AS LEVEL GEOGRAPHY RIVERS (FLOODS) HYDROLOGY AND FLUVIAL GEOMORPHOLOGY

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1. What

is a flood?

A flood is a natural hazard that occurs when a

river carries too much water that the banks cannot hold it hence it bursts. The

extent to which the river exceeds its limits determines the severity of the

flood.

2. What

causes flooding?

Flooding may be caused by both human and

physical factors determine whether a place is more prone flooding than others.

Ø Natural Causes

·

Nature of the drainage basin –Some drainage basins are more likely to flood

than others die to the relief, vegetation, soil depth, rock type, drainage

density.

·

Rock Type – Rock that is more permeable allows more

infiltration and ground storage to take place hence there are fewer surface run

offs.

·

Soil Depth – The deeper the soil is the more water is

absorbed which results in less run off.

·

Vegetation - In areas where there is more vegetation

cover, there are more levels of interception and evapotranspiration which

reduces the risk of flooding.

·

Slope – Gentler slopes means more water is absorbed

and less run offs therefore reduced risk.

·

Drainage Density - Where the drainage density is high, there is

a shorter lag time and an increased risk of flooding.

·

Low lying parts of active floodplains e.g. in Bangladesh, 110

million people are living on the floodplain of the Ganges and Brahmaputra

rivers are relatively unprotected as the floods caused during the monsoons

usually cover 20-30 % of the flat delta hence in high floods, nearly half of

the country can be flooded leading to over 1500 deaths.

·

Earthquakes – Seismically unstable terrain increases flood

risks.

·

Climatic factors – Excessive levels of precipitation that occur

over a long period of time would lead to saturation of the soil. This is

because there will be increased amounts of water on the ground surface leading

to increased overland flow. For example

the heavy autumn and winter rains in the UK are due to the low pressure weather

systems (deep depressions that cover a wide area and are long lasting). Intensive precipitation over a short

period of time especially when the ground surface was extremely dry for a long

period of time would lead to the water passing the rate of infiltration hence

the ground cannot soak up the rainfall quickly enough therefore more water will

reach the river than it would under normal circumstances. E.g. Monsoon rainfall

in India and Pakistan during summer. Melting

of snow when the subsoil is still frozen leads to reduced infiltration e.g.

in the Alpine and Arctic areas, the melting snow is responsible for widespread

flooding.

Ø Human

Causes of Flooding

There

are the same factors that affect a storm hydrograph e.g. land use,

urbanisation.

·

Flood-intensifying conditions which alter the

drainage basin response to a given storm e.g. urbanisation – Urbanisation (an increase in the proportion of

a country’s population living in urban areas) increases the frequency and

magnitude of flooding by:

i.

Increasing

the creation of impermeable surfaces e.g. roofs, roads (concrete and tarmac),

car parking and pavements leads to less infiltration therefore shorter lag

time.

ii.

Increasing

and speeding up drainage density by the construction of artificial conduits e.g.

underground sewers.

iii.

Impending

and constricting river channel flow by building alongside or in the river e.g.

a bridge supports or riverside facilities which reduce the carrying capacity

and slows down discharge.

iv.

Straightening

of channels to increase the velocity which leads to flooding downstream.

v.

Land

use e.g. for deforestation (greater risk of soil erosion leads to less

interception), ploughing and overgrazing

increases flood run off, reduces interception and decreases channel

capacity due to increased deposition in the channel which eventually blocks the

channel and increases the risk of flooding.

·

Climate change – Global warming has increased the frequency of

flooding. This is because of increase in the average sea level temperature rise

has led to the increased frequency and magnitude of tropical storms especially

in the Caribbean. These storms bring heavy rainfall along the coastlines for

example Hurricane Katrina in 2005 led to flooding in New Orleans. Higher

temperatures will also lead to increased evaporation leading to more

precipitation, melting of polar ice caps, rise in sea levels which all increase

the risk of floods.

3.

What are recurrence intervals?

