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Sediment transport and deposition

Second year lecture on Geomorphological Processes module

Dawn Theresa Nicholson

on 30 October 2013

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Transcript of Sediment transport and deposition

Slope development
Erosion rates
Erosion control
Sediment transport and deposition
Infiltration excess
Saturation excess
Stream bed and suspended load
Hillslope wash processes
Mass movement
Hillslope wash processes
Rainsplash and overland flow
Subsurface flow
Rilling, gullying and badlands
Solute processes
And finally...
The movement of sediment parallels the movement of water
Lateral displacement
Selective sorting
Resistance of soil
Cover (vegetation)
Rates 75 to 340mm/h
Max size 5mm
Terminal velocity
Index of Erosivity IE30
Characteristics of rain
Originally described by Horton (1933)
Laminar flow
Uniform thin layer
Rarely observed
Concentrations around obstructions, irregularities
Occurs where water which cannot infiltrate ponds on the surface and runs downslope
Thin, impermeable soil
Low water storage capacity
Absence of vegetation
Freezing and compaction
High intensity or prolonged rainfall
Infiltration cannot occur because the soil is already saturated – there is no space left for more water
Impermeable soils
During periods of prolonged rainfall
Humid tropics
'Contributing areas’ occur – where overland flow contributes directly to stream channels (these expand during the period of prolonged rainfall)
A. Overland flow from saturated areas close to the stream
channel (saturation overland flow)
B. Direct overland flow (Horton overland flow) contributed
from nearly the whole basin area
C. Water moving downslope to the stream through the
unsaturated zone of the soil (subsurface stormflow)
D. Increased groundwater discharge into the stream
E. Rain falling directly into the stream
Qn: In a barren rocky catchment, most water appearing as streamflow during storms gets to the stream by which of the following methods:
How does sheetflow actually move sediment?
Soil detached by raindrop impact
Detachment also by runoff but..... Hjülstrom
Runoff velocity and turbulence increase as slope steepens and depth of flow increases
Overland flow velocities commonly 0.015 to 0.3m/s
Depths normally only a few mm
Once entrained – particles remain in suspension until depositional velocity occurs (Hjülstrom)
Some semi-arid areas - sheetwash = 98% of all sediment production (Emmett 1978)
Pipeflow occurs as subsurface stormflow through pores, macropores or cracks in soil or sediment
Diameters 0.02 to >1m
Length can be >1km
Flow velocity comparable to surface overland flow
Major contributor to peak flow during storms
Up to 50% total storm discharge
Strong enough for roof support
Weak enough for erosion
Predisposed to cracking
Impermeable layer(s)
Highly variable or seasonal rainfall
Reduced vegetation cover
Topographic position
Requirements for pipeflow
Characteristics of pipeflow
Many environments
Important hydrological process and erosion mechanism
Major runoff generator in humid environments
Badlands, loess and loessic colluvium
Occurrence of pipeflow
Piping near Beni Boufrah in the Rif Mountains (Morocco)
Piping in Mexico
Severe erosion on a citrus plantation in Belize
Piping on a hill after just one year of cultivation in Malawi
Micro-sand and gravel pillars due to rainsplash
Rainsplash on sandy topsoil, North carolina
Notice the 'caps' resting on these sand pillars
IEOF along tobacco croprows, North Carolina
Problems of sediment deposition
Thalweg rill and fan deposition on agricultural field, Oxfordshire, UK
Accumulation of sediment from upslope erosion at Billinge Hill in Merseyside
Adversely affects crop yield
Sedimentation in rivers, reservoirs and watercourses
Knock-on effects for flood management and water quality
Impact on other users (e.g. fishing, extraction, recreation, hydro-power)
Increased dust blow
Reduction in value of land
Rilled slope in the Zin Valley Badlands, central Negev, Israel; note integrated rill networks and discontinuous parallel rills.
Gully erosion at Tabernas in southern Spain
Parallel rills and piracy
Characteristics of rills
Micro-channels with cross-sectional width of a few cm to tens of cm
Usually discontinuous and unconnected to stream channel system
Often temporary (ephemeral)
Rills usually straight (not meandering)
Development of rills
Parallel sets
Micro-piracy and cross-grading
Greater spacing downslope
Enlargement downslope
Wider and shallower downslope
Absence of vegetation (e.g. arid and semi-arid regions)
Steep and disturbed ground
Deeper flow and turbulence
Collapse of pipe roofs
Key sediment transport on bare slopes
Destroyed by creep or coalescence (piracy)
Two types of rainfall; (i) high intensity, and (ii) prolonged rainfall
Formation of rills
Eroded peat gully from Kinderscout, UK
Massive gully triggered by saturated slope failure induced by pipeflow
V-shaped gully in valley fill, Morocco
Rills and gully in cultivated field on sandy soils near Bridgnorth (R. Severn catchment)
Gully in cultivated field in Malawi
Parallel rills and gullying in Mexico
Large rill (small gully?) in a cultivated field in Severn catchment
Characteristics of gullies
Formation of gullies
Steep sides, steeply sloping head scarp, >0.3m wide and >0.6m deep (Brice 1966).
Common in loess, volcaniclastic materials, shales and loose debris
High sediment discharge
Can become semi-permanent
Rills develop into gullies
Breaks of slope or in vegetation cover
Triggered by increase in surface runoff
Or reduced flood capacity
Often begin as pipes or 'master' rills
"A recently extended drainage channel transmitting ephemeral flow"
Rilled slope in the Zin Valley Badlands, central Negev, Israel; note integrated rill networks and discontinuous parallel rills.
An example of badlands from Wyoming
Fully dissected badlands from Morocco (note high drainage density, steep slopes and narrow interfluves)
Zin Valley, central Negev
Gullies and badlands in the Tabernas Basin, southern Spain
High drainage density, rugged topography, high steep slopes, narrow interfluves
Intense fluvial erosion of weak rock (e.g. shale) on bare, unprotected surfaces
Occur in semi-arid regions
Kärkevagge: Classic geomorphic investigation of a glaciated valley
Anders Rapp 1927-1998
Weathered material carried in solution
Difficult to determine volume of material removed
Solute analysis of streams
Surface lowering (?processes)
Strong effect in limestone
Single large earth slide
Many rockfalls over nine years
Largest losses by solute transport
Next week
Wednesday 9-10am in E143 as usual (Ian)
11-1pm in E143 .......for field trip preparation
Attendance essential
Energy driving erosion: E.g. rainfall, wind speed, slope gradient
Resistance to erosion: E.g. properties of the sediment, infiltration capacity, moisture content
Protection from erosion: E.g. cropping regime, vegetation cover, engineering structures, conservation measures
WHY control erosion?
Land value
Crop yield
Erosion trigger
Land lowering
Traditional gully stabilisation in Mexico
Physical barriers (terraces) to intercept or slow water flow
Vegetative filter strips (buffers) are barriers to intercept or slow water flow
Narrow vegetative filter strips
Prickly pear crop lines in Tunisia (stabilise soil and act as wind break)
Ridges in soil in Tunisia in between olive trees (effective water and soil storage)
Ridges on a fan in Tunisia – hollows at back collect water
Engineered approach in a gully system, Spain
A permaculture approach to soil erosion control in Costa Rica
Overland flow
Solute flow
Erosion assessment and control
Landform evolution
Under what conditions might you expect it to occur?
Seedpool loss
Flood risk
Water quality
Dust blow
Full transcript