Lecture1 Conduct

8/12/2019 Lecture1 Conduct

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WR 322 RIVERS AND

RESERVOIRS

By

NDOMBA, P.M., PhD.

Lecturer

Water Resources Engineering DepartmentRoom C3, Block C1, or CoET Admin Block Room 31

Contact: Anytime during official hours

Venue: B2-2, B3-1


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LECTURE 1 - INTRODUCTION

WR 322 COURSE CONDUCTCourse Objectives

Delivery format

Course ContentDesign exercises

Assessment

Reference

OVERVIEWRivers

Reservoirs


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COURSE OBJECTIVES

To provide the student with knowledgeof

river engineeringand reservoir

operations. Students should be able to

design simple river structures and

understand their performance under

different conditions


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EXPECTED OUTPUT

Students will be able to design reservoirs

and river structures, understand their

performance under different conditions of

flow


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PREREQUISITES

WR 321: Fundamentals of Engineering

Hydrology

WR 212: Open Channel Hydraulics


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DELIVERY FORMAT

2 hrs lecture per week (30 hours)

A visit to rivers and reservoirs or

construction sites and laboratory

demonstrations of different performance

conditions.


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COURSE CONTENTS

RIVERS

Classification and definitions, data needs for rivers

analysis;

Meandering in rivers, its causes, factors controllingmeandering, remedial measures, river training works;

Stage-discharge relationship in rivers, flood routing,

flood characteristics, stage and discharge forecasting;

Aggradation and degradation in rivers beds, causes,regime channels, sediment routing and analysis.

Contd.


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COURSE CONTENTS

RESERVOIRS

Definition, planning purposes, classification, planning:

design data studies, topographical, geological,

hydrological and reservoir data, selection ofconstruction site;

Analysis of water availability, mass curve analysis,

determination of yield and storage capacity,

identification of water deficit and drought periods,

flood routing through a reservoir, sediment storage

requirements, sediment rate, distribution of sediment,

useful life of a reservoir;

Reservoir operation studies


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REFERENCES

Linsley, R.K. Jr., et al. Hydrology for

Engineers, McGrawHill Co.

Webber N.B. Fluid Mechanics for Civil

Engineers.

Jansen P Ph et al (1978). Principles if

River Engineering. Pitman Publishing

Limited


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OVERVIEW


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OVERVIEW

RIVERS

RESERVOIRS


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Definition of a river

A riveris a natural watercourse, usually

freshwater, flowing toward an ocean, a lake,

a sea or another river.

In a few cases, a river simply flows into the

ground or dries upcompletely before

reaching another body of water.

Small rivers may also be called by several

other names, including stream, creek, brook,rivulet, and rill;


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Position of a River in Hydrological Cycle

A river is part of the hydrological cycle. Water within a river is generallycollected from precipitationthrough surface runoff, groundwaterrecharge,springs, and the release of stored water in natural ice and snowpacks (i.e.,from glaciers).


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Classification Youthful river

a river with a steep gradientthat has very few tributariesand flows quickly. Its channels erode deeperrather thanwider.

Mature river a river with a gradient that is less steepthan those of

youthful rivers and flows more slowly. A mature river isfed by many tributariesand has more discharge than ayouthful river. Its channels erode widerrather thandeeper. (eg., Mississippi River)

Old river a river with a low gradientand low erosive energy. Old

rivers are characterized by flood plains. (eg., Nile River)

Rejuvenated river a river with a gradient that is raised by tectonicuplift.


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Use of rivers

Navigation: navigational requirements are easily

determined; a constant discharge, as high as

possible, should be the aim.

Hydropower: hydropower requirements are morecomplex. They are fully compatible with

navigational requirements only if water releases

from the reservoir are that of a base load station.

If they are not, some compromise for amultipurposedevelopment including hydropower

and navigation would be needed


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Use of rivers

Water supply

Irrigation: irrigation requires the release of waterwhen needed for the crops. It can be fully inharmony with hydropowerneeds when the

irrigation intake is somewhere downstream ofthe reservoir and it does not exceed the lowesthydropower discharge. Irrigation withdrawswater from a river and will therefore always be acompetitor of navigationbut it depends on thesize of the projects whether this water allocationcompetition is serious.

Planning, legislation


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Rivers patterns

Rivers patterns (fluvial styles) have been

classified in the classical paper by Leopold

& Wolman (1957) as braided, meandering

and straight. This classification isessentially based on the shape of the river

channel, as it appears on aerial

photographs. Later, Smith (1983) addedanastomosed rivers as a distinct fluvial

style.


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Meandering rivers

Meandering rivers

typically have onechannel that is

sinuously curved.


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Straight rivers

Straight rivers have a

low sinuosity index

(the ratio: channel

length/distanceacross the channel

belt). They generally

lack the typical

features of

meandering rivers,like point bars.


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Straight channels

Straight Channels - Straight stream channels are rare.Where they do occur, the channel is usually controlledby a linear zone of weakness in the underlying rock, likea fault or joint system. Even in straight channelsegments water flows in a sinuous fashion, with the

deepest part of the channel changing from near onebank to near the other (next slide). Velocity is highest inthe zone overlying the deepest part of the stream. Inthese areas, sediment is transported readily resulting inpools. Where the velocity of the stream is low, sediment

is deposited to form bars. The bank closest to the zoneof highest velocity is usually eroded and results in acutbank.


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Straight channels


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Anastomosing river:

Anastomosing rivers are

characterized by multiplechannels that are laterally

stable. The channels enclose

flood basins.


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Meandering rivers

Meandering rivers shift theirpositions across the valleybottom by depositingsediment on the insideof

bends while simultaneouslyeroding the on the outerbanksof the meander bends.

