GEOGRAPHY AS REVISION

Physical: Rivers, Floods and Management

KEY WORDS• Precipitation; All forms of moisture that reach the Earth’s surface.

• Evapotranspiration; The loss of water from a drainage basin into the atmosphere.

• Surface Storage; The total volume of water held on the Earth’s Surface.

• Groundwater Storage; The storage of water underground in permeable rock strata.

• Infiltration; The downward movement of water into the soil surface.

• Percolation; The gravity flow of water within soil.

• Overland Flow; The movement of water over the surface of the ground. Often because the ground is saturated or

frozen, or where precipitation is too intense for infiltration to occur.

• Throughflow; The movement of water downslope within the soil layer.

• Groundwater Flow; The deeper movement of water through underlying rock strata.

• Hydrograph; A graph showing, for a given point on a river, the discharge, depth and velocity with respect to time.

It’s a graphical representation of river discharge during a storm or flood event.

• Base Flow; Water that reaches the channel largely through slow Throughflow and from permeable rock below the

water table.

• Storm Flow; Water that reaches the channel largely through runoff. The may be a combination of overland flow

and rapid Throughflow.

• Hydraulic Radius; The ratio of the cross sectional area of the channel and the length of its wetted perimeter. =

Cross sectional area/wetted perimeter.

• Wetted perimeter; That portion of the perimeter of a stream channel cross section that is in contact with the water.

• Cross Sectional Area; The total length of the bed and the bank sides in contact with the water in the channel.

KEY WORDS• Attrition; The reduction and rounding of particles of sediment carried in water by repeated

collision with each other and the shore.

• Abrasion/corrasion; The wearing away of the shoreline by sediment carried by the waves.

• Corrosion; Includes the dissolving of carbonate rocks in sea water and the evaporation of salt

crystals which expand on formation and help the rock to disintegrate.

• Sinuosity; The curving nature of a meander. =Actual Channel Length/Straight Line Distance.

• Isostatic; Changes in sea level resulting from the rise and fall of land masses.

• Eustatic; Changes in sea level induced by variations in the amount of water in the oceans.

• Drainage Density; The number of tributaries in a drainage basin.

• Negative Feedback; Increase in one thing, causes a decrease in another.

Positive Feedback; Increase in one thing, causes an increase in another.

• River Capacity; Total amount of load that a river can carry at a given time. Increases as

discharge increases. Increases down stream.

• River Competence; The maximum mass of material a river can transport. Will be higher in times

of flood. Increases downstream.

THE DRAINAGE BASIN HYDROLOGICAL CYCLE: THE WATER

BALANCE

• Drainage basin is an area of land bounded by its watersheds

and the ocean. Any precipitation falling beyond its watershed

will drain into a neighbouring basin.

• Inputs Transfers/flows Stores Outputs

• The balance between water inputs and outputs of a drainage

basin may be shown as a water budget graph.

• P= Q + E ± Changes in storage. [P= Precipitation; Q=Run Off;

E=Evapotranspiration]

INPUTS, FLOWS, STORES AND OUTPUTSInputs Flows Stores Outputs

Precipitation Surface Runoff Evaporation River

Infiltration Transpiration

Throughflow Interception

Percolation Surface Storage

Through fall Stem Flow Soil Moisture

Groundwater

Channel Storage

Vegetation

WATER BUDGET GRAPH

Water in excess,

evapo-transpiration

is lower than rainfall.

Due to water being

used in the summer

months, water

‘recharges’ the soil

during this period.

This is where

vegetation depletes

the soil of water

content.

THE STORM HYDROGRAPH

• Factors that affect river discharge.

• Rainfall

• Vegetation

• Geology

• Soils

• Relief (Gradient of the surrounding land)

• Urbanisation

• Basin shape and size

• History of previous flooding

FACTORS THAT AFFECT FLOODING• Amount and type of vegetation

• Temperature can affect the rate or transpiration and evaporation

• Gradient of slopes; steeper means more surface run off, as less time to infiltrate the soil (The relief)

• Rock type and structure

• Presence and type of soil

• Break in slopes, where water can build up

• Rainfall (Affected by passing of low pressure systems)

• Levels of urbanisation

• History of flooding

• Basin shape and size

• Where rivers confluence

• Must always link to how infiltration occurs, what flows will increase decrease.

