Runoff and Erosion

Surface water excess

• the free water on the soil surface whenever the water supply rate exceeds the infiltration rate

Surface storage capacity

• the volume of water per unit area which can be held on the soil surface before runoff begins

Surface runoff

• the amount of water that flows downslope along the surface – overland flow – channel flow or stream flow

Good or bad?

• • • p. 286 “Uncontrolled runoff is never desirable…” Quantity vs. quality Runoff inducement – mechanical treatments – chemical treatments

El Mustaqbal is a Bedouin school located c 10km southeast to Beer Sheva. The school maintains a small garden (Bustan) based on rain water harvesting.

Runoff reduction

Runoff Prediction

• • Curve Number Method – Developed by Soil Conservation Service (now NRCS) – Purely empirical – Widely used Mechanistic models – Rainfall, soil properties, and land use must be known – First, simulate infiltration – Second, simulate overland flow process – Example: Water Erosion Prediction Project (WEPP)

•

Water Erosion Prediction Project (WEPP)

Developed by the Agricultural Research Service, National Soil Erosion Research Laboratory • Infiltration simulation driven by the Green-Ampt model • Hydraulic conductivity in the model is influenced by tillage, crusting, surface cover, and storm precipitation amount.

• Runoff predicted by surface water excess • Online version prototype: http://milford.nserl.purdue.edu/ NSERL, West Lafayette, Indiana

Reading assignment

• • Soil erosion, p.287-295 & p. 359-361 An Urgent Appeal for Soil Stewardship

A farmer and his two sons during a dust storm in Cimarron County, Oklahoma, 1936. Photo: Arthur Rothstein

Goodwell, Oklahoma, June 4, 1937 Lubbock, Texas, October 17, 2011

Photo source: http://www.greatmirror.com

Plant and Soil Sciences Distinguished Speaker Series D R . B OB S TEWART D IRECTOR W EST T D EXAS RYLAND A A&M U GRICULTURE NIVERSITY I NSTITUTE “Global Agricultural and Environmental Issues” Monday, November 5, 2012 FAPC, Room 201 Welcome reception with refreshments, 3:00 p.m.

Lecture, 3:30 p.m. Ken Burns’ The Dust Bowl: A Special Advanced Screening & Community Conversation Featuring Congressman Frank Lucas Oklahoma State University, 315 Student Union, Student Union Theater 6:45 p.m., Monday, November 5, 2012

Soil erosion stages

• • • Detachment Transport Deposition

Detachment

• • Water – Raindrop impact – Runoff scour Wind – gusts

Detachment

• • • Water – Raindrop impact – Runoff scour Wind – gusts Depends on: – surface cover, soil strength, rain intensity, wind or water flow velocity, etc…

• Water – Sheet – Rill – Gully

Transport

Source: Soil Erosion and Its Control, Q.C. Ayres, 1936, McGraw-Hill

Source: http://cst.cmich.edu/users/Franc1M/2GEO334/lectures/erosion.htm

Source: http://www.dot.ca.gov/hq/esc/geotech/photos/south/erosion.htm

Creep in Barnes Co., ND Slump near Bismark, ND Same slump nine months later.

Source: http://www.ndsu.edu/nd_geology/nd_mass_wasting /index_mass_wasting.htm

http://landslides.usgs.gov/research/other/centralamerica.php

A massive landslide occurred in the Las Colinas neighborhood of Santa Tecla, El Salvador, Central America as a result of the M=7.6 earthquake of January 13, 2001.

• •

Transport

Water – Sheet – Rill – Gully Wind – Surface creep (d > 0.5 mm) – Saltation (0.1 < d < 0.5 mm) – Suspension (d < 0.1 mm)

• • •

Transport

Water – Sheet – Rill – Gully Wind – Surface creep (d > 0.5 mm) – Saltation (0.1 < d < 0.5 mm) – Suspension (d < 0.1 mm) Depends on: – flow velocity, particle size, particle density

Reading assignment

• “Mechanical Analysis” p.45-47

Deposition

• • • Initiated by a decrease in the flow velocity Approximated by Stokes’ Law: – the settling velocity of a spherical particle is proportional to its radius squared (r 2 ) To apply we assume: – soil particles are spherical – the suspension is dilute enough that the particles do not interact with each other – the fluid flow is laminar (not turbulent)

Stokes Law

• • Drag force  = viscosity

F d

 6 

ru

Force of gravity

F g

 4 3 

r

3  

s

 

f



g

• • At terminal velocity Solve for velocity

d

2

g

 

s u

  18  

f



F g



F d

http://www.answers.com/topic/stokes-law

Deposition example

• A pulse of sediment laden runoff is delivered to a pond. About how long will it take for soil particles with diameters of the following sizes to settle to a depth of 1 m?

– – – 5 x 10 -2 mm (fine sand) 5 x 10 5 x 10 -3 -4 mm (silt) mm (clay)

u



d

2

g

 

s

18   

f

   s = 1 x 10 -3 kg m = 2650 kg m -3 -1 s -1

Sediment transport and deposition

• • • Major issue in streams, reservoirs, and coastal areas Major driver for spatial variability in floodplain soils Management concern for agriculture, construction, and engineering http://blackwarriorriver.org/siltation-sedimentation.html

Reading assignment

• Redistribution of soil moisture – p. 297-303