Rivers, Floods & Management

1) Drainage basin characteristics

i) A larger drainage basin can carry more water, and will have a greater peak discharge

ii) Smaller basins will have a smaller lag time as the journey is shorter

2) Rock type

i) IMPERMEABLE V.S NON IMPERMEABLE = infiltration, surface runoff, lag time 

3) Vegetation; lag time & peak discharge 

i) LOTS VEGETATION = throughfall and infiltration 

4) Precipitation; peak discharge

i) INTENSE RAINFALL = bigger peak discharge 

5) Weather; lag time and peak discharge

HOT & DRY OR FREEZING = HARD GROUND = NO INFILTRATION

1) HYDRAULIC ACTION

- hydraulic pressure breaks off material from river banks 

2) ABRASION

- rocks scrape and rub against the bed. 

3) ATTRITION

- rocks smash into each other = become more rounded

4) CAVITATION

- air bubbles implode; break off rock in bank

5) CORROSION

- rocks dissolving from chemical processes

TRACTION, SALTATION, SUSPENSION, SOLUTION

LARGEST -------------------------------> SMALLEST

boulders      rocks            particles         solution

Deposition occurs due to...

1) reduced discharge

2) less energy

3) cannot transport material, so must be dropped 

CAPACITY IS THE TOTAL LOAD A RIVER CAN TRANSPORT
COMPETENCE IS THE MAXIMUM SIZE THAT CAN BE TRANSPORTED 

UPPER COURSE

Vertical erosion via abrasion and hydraulic power.The rough channel causes turbulence, causing large, angular bedload to be carried, resulting in great vertical erosion.

Transporting mainly largre particles via traction and saltation. 

Little deposition occurring; largest paritcles depositing on river bed

MIDDLE COURSE

Lateral erosion via abrasion. Attrition happens here

Transporting smaller materials via suspension 

Depositing sand and gravel across the flood plain 

LOWER COURSE

Despite velocity being highest, the least erosion happens at this stage, due to friction caused by turbulence absorbing the rivers energy

Transporting small materials such as silt and clay via suspension and solution 

Depositing smaller particals such as sand on the rivers mouth, of which absorbes the rivers energy

River velocity is influenced on the gradient, discharge and channel characteristics...

LOTS of the river's energy is used to overcome the friction of the channel;the HR is a measure of efficiency that tells you how the stream is affected by the channel characteristics:

1) wide & shallow characteristics would mean that there is a lot of friction to overcome = LOW HR - LOW VELOCITY

2) Narrow & deep characteristics would mean that there is less friction to overcome = HIGH HR - HIGH VELOCITY

Channel roughness also affects the velocity:

I) a rough channel will slow down the stream because they cause more friction. UPPER COURSE 

II) a smooth channel will absorb less energy via friction, allowing the stream to proceed at a fast velocity. LOWER COURSE

LONG PROFILE; a diagram to show how the rivers gradient has varied as you go downstream

CROSS PROFILE; a diagram to show the cross-section(insides) of parts of the river channel

The cross section is a diagram showing how each stages of the river is shaped

The long profile is a diagram showing how each stage of the rivers gradient varies

UPPER STAGE will have a:

- V SHAPED cross section; lots of vertical erosion 

-STEEP LONG PROFILE; more erosion than deposition

MIDDLE STAGE will have a: 

- U SHAPED cross section; mix of lateral and vertical erosion 

-MIXED LONG PROFILE; mix of deposition and erosion 

LOWER STAGE will have a:

-FLAT cross section; flat floodplain caused by deposition 

-FLAT LONG PROFILE; deposition emphasis 

Waterfalls; fluvio landform made where hard rock meets soft rock

1) soft rock is eroded by stream

2)stream undercutts into hard rock(resistant rock broken down by stronger, friction-free stream)

3)waterfall is made, with a plunge pool underneath 

Potholes; small circular hollows in the river bed

1)abrasion from turbulence in river bed causes material to rub and scrape out holes

Rapids; 'mini waterfalls' 

Meanders: combination of erosion and deposition 

1)erosion occurs in 'pools' and deposition occurs in 'riffles'

2)the sinuosity increases as the flow becomes bias towards 'pools' 

-a meaner with 'no neck' is known as a oxbow lake

Braiding: division of river to get rid of heavy load

1)River's velocity and energy falls 

2)Load becomes too heavy too carry; river divides into smaller streams while depositing material

3)Thin channels eventually rejoin to form a single channel 

Delta: division of river to meet sea

1)River's velocity and energy falls when it meets the sea(absorbs)

2)Channels deposit their load, untill the deposited material builds up and rises above the sea level

3)This slightly blocks the mouth of the river, meaning the channel has to braid into smaller streams to reach the sea.

