geography weathering, mass movement, rivers

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  • Created by: charlie
  • Created on: 24-10-13 16:26

definitions

denudation: general lowering of the earths surface due to processes over the long term  

  • processes: EROSION, WEATHERING, MASS MOVEMENT 

erosion: breakdown and transport of rocks 

  • agents: RUNNING WATER, WAVES, ICE, WIND 

weathering: breakdown of rocks IN SITU 

  • processes: PHYSICAL, CHEMICAL, BIOLOGICAL MEANS 

mass movement: process soil, sand, regolith (broken parts of rock- no humus) and rock move downslope as mass, under gravity 

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weathering- rates + adv/disadv

depends on :

  • climate (rainfall/temp.) -expeansion + contraction 
  • amount of vegetation - detritivores inc. acidity of soil 
  • chemical make up of rock - feldspar is weakness in granite 
  • rock structure (bedding planes + joints) - cracks allow water/gases to get in 

adv: 

  • helps create deep fertile soils 
  • create fantastic landforms for tourists (pot holes, cave systems, limestone pavements, stalactites+ stalagmites, swallow holes, dolines, shakeholes) 

dis adv:

  • creates unstable slopes 
  • damages historical buildings, monuments, statues 
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weathering classifications- physical

"physical break up of rocks owing to internal + external stresses caused by weathering agents"

FREEZE THAW: 

  • breakdown of rocks due to water freezing in cracks- expanding (9.05%)- exerts pressure of 2115 kg/cm2 downwards 
  • most effective in jointed rocks (granite) + alternating cycles cause rock to split + break 

PRESSURE RELEASE:

  • igneous rocks(granite) formed under pressure- weathering + erosion overlying rocks- underlying rocks pressure released + expand - fractures appear + vulnerable to weathering agents 
  • creates dialation joints + cracks parallel to surface + sheeting of rock layers 

EXFOLIATION/INSOLATION:

  • large temp change- heating + cooling (expand + contract)- stresses in rock layers- outer layers heat + coo quicker than inner- outer layers peel off 
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weathering classifications- chemical

"processes which lead to decomposition/breakdown of solid rocks by chemical reactions"

CARBONATION:

  • significant in areas dominated by limestone + chalk - creates limestone pavements + caves
  • rain+ CO2 - weak carbonic acid - insoluble CaCO - soluble CaCO2 + runs away in (aq) 

OXIDATION 

  • oxygen dissoves in water -reacts with rock minerals- (especially iron) forms oxides+hydroxides
  • weakens + disintergrates rocks + discolours 

HYDROLYSIS

  • water+ rocks minerals = insoluble precip. 
  • e.g. feldspar (granite) ---> kaolinite (china clay) 
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weathering classifications- biological

"group of procesess produced by actions of plants+animals"

ROOT ACTION

  • plant roots widen rock cracks, increasing their disintegration rates.

EXCAVATION

  • animals may excavate (dig) into soil or rock.

ACIDIFICATION

  • carbonation - plant canopies (especially coniferous ones) + organic matter inc the acidity of water 
  • equals more chemical weatherig processes such as carbonation. (ie. Pine forest)

RAHNS INDEX 

  • 1-8 legibility scale on gravestonesto calculate rate- text depth, N or S facing, width of stone 
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mass movement - flows

  • both flows characterized by materials no larger than sand particles 

MUDFLOWS: 

  • FASTER + fluid end of spectrum - constitency can vary 
  • just below the speed + wetness of a river 

EARTHFLOWS 

  • THICKER in consistency 
  • involve deeper material movements 
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mass movement - slides

  • very seasonal 

LANDSLIDES 

  • "movement of any mass of material, rock or regolith along a slip plane (icluding rotational slumps + slides) " 
  • high magnitude + low frequency + very life threatening 
  • slip plane is weak (concave) 
  • inc. rainfall, little veg. + low evap causes rotational slumps in permeable rocks (sanstone) leaving crescent shape scar 
  • C.S: aberfan 21/10/66 - 
  • heavy rain, sandstone + coal, steep, spoil heap no.7 built above spring removing clay 
  • 147 dead, 116 pantglas junior school 

ROCKSLIDES 

  • effect individual masses of rock - dry and FASTER 
  • large scale movements influence large parts of slope 
  • bedding + jointing influences rock detachment + slide planes 
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mass movement - heave

  • downslope movement caused by expansion + contraction 

SOIL CREEP 

  • steep grasses slopes- turf only binded to top layer soil- lower layer moves 
  • if moist inc. weight + inc. friction - particles move past each other 
  • controlled by -particle size -particle shape -MAV 

SOLIFLUCTION (soil creep plus)

