Rivers, Floods and Management

What is the global hydrological cycle and what type of system is it?

continuous cycle of water between oceans and the atmosphere, returning it to river via precipitation. closed system (no inputs or outputs).

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Give an e.g of a local hydrological cycle. what is it? type of system?

Drainage basin. Area of land around river where rain falls & flows into river. open system

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why is the drainage basin an open system?

water enters system as precipitation & leaves by evap, trans,& river discharge.

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what is a watershed and what does it show?

boundary of a drainage basin. Any ppt. falling beyond it enters a different drainage basin.

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watertable

top surface of the zone of saturation

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Percolation

Water seeping down soil into the water table.

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groundwater flow

water moving through permeable rocks in ground below water table

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Base flow

groundwater flow that feeds rivers via river banks & beds

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Interflow

water moving downhill through permeable rock above water table

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What is channel flow also called?

river's discharge

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what does the water balance show?

the balance between inputs & outputs.

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Explain the water balance (3)

1) water surplus in wet seasons, precipitation exceeds evapotranspiration. Ground stores saturated-> more runoff-> more discharge-> river levels rise 2) drier seasons-> more et.-> less water storage (depleted) 3) ground storage recharged i.e autumn

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River discharge & what its measured in?

volume of water flowing in river per sec. Cumecs (m3/s)

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river discharge is affected by...

1) ppt. 2) temp- hotter-> lower discharge (more evap) 3) Removal of water from river (abstraction)

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how do you show river discharge over time

hydrographs (storm hydrograph to show driver discharge during storms)

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Physical factors that affect storm hydrographs: DRAINAGE BASIN CHARACTERISTICS affect lag time & peak discharge

1. - larger-> catch more ppt-> higher peak discharge than smaller ones. smaller-> shorter lag time, shorter distance. 2) steepness... 3) circular basins-> flashy h than long narrow basins, lots of w will reach measuring point at same time.

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More factors: AMOUNT OF WATER ALREADY IN DRAINAGE BASIN affects lag time

saturated soil-> increase runoff. Its quicker than through flow or baseflow-> so reaches river quicker, reducing lag time

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More factors: ROCK TYPE affects peak& lag time

Impermeable rock-> increase surface runoff-> decreased lag time. Peak discharge increases, as more water enters river quicker

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More factors: VEGETATION affects lag time & peak discharge

1. Interception-> lag time increased 2. Evapotranspiration-> peak discharge reduced

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More factors: PRECIPITATION affects peak discharged

1. Intense storms -> more ppt-> higher peak discharge. 2. Type of ppt, snow-> long lag time

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More factors: TEMP affects lag & peak discharge

Hot dry & cold freezing conditions-> hard surface-> more runoff-> short lag time-> greater peak discharge. hot-> incr et. -> les water reaches river-> reduced peak discharge

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How does human activity affect the hydrograph?

1. Impermeable surfaces-> runoff-> short lag time-> greater peak discharge. 2. Man made drains-> can't evaporate/ infiltrate-> short lag time-> greater peak d

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*What are the 5 ways of river erosion

1.Hydraulic action... 2. Abrasion- eroded rock scrape against bed & banks (common) 3. Attrition- smashing of rocks-> small & rounded. DOESN'T ERODE BED/BANKS 4. Cavitation- air bubbles in turbulent stretches of water collapse-> shockwaves.

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4 transportation processes

1. solution- dissolved material. 2. Suspension- carried material. 3. Saltation- material bounces (common). 4. Traction- rolls

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What is the rivers bedload

material transported by saltation or traction

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5 ways rivers energy is reduced?

1. reduced rainfall-> less discharge -> rivers slows down ->less energy. 2. evap./abstraction-> less discharge 3. friction 4. when river is forced to slow down e.g. before a narrow section of the channel-> loses energy 5. meets sea

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Capacity

total load river can transport at given point

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Competence

max. particle size river can carry at given point

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What does the Hjulstrom curve show (2)

The link between velocity & competence AND how erosion, transportation & deposition changes with velocity

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Hjulstrom curve- What does the CRITICAL EROSION VELOCITY CURVE SHOW

The minimum velocity needed (for river) to erode & transport material of different sizes. Higher velocity needed to erode than to transport

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Hjulstrom curve- What does the MEAN SETTLING VELOCITY CURVE show

Competence of the river at different velocities

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What is the long profile (LP) of a river

shows the gradient of a river channel along its course (source to mouth)

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LP: Base level?

