The intensity of rainfall is the rate at which the rain is falling and it is expressed in cm/hr. For example, during a particular event of rainfall occurred for 10 minutes the quantity of rainfall is 2 cm, then the intensity of rainfall of that rain event is 12 cm/hr. 5 cm/ hr intensity of rainfall means an average rainfall rate of 5 cm per hour duration. The rainfall particulars are recorded with either non-recording rain gauges or automatic recording rain gauges or by Meteorological Department (IMD).

 Rainfall Recording Non-recording gauges

In non-recording gauges the rainfall for the past 24 h is measured and recorded as cm of rainfall during the last 24 hours. These data give only average intensity and not the actual intensity of rainfall of the rain event, which might have last for only 10 to 15 minutes. This type of rain gauges does not record the rain but only collect the rain. Symon’s type instrument is most commonly used. The rain gauge consists of a collector, with a gun metal rim, a base and a polythene bottle. The collector and the base are made of Fibre Glass Reinforced (FRP). The collectors have aperture of either 100 cm² or 200 cm² area and are so made that they are interchangeable. The polythene bottles are of three sizes having capacities of 2, 4 and 10 litres of water respectively. The rain gauge should be fixed on a masonry or concrete foundation 60 x 60 x 60 cm sunk into the ground. The base of the gauge should be embedded in the foundation, so that the rim of the gauge is exactly 30 cm above the surrounding ground level. The rim of the gauge should be kept perfectly level. The horizontality should be checked with a spirit level laid across the rim.

At the time of recording rainfall, remove the funnel of the rain gauge and take out the polythene bottle. Place the measuring jar in an empty basin and slowly pour the contents of the receiver into the measuring jar taking care to avoid spilling. If, however, any water is spilled into the basin, add it to the water in the measuring jar before arriving at the total amount collected. While reading the amount of rain, hold the measuring jar, upright between the thumb and the first finger or place it on a table or other horizontal surface. Bring the eye to the level of the water in the measure glass and take the reading of the bottom of the meniscus or curved surface of the water. The amount of rainfall should be read in millimeters and tenths. It is extremely important to note that the correct type of measuring jar appropriate to the type of rain gauge funnel in use should be used for measuring the amount of rainfall, to avoid errors in the results. Enter 0.0 for no rain and a ‘t’ (meaning trace) for rainfall below 0.1 mm.

The collector of the rain gauge the receiving bottle and measuring cylinder are always kept clean. They should be emptied regularly of sediment or other material that may have fallen into them and cleaned periodically. The grass around the gauge should be kept short. No shrubs or plants should be allowed to grow around the gauge.

Automatic Recording gauges

Automatic ram gauges record continuously the cumulative amount of water with time on a graph paper. After the collection in the bottle has recorded 10 mm of rain, the bottle gets emptied and the line representing cumulative rainfall vertically falls down. Hence, while estimating the amount of rain fallen during any time interval, this fact must be kept in view. The graph of the automatic rain gauge shows the time taken for each 10 mm of rain. The total rainfall in any particular hour can be obtained from the graph.

Recording type of rain gauges are those which can give a permanent automatic rainfall record without bottle reading. In this type of rain gauges, a man need not go to the gauge to measure or read the amount of rain fallen. A mechanical arrangement by which the total amount of rain fallen, since the record was started, gets recorded automatically in graph paper. Thus the gauge forms a record of cumulative rain vs time in the form of a graph and which is known as the mass curve of rain fallen. The curve will also help in indicating the times of onset and cessation of a rain and its duration.

The slope of the curve gives the intensity of rainfall for any given period.

                                            do     _(P2—

The intensity of rainfall,            — (_{1, ii})

Since such gauge represent the cumulative rain, they are called as integrating rain gauges. There are three types of recording rain gauges. They are

1.  Tipping bucket gauge,
2. Weighting type,
3. Floating type.

Tipping bucket gauge

The arrangement of a tipping bucket rain gauge. In this type of rain gauge, the rain­water is collected in the collector and then passed through a funnel. The funnel discharges the water into a two compartment bucket. If 0.1mm of rainwater gets filled up in one compartment, the bucket tips emptying in to a reservoir and moving the second compartment into place beneath the funnel. The tipping bucket completes an electric circuit, causing a pen to mark on a revolving drum. These types of gauges are generally in hilly and inaccessible areas, where they can supply measurements directly to control room. No graph paper or drum is installed in the gauge and the rainfall measurements are directly recorded at the control room.

Weighting type gauge

This type of gauge weights the rain which falls into a bucket placed on the platform of a spring or a lever balance or any other weighting mechanisms as shown in fig. When the weight of the bucket increases, that helps in recording the increased quantity of rain with time by moving a pen on a revolving drum.

