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7/30/2019 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
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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
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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
Estimation of average rain fall (precipitation)
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
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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
Estimation of average rain fall (precipitation)
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
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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
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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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Thank you
Questions….
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