Movement by water Detachment Water Erosion Transport Deposited Water Erosion and Sedimentation Soil Erosion by Water Raindrop impact and the three phases of erosion Classified based on nature and extent of soil removal Sheet erosion removal of thin layer of soil over whole surface Rill erosion erosion channels small enough to be eliminated removed by normal tillage operations 1 Soil Erosion by Water con t Gully erosion channels can not be erased by ordinary tillage Streambank erosion banks of perennial streams Rain simulator 2 Sedimentation Common types of erosion damage Soil loss nutrient losses Textural changes coarse grains left sand Structural Damage exposes less friable less permeable subsoil breaks aggregates crust suspended clay infiltrates into pores Plant 3 Common types of erosion damage Productivity loss Field dissection gullies Engineering structural damage Water pollution Sedimentation Agents active in water erosion A Fall raindrops Kinetic energy E mv2 E kinetic energy ergs m mass of falling body g v velocity of fall cm sec Agents active in water erosion Predicting rainstorm energy A Fall raindrops Con t Air friction slows the fall of the water drops terminal velocity maximum speed Larger drops fall faster diameter of rain 0 25 mm to 7 0 mm Wischmeier father of water erosion prediction and Smith 1958 e 118 9 87 3 log10I e total energy in units of 103 joules ha for each mm of rainfall I rainfall intensity mm hr joule 1 36 foot pound erg 10 joules Vegetation Wind component Can add a horizontal velocity component to rain drop fall reduces air resistance Increases velocity Increases kinetic energy Interrupts raindrops and reduces velocity Mulch reduces impact 4 Raindrops Breaks aggregates Removes small soil grains Compacts and puddles the surface layer of soil Agents Active in Water Erosion con t B Running water con t Runoff occurs when the intensity exceeds the soil infiltration rate Greater depth of flow Velocity is higher and erosive energy is greater Slope and Runoff and Erosion Agents Active in Water Erosion con t B Running water Overland flow or runoff Energy kinetic energy erosive force Soil grains saltating in stream of water jumping along the bed Agents Active in Water Erosion con t B Running water con t Slope and Runoff and Erosion Features of slope affecting velocity and amount of runoff Slope gradient Slope length Slope shape Slope aspect Hydraulic radius con t R A P is less than D A W R hydraulic radius m D average depth of flow W width of flow on surface P wetted perimeter on soil surface of channel bottom m A cross sectional area of flow in a channel m2 Hydraulic radius R W D P R hydraulic radius m D average depth of flow W width of flow on surface P wetted perimeter m A cross sectional area of flow in a channel m2 5 Manning s Formula V R2 3 S1 2 h V average velocity of flow m sec R hydraulic radius m S slope of channel m m H coefficient of roughness Stokes Law rate of particle drop V 2 ds df gr 9n V velocity of fall cm sec ds density of the solid g cm3 df density of the fluid g cm3 n viscosity of fluid poise g acceleration of gravity c sec2 r radius of particle cm Gravity landslides Soil particles suspended in water at 20 C Assumed ds 2 65 g cm3 df 0 998 g cm3 n 0 01005 poise g 980 cm sec2 V 35 800 r2 cm sec 20 cm V cm sec t sec Soil properties effect erodibility Soil texture Sand heavy hard to transport Clay once separated is easy to move Soil properties effect soil erodibility con t Soil structure Clay acts as a cementing and aggregating agent Aggregation impedes movement Texture a point high clay higher aggregates to 6 Soil properties effect soil erodibility con t Soil structure con t Type of clay Tropics hydrous oxides of Fe Al amorphous 1 1 clays kaolinite more aggregates 2 1 clays montmorillonite and illite less aggregates Soil Structure con t Organic matter Cementing agent lime holds particles in aggregates Cropping history native vegetation Vegetation and water erosion Fallow corn wheat bluegrass least erosive Columbia Missouri Principles of water erosion control Reduce raindrop impact runoff volume and velocity Increase the soil s resistance to erosion Management practices that effect one or more of these principles will help to control water erosion Reduce 7