Slope Hydrological Processes 1
- Mainly caused by athropogenic activities
o Affects physical profile and characteristics
o Bring in a great impact on hydro-geomorphological processes in hillslope environment Slope Processes
1) Fluvial slope processes - Caused by rainfall mainly
- Rainsplash, overland flow, subsurface flow
o Highly influenced by rainfall, rate of water entry, rate of moisture transmission evapotranspiration, etc.
2) Mass movement processes - Gravitationally induced
- Rockslides, landslides, mudflows, etc. - Take place during heavy rainstorms
- Caused by slope regradation, lead by accumulation of rainwater.
Canopy Interception
Interception: Deciduous trees commonly intercept 20 to 30% of the falling rain
- Throughflow: controlled by plant leaves and stem density, type, intensity and duration of precipitation - Stemflow: Flow of water through stems to the ground
Infiltration and Interflow
- Infiltration: Absorption and vertically downward movement of precipitation
- Depression storage: Accumulation of water in the soil, in minor surface depression (lower region) - Overland flow: Surface runoff on slope
Fluvial slope processes
Rainspliash
- Raindrop posses high kinetic energy by virtue of their mass and velocity - Create thin surface crust
- Blocks soil opening and causes the soil to be impermeable Overland flow
- Infiltration-excess flow : (Hortonian overland flow) Overflow occurs when rainfall intensity exceeds the infiltration capacity of the land surface
o Common in high-intensity rainfall environments with
Thin impermeable soils
Little vegetation
Engineering slopes with compact materials
- Saturation- excess flow : Soil layer is fully saturated with water at which the soil is unable to absorb anymore water
o Tends to occur in humid environments with thin soil profiles o Initial water content is high
Subsurface flow
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- Subsurface flow: lateral movement of water downslope below soil surface
- Macropore or pipe flow is a type of subsurface flow where water flows along the macropores, soil pipes and underground tunnels created by animals (earthworms)
- Critical factor influencing the stability of the hillslope as the soil mass movement is highly affected by the discharge of water from the soil mantle
Factor affecting Slope Stability and Landslides
Geology
- Structure of lithological setting is crucial in affecting the stability of slope
- Downslope dipping planes destabilizes the slope by causing obstruction of infiltration of water and root penetration.
- In Singapore, the landform that is the most susceptible(sensitive) to slope failure is Jurong Formation
Terrain
- Slope steepness will also bring direct effect to the slope stability as landslides are gravitationally induced
Vegetation
- Vegetation can provide cohesion to soil strength through root systems - Protective layers against rainfall erosion
- Hydrologically
o Evapotranspiration and interception by plant can reduce the water content in soil, preventing runoff generation
o Maintain soil porosity and permeability, improves infiltration and delays the onset of surface runoff on the slope
o Decreases velocity of runoff o Limit rainsplash effects - Mechanically
o Roots reinforce soil by binding the soil
o Bond unstable soil particles, mantles to stable subsoils or substrata o Provide cohesion to both organic layer and lateral layer of soils o Counteract downslope shear forces
o Reduce sediment washed away from slopes
Slope Modification in Singapore
- Slope grading (cut and fill)
o Cutting or steepening of slopes o Changes gradient, length and profile
Affects the hydrological processes on the slope
Increases runoff velocity
o Natural slope structure: Convex-Straight-Concave
Modification to linear, planar o Shortening length of slope
Disruption of subsurface flow pathways
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- Resurfacing
o Replace topsoil layer by building concrete surfaces o Does not have topsoil layer, removed during grading
o Soils are unable to support diverse plant life, lead to erosion
Stabilization of Slopes
- Depends on engineering and structural control methods
o Building toe-walls, rock, buttresses, gabion walls, steel or wooden pilings, concrete covers, gravity retaining structures, etc
- Maintain soil stability by using their weight to counteract external forces, but are usually unnatural looking, do not blend with surrounding landscape
- Biotechnical stabilization
o Combining mechanical structures and biological element to maintain stability of slope o More cost-effective
o Blend better with surrounding landscape
Soil Erosion
- Normal slope erosion – slow removal of soil o Inevitable and universal
- Detachment and transport of the topsoil through physical action of wind and water acting on the soil surface
- Soil erosion occurs when
Erosion Rate≫ Soil formation Rate - Human activities often become the main factor of this phenomenon
Rills & Gullies
- Rills
o Produced by sustained concentrated flow
o Occurs on steep slopes with poor vegetative cover - Gullies
o Formed from rills
o Form permanent part of a channel network o More fluvial than slope processes sometimes
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Soil Erosion
Erosivity [Potential to cause
erosion]
Rainfall Factors Drop size, velocity, intensity, frequency,
duration
Hillside Hydrology Overland flow vs Subsurface flow