HOW VEGETATION AFFECTS THE HYDROLOGICAL

CYCLE• Interception

• Water absorption through roots

• Evaporation from leaves

• Stems flow and Dead leaves add organic matter to soil

Forms barriers to surface run off

• Affect infiltration.

THE LONG PROFILE

• Elevation decreases towards the sea.

• The base level is the lowest point to which a river can erode.

• A model of what a river could look like, with mainly vertical erosion at the upper level

and lateral erosion in the mid and lower courses, due to the river discharge at these

locations and the subsequent energy the river has to erode.

CHANGING CHANNEL CHARACTERISTICS

Upper Course: Vertical erosion, Lakes, Waterfalls, Potholes. Overland flow makes

lakes. Channel becomes deeper waterfalls become rapids.

Middle Course: Lateral erosion, transportation, floodplains, meanders, truncated

spurs, river cliffs and smoothing gradient; flood plans begin to develop.

Lower Course: Transportation and Deposition. Channel is large. Deltas, Levees,

Bluffs, and Meanders. Flood plan increases in size.

FLUVIAL EROSION

• Abrasion/corrasion

• River picks up materials, then rubs it against river bed and banks.

• Most effective during times of flood

• Main method of both vertical and horizontal erosion

• Attrition

• Pebbles collide with one another as they move down the river.

• Break into smaller pieces.

• Become rounded in appearance.

• Hydraulic Action

• Force of water hits the banks and pushed into cracks. Air in the cracks

becomes compressed, increasing the pressure which may cause collapse.

WEATHERING

• Freeze Thaw

• Water enters cracks, freezes and expands. Ice thaws. Weakens rocks.

Most effective in areas where the temperature fluctuates around 0 C

during the diurnal range.

• Biological Weathering

• Growing trees or animals dislodge or break up rock.

• Chemical Weathering

• Chemicals break down rocks into smaller pieces which can fall into the

river. Acid rain on limestone is an example.

FLUVIAL TRANSPORTATION• Suspension

• Small particles, such as clay or silt, are carried in suspension.

• More turbulence/ higher velocity, the more particles that can be picked up.

• Accounts for the largest percentage of the load of a river.

• Solution• Acids that have dissociated within the water can dissolve certain types of rock,

such as limestone.

• Generally a minor process.

• Bed Load• If particles are large enough for the current not to be able to pick them up, they will

move in one of two ways:• Saltation: Bounce along the river bed.

• Traction: Rolling or sliding along the river bed; only in times of great flood.

• 90% of river load from movement of material down slopes of the river valley through weathering and mass movement.

• 10% comes from erosion of bed and banks.

FLUVIAL DEPOSITION

• Happens when the velocity of a river is reduced

• Velocity could be reduced by:

• Low rainfall

• River entering sea or lake

• Water becoming shallower

• Increase in load

• River overflowing its banks, depositing materials on the floodplain.

HJULSTROM CURVE• A graph that shows the relationship

between the velocity of a river and the particle sizes that may be transported.

• Higher velocity: materials transported

• Lower velocity: materials deposited.

• Larger heavier sediment needs a higher velocity for movement.

• Silt and clay need a higher velocity that expected because they are cohesive. Once picked up its not put back down.

• Natural channels, it’s more complex. Larger boulders may prevent smaller ones from being picked up.

BRADSHAW MODEL

• Suggests that velocity, discharge, cross sectional area and hydraulic radius all increase with distance downstream.

• As a river travels down its course from the source in the mountains to the low lying floodplains towards its mouth, the nature of the valley, the channel, the flow of water and the sediment transported all change.

• These changes are reflected in the Bradshaw model.

POTHOLES

• Circular depression on the river bed carved out of solid rock.

• Formed by a drilling action as pebbles are caught in eddy

currents and whisked around within a small natural crack or

hollow.

• As time passes, the drilling action enlarges the hollow to form

a pothole.

