Grassed water ways

Grassed waterways and their Design

1. Subject
Soil and Water Conservation
Engineering
Topic
Grassed Water-ways
&
Their Design
by
Dr. Sanjay Singh Chouhan
Assistant Professor
JNKVV-College of Agriculture
Powarkheda, Hoshangabad

2. Grassed Waterways
•Grassedwaterwaysarenaturalorman-made
constructedchannelsestablishedforthe
transportofconcentratedflowatsafe
velocitiesfromthecatchmentusingadequate
erosionresistantvegetationwhichcoverthe
channels.
•Thesechannelsareusedforthesafedisposal
ofexcessrunofffromthecroplandstosome
safeoutlet,namelyrivers,reservoirs,streams
etc.withoutcausingsoilerosion.
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3. •Terracedandbundedcroplands,diversionchannels,
spillways,contourfurrows,etc.fromwhichexcess
runoffistobedisposedof,preferablyuseconstructed
grassedwaterwaysforsafedisposaloftherunoff.
•Thegrassedwaterwaysoutletsareconstructedpriorto
theconstructionofterraces,bundsetc.becausegrasses
taketimetogetestablishedonthechannelbed.
•Generally,itisrecommendedthatthereshouldbeagap
ofoneyearsothatthegrassescanbeestablished
duringtherainyseason.
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4. Purpose of Grassed Waterways
1.Grassedwaterwaysareusedasoutletstopreventrillandgully
formation.
2.Thevegetativecoverslowsthewaterflow,minimizingchannel
surfaceerosion.
3.Itcansafelytransportlargewaterflowstothedownslope.
4.Thisbestmanagementpracticecanreducesedimentationof
nearbywaterbodiesandpollutantsinrunoff.
5.Thevegetationimprovesthesoilaerationandwaterquality.
6.Thewaterwayscanalsoprovideawildlifehabitat.
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5. Design of Grassed Waterways
Thedesignsofthegrassedwaterwaysare
similartothedesignoftheirrigationchannels
andaredesignedbasedontheirfunctional
requirements.Generally,thesewaterwaysare
designedforcarryingthemaximumrunofffor
a10-yearrecurrenceintervalperiod.
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6. Design Procedure
1.Datacollection
A.Watershedareaalongwithinformationonsoilcharacteristics,
cropcoverandtopography.
B.Peakrunoffrateofthewatershed.
C.Gradeoftheproposedwatershed.
D.Informationonvegetalcoverexistingatthesideforroughness
coefficientcalculation.
E.Erodibilityofthesoil.
F.Allowableflowvelocityasperconditionofwaterways.
G.Allowancetobeprovidedtothecross-sectionofwater-waysfor
compensatingthespace,occupiedbythevegetation.
H.Additionaldepthas“Freeboard”tobeaddedinthedepthof
waterwaystoeliminatethechanceofover-tapping.
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College of Agriculture, Powarkheda

7. Size of Waterway
•The size of the waterway depends upon the
expected runoff. A 10 year recurrence interval is
used to calculate the maximum expected runoff to
the waterway.
•The cross-sectional area is calculated using the
following formula:
Q = a x V
where,
a= cross-sectional area of the channel
Q= expected maximum runoff
V= velocity of flow
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8. Shape of Water Way
•Theshapeofthewaterwaydependsuponthe
fieldconditionsandtypeoftheconstruction
equipmentused.Thethreecommonshapes
adoptedaretrapezoidal,triangular,and
parabolicshapes.
•Incourseoftimeduetoflowofwaterand
sedimentdepositions,thewaterwaysassumean
irregularshapenearingtheparabolicshape.
•Ifthefarmmachineryhastocrossthe
waterways,parabolicshapeortrapezoidalshape
withveryflatsideslopesarepreferred.

9. •disthedepthofwaterflow
•bisbottomwidth
•tisthetopwidthofmaximum
waterconveyance
•Tistopwidthafterconsideringfree
boarddepth
•(D-d)isthefreeboard
•slope(z)isc/d
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10. Channel Flow Velocity
•Thevelocityofflowinagrassedwaterwayisdependentontheconditionofthe
vegetationandthesoilerodibility.
•Itisrecommendedtohaveauniformcoverofvegetationoverthechannelsurfaceto
ensurechannelstabilityandsmoothflow.
•Thevelocityofflowthroughthegrassedwaterwaydependsupontheabilityofthe
vegetationinthechanneltoresisterosion.
•Eventhoughdifferenttypesofgrasseshavedifferentcapabilitiestoresisterosion;an
averageof1.0m/secto2.5m/secaretheaveragevelocitiesusedfordesign
purposes.
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11. Recommend Velocities of Flow in a Vegetated Channel
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Type of Vegetation Cover
Flow velocity, (m/s)
Type Magnitude
Spare green cover Low velocity 1-1.15
Good quality cover Medium velocity 1.5-1.8
Excellent quality cover High velocity 1.8-2.5

