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SoilErosion_WEP - Hillslmpe Erosion by 1'i’ilate-r In the...

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Unformatted text preview: Hillslmpe Erosion by 1'i’ilate-r In the earl}r 1940’s, experimental work by W. D. Ellison demonstrated the role of raindmps in splashing soil particles into motion. Earlier work in Europe and the United Slates had pointed out that the raindrops eomposing a storm of moderate size and inlenaily [emanate tremendous amount:: of kinetic energy by virtue of their mass and their velmity of fall {kinetic energy — 115 X mass X velocitfi}. Ellison showed that a signifieanl part OE'this energj,r mobilizes soil, and is the major proeess initiating soil erosion at both the gonlogiealljr normal and accelerated rates In areas subject to Horton runoff. He took high-speed photographs of the rainsplash erosion. as shown in Figure [5—4. It is possible to see elearly from the photographs that the impael ul‘ the drop generates a small explosion of soil and water. Large soil aggregates are dispersed and the smaller particles are splashed Figure l5--'l Highsspeed photograph ot'a raindrnp impact on a soil surface. Soil particles are splashed in all directions. but those traveling downhill rntwc farther than those splashed uphill. tL'SIlA —5oi| {‘onsenation tiers-ice} over several feet. The soil particles splashed downhill tras'ei farther than those that more uphill. causing a net downslope transport of soil. Repeated billions of times per rainstorm. this downslope transport is what we measure as rrtt'nspt'usl: erosion. Alter centuries ol‘ concern the Fundamental cause ol‘ soil erosion finalis- became apparent from Ellison‘s work. To appreciate this process. you mas- recall occasions wlten you have walked in the desert or on a construction site during a hears- rainstorm. ‘rour trouser legs record a profile of the heights to which soil particles can he splashed by raindrops. Alter the rainstorm each area of soil protected by a pebble stands up on a small earth pillar. indicating the depth of soil mos-ed by rainsplasb. Rainsplash erosion is particularly important on steep slopes ties-old ot‘ vegetation. just the conditions created during parts ol" the year on agricul- tural fields. toad cuts. and mining and construction sites. A tltiek 1vegetation coeer intercepts virtually all the kinetic energy of rainfall tsee Figure Iii-fit and herein lies the critical and dominant role of vegetation in reducing soil erosion. Man;-r cultts-ated crops leave the soil esposed to raindrop impacts [Figure 15—61. In general. the denser the vegetative cot-er. the lower the rate ol'erosion: this factor. which is controlled in tttrn by climate and land use. dominates the ell'eets ol' all other controls such as raint'all energy and hill- slope gradient. In Fact. the area of the United States that experiences the heaviest rainfalls. namely the southeast. also has one of the lowest geologic rates ol'crosion becaUse ot' the thick natural forest cover. When the forests ot‘ the region were cleared for planting corn. tobacco. and cotton in the late if t’ Hrt'ilsi'rtgm- l’rrtt '1 ' site I.' Figure I55 Thick ground cover of guns and ulnwr. A [hick vegetation 01}ch protects 1hr: mil very L-mdunuy Tram th: klnctit. cum-.1123 "I" raindrops. Few if any drops mach 1J1: 50]]. Lin-mil}r hul rathcr drop rm": 3. Small thghL ultcn on“; urgun'u: mm. {USDA Sui] Cnnscnala'tm Service.) Figure 15-6 Unch a corn crop but: gmqu i5 :xpflfled tfl raindrop J'Tl'lpilcl and Shmmnsh. I|1 lh'rs picrurc m-crland flnw 1's rumflng. 0H' 3 hlllilnp: on a Maryland Farm. {USDA— Snll Curmcn'aliun Hewitt} Figure I55 Thick ground cover of guns and ulnwr. A [hick vegetation 01}ch protects 1hr: mil very L-mdunuy Tram th: klnctit. cum-.1123 "I" raindrops. Few if any drops mach 1J1: 50]]. Lin-mil}r hul rathcr drop rm": 3. Small thghL ultcn on“; urgun'u: mm. {USDA Sui] Cnnscnala'tm Service.) Figure 15-6 Unch a corn crop but: gmqu i5 :xpflfled tfl raindrop J'Tl'lpilcl and Shmmnsh. I|1 lh'rs picrurc m-crland flnw 1's rumflng. 