Physical Geology

Geological agents are natural forces that wear down, move and rebuild the earth's surface. They are:

1. Running water (rivers), the most important agent.
2. Groundwater, working below the surface.
3. Glaciers (ice).
4. Wind.
5. Sea waves, tides and currents.
6. Gravity (mass movement), volcanism and organisms (including humans).

Each agent does three kinds of work: erosion (wearing away), transportation (carrying) and deposition (laying down), which together make a landform.

Geological work of the agents

• Running water is the most powerful agent: it makes valleys, carries sediments and builds the floodplain and delta.
• Glaciers move slowly but carry huge loads and make U-shaped valleys.
• Wind acts mainly in dry, sparsely vegetated areas.
• Groundwater works mainly by solution in limestone.
• Sea shapes the coast.

The geological work of wind is called aeolian action. It is strong in deserts and dry coasts where there is little vegetation, loose sand and strong wind. It has three parts:

1. Erosion
   • Deflation: lifting and removal of loose fine particles.
   • Abrasion (sand blasting): grains carried by wind cut and polish rocks.
   • Attrition: grains wear each other down to rounded, frosted shapes.
2. Transportation: fine dust moves in suspension for long distances; sand moves by saltation (bouncing, the main way) and coarse grains by surface creep.
3. Deposition: when the wind speed falls or hits an obstacle.

Erosional landforms

• Deflation hollows (blowouts): wind lifts loose fine particles and lowers the ground, e.g. Qattara depression.
• Mushroom (pedestal) rocks: sand abrades the base of a rock more than its top.
• Yardangs: streamlined ridges of rock parallel to the wind.
• Ventifacts: pebbles with polished, flat faces cut by sand blast.
• Zeugen: tabular ridges of hard rock over softer layers.
• Desert pavement: gravel left after fine particles are blown away.
• Inselbergs: isolated residual hills.

Depositional landforms

• Sand dunes: barchan (crescent shaped, horns point downwind), transverse, longitudinal (seif) and parabolic dunes.
• Ripple marks: small ridges on sand surfaces.
• Loess: thick deposit of fine, windblown silt, uniform, porous and buff in colour (northern China). It collapses when wetted, which matters in foundation design.

Weathering is the in-place breakdown and decay of rocks at or near the surface by the action of the atmosphere, water and organisms. Erosion is the removal and transport of the loosened material by agents like water, wind and ice.

Weathering is the breakdown of rocks in place by physical and chemical processes.

Physical (mechanical) weathering breaks rock into smaller pieces without changing its mineral composition. Main processes:

• Frost wedging: water freezes in joints and expands about 9%, prying the rock apart. Common in the high Himalaya.
• Thermal expansion and contraction (insolation): repeated day-night heating and cooling stresses the surface minerals.
• Exfoliation (unloading): removal of overlying rock releases pressure, so sheets peel off massive rocks such as granite.
• Salt crystallisation: salts growing in pores push the grains apart (arid and coastal areas).
• Biological action: plant roots widen cracks; burrowing animals expose fresh rock.

Chemical weathering changes the minerals of the rock into new, more stable minerals (mostly clay) and soluble salts. It is fastest in warm, wet climates. Main processes:

• Solution: soluble minerals such as rock salt and gypsum dissolve in water.
• Carbonation: CO$_2$ in rain forms carbonic acid, which dissolves limestone: $CaCO_3 + H_2O + CO_2 \rightarrow Ca(HCO_3)_2$. This produces karst.
• Hydrolysis: feldspar reacts with water to form clay: $2KAlSi_3O_8 + 2H^+ + H_2O \rightarrow Al_2Si_2O_5(OH)_4 + 4SiO_2 + 2K^+$ (orthoclase to kaolinite).
• Oxidation: iron minerals combine with oxygen and turn into rust-coloured oxides, e.g. $4FeO + O_2 \rightarrow 2Fe_2O_3$.
• Hydration: minerals take up water and swell, e.g. anhydrite to gypsum, hematite to limonite.

Differences

Running water (rainwater, streams and rivers) is the most important agent of landscape change. Its work has three parts.

1. Erosion

Done by hydraulic action (force of water), abrasion (carried load scrapes the bed), attrition (load wears itself) and solution. Erosion is vertical in the youth stage and lateral in old age.

2. Transportation

• Solution: dissolved salts.
• Suspension: clay and silt.
• Saltation: sand bouncing along the bed.
• Traction: pebbles and boulders rolled along the bed.

3. Deposition

Occurs when velocity falls (gentle gradient, river mouth, floods).

Landforms

Erosional work of a river is done by hydraulic action, abrasion, attrition and solution. Features:

• V-shaped valley: vertical cutting in the youth stage, in the hills.
• Gorge and canyon: deep, narrow valley in hard rock, e.g. the Kali Gandaki gorge.
• Waterfall and rapids: hard rock lying over soft rock; the plunge pool forms at the base.
• Pot holes: circular holes drilled in the bed by swirling pebbles.
• River terraces: step-like old floodplain levels left after the river cuts down again.
• Meanders and ox-bow lakes: side cutting in the old stage.
• Interlocking spurs: ridges between bends of a young river.

Depositional work occurs when the velocity and load capacity fall (at gentle slope, at a mouth, in floods). Features:

• Alluvial fan: cone-shaped deposit where a mountain stream meets a plain, e.g. the Bhabar zone at the foot of the Siwaliks.
• Floodplain: broad flat valley floor built by flood deposits.
• Natural levees: low ridges of coarse silt built along the banks during floods.
• Braided bars and channel (point) bars: sand and gravel bars inside the channel.
• Delta: fan-shaped deposit at the river mouth (Ganges-Brahmaputra delta).
• Ox-bow lake and meander scars: cut-off bends.