• Commonly found below waterfalls or rapids, where hydraulic

action is a significant process.

RAPIDS

• Small series of waterfalls associated with very disturbed

turbulent water.

• Forms when rocks or varying resistances cut across a valley

and erosive process create a series of steps.

WATERFALLS

• Sudden step in a river’s long profile.

• Often the result of a tougher, more resistant band of rock

cutting across the valley.

• Unable to erode the rock at the same rate as neighbouring

rocks, a step is formed and a waterfall results.

• Over time, the river cuts backwards into the resistant rock

causing the waterfall to retreat backwards. Forming a narrow,

steep sided gorge.

GORGES

• A narrow valley between hills or mountains. Typically with

steep rocky walls and a steam running through it.

• Waterfalls are not uncommon.

MEANDERS• Sudden bends in the course of a river.

• Form when areas of alternating pools, deep water and riffles (Shallow water) develop at equally spaced intervals

along a stretch of a river.

• The distance between pools is usually 5-6 times the width of the river bed.

• Because water is deeper in pools, the river is more efficient when passing over them.

• The energy and erosive power is therefore increased when passing over these areas. On the other hand, the river

is less efficient when passing over riffles as there is more friction, causing the river to lose energy.

• This combination of the river gaining and losing efficiency at different intervals causes the river’s flow to become

uneven, and maximum flow is concentrated on one side of the river. As the water speeds up, turbulence increases

in and around pools.

• This leads to corkscrew-like currents in the river called helicoidal flow.

• These helicoidal currents spiral from bank to bank, causing more lateral erosion (Abrasion and hydraulic action)

and deepening of the pools – river cliff.

• This leads to the increased amount of eroded material being deposited on the inside of the next bend, where the

river loses energy- slip off slope.

• Combination of erosion and deposition exaggerated bends until large meanders are formed.

• These combined processes also give the meanders an asymmetric cross-section.

BRAIDING

• When a river subdivides into smaller streams

• In between these channels, small islands of deposited

sediment will form.

• Braiding occurs when there is a sudden decrease in energy

causing deposition of large amounts making the channel

subdivide.

LEVEES

• Natural embankments of sediment formed when the river

floods and deposits it.

FLOODPLAINS

• Flat area of land bordering a river that is subjected to periodic

flooding.

• It’s made up of silts and sands which have been deposited

over many years.

• As a river floods its velocity is dramatically decreased, causing

energy to be reduced and a bulk of sediment left floating in a

thin sheet of water on the flood plain.

• Once the water evaporated, a fresh layer of alluvium will be left

behind, this is what makes a flood plain so fertile and good for

farming upon.

DELTAS

• Formed when a river enters a sea or lake losing velocity,

energy, capacity and competency causing a large amount of

sediment to be deposited as a delta.

• Rivers flowing over deltas tend to break up into many smaller

channels called distributaries.

• Two types of delta are Arcuate (Fan shaped) and bird food

(Shaped like a bird’s foot).

RIVER TERRACES

• Remnants of the former floodplain prior to rejuvenation.

• Terraces create steps in the valley cross profile.

INCISED MEANDERS

• A meander that lies at the bottom of a steep walled canyon

• Most often occurs at an existing meander after the

rejuvenation of a river.

• There is then severe downwards erosion creating a steep walled canyon.

• Two types of incised meander:

• Entrenched: have a symmetrical cross section resulting from very rapid

incision by the river of the valley sides being made of hard, resistant rock.

• Ingrown: formed when the incision or uplift is less rapid and the river may

shift laterally, producing an asymmetrical cross section shape.

RAPIDS

• Found where there is a sudden increase in the slope of the

slope of the channel or where the river flows over a series of

gently dipping harder bands of rock

• As the water becomes more turbulent its erosive power

increases.

WATERFALLS

• Found where there are marked changes in geology in the river

valley.

• Where resistant rocks are underlain by less resistant beds,

the plunge pool at the foot of the fall experience the force of

the swirling water around the rocks, leading to more erosion

• This undercuts the beds above, leaving them overhanging and

prone to collapse

• The waterfall therefore retreats upstream.