0H' 3 hlllilnp: on a Maryland Farm. {USDA— Snll Curmcn'aliun Hewitt} eighteenth and early nineteenth centuries. however. the result was enta- strophic accelerated erosion. Parts of Brazil. the it'lalagasfgr Republic. Malawi. and many other tropical and subtropical regions have sulferetl the same fate. Another important factor controlling the intensity of rainsplash erosion is the resistance of soil to dispersal. High organic content and moderate amounts ol'elag.r and calcium seem to he the factors promoting the develop- ment of stahle soil aggregates. Si]: and sand are more easin splashed. and man}- tillage operations break down the aggregates to the detriment of soil resistance. The factors controlling resistance to splash are mirrored by Bljl'illl {I963}. M detained in Chapter 9, in areas where rainfall intensity,r exceeds the infiltration capacity of the soil. water accumulates on the land surface and runs downslnpe as an irregular sheet. The depth and velocity of this water increase downsiope as more water is generated by precipitation excess. until the force applied to the soil by the water is sufficient to exceed the resis- tance ol'soil to erosion. The intensity of erosion depends on the product of water depth and hillsiope gradient. it mag.r increase or decrease with distance downslopc depending on the combination of water depth and hillsiopc gradient along the hillslnpe profile. it mmmon situation is for erosion to increase with distance as the gradient steepens dowuslopc. and then for erosion to decrease. or even for some deposition to take place on the gentle footslope. These relationships are summarized in Figure 15-1. Our picture F e it Belt of no 1 F J R. Erosion decreasing shutout-h : Erosion or deposition emdrm : ind-easing occurring F I . '- / ’ Zf: II I; i I 2- 'L/ if %7 ' Z agar)???" 19/594,; / / / / my! I _ .1 .r/,- //. s pep? , , y/fie p/fi/// // - I" - _.r emf/essays ///5/C”JrZs/ficé/ e/a Figure 15-? The relative magnitudes ofthe ecu-din! force ofsheetwesh {F} and the resistance {R} of the soil so creation determine the width of a belt of no shmwmhflosinnnearlhempol'altil sheetweshemsiondependsonfltcuuiw Ft’lr'hiclisptoportimalmthe product ei'hiltshpegadient and water depth}. Nine titer rainsplestt erosio- three oerurintbebellofnoshecmlshmion. 5:3 Harrier: Processes Fla-Int“ Ahflhkcouadcrgoapnpidnmmmdshmwnhmmuamoh ofbta-ip gum; mmpmdhummp Ila-r mots-oil sunning undct each that bdtlh‘lhfllbtrhl .‘hxhhnapradwuflonh-nomcnnhnlotIant tapioll dining Ilic Lot 3cm. In the Wound. thc soil our“ in“ opt of Input mfacc wash. of a uniform slice! Hr n'atcr iii an idcait'zntion. In fact. tht: shcct Domains tin}- 5trcatns and thrcada ol' watcr that am slightly dccpcr and l'aattrl‘ than tltc ai-cragc. The}; moi-t.- hack and forth amiss thc hilhlopc during the rain- storm and tngcthct with rain-splash atc rct-ponsihlc tor mott ol' thc erosion and tramptntanon ot' mi! from tho hillflup-c. Wu.- nill rcrcr to this opinion [um-$5. htmcvcr. an aha-town caution. Manse the act cll'ccl. of tho mow:- tncnt ol‘thosc thrcacia. through many rainstonns is to rcntovc a morn or too. uniform dcpth of soil from the hiiislopc. flt‘tcn tho lflm ahcotwaah cnnion cncotnpam rainipiaah ctocion hccanw tho two arc ditticult to Mpamtt.‘ in thc field. but wc hat-c choscn to dixcnw thcrn scparatclp hcrc. Tttc cfi‘ccti'. of oomhtncd 5hcctwash and l'ilil'h'piiflh crotion arc apparcnt in Fignrc tins. Some at tho oontrots ol‘ I'ilil'iSphhh cmaion also att‘cct ahcctwaah crow-ion. Rainfall 'tntcnaitg:r tictcrrninca the amount and ran: ol‘ precipitation cacti-as and thcroftn'c of the amount and ticpth or runoil'. chctation supplies otpa nic matth to thc soil. pnwitlcs fat-oratiic conditions for soil microfauna. which with plant roots prornotc lotwc lop-mil wilh a lush infiltration capacity and lint talc: ol' ahat'ctwaxh. Sui'l tchturc 'ts anotth dctcnninant ol‘infiltra- lion capacity. as or; ammo in Chaptcr ii. The mistam'c or soil to mansion is anothcr important rant-tow. though onc that is not dearly andomood. Largo mil particica are ntotc dillicolt to moi-c than Iincr onus. hot that ccry Iincat trill and clay particlox arc morn ditticult to mow than sand. A 3.9m! soil structurc in which particch arc hold togctlicr Fla-Int“ Ahflhkcouadcrgoapnpidnmmmdshmwnhmmuamoh ofbta-ip gum; mmpmdhummp Ila-r mots-oil sunning undct each that bdtlh‘lhfllbtrhl .