Lowest point river can erode to (usually sea level)

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LP: Whats a graded profile

As erosion and deposition is said to be balanced overtime the long profile will change from an uneven to a smooth curve (but this hardly ever happens) .

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how does the energy of a river vary at upper, middle & lower stage

U- steep gradient & well above sea level-> so lots of potential (energy can be converted into other forms) . M- lower gradient, potential energy turned to kinetic energy-> river gains velocity. L-little potential energy but lots of k energy-> faster

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Characteristics of an efficient river

high velocity, high discharge, less friction

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What is efficiency measure in? what is it? what does it show?

Hydraulic radius- the channels cross section area DIVIDED by the length of its wetted perimeter(ttl length of bank & river bed that's in contact with water). Larger hydraulic radius -> more efficient the river

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How does the wetted perimeter effect the river?

contact between water & wetted perimeter -> friction-> energy loss-> slows river down

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A larger hydraulic radius means what? (wetted perimeter)

less water in contact with wetted perimeter-> less friction->reduces energy loss-> velocity & discharge increased

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How does channel roughness effect affect efficiency?

Increases the wetted perimeter-> more friction-> reduces efficiency,velocity & discharge. Incr in channel roughness =incr in turbulent flow-> more effective at picking up material from river bed than smooth flow-> so greater erosion

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What is the difference between channel roughness at upper & lower stage and how does this effect discharge & velocity?

upper- channel roughness greatest. Although gradient is steep, the river loses lots of energy to friction, so discharge & velocity is lowest. Lower-smooth banks & bed-> less friction-> less energy lost-> discharge & velocity highest

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Waterfall formation

Where hard rock meets soft rock. Softer rock gets eroded more-> 'step'.Water flowing over step increases speed d.t less friction-> greater erosive power-> soft rock further erodes & hard rock undercut. -> Collapses-> plunge pool(abrasion)->gorge

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Potholes formation (small circular hollows)

d.t abrasion as turbulence swirls a rivers bedload in circular motion-> rub & scrape out holes

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Meander formation

1.Where there's alternating pools & riffles at equally spaced intervals. 2.Greater energy & erosive power in pools (deep),energy lost over riffle (shallow) d.t friction.3.Max flow concentrated to one side d.t spacing between riffles & pools. ...

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... 4.Turbulence increases in & around pools as the water speeds up, so flow begins to twist & coil-> 5. helicoidal flow-> spiral from bank to bank between pools. 6. Helicoidal flow-> more erosion & deepening of pools.

... also causes eroded material to deposit inside of the next bend, where river lose energy. 7.Both erosion & deposition exaggerate the bend until large meanders are formed-> creates asymmetric cross section.

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Oxbow lakes

neck of meander broken through often during flooding & deposition dams off loop

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Braiding

when river carries lots of eroded material and rivers speed drops or load becomes too heavy, sediment is deposited in channel. River divides into many small channels that rejoin

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Levees

natural raised embankments formed when river overflows its banks & deposited

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Deltas

when river reaches sea, energy is absorbed by sea. so river deposits load-> build up on sea bed until it rises above sea level, partially blocking mouth. River braids into several distributeries to reach sea-> delta

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Flood plains

flat land either side of river that gets flooded. When river overflows there's an incr in wetted perimeter & reduction in hydraulic radius. It increases friction-> reducing velocity -> deposition of fine silt & sand

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What is river rejuvenation?

lowering of base level- by CRUSTAL UPLIFT or FALL OF SEA LEVEL. Drop in base level increases vertical erosion potential. long profile of river increased & a knickpoint (often a waterfall) marks junction between original & new base level

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What are the landforms associated with rejuvenation?

River terraces- former flood plains that have been left above the level of present day flooding following increased vertical erosion. Incised meanders- river keeps its meandering course as vertical erosion increases.= deep, wide,steep.

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Physical factors that incr risk of flooding (7)

1. Heavy rainfall 2. Snow melt 3. Sparse/lack of veg 4. impermeable ground 5. Circular drainage basin 6. High drainage density 7. steep slopes (EXPLAIN ALL)

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Human factors that incr risk of flooding (5)

1. Urbanisation (impermeable, gutters/drains) 2. Deforestation( reduces ET, soil is loose & eroded by rainwater-> carried to river-> raises river bed-> channel capacity reduced -> can easily flood). 3.Flood management strategies e.g dams can fail

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Human factors continued...

