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: irfan26200@yahoo.com)

2. Topics to be Covered
• Grassed water ways and design of grass
water ways wind erosion: mechanisms of
wind erosion, types of soil movement,
principles of wind erosion, control and
control measures.

6. Grassed waterways are open channels protected
with suitable grasses or shrubs. It is simply a
drainage channel.
It is constructed along slope and act as outlet for
terraces or graded bunds. Its size depends on
expected run off.
It is just like irrigation channel.

7. • Vegetated waterways are built to protect
soil against the erosive forces of
concentrated runoff from sloping lands.
• By collecting and concentrating overland
flow, waterways absorb the destructive
energy which causes channel erosion and
gully formation..

8. Grassed Waterways

10. Purpose of Grassed Waterways
• Grassed waterways are used as outlets to
prevent rill and gully formation.
• The vegetative cover slows the water flow,
minimizing channel surface erosion.
• These channels are used for the safe
disposal of excess runoff from the terraced
and bunded crop lands, to some safe
outlet, namely rivers, reservoirs, streams
etc. without causing soil erosion.

12. • Grassed waterways:
natural
man made
• These constructed channels are
established for the transport of
concentrated flow at safe velocities from
the catchment using adequate erosion
resistant vegetation which cover the
channels.

13. • The grassed waterways are constructed
prior to the construction of terraces, bunds
etc. because grasses take time to get
established on the channel bed.
• Generally, it is recommended that there
should be a gap of one year so that the
grasses can be established during the
rainy season.

14. Photo 1: Severe soil erosion and sediment deposition

15. Photo : Successfully established and maintained grassed waterway
Well designed and maintained grassed waterways
can be an important tool for maintaining soil quality
and productivity.

17. Units of Measurement of Water

18. • Water can be measured on volume basis
when it is at rest.
• Eg.: litres, milliliters, ha-cm, ha-m, cubic
metre, etc (depending upon the
magnitude and purpose of
measurement).
• Large quantity of water present in a
reservoir may be measured in m3.
• Volume of water applied to a field can be
suitably measured as ha-cm.
• Volume=Area x depth of water

19. • Water can be measured on discharge
basis when it is in motion.
• Eg.: litres per second (lps), litres per
minute (lpm), cubic metre per hour (m3/hr)
cubic metre per minutes (m3/min, etc.
• It is the volume of water passing through a
section per unit time.
• Q=V/t
• V=Q X t

20. Shape of Waterways
The shapes of grassed waterways are of
three types:
(1) Trapezoidal
(2) Triangular
(3) Parabolic
If the farm machinery has to cross the
waterways, parabolic shape or trapezoidal
shape with very flat side slopes are
preferred.

22. Parabolic Shaped Waterway
• A parabolic shaped waterway represents a
natural channel.

23. The factors that affect the selection of shape
of waterways are:
(a) Construction equipment availability
(b) Velocity of flow
(c) Grade of the waterway
(d) type of grass cover

24. Construction Equipment Availability
• The equipment available for construction
of the waterway is one of the main factors.
• Trapezoidal shaped channel can easily be
constructed with the blade type machines.
• Trapezoidal cross section is widely used
where the waterway is artificially
constructed as terrace outlets.

25. • Trapezoidal and triangular cross-section of
channel normally take the form of
parabolic section either due to siltation at
the bottom or due to scouring of the soil
from its bottom and sides with passage of
time.
• That is why parabolic shape of grassed
waterway is generally considered as most
economical and also a more stable cross
section.

26. Velocity of Flow
 The permissible flow velocity in the
grassed waterway depends upon the type
and condition of vegetation and its density
to resist the erosion.
 A uniform vegetative cover in the
waterway is important to provide a better
channel stability and also to decide the
permissible flow velocity.
Permissible velocity in grassed waterways
varies according to the vegetative growth.

31. • The approximate values of permissible
flow velocity for different grassed cover
are given as:
• Permissible Velocity
(m/s) Cover Condition
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, for an average condition of
grassed cover and channel section, a flow
velocity from 1.5 to 2.0 m/s is used for
design purposes.