PROCESS AND IMPACT OF REJUVENATION

• Causes the river to increase its down cutting activity.

• Isostatic or Eustatic change result in increased amounts of energy available to the river for erosive processes, because it has to go down further to reach the base level.

• The river is working towards having a smooth gradient down its course.

• Knick points are breaks in gradient along the profile, usually marked by rapids or waterfalls. This signifies the current progress of regression.

Original Graded

ProfileFirst Re-graded

ProfileSecond Re-graded

Profile

MAGNITUDE-FREQUENCY ANALYSIS OF FLOOD

RISK• T= n+1/m [T = recurrence interval; n = number of years of

observation; m = rank order]

• Is a probability based on historic values, does not mean that

floods will occur more or less frequently.

SOFT ENGINEERING STRATEGIES

• Reforestation

• Forecasts and warnings

• Land use zoning

• River restoration schemes

• Wetland and river bank conservation

HARD ENGINEERING STRATEGIES

• Flood embankments• Artificial raising or straightening of channels allowing a greater capacity for water.

Also has the consequence of increasing the velocity of the water. May cause

problems down stream.

• Channel enlargement• This involved dredging and the removal of large boulders from the river bed. It

increases channel efficiency and reduced roughness. River velocity increases.

• Flood relief channels• Take excess water around a settlement.

• Dams• Allows water to be stored temporarily in a reservoir the rate at which water passes

through into the river can be controlled.

• Removal of settlements

CASE STUDY 1: PHYSICAL AND HUMAN CAUSES

OF FLOODING; HULL 2007• Physical:

• Unusually high level of rainfall (44% of annual rainfall in just two weeks)

• Occurred in early summer; meant that the soils were saturated

• Hull is in a low lying position

• Tributaries of the rivers Severn, Humber and Thames flowed faster than they have ever done since meters were installed.

• Human:• Drainage system overwhelmed

• Modern drainage system; more efficient at draining water towards river than other UK systems

• No rainfall/flood warning systems were in place

• No appropriate protection against flooding

• Concerns over whether the pumping station was sufficient

CASE STUDY 2: PHYSICAL AND HUMAN CAUSES

OF FLOODING; CHINA 2011• Affected 12 provinces of China

• Physical:• Months of drought destroyed crops before the floods hit, increasing surface run off.

Ground harder to penetrate.

• Erosion has increased the likelihood of flash floods

• Relentless heavy rains struck central and southern China

• Several major rivers swelled and burst their banks

• Human:• Deforestation resulted in flood and landslide problems

• Large scale urbanisation resulted in increased surface run off and decreased interception by vegetation.

• Most of this occurred on floodplains

• Large areas of land already water logged due to high levels of farming (paddy fields). Resulted in reduced infiltration.

CASE STUDY 3: IMPACTS OF FLOODING;

TEWKESBURY• Social:

• 10,000 properties damaged

• 20,000 people directly affected

• Elderly residents terrified at the rate of the water level increasing in their homes.

• Economic:• Local infrastructure devastated

• People unable to work for an extended amount of time

• Large amounts of insurance claims made

• National government had to step in to deliver aid due to the socioeconomic status of the people living in the area

• £140 million in damage

• Environmental:• Mostly built up area, so very little environmental damage.

CASE STUDY 4: IMPACTS OF FLOODING;

PAKISTAN; SINDH PROVINCE• Social:

• 5000 schools damaged, social implications

• Schools not rebuilt, meaning that children are not getting an education, meaning that they may not be able to get higher paid jobs

• Thousands of deaths

• Water Bourne diseases likely to spread

• Economic:• 5.3 million jobs lost

• Estimated GDP growth rate of 4% prior to floods was lowered to between -2% and -5%. Pakistan unlikely to meet IMF’s target repayment, meaning that its existing $55 billion external deficit will grow• May have been a factor in deciding the amount of aid that the Sindh province received

• Infrastructure devastated: 2,422 miles of highway and 3,508 miles of railway damaged or destroyed. Repairs to cost $289 million

• Public building damage estimated at $1 billion

• Hundreds of square miles of farmland lost