Stages of a river

River channels are classified by their plan shape (pattern) as:

 Straight:       ----------------------
                 ======================

 Meandering:      ___      ___
                 /   \    /   \
              __/     \__/     \__

 Braided:         ___   ___   ___
                 /   \_/   \_/   \
                 \___/ \___/ \___/

1. Straight channel: nearly straight, sinuosity less than 1.05. It is rare and short, found in steep, narrow, bedrock valleys. Flow is fast; the thalweg still swings from bank to bank.
2. Meandering channel: single, sinuous channel with sinuosity above 1.5. It occurs on gentle slopes in fine, cohesive sediment. Erosion occurs on the outer (concave) bank and deposition on the inner (convex) bank as point bars. Cut-offs form ox-bow lakes.
3. Braided channel: many small channels dividing and rejoining around bars. It occurs on steep slope, heavy bed load, easily erodible banks and flashy flow, e.g. the Koshi, Narayani and Karnali rivers below the hills.
4. Anastomosing channel: several stable, interconnected channels separated by vegetated islands, on very low slopes.

Engineering significance (hydropower, bridges and river training)

• Straight reach: stable, so good for a headworks, intake, weir or bridge site. Bedrock gorges give a strong abutment and a high head.
• Meandering reach: banks migrate, so bridges, intakes and embankments need bank protection and spurs. Bend flow causes scour on the outer bank, so the intake should be placed on the outer bank side (to avoid sediment) and structures kept away from migrating bends. Cut-offs can leave an intake dry.
• Braided reach: unstable, shifting channels and high bed load. Bridges need long spans and guide bunds; intakes silt up; embankments are at risk. Needs river training works.
• General: study of channel type guides selection of dam, weir, intake and bridge sites, estimates of sediment load, flood level and scour depth, and the design of bank protection.

A volcano is an opening in the earth's crust through which molten rock (magma), gases and rock fragments are thrown out to the surface; the cone-shaped hill built round the vent is also called a volcano.

Volcanoes are classified in two ways.

1. According to activity

• Active: erupting now or erupted in recent history, e.g. Mount Etna, Mauna Loa.
• Dormant: quiet for a long time but may erupt again, e.g. Mount Fuji.
• Extinct: no eruption in historic time and no chance of future eruption.

2. According to form and eruption (shape)

• Shield volcano: broad and gently sloping, built by runny basaltic lava, e.g. Mauna Loa.
• Composite (strato) volcano: steep cone built by alternate layers of lava and ash, with explosive eruptions, e.g. Mt. Fuji, Vesuvius.
• Cinder (scoria) cone: small steep cone built of loose ash and cinders.
• Lava dome (plug): thick viscous lava piles over the vent.
• Caldera: large depression formed by collapse after a big eruption, e.g. Crater Lake.
• Fissure eruption: lava flows from long cracks and builds a lava plateau, e.g. Deccan Traps.

Mass movement is the downslope movement of soil, rock or debris under gravity, without a flowing medium like a river or glacier carrying it.

Varnes classification (1978) is based on the type of movement and the material (rock, debris, earth):

• Fall: free fall of detached blocks from a steep face.
• Topple: forward rotation of a block about a lower point.
• Slide: movement along one or more slip surfaces (rotational or translational).
• Spread: extension of a soil or rock mass above a softer layer.
• Flow: the mass moves like a viscous fluid, often water-saturated.
• Creep: very slow, continuous movement.

Darcy's law states that the rate of flow of water through a porous medium is directly proportional to the hydraulic gradient and to the cross-sectional area normal to the flow.

where $Q$ is the discharge (m$^3$/s), $K$ is the coefficient of permeability (hydraulic conductivity, m/s), $i = \Delta h / L$ is the hydraulic gradient, and $A$ is the total cross-section area (m$^2$). The Darcy velocity is $v = Q/A = K i$. The actual seepage velocity is $v_s = v/n$, where $n$ is the porosity.

   h1 ->  _________ L _________
         |   soil sample (A)   |-> Q
   h2 ->  ---------------------
   head loss  dh = h1 - h2

How water flows underground

Rain infiltrates, moves down through the unsaturated zone, and reaches the water table. In the saturated zone it moves slowly (cm/day to m/day) through pores and fractures, from high to low hydraulic head, until it comes out at springs, rivers or wells.

Applicability

• Valid for laminar flow in saturated, homogeneous and isotropic media, with Reynolds number $R_e$ less than about 1 (up to 10).
• Not valid for very coarse gravel or fractured rock with turbulent flow, very high gradients, or very fine clay at very low gradient.

Example: $K = 1\times10^{-4}$ m/s, $i = 0.02$, $A = 50$ m$^2$ gives $Q = 1\times10^{-4}\times0.02\times50 = 1\times10^{-4}$ m$^3$/s.

Answer: $Q = 1\times10^{-4}$ m$^3$/s, i.e. 0.1 litre/s.

Weathering is the in-place disintegration (physical) and decomposition (chemical) of rocks at or near the earth's surface by the action of atmospheric agents, water and organisms, without transport of the material.

Factors controlling weathering

1. Climate: temperature and rainfall. Frost action dominates in cold areas; chemical weathering in hot, humid areas.
2. Rock type and mineral composition: quartz is very resistant; olivine, feldspar and calcite weather quickly.
3. Rock structure: joints, bedding and faults let water and air enter.
4. Topography (relief): steep slopes remove debris and expose fresh rock; gentle slopes keep a thick soil cover.
5. Vegetation and organisms: roots, bacteria and humic acids speed up weathering, but dense cover also protects the surface.
6. Time: the longer the exposure, the deeper the weathering.
7. Grain size and porosity: coarse or porous rocks have more surface for reaction.

A landslide is the downslope movement of rock, soil or debris. Rock slope instability is failure of rock blocks along discontinuities (plane, wedge, toppling).

Preventive and mitigative measures reduce driving forces or increase resisting forces.

