Hydrographic Survey (Rain Gauging) By D.M Siddique

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Date: 08/11/2012

Hydrographic SurveyingLecture 18 : Hydrographic Survey

Rain 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

1. Rain au in :

The amount of rainfall in the catchment area of a basin or a river isgenerally recorded. This helps to predict the probably run-off

through the basin or river

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Hydrographic Survey

2. River gauging:

River gauging involves the measurement of discharge of a river and

the establishment of a gauge post on one of its banks.

This is done to directly read the Highest Flood Level (HFL) so that

the warning may be given to surrounding areas.

Measurement of river discharge require surveys for river cross-

sectional area and flow velocity

3. Marine survey:

It involves marking of shore line at high and low tides, determining

depth of water around the harbor by echo-sounder, current and tide

observations, and preparing of maps etc.

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RAIN GAUGING

Necessity

The amount of rainfall (precipitation) in the catchment area of a

basin or a river is generally recorded. This helps to predict the

probably run-off through the basin or river.

Thus, any probably flood in the downstream area may be forecasted

in case there is heavy rain, and necessary precaution may be taken

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Location of site for rain gauge station

The recommended network of rain gauge depend upon the

topography of the catchment area.

Generally, one rain gauge for every 500 sq.km in plane area and onefor every 150 sq. km in hill areas are considered appropriate.

To get the proper rain fall recorded from the catchment are, the

stations should be so selected that they may fully cover the basin



RAIN GAUGING

Measurement of rainfall

Generally there are two types of rain gauges

1. Non-Recording rain gauges

Cylindrical Rain Gauges and Ordinary Rain Gauges

2. Recording rain gauges

Si hon Rain Gau es

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Tipping Bucket Rain Gauges

Note: Rainfall is measured as depth of water on the level surface



RAIN GAUGING

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RAIN GAUGING

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RAIN GAUGING

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Rain Gauging

Estimation of average rain fall (precipitation)

1. Arithmetic mean method

2. Theissen polygon method 3. Isohyetal method

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Rain Gauging


1. Arithmetic mean method

Calculate the average of the precipitation

records from all the rain gages in a

watershed (include any on the watershed

boundar as well

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Best to have the gages distributed

throughout the watershed (unfortunately,

not always the case in real life)

Use only the rain gages within and on thewatershed boundary - not those outside the

boundary - to calculate the average



Rain Gauging

Estimation of average rain fall

(precipitation)

2. Theissen polygon method

Advantage: allows for the uneven

distribution of rain gages

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Rain Gauging


3. Isohyetal method

In Greek, iso means equal and hyet means

rain

An isohyet is a line of equal rainfall

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n so ye a map s ows nes o equa

elevation drawn the same way that a topo

map is drawn

Between isohyet lines is a given change in

precipitation (isohyet interval -similar to a

contour interval



Rain Gauging

Estimation of runoff

Rational Formula

Where

= Peak rate of runoff in cubic feet er second

CIAQ =

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C=Runoff coefficient, an empirical coefficient representing arelationship between rainfall and runoff

I=Average intensity of rainfall (inch/hr)

A=drainage area in acres



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=



Thank you

Questions….

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Hydrographic Survey (Rain Gauging) By D.M Siddique

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