Floating type gauge

In this type of gauge, the rise of gloating body due to increasing rain catch helps in lifting the pen point, which goes on recording the cumulative rain with time in a graph paper wrapped round a rotating drum. Nowadays various types of floating type recording rain gauges are available. Natural Syphon recording rain gauge is widely used in India.

The rainwater entering the gauge at the top of the cover is led via the funnel to the receiver, consisting of a float chamber and a siphon chamber. A pen is mounted on the stem of the float, and as the water level in the receiver rises, the float arises and the pen records, on a chart wrapped round a clockwise rotating drum, the amount of water in the receiver at any instant. The rotating drum completes one revolution in 24 hours (one day) or sometimes in 7 days. Syphoning occurs automatically when the pen reaches the top of the chart, and as the rain continues, the pen rises again from the zero line of the chart. If there is no rain, the pen traces a horizontal line from where it leaves off rising.

The siphon recording rain gauge is an instrument designed for continuous recording of rainfall. In addition to the total amount of rainfall, the onset and cessation of rain (and therefore the duration of rainfall) are recorded.

The gauge should be installed in such a way that the rim of the funnel is horizontal and set at a height of exactly 75 cm above ground level. For setting the pen at the zero mark, pour sufficient water into the receiver till the pen reaches the top and water siphons out. After all the water is drained out, the pen should be on the zero line; if not, it should be adjusted.

Rainfall enters the gauge at the top via a funnel and passes through a receiver consisting of a float chamber and a siphon chamber. A pen is mounted on the stem of the float, and as the water level rises in the receiver, the float rises and the pen records the level of water in the chamber on a chart wrapped round a clockwise rotating drum. The rotating drum completes one revolution in 24 hours (one day) or sometimes in 7 days. Siphoning occurs automatically when the pen reaches the top of the chart, at the 10 mm mark, and then the pen comes down to the zero line of the chart. The pen rises again with the onset of rainfall. When there is no rain, the pen traces a base horizontal line of the chart.

The siphon is arranged concentrically so that the long discharge tube being surrounded by the shorter siphon chamber and is directly connected to the float chamber. A glass piece is placed over the joint of these tubes and the passage connecting two tubes at this joint is of almost capillary dimensions, but the sectional area is large enough to discharge the water collected in the receiver with enough speed. When the upper end of the water level falls to a certain depth, the siphon ceases to act, the water column is broken at a definite stage by a bubble of air which gets into the capillary and freedom from dribbling is thus ensured. There is just sufficient water to float the float after siphoning.

The chart should be changed daily (in India at 08 30 IST) as a routine observation irrespective of the rainfall occurrence. The observer should see that the pen trace matches the base horizontal line of the chart without an error after every siphoning operation. The instrument should be checked daily once for correct siphoning operation.

Duration of Rainfall

The duration of rainfall is the time period for which the rain event occurs at that given intensity of rainfall. From the historic records of the automatic rain gauge station (of graphs) for 30 to 50 years the intensity of rainfalls for different time intervals such as 5 minutes, 10 minutes, 15 minutes, 20 minutes, 60 minutes, etc., could be obtained.

Frequency of Rainfall

The frequency of rainfall is the number of times the rainfall of a particular intensity and duration occurred in the past based on the historic records. Frequency of rainfall is also know as the recurrence interval of a particular rainfall.

 Time of Concentration

During a rainfall only a certain amount of water from the adjacent area will reach the outlet initially, but after sometime the rain water from the entire area of the catchment will start reaching the outlet and the runoff rate would become maximum and equal to the rate of rainfall. The period after which the entire area will start contributing to the runoff is called the time of concentration. Time of concentration generally consists of two parts: viz., the inlet time and the channel flow time.

The inlet time (Ti) is the time taken by the water to flow overland from the critical point up to a point where it enters the drain mouth.

Ti is inlet time in hours,

L is the length of overland flow in km,

H is the total fall in level from the critical point to the mouth of the drain.

The channel flow time is the time taken by the water to flow from the mouth of the channel to the considered point. This is obtained by dividing the length of the drain by the velocity of flow in the drain.

Unless otherwise determined by overland flow charts or nomographs, the Time of Concentration (T_c) for inlets of storm water collection systems may be used as follows as furnished in Table

Time of concentration for different areas

 At no time the Time of Concentration shall be greater than 30 minutes for design of storm inlets. The runoff resulting from a rainfall having duration equal to the time of concentration will be the maximum and that duration of rainfall is called the critical rainfall duration.

Design Storm Duration

Although the design storm must reflect required levels of protection, the local climate, and catchment conditions, it need not be scientifically rigorous. It is more important to define the storm and the range of applicability fairly precisely to ensure safe, economical and standardised design.

Two types of design storm are recognised: synthetic and actual (historic) