1 grassed water ways

Useful for B. Tech Agril Engg and B Sc. (Ag.) students

1. Grassed Waterways and Their Design
Md. I. A. Ansari
Department of Agricultural
Engineering
BAU, Kanke, Ranchi
(e-mail: [email protected])

2. Topics to be Covered
•Grassedwaterwaysanddesignofgrass
waterwayswinderosion:mechanismsof
winderosion,typesofsoilmovement,
principlesofwinderosion,controland
controlmeasures.

6. Grassedwaterwaysareopenchannelsprotected
withsuitablegrassesorshrubs.Itissimplya
drainagechannel.
Itisconstructedalongslopeandactasoutletfor
terracesorgradedbunds.Itssizedependson
expectedrunoff.
Itisjustlikeirrigationchannel.

7. •Vegetatedwaterwaysarebuilttoprotect
soilagainsttheerosiveforcesof
concentratedrunofffromslopinglands.
•Bycollectingandconcentratingoverland
flow,waterwaysabsorbthedestructive
energywhichcauseschannelerosionand
gullyformation..

8. Grassed Waterways

10. Purpose of Grassed Waterways
•Grassedwaterwaysareusedasoutletsto
preventrillandgullyformation.
•Thevegetativecoverslowsthewaterflow,
minimizingchannelsurfaceerosion.
•Thesechannelsareusedforthesafe
disposalofexcessrunofffromtheterraced
andbundedcroplands,tosomesafe
outlet,namelyrivers,reservoirs,streams
etc.withoutcausingsoilerosion.

12. •Grassedwaterways:
natural
manmade
•These constructedchannelsare
establishedforthetransportof
concentratedflowatsafevelocitiesfrom
thecatchmentusingadequateerosion
resistantvegetationwhichcoverthe
channels.

13. •Thegrassedwaterwaysareconstructed
priortotheconstructionofterraces,bunds
etc.becausegrassestaketimetoget
establishedonthechannelbed.
•Generally,itisrecommendedthatthere
shouldbeagapofoneyearsothatthe
grassescanbeestablishedduringthe
rainyseason.

14. Photo 1: Severe soil erosion and sediment deposition

15. Photo : Successfully established and maintained grassed waterway
Welldesignedandmaintainedgrassedwaterways
canbeanimportanttoolformaintainingsoilquality
andproductivity.

17. Units of Measurement of Water

18. •Watercanbemeasuredonvolumebasis
whenitisatrest.
•Eg.:litres,milliliters,ha-cm,ha-m,cubic
metre,etc(dependinguponthe
magnitude and purpose of
measurement).
•Largequantityofwaterpresentina
reservoirmaybemeasuredinm3.
•Volumeofwaterappliedtoafieldcanbe
suitablymeasuredasha-cm.
•Volume=Areaxdepthofwater

19. •Watercanbemeasuredondischarge
basiswhenitisinmotion.
•Eg.:litrespersecond(lps),litresper
minute(lpm),cubicmetreperhour(m3/hr)
cubicmetreperminutes(m3/min,etc.
•Itisthevolumeofwaterpassingthrougha
sectionperunittime.
•Q=V/t
•V=QXt

20. Shape of Waterways
Theshapesofgrassedwaterwaysareof
threetypes:
(1) Trapezoidal
(2) Triangular
(3) Parabolic
Ifthefarmmachineryhastocrossthe
waterways,parabolicshapeortrapezoidal
shapewithveryflatsideslopesare
preferred.

22. Parabolic Shaped Waterway
•Aparabolicshapedwaterwayrepresentsa
naturalchannel.

23. Thefactorsthataffecttheselectionofshape
ofwaterwaysare:
(a)Constructionequipmentavailability
(b)Velocityofflow
(c)Gradeofthewaterway
(d)typeofgrasscover

24. Construction Equipment Availability
•Theequipmentavailableforconstruction
ofthewaterwayisoneofthemainfactors.
•Trapezoidalshapedchannelcaneasilybe
constructedwiththebladetypemachines.
•Trapezoidalcrosssectioniswidelyused
wherethewaterwayisartificially
constructedasterraceoutlets.

25. •Trapezoidalandtriangularcross-sectionof
channelnormallytaketheformof
parabolicsectioneitherduetosiltationat
thebottomorduetoscouringofthesoil
fromitsbottomandsideswithpassageof
time.
•Thatiswhyparabolicshapeofgrassed
waterwayisgenerallyconsideredasmost
economicalandalsoamorestablecross
section.

26. Velocity of Flow
Thepermissibleflowvelocityinthe
grassedwaterwaydependsuponthetype
andconditionofvegetationanditsdensity
toresisttheerosion.
Auniformvegetativecoverinthe
waterwayisimportanttoprovideabetter
channelstabilityandalsotodecidethe
permissibleflowvelocity.
Permissiblevelocityingrassedwaterways
variesaccordingtothevegetativegrowth.

31. •Theapproximatevaluesofpermissible
flowvelocityfordifferentgrassedcover
aregivenas:
•Permissible Velocity
(m/s) CoverCondition
0.9 to 1.2 Sparse grass cover
1.5 to 1.8 Good grass cover
2.0 to 2.5 Sod of excellent cover
•However,foranaverageconditionof
grassedcoverandchannelsection,aflow
velocityfrom1.5to2.0m/sisusedfor
designpurposes.

32. Grade of the Waterway
Normally,achannelgradeof
approximately5%isrecommendedfor
vegetatedwaterways.
Agradeofmorethan10%isnot
recommended,asitislikelytobecome
erosive.
Vegetatedwaterwaysaregenerally
constructedalongthedirectionofthe
slope.