The recurrence interval is the regularity

which particular levels and size of floods occur. Once it is calculated, it

indicates the number of years which a flood of a particular size is expected. Small

floods are expected to occur more regularly than larger floods but are less

damaging. Nonetheless, it is a probability based on previous historic evidence

and does not mean floods will occur more or less frequently.

The recurrence interval can be

calculated by the formula: T=n+1m where

T is the recurrence interval, n is the number of years on record and m is the ranking of the flood relative

to all the other floods on record for a specific river.

For

example, a flood with a discharge of 200m3s-1 occurred

at some point in the river’s past. Out of a data set spanning 199 years5, this flood was the 2nd largest

in terms of discharge. Using the formula, this means that a flood of this size

is expected to occur once every 100 years (199+12). We’d describe it as a 1 in

100 year flood. https://geographyas.info/rivers/flooding/

All

in all, the recurrence interval is not the most reliable as it is based on

averages which can have anomalies such as the fact that flood patterns can be

unpredictable as they change all the time.

4.

How else can floods be predicted?

The

Environmental Agency constantly monitors rivers and records of floods. Except

for the recurrence interval, floods can also be predicted by using the Risk

Assessment for Strategic Planning (RASP) which is a method that improves flood

warning and prediction. This can be done by weather radar which shows the likely and actual precipitation

approaching an area hence it improves the rainfall and snow pack estimates.

Automatic rain gauges also help in the prediction because it helps

in collecting the meteorological information and maps the channels with the

amount of rainfall that has fallen in the catchment area.

River gauges record the discharge along a river therefore

improving the flood warnings as there will be more complete and timely

information of hydrological information that can be shared.

http://www.slideshare.net/anicholls1234/flood-prediction-andwarning

5.

How do floods affect places?

Boscastle, UK, 2004

Boscastle is a village and fishing port located on the north coast of Cornwall, South West of England

and is home to around 2000 people.

River Valency flows directly through the towns and meets River Jordan at a

confluence in the town which leads to huge amounts of water flowing when

discharge increases. The river walls

are steep made of impermeable rock and shale.

The

Boscastle flood of 2004 occurred on Monday 16th August 2004 when 200.2 mm of rainfall (3

normal months of rainfall) was recorded in only four hours.

Causes

Ø Physical

and Human causes

·

Hurricane

Alex and the convectional rainfall triggered the intense heating of the ground

which led to heavy rainfall that fell over Bodmin Moor, which is an area made

of impermeable shale which increased the surface run off due to a lack of

infiltration and interception (sparse vegetation) hence the volume of River

Valency and Jordan increased.

·

The

ground was already saturated by previous rainfall which reduced infiltration

and increased surface run off therefore most of the water swept downhill

between the river walls that resulted in the water reaching 3 m in height that

swept through the village.

·

The

surrounding areas of River Valency and Jordan have high relief and steep slopes

therefore it encourages surface runoff.

·

The

floodplains around the rivers had been urbanised which reduced infiltration and

increased surface runoff.

·

River

Valency had its channel walled off therefore it prevented the channel from

adjusting to the increased discharge and limited its efficiency which

inevitably caused the flood.

Effects

Ø Social

·

Some

people suffered from hypothermia and broken bones as people had to seek refuge

on higher grounds such as rooftops. 120 people were rescued from rooftops.

·

Houses

became flooded and silt, sewage and debris were deposited inside of them which

left many people homeless. Around 1000 people’s homes, businesses and cars were

swept away.

·

There

was no water and power supply. The power has to be switched off to protect

rescuers and survivors from electrocution.

Ø Economic

·

Potential

increase in home insurance due to the extensive damage caused but because

floods are so rare hence it was unlikely. Claims came in between £15,000 and

£30,000 per property.

·

Tourism

was a main source of income (nearly 90% of the economy was dependent) but after

the flood people were less willing to travel to Boscastle because of the

potential risk of another flood. This led to sever loss in revenue.

·

76

cars were carried through the village by a torrent to the sea because of the

low lying nature of the town’s car park.

·

The

“lower bridge” was destroyed by the debris that blocked it.

Ø Environmental

·

Due

to the sewage burst, raw sewage was washed out to the sea and in the rivers.