‘hxhhnapradwuflonh-nomcnnhnlotIant tapioll dining Ilic Lot 3cm. In the Wound. thc soil our“ in“ opt of Input mfacc wash. of a uniform slice! Hr n'atcr iii an idcait'zntion. In fact. tht: shcct Domains tin}- 5trcatns and thrcada ol' watcr that am slightly dccpcr and l'aattrl‘ than tltc ai-cragc. The}; moi-t.- hack and forth amiss thc hilhlopc during the rain- storm and tngcthct with rain-splash atc rct-ponsihlc tor mott ol' thc erosion and tramptntanon ot' mi! from tho hillflup-c. Wu.- nill rcrcr to this opinion [um-$5. htmcvcr. an aha-town caution. Manse the act cll'ccl. of tho mow:- tncnt ol‘thosc thrcacia. through many rainstonns is to rcntovc a morn or too. uniform dcpth of soil from the hiiislopc. flt‘tcn tho lflm ahcotwaah cnnion cncotnpam rainipiaah ctocion hccanw tho two arc ditticult to Mpamtt.‘ in thc field. but wc hat-c choscn to dixcnw thcrn scparatclp hcrc. Tttc cfi‘ccti'. of oomhtncd 5hcctwash and l'ilil'h'piiflh crotion arc apparcnt in Fignrc tins. Some at tho oontrots ol‘ I'ilil'iSphhh cmaion also att‘cct ahcctwaah crow-ion. Rainfall 'tntcnaitg:r tictcrrninca the amount and ran: ol‘ precipitation cacti-as and thcroftn'c of the amount and ticpth or runoil'. chctation supplies otpa nic matth to thc soil. pnwitlcs fat-oratiic conditions for soil microfauna. which with plant roots prornotc lotwc lop-mil wilh a lush infiltration capacity and lint talc: ol' ahat'ctwaxh. Sui'l tchturc 'ts anotth dctcnninant ol‘infiltra- lion capacity. as or; ammo in Chaptcr ii. The mistam'c or soil to mansion is anothcr important rant-tow. though onc that is not dearly andomood. Largo mil particica are ntotc dillicolt to moi-c than Iincr onus. hot that ccry Iincat trill and clay particlox arc morn ditticult to mow than sand. A 3.9m! soil structurc in which particch arc hold togctlicr by organic material and cations into a porous aggregate is known to resist shcctwash erosion as it resists dispersal by rainsplash. It moderate amount of clay in the soil binds sand particles together, but high clay content lowers the infiltration capacitglr and promotes runoff. tiilislope gradient afiects sheetwash erosion by increasing the erosive force the runofl' applies to the soil surface and hillslope length is important because. as indicated in Figure li—“l', the depth of overland flow increases with the distance downslope and its abilityr to erode is proportional to its depth. At the top of a hillslope, the depth of the sheet is not sufficient for the eroding force to overtitrme the resistance of the soil to erosion. There will he a zone near the top of the slope in which sheetwash erosion cannot Occur. though rainsplash erosion can. The relative importance of rainsplash and the force applied by sheetwasb changes with the distance downslope. Cln teen,r long hillslopcs with lowr infiltration rates. sheetwash usually becomes the dominant process~ although rainsplash is still exceedingly important in mobilising soil which is then transported by the sheetwash. The combination of the two processes can be appreciated by closelyr observing surface runoff during a lull in a storm when abundant water is moving over the surface. but onlyr an occasional large raindrop falls. The drop passes through the sheet of overland flow and splashes soil up into the flow. A small. faint plume of sediment can be seen drifting downslopc with the sheetwash. If the minute streams of water out separate channels, as shown in Figure IS—El'. the process is known as rill erosion. The concentration of runoff in Figure 15-9 Kill Unbliifl mi an agricultural hold in Illlnoix. tL'SDA —Sanil CUTIWB'ELIIHTI ficrs‘ict‘J these small channels causes an increase in the etfieieney and intensity ol‘soil removal. Cln agricultural fields. tillage operations may obliterate the rills each year, causing their significance to he overlooked. If the rills become engraved into the land surface to depths of more than about one foot. and especially if they are not obliterated by cultivation and do not migrate back and forth across the hihslorpe. they are generally classi- fied as gullies [Figure 15-11)}. Gall].