Chapter one. River morphology.pdf

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1. Love and peace for our country , Ethiopia!!
CHAPTER ONE: RIVER MORPHOLOGY
By inst. AmanuelB. 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
1.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.Landdevelopments
❖Landclearance
❖Infrastructurebuilding
❖Landusechange
b.Riverdevelopments
❖Hydraulicstructures
❖Riverchannelization
❖Gravelandsandmining
River morphologyis the study of the forms and patternsof
rivers and theprocessesthat 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 planformgeometry
➢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
Totalareafromwhichsurfacerunoffflowstoagivenpointofconcentrationis
calledawatershed,acatchmentarea,adrainagebasin,oradrainagearea.
Henceawatershedisalwaysconnectedtoacertainpointofconcentration,the
lowestpointoftherespectivebasin.

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
➢WatershedShape
✓Watershedshaveaninfinitevarietyofshapes;
✓Notuseddirectlyinhydrologicdesign
Hypotheticalwatersheds.(a)Ellipse:side;(b)triangle:center;(c)
square:Comer;(d)circle;(e)triangle:vertex;(f)rectangle;(g)ellipse:
end.

11. Cont’d
Inspiteoftheinfinitenumberofpossiblewatershedshapes,themajoritymay
neverthelessbeusuallyreducedtothreemainconfigurationgroups(Fig.below):
✓elongatedwatershed,
✓broadwatershed,and
✓fan-shapedwatershed
Fig. watershed shapes
EffectofWatershedShapeontheRunoffFlow
Therearemanyfactorswhichinfluencethehydrauliccharacteristicsofawatershed
asapartofanoverallfluvialsystem.Thesefactorsmaygenerallybesummarizedas
follows:
✓Watershedgeometry-which,amongothers,includesarea,shape,location,
lengthofstreams,averagemain-streamslope,drainagedensity,etc.
✓Soilcharacteristics-typeofsoil,grain-sizedistribution,texture,erosivity,etc.

12. Cont’d
✓Vegetationcover-typesanddistribution,interception,transpiration,etc.
✓Hydrology-infiltrationrate,groundwater,peakdischarges,typeofflow
(perennialorephemeral),yearlyhydrographs,etc.
✓Geology-structure,bedrockandsurface-soiltypesanddistribution,etc.
✓Climate-temperature,precipitationtype,seasonaloccurrenceandduration,
frequency,etc.
✓Sedimentyield-erosionandtransportmechanism,sourcesof,etc.
✓Humaninfluence-degreeandtypeofdevelopment,constructionactivity,
deforestation,etc.
➢WhileSoilcharacteristicsandhumaninfluencehaveapreponderantinfluence
onthevolumeoftherunofffromagivenwatershedarea.
➢Geometriccharacteristicsaffecttheformofthehydrographandthepeak
discharge.

13. Cont’d
Figure;Effectofgeometricshapeofwatershedonhydrographand
peakdischarge
LongitudinalProfileandMeanSlopeofstreams
❑Longitudinalprofilealongthemainstreamofthewatershedwill
oftenprovide:-
✓Valuableinformationaboutstreamproperties
✓Extensionofvariouspartsofstreamsinrespectoftheelevation
❖LongitudinalProfilehasthreezones

14. Cont’d
ElementsofHydrometricMeasurements
Understandingofthemorphologyandbehaviorofnaturalwatercourseshasbeengainedor
derivedfrommanylong-termobservationsandmeasurements.
Generallythosemeasurementscanbeclassifiedintothefollowinggroups:-
1.Velocitymeasurements, 4.Depthsounding
2.Water-levelmeasurements,5.Dischargemeasurements
3.Sedimenttransportmeasurements
1.Water-levelmeasurements
✓StaffGauges
Staff gauge

15. Cont’d
✓Suspended-WeightGauge
✓AutomaticGauge
2.DepthSounding
✓SoundingRod
✓EchoSounding
3.VelocityMeasurements
✓Floats
✓Currentmeters
4.DischargeMeasurement
Thefinalscopeofallthehydrometricmeasuringmethodsisgenerallytoobtaina
fairlyaccurate,economicallyandtechnicallyacceptableestimateofthedischarge
atagivenflowsection.