32. Grade of the Waterway
Normally, a channel grade of
approximately 5% is recommended for
vegetated waterways.
A grade of more than 10% is not
recommended, as it is likely to become
erosive.
Vegetated waterways are generally
constructed along the direction of the
slope.

33. Grass Cover
• The grass cover increases the factor of
roughness which reduces the velocity of
flow and the channel hydraulic capacity,
and along with it, the velocity is made safe
(non-erosive) for the runoff to pass
through the channel.

34. Design of Grassed Waterways
The designs of the grassed waterways are
similar to the design of the irrigation
channels or drainage channels.
Generally, these waterways are designed
for carrying the maximum runoff for a 10-
year recurrence interval period.

35. • Waterways can be shorter in length or
sometimes, can be even very long.
• For shorter lengths, the estimated flow at
the waterways outlets forms the design
criterion, and for longer lengths, a variable
capacity waterway is designed to account
for the changing drainage areas.

36. The waterway is to be given greater cross-
sectional area towards the outlet as the
amount of water gradually increases
towards the outlet.
The cross-sectional area is calculated using
the following formula:
Q=Av
where, A = cross-sectional area of the
channel,
Q = expected maximum runoff, and
v = velocity of flow.

37. Procedure for the design of the channel
parameters comprises of the following
steps. This procedure is an iterative one.
Step 1: Assume the value of flow depth and
calculate the channel cross sectional area
(A), wetted perimeter (P), hydraulic radius
(R) and top width (T):

38. Parameters of Open Channels
• Wetted Perimeter (P) :The Length of
contact between Liquid and sides and
base of Channe.l
• Hydraulic Radius (R): If cross sectional
area is A, then R = A/P.
• Depth of flow section (d) : depth of flow
normal to the direction of flow.

40. • Freeboard: Vertical distance between the
highest water level anticipated in the design
and the top of the retaining banks. It is a
safety factor to prevent the overtopping of
structures.
• Side Slope (Z): The ratio of the horizontal
to vertical distance of the sides of the
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. • Step 2: Determine the mean velocity of
flow by using Manning’s formula which
states that:
• R= hydraulic radius of the channel section
(m)
• S = channel grade
n = Manning’s roughness co-efficient (for
vegetated waterways, n= 0.04)
• Step 3: Determine the discharge rate Q =
Av (m3/s) through the channel.

50. Location of Grassed Waterways
• The grassed waterways should be located
at that point where the water can drain into
the waterways from all sides.
• The places which require a minimum cost
of shaping are also preferred for location
of waterways.
• When suitable natural depressions are not
available, the waterway should preferably
be located along the field boundaries.

51. Construction of the Waterways
• Shaping (Soil Digging)
• It is advantageous to construct the
waterways at least one season before the
bunding. It will give time for the grasses to
get established in the waterways.
• First, unnecessary vegetation like shrubs
etc. are removed from the area is marked
for the waterways.
• The area is then ploughed if necessary
and smoothened.

53. • The shaping of the waterway should be
done as straight and even as possible.
• Any sudden fall or sharp turn must be
eliminated, except in the area where the
structure is planned to be installed in the
waterway.
• In addition, the grade should also be
shaped according to the designed plan.

54. 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.
• Perennial (species that live longer than one
year) grasses and shrubs are recommended as
they require less maintenance and cost less in
the long-term.
• Generally, the rhizomatous grasses are
preferred for the waterway, because they get
spread very quickly and provide more protection
to the channel than the brush grasses.

57. • In large waterways, the seeding is cheaper
than the sodding.
• Therefore, the seeding should be
preferred for grass development.
• Immediately after grass planting, the
waterways should not be allowed for runoff
flow.

58. Maintenance
• Overall, it should always be remembered
that the waterways are an integral part of
watershed conservation or land treatment
system.
• If they fail to handle the peak discharge
due to lack of proper maintenance, then
the prolong flow of runoff through them
can develop gullies in the area.

59. The maintenance of waterways can be taken
up using the following process:
 Grassed waterways should not be used as
footpaths, animal tracks, or as grazing
grounds.
Frequent crossing of waterways by wheeled
vehicles should not be allowed.
Newly established waterways should be kept
under strict watch.
The large waterways should be kept under
protection with fencing.