1. Surface and subsurface drainage: catch drains, lined channels, horizontal drains and drainage tunnels lower the pore-water pressure. It is the cheapest and most effective step.
2. Slope geometry change: reduce the slope angle, make benches, or remove load from the head.
3. Toe loading and retaining structures: gabion walls, retaining walls, buttresses, and toe berms.
4. Rock reinforcement: rock bolts, cable anchors, soil nails, shotcrete with wire mesh, and rock dowels.
5. Bio-engineering: grass, shrubs and trees (vetiver, bamboo) give root binding and cut surface erosion. Widely used in Nepal.
6. Rockfall control: catch ditches, rock-fall barriers, and trimming loose blocks.
7. Slope protection: stone or concrete lining, geotextiles, and erosion-control mats.
8. Avoidance, monitoring and planning: hazard zoning, route realignment, and instruments (inclinometer, extensometer, piezometer) with early warning.
9. River training at the toe: spurs and check dams to stop undercutting.

A glacier is a large mass of ice formed by compaction and recrystallisation of snow, which moves slowly downslope under its own weight. Types: valley (alpine) glaciers, as in the Nepal Himalaya, and continental ice sheets.

Erosional landforms

Glaciers erode by plucking (ice freezes to the rock and pulls out blocks) and abrasion (rock fragments in the ice scrape the bed).

• Cirque (corrie): armchair-shaped hollow at the head of a glacier, made by plucking and abrasion; a lake in it is a tarn.
• Arête: knife-edge ridge between two cirques.
• Horn (pyramidal peak): sharp peak left when cirques cut back on all sides, e.g. Matterhorn.
• U-shaped valley (glacial trough): the glacier widens and deepens a river valley.
• Hanging valley: side valley left high above the main trough; it often ends in a waterfall.
• Fjord: glacial trough drowned by the sea.
• Roche moutonnée, striations and grooves: smoothed and scratched bedrock surfaces.
• Rock basins and tarns.

Depositional landforms

Material carried by ice is called till (unsorted, unstratified clay, sand, pebbles and boulders). Features:

• Moraines: lateral, medial, terminal (end) and ground moraines.
• Erratics: large boulders carried far from their source rock.
• Drumlins: smooth, elongated hills of till, shaped like an inverted spoon.
• Eskers: long, winding ridges of sand and gravel left by streams under the ice.
• Kames and outwash plains: mounds and flat areas of sorted sand and gravel built by meltwater.
• Kettle holes: depressions left where buried ice blocks melted.

  Section of a glaciated valley
  \   /\        /\   arete   /
   \_/  \______/  \_ cirque /
        U-shaped valley floor

The geological cycle is the continuous, slow, cyclic process by which the rocks of the earth's crust are formed, destroyed and re-formed into other kinds of rock. It has no start or end. It links the rock cycle, the hydrological cycle and plate tectonics.

 Magma --cooling--> Igneous rock
   ^                    |
 melting         weathering, erosion
   |                    v
 Metamorphic <--heat,-- Sediments
 rock         pressure     |
   ^                       |compaction,
   |                       v cementation
   +------heat,--------Sedimentary rock
        pressure

• Magma cools to igneous rock.
• Weathering, erosion, transport and deposition give sediments, which become sedimentary rock by lithification.
• Heat and pressure change any rock into metamorphic rock.
• Deep burial and melting return the rock to magma.

Uplift brings deep rocks back to the surface. Engineers use this idea to understand why different rock types occur at a site.

A weathering profile is the vertical section from fresh bedrock up to the ground surface, showing zones of increasing degree of weathering.

 Zone         Description
 ---------------------------------------
 VI  Residual soil   (fully altered, no rock fabric)
 V   Completely weathered (soil-like, fabric seen)
 IV  Highly weathered (more than half is soil)
 III Moderately weathered (less than half soil)
 II  Slightly weathered (discoloured joints)
 I   Fresh rock

• The classification is that of ISRM/BS 5930 (grades I to VI).
• Thickness depends on climate, rock type, structure and topography; it is deep in warm humid regions (tens of metres in the Siwaliks and Terai fringe).
• Strength, bearing capacity and the cut-slope angle fall as the grade increases.
• It is important for foundation depth, tunnel portals and slope design, because the weathered zone is weak, permeable and easily eroded.

River

Erosional: V-shaped valleys, gorges, waterfalls and rapids, potholes, meanders, river terraces. Depositional: alluvial fans, floodplains, levees, point bars, braided bars, ox-bow lakes, deltas.

Erosional work of a river is done by hydraulic action, abrasion, attrition and solution. Features:

• V-shaped valley: vertical cutting in the youth stage, in the hills.
• Gorge and canyon: deep, narrow valley in hard rock, e.g. the Kali Gandaki gorge.
• Waterfall and rapids: hard rock lying over soft rock; the plunge pool forms at the base.
• Pot holes: circular holes drilled in the bed by swirling pebbles.
• River terraces: step-like old floodplain levels left after the river cuts down again.
• Meanders and ox-bow lakes: side cutting in the old stage.
• Interlocking spurs: ridges between bends of a young river.

Depositional work occurs when the velocity and load capacity fall (at gentle slope, at a mouth, in floods). Features:

• Alluvial fan: cone-shaped deposit where a mountain stream meets a plain, e.g. the Bhabar zone at the foot of the Siwaliks.
• Floodplain: broad flat valley floor built by flood deposits.
• Natural levees: low ridges of coarse silt built along the banks during floods.
• Braided bars and channel (point) bars: sand and gravel bars inside the channel.
• Delta: fan-shaped deposit at the river mouth (Ganges-Brahmaputra delta).
• Ox-bow lake and meander scars: cut-off bends.

Glacier

• Cirque (corrie): armchair-shaped hollow at the head of a glacier, made by plucking and abrasion; a lake in it is a tarn.
• Arête: knife-edge ridge between two cirques.
• Horn (pyramidal peak): sharp peak left when cirques cut back on all sides, e.g. Matterhorn.
• U-shaped valley (glacial trough): the glacier widens and deepens a river valley.
• Hanging valley: side valley left high above the main trough; it often ends in a waterfall.
• Fjord: glacial trough drowned by the sea.
• Roche moutonnée, striations and grooves: smoothed and scratched bedrock surfaces.
• Rock basins and tarns.

