Water Erosion NREM 461 Dr. Greg Bruland I. Water Erosion A.Basic phases 1. Detachment: individual grains separated from soil mass/matrix 2. Transportation: detached grains transported over land surface 3. Deposition: soil grains deposited in new sites 2 B.Types of Water Erosion 1. splash: loosening & splattering of small soil particles caused by raindrops impacting a wet soil surface a. 3 2. sheet or interrill: removal of thin layer of soil over an entire soil surface caused by water flowing across soil surface a. b. 4 3. rill: occurs during/after rain or when snow melts, & involves concentration of flowing water into small channels (<0.3 m) that are more turbulent & have greater scouring action than sheet flow a. 5 4. gully: in areas steeper slopes, flow of water from rills concentrated, forming deep channels a. b. c. d. active: walls w/ no veg; inactive: walls w/ veg Australia 6 Stages in the development of a gully on a hillside (Morgan 2005, Leapold et al. 1964) 7 Gully erosion, Moloka‘i (Photo: J.B. Friday)8 Gully erosion in Morocco 9 Gully erosion in Madagascar (www.wildmadagascar.org) 10 11 5. streambank: removal of soil from banks of running streams a. 12 6. coastal/shoreline: erosion of ocean, lake, reservoir shores by action of waves resulting from wind, tides, currents, storm events, & boat traffic a. b. Gulf coast erosion, Alabama Coastal erosion in Pacifica, California 13 Shoreline erosion in New South Wales, Australia 14 These pictures were taken in Shishmaref, Alaska, during a storm in 2003. 2 hrs separate the first photo (left) from the second (right). For reference, red arrows mark the barrel. 15 16 C. Active Agents in Water Erosion 1. Falling Raindrops a. Energy of falling raindrops i. kine tic energy o f a fa lling bo dy ca lcu la te d as: E = ½ mv2 m = mass of fallinggyg body g v = velocity of fall cm/s E = kinetic energy (ergs) (g cm2/s2) 17 b. Relationship between drop diameter & terminal velocity i. Larger drops have higher terminal velocities ii. ↑ drops → ↑ speed → ↑ impact → ↑ detachment iii. all drops at terminal velocity after falling 9 m (30 ft) 18 c. Effect of raindrops i. Breaks peds & clods into smaller aggregates & idiidlindividual par tilticles ii. Moves particles into new locations iii. Compacts & puddles surface soil layer, ↓ infiltration & ↑ runoff 19 d. Rainstorm intensity & energy i. avg. drop size ↑ as storm intensity ↑ ii. threshold intensity at which rainfall becomes erosive = 25 mm hr-1 iii. re ltilations hibthip between KE&KE & ra ifllitinfall intens it(I)ity (I): KE = KE = total energy, ft-ton/ac-inch for each inch of rainfall I = rainfall intensity in inches/hr up to 3 in/hr 20 iv. Rainfall intensity in temp vs trop climates (Hudson ‘81) Wha t is thres ho ld in tens ity a t which rain becomes erosive? 21 e. Wind & raindrop energy i. wind adds horizontal-velocity component to raindrops & ↓ air resistance ii. 3 mm diameter raindrop falling in a wind at 30o angle has a ve loc ity 17% grea ter, KE 36% grea ter than same raindrop falling vertically f. Vegetative cover i. Vegetation acts as buffer bet. atmosphere & soil 1. aboveground: stems, leaves absorb energy of falling raindrops, protect soil 2. belowground: roots, contribute to mechanical stthfiltrength of soil 3. 22 23 f. Slope i. Raindrops more responsible for sheet erosion than surface flow of water ii. Raindrops falling on level bare soils, w/o wind, sppqylatter equally in all directions , soil removed matched by soil redeposited from another area iii. Raindrops fall on sloping land, more of splash goes downhill than uphill 1. 60-75% of splash downhill on a 10% slope 24 2. Running Water a. Runoff is important cause of erosion b. Runoff classification: sheet/prechannel (3-5 mm depth) or channelized flow (5 mm - >3 m) c. KE of rain vs runoff (KE = ½ m v2) Rain Runoff Mass Assume mass of fllfalling rain is R Assume 25% runoff, mass of RO = R/4 Velocity Assume term. vel. of 8 m s‐1 Assume speed of surf flow =1 m s‐1 KE 25 d. Runoff occurs when rain falls faster than soil can absorb it i. Does all precipitation generate runoff? 26 e. Energy of running water i. Laminar flow: water slowly passing over land in thin film w/ low energy ii. Turbulent flow: fast flowing water moving w/ sudden changes in both horizontal & vertical velocities 27 f. Streams with abrasive materials have > power to cause erosithtion than streams w /l/ clear wa ter 28 g. Stream velocity predicted w/ Manning’s Eqn V = 1.5 R2/3 S1/2 V = avg flow vel., m/s η R = hydraulic radius, m S = slope % or o η = coefficient of surf. roughness R = A/P A = P = 29 h. Effect of slope on surface water i. gradient: vertical fall per horizontal distance steeper slopes mean faster speed of water moving downhill & ↑ erosive force of water ii. length: distance from crest of hill to point where deposition starts shorter slopes usually have greater erosion per unit area, 30 i. slope shape: straiggpht, convex, concave, complex i. convex upper part of slope is dry, erosive area ii. concave lower part is moist, depositional area iii. complex: convex at the top, concave at the bottom iv. Nearly all slopes in the tropics are 31 j. slope aspect: the direction the slope faces i. greater erosion on south-facing slopes. Why? South vs North facing slopes in Colorado 32 D. Soil Properties & Erodibility 1. Texture: a. Sands: easily detached b.c. they lack cohesiveness, difficult to transport b.c. they are large, heavy b. Clays: difficult to detach b.c. they stick together, easy to transport b.c. they are small, light c. Silts: itinterme ditdiate 33 2. Organic matter: a. improves aggregate stability, water holding capacity, fertilityyg & plant growth, microbial activit y, & bioturbation 34 3. Cation Content a. Soils w/ high base cation content are more stable as cations contribute to bondinggggg of aggregates 4. Mineralogy a. Kaolinite (1:1), halloysite (1:1), chlorite (2:1:1), micas (2:1) don’t expand on wetting, resist erosion b. Smectite (2:1), vermiculite (2:1) expand w/ wetting & are highly erodible 35 E. 3 ma in w ays to reduce w ater er osi on 36.