GG2021: GEOMORPHOLOGY

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GG2021 GEOMORPHOLOGY

• Landform process and landscape history
• Course Material http//www.gg.rhul.ac.uk/Thompson
  /teaching.html

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GG2021 GEOMORPHOLOGY

• Non Assessed Assignments
• Essay ( to be set 24th October), deadline 4th
  December
• Three Google Earth Practicals
• Assessed Coursework 33
• One Assessed Essay, 3000 words (Spring Term)
• Examination (May 2008) 67
• Answer THREE questions from NINE

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GEOMORPHOLOGY

• the science concerned with the form of the
  landsurface and the processes which create it
• the relationship between landforms and the
  processes currently acting on them
• .consideration of past events that may have
  helped to shape the landscape
• To a significant extent, then, geomorphology is
  a historical science
• (Summerfield, 1991)

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Geomorphology
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Geomorphology
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Geomorphology Links to allied disciplines

• Geology Machanisms rates of tectonic uplift
• Sedimentology reconstruction of past erosional
  events from sedimentary sequences
• Hydrology frequency intensity of runoff
  flooding
• Climatology Effects of climatic change on
  nature and intensity of geomorphic processes
• Pedology Effect of soil properties on slope
  stability
• Biology Vegetation controls on erosion rates
• Engineering techniques for analysis of slope
  stability.
• (After Summerfield, 1991)

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The Global Hydrological Cycle(Figures x1000km3)
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Hillslope Hydrology
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Quickflow
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Runoff River/Slope Linkage

• Quickflow
• Storm runoff in rivers
• Baseflow
• Low flow, between significant rainfall-runoff
  events in rivers.

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The River Flow Hydrograph
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River levels on the Medway downstream of Yalding
rainfall at Dunks Green Oct Nov 2000
Quickflow
Baseflow
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Hillslope Hydrology Soil Water Zones
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Soil Moisture Content

• Water Content Measurement
• Weigh wet soil, dry the soil and take weight
  difference.
• Quantifying soil moisture
• Volumetric Moisture Content Volume of water
  associated with given volume of dry soil (m3)
• Mass Water Content mass of water for a given
  mass (eg 1kg) soil

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Volumes of air/water for 100g of a well
granulated silt/loam (Brady, 1990)
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Properties of Water relevant to soil moisture
retention movement

• Water Molecule H-O-H is a POLAR MOLECULE.
• Asymmetrical
• Polarity hydrogen end ve charged, oxygen end
  ve.
• Electrostatic ion attracted to ve charged clay
  surfaces.
• Water molecules are also attracted to each other
  they share hydrogen atoms in a HYDROGEN
  BONDthus high boiling point.

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Soil water retention Hydrogen Bond Effects

• Water retention can be by
• ADSORBTION (ADHESION)- between water molecules
  and solid particle surface
• Or
• COHESION Attraction between water molecules.
  This force also results in Surface Tension that
  produces inward forces at a soil-air interface.
  It causes Capillarity.
• These forces also control soil water movement.
• They also cause plastic behaviour in Clays (See
  Mass Movement session)

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Capillary Forces

• Due to 1) Adhesion 2) Cohesion.
• Capillarity is greater in a low radius tube (soil
  pore).
• Capillary rise is inversely proportional to tube
  diameter, and directly proportional to surface
  tension.
• H 2T/rdg
• for Water H0.15/r
• H height of rise, T surface tension, rradius
  of tube, ddensity of liquid, ggravity.

r
Small pores hold water more strongly than large.
Small pores fill first on wetting and drain last
on drying.
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Soil Water Energy movement Total Potential
Source Brady (1990)
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Elements of soil moisture energy.

• Soil water movement is related to its FREE
  ENERGY.
• Soil water moves from areas of HIGH to LOW free
  energy. Energy controlled by gravity (height) and
  water content (wetness) of soil.
• Components
• Matric Potential Due to adhesion capillarity.
  Suction force in unsaturated conditions.
• Osmotic Potential Attraction of dissolved ions
  for the water.
• Gravity Potential Pulls water DOWNWARD.

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Soil Water Potential

• TOTAL SOIL WATER POTENTIAL Gravitational
  Potential Matric Potential Osmotic Potential
• TERMINOLOGY! Matric potential pressure
  potential matric suction soil moisture tension.

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GRAVITY POTENTIAL

• Attracts water to the centre of the Earth.
• Gravity Potential g x h
• (g acceleration of gravity, h height of soil
  water above a reference elevation)
• Water will always move vertically down a soil
  profile when gravity alone is acting.
• g is constant so measured by height above datum
  (h). Eg height ASL, height above slope base in
  m.

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MATRIC POTENTIAL

• Matric Potential is caused by CAPILLARITY
  ADHESION.
• Net effect of these processes is to REDUCE soil
  water free energy compared to un-adsorbed water
  (Brady).
• Unsaturated Conditions Matric Potential -VE
  (Suction)
• Saturated Conditions Matric Potential 0.0
• Groundwater or Springs Matric Potential VE
  (Pressure)
• Matric Potential DIFFERENCES within a soil will
  lead to water movement. May be enough to
  overcome GRAVITY.
• Water moves from WETTER (High energy potential)
  to DRIER (low energy potential) zones.

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Osmotic Potential

• Due to presence of SOLUTES in soil water. They
  reduce free energy of the water as the dissolved
  salts attract the water molecules (Brady).
• Osmotic potential has LITTLE effect on water
  movement in soils but is essential for root
  uptake of water by plants.

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Matric Potential measurement
Tensiometer

• Tension or Suction is an expression of soil water
  (matric) potential (Brady, 1990).
• Measured by Field Tensiometers.
• Fill tensiometer with water and place in soil.
• Water is drawn through tip into the soil until
  potential in tensiometer the soil is the same.
• Pressure is read on gauge.

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Using Tensiometers to measure matric suction
Installed tensiometers pressure gauges visible
Installing tensiometers Note that a nest of
several instruments will allow measurements which
will indicate water movement
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Quantifying Matric Potential

• Matric Potential is expressed in terms of
  atmospheric pressure at sea level (Bar) or in
  terms of height of a water column whose weight
  equals the potential being considered.
• Ht Water Column (cm) Soil Water Pot
  (bars) Potential (MPa)
• 0 0 0
• 10.2 -0.01 -0.001
• 102 -0.1 -0.01
• 306 -0.3 -0.03
• 1020 -1.0 -0.1
• 15300 -15 -1.5
• 31700 -31 -3.1

DRIER
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Soil Matric Potential Curve (Characteristic)
Shows how much water the soil can hold at
different suctions. Related to pore size.
Clays hold more water at given potential as the
pores are small, hence high adhesion surface
area higher than coarser loams or sands. Clays
retain water at lower potentials( to right).
Sand Moisture levels fall quickly at high
potentials as pores are large and
capillarity/adhesion element is thus low
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Soil Matric Potential curve in loam soil.Source
Brady (1990)
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Soil Water Movement

• SATURATED FLOW ALL pores full of water.
• Flow of water between two points in a soil is due
  to a DIFFERENCE in soil water total potential.
• UNSATURATED FLOW
• Flow in pores partially filled with water.
  Larger pores often air filled and thus
  non-conducting. Smaller pores are water
  filled.

HIGH POTENTIAL
Water Flow
LOW POTENTIAL
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Water Movement in soils

• SATURATED CONDITIONS
• 1) TOTAL POTENTIAL DIFFERENCE( HYDRAULIC
  GRADIENT) - F
• 2) HYDRAULIC CONDUCTIVITY The ability of the
  soil matrix to allow water movement. Total flow
  rate in soil pores is proportional to 4th power
  of the pore radius. Units Length/Time eg m/sec
• DARCYS LAW
• Rate of Soil Water Movement (V)
• Velocity Ksat x F
• Ksat Saturated Hydraulic Conductivity of the
  soil matrix
• Matric Potential is ZERO so Hydraulic gradient
  Height Difference
• Ksat depends on
• Size of soil pores
• Configuration of soil pores eg tortuosity
• Frictional properties of the soil particle
  surfaces

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Porosity total volume of rock or soil
represented by voids.
Well sorted- permeable clasts
Well Sorted-impermeable clasts
Solutional voids in soluble carbonate rocks
High Ksat
Fractures
Voids part filled with cements
Poorly sorted clasts
Low Ksat
Source www.geo.vu.nl/.../3-mineralsrocks/porosit
y1.htm
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Pore structure hydraulic conductivity
Tortuosity Friction
High Ksat
Lower Ksat
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Hydraulic Conductivity

• At HIGH matric potential (eg WET CONDITIONS)
• 1. Hydraulic Conductivity of Sandy Loam gt Clay.
• At LOW matric potential (eg DRY conditions)
• 2. Hydraulic Condutivity of Clay gt Sandy Loan
• Why?

Sandy Loam
Clay
WET
DRY
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Water Movement in soils

• UNSATURATED SOILS
• Macropores are air-filled and non-conducting.
  Finer pores are water filled and conducting so
  friction is higher and velocity of flow is lower.
• F TOTAL POTENTIAL DIFFERENCE (HYDRAULIC
  GRADIENT) HEIGHT DIFFERENCE MATRIC POTENTIAL
  DIFFERENCE
• Darcys Law
• V Kunsat x (F)
• Kunsat unsaturated hydraulic conductivity
  (units m/sec)

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Soil water movement in unsaturated conditions
WET SOIL High Matric Potential Low suction
DRIER SOIL Low matric potential High suction
..if matric potential exceeds gravity
Water movement
Water movement
DRIER SOIL Low Matric Potential High suction
WET SOIL High Matric potential Low suction
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Unsaturated water flow in Soils

• Rate of water movement from moist to dry soil
  with THREE moisture levels (See Brady 1990).
• Higher the soil moisture content the greater the
  potential gradient and soil water movement is
  MORE RAPID.

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Soil water retention

• Water retention can be by
• ADSORBTION (ADHESION)- between water molecules
  and solid particle surface
• Or
• COHESION (Between water molecules)