  • takes place in permafrost regions (permanently frozen) 
  • heave up to 40 cm 
  • moist ground undergoing volumetric expansion when frozen 
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mass movement - falls

  • movement off steepest bare rock slopes (40 degrees +) 
  • fragments fall + bounce 

responsible:

  • thermal or freezing expansion 
  • water pressure in pores or joints 
  • pressure release jointing 
  • chemical activity 
  • very seasonal due to chlimate related factors above 
  • maximum in spring/ atumn when most water present 
  • can be triggered by natural disasters 
  • FASTEST MASS MOVEMENT 
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drainage basin INPUTS + STORES

"area drained by one river and its tributaries. water + sediment are moved downslope by river from highland to sea or lake"

INPUTS:

  • only precipitation in form of : rain, snow, hail,sleet...
  • snow - delayed response (winter inc. input dec. evap. + interception)
  • INTENSITY - in short space of time river system + basin struggle to cope 
  • ROBERT E HORTON - overland flow occurs when input > infiltration  
  • DURATION - precip. long time = soil saturated + water table at surface + high surface run off

STORES (in sequence)

  • INTERCEPTION (vegetation - evergeren VS deciduous   broad leaf VS small leaf    natural VS manmade + other surfaces above groun catch precip.)
  • SURFACE STORAGE (puddle to large lake, happens more in impermeable ground) 
  • SOIL WATER STORAGE (essential for plant growth) 
  • GROUNDWATER STORAGE (permeable rocks- large stores of groundwater called aquifers - top surface called water table) 
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drainage basin FLOWS

FLOWS 

  • THROUGHFALL - rain misses plant or not intercepts 
  • STEMFLOW - rain drops collect and trickle down plant trunk or stem 
  • LEAFDRIP- drips fall off end of leaves 

INFILTRATION (e.g coarse sand = rapid infiltration clay based soil = slow infiltration)

  • movement of water from above surface into soil 
  • depends on: -amount of water in soil   -porosity + structure of soil   -type + extent of vegetation (helps as provides pathway for water to enter soil)
  • PERCOLATION - movement of water down through soil into permeable rock down water table 
  • OVERLAND FLOW (FASTEST) - type of surface run off in heavy rainfall across impermeable/saturated surfaces - very fast causing rising limb on hydrograph 
  • THROUGH FLOW - above water table- movement of soil downslope below surface down towards river at the valley floor. if appears at surface on a valley side a spring will form 
  • GROUND FLOW/BASEFLOW - under water table- water moves downslope+contributes water to river in valley floor-water table meets surface at valley side a spring forms
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drainage basin OUTPUTS

RIVER RUN-OFF 

  • river channels are stores + flows - water stored in river as it flows 
  • water can be stored in lake for years or water carried to sea + output from drainage basin 

EVAPORATION 

  • liquid water evaporates into vapour + leaves drainage basin to atmosphere 

TRANSPIRATION 

  • water enters plant roots from soil (store) - travels up stem- leaves leaf via stomata 
  • occurs as part of the process of photo. + greater during growing season (summer in UK)
  • evap. + trans. both transfer water to atmosphere + combined as EVAPOTRANSPIRATION 
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rivers

  • WATERSHED - boundary between 2 drainage basins 
  • SOURCE - river originates 
  • MOUTH - enters sea/lake or leaves drainage basin 
  • DISTRIBUTARY - river splits to smaller streams 
  • TRIBUTARY - smaller streams join river 
  • CONFLUENCE - point where tributary + river meet 
  • EPHEMERAL CHANNEL - grass bent over (rarely occupied only in major floods)
  • BLUE LINE SYSTEM - rivers shows on a map 
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drainage basin URBAN AREAS

STORES :

  • interception - houses and settlements 
  • soil moisture 
  • groundwater 

FLOWS

  • drip 
  • infiltration 
  • percolation 
  • throughflow - extremely quick due to STORM DRAIN 
  • baseflow - very little due to STORM DRAIN 

% of water reaching river much higher + faster = FLASHY RESPONSE 

very little evap 

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C.S Boscastle flood

  • august 2004 

input

  • a months rain (200mm) in one hour 
  • remains of hurricane Alex 
  • peak discharge 140m3/s between 5:00 and 6:00

basin 

  • steep valley + thin soils accel. rainfall + concentrated on village 
  • actecedent conditions- saturated land + impermeable shale (clay) meant no infiltration 
  • circular basin + little vegetation funneled water in (small bridge area) 

effects 

  • 'brown willy effect' highest point (420m) vapour rises + cools forming clouds + precip. 
  • roads blocked (no emergency exit) people trapped + homeless
  • 6 weeks of the tourist season lost - witchcraft museum 
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C.S Boscastle flood defences

BRIDGE WIDENED 

  • railings above it to allow water to flow over 
  • larger volume can flow under 

LANDSCAPING 

  • tributaries + vegetation + embankment lowers flood risk 

DREDGING 

  • allows extra capacity 

STORM DRAIN (CULVERTS) 

  • as soon as water hits basin it travels through + out 

RENOVATED BUILDINGS 

  • raised up + grooves allow boards to be placed in as flood defence system 
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downstream changes in river system

BRADSHAW:

  • "river system is an organised/ordered transport system for water + debris. in general rivers are at or close to equilibrium."

