3 Erosion and Sediment Transport - 3.2 Water erosion processes

Question 1

Water Erosion Processes with image

Answer 1

rain
DETACHMENT

TRANSPORT
surface runoff

DEPOSITION
water body

Question 2

Soil erosion

Answer 2

the wearing away of the land surface by physical forces such as rainfall, flowing water, wind, ice, temperature change, gravity or other natural or anthropogenic agents that abrade, detach and remove soil or geological material from one point on the earth’s surface to be deposited elsewhere.

Question 3

Characteristics of Soil Erosion

Answer 3

 Variable in intensity, scale and nature
– soil erosion driven by nature as well as human activities
– soils vary in their erodibility (susceptibility to erosion)

 Episodic – very often event-driven, e.g.
– heavy rainfall
– tillage operations

 Leads to conversation of soil into sediment
– detachment of individual particles from soil aggregate
– transport of particles by erosive agents (wind, water)
– deposition of sediment to form new soils or to fill lakes and reservoirs

 Intake into aquatic ecosystems

Question 4

Anthropogenic Driving Forces

Answer 4

 Human/climate-induced changes in soil
– loss of organic carbon, structure

 Tillage and other mechanical influences (agriculture)
– soil compaction by heavy machines
– logging, esp. clear cutting
– improper farming
– livestock

 Traffic
– off road vehicles
– intense foot traffic

Question 5

Some Figures

Answer 5

 Average erosion rates
– 1 mm soil loss equals 13-16 t/ha/yr
– Soil genesis rate: 1/10 – 1/100 of soil erosion rate
– Study by Wilkinson and McElroy, 2007:
# Avg. rate for 542million years (Phanerozoic) = 0.42 t/ha/yr
# Avg. rate in Pliocene (most erosive period) = 1.36 t/ha/yr
# Current riverine flux to global oceans = 1.78 t/ha/yr
# Erosion from present day farmland = 6.36 t/ha/yr
– Rhine (Central Europe): 0.2 t/ha/yr erosion, 2.7 * 106 t/yr solid load
– Huang He (China): 24.8 t/ha/yr erosion, 1.6 * 109 t/yr solid load

 Extreme Events
– Storms can erode 50-100+ t/ha/single event

 Tolerable erosion rates
– < 1 t/ha/yr as a precaution, 1.5 t/ha/yr might be acceptable

Question 6

Wind Erosion Process

Answer 6

0.1 – 1 mm (fine and medium sand) saltation, “jumping” over short distance, dissolving other particles when clashing

Wind velocity dislodges loose soil particles that become airborne until velocity reduces

# 0.001 – 0.1 mm (clay and silt)
suspension, “dust storm”, high
and longer distance transport

> 1 mm (coarse sand) rolling, “creep” on the soil surface

Question 7

Water Erosion – Deposition Process

Answer 7

See diagram on slide 17

Question 8

Water Erosion Process

Answer 8

1. Kinetic energy of raindrop splash detaches soil particles.
   – Example:
    Ekin = 0.5 m v²
    Velocity of falling rain drops v= 8 m/s
    20 mm rainfall over 1 ha land surface
    Ekin = 0.5 * 200,000 kg * 64 m²/s² = 6,400,000 J = 6.4 MJ
   – Particles can be moved approx. 0.6 m vertically and 1.5 m laterally (heavy rainfall events).
2. Physical breakdown of soil aggregates by unimpeded raindrops:
   – choking of soil by the washing of fine particles into the interstices of larger particles in the surface layers (puddle erosion)
   – reduction of infiltration rates (increase of flood risk due to larger fraction of fast runoff components)
3. Washing of clay particles through gravels or sands to accumulate at lower depth in the soil profile (vertical erosion).
4. Surface flow transports fine particles down-slope and detaches more particles.
   – Example (from above):
    50 % runs off at v = 1 m/s
    Ekin = 0.5 * 100,000 kg * 1 m/s = 50,000 J
   – Movement by splash is more important than by runoff.
   – Surface runoff originates from infiltration excess, saturation excess or inflow from neighboring/uphill areas
5. With decreasing tractive force most of the particles are deposed at the lower part of the slope.

Question 9

Forms of Soil Erosion by Water

see image on slide 21-24

Answer 9

 Sheet or interrill erosion
– Areal, shallow overland flow moving down-slope

 Rill erosion (anak sungai)
– linear, flow in small channels (e.g. plough furrows, concave slope
hollows) erodes side and bottom of rill
– concentrated overland flow, emersion of interflow < 10 cm to 40 cm

 Gully erosion (erosi selokan)
– water concentrates in channels too deep to smooth over by tilling (> 40 cm)
 Stream or river bank erosion

Question 10

Stream Bank Erosion

Answer 10

Wave motion in streams, rivers and lakes cause slumping of bank material, which is then transported downstream as sediment.

Question 11

Deposition: Crevasse Splay (Special Case Example)

Answer 11

 Overloaded stream breaks a natural or artificial levee and deposits sediment on a floodplain.

 This breach can cause large deposits that spread in a pattern similar to that of a river delta. In other words, coarser sediment is deposited first, close to the breach while finer sediment is carried farther to the edges of the crevasse splay.