Channel straightening

increases gradient water drains quickly,
disadvantages - results in more sever flooding down river.
expensive
reduces soil moisture
also slow down rain water going towards sea
disadvantages:
requires large areas of land that will be unavailable for other uses.

deepen+widen channel:
excavation + dredging equipment increase channel size. concrete slabs help reduce friction on river bed improving efficiency of flow. increases discharge flor through channel.
disadvantages:
expensive
creates problems downstream due to increased discharge

artificial levees:
build up river banks using earth + rock increases discharge capacity.

discharge:
expensive
can create problems downstream if breached = massive flooding as river level is much higher than surrounding flood

overflow basins:
designated area of floodplains allowed to flood

dams:
built along tributaries and the main river channel less water released into river during periods of heavy rain. this prevents discharge from rising too much. reservoir levels are increased instead.
Disadvantages:
expensive but can help produce electricity and provide regular water supply.

Case study - mississippi

Mississippi - the Facts

The river basin is the fifth largest in the world

It is the third longest river in the world behind the Nile & the Amazon

The river discharges 584 million tonnes of sediment a year

The flood plain is 200km wide at its widest point

The Mississippi flows through 10 states

The river carries 13% of all freight traffic in the USA

Its main tributaries are the R. Ohio, R. Kansas, R. Missouri & the Red River



The Causes of the 1993 Flood

Floods are normal in the mid-west - usually arriving in the spring when rain and snowmelt fill the streams & rivers that drain the upper Mississippi Basin

In 1993 as normal this happened - the soil was still saturated from spring rains. Normally this is followed by dry weather &has done so for the last 20 years

In 1993 Atmospheric conditions conspired to bring further torrential rains to the Mississippi Basin




a. A Jet stream swung South bringing Cool dry air
b. Warm air moved North causing Thunderstorms
c. Two high pressure systems developed blocking any movement of the thunderstorms
d. The rains continued throughout May, June and July.



Human Causes of the 1993 Floods

Urbanisation of the Flood Plain - reducing infiltration rates etc

Poorly built non-federal levees

The development of unsuitable sites for development

The channelisation of the river - especially at St Louis

The 1993 Flood Fact File

Primary Effects

50 people died

62,000 families were evacuated

72,000 homes were flooded

70% of levees were damaged

55 towns were wrecked

6 million acres of farmland was flooded



Secondary Effects

River traffic halted for several months

Crop losses were put at $2.6 billion

Insurance pay-outs reached $12 billion in property alone

Stagnant water attracted mosquitoes and rats and there was a threat of disease

Electricity lines collapsed leaving many towns without power





Holding back the tide with sandbags

Transport - Roads and Bridges affected





Response to Flooding on the Mississippi

Dams & Reservoirs - 6 huge reservoirs have been built along the River Missouri to store excess water with a further 19 along the Tennessee River and its tributaries

Afforestation - The Tennessee Valley Authority has been increasing tree cover to delay run off into rivers

Levees - Have been strengthened with concrete mattresses to reduce erosion of the river banks

Diversionary Spillways - These are overflow channels which can store excess water in times of flood and release it after the risk of floods have passed

FEMA - The Federal Emergency Management Agency has published risk assessments and encourages at risk settlements to move off the flood plain

Flood Forecasting - The National Weather Authority are now responsible for flood warnings along the river

There are of course more - remember to look at your own case study notes

Mississippi

1993 - mississippi burst its banks.
150 levees collapsed
dam burst - bridges closed
by July mississippi spread between 10 - 25 km

Effects:
9 states affected
23million acres covered 48 killed
26.5m sandbags used
70000 evacuated
final damage cost 10 billion US dollars (25% crop losses)
Town of valmeyer abandoned
River closed for 2 months (15% freight uses river)

Climate prior flood:
Heavy rain due to high pressure over bermuda
torrential rain June/July added to water level most water in 2 months since 1895

Taming mississippi:
- Vital Transport route
- Wing dykes
- Straightening
- 1600km of levees
- Many dams
- Much of river lined with concrete slabs

engineers caused floods
1993 worst flood ever despite engineering

- shortening the river caused it to flow more quickly increasing erosion
- restricting flow inside levees speeded up flow and increased pressure on levee. 1993 a large no. of levee's broke
- altering natural flow created bigger flood despite less water in the river than previous floods
- Increasing energy in river reduced sediment movement.

Causes of floods

- heavy Rainfall
- snow melt and heavy rainfall
- Deforestation
- Urbanisation - Garbage
- Soil Saturation

Useful Terminology

Delta
A large fan-shaped area of silt deposits found at the mouth of a river where it enters a sea or lake. A triangular area of deposited silt or alluvium at the mouth of a river. It will occur where a river carries a heavy load and empties into a shallow sea. As the area of silt increases the river tends to divide into many smaller channels.

Lagoon
A body of shallow water of varying salinity wholly or partially separated from the sea by some form of barrier, such as a sand bank (or reef).

Bottomset beds
Layers of the finest load (clay) that is deposited at the seaward edge of the delta. These particles are transported by suspended load being carried out the furthest by the river and are deposited when the river has little or no remaining energy at the seaward edge of the delta

Foreset beds
Layers of bedload, including the largest sediments (pebbles & sand) that roll along the river channel. The bedload rolls over the edge of the delta and builds up a steep angled layer on top of the bottom set beds as the main delta advances. These course materials at the advancing front of a delta form the bulk of the delta.

Topset beds
Horizontal layers of smaller sediments (silt & clay) that overlay the foreset bed. The suspended load settles out into horizontal beds over the top of the delta.

Class notes

flood hydrographs

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ACTIVITIES PAGE 15:

1a) on picture.
b)i - February. ii - February.
c) Because there is more rain in the winter than in the summer.
d) Because while there was no rain the water was still running off.

2.a)i - 5 hours.
ii - 4 to 5 am.
iii - 9 hours.

b)i - The time it takes to begin to discharge.
ii - Because the next storm stops it from falling.
iii - Because there was more precipitation and because it went on for longer.


c) The water would have over flown over the banks and flooded the surrounding area.