The brownregions indicatepoint bars(areas ofdeposition).

The red linerepresents thethalweg(the deepest part ofthe stream).


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Meandering channels


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Meandering rivers

Meandering Channels - Because of the velocity structureof a stream, and especially in streams flowing over lowgradients with easily eroded banks, straight channels willeventually erode into meandering channels. Erosion will

take place on the outer parts of the meander bendswhere the velocity of the stream is highest. Sedimentdeposition will occur along the inner meander bendswhere the velocity is low. Such deposition of sedimentresults in exposed bars, called point bars. Because

meandering streams are continually eroding on the outermeander bends and depositing sediment along the innermeander bends, meandering stream channels tend tomigrate back and forth across their flood plain.


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The effect of a curved channel on

water flow


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Braided rivers

A stream system so overloaded with sediment that there are manydividing and rejoining channels

complex tangle of converging and diverging stream channelsseparated by sand bars or islands. Characteristic of flood plainswhere amount of debris is large in relation to discharge

River in which the main channel is braided with multiple paths thatsplit and join frequently. Usually a gravel or sand bed stream

Where a river is forced to divide into several channels with banks orislands separating the channels

A braided river channel consists of a network of smaller channelsseparated by small and often temporary islands called braid bars.Braided streams are common wherever a drastic reduction in streamgradient causes the rapid deposition of the stream's sediment load.Braided channels are also typical of river deltas.


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Braided channels

Braided Channels - In streams having highly variabledischarge and easily eroded banks, sediment getsdeposited to form bars and islands that are exposedduring periods of low discharge. In such a stream thewater flows in a braided pattern around the islands and

bars, dividing and reuniting as it flows downstream. Sucha channel is termed a braided channel. During periods ofhigh discharge, the entire stream channel may containwater with the islands covered to become submergedbars. During such high discharge, some of the islandscould erode, but the sediment would be re-deposited asthe discharge decreases, forming new islands orsubmerged bars. Islands may become resistant toerosion if they become inhabited by vegetation.


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Braided channel


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Movement of a meandering

channel over time


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Irrigation

Power House

River

Dam

Rivers, Reservoirs and use


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RESERVOIRS

Whatever the size of a reservoir or the ultimate

use of the water, the main function of a reservoir

is to stabilize the flow of water, either by

regulating a varying supply in a natural streamor by satisfying a varying demand by the

ultimate consumers.

Since the primary function of reservoirs is to

provide storage, their most important physicalcharacteristic is storage capacity.


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A reservoiris, most broadly, a place or hollow vessel where fluidis kept in

reserve, for later use.

Most often, a reservoir refers to an artificial lake, used to store waterfor various

uses. Reservoirs are often created by building a dam, usually out of cement,

earth, rock, or a mixture.Once the dam is completed, a streamis allowed to flow behind it and eventually

fill it to capacity.


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Reservoirs

Discharge control is the redistribution ofdischarge time-wise and it is effected by theconstruction of reservoirs in the river valleys.

A reservoir fills up when the inflow - fromprecipitation, snow melt, subsurface flows orother upstream sources such as upstreamreservoirs exceeds the required outflow, and itempties when the required release exceeds the

inflow.

A reservoir acts as a buffer between supply anddemand


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The long-term average (inflow) is of coursedetermined by precipitation, and therefore thetotal demand is restricted by the existinghydrological possibilities.

The flow into a reservoir is of a stochastic naturebecause precipitation is of that nature.

There is therefore always an uncertainty withregard to the amount of future inflowand hencethe outflow cannot be regulated to the extend ofthe maximum possible benefit.


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The best that can be achieved is the maximum

probable benefit.

Reconciling demandwith an inalterable but

uncertain supplythrough the optimum designand operation of reservoirsis the main subject

on water resources allocation

There are two main types of demand

An increase in the lowest discharges: eg.hydropower,

navigation and irrigation

A decrease in the highest discharges: eg. Flood

control


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Reservoir

Water from a reservoir is releasedeitherover the top of the dam (spillway) orthrough outlets below the reservoir water

level. The capacityof a spillway increases

rapidly with rising reservoir water level

The spillway or the outlets may beprovided with gates to regulate the outflowwithin specified limits.


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Reservoir survey

Reservoirsurvey byDGPS andDigital echosounder:

Verificationdata collectiontechnology

High technology:

improves precisionand accuracy ofmeasurements/computed accumulatedsediment volume inNyM reservoir

Source:Ndomba (2005)


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River engineering

River engineeringis the process of planned humanintervention in the course, characteristicsor flow of ariverwith the intention of producing some definedbenefit.

People have intervened in the natural course and

behaviour of rivers since before recorded history - tomanage the water resources, to protect against floodingor to make passage along or across rivers easier.

From Romantimes, rivers have been used as a sourceof hydropower.

From the late 20th century, river engineering has hadenvironmentalconcerns broader than immediate humanbenefit and some river engineering projects have beenconcerned exclusively with the restoration or protectionof natural characteristics and habitats.


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River engineering

Before undertaking any river engineering,it is most important that the physicalcharacteristics of the river is investigated,

for these vary greatly in different rivers,being dependent upon the generalconfiguration of the land, the nature of thesurface strataand the climateof the

country which the rivers traverse.Legislation in many countries requires anenvironmental impact assessment.


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Reservoir operation

As stated above, the aim of reservoir

operation is to achieve the maximum

probable benefit for the users.

When trying to attain this objective, a

reservoir operator is continuously faced

with the question:

Whether to release an amount of water orkeep it in storage


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THANKS!

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