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DesignofGullyErosionControl
Practices
Reading10-2thru10-6
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DesigningaPractice
?Example
–Grassedwaterways
–Diversions
–Dams
?Gradestabilizationstructures
?Sedimentcontroldams
–DropStructures
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Designingapractice
?Sizeandengineerthepracticeto
eitherholdtherunoffwaterback,
releaseitslowlyAND/ORcarrythe
runoffwatersafely
–Pipesize,spillwaywidth/height,depthand
widthsofwaterway…
3
8’
4
11’
5
Diversion
channel
berm
runoff
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?Howmuchrunoffwater?
–Volume
–peakdischarge(CFS)
?Cubicfeetpersecond
7
0
10
20
TIME
CF
S
Example-PeakDischarge-gully
8
910
1112
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Real-timeStreamData
http://waterwatch.usgs.gov/
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Factorsyouneedtoknowtodetermine
howmuchrunoffwatertohandle
1)Watershed:drainageareasize
–Largerthewatershed,largerthepractice
components
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Whatisawatershed?
?Surfacewater(runoff)
“Adelineatedareaoflandthatcontributes
runoff(drains)toacertainpointof
interest.”Waterflowsdownhill!
Inworkingwithwatershedrunoffyouneed:
1)Identifythewatershedboundaryandsize
Survey
Topographicmaps
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Otherfactorsneedtoknowforhow
muchrunoffwatertohandle
?Acresofeachlandusewithinthe
watershed
–Runofffrom:
concrete>cropland>pasture>hayland>forest
?Conditionoflanduse
–Heavilygrazedpasture>properlygrazed
pasture.
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2)SoilsInformation
–Hydrologicsoilgroups
?A,B,C,D
?SoilSurvey
?A-highinfiltrationrate,lowrunoffpotential
?D-lowinfiltrationrate,highrunoffpotential
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?Usethesefactorstodetermine
“RunoffCurveNumber(CN)”
?WeightedAverageforthewatershed
?HighertheCN:
–Morerunoff
–Quickertimeofconcentration
2324
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4
12
20
12
260
996
732
1540
352848
Weightedcurvenumber3528/48=74
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3)Slopewithinlanduse
–Fordesign,useaverageslopewithinwatershed
4)Rainfall
–needrainfallamountstoknowhowmuchrunoff
?Storms
?Worsecasescenario,dependingonpractice
?Lookatamountofrainina24hrperiod
?Basedonfrequencies:
–1,2,5,10,25yearfrequencies
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RainfallinGreeneCo.
Years1251025
Inches/
24hrs
3.23.94.95.76.8
Source:GreeneCountyStormWaterDesignStandards
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PracticeDesignandstormfrequency
?DiversionsandGrassedwaterways:10yrs
?Dams:Depends!
–2yearsforprincipalspillway
–25yearsonemergencyspillway
?DropStructures
–10or25years
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MorePiecesofthePuzzle
?Timeofconcentration
–Thetimeittakesforrunofftotravelfrom
distantpointofwatershedtotheoutlet
–Ifsamesizewatershed,theshorterdistance
willhavehigherpeakdischarge(CFSof
runoff)gettingtooutlet(pointofinterest).
–Tablesorequation:flowlength,slopeof
watershed,curvenumber
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?InitialAbstraction
–Accountforlossesbeforerunoffbegins
(infiltration,losttoevaporation,interceptedby
vegetation,retainedinsurfacedepressions)
–Table
?considerscurvenumber
?higherthecurvenumberthequickertorunoff,
thusalower“initialabstraction”
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?Usingtables,charts,calculations…come
upwithhowmuchrunoff(volume)and
howfastrunoffwillgettothepointof
interest(wherepracticewillbe).
?Sizeandengineerthepracticetohandle.
–Pipesize,spillwaywidth/height,depthand
widthsofwaterway…
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