increases, the drainage area increases Watershed Slope – S Flood magnitudes reflect the momentum of the runoff. Slope is an important factor in the momentum. Watershed slope reflects the rate of change of elevation with respect to distance along the principal flow path.

10. Cont’d Watershed Shape Watersheds have an infinite variety of shapes; Not used directly in hydrologic design Hypothetical watersheds. (a) Ellipse: side; (b) triangle: center; (c) square:Comer ; (d) circle; (e) triangle: vertex; (f) rectangle; (g) ellipse: end.

11. Cont’d In spite of the infinite number of possible watershed shapes, the majority may nevertheless be usually reduced to three main configuration groups ( Fig.below ):   elongated watershed, broad watershed, and fan-shaped watershed Fig. watershed shapes Effect of Watershed Shape on the Runoff Flow  There are many factors which influence the hydraulic characteristics of a watershed as a part of an overall fluvial system. These factors may generally be summarized as follows: Watershed geometry - which, among others, includes area, shape, location, length of streams, average main-stream slope, drainage density, etc. Soil characteristics - type of soil, grain-size distribution, texture, erosivity , etc.

12. Cont’d Vegetation cover - types and distribution, interception, transpiration, etc. Hydrology - infiltration rate, groundwater, peak discharges, type of flow (perennial or ephemeral), yearly hydrographs, etc. Geology - structure, bedrock and surface-soil types and distribution, etc.  Climate - temperature, precipitation type, seasonal occurrence and duration, frequency, etc. Sediment yield - erosion and transport mechanism, sources of, etc. Human influence - degree and type of development, construction activity, deforestation, etc. While Soil characteristics and human influence have a preponderant influence on the volume of the runoff from a given watershed area. Geometric characteristics affect the form of the hydrograph and the peak discharge.

13. Cont’d Figure; Effect of geometric shape of watershed on hydrograph and peak discharge Longitudinal Profile and Mean Slope of streams Longitudinal profile along the main stream of the watershed will often provide:- Valuable information about stream properties Extension of various parts of streams in respect of the elevation Longitudinal Profile has three zones

14. Cont’d Elements of Hydrometric Measurements Understanding of the morphology and behavior of natural watercourses has been gained or derived from many long-term observations and measurements. Generally those measurements can be classified into the following groups:- 1.Velocity measurements, 4. Depth sounding 2. Water-level measurements, 5. Discharge measurements Sediment transport measurements Water-level measurements Staff Gauges Staff gauge

15. Cont’d Suspended-Weight Gauge Automatic Gauge 2. Depth Sounding  Sounding Rod Echo Sounding 3. Velocity Measurements Floats Current meters 4. Discharge Measurement The final scope of all the hydrometric measuring methods is generally to obtain a fairly accurate, economically and technically acceptable estimate of the discharge at a given flow section.

16. Cont’d 5. Sediment Transport Measurements The total sediment load of a stream consists of two parts:  Suspended load (discharge), carried by the water across a stream section above the bed layer. It may contain sand and finer fractions, such as silt and clay. Bed load (discharge), sediment that is carried across a stream section inside the bed layer or close to it. Particles move by means of small jumps (saltation), rolling or sliding. 1.3 Alluvial Streams May be defined as an open conduit, with geometric dimensions - cross section, longitudinal profile and slope –changing with time, Are virtually free to adjust their dimensions and shape in response to changing hydraulic conditions of flow;

17. Cont’d Most parts of the stream bed and its banks are composed of the material transported by the stream. The morphological processes which are responsible for the formation and development of alluvial streams are: ( i ) Erosion, (ii) Transportation, and (iii) Deposition. Erosion: is the process by which soils and minerals are detached and transported. Erosion can take place both in the channel and the watershed. Channel erosion – due to channel flow On the watershed –due to overland flow Splash erosion Sheet erosion Rill erosion Gully erosion

18. Cont’d Splash erosion: When rain drop strikes bare soil, it breaks up the soil aggregates and separates the fine particles from heavier soil particles. These particles are then transported with the surface runoff. Sheet erosion: The thin sheets of water that forms at the beginning of rainfall carries loose materials as it runs over the land towards the waterways. Rill erosion: As the sheet flow begins to concentrate on the land surface, the kinetic energy of the concentrated flow begins to cut small channels called rills And is capable of detaching and transporting soil particles.

19. Cont’d Gully erosion: As rills become deeper and wider, gullies are formed. They are capable of transporting large quantities of sediment since the flow rates are greater. Channel erosion It occurs when bank vegetation is disturbed Or when the flow rate in the stream is increased beyond the critical point Where bed material particle movement is initiated These changes destroy the geomorphic equilibrium of natural streams And cause channel erosion to begin

20. Cont’d Stable stream channel There is no objectionable Silting/aggradation (gradual rise in channel bottom over entire length), Scouring (erosive deformation of the channel), Or sedimentation (increase and advancing forward of the volume of sediment deposited). When these processes occur in excessive amount the channel becomes unstable and its natural balance is destroyed. The channel functions by transporting sediment out of the watershed by means of its flow. The total load of sediment transported by the channel is made up of the bed-material load and the wash load.

21. Cont’d …. Assignment-1 (10%) 1. How rivers are formed? 2. Discuss briefly the Advantage and disadvantage of a rivers? 3. What is the relationship between river and human civilization? 4. List some countries formed near a river or ocean. 5. List some towns formed near ocean, lake or river in Ethiopia. 6. Explain self adjustment of channel cross section with example. 7. Write short notes about cross sectional and meandering index.

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