revision on river power point

8/14/2019 Revision on River Power Point

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REVISION ON RIVER


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Velocity

Discharge

Factors

Flows

Helicoidal

Laminar

Turbulenceprocesses

ErosionTransportation

Landforms

Waterfall

Braided channel

Gorges meanders

delta

floodplain

Flooding

Hjulstrom curve


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Velocity & discharge

Qn. 1. Define what is velocity &discharge.

Velocity: mean speed of flow overtime(ms2) at a

point in a river channel or over a setdistance.

Discharge: Volume of water passing a

particular


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Qn. 2. State and describe three factorsaffecting

velocity.

(i) Channel shape in cross-section

(ii) Channel steepness

(iii) Roughness of river beds and banks


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River flow

Qn 1. Describe what is meant by Helicoidal,Turbulence and Laminar flow. Usediagrams to explain them.

Laminar flow : horizontal movement of water(rarely

found), common in lava flow.

Turbulent: a series of erratic eddies, both

vertical andhorizontal, in a downstream direction.


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Helicoidal flow: a corkscrewmovement, in a

meander, it is responsible for movingmaterial

from the outside of one meander bendand

depositing on the inside of the nextbend.


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Process

Qn. 1. Describe four ways of rivererosion.

Corrasion: occurs when the riverpicks up materials and rubs it alongits bed and banks, wearing themaway by abrasion, effective during

flood.

Attrition: As bedload moved

downstream, boulders collide with


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Hydraulic action: The sheer force of

thewater as the turbulent current hits

banks

(outside of meander) pushes waterinto cracks.

The air in cracks compressed, pressure

increasedand in time bank will collapse.

Solution/corrosion: It is related to

t


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n. . ent y an escr efour ways how a river

transports its load Saltation: when pebbles, sand and gravel(bedload) are lifted up by current and bouncedalong the bed in a hopping motion.

Traction: when largest boulders and cobbles

(bedload) roll or slide along the bed. Suspension: very fine particle such as clay and

silt (suspended load) are dislodged and carriedby turbulence in a fast flowing river.

Solution: water flowing within a river channelcontains acids (e.g. carbonic acid fromprecipitation) dissolve the load such aslimestone in running water and removed in

solution.


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Landforms

Define each terms and describe usingdiagrams

how they are formed, give examples.

(iii)Waterfall and gorges

(iv)Meander

(v)Braided channel(vi)Floodplain

(vii)Delta


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Waterfall & Rapids

Waterfall: A waterfall form when ariver, after

flowing over relatively hard rock meetsa band

of less resistant rock flow over theedge of a

plateau. Over a period of years, theedges of this

shelf will gradually break away and thewaterfall


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Rapid:

Rapids develop where the gradient ofthe river

bed increases without a sudden break

of slope(as in a waterfall) or where stream

flows over

a series of gently dipping bands ofharder

rock. Rapid increase the turbulence of

a river


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Meanders

Meanders are bends in the course ofthe river

channel. They often begin to appear asa river

approaches its middle course and thegradient

of channel becomes less steep.


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-Meanders are a result of helicoidalflow in

Which the fastest current (thalweg)spiral

downstream in a corkscrew fashion.

-This movement results in erosion inoutside

bend of a meander to form river cliffand

deposition on inside bend to form slip-off slope.

-


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Braided channel

What and how is it formed?A braided stream has islands/eyots of

deposited

Coarser materials such as sand and gravelwithin

the channel, formed due to variation indischarge such

as in semi-arid environment and temperateclimate.

High discharge due to heavy rainfall and

snowmelt thus


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A

BCDE

F

Name the features

Marked A-F


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Delta

It is composed of fine sediment whichis

deposited when a river losses energyand

competence as it flows into an area ofslow

moving water such as a lake or sea.


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Types of delta

(i)Arcuate (fan-shaped delta)

having rounded, convex outer margine.g. Nile,

(ii)Cuspate (tooths delta): wherematerial

Brought down by a river is spread out

evenly oneither side of its channel. E.g. Tiber

(iii)Birds foot: where the river has

many


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Floodplain

A floodplain is a mostly flat area ofland

bordering a river subjected to periodic

flooding.It is made of silts and sands which

have been

deposited over many years by theriver.


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LEVEES:

When river overflows its banks, the increase

in frictionproduced by the contact with the floodplain

causes

material to be deposited. The coarsestmaterial is

dropped first to form a small, naturalembankment

(levee) alongside the channel. Duringsubsequent

periods of low discharge, further depositionwill occur


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Flooding

1. Describe how flooding can bepredicted

2. State main causes of flooding

3. List down effects of flooding

4. State ways how flooding can be

reduced5. How human actions affect flows

within a river channel.


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Prediction of flooding

Recurrent interval: Statisticalstudies can be undertaken toattempt to determine the probabilityand frequency of high discharges ofstreams that cause flooding.

Flood hazard mapping : Mapsconstructed based on aerialphotographs and satellite images ofprior floods to help to determine theaffected areas.


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3. Forecasting: forecast when floodingwill occur base on correlationbetween amount of rainfall, degree

of ground saturation, degree ofpermeable soil, and amount ofvegetation so to give early warning

for people.


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Main causes1. Torrential rainfall produce by

convectional thunderstorm, tropicalcyclones or monsoon.

2. River flooding occurs when a riverchannel no longer able to containwater flowing within it, causes:Heavy rainfall, silting, landslide,

increase storm flow due tourbanisation, reduction in channelsize etc.

3. Coastal flooding: Low-lying coastal


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Human causes:

1. Deforestation: remove forestreduced interception and increaselandslides, soil erosion and overlandflow. Deposition in river channel

caused raised in river bed risk toflooding.

3. Dam building: Farraka Dam in India1971, raised river bed, dry seasondam reduces discharge and

encouraging sedimentation.


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3. Global warming: Rise sea level,surface water

on floodplain took longer to infiltrate.

4. Urbanisation: roads and

embankmentobstacles to free drainage of water.


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Effects of flooding

Bangladesh: flood 1998, over 1000people killed,

million homeless, large farmland and

properties was washed away, shortageof

drinking and food, respiratoryinfections

affecting large number of people-diarrhoea


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Ways to reduce

1. Raised river banks-reduce floodingsince hold more water butexpensive to straightened and

heighten.2. Dams and reservoirs-multipurpose

use but high cost to construct andmaintain and lead sedimentationover time.


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3. Straightening river channels-reduceflooding

in area but lead flooding downstream

4. Warning system-less flood damage,

propertyloss & deaths but little use if people

ignore,

need good communication networks


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How human action affectflows in a river channel?

Building dams, dredging, abstractionsurface and

underground water, irrigation etc-effect flows

and storage.


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Hjulstrom curve

Qn 1. Define what is Hjulstrom curve

Qn.2. Name type of sediment that can be eroded andtransported at

low velocity.

Qn. 3. Describe velocity to transport and erode silt andclay and

boulders. Why?

Qn. 4. Name the sediments that can be transported atany/all velocity.

Qn. 5. Define competence and capacity, critical velocity& settling velocity.

revision on river power point

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