Revision On River Power Point

REVISION ON RIVER

Factors Hjulstrom curve Veloci ty

Flooding Braided channel delta Waterfall

Helicoidal

Discharg e Flows

Laminar

floodplain Landforms

Gorges

meanders

process es Transportation

Turbulence Erosion

Velocity & discharge Qn. 1. Define what is velocity & discharge. Velocity: mean speed of flow over time(ms2) at a point in a river channel or over a set distance. Discharge: Volume of water passing a particular

Qn. 2. State and describe three factors affecting velocity. (i) Channel shape in cross-section (ii) Channel steepness (iii) Roughness of river beds and banks

River flow Qn 1. Describe what is meant by Helicoidal, Turbulence and Laminar flow. Use diagrams to explain them. Laminar flow : horizontal movement of water (rarely found), common in lava flow. Turbulent: a series of erratic eddies, both vertical and horizontal, in a downstream direction.

Helicoidal flow: a corkscrew movement, in a meander, it is responsible for moving material from the outside of one meander bend and depositing on the inside of the next bend.

Process Qn. 1. Describe four ways of river erosion. • Corrasion: occurs when the river picks up materials and rubs it along its bed and banks, wearing them away by abrasion, effective during flood. • Attrition: As bedload moved downstream, boulders collide with

•

Hydraulic action: The sheer force of the water as the turbulent current hits banks (outside of meander) pushes water into cracks. The air in cracks compressed, pressure increased and in time bank will collapse. •

Solution/corrosion: It is related to

Qn. 2. Identify and describe four ways how a river transports its load • Saltation: when pebbles, sand and gravel • • •

(bedload) are lifted up by current and bounced along 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 carried by turbulence in a fast flowing river. Solution: water flowing within a river channel contains acids (e.g. carbonic acid from precipitation) dissolve the load such as limestone in running water and removed in solution.

Landforms Define each terms and describe using diagrams how they are formed, give examples. (iii)Waterfall and gorges (iv)Meander (v)Braided channel (vi)Floodplain (vii)Delta

Waterfall & Rapids Waterfall: A waterfall form when a river, after flowing over relatively hard rock meets a band of less resistant rock flow over the edge of a plateau. Over a period of years, the edges of this shelf will gradually break away and the waterfall

Rapid: Rapids develop where the gradient of the river bed increases without a sudden break of slope (as in a waterfall) or where stream flows over a series of gently dipping bands of harder rock. Rapid increase the turbulence of a river

Meanders Meanders are bends in the course of the river channel. They often begin to appear as a river approaches its middle course and the gradient of channel becomes less steep.

-Meanders are a result of helicoidal flow in Which the fastest current (thalweg) spiral downstream in a corkscrew fashion. -This movement results in erosion in outside bend of a meander to form river cliff and deposition on inside bend to form slipoff slope.

Braided channel What and how is it formed? A braided stream has islands/eyots of deposited Coarser materials such as sand and gravel within the channel, formed due to variation in discharge such as in semi-arid environment and temperate climate. High discharge due to heavy rainfall and snowmelt thus

E Name the features Marked A-F

D

C

B A

F

Delta It is composed of fine sediment which is deposited when a river losses energy and competence as it flows into an area of slow moving water such as a lake or sea.

Types of delta (i)Arcuate (fan-shaped delta) having rounded, convex outer margin e.g. Nile, (ii)Cuspate (tooth’s delta): where material Brought down by a river is spread out evenly on either side of its channel. E.g. Tiber (iii)Bird’s foot: where the river has many

Floodplain A floodplain is a mostly flat area of land bordering a river subjected to periodic flooding. It is made of silts and sands which have been deposited over many years by the river.

LEVEES: When river overflows its banks, the increase in friction produced by the contact with the floodplain causes material to be deposited. The coarsest material is dropped first to form a small, natural embankment (levee) alongside the channel. During subsequent periods of low discharge, further deposition will occur

Flooding 1. Describe how flooding can be predicted 2. State main causes of flooding 3. List down effects of flooding 4. State ways how flooding can be reduced 5. How human actions affect flows within a river channel.

Prediction of flooding • Recurrent interval: Statistical studies can be undertaken to attempt to determine the probability and frequency of high discharges of streams that cause flooding.  • Flood hazard mapping : Maps constructed based on aerial photographs and satellite images of prior floods to help to determine the affected areas.

3. Forecasting: forecast when flooding will occur base on correlation between amount of rainfall, degree of ground saturation, degree of permeable soil, and amount of vegetation so to give early warning for people.

Main causes

1. Torrential rainfall produce by convectional thunderstorm, tropical cyclones or monsoon. 2. River flooding occurs when a river channel no longer able to contain water flowing within it, causes: Heavy rainfall, silting, landslide, increase storm flow due to urbanisation, reduction in channel size etc. 3. Coastal flooding: Low-lying coastal

Human causes: 1. Deforestation: remove forest reduced interception and increase landslides, soil erosion and overland flow. Deposition in river channel caused raised in river bed risk to flooding. 3. Dam building: Farraka Dam in India 1971, raised river bed, dry season dam reduces discharge and encouraging sedimentation.

3. Global warming: Rise sea level, surface water on floodplain took longer to infiltrate. 4. Urbanisation: roads and embankment obstacles to free drainage of water.

Effects of flooding Bangladesh: flood 1998, over 1000 people killed, million homeless, large farmland and properties was washed away, shortage of drinking and food, respiratory infections affecting large number of peoplediarrhoea

Ways to reduce 1. Raised river banks-reduce flooding since hold more water but expensive to straightened and heighten. 2. Dams and reservoirs-multipurpose use but high cost to construct and maintain and lead sedimentation over time.

3. Straightening river channels-reduce flooding in area but lead flooding downstream 4. Warning system-less flood damage, property loss & deaths but little use if people ignore, need good communication networks

How human action affect flows in a river channel? Building dams, dredging, abstraction surface and underground water, irrigation etceffect flows and storage.

Hjulstrom curve Qn 1. Define what is Hjulstrom curve Qn.2. Name type of sediment that can be eroded and transported at low velocity. Qn. 3. Describe velocity to transport and erode silt and clay and boulders. Why? Qn. 4. Name the sediments that can be transported at any/all velocity. Qn. 5. Define competence and capacity, critical velocity & settling velocity.