33. Grass Cover
•Thegrasscoverincreasesthefactorof
roughnesswhichreducesthevelocityof
flowandthechannelhydrauliccapacity,
andalongwithit,thevelocityismadesafe
(non-erosive)fortherunofftopass
throughthechannel.

34. Design of Grassed Waterways
Thedesignsofthegrassedwaterwaysare
similartothedesignoftheirrigation
channelsordrainagechannels.
Generally,thesewaterwaysaredesigned
forcarryingthemaximumrunofffora10-
yearrecurrenceintervalperiod.

35. •Waterwayscanbeshorterinlengthor
sometimes,canbeevenverylong.
•Forshorterlengths,theestimatedflowat
thewaterwaysoutletsformsthedesign
criterion,andforlongerlengths,avariable
capacitywaterwayisdesignedtoaccount
forthechangingdrainageareas.

36. Thewaterwayistobegivengreatercross-
sectionalareatowardstheoutletasthe
amountofwatergraduallyincreases
towardstheoutlet.
Thecross-sectionalareaiscalculatedusing
thefollowingformula:
Q=Av
where,A= cross-sectional area of the
channel,
Q= expected maximum runoff, and
v= velocity of flow.

37. Procedureforthedesignofthechannel
parameterscomprisesofthefollowing
steps.Thisprocedureisaniterativeone.
Step1:Assumethevalueofflowdepthand
calculatethechannelcrosssectionalarea
(A),wettedperimeter(P),hydraulicradius
(R)andtopwidth(T):

38. Parameters of Open Channels
•WettedPerimeter(P):TheLengthof
contactbetweenLiquidandsidesand
baseofChanne.l
•HydraulicRadius(R):Ifcrosssectional
areaisA,thenR=A/P.
•Depthofflowsection(d):depthofflow
normaltothedirectionofflow.

40. •Freeboard:Verticaldistancebetweenthe
highestwaterlevelanticipatedinthedesign
andthetopoftheretainingbanks.Itisa
safetyfactortopreventtheovertoppingof
structures.
•SideSlope(Z):Theratioofthehorizontal
toverticaldistanceofthesidesofthe
channel.

42. Trapezoidal grassed waterways.

44. Where,
b = bottom width (m)
d = channel depth (m)
Z = e/d =side slope (horizontal:
vertical) of trapezoidal channel

45. Triangular grassed waterways.

49. •Step2:Determinethemeanvelocityof
flowbyusingManning’sformulawhich
statesthat:
•R=hydraulicradiusofthechannelsection
(m)
•S=channelgrade
n=Manning’sroughnessco-efficient(for
vegetatedwaterways,n=0.04)
•Step3:DeterminethedischargerateQ=
Av(m
3
/s)throughthechannel.

50. Location of Grassed Waterways
•Thegrassedwaterwaysshouldbelocated
atthatpointwherethewatercandraininto
thewaterwaysfromallsides.
•Theplaceswhichrequireaminimumcost
ofshapingarealsopreferredforlocation
ofwaterways.
•Whensuitablenaturaldepressionsarenot
available,thewaterwayshouldpreferably
belocatedalongthefieldboundaries.

51. Construction of the Waterways
•Shaping(SoilDigging)
•Itisadvantageoustoconstructthe
waterwaysatleastoneseasonbeforethe
bunding.Itwillgivetimeforthegrassesto
getestablishedinthewaterways.
•First,unnecessaryvegetationlikeshrubs
etc.areremovedfromtheareaismarked
forthewaterways.
•Theareaisthenploughedifnecessary
andsmoothened.

53. •Theshapingofthewaterwayshouldbe
doneasstraightandevenaspossible.
•Anysuddenfallorsharpturnmustbe
eliminated,exceptintheareawherethe
structureisplannedtobeinstalledinthe
waterway.
•Inaddition,thegradeshouldalsobe
shapedaccordingtothedesignedplan.

54. Grass Planting
•Aftershapingthewaterwaychannel,theplanting
ofgrassesisveryimportant.
•Prioritiesshouldalwaysbegiventothelocal
speciesofgrasses.
•Perennial(speciesthatlivelongerthanone
year)grassesandshrubsarerecommendedas
theyrequirelessmaintenanceandcostlessin
thelong-term.
•Generally,therhizomatousgrassesare
preferredforthewaterway,becausetheyget
spreadveryquicklyandprovidemoreprotection
tothechannelthanthebrushgrasses.

57. •Inlargewaterways,theseedingischeaper
thanthesodding.
•Therefore,theseedingshouldbe
preferredforgrassdevelopment.
•Immediatelyaftergrassplanting,the
waterwaysshouldnotbeallowedforrunoff
flow.

58. Maintenance
•Overall,itshouldalwaysberemembered
thatthewaterwaysareanintegralpartof
watershedconservationorlandtreatment
system.
•Iftheyfailtohandlethepeakdischarge
duetolackofpropermaintenance,then
theprolongflowofrunoffthroughthem
candevelopgulliesinthearea.

59. Themaintenanceofwaterwayscanbetaken
upusingthefollowingprocess:
Grassedwaterwaysshouldnotbeusedas
footpaths,animaltracks,orasgrazing
grounds.
Frequentcrossingofwaterwaysbywheeled
vehiclesshouldnotbeallowed.
Newlyestablishedwaterwaysshouldbekept
understrictwatch.
Thelargewaterwaysshouldbekeptunder
protectionwithfencing.