Material carried by ice is called till (unsorted, unstratified clay, sand, pebbles and boulders). Features:

• Moraines: lateral, medial, terminal (end) and ground moraines.
• Erratics: large boulders carried far from their source rock.
• Drumlins: smooth, elongated hills of till, shaped like an inverted spoon.
• Eskers: long, winding ridges of sand and gravel left by streams under the ice.
• Kames and outwash plains: mounds and flat areas of sorted sand and gravel built by meltwater.
• Kettle holes: depressions left where buried ice blocks melted.

Factor of safety (FS) is the ratio of the forces (or shear strength) resisting failure of a slope to the forces (or shear stress) driving it.

• $FS > 1$: stable; $FS = 1$: limiting equilibrium; $FS < 1$: failure.
• Usual design values are 1.3 to 1.5 for permanent slopes, and 1.1 to 1.2 for temporary cuts (higher when failure would be disastrous).
• For a planar rock slide along a plane of dip $\psi_p$ with weight $W$, cohesion $c$, area $A$ and friction angle $\phi$:

Example: $W = 1000$ kN, $\psi_p = 30^\circ$, $\phi = 35^\circ$, $c = 10$ kPa, $A = 20$ m$^2$. Resisting $= 200 + 606.4 = 806.4$ kN; driving $= 500$ kN. Answer: $FS = 1.61$ (stable).

 ground  ~~~~~~~~~~~~~~~~
 unconfined: water table .  (sand)
 --------------------------
 clay (aquiclude)
 confined: sand under pressure
 clay (aquiclude)

A geological agent is a natural force or medium (running water, wind, glacier, groundwater, sea waves and gravity) that erodes, transports and deposits earth material and so changes the surface.

Geomorphic features produced by running water

Erosional work of a river is done by hydraulic action, abrasion, attrition and solution. Features:

• V-shaped valley: vertical cutting in the youth stage, in the hills.
• Gorge and canyon: deep, narrow valley in hard rock, e.g. the Kali Gandaki gorge.
• Waterfall and rapids: hard rock lying over soft rock; the plunge pool forms at the base.
• Pot holes: circular holes drilled in the bed by swirling pebbles.
• River terraces: step-like old floodplain levels left after the river cuts down again.
• Meanders and ox-bow lakes: side cutting in the old stage.
• Interlocking spurs: ridges between bends of a young river.

Depositional work occurs when the velocity and load capacity fall (at gentle slope, at a mouth, in floods). Features:

• Alluvial fan: cone-shaped deposit where a mountain stream meets a plain, e.g. the Bhabar zone at the foot of the Siwaliks.
• Floodplain: broad flat valley floor built by flood deposits.
• Natural levees: low ridges of coarse silt built along the banks during floods.
• Braided bars and channel (point) bars: sand and gravel bars inside the channel.
• Delta: fan-shaped deposit at the river mouth (Ganges-Brahmaputra delta).
• Ox-bow lake and meander scars: cut-off bends.

Geological agents are natural forces that wear down, move and rebuild the earth's surface. They are:

1. Running water (rivers), the most important agent.
2. Groundwater, working below the surface.
3. Glaciers (ice).
4. Wind.
5. Sea waves, tides and currents.
6. Gravity (mass movement), volcanism and organisms (including humans).

Each agent does three kinds of work: erosion (wearing away), transportation (carrying) and deposition (laying down), which together make a landform.

Geological cycle

The geological cycle is the continuous, slow, cyclic process by which the rocks of the earth's crust are formed, destroyed and re-formed into other kinds of rock. It has no start or end. It links the rock cycle, the hydrological cycle and plate tectonics.

 Magma --cooling--> Igneous rock
   ^                    |
 melting         weathering, erosion
   |                    v
 Metamorphic <--heat,-- Sediments
 rock         pressure     |
   ^                       |compaction,
   |                       v cementation
   +------heat,--------Sedimentary rock
        pressure

• Magma cools to igneous rock.
• Weathering, erosion, transport and deposition give sediments, which become sedimentary rock by lithification.
• Heat and pressure change any rock into metamorphic rock.
• Deep burial and melting return the rock to magma.

Uplift brings deep rocks back to the surface. Engineers use this idea to understand why different rock types occur at a site.

Weathering

In-place breakdown of rocks by physical, chemical and biological processes.

Erosion

Removal and transport of weathered material by agents such as water, wind and ice. Weathering prepares the rock; erosion carries it away.

Volcanism

Volcanism is the process by which molten rock (magma), gases and fragments move up from the interior and erupt on the earth's surface (or are intruded in the crust).

• Causes: pressure release or fault openings at plate margins (subduction zones, mid-ocean ridges) and hot spots.
• Products: lava, gases, pyroclasts (ash, lapilli, bombs).
• Types of eruption: central (explosive, with a vent) and fissure (quiet, with long cracks).
• Landforms: cones, calderas, lava plateaus (Deccan).
• Hazards: ash fall, lava flows, mud flows (lahars), poisonous gases.

Erosion is the wearing away of the earth's surface by the removal of weathered material by agents such as running water, glacier, wind, sea waves and groundwater.

• Mechanisms: abrasion (scraping by carried load), attrition (particles wear each other), hydraulic action (force of water), corrosion/solution (chemical dissolving), deflation (wind lifting fine particles) and plucking (glacier pulling out blocks).
• Types: fluvial, glacial, aeolian, marine, and groundwater erosion.
• Factors: rock hardness, slope, rainfall, vegetation cover and human activity.
• Engineering effects: scour at bridge piers, bank cutting, gully formation, siltation of reservoirs, and loss of fertile soil. In Nepal the young, steep Siwalik and Mahabharat slopes erode very fast.