UPPER COURSE

  • inefficient- high friction (low volume) + high PE 
  • water moves in all direction 

MIDDLE/LOWER COURSE 

  • slighty HIGHER VELOCITY as tributaries meet + more efficient flow 
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UPPER COURSE EROSION LANDFORMS - interlocking spur

  • v-shaped valley formed by constant movment of river channel as result of PE (gravity) and KE (downhill)
  • + hydraulic action + abrasion channel deepended 
  • soil creep + mass movement cause vertical erosion resulting in further deepening 
  • takes most effiecint + less energy consuming routes + therefore bends around objects 

E.G. - Ashes hollow - tibutary of the river severn 

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UPPER COURSE EROSION LANDFORMS - potholes

  • hard rock sediments flowing freely along river bed trapped in small depressions + swirl 
  • abrasion, turbulent flow around + over object causes sediments to wedge further + widen eventually a vortex or eddy is formed
  • CAVITATION - bubbles collapse where water is broken up + exert pressure on rocks as shockwave loosening material 
  • takes place in porous limestone rocks - small cracks/fissures makes sediments cracked 

e.g river clyde 

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UPPER COURSE EROSION LANDFORMS - rapids + waterfal

RAPIDS - high force + low force

  • downstream from waterfall where is has once stood 
  • happen at knick point of rejuvanating rivers where bed is at 2 diff. levels 
  • can also be due to hard rock standing above soft rock 
  • large jagged bed load create low efficiency therefore turbulence is name "white water" 

WATERFALLS 

  • result of differential erosion - softer rock erodes faster creating step 
  • HYDRAULIC ACTION pronounces step 
  • water forced over at high speeds creating plung pool - headwardly erodes rock underneath 
  • top heavy rock falls then used as tool of abrasion + hydraulic action 

e.g. high force waterfall, river tees NE England 

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bradshaw's model

as you go DOWNSTREAM things that INC. are: 

  • discharge - (volume of water passing through cross section per unit of time m3/s)
  • load quantity 
  • occupied channel width (distance across actual channel measured at waters surface) 
  • channel depth (height from water surface to bed) 
  • average velocity (speed of water flow measured 40% up) 

things that DEC. are:

  • load particle size 
  • channel bed roughness 
  • slope angle (gradient)
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velocity

THALWEG 

  • place of maximum velocity : 
  • just below suface + mid point between two banks (air resistance and friction) 
  • ISOVELS = lines of equal velocity 

AVERAGE VELOCITY = 40% up from bed 

  • inc. due to "F" dec. + smaller smoother load 
  • despite PE decreasing 

A.V shown by HYDRAULIC RADIUS (cross sectional area/wetted perimeter) 

to get largest hydraulic radius = largest C.S.A/smallest W.P

MEAURE A.V = measure passage of object over 10m then times by 0.8 

  • use flowmeter record 1 min (counts/min) 
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measurements of discharge

DISCHARGE (m3/sec)(cumecs)(q) = CROSS SECTIONAL AREA(m2)  x  VELOCITY (m/s)

method: 

  • stretch measuring tape between 2 banks 
  • peg tape so tort 
  • measure channel depth + water depth at regular intervals (e.g.25 cm)
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river transport

rivers perform 3 main functions : 

  • EROSION 
  • TRANSPORTATION
  • DEPOSITION

bedload = particles moves or rolls + always in contact with bed 

how it happens:

  • particle rocks on bed + lifts slightly into faster flowing water 
  • rolling action rocks off corners + gets pushed forward 

saltation happens by bedload:

  • other particles pushed into motion due to impact of intial particle = "BEROULLI EFFECT"

suspension load:

  • bedload + saltation load become suspension load as velcoity inc. prevalent in lower course 
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hjulsrom curve

  • particles smaller then 0.2 mm difficult to pick up (miscroscopic + platelet shape means water moves over) creating no variation in velocity therefore no lift.
  • large SA:VOL +ve charges means particles cohesive + stick together 
  • CLAY 0.001mm stays in suspension  + floculates at sea (fresh wate meets salt water)
  • coarse/ sand materials >0.1 mm are deposited due to weight + size
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