Test: Measurement of Rainfall - Civil Engineering (CE) MCQ

The rainfall is also known as precipitation, is a natural process of atmospheric vapor into water. Atmospheric vapor are nothing but clouds which are contamination of air and water. When the temperature goes below dew point then precipitation takes place.

The rate of rainfall is expressed in centimeters of water during a given period of time. One centimeter rainfall is the quantity of water collected on a certain area due to rainfall which becomes one centimeter in height.

The annual rainfall at any given station varies irregularly from year to year. The range of this variation marks the reliability of the rainfall and has great importance in the design of storage reservoirs.
 

The relation between the area of rainstorm and its average intensity is used in assessing the amount of rain which may be expected to fall upon a catchment area within a given period. The intensity of rainfall is equally important in the design of spillways during heavy rain periods.

All forms of precipitation are measured on the basis of vertical depth of water which would accumulate on the level surface if all the precipitation remained where it fell. The rainfall is usually measured with rain gauges.

Percolation refers to movement and filtering of fluid from porous material. Usually the water which falls on the ground is filtered into the earth surface and is stored under the surface of earth. And this water also improves the ground fertility.

The base of the gauge is permanently fixed into a concrete block at a site where rainfall is to be measured. The precaution is taken during fixing, to level perfectly. The gauge is fixed in the block in such a way that the top of the gauge will be 30 cm above the natural surface level.

The rotating drum is kept rotating continuously with the help of electric motor. The drum makes generally one rotation during 24 hours. On the rotating drum the graph paper is placed throughout its rotation the recording is made.

During summer, the evaporation loss from free water surface is considerably large and this evaporated water finds room in the air mass. The water holding capacity of air in the form of vapor is also considerably large in hot weather.

The rainfall varies widely from one part of the world to another, ranging from desert regions to the hilly regions where the average annual rainfall may be over 1100cm. in some regions, the seasonal variation is very slight and monthly rainfalls are relatively uniform.

The intensity of rainfall is expressed as an amount of precipitation in a stead period. During heavy rainstorm the intensity of rainfall varies widely from minute to minute and can be measured only with a continuously recording gauge.

The intensity of rainfall is given by,
I = R/(T+C)
Where, I = intensity in inches/hr.
T = Duration of rain storm in hours
R & C = these are constants quoted by different authorities for different areas of world.
The intensity also depends on the area selected for the measurement.

There are two types of rain gauges. They are Non-recording type and recording type. Non-recording type do not record the depth of rainfall but only collect the amount of rainfall and recording type records the depth of rainfall in mm or cm.

The non-recording gauge usually consists of a standard funnel discharging into a receiver large enough to hold the maximum possible day’s rainfall. The funnel and receiver are placed in a metal casing with suitable packing.

These are rain gauges which can give a permanent, automatic rainfall record in the form of a pen mounted on a clock driven chart. From the chart intensity or rate of rainfall in cm per hour. The recording is done by an attached siphon.