Erodibility [ Resistance to entrainment &
transport]
Soil Properties Particle size, infiltration capacity
Topography Slope inclination,
length
Intensity of Management Degree of protection,
vegetation, land use
Factors Affecting Soil Erosion
- Erodibility
o Resistance of slope materials to entrainment and transport - Erosivity
o Potential forces of the slope to cause soil erosion Rainfall Factor & Hillside Hydrology
- Intensity and duration of rainfall affects the soil erosion - Excess rain will cause surface runoff, causing overland flow - Affects hillside hydrology
o Steeper slope encourages downfall of materials in soil
Soil Properties
- Soil texture, infiltration capacity, organic and chemical constituents of soil
- Soil Texture
o Soil with large particle size are less subjective to erosion
o Greater force is needed to detach them o Silts and sands are very sensitive to erosion
o Clay particles are resistant to erosion because it can:
Lowering permeability due to increased water retention
An increase in cohesion
Lower the angle of friction - Infiltration capacity
o Maximum rate at which soils can absorb water
o Influenced by the pore size of soil, formation of soil profile o Cohesiveness of soil reduces, infiltration capacity REDUCES!!!! - Chemical constituents
o Soils with kaolinite content has lower percentage of volume chang o More resistant to erosion
Topography
- Surface gradient affects the runoff and sediment transport
- High gradient – High runoff rates, ender high shear stress, high sediment transport capacity o This will give rise to low infiltration capacity and steep slope
- Amount of erosion per unit area increase 2.8 times if the slope is doubled - Increases water velocity, allowing rainsplash to occur
- Curvature of slope, surface wash is more severe on convex slope than on concave slope o Soil at convex slope dries out readily, contains less humus than concave slope
Less stable, easily disrupted by raindrops
o Convex slope, insufficient water accumulates to form thick surface o Humus and soil materials are being removed by water
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Intensity of Management
- Vegetation acts as breakers to dissipitate the kinetic energy from falling raindrops so that plant absorb that energy instead of dissipating it to the soil
- Forests play an important role in preventing soil erosion because: o Multiple layers of undergrowth and canopy layer
- It decreases volume of rain reaching forest floor, but doesn’t alter the kinetic energy o Coalescence of leaves eventually produces larger raindrops
- Forest floor plays a more important role
o Presence of leaf litter and humus can reduce the effect of rainsplash
Universal Soil Loss Equation (USLE)
A = R × K × L × S × C × P A: annual soil loss per unit area
R: rainfall-runoff erosivity factor K: Soil erodibility factor
L and S: Terrain factors of slope gradient (S) and length (L) C: cropping factor or vegetational cover
P: Soil conservation factor
Soil Erosion in Urban Development of Singapore
- Rainstorms in Singapore have short duration but high intensity - Construction sites often leave loose materials
- Compacted soil in Singapore promotes soil erosion, leads to lower infiltration rate - Modified slopes in general have higher gradient and more compact
- Trampling trails
o Trampling over grass-covered walkway : causes compaction of soil, killing the grass cover o Caused by shortcut paths, momentarily trampling of certain spots of grass patches o Repeated actions create trails and erosion patches
- Burning of topsoil: Bush fire, bonfire, etc.
- Planking: Placing of large, flat materials on grass surface for advertisement, etc. - Dry weather
o Reduces the moisture content of the soil o Reduces cohesiveness of soil
- Leaf drip and stemflow
o Extent of soil erosion is a function of roughness of tree, species type and intensity of rainfall
Effects of Soil Erosion
On site effects
- Landscape deformation
o Formation of rills, gullies and mass movements - Reduced thickness of soil due to soil loss
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- Reduces nutrient content of soil - Affects productivity of soil
- Soil loss Economic loss (buy soil from Riau for S$15/cm3)
Off-site effects
- Initial transport
- Subsequent deposition of sediment (Sedimentation and Silation) o Disrupts water-based activities
o High Cost for recovery measures (S$1.5 Million per annum in cleaning up drainage system) - Silation of water bodies
o Reduces water quality
o Results in higher total suspended soils in the water
o Bring out low dissolved oxygen capacity, increases CO2 content ( making water more acidic)
Countermeasures
- Drains
o Create efficient sediment delivery system - Revegetation
o Only temporary, as long as there is trampling on grass grounds, the soil erosion will recur - Turn Trampling trails into walkway
o This will prevent trampling in the proximity of walkway
Soil Erosion Quantifications
Usually is hard to quantify because it is a slow process - Erosion Pins
o Measure of soil surface level changes o We do not know WHEN the soil erode
o But we do know HOW MUCH does the soil erode - Tree roots
o Measuring the height of the exposed tree
o Estimating the age of the trees from tree rings count
o We can know HOW MUCH and HOW LONG the soil takes to erode - Erosion Plots
o Bounded small area with various sizes o Small watershed
o Measure the degree of erosion
