Chapter 2.pdf

Measurement of river Stage, depth, velocity
HWRE-3122
Mengistu .Z (MSc in Hydraulic Engineering )
Lecturer @ Hydraulic and Water Resources Engineering department
Mizan Tepi university
Email: mengistu.zantet@gmail.com
mengistuzantet@mtu.edu.et
P.O.Box: 260
Tepi, Ethiopia
03-Dec-22 1

2.1 General aspects of Measurement of river Stage, depth, velocity
1) Stage measurements
2) Types of Gauges
3) Stage data analysis,
4) Velocity measurement
 current meter method,
floating method,
ultrasonic method.
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Stage Measurement
Stage measurements are mainly made with gauges
Stage measurement can be inaccurate if there is scouring.
Basically there are two modes of stage measurements.
The first is discrete stage measurements using manual gauges,
and the second is continuous stage measurements using
recorders.
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1) Manual staff Gauges
The simplest way to measure river stage is by means of a
staff gage.
A staff gauge is vertically attached to a fixed feature such
as a bridge pier or a pile
a) Sectioned Staff Gauges:
 A series of posts each overlapping.
The height above a certain datum is measured.
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lecturer@ Hydraulic and water
resources Engineering Department 4

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 A certain datum level is on the bridge and a string
having a weight electrically connected is lowered to
the water surface to get the depth of water surface.

c) wire gauge
consists of a reel holding a length of light cable with
a weight affixed to the end of the cable.
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Cont.….
Advantage of Manual staff
Gauges
are simple and
inexpensive
Disadvantage of
Manual staff Gauges
are impractical in small or flashy
streams
read frequently to get a continuous
curve of the stream flow
the peak stage may be missed
when it occurs between the
observations.
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2. Automatic Stage Recorders
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Automatic stage recorders overcome this basic objection of
manual staff gauges and find considerable use in stream-flow
measurement practice.
Two typical automatic stage recorders are described below
a) Float Gauge:
b) Bubble Gauge

A) Float Gauge:
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 Float movement fluctuates with change in stage and
this is recorded by a chart.
 In hydrologic measurements, both the big and low
flows are measured within the chart

B) Bubble Gauge
Gauge in this gauge compressed air or gas is made to bleed out at
a very small rate through an outlet placed at the bottom of the river
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Advantages of bubble over a float operated water
stage recorder listed as
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a) there is no need for costly stilling wells;
b) a large change in the stage, as much as 30 m, can be measured;
c) the recorder assembly can be quite far away from the sensing
point; and
d) due to constant bleeding action there is less likelihood of the
inlet getting blocked or choked.

2.Stage data analysis
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 The stage data is often presented in the form of a plot of
stage against chronological time known as stage hydrograph

Importance of Stage data
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A. Determination of stream discharge,
B. Importance in flood warning and flood protection
works.
C. To estimate the design peak river stages for use in the
design of hydraulic structures, such as bridges, weirs,
etc.

Measurement of Velocity
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For the measurement of discharge, the mean velocity is
required at various vertical lines
The following methods are commonly used:
1. Float method
2. Current meter method

1.Velocity Measurement by Float
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In this method, a straight and uniform reach of the river is
selected for the float to travel. Velocity = distance/time
 The time t taken by a float to travel a certain distance L is
measured.

The surface velocity(Vs) is equal to 1.2(average Velocity, V)
i.e. Vs = 1.2V and V = 0.8 Vs.
There are three types of float commonly used in practice
1. Surface floats
2. Sub-surface floats of Double floats
3. Velocity rods or rod floats.
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Cont.…
 Surface floats: They are made up of light material for
example cork. Any object that can float can be used as surface
float.
 Sub-surface float or Double float: It consists of a lighter
surface float attached by a cord to a hollow metal sphere or a
cylinder which is another subsurface float.
 Rod Float: A velocity rod or a rod float consists of a hollow
metal tube or a wooden rod.
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Restriction of floats
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Used when high accuracy is not required
 Used at uniform cross-section and free of surface
disturbances
The test section of the channel length is measured at the
beginning, midpoint and end
A float is released at sufficient distance upstream to attain
the stream velocity before it enters the test section
 The time when the float passes each section is recorded
and the average velocity is determined
 Velocity = distance/time

Velocity Measurements by Current Meters
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The most commonly used instrument in hydrometery to
measure the velocity at a point in the flow cross-section
is the current meter.
Consists of a rotating element which rotates due to the
reaction of the stream current with an angular velocity
proportional to the stream velocity.

 There are mainly two types of current meters which are in
common use, namely:
1. cup type and
2. Propeller type current meter
 The principle involved in both the meters is that the
water flowing past the rotating element of the meter makes it
revolve due to the unbalanced drag force acting on it and the
speed of the rotating element is directly proportional to the
velocity of water.
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V = a + bN where N is the rotation of the propeller (revs
per sec) a and b are coefficients determined by calibration in an
experimental flume.
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Velocity measurement by Ultrasonic method
 Another method to measure velocity of a stream is the
use of ultrasonic sound wave moving in water from one
side of the channel to other.
 Two transducers capable of emitting and recording
sound waves are placed at the same reduced level on
either side of the river banks, making an angle with
respect to the river bank.

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Cont..
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When the transducer P2 sends ultrasonic signals, it is
received by P1.
The signals move with their own velocity in the water
medium plus the velocity of water favoring it.
When P1 sends signals, it is received by P2 with
velocity of water opposing it.

Cont..
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Where
• L is the distance between P1 and P2
• t1 and t2 respectively represents the times taken by sound
wave to reach from P2 and P1 and from P1 to P2.

Cross-sectional Area (A)
In wide and/or irregularly shaped channels, the
cross-sectional area is divided into several segments.
 The area of each segment is the product of the
width of the segment and its average depth
1. Rectangle area = d* w
2. Trapezoid area = (
𝑑1+𝑑2
2
) *w
3. Triangle area =
𝑑∗𝑤
2
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• The areas of the segments are summed to determine the
total cross-sectional area
A= 𝑎
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Determination of Discharges
• The cross-sectional area of flow is then multiplied
by the average velocity to get the flow rate
Q =AV
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03-Dec-22

THE END
03-Dec-22

Chapter 2.pdf

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