This document discusses grassed waterways, which are natural or man-made channels covered in erosion-resistant grasses that are used to drain surface water from an area. Grassed waterways control soil erosion from runoff by lining the channel with grasses. Their design requires data on watershed characteristics, soil type, and topography to calculate peak runoff rates. Their shape depends on construction equipment availability, flow velocity, grade, and grass cover. Design involves determining peak runoff, assuming a channel cross-section, calculating velocity and capacity using Manning's equation, and iterating assumed dimensions until capacity matches runoff rates.

1. GRASSED WATERWAYS
Lecture-9
Dr. Ajay Singh Lodhi
Assistant Professor
College of Agriculture, Balaghat (M.P.)
Jawahar Lal Krishi Vishwa Vidyalaya, Jabalpur (M.P.)

2. GRASSED WATERWAYS
 Grassed waterways are the natural or man made water courses, covered with
erosion resistant grasses, used to dispose surface water from the area.
 The use of grasses in the section of waterway, acts as a lining material to control
the problem of soil erosion, caused by haphazard runoff flow through the section.
 The grassed waterways are constructed along the slope of the area.
 Apart from disposing the runoff from area, these waterways also act as an outlet
for the terraces or graded bunds.
 Waterways are counted as important tool for removing surplus water from the
terraced field, and for erosion control work.
 The grassed water ways should be fully established with grasses before water is
turned into them. In other words, in the area the waterways should be ready to
hold water before bunds, terraces, or diversions etc. are being constructed.

3. DATA REQUIRED FOR DESIGNIG OF WATERWAYS
 Watershed area along with the informations on soil characteristics, crop cover and
topography. This data is basically required for computing the peak runoff rate of the
watershed on the basis of which the grassed water ways are designed.
 Grade of the proposed waterway (in percent). It is fixed by accounting the elevation of
outlet.
 For selecting the roughness coefficient, the information on vegetal cover adopted to the
site, is also needed.
 Erodibility of the soil of grassed waterway. It is required to predict the soil erosion likely
to made through the waterway, till the vegetations or grasses get fully established in the
cross-section.
 Allowable flow velocity suitable to the condition of waterway.
 Allowance to be provided to the cross-section of waterway for compensating the space,
occupied by the vegetations.
 Additional depth as 'free board' to be added to the depth of waterways for removing the
chances of overtopping from the top of the waterway.

4. Factors Affecting the Waterways
 The shapes of grassed waterways are of three types:
 Trapezoidal
 Triangular
 Parabolic

5.  A parabolic shaped waterway represents a natural channel.
 In normal course of water flow, the trapezoidal and triangular sections become
parabolic in shape due to deposition of sediments over the channel section and
bank erosion.
 The factors that affect the selection of shape of waterways are:
 Construction equipment availability
 Velocity of flow
 Grade of the waterway
 The type of grass cover

6. 1. 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
provided that the design bottom width of the channel is greater than the minimum width
of the cut of the equipment.
 If the design bottom width of the trapezoidal channel is narrower than the width of the
mower swath, then construction is not possible, because neither equipment can move
nor desired width of the waterway can be achieved.
 Similarly, triangular and parabolic shaped channel with side slope of 4:1 or flatter can
easily be constructed by using suitable equipment.
 From construction point of view, the trapezoidal cross section can easily be constructed
and widely used where the waterway is artificially constructed as terrace outlets along
the boundary line.
 Trapezoidal and triangular cross-section of channel, after some time, is liable to 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. That is why parabolic shape of grassed waterway is
generally considered as most economical and also a more stable cross section.

7. 2. 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.
 The approximate values of permissible flow velocity for different grassed cover are
given as:
 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. In grassed waterways, the average flow
velocity is always higher than the actual velocity near the bed, as surface roughness
is greater.

8. 3. 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.
4. 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.
 The value of Manning’s roughness factor (n) is not constant for any given species of
grass, but varies with the depth and the velocity of flow and the submergence level
of grass.
 When the depth of flow is less, the water seeps through the stems of the grass,
which reduces the flow velocity considerably as the resistance to the flow is very
high with a high value of ‘n’.
 The value of roughness coefficient ‘n’ usually taken as 0.04 for design of
grassed waterways.

9. Design of Grassed Waterways
In cases, where the shape of the waterway, the carrying capacity, and the slope of the
bed are known, the procedure for the design of the channel parameters comprises of
the following steps.
Step 1: Determine the peek runoff rate, generated from the area which is needed to
drain through the waterway. The area to be drained (A) may be obtained from the
contour map. The peak runoff rate (Q) is estimated using the rational formula given as
under

10. Design of Grassed Waterways
In cases, where the shape of the waterway, the carrying capacity, and the slope of the
bed are known, the procedure for the design of the channel parameters comprises of
the following steps.
Step 2: Assume the value of flow depth and calculate the channel cross sectional area
(A), wetted perimeter (P), hydraulic radius (R) and top width (t)
(A) For trapezoidal channel section
Where: b = bottom width (m), d = channel depth (m)
Z = e/d =side slope (horizontal: vertical) of trapezoidal channel

11. (B) For Triangular channel section
Where: d = depth of channel (m)

12. (B) For Parabolic channel section
Where: d = depth of channel (m)

13. Step 3: Determine the mean velocity of flow by using manning’s formula which states
that
Where,
R= hydraulic radius of the channel section (m)
S = channel grade
n = Manning’s roughness co-efficient (for vegetated waterways, n= 0.04)
Step 4: Determine the discharge rate Q = Av (m3/s) through the channel.
Step 5: Check if the velocity is safe, and the carrying capacity of the channel is within
the permissible range. (Computed capacity of waterway is equal or nearly equal to the
peak runoff rate).

14. Step 6: If it is observed that the velocity is unsafe, and the carrying capacity is not
within the permissible range, and then repeat the process with another set of assumed
value in step (1), till the carrying capacity is found to be within the permissible range.
Step 7: A free board of 15 cm is then added to the assuming channel depth as

15. Thank You