Factors of weathering

1. Climate: temperature and rainfall. Frost action dominates in cold areas; chemical weathering in hot, humid areas.
2. Rock type and mineral composition: quartz is very resistant; olivine, feldspar and calcite weather quickly.
3. Rock structure: joints, bedding and faults let water and air enter.
4. Topography (relief): steep slopes remove debris and expose fresh rock; gentle slopes keep a thick soil cover.
5. Vegetation and organisms: roots, bacteria and humic acids speed up weathering, but dense cover also protects the surface.
6. Time: the longer the exposure, the deeper the weathering.
7. Grain size and porosity: coarse or porous rocks have more surface for reaction.

Volcanism (vulcanicity) is the set of processes by which magma, gases and fragmented rock rise from the earth's interior and are erupted on the surface through a vent or fissure, or are intruded into the crust.

• Volcanism produces lava, gases and pyroclasts, and builds cones, calderas and lava plateaus. Associated features are hot springs, geysers and fumaroles.

Chemical weathering

Chemical weathering changes the minerals of the rock into new, more stable minerals (mostly clay) and soluble salts. It is fastest in warm, wet climates. Main processes:

• Solution: soluble minerals such as rock salt and gypsum dissolve in water.
• Carbonation: CO$_2$ in rain forms carbonic acid, which dissolves limestone: $CaCO_3 + H_2O + CO_2 \rightarrow Ca(HCO_3)_2$. This produces karst.
• Hydrolysis: feldspar reacts with water to form clay: $2KAlSi_3O_8 + 2H^+ + H_2O \rightarrow Al_2Si_2O_5(OH)_4 + 4SiO_2 + 2K^+$ (orthoclase to kaolinite).
• Oxidation: iron minerals combine with oxygen and turn into rust-coloured oxides, e.g. $4FeO + O_2 \rightarrow 2Fe_2O_3$.
• Hydration: minerals take up water and swell, e.g. anhydrite to gypsum, hematite to limonite.

Running water

Erosional work of a river is done by hydraulic action, abrasion, attrition and solution. Features:

• V-shaped valley: vertical cutting in the youth stage, in the hills.
• Gorge and canyon: deep, narrow valley in hard rock, e.g. the Kali Gandaki gorge.
• Waterfall and rapids: hard rock lying over soft rock; the plunge pool forms at the base.
• Pot holes: circular holes drilled in the bed by swirling pebbles.
• River terraces: step-like old floodplain levels left after the river cuts down again.
• Meanders and ox-bow lakes: side cutting in the old stage.
• Interlocking spurs: ridges between bends of a young river.

Wind

• Deflation hollows (blowouts): wind lifts loose fine particles and lowers the ground, e.g. Qattara depression.
• Mushroom (pedestal) rocks: sand abrades the base of a rock more than its top.
• Yardangs: streamlined ridges of rock parallel to the wind.
• Ventifacts: pebbles with polished, flat faces cut by sand blast.
• Zeugen: tabular ridges of hard rock over softer layers.
• Desert pavement: gravel left after fine particles are blown away.
• Inselbergs: isolated residual hills.

Glacier

• Cirque (corrie): armchair-shaped hollow at the head of a glacier, made by plucking and abrasion; a lake in it is a tarn.
• Arête: knife-edge ridge between two cirques.
• Horn (pyramidal peak): sharp peak left when cirques cut back on all sides, e.g. Matterhorn.
• U-shaped valley (glacial trough): the glacier widens and deepens a river valley.
• Hanging valley: side valley left high above the main trough; it often ends in a waterfall.
• Fjord: glacial trough drowned by the sea.
• Roche moutonnée, striations and grooves: smoothed and scratched bedrock surfaces.
• Rock basins and tarns.

Underground water

Groundwater erodes mostly by solution, working on soluble rocks (limestone, dolomite, gypsum). The landscape is called karst topography. Features:

• Sinkholes (dolines): closed depressions made by dissolution or by collapse of a cave roof.
• Caves and caverns: underground hollows enlarged along joints and bedding planes.
• Sinking streams and underground rivers, springs.
• Lapies (karren): grooves and ridges on exposed limestone.
• Uvala and polje: large, compound sinkholes and wide valley-like basins.
• Natural bridges and dry valleys.

Groundwater erodes mostly by solution, working on soluble rocks (limestone, dolomite, gypsum). The landscape is called karst topography. Features:

• Sinkholes (dolines): closed depressions made by dissolution or by collapse of a cave roof.
• Caves and caverns: underground hollows enlarged along joints and bedding planes.
• Sinking streams and underground rivers, springs.
• Lapies (karren): grooves and ridges on exposed limestone.
• Uvala and polje: large, compound sinkholes and wide valley-like basins.
• Natural bridges and dry valleys.

Seawater (waves, tides, currents)

• Erosional: sea cliffs, wave-cut platforms, sea caves, arches, stacks and stumps, headlands and bays.
• Depositional: beaches, spits, bars (baymouth bars), tombolos, barrier islands, lagoons and deltas.

Groundwater

• Erosional: sinkholes, caves, karst valleys, lapies and springs (in limestone areas).
• Depositional: stalactite, stalagmite, travertine and tufa.

  Sea cliff section
       |  cliff
       |   \
  _____|    \___ wave-cut platform
  notch ^ wave erosion

Karst topography is the irregular landscape of sinkholes, caves, disappearing streams and springs formed by groundwater dissolving soluble rock, mainly limestone and dolomite (named after the Karst region of Slovenia). It needs soluble rock, a joint system, and rainfall. In Nepal it is found in Lesser Himalayan limestone, e.g. Pokhara and Mahendra cave areas. It is a problem for dams and tunnels because of leakage and collapse.

A cirque (corrie) is a bowl or armchair-shaped hollow with steep back walls, at the head of a glacial valley, formed by frost action, plucking and abrasion where the glacier begins. After the ice melts, it often holds a small lake called a tarn. Cirques cutting back to back form arêtes and horns.

Both are cave deposits (speleothems) of calcite formed when water dripping through limestone cave roofs loses CO$_2$ and deposits calcium carbonate.

• Stalactite: an icicle-shaped cone hanging down from the cave roof; it grows downward from drip-water.
• Stalagmite: a cone rising up from the floor, built where drops fall.
• When both join, they form a column (pillar).

