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Soil erosion:
Factors influencing soil erosion:
Erodibility of soil measures how resistance is the slope material to entrainment and transport.
Soil properties:
Soil texture – coarse and fine
Infiltration capacity – affected by size of pores, stability of pores and the forms of slope profile.
Chemical and organic constituents of the soil – affects the stability of aggregates.
Topography of the slope:
Slope steepness – velocity of runoff, forming of thin film of water, unidirectional rainsplash erosion.
Slope length – accumulation of water for concentrated surface flow.
Slope curvature – convex more prone to erosion
Soil aggregate dries rapidly less stable
Insufficient accumulation of water for thin film to form
Water flows away from slope rapid with no deposition of sediments.
Intensity of management:
Vegn acts as breakers to dissipate ke of rain. Energy absorbed by plants
Imparts roughness to surface flow, lower the velocity and eroding power.
Forests – multi-layered structure and canopy to intercept the fall of rain. And leaf litter to minimize rain splash erosion.
Erosivity of soil which refers to the potential forces for erosion to occur:
Rainfall factors: intensity, duration, frequency, size of rainfall.
Hillslope hydrology: - overland flow or subsurface flow of precipitation. Determined by soil properties (texture) and slope profiles)
Higher infiltration less overland flow less erosion effects.
Soil erosion in urban Singapore:
1. High intensity rainfall.
2. Small scale construction and renovation.
3. Compacted soils.
4. Modified slope of higher angle and compaction.
5. Trampling trails and erosion patches by human.
6. Topsoil burning.
7. Planking.
8. Periods of dry weather spell.
9. Leaf drip and stem flow from trees.
10. Shading.
Effects of soil erosion:
Onsite:
1. Land deformation: rills, gullies, mass movement, hollows. Rainpillars
2. Soil loss.
3. Nutrients loss.
4. Lower productivity of soil.
Offsite:
1. Extensive drainage network sediments washed to rivers and water bodies.
2. Infilling of river beds flash floods, lower water storage capacity, and lower quality of water.
3. High economic costs for recovery measures (cleaning up drainage and improvement projects.)
4. Affect aquatic life: sediment in water lower oxygen content and increase carbon dioxide. More acidic.
Green space and their functions:
1. Preserve biodiversity for its potential as sources of raw materials, food and medicine.
2. Watershed protection:
a. Rainforest purifies the rainwater by removing heavy metals and nutrients.
b. Widespread root networks stabilize soil which acts as filter as water percolates through soil mantle.
c. Canopy traps rain (interception) and slows down surface runoff
d. Deep penetration of roots maintain underground water table at constant level.
3. Prevent urban heating and provide pollution control
a. Direct shading
b. Evapotranspiration lower surrounding temperature.
c. Foliage traps dust and air borne particles.
d. Lower ambient temperature minimizes vertical thermal air movement.
e. Green lungs - Photosynthesis remove CO2 and replenish oxygen
f. Buffer against noise.
4. Carbon storage and sequestration.
a. Indirectly, less energy in cooling surrounding. Cooling brought about by urban greens.
b. Fixate carbon in air by photosynthesis, convert CO2 to oxygen or store in plant tissues, that is only reached upon decomposition
c. Indirectly fixate carbon in soil by holding the soil with the roots, prevent erosion of the soil.
5. Recreational and sense of identity.
a. Recreational experience and quality green infrastructures.
b. Sense of identity.
c. Heritage roads for sense of nostalgia.
d. People identify with the culture and heritage of these places.
6. Aesthetics
a. Flowering plants grown to break monotony of urban buildings.
Habitat fragmentation:
1. Constrict wildlife movement across fragmented zones.
2. Lengthen edge effects
a. More prone to external forces
b. More tree falling
3. Structural changes to habitat
a. More sunlight reaching forest floor.
b. More undergrowth vegetation
c. Wildlife has to adapt fast to changes.
4. Extinction of flora and fauna.
a. Need to protect the surrounding buffer areas or ‘interim green’ to reduce ecological pressures on the deeper inner forest.
5. Walk boards and trails introduced.
a. Deeper access by human.
b. More species disturbance and possibility of proaching activities.
c. Trampling by human within forest soil compaction aeration and drainage problems.
d. Rills and gullies formation
6. Some other issues regarding nature reserves and green space
a. Removal of leaf litter which can be a source of nutrients.
b. Shading.
c. Etc.
Soil compaction!!!!
Impacts on soil aeration and plant growth
And watershed protection.
Major soil stresses – soil compaction and aeration
Compaction during construction – heavy machinery
Due to heavy human traffic and trampling over green areas
Risk highest when soil is wet
Compaction destroys pore space and increases bulk density
Poor aeration affects activity of soil organism.
Slower decomposition of organic matter and hence release of nutrients
Less fertile soil.
Shallow root penetration.
Less intake of water and nutirientsless healthy trees.
Contributes to urban storm water and water resource problems
High runoff coefficient of compacted surface.
Greater surface flow more runoffs of pesticides and nutrients into rivers and water bodies.
Compaction worsens soil which is already poor in filtration and water retention.
Easy generation of concentrated water flow slope erosion and expose roots.
Sediment washed off into rivers and water bodies, affecting water quality.
Need to limit the amount of erosion and sediment generated during construction by minimizing soil compaction.
