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Date: 20/11/2012
Hydrographic SurveyingLecture 19 : Hydrographic Survey
River Gauging
Dr. Mohsin Siddique
NU-FAST Lahore Campus
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Hydrographic Survey
Hydrographic sur vey includes the following
1. Rain gauging
2. River gauging
3. Marine survey
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River Gauging
Basic objective
Determine area of flow and
Determine velocity of flow
For a channel
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Q=discharge
A=area of flow
V =average velocity of flow
AV Q =
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RIVER GAUGING
Necessity
River Gauging involves he measurement of discharge of a river and
the establishment of a gauge post on one of its banks.
This is done directly to read the Highest Flood Level (HFL)so that a
warning may be given to surrounding areas for any precautionary
measures to be taken if necessary.
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In addition the river gauging is undertaken for thefollowing;
To fix the number of spans of road and railway bridges, so that
unexpected highest flood may not cause any damage to the
structure. To fix the height of the guide bank
To fix the spillway level and height of the dam or barrage
To design the cross-drainage works when a canal crosses a river
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RIVER GAUGING-SELECTION OF
DISCHARGE SITE
The following points should
be remembered while
selecting a discharge site
1. The river should be
straight for a minimum length
of about 500m or four times
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its width
2. The section of dischargesite should be well defined
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River Gauging-Fixing the Gauge Post
Gauge posts are wooden posts of
section 10cmx5cm and length 2m,
graduated in meters and fractions
of meters so that a minimumreading of up to 0.01m can be
taken.
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The posts are fixed in series on aconcrete foundation along the
slope of the river bank
The bottom post is assumed to beat zero and other posts are
graduated accordingly
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River Gauging-Fixing the Gauge Post
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River Gauging-Fixing the Gauge Post
The RL of the zero mark of the starting gauge post is determined by fly
leveling from a bench mark. Then by simple noting the reading on the gauge
the RL of the water level can be predicted
For example, suppose the RL of the zero mark is 155.25m. So, when the
gauge reading is 1m, the water level is at RL 156.25m, and when the reading
is 3m, the RL of the water level is 158.25m and so on…
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River Gauging-Measurement of Depth of
Water (sounding)
1. Sounding Rod
It is wooden of bamboo pole of 5cm diameter and 2.5m length. It isprovided with a disc of 15cmdiameter at the bottom.
It is graduated in meters and 1/10th
of a meter.
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measuring the depths of smallrivers of the order of 2m. Tomeasure depth, the sounding rod isslowly immersed vertically from aboat, at the required place, so that
the base plate just touches the bedof the river.
The depth of water is then notedfrom the graduation
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River Gauging-Measurement of Depth of
Water (sounding)
2. Sounding cable
When the depth is more than 2m
and water current is high, the
sounding rod is not practicable. In such situation, a cable or rope
released slowly from a boat by
means of a pulley system.
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The cable consists of a counterweight of about 5kg. When the
weight just touches the bottom of
the river, a mark is made on the
cable exactly at the water level.
Then the depth is measure by tape.
(Alternatively the rope may have
tags to assist in depth
measurements)
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River Gauging-Measurement of Depth of
Water (sounding)
3. Echo Sounder
Echo sounder is used to measure
the depth of large rivers, in excess
of 10m depth and those of seas. It is an electrical instrument, in
which a sound pulse from the
surface of water is sent towards
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the bottom of the river or sea. The sound waves are reflected
back from the bed in the form of
an echo which is
arrested(received) by the receiver.
There is automatic record of
onward and backward travel of
sound wave
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River Gauging-Measurement of Depth of
Water (sounding)
3. Echo Sounder
Taking the velocity of
sound in water to be
approx 1470m/s, thedepth of water can be
computed.
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Generally the ecosounder is placed just at
the water level to
eliminate transmission
loss and obtain accurate
results.
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River Gauging-Measurement of Flow
Velocity
Velocity of flow can be measured by the following methods
1.The surface float method
2. The sub surface float method
3. The velocity rod method
4. The pitot tube method
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5. The current meter method.
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River Gauging-Measurement of Flow
Velocity
1.The surface float method
Surface floats are made of cork,
and can easily float on water.
The are generally in he form of 10cm cubes.
The floats are painted red or white
and have a small fla attached on
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top
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River Gauging-Measurement of Flow
Velocity
Velocity is measured in the following way
(a). At the discharge site a rope is stretched between two poles fixed on
both banks of the river and water section is divided into severalcompartments by hanging tags from rope
(b). The velocity of each compartment is measured by the float which is
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re ease s g y a ea o c s . w en e oa us crosses e sec on, e
stop watch is started. When it crosses c/s 2, the stop watch is closed. Thetime taken by the float to cover the ‘run’ (known distance) is noted, andfrom this velocity is measured.
© Similarly the velocity of all compartments is measured and the average of the these is taken as the mean velocity. But this velocity representsonly surface velocity and is not the actual mean velocity of theriver.
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River Gauging-Measurement of Flow
Velocity
2.The sub surface floatmethod
The sub-surface float is a hollowcylinder which is attached by acord to a surface float.
The position of a sub-surface floatis adjusted according to the depthof the river.
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Generally, it is kept 0.2D meters(where D is depth of water) abovethe bottom.
In this case, the measured velocityis equal to the mean velocity
The procedure of measurement isthe same as that in the surface floatmethod
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River Gauging-Measurement of Flow
Velocity
3.The Velocity Rod Method
The velocity rod is made of hollow
metal or wood.
Its diameter is about 2.5m to 5m.
A weight is provided at the bottom
of the rod to keep it vertical.
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.
The rod is generally submerged to
the extent of 0.6D.
The procedure of measurement is
exactly similar to that in surface
float method
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River Gauging-Measurement of Flow
Velocity
4.The PitotTube Method
The pitot tube is a glass tube bent
at an angle of 90o at the lower
end. The upper end is open and is kept
above the water level.
The lower end is in the form of
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nozzle, and is directed upstream. Due to the velocity of water, the
water column rises in the right
limb (say rise is h).
The mean velocity (V ) of flow isdetermined according to
Bernoulli’s theorem as
ghV 2=
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River Gauging-Measurement of Flow
Velocity
5.The Current meter method
The current meter is commonlyused for measuring the velocity of water.
It consists of the followingcomponents
1. A tail which keeps the meter in
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2. A system of conical bucketsfitted to a spindle which rotatesdue to current
3. A counter weight which keepsthe meter vertical
4. A headphone for registering thesound produced by the spindle atevery revolution
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River Gauging-Measurement of Flow
Velocity
The current meter is lowered to adepth of 0.6D by the suspensioncable. The system of conicalbuckets is kept on the upstream
side and the tail on thedownstream side.
Due to the velocity of water, thesystem of conical buckets rotates
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a out t e spin e.
The number of revolutions per unittime are record eitherautomatically or manually.
The velocity is then obtained by
the rating table provided by themanufacturer.
This gives the mean velocity of flow
baN V +=
V= mean velocity
a & b= Coefficients
N= no. of revolution per unit time
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Thank you
Questions….
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