   ______________ roof
     \/  \/ \/   stalactites
      .    .
      .    .
     /\   /\     stalagmites
   ____________ floor

An ox-bow lake is a crescent-shaped lake formed when a meander loop of a river is cut off from the main stream.

1. In the old stage, a meander bends more and more as the outer bank is eroded and the inner bank gets deposits.
2. In a flood, the river cuts through the narrow neck of the loop (neck cut-off).
3. Deposits seal the ends of the abandoned loop, leaving a lake.

    ____            ____
   /    \          /    \
  ( loop )  ->   river    (ox-bow)
   \____/       cut-off

It slowly fills with silt and becomes a swamp or meander scar. Such areas of the Terai show soft, wet soil.

A flood is a rise in river discharge and water level so that the water overflows its banks and covers the surrounding floodplain. It occurs from heavy rainfall, snow or glacier melt, GLOF, landslide dam breach or dam failure.

River channels are classified by their plan shape (pattern) as:

 Straight:       ----------------------
                 ======================

 Meandering:      ___      ___
                 /   \    /   \
              __/     \__/     \__

 Braided:         ___   ___   ___
                 /   \_/   \_/   \
                 \___/ \___/ \___/

1. Straight channel: nearly straight, sinuosity less than 1.05. It is rare and short, found in steep, narrow, bedrock valleys. Flow is fast; the thalweg still swings from bank to bank.
2. Meandering channel: single, sinuous channel with sinuosity above 1.5. It occurs on gentle slopes in fine, cohesive sediment. Erosion occurs on the outer (concave) bank and deposition on the inner (convex) bank as point bars. Cut-offs form ox-bow lakes.
3. Braided channel: many small channels dividing and rejoining around bars. It occurs on steep slope, heavy bed load, easily erodible banks and flashy flow, e.g. the Koshi, Narayani and Karnali rivers below the hills.
4. Anastomosing channel: several stable, interconnected channels separated by vegetated islands, on very low slopes.

River channel morphology is the study of the shape, pattern and cross-section of river channels, including width, depth, slope, plan form (straight, meandering, braided) and bed material, and how they change with time.

Engineering significance of the meandering channel

Meanders grow with time: erosion on the outer bank and deposition on the inner bank move the channel sideways and downstream.

        ___
   ___ /   \   outer bank: erosion (scour)
  /   \     \  inner bank: deposition (point bar)
 /     \___/

1. Bridges: the channel can shift away from the span, or attack the abutments. Bridges should be built at straight reaches and long approach embankments or training works are required.
2. Intake and headworks: place the intake on the outer (concave) bank, where water is deep and sediment is less. The convex bank gets silted.
3. Bank protection: outer banks need revetments, spurs, gabion and boulder pitching.
4. Embankments and settlements: land and buildings on the outer bank are lost to erosion; set-back distances are needed.
5. Flood level: meanders reduce the slope and the carrying capacity, so floods rise and spread.
6. Cut-off and straightening: natural cut-offs change river length, level and slope; engineers sometimes cut them artificially to lower flood levels, but this causes upstream degradation.
7. Aggregate and groundwater: point bars give sand and gravel; old channels and ox-bows give saturated and soft soils.
8. Navigation and water supply are also affected.

Types of rivers in Nepal

1. Antecedent (Himalayan) rivers: the Koshi, Gandaki (Narayani) and Karnali, whose sources are in the snow of the Himalaya or the Tibetan plateau. They cut deep gorges through the rising Himalaya and carry water all year. Snow and glacier fed.
2. Rivers from the Mahabharat range: the Bagmati, Kamala, Rapti, Babai, West Rapti; fed by monsoon and springs, with some low-season flow.
3. Rivers from the Siwalik (Churiya): small, seasonal and flashy; dry in winter, severe floods in the monsoon, high sediment load.

By pattern, rivers are also divided into straight, meandering and braided.

Features developed by the rivers

Erosional work of a river is done by hydraulic action, abrasion, attrition and solution. Features:

• V-shaped valley: vertical cutting in the youth stage, in the hills.
• Gorge and canyon: deep, narrow valley in hard rock, e.g. the Kali Gandaki gorge.
• Waterfall and rapids: hard rock lying over soft rock; the plunge pool forms at the base.
• Pot holes: circular holes drilled in the bed by swirling pebbles.
• River terraces: step-like old floodplain levels left after the river cuts down again.
• Meanders and ox-bow lakes: side cutting in the old stage.
• Interlocking spurs: ridges between bends of a young river.

Depositional work occurs when the velocity and load capacity fall (at gentle slope, at a mouth, in floods). Features:

• Alluvial fan: cone-shaped deposit where a mountain stream meets a plain, e.g. the Bhabar zone at the foot of the Siwaliks.
• Floodplain: broad flat valley floor built by flood deposits.
• Natural levees: low ridges of coarse silt built along the banks during floods.
• Braided bars and channel (point) bars: sand and gravel bars inside the channel.
• Delta: fan-shaped deposit at the river mouth (Ganges-Brahmaputra delta).
• Ox-bow lake and meander scars: cut-off bends.

  Mountain      Foothill        Terai
   gorge  ->  braided fan  ->  meanders
  V-valley     (Bhabar)        ox-bow, levee

Engineering significance

• Hydropower: gorges, high gradient and discharge in the hills; narrow bedrock valleys for dams and a high head, but high sediment load wears turbines and fills reservoirs.
• Bridges and roads: terraces make good road and bridge locations; braided Terai rivers need long spans and guide bunds.
• Flood control: levees, embankments, and spurs for the meandering and braided reaches, e.g. Koshi barrage.
• Construction materials: river-bed sand, gravel and boulders.
• Water supply and irrigation: alluvial groundwater and canals.
• Hazards: landslide dams, GLOF, bank erosion, and shifting channels.

