Mass Movement: Factors and Processes

• Mass movement involves the downslope movement of broken-down rock or material due to gravity, causing disruption and posing dangers by rapidly affecting large areas.

Introduction to Mass Movement

• Regolith, consisting of loose and weathered rock, soil, and mud, is moved downslope by gravity in the process of mass movement.

Factors Affecting Mass Movement Speed

• Natural Processes
• Tectonic Activity
• Human Activity

Natural Processes Explained

• Gradient: Steeper slopes lead to faster mass movement, while gentle slopes result in slower movement.
• Water Content: Higher water content in regolith accelerates mass movement, especially after heavy rainfall when soil becomes saturated and acts as a lubricant.
• Slope Material: Consolidated materials like solid rock resist mass movement, while unconsolidated materials like sand or boulder clay are more prone to it.
• Vegetation: Vegetation roots bind soil, reducing mass movement; deforestation destabilizes slopes.

Tectonic Activity Explained

• Volcanic Activity: Releases material downslope, causing lahars and mudflows.
• Earthquakes: Trigger mass movement through vibrations and surface changes, leading to ground instability.

Human Activity Explained

• Human Activity: Construction, deforestation, and poor farming practices destabilize land, leading to mass movement.
• Road Construction: Undercuts slopes during foundation digging, reducing stability.
• Deforestation & Poor Farming: Weaken soil structure, increasing the likelihood of mass movement.

Types of Mass Movement

• Slow movements
• Moderate movements
• Fast movements

Slow Mass Movement

• Soil Creep
• Solifluction

Soil Creep Explained

• Soil creep is the slowest form of mass movement, occurring at a maximum rate of 0.5cm per year on gentle slopes between 10 and 15 degrees.
• Freeze-thaw action and wetting/drying cycles are key processes in soil creep.
• Movement is greatest near the surface due to less compaction.
• Thick vegetation areas experience more soil creep as roots bind the regolith.
• Evidences of soil creep:
  • Terracettes
  • Tilted Vertical Objects
  • Curved Tree Trunks
  • Collapsed Stone Walls
  • Cracked Roads and Pavements

Solifluction Explained

• Solifluction occurs in high-altitude, frozen-soil environments
• During summer, surface soil thaws but deeper layers stay frozen
• Rainfall saturates the thawed soil, which cannot drain through the permafrost
• Saturated soil flows slowly downslope due to gravity, forming thin folds

Moderate Mass Movement

• Earthflow
• Slumping

Earthflows Explained

• Earthflows occur on hills after heavy rain where regolith lies over impermeable rock.
• Saturated soil caused by heavy rainfall loses friction and flows downslope.
• Deforestation increases likelihood as roots no longer bind soil.
• The saturated soil is no longer sheltered from the rain.
• Evidence: Small bulges or torn vegetation.

Slumping Explained

• Slumping occurs on steep slopes of unconsolidated material when the base is removed
• A large block of regolith slides downwards due to gravity in a slump rotational slide
• Road construction or undercutting by water can trigger slumping
• Evidences of slumping can be seen along the boulder clay cliffs such as Cregg Beach in Lahinch, Co. Clare.
• A steep, crescent-shaped cliff forms on the upper slope point.

Fast Mass Movement

• Rockfalls
• Landslides
• Mudflows
• Lahars
• Bogbursts
• Avalanches

Rockfalls Explained

• Rockfalls occur on steep slopes when solid rock blocks break free and fall at high speeds.
• Freeze-thaw action typically causes this weathering
• Over time, rock accumulates at the base to form scree fields.

Landslides Explained

• Landslides occur in mountainous and coastal areas when large rock sections fracture
• They are common on sedimentary rock with fractures along bedding planes
• Loose regolith slides off due to gravity, often after rainfall reduces friction
• Coastal erosion and seismic activity can also trigger landslides.

Mudflows Explained

• Mudflows occur when soil is severely saturated by heavy rainfall
• They are most common on steep slopes with soil over impermeable rock Soil with over 30% water flows downslope due to reduced friction at speeds up to 100km/h.
• Flows into streams and ponds increases speed, and velocity
• High water content reduces viscosity, increasing speed and distance.
• Debris accumulation increases during flow.

Lahars Explained

• This is the deadliest form of a mudflow and occurs as a result of volcanic eruption
• Ash and lava cause snow to melt and mix with water, becoming boiling mud
• The mixture moves rapidly down mountain slopes, destroying anything in its path
• Lahars and mudflows near towns and villages can result in high loss of life.
• Example: 1980 Mt. St Helens eruption.

Bogbursts Explained

• These occur when saturated peat along blanket bog slopes moves downslope due to gravity
• Heavy rainfall saturates the peat, causing it to flow downslope.
• Human activity, especially during wind farm construction, can cause bogflows
• Common in mountainous regions of western Ireland, such as blanket bogs.

