Gully Erosion, Sand Dunes, and Mass Movement

1. Gully Erosion, Sand Dunes, andMass Movement

2. 4.1 Introduction • Gullies are severe stage of water erosion. • Sand dunes are the results of severe wind erosion and deposition. • Mass movement is occurred on steep land which is mainly caused by gravity agent. Watershed management-- 13

3. 4.1 Introduction • Poor land use practices and poorly planned development activities can cause these problems. • The greatest economic loss from gully erosion, wind erosion, and mass movement frequently occur in downstream and down wind, or down slope from the problem area. Watershed management-- 13

4. 4.2 Gully Erosion • Gully erosion is the detachment and removal of soil by concentrations of flowing water, sufficient to cause the formation of channels that could not be passed by normal cultivation tools. • The flow in gully is almost ephemeral • Gully often occur on area that have a low density vegetation cover and highly erodible soils. Watershed management-- 13

5. 4.2 Gully Erosion The causes of gully erosion • In engineering terms the cause of gully erosion is the breakdown of a state of metastable equilibrium in the stream or watercourse. Increasing the floods Decreasing channel capacity Watershed management-- 13

6. Classification of gully erosion

7. 4.2 Gully Erosion

8. 4.2 Gully Erosion

9. 4.2 Gully Erosion

10. 4.2 Gully Erosion Control of gully erosion Principles of gully control Control by vegetation In gully control a bag of fertilizer is more effective than a bag of cement Structures for gully control Temporary structures Permanent structures Watershed management-- 13

11. 4.3 Sand Dunes • Sand dune is mounds of erosion materials that exist independently of any fixed surface feature and are capable of movement from place to place. Watershed management-- 13

12. 4.4 Mass Movement • Mass movement is the rapid down slope movement of finite masses of soils, rock, and debris that driven by gravity. Falls / Slumps 崩塌 Land slides / Avalanches 滑坡 Debris torrents 泥石流 Earthflows 泻溜 Creep 蠕动 Watershed management-- 13

13. 4.4 Mass Movement • The stability of the soil mass on a hillslope in respect of mass movement can be assessed by a safety factor (F): F = s/ The slope is stable if F > 1 The slope will failure if F <=1 F = [c + ( zcos2 - u) tan]/ [z sin cos] Watershed management-- 13

14. 4.4 Mass Movement F can be defined by Skempton and Hutchinson, 1969: F = [c + ( zcos2 - u) tan]/ [z sin cos] Where c = the effective cohesion of the soil,  = the unit weight of soil, z = the vertical depth to the failure plane,  = the slope angle and  = the effective angle of internal friction of soil. Watershed management-- 13