Morphological features of river channels

Erosional work of a river is done by hydraulic action, abrasion, attrition and solution. Features:

• V-shaped valley: vertical cutting in the youth stage, in the hills.
• Gorge and canyon: deep, narrow valley in hard rock, e.g. the Kali Gandaki gorge.
• Waterfall and rapids: hard rock lying over soft rock; the plunge pool forms at the base.
• Pot holes: circular holes drilled in the bed by swirling pebbles.
• River terraces: step-like old floodplain levels left after the river cuts down again.
• Meanders and ox-bow lakes: side cutting in the old stage.
• Interlocking spurs: ridges between bends of a young river.

Depositional work occurs when the velocity and load capacity fall (at gentle slope, at a mouth, in floods). Features:

• Alluvial fan: cone-shaped deposit where a mountain stream meets a plain, e.g. the Bhabar zone at the foot of the Siwaliks.
• Floodplain: broad flat valley floor built by flood deposits.
• Natural levees: low ridges of coarse silt built along the banks during floods.
• Braided bars and channel (point) bars: sand and gravel bars inside the channel.
• Delta: fan-shaped deposit at the river mouth (Ganges-Brahmaputra delta).
• Ox-bow lake and meander scars: cut-off bends.

Definitions

• Hydrological cycle: continuous circulation of water between the oceans, atmosphere and land: evaporation, condensation, precipitation, runoff, infiltration and groundwater flow back to the sea.
• Porosity ($n$): ratio of the volume of voids to the total volume of the rock or soil, $n = V_v/V$. It measures the water storage capacity.
• Permeability: the ability of a material to transmit fluid through its interconnected pores. Gravel is highly permeable; clay is almost impermeable.
• Aquifer: a saturated rock or soil formation that stores and yields useful amounts of water, e.g. sand, gravel, fractured rock.
• Aquiclude: a saturated formation that stores water but cannot transmit it in useful amounts, e.g. clay, shale.
• Piezometric level: the height to which water rises in a well drilled into a confined aquifer; the imaginary surface joining these levels is the piezometric (potentiometric) surface.

Morphology of river channel

River channel morphology is the form of the channel: its cross-section, long-profile, slope and plan form. By plan form, channels are straight, meandering, braided or anastomosing. In the long profile, a river changes along its length from a steep, narrow, V-shaped upper course (erosion) through a middle course to a gentle, wide lower course with meanders and floodplain (deposition).

River channels are classified by their plan shape (pattern) as:

 Straight:       ----------------------
                 ======================

 Meandering:      ___      ___
                 /   \    /   \
              __/     \__/     \__

 Braided:         ___   ___   ___
                 /   \_/   \_/   \
                 \___/ \___/ \___/

1. Straight: nearly straight, steep, in rocky hills.
2. Meandering: sinuous loops on gentle slopes; outer bank erosion, inner bank deposition.
3. Braided: many channels around bars with heavy bed load, e.g. Koshi in the Terai.
4. Anastomosing: stable multiple channels with vegetated islands.

Definitions

• Porosity ($n$): ratio of the volume of voids to the total volume of the rock or soil, $n = V_v/V$. It measures the water storage capacity.
• Permeability: the ability of a material to transmit fluid through its interconnected pores. Gravel is highly permeable; clay is almost impermeable.
• Aquifer: a saturated rock or soil formation that stores and yields useful amounts of water, e.g. sand, gravel, fractured rock.
• Aquiclude: a saturated formation that stores water but cannot transmit it in useful amounts, e.g. clay, shale.
• Piezometric level: the height to which water rises in a well drilled into a confined aquifer; the imaginary surface joining these levels is the piezometric (potentiometric) surface.

Volcanism (vulcanicity) is the set of processes by which magma, gases and fragmented rock rise from the earth's interior and are erupted on the surface through a vent or fissure, or are intruded into the crust.

A volcano is a vent in the crust through which magma, gases and ash erupt, and the hill or mountain built around it.

Positive topography developed

• Cinder (scoria) cone: small steep cone of loose ash and cinders.
• Composite (strato) cone: large steep cone of alternate lava and ash, e.g. Mt. Fuji.
• Shield volcano: broad low dome of fluid basalt, e.g. Mauna Loa.
• Lava dome (plug dome): steep mound of viscous lava.
• Lava plateau: wide flat area from fissure flows, e.g. Deccan Traps.
• Ash and tuff plains, volcanic necks, dykes and sills exposed by erosion.

Negative forms are craters, calderas and lava tubes.

Volcanic materials ejected through the vent are of three kinds.

1. Gases and vapours: water vapour (most), CO$_2$, SO$_2$, H$_2$S, HCl, nitrogen.
2. Liquid: lava (molten rock at the surface). Basic (basaltic) lava is runny; acid (rhyolitic) lava is thick.
3. Solids (pyroclasts):
   • Volcanic dust and ash: less than 2 mm
   • Lapilli: 2 to 64 mm
   • Volcanic bombs and blocks: greater than 64 mm
   • Pumice, scoria, tuff, and volcanic breccia

A landslide is the sudden downslope movement of a mass of rock, soil or debris along a distinct slip surface. Slope failure is a broad term for any failure of a natural or artificial slope, including falls, topples, slides, flows and spreads, whenever the shear stress exceeds the shear strength.

A landslide is the downward and outward movement of slope-forming rock, soil or debris along a slip surface under gravity.

   crown (undisturbed ground)
   ======+
         |  <- main scarp
         +-----+  head
          . . . +--------+
          .  slip surface  +---+
          .  body (moved)   . toe
          . . . . . . . . . .+---> foot

Main parts:

1. Crown: undisturbed material above the main scarp.
2. Main scarp: steep surface at the upper edge, formed by the slide.
3. Head and minor scarps: top of the moved mass and small steps within it.
4. Slip (rupture) surface: the surface on which movement takes place.
5. Body (displaced mass): material above the slip surface.
6. Toe and foot: lower margin of the moved mass, and the part beyond the toe of the rupture surface.
7. Flanks: the sides of the slide.

A landslide is the downward and outward movement of rock, soil or debris down a slope along a slip surface, under gravity. It occurs when the driving shear stress exceeds the shear strength of the slope material.