·

Local

wildlife habitats were destroyed.

https://geographyas.info/rivers/flooding/

Responses

Ø Short

Term

·

A

search and rescue team was issued a few hours after the flood which lasted

until 2.30 am the next day hence 150 people (120 from the rooftops) were saved

by search and rescue operations.

·

Lifeboats

searched the harbour fearing people may have swept out to sea.

·

North

Cornwall Council provided accommodation for 11 tourists who were unable to

return home while the night of the disaster. While the night of the disaster,

100 people sought refuge in the Camel ford leisure centres.

·

A

cleanup operation was set up to oversee sewage, road, water and electricity

systems.

Ø Long

term

·

In

2006, donations summed up to £800,000 which was used in a flood scheme that

would incorporate the sewage and drainage system, deepen and widen the riverbed

by 6 feet to accommodate more water and to handle an increase in discharge, and,

to raise the Boscastle car park. . The

Environmental Agency built new flood defences in 2006 at the source and the

river base of Boscastle. They also removed the debris and sediments from the

source.

·

Reconstruction

was delayed to 2005 as the council was waiting for hydrologists to find the

recurrence interval of the flood. But by early 2005 power and water was

restored.

·

The

remains of the “lower bridge” that triggered a 3m wave were demolished and

replaced with a larger bridge that would be more difficult to block with

debris.

·

Environmental

footprint was taken more seriously by the locals as they believed that climate

change was exacerbated by human activity which caused the flood therefore all

the new construction was eco- friendly e.g. with insulation.

6.

How can floods be prevented?

a)

Loss Sharing – This includes disaster aid and insurance.

Disaster aid is the financial and economic resources that are given to an area

after a disaster e.g. money, staff, equipment, aid and technical assistance. An

example of loss sharing would be insurance which is common in MEDC’S.

b)

Event Modification – The adjustments include environmental control

and hazard resistant design. This kind of physical control of floods depends on

two measures: flood abatement and flood diversion.

c)

Flood abatement – Abatement measures aim to reduce the

possibility of flooding by managing land use upstream and decreasing the amount

of runoff thereby reducing the peak flow. This includes a variety of methods

such as:

·

Afforestation

(requires substantial areas in the catchment to be planted with trees.

·

Reforestation to increase interception and

evapotranspiration and reduce runoff as well as reducing silting of the river

channels as the soil will be held together.

·

Contour

ploughing or terracing to reduce to runoff and soil loss, reducing the amount

of bare soil to avoid excessive run off by reseeding of sparsely to increase

evaporative losses.

·

Comprehensive

protection of vegetation from forest fires, overgrazing and deforestation.

·

Allowing

rivers to meander.

·

Reduce

the amount if sediment and debris washed into the channel.

·

Construction,

restoration and excavation of backwaters, small waters and sediment holding

areas to accommodate flood water.

d)

Flood diversion – This includes the construction of levees,

reservoirs and the modification of river channels.

·

Levees are used to divert and restrict water to low

value land on the floodplain.

·

Channel improvements such as channel enlargement will increase the

carrying capacity of the river but the channel becomes clogged with weeds.

·

Reservoirs store excess rainwater in the upper drainage

basin. However this is only appropriate in small drainage networks.

Nonetheless, the reservoirs create a water supply area.

·

Intercepting channels divert parts of the flow away which allows the

flow to be used for agricultural purposes.

e)

Land use planning – This is the mapping of areas that are of a

greater risk of flooding and once this is discovered, low value buildings can

be developed. However, the poor tend to find it hard to evacuate from this area

as it is not always possible to change land uses hence they choose to live on

the land.

f)

Flood proofing (building design) - This is making property less vulnerable to

flooding and the damage after. It may be temporary e.g. using sandbags of

permanent e.g. buildings with flood proof walls. This will ensure that only low

value land uses like car parks occupy ground lives and the potential damage

from floods reduced. However temporary flood proofing is only useful for minor

flood problems and some people may not be willing or able to afford new

advancements to their houses.

.http://greenfieldgeography.wikispaces.com/Floodplain+management