Erosion and Sediment Concepts

1. Erosion and Sediment Concepts Slides with this background are From Wiley Science

2. Streams as Geological Agents • Streams are vital geologic agents. • Streams carry most of the water that goes from land to sea (essential part of the hydrologic cycle). • Streams transport billions of tons of sediment to the oceans each year. • The load is the sediment and dissolved matter the stream transports • Load (expressed in kilograms per cubic meter). • Dissolved matter generally does not affect stream behavior.

3. “Erosion is a process of detachment and transport of soil particles by erosive agents.” Ellison, 1944 Erosive Agents Raindrop impact Overland flow surface runoff from rainfall Bed and bank turbulence in streams EROSION

4. TYPES OF EROSION • Interrill and rill (sheet-rill) • Ephemeral gully • Permanent, incised (classical) gully • Stream channel • Mass movement • Geologic

5. EROSION IS A CONCERN • Degrades soil resource • Reduces soil productivity • Reduces soil organic matter • Removes plant nutrients • Causes downstream sedimentation • Produces sediment which is a pollutant • Produces sediment that carries pollutants

6. Physical factors leading to high soil erosion • Intense storms • May occur anywhere • Likelihood may increase with climate change • Sparse vegetation and ground cover • Particularly in association with intense storms • Silt-rich soils • Tendency to crusting, giving high runoff when bare • Mechanically weak and so highly erodible • Steep slope gradients • But steepest slopes may be on resistant soil or rock • Combination of steep slopes and weak soils most common on marginal lands • Large collecting area for runoff • Often related to flow convergence rather than slope length www.geog.leeds.ac.uk/people/m.kirkby/Presentations...

7. Erosion by Running Water (1) • Erosion by water before a distinct channel has formed occurs in two ways: • By impact as raindrops hit the ground. • By overland flow during heavy rains, a process known as sheet erosion. • The effectiveness of raindrops and overland flows in eroding the land is greatly diminished by a protective cover of vegetation.

8. Factoid: large raindrops fall at 30 km/hr ! Erosion mechanics • Detachment • Transport • Deposition Most hillslope erosion is initiated by the impact of raindrops, NOT by the flow of running water

9. Erosion by Running Water (2) • The ability of streams to erode is influenced by the way water moves through a stream channel. • If the velocity is very slow the water particles travel in parallel layers, a motion called laminar flow. Seldom occurs in natural streams except in the boundary layer • With increasing velocity, the movement becomes turbulent flow. • The ability of a stream to pick up particles of sediment from its channel and move them along depends largely on: • Turbulence. • Velocity of the water.

11. The Stream’s Load • The solid portion of a stream’s load consists of two parts: • The bed load: the coarse particles that move along the stream bed. • The suspended load: fine particles that are suspended in the water. • Wherever they are dropped, these solid particles constitute alluvium. • Streams also carry dissolved substances called the dissolved load. • These are chiefly a product of chemical weathering.

12. Stream Load Fluvial propcesses Strahler, A. and Strahler, A., 2004. Physical Geography. Wiley, NY.

13. Bed Load • The bed load generally constitutes between 5 and 50 percent of the total load of a stream. • Particles move discontinuously by rolling or sliding at a slower velocity than the stream water. • The bed load may move short distances by saltation (series of short intermittent jumps). • Coarse-grained sediment is concentrated where the velocity is high. • Finer-grained sediment is found in zones of progressively lower velocity.

14. Lane (1955) diagram showing relationship between Sediment load, water discharge, sediment size and channel slope

16. Sediment Yield (1) • Sediment yield is a function of: • Rock type and structure. • Local climate. • Relief and slope. • Because of storage differences, smaller basins tend to show higher yields than larger basins on unit area basis. • The greater the precipitation the greater the potential erosion. • In moist regions, plant roots tend to anchor the soil, thereby curtailing erosion. • In temperate eastern North America and Western Europe, vegetation cover is more or less continuous and erosion rates are low.