Causes

Natural causes

• Steep slope, weak or weathered rock, and adverse orientation of joints and bedding.
• Heavy rainfall and rise of the groundwater table (increase of pore-water pressure).
• Earthquake shaking and volcanic activity.
• Undercutting of the toe by river or wave erosion.
• Loss of vegetation due to forest fire.

Man-made causes

• Cutting of the slope or toe for roads (the main cause in Nepal).
• Loading the top by buildings, dumps and reservoirs.
• Deforestation and poor drainage, leaking pipes and irrigation canals.
• Blasting and vibration, excessive mining or quarrying.

Techniques to determine the slip surface

The slip (failure) surface is the base of the moving mass; its depth and shape are needed to design stabilisation.

1. Surface mapping: location of the main scarp, cracks, bulges at the toe, and tilted trees or poles. It gives the approximate outline and shape of the slide.
2. Test pits and trenches: excavation exposes shear surfaces directly, shown by polished, slickensided or remoulded clay layers. Suitable for shallow slides.
3. Boreholes and core logging: drilling through the slide shows the change from disturbed to intact material; soft and sheared zones, loss of core, and different moisture identify the slip zone.
4. Inclinometer: a casing in a borehole is read at intervals; the depth at which the casing bends or shifts gives the slip surface and rate of movement. It is the most reliable method.
5. Slip indicators and shear strips: a flexible tube, a wire or a column of rings or beads in a borehole is sheared at the slip depth.
6. Time domain reflectometry (TDR): a coaxial cable in a borehole gives a signal change when it is sheared.
7. Geophysical surveys: seismic refraction, electrical resistivity imaging and ground penetrating radar show the contrast between the slide mass and the stable ground beneath.
8. Piezometers: water pressure readings help locate the water-bearing zone along the slip surface.
9. Back analysis: a trial slip surface is found by stability analysis for $FS = 1$ using the known strength and geometry.

Using more than one method gives better confidence.

Note on the statement: the standard teaching is the other way round: all landslides are mass movements, but not all mass movements are landslides. Landslide is only one group within the larger family of mass movement, as shown below. The reasoning is as follows.

• Mass movement is any downslope movement of soil, rock or debris under gravity. It includes falls, topples, slides, spreads, flows and creep (Varnes, 1978).
• Landslide (in the strict sense) means sliding along a defined slip surface (rotational or translational slide). In popular and common usage "landslide" is used for nearly all slope movements.
• Thus every landslide is a form of mass movement, because it is a gravity-driven downslope movement.
• Some mass movements are not landslides: creep (very slow, no distinct slip surface), rock fall, topples, debris flow and mud flow (move like a fluid) and solifluction.

   MASS MOVEMENT
   +---------------------------------+
   | fall  topple  flow  creep spread|
   |        +------------+           |
   |        | LANDSLIDE  |           |
   |        | (slides)   |           |
   |        +------------+           |
   +---------------------------------+

So landslide is a subset of mass movement; the reverse is not true.

A debris flow is a rapid flow of a water-saturated mixture of soil, rock fragments, boulders and organic matter down a steep channel, under gravity. It contains 50-80% solids by volume and behaves like wet concrete.

• Causes: intense, prolonged rainfall; abundant loose debris from old landslides; steep gullies; sudden release of landslide dams or glacial lakes; earthquakes.
• Features: three parts: initiation zone (source), transport channel with levees, and a deposition fan at the foot. Boulders concentrate at the front.
• Speed: from 1 m/s up to over 10 m/s, so it is destructive.
• Occurrence: frequent in the Siwalik and Lesser Himalaya during the monsoon.
• Control: check dams, debris basins, channel lining, deflection walls, planting, early warning, and keeping settlement away from fans and gully mouths.

Here $\psi_f$ is the slope face angle, $\psi_p$ the dip of the plane, $\psi_i$ the plunge of the intersection line and $\phi$ the friction angle.

The angle of friction ($\phi$) is a measure of the shear strength due to friction on a surface or in a soil or rock. It is the angle whose tangent is the ratio of shear stress to normal stress at failure on a surface with no cohesion:

With cohesion $c$ the Mohr-Coulomb equation is $\tau = c + \sigma_n \tan\phi$.

• Basic friction angle $\phi_b$: from smooth, clean saw-cut rock surfaces (25 to 35 degrees).
• Residual friction angle $\phi_r$: after large displacement; lower than the peak value. Clay-filled joints can have $\phi_r$ of 10-20 degrees.
• Peak friction angle: includes roughness: $\phi_p = \phi_b + i$, where $i$ is the roughness angle.
• Angle of repose: the steepest angle of a loose heap that stays stable, about $\phi$ for dry granular material.

Example: $\tau = 140$ kPa at $\sigma_n = 200$ kPa gives $\phi = \tan^{-1}(0.7) = 35.0^\circ$.

A slope with a discontinuity dipping steeper than $\phi$ and daylighting in the face can slide.

Stability of a rock slope depends on how the discontinuities are oriented relative to the slope face (dip and dip direction), and on the friction angle $\phi$ of the discontinuities.

 UNSTABLE (dip out of slope)    STABLE (dip into slope)
      \                              /
       \ bedding                    /  bedding
  ______\______                ____/_______

In road cuts, the cut slope should not be steeper than the dip of bedding or foliation when these dip out of the slope. This is investigated using a stereonet (kinematic analysis) before design.

Permeability is the property of a rock or soil by which it allows fluid (water) to pass through its interconnected pores, fractures and voids. Its measure is the coefficient of permeability $k$ (or hydraulic conductivity $K$, in m/s). Gravel and clean sand are highly permeable; clay and unfractured granite are practically impermeable.

Permeability (intrinsic permeability, $k$) is a property of the porous medium alone, independent of the fluid. Hydraulic conductivity ($K$) depends on both the medium and the fluid (its density and viscosity).

In many civil engineering books the term "coefficient of permeability" is used for $K$ in m/s.