4. Agriculture- overgrazing-> less veg-> less interception/ET. Soil erosion. 5. Climate change- could = incr rainfall & more storms -> incr flooding

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Social impacts of flooding (5) -ve

1. death 2. sewage contamination-> lack of clean water. 3. contaminated water-> diseases (diarrhoea) 4. Damaged possessions 5. Property damaged->homeless

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Economic impacts of flooding (5) -ve

1. Closure of businesses and power supplies are affected .2.Rescue work & repairs-> costly. Insurance premiums rise after floods. 3. incr unemployment as businesses shut down 4. infra & transport destroyed 5. crops destroyed-> food price may incr

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Env. impacts of flooding (4) 2 +ve & 2 -ve

1. river contamination 2. River banks eroded 1. River sediment carried onto floodplain-> more fertile 2. Wetlands can be created e.g marshes & ponds-> habitats

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How is flood frequency calculated & how is it worked out?

Predicted using FLOOD RETURN INTERVAL (RI). See the history of floods and work out the interval.

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What method is used to choose which places are protected?

Cost-benefit analysis

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What is the difference between hard & sot engineering?

Hard- man-made structures to reduce floods. Soft- use knowledge of whole river basin & work with nature

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Give 4 types of hard engineering

Dams, Diversion spillways, Channel straightening, Levees

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Dams how it works & Advantages

Huge walls built across rivers, reservoir (artificial lake) formed behind it, prevents flooding down stream & releases it at a steady flow throughout yr. ADV- 1.Often have turbines-> elec 2. Recreational activities e.g sailing 3. Irrigation.

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Dams disadvantages

1.v.expensive 2.Destroys farmland & Ppl forced to move 3.Habitats lost 4.Traps sediments->dam can fail & can cause further erosion downstream d.t lack of protective deposited sediments 5.Affect wildlife e.g prevents salmon migrating upstream to breed

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Channel straightening how it works & Advantages

Meanders removed by building artificial cut-throughs-> so water flows faster -> reduces flooding as water drains down stream quicker and doesn't build up too much. ADV- shortened, so less time to navigate river.

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Channel straightening disadvantages

1. flooding may happen downstream instead as flood water carried there faster 2. More erosion downstream d.t flows faster 3. Disturbs wildlife

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Levees (how it works, Adv & Disadv)

Embankments built along river-> incr river capacity-> so floods less often. ADV- 1.Allows flood plain to be built upon DISADV- 1. Quite expensive, 2.Risk of severe flooding if breached

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Diversion spillways (how it works, Adv & Disadv)

Takes water elsewhere. Water normally diverted around important area or in another river. Discharge reduced-> prevent flooding. Have gates-> controllable. D- 1.rejoining itself/another river=floods 2.If overwhelmed-> floods areas not used to flooding

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Give 5 soft engineering methods

Land use management, Wetland & river bank conservation, River restoration, Alteration of urban surfaces & Flood warnings/ weather forcast

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Land use management- how it works & Advantages

Planning restrictions prevents construction of buildings/roads. Floodplain is restricted to things like parks, playing fields & allotments. More water can infiltrate-> less runoff-> floods less likely. ADV- 1.No damage to new buildings 2. recreation

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Land use management -disadvantage

1.Restricts dev- problematic esp.when housing is short 2. Cant be used in already urbanised areas

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Wetland & riverbank conservation- how it works

wetlands store water-> reduces flooding downstream. conserving them-> natural protection from flooding. Afforestation along riverbank-> incr interception & lag time-> reduce discharge & floods

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Wetland & riverbank conservation Adv & Disadv

1.veg- protects from soil erosion 2. veg-> habitats 1. less land available for farming

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River restoration -how it works & Adv

Making river more natural e.g removing man made levees. so flood plain can then flood naturally-> decreasing discharge & floods. ADV- 1.Little maintenance needed, as river is left in natural state 2. Better habitat.

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River restoration Disadvantages

Local flood risk can increase esp if nothing done to prevent major flooding

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Alternation of urban surfaces (how it works, Adv & Disadv)

Building porous pavements/soakaways -> incr infiltration-> incr lag time -> reduce discharge & flooding. ADV- Any pollutants in water filtered out by soil before reaching channel. D- Expensive

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Whether forecast & flood warnings (how it works, Adv & Disadv)

Env Agency monitors weather, rainfall & river discharge. warnings via radio, tv etc. A- Quick evacuation-> saves lives, Can move possessions & Use sandbags to stop damage.D- Not all have access, May happen too fast, May be ignored if wrong in past

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Why is soft engineering more sustainable than hard engineering?

Cheaper, Doesn't interrupt natural processes- integrates with natural env & creates areas like wetlands-> habitats for wildlife, Lower economic cost & env impact

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Rivers, Floods and Management

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