- erosion produces incisions ranging in size from a foot in depth and width and a few feet long to several tens of feet deep. hundreds of feet wide and miles long. (ieomorphologists do not yet understand exactly hov.r rilis and gullies form and remain stable, but they are generally associated with very intense water erosion under the most severe oombinations of the oontrolling factors listed above. The amount of soil exported by gully erosion is usually small by omnpar— ison with that removed by rainsplash and sheetwash erosion. but the process can do great damage to an individual piece of land. In a heavily cut and grazed region in northern Kenya, gully erosion measured by Duane has mobilized only I percent as much sediment as sheet erosion. Leopold et a]. [Who] found that gully groth supplied only L4 percent of the sediment yield of a small EIWJBd catchment in a semi-arid area near Santa Fe. Rain- splash and sheetwash erosion supplied 913 percent and soil creep EL? Figure lS—llll {tally erosion after land was cleared and loaded for housing construction. In a single winter the main gully was eat to a depth oft; feet and an average vothh of 501m and was more than SLI'J feet long. Juanita Creek basin: Kirkland. Washington. {USDA—Soil Conservation Service.) Figure [5-11 Coma-notion sites undergo intense shoemash and gully erosion because the vegetative oover is removed and the sortaee is continually stocpencd. churned. and compacted. [Hansen] Conservation ServieeJ percent. Brune {1950} measured gully contributions of 3.4 to 45] percent In an agricultural basin in Illinois; sheet erosion supplied 19—96 peroent. with the remainder coming from erosion in the valley bottom. and probably from soil creep. Glymph [1957} compared gully and sheetwash erosion at many localitiw in the agricultural lands of the east and central United States. Gullies supplied from I} to 8‘} percent of the total sediment yield. but most ol‘ the high values came {rorn one region of Mississippi where the pmcess is particularly severe. Seventy-five percent of Ulymph’s values were less than 343 percent. We do not yet have any quantitative estimates of the contributions of rill and gully erosion on urban mnsttuctlon sites. road cuts. or mined areas and spoil heaps, where conditions favor all types of water erosion. As we will show in the next section. rates of soil loss from such areas are very high bewase the vegetative cover is removed. the permeable topsoil is removed. and the dense subsoil is exposed. compacted. and graded into steep slopes and is frequently disturbed by heavy machinery. Some of the conditions encountered on these sites are shown in Figure 15-11. Mined areas and their spoil heaps sufi‘er rapid water emsion for the same reasons. Even in formed and agricultural areas. roads are important sources of sedintent and need constant care to minimize damage to stream systems {Figure 15-12). More will be said in Chapter 11" about the efiect of urbaniza- Lion and other types of land use on the sediment yields of drainage basins. Figure 15-12 Sediment washing from a mud road in eastern Kenya. Rates of Water Erosion Because of the difl'ieuities of separating rainsplash. sheetwash. and rill erosion. their efl'eets are usually measured together by monitoring the amount ofsedimertt lost from hillside plots like those shown in Figure 1543. or from small wtehrnents. Another method of obtaining the same data is by repeated measurement or the exposure of stakes or nails (Figure 1314]. A detailed discussion of techniques for evaluating various kinds of erosion is provided by Leopold el