hydrographic survey (river gauging) by d.m siddique

7/30/2019 Hydrographic Survey (River Gauging) By D.M Siddique

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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



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



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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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




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)




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




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.



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.




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




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.




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




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




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=




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




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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hydrographic survey (river gauging) by d.m siddique

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