Floodplains: an area of land adjacent to rivers thats succectible to flooding

Levees: natural, raised embankments formed as the river overflows onto its banks 

Long periods of heavy rainfall is the main cause of flooding 

There are physical factors that increase the risk of flooding...

1)(lack of) Vegetation; less throughfall; faster discharge increasing flood risk 

2)Impermeable ground; less interception; faster discharge; increasing flood risk

3)High hydraulic raidus; less friction; faster discharge; increasing flood risk

There are Human factors that increase the risk of flooding...

1)Urbanisation; impermeable; fast runoff & no slow interception; increasing flood risk

2)Deforestation; no evaportranspiration/interception/throughflow; increase discharge & flood risk

3)Climate change; increase in extreme conditions; heavier storms; increasing flood risk

There are Social, Political, Environmental, Economical & Demographical impacts of flooding...

S --> death, contaminated water, loss of place of tradition

P --> Increased pressure to invest in the short term(rescue and recover) and long term

E --> sewage and rubbish in flood-waters pollute rivers

E --> Loss of jobs(housing,industry), expensive for economy, insurance for locals rise

D--> Death tolls(can be high), unviable housing(destroyed) 

3 Causes for the Bangladesh flood...

1)Bangladesh has a monsoon climate; 80% of the yearly rain falls over 1/4 of the year

2)Bangladesh consists on low-layer 

3)The melting snow from the himalayas increased the Bramuhtas discharge 

The Effects of the flood...

S -> 2000 deaths

P -> Increased national debt 

E -> Rivers were pollutted with debris. +) flood deposited fertile silt on flood plain

E -> Cost Bangladesh $1Billion; loss of primary and secondary industries

D -> Over 100'000 houses destroyed; 25million people made homeless 

The damage had a less absolute economic impact, but the relative impact was much greater, meaning bangladesh, a poor nation were more heavily hit bt this than...  

The causes of the flooding were due to...

1)heavy torrential rainfall; 36hours, with over 200mm of water 

2)human development; many impermeable and deforested areas 

3)drains, sewage systems overflowed. ground became unsaturated 

The effects of the flooding was...

S -> 3 deaths; 70'000 houses had lost electrical power; community network destroyed 

P -> Increased pressure to invest 

E -> Rivers were polluted with debris

E -> cost £100Million to repair damage; 350 business shut down

D -> 3000 people made homeless

The relative economic damage was much less than Bangladesh, despite the absolute economic damage being greater than the Bangladeshs'

There are hard engineering schemes used to prevent flooding:

Hard engineering: time and cash expensive but more effective and durable

1) Dams; huge walls built across a river, with a resevoir that has a controlled flow

+bipurpose(provide electricity, bridge); used for recreational activities 

-very expensive; affect wildlife; will only protect upper stages of the river; lower stages left vunerable 

2) Levees; artificial channels taking water elsewhere

+allow floodplain to be developed 

-expensive; risk of flooding 

Soft engineering strategies are also used to prevent floodings...

1) Land management; building restrictions; with the exception of parks and greenland development(more soil; infiltration; less discharge; less flood risk)

+impact of flooding is reduced; provides recreational opportunities 

-restricts economical development; cannot be used in places that are allready developed 

2) River restoration; making the river more natural; removing man made leevees e.g. allow floodplain to flood naturally, restricting further flooding 

+little maintainence needed; impact will be minimal

-local flood risk may increase

Weather forecasts and flood warnings can also be used to reduce effect of flooding

+Little cost 

-Flash floods may happen too fast; previous innaccuracies may cause people to neglect the warnings

Three gorges dam;

located on the yangtze river -> experieinced 5 floods in the past century 

resevoir has been built; catches any floodwater and slowly releases it -> stoes 22km of water

dam ultises HEP; Largest in the world -> turning 26 turbines

+bipurpose ^^^ -> turbines supply 3% of china's energy demand

+reduce flood risk from every 10 years to every 100 years

-2 million people will have to relocate as the resevoir becomes larger 

-destroy habitats & wildlife

-increase floodings in tributaries downstream

albrighton; town in south east england; on the floodplains of river thames -> succectible to flooding

7 floods over last century

land use management & flood warnings

+cost effective

+minimise impact

+allow environmental aestheticall enhancement

-does not essentially stop flooding/flood risk

-can not measure sucess ^

-restricts economicall growth