al. {igfifij and by Dunne HEW}. Thousands of measurements of rates of soil loss have been made from hillslopes with different gradients, length. vegetative savers, and soil eon— servation techniques. We make no attempt to summarize this literature. except to present a few representative measurements under dilTerent types of land use. They simply emphasize the overriding importance of vegetative cover and therefore of land use. Figure li-lfita] is perhaps the most Ilivider quoted summary of soil loss from agricultural plots in the midwestern United States. Figure lfi-lfith} shows a similar compilation from studies in Tanzania. The results speak liar themselves in view of our foregoing dis.- eussion. A sample of data is listed in Table I5—l. figure 15-13 Experimtniul pluh 2hr mausun'ng runflfi' and soil CTflfiil'ITI Lind.“ Furiuui- EmE‘IS and cultivaunn prawns a: MmJb-um Ncu- June}: The 111;: lamks Trap Ihi‘ watcr and HflJil'ntl'll Hugh! in the tfnugh HL lhl: hulluln of filth plfll {USU-"n Sail CUT-IMH- wutjun Sen-mm Figure 35-M- Fmfimn Flflfi. an a hillsidx: near Sana] Fe. New Mexico. Nails and Wafihtrh' an: phL-cd in fines an tin: slopc. and th: hmghl r‘rm-n [he mp of 1m nail to :he washer |5 measumd am‘maH}I with :1 millimclrr .IiL'aJc. k Soil inst by erosion Items per acre] W Water luel by runvncfl' [percent of rainfallj - _ .El} tens ' .122 .04 tons 6.5% 3-1.6 Inns .+:._— .:. 59.2 tans Baretl‘ailew} l3} Figure 5-15 Results from plot studies of runefi' and erosion under various types of land use. {at} Midwestern Llnilfil Sluice. [Sail Ctmxtrvafinn Sen-let} {1}] Mpwttpwn. Tunmnin. {From Rapp er al. 19H.) I'M 15-]. Sum: measurements elf unl lesa. l'rnm hillside pints. 5011 1:055 {TOHSIM'REIvn} [I TGNNEIHEITT tRE ] LAND use chTmN = 0.4414 I'thIM'RE HULIRCF. PURE-ST Primeval Oklahoma [IUD] Smith and Stanley U965} Hunted arm'uall}r Oklahoma 111] Smith and Stamev {1965‘} Primeval North {Interline 0.1112 Smith and Blarney “9195] Primele Kenya 0119 Dunne. unpubliahetl Burned semiannunfl}: North Carolina 3118 Smith and Stumey {[9155] Second growth Ivory {Toast 0.40 Nye anfi Greenland {1950} 'anun-rilanel1 protected Texas 0.135 Smith and Smmey {1965} wall-dlflfld.‘ burned Texas 0.35 Smith and Stanley {1965} annually iii"rtetrlltmnil1 protected Ohio [LDI Smith and Blame}; {1965} Woodland, protected North Carolina [LBS Dils ["5533 rmtr'nued TM '54. t'mmurd LAND lihE AGHK'IEIJHHE. {'UIJ lVATI-JII Gnmtnum Bluegrass Alfalfa Clover and gun Bermuda gnu Flume gram Iiuyiand Hayiand Tmpicnl PCNI'II'Iill-I yams Truphul imam Hayluntl Tmpical pattIurt: 5mm“ Dubgrus Urns:- AGII'I. lJLTUlh ('IO'PLAN I}! Ran: fallurw Ban.- rllluw Ban: I'atlluw Barn I'nllnw Ban.- fitth- Maize (mm: Mail: Mail: Mair: Maia- Milli}! Hill rice Hill rice Ilill nut Hill rice Ari-tat grain Coup-taut Urid 5min Peanuts IJ'K'ATIHH Midtt-fltcm [1.5. Midwcttcrn [3.5. vttginia 5W. Unitctl Suits- {Infirgih Washinglnn Earth Cum-HM Puma Rim Put-rm Rim lndta India lndta India Georgia ‘I'nnntnin Iii-arty Cum Midwestern US. India indin India hitdtn-cntt-rn LLS. Rhodes“ hfitltwnltm L'.5. Tu nianiu lndin India hut Gullah India India India indI-‘t “III. Ill'k‘i: [IlINSIrAleITII I Ittsutii'ln‘t'nttr: '- 0.4474 'tnhsn'tcnl. 9.92 4134 (“33 1.115 CLO! 410? noon [0 0.29 Illfll 13.03 [LEI LII} 0. IE L05 5.30 0.20 {1.41 |.4- LE- [Ell 59.2 H Hail-lint 4.1 "154 I156 2.1-4.5 '33.! 3M: L0 931 | [.2 {14- Hi 5.5 I .9 21.0 (it! Smith and Stamuy {Haiti Smith and Ethnic} [I'Ntl‘i Smith and Sumcy {lilfifii Smith and Slamcy “96$; Bunch 1 I965! Smith and Stanley “9155) Smith and fitantey i I965} Smith and Elamty {I965} Smilh and Emmett 1 I965} h'a-tudc'miah :1 II. [19:55] Vatudtwatnh et al. {I965} Italian-nt- and Rm “969] Linich Natinmilvfin Bnmctt “965i Rnpp ttl til. “912i Dubitt {[959} Bcnnnll [[939] United Matias-Lt H95” Bntlnnar and RH?“ ([969) Vasudcvttiith :1 nl. ilih‘rfi} limit-on :1 al. “9631' Human and Jackson “959:! Ht:an 1 H.391! Rnpp H in. tlifll} Batu-mar and RN “WI Vusutlct-ninh ut ul. {[915...
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