Class lectures on hydrology by Rabindra Ranjan Saha, Lecture 8
1. Lecture-8
(2) Moving – Boat Method
Standard current meter technique is not suitable for measuring
velocity in case of Moving Boat Method. In such circumstances
moving boat techniques is useful. In this method a special propeller
–type current meter which is free to move about a vertical axis is
towed in a boat at a velocity vb at right angles to the stream flow.
Procedure
The boat is started from the water edge and aligned to go across the
cross-sectional line (Figure 5- 1). When the boat is in sufficient depth
of water, the instruments are lowered. The echo-depth recorder and
current meter are commissioned. A button on the signal processor
when pressed marks a distinctive mark line on the depth vs. time chart
of the echo-depth recorder. Further, it gives simultaneously a sharp
audio signal to enable the measuring party to take simultaneous
readings of the Resultant velocity vR and the inclination Ө.
1
2. A large number of such measurements are taken during
the transverse of the boat to the other bank of the river.
The operation is repeated in the return journey of the
boat.
Lecture-8(contd.)
Markers for Alignment
Verticals
Flow
Wi+1 yi+1
•
•
•
•
• yi -1
•
•
•
•
yi
•
•
Section Line
ө
vb vR
vfWi-1
Wi
Boat
Figure 5-1 : Moving-Boat method
SLIDE-4
2
Figure5-1
3. It is most important to keep the boat aligned along the cross
sectional line. Typically, a river of about 2 km stretch takes
about 15 minutes for one crossing. A number of crossings are
made to get the average value of the discharge. The velocity
thus measured is surface velocity.
The surface velocities are converted to average velocities
across the vertical by applying a coefficient as ,
v = Kvs.
The value of K may be taken as 0.95
The depths yi and time intervals ∆t are read from the
echo- depth recorder chart.
Lecture-8(contd.)
3
4. Discharge Calculation
The boat velocity is vb ,the flow velocity vf then the resultant
velocity vR . If the flow velocity is vf the meter will align itself in the
direction of the resultant velocity vR making an angle Ө with the
direction of the boat as shown in the figure above. Further, the
meter will register the velocity vR. If vb is normal to vf .
vb = vR cos Ө and vf = vR sin Ө
Lecture-8(contd.)
If the time of transit from Wi t1 to Wi +1 t2 i.e.
two verticals time interval is (t2 – t1)= ∆t, then the
width between the two verticals is
W = vb ∆t = vR cos Ө ∆t
4
5. We know discharge, Q = Av
For elementary section discharge between two verticals = ∆ Qi
∆ Qi = ∆ Ai vi, where i = 1 ,2, 3, 4....... N
Qi ={(Wi+1) (yi + yi+1)/2} vf
∆ Qi ={(vR cos Ө ∆t) (yi + yi+1)/2} * vR sinӨ
∆ Qi ={(vR
2 sinӨ cos Ө ∆t)} * {(yi + yi+1)/2}
Lecture-8(contd.)
5
6. Lecture-8(contd.)
Thus by measuring the depths yi , velocity vR and Ө in
a reach and the time taken to cross the reach ∆t, the
discharge in the sub area can be determined. The
summation of the partial discharges ∆Qi over the
whole width of the stream gives the stream discharge:
Q = ∆ Q1 + ∆ Q2 +∆ Q3 +∆ Q4+ ∆ Q5 +.....
Q = ∑∆Qi ....................Eq 8-1
6
7. Lecture-8(contd.)
Example 5-4
In the moving boat method of discharge measurement the
magnitude (VR ) and direction ( ) of the velocity of the stream
relative to the moving boat are measured. The depth of the stream
is also simultaneously recorded. Estimate the discharge in a river
that gives the following moving - boat data. Assume the mean
velocity in a vertical to be 0.95 times the surface velocity measured
by instrument.
Section VR
(m/s)
(degrees) Depth
(m)
Remark
0 - - - Right bank
1 1.75 55 1.8
ө is the angle made by VR with the
boat direction and Channel Width 75 m.
2 1.84 57 2.5
3 2.00 60 3.5
4 2.28 64 3.8
5 2.30 65 40
6 2.20 63 3.8
7 2.00 60 3.0
8 1.84 57 2.5
9 1.70 54 2.0
10 - - - Left bank
DATA TABLE
7
12. 12
Lecture-8(contd.)
RUNOFF AND RAINFALL RELATIONSHIP
Runoff and Rainfall
Runoff
The draining or flowing off of precipitation from a
catchment area through a surface channel is called
runoff.
Overland flow
The excess precipitation ( = total precipitation – evapo
transpiration, initial loss, infiltration, and detention-
storage requirements) moves over the land surface is
called overland flow.
13. 13
Lecture-8(contd.)
Surface runoff
The flow that travels all the time over the surface as overland
flow and through the channels as open-channel flow and
reaches the catchment outlet is called surface runoff.
Types of runoff
(1) Direct Runoff:
The part of runoff which
enters the stream
immediately after the
precipitation. It includes
surface runoff, prompt
interflow and precipitation
on the channel surface.
(2) Base flow /runoff
The delayed flow that
reaches a stream as
groundwater flow is
called base flow.
Delayed interflow is
included in this
category.
Two Types
14. 14
Lecture-8(contd.)
Rainfall
The total precipitation that falls upon the land surface per unit
time is called rainfall.
virgin flow :Stream flow unaffected by works of man, such as
structures and storage and diversion on a stream is called virgin
flow. When there exist storage or diversion on a stream , the flow
in the down stream channel is affected by structures and hence
does not represent the true runoff unless corrected for storage
effects and the diversion of flow and return flow. Hence Virgin
flow:
Rv + Vr = Vs + Vd
Rv = Vs + Vd- Vr .........................Eq 6-1
where, Rv = virgin flow; Vs = volume of flow Storage
Vd = volume diverted out of stream
Vr = volume returned to the stream
15. 15
Lecture-8(contd.)
Example 6-1
The following table gives values of measured discharges at a
stream-gauging site in a year. Upstream of the gauging site a weir
built across the stream diverts 3.0 and 0.50 Mm3 (million m3) of
water per month for irrigation and for use in an industry
respectively. The return flows from irrigation is estimated at 0.80
Mm3 and from the industry at 0.30 Mm3 reaching the stream
upstream of the gauging site. Estimate the virgin flow. If the
catchment area is 120 km2 and the average annual rainfall is 185
cm. Determine the runoff—rainfall ratio.
Month 1 2 3 4 5 6 7 8 9 10 11 12
Gauged
flows
(Mm3 )
2.0 1.5 0.8 0.6 2.1 8.0 18.0 22.0 14.0 9.0 7.0 3.0
Data Table
16. 16
Lecture-8(contd.)
Solution:
Given,
Stream diverts water per month for irrigation.
= 3.0Mm3 + 0.50 Mm3= 3.50 Mm3
The return flows from irrigation
= 0.80 Mm3 + 0.30 Mm3 = 1.10 Mm3
Rainfall = 185 cm
Catchment area, A = 120 km2
To be calculated :
Runoff- Rainfall Ratio
We know,
Rv = Vs + Vd - Vr ......................... Eq-1
where,
Rv = virgin flow
Vs = volume of flow measured
Vd = volume diverted out of stream= 3.50 Mm3
Vr = volume returned to the stream= 1.10 Mm3