16. Cont’d
5.SedimentTransportMeasurements
Thetotalsedimentloadofastreamconsistsoftwoparts:
➢Suspendedload(discharge),carriedbythewateracrossastreamsectionabove
thebedlayer.
✓Itmaycontainsandandfinerfractions,suchassiltandclay.
➢Bedload(discharge),sedimentthatiscarriedacrossastreamsectioninsidethe
bedlayerorclosetoit.Particlesmovebymeansofsmalljumps(saltation),
rollingorsliding.
1.3AlluvialStreams
➢Maybedefinedasanopenconduit,withgeometricdimensions-crosssection,
longitudinalprofileandslope–changingwithtime,
➢Arevirtuallyfreetoadjusttheirdimensionsandshapeinresponsetochanging
hydraulicconditionsofflow;

17. Cont’d
➢Mostpartsofthestreambedanditsbanksarecomposedofthematerial
transportedbythestream.
➢Themorphologicalprocesseswhichareresponsiblefortheformationand
developmentofalluvialstreamsare:
(i)Erosion,
(ii)Transportation,and
(iii)Deposition.
oErosion:istheprocessbywhichsoilsandmineralsaredetachedand
transported.
oErosioncantakeplacebothinthechannelandthewatershed.
❖Channelerosion–duetochannelflow
❖Onthewatershed–duetooverlandflow
✓Splasherosion
✓Sheeterosion
✓Rillerosion
✓Gullyerosion

18. Cont’d
➢Splasherosion:
✓Whenraindropstrikesbaresoil,itbreaksupthesoilaggregatesandseparates
thefineparticlesfromheaviersoilparticles.
✓Theseparticlesarethentransportedwiththesurfacerunoff.
➢Sheeterosion:
✓Thethinsheetsofwaterthatformsatthebeginningofrainfallcarriesloose
materialsasitrunsoverthelandtowardsthewaterways.
➢Rillerosion:
✓Asthesheetflowbeginstoconcentrateonthelandsurface,
✓thekineticenergyoftheconcentratedflowbeginstocutsmallchannelscalled
rills
✓Andiscapableofdetachingandtransportingsoilparticles.

19. Cont’d
➢Gullyerosion:
✓Asrillsbecomedeeperandwider,gulliesareformed.
✓Theyarecapableoftransportinglargequantitiesofsedimentsincethe
flowratesaregreater.
Channelerosion
✓Itoccurswhenbankvegetationisdisturbed
✓Orwhentheflowrateinthestreamisincreasedbeyondthecritical
point
✓Wherebedmaterialparticlemovementisinitiated
✓Thesechangesdestroythegeomorphicequilibriumofnaturalstreams
✓Andcausechannelerosiontobegin

20. Cont’d
Stablestreamchannel
❖Thereisnoobjectionable
✓Silting/aggradation(gradualriseinchannelbottomoverentirelength),
✓Scouring(erosivedeformationofthechannel),
✓Orsedimentation(increaseandadvancingforwardofthevolumeofsediment
deposited).
✓Whentheseprocessesoccurinexcessiveamountthechannelbecomesunstable
anditsnaturalbalanceisdestroyed.
✓Thechannelfunctionsbytransportingsedimentoutofthewatershedbymeans
ofitsflow.
✓Thetotalloadofsedimenttransportedbythechannelismadeupofthebed-
materialloadandthewashload.

21. Cont’d….
Assignment-1(10%)
1.Howriversareformed?
2.DiscussbrieflytheAdvantageanddisadvantageofarivers?
3.Whatistherelationshipbetweenriverandhumancivilization?
4.Listsomecountriesformednearariverorocean.
5.Listsometownsformednearocean,lakeorriverinEthiopia.
6.Explainselfadjustmentofchannelcrosssectionwithexample.
7.Writeshortnotesaboutcrosssectionalandmeanderingindex.