12. Permissible Velocity
of Flow on Different
Types of Soil
(Source: V.V.N. Murty)
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Type of soil
Permissible velocity, (m/s)
Clean waterColloidal water
Very fine sand 0.45 0.75
Sandy loam 0.55 0.75
Silty loam 0.60 0.90
Alluvial silt without
colloids
0.60 1.00
Dense clay 0.75 1.00
Hard clay, colloidal1.10 1.50
Very hard clay 1.80 1.80
Fine gravel 0.75 1.50
Medium and coarse
gravel
1.20 1.80
Stones 1.50 1.80

13. Length of Waterway
It should be as long as the velocity of runoff flow is in safe limit and erosion is not very
prominent. Generally a length of 30 to 40 m is considered.
Width and Depth
On large cultivated field widthof grassed waterway may be 20 to 60 m and on a small field it
may be 1 to 2m.
The depthof waterways affect the field operation, therefore it should be taken accordingly
site condition. A freeboardof 10 to 20cm should be added in designed depth for safety of
waterway.
Grade of waterway
It should not exceed 10%, in normal course the grade within 5% is generally preferred.
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14. Construction Procedure and
Maintenance of Grassed
Waterway
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15. 15
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College of Agriculture, Powarkheda
Step-1:Shaping(SoilDigging)
•Theshapingofthewaterwayshouldbedoneasstraightandevenaspossible.
•Anysuddenfallorsharpturnmustbeeliminated,exceptintheareawherethe
structureisplannedtobeinstalledinthewaterway.
•Inaddition,thegradeshouldalsobeshapedaccordingtothedesignedplan.
•Also,thestonesandstumpswhicharelikelytointerferewiththedischargerate
mustberemoved.
The construction of grassed waterways is carried out using the following steps

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College of Agriculture, Powarkheda
Step-2: Grass Planting
•After shaping the waterway channel, the planting of grasses is very important.
•Priorities should always be given to the local species of grasses.
•The short forming grasses are more preferable as compared to the tall bunch type
grasses.
•In large waterways, the seeding should be preferred for grass development.
Step 3: Ballasting
•Ballasting is done in those localities where rocks are readily available adjacent to the
sites and waterway gradient is very steep.
•Ballasting is generally recommended for the waterways in the small farms.

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JNKVV
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College of Agriculture, Powarkheda
Step 4: Placing of Structure
•Structures(drop)areessentialifthereissuddenfallinthewaterwayflowpath.
Becauseunderthissituation,thereisapossibilityofsoilscouringduetofalling
ofwaterflowfromahigherelevationtoalowerelevation.Foreliminatingthis
problem,theconstructedstructuremustbesufficientlystrongtohandlethe
designedflowssuccessfully.

18. Maintenance
•Thegrassesgrowninwaterwayshouldalwaysbekept
shortandflexible,sothattheyshingleaswaterflows
overthem,butdonotlodgepermanently.Forthis
purpose,thegrassshouldbemowedtwotothreetimes
inayear.
•Thewaterwayshouldnotbeusedasaroadfor
livestock.
•Afterthevegetativecoverisestablishedandrunoff
passesthroughthemforalongtime,alightapplication
offertilizershouldbedonebecausetheflowingrunoff
removestheplantfoodfromthesoilofwaterway.
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19. •Ifwaterwaysaretobecrossedbytillageimplements,theyshouldbedisengaged,plough
shouldbeliftedanddiscstraightened.
•Itisalsoessentialthatifthereisanydamageofthewaterway,itshouldbequicklyrepaired
sothatthedamagemaynotenlargeduetorainfalls.
The maintenance of waterways can be taken up using the following process.
a)The outlets should be safe and open so as not to impede the free flow.
b)Grassed waterways should not be used as footpaths, animal tracks, or as grazing grounds.
c)Frequent crossing of waterways by wheeled vehicles should not be allowed.
d)Newly established waterways should be kept under strict watch.
e)The large waterways should be kept under protection with fencing.
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College of Agriculture, Powarkheda
f)Waterwaysmustbeinspectedfrequentlyduringfirsttworainyseasons,after
construction.
g)Ifthereisanybreakinthechannelorstructures,thentheyshouldberepaired
immediately.
h)Thebushesorlargeplantsgrowninthewaterwayshouldberemoved
immediatelyastheymayendangerthegrowthofgrasses.
i)Thelevelofgrassinwaterwayshouldbekeptaslowanduniformaspossibleto
avoidturbulentflow.

21. Thanks for listening…..
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