Avalanches Explained

• Avalanches are one of the fastest and most deadly forms of mass movement
• Rock avalanches
• Snow avalanches

River Processes

• Rivers shape the landscape through erosion, deposition, and transportation
• River effects are determined by discharge, velocity, and flow

Parts of a River Basin

Drainage Patterns

• Drainage patterns are influenced by landscape and rock types
• Dendritic
• Trellis
• Radial
• Parallel
• Deranged

Dendritic Explained

• The most common type of drainage pattern in Ireland
• Resembles a tree in winter

Trellis Explained

• Tributaries meet the main river at right angles

Radial Explained

• Tributaries flow from a circular or oval-shaped mountain

Perpendicular Explained

• Rivers flow alongside each other without joining

Deranged Explained

• Tributaries flow in a disorganized or aimless manner

River Processes

• Erosion
• Transportation
• Deposition

River Erosion

• Hydraulic Action: Sheer force of water enlarges joints, breaks off fragments, and washes away loose material.
• Abrasion: River load hits the bed and banks, loosening other rocks and soil.
• Attrition: Rocks collide and are worn down into fine particles called alluvium.
• Solution: Soluble rocks dissolve in the water.

River Transportation

• Solution
• Suspension
• Saltation
• Traction

River Transportation Explained

• Solution: Dissolved soluble rocks are carried.
• Suspension: Clay and silt particles are carried as a suspended load, giving it a brown, muddy colour.
• Saltation: Small pebbles and stones bounce along the riverbed.
• Traction: Heavy materials are dragged along the riverbed, requiring a lot of energy.

River Deposition

• A River Loses Its Velocity
• A River Loses Its Volume:
• Increased Load

River Deposition Explained

• Velocity Loss: Occurs at the old stage or due to human interference (dams)
• Volume Loss: Occurs at distributaries, during hot weather (evaporation), or due to irrigation
• Increased Load: Tributaries increase sediment, clogging the channel

Stages of a River

• Upper Course/Youthful Stage
• Middle Course/Mature Stage
• Lower Course/Old Stage

Landforms on the Upper Course of the River

• V-Shaped Valleys
• Interlocking Spurs
• Waterfalls

V-Shaped Valleys Explained

• V-shaped valleys are formed as a result of vertical river erosion that occurs due to gravity
• Potholes forming along the riverbeds also deepen the V-shaped valley

Interlocking Spurs Explained

• Interlocking spurs are upland areas that lock into each other in the valley caused by differential erosion
• The river flows around the harder rock forming a zig-zag pattern

Waterfalls Explained

• A waterfall is a vertical drop in a riverbed formed where hard rock lies upstream of soft rock
• The soft rock erodes faster than hard rock
• Turbulence creates eddy currents that deepen hollows in the riverbed by hydraulic action and abrasion forming hollows
• As the water flows over, the elevated step is deepened.
• The force of the flowing water carves out a plungepool at the base and undercuts into the back wall.
• Hard rock collapses, causing waterfall to retreat upstream (headward erosion), forming a gorge.

Landforms in the Middle Course of the River

• Meanders
• Alluvial Fans and River Braids

Meanders Explained

• Meanders are S-shaped loops that develop due to lateral erosion, with erosion on the outer bank and deposition on the inner bank.
• Increased deposition forms an uneven bed of riffles and pools, resulting in bends in the river
• Continuous erosion and deposition form point bars, move the meander sideways, and create bluff lines
• Found in most rivers across Ireland

Alluvial Fans and River Braids Explained

• Alluvial fans are built when the river deposits sediment on the riverbed above water level, causing river to split into narrow steams called river braids

Landforms in the Lower Course of the River

• Floodplain
• Levee
• Ox-Bow Lakes
• Deltas

Floodplain Explained

• The gradient is almost flat
• It is a wide, flat floor of a river valley over which the river floods in times of rainfall
• The floodwater alluvium is then sorted, causing larger items to deposit closer to the bank and smaller material to deposit further along the floodplain
• Bluff lines mark the farthest point that floodwater reaches
• Fertile floodplains are used for agricultural production

Levee Explained

• A levee is a natural ridge of material deposited along the banks of the river and formed by repeated flooding
• Levees are natural flood barriers that become more heightened due to deposition during summer months
• The floodwater is unable to drain back through the alluvium levee causing backswamps made of marshland to develop on the floodplain

Ox-Bow Lakes Explained

• An oxbow lake is a horseshoe-shaped lake that forms when the river cuts off a meander over time
• Water cuts though the narrow strip of land and straightens the river channel, separating it from the main channel
• Depositions then sealed off the ends of the meander to form a stagnant lake
• It eventually dries and is referred to a meander scar or mort lake

Deltas Explained

• A delta is a flat area of sediment deposited at the mouth of a river
• Lacustrine deltas form where rivers flow into lakes, while marine deltas flow where rivers flow into sees
• Sediment is deposited due to the river slowing down and the mixing of salt river water
• Marine deltas are created when the rate of deposition is greater that the rate of erosion
• The mouth of the river becomes clogged with sediment
• The shapes are formed dependant of the current and waves
  • Arcuate Deltas
  • Bird's Foot Deltas
  • Cuspate Deltas

Human Interference with Rivers

• The construction of dams
• The construction of canals
• The construction of artificial levees

Construction of Dams for HEP

Construction of Canals for HEP, Transportation and Flood Control

Construction of Levees for Flood Control