17. Sediment Yield (2) • In drier regions, reduced precipitation limits vegetation, making the land vulnerable to erosion. • Areas receiving abundant precipitation may actually experience less erosion than some relatively dry regions, • Fields measurements suggest that some of the greatest local sediment yields are from desert landscapes. • Some of the highest measured sediment yields are from basins that drain steep mountains along plate boundaries. • Monsoon regions of southeastern Asia receive abundant precipitation that generates high runoff.

18. Sediment Yield (3) • In southern Alaska and the southern Andes, large active glaciers contribute to high sediment yields. • Rocks that are highly jointed or fractured are more susceptible to erosion than massive rocks,. • The clearing of forests, cultivation of lands, damming of streams, construction of cities, and numerous other human activities also affect erosion rates and sediment yields.

19. Langbein and Schumm Model Slopeeroson From Ritter et al., 1995; After Lanbein and Schumm, 1958

20. Universal Soil Loss Equation • Most widely used model • Empirically based • Watch your units! • USLE still hidden in some more complex models • Various more complex models now available • WEPP Watershed Erosion Prediction Project

22. USLE - Universal Soil Loss Equation A =RKLSCP = Annual soil loss rate Hydrologic cycle factor • R= rainfall erosivity • K= soil erodibility • L = slope length • S = slope steepness • C = cover and management • P = erosion-control practices Soil/topography-related factors Land management factors Revised (RUSLE) and Modified (MUSLE) versions also exist http://cropandsoil.oregonstate.edu/classes/css305/lectures/Chpt13_erosion_short.pdf

24. http://snr.osu.edu/current/courses/NR675/transport_soil.pdf

25. http://snr.osu.edu/current/courses/NR675/transport_soil.pdf

26. R-factor Rainfall erosivity • R = EI for a given storm • E is the kinetic energy of the storm • I is the maximum 30-minute intensity of the storm • EI is calculated for each storm and then summed to get the annual erosivity index • Figure 9.6, p 260 in your book show R for the U.S.

27. K: soil erodibility factor A = RKLSCP • Infiltration capacity • Structural stability • Particle cohesion • cementation by Organic Matter and clays • Particle mass (2.0 g/cm3 for bauxite, 5.3 for hematite, 7.6 for galena) http://cropandsoil.oregonstate.edu/classes/css305/lectures/Chpt13_erosion_short.pdf

28. Soil properties resulting in low K values (less erosion) • High organic matter content • Non-expansive clays • Strong granular structure • “stoniness” & macropores • Nomograph in figure 9.8, p. 264 in your book to get K value • Nomgraph uses values of: % silt and very fine sand, % sand, % organic matter, soil structure, and soil permeablity

29. Soil erodibility factor for various soil textures

30. Topographic factor - LS • More erosion from steep, longer slopes • Everything compared to a 9% slope of 72.6 feet • Figure 9.10, p 265 in your book

31. Cover Management Factor, C • Vegetation cover • Crop sequence • Productivity level • Length of growing season • Tillage practices • Residue management • Temporal distribution of erosion events

32. Ag systems: C based on crop rotation and tillage practices • Forest and range systems: C based on vegetation density, (% ground cover) and litter/duff condition • Table 9.1, p 260 in your book shows C factors for non-ag systems • Table 9.2, p 261 shows an example of C factors for Ohio crops

33. 1. Mulch as a means to reduce erosion A = RKLSCP C =cover and management http://cropandsoil.oregonstate.edu/classes/css305/lectures/Chpt13_erosion_short.pdf

34. A few practices to reduce soil loss caused by timber production • Tree removal: cable not skidder • Scheduling: when dry or frozen. snow great. • Road design: 99% of soil loss avoided by gravel, planting grasses on road cuts • Buffer strips: 1.5 times the height of the tallest trees http://cropandsoil.oregonstate.edu/classes/css305/lectures/Chpt13_erosion_short.pdf

35. Erosion Control Practice Factor- P • Often this factor = 1, no practice in place • Contouring • Strip cropping • Terracing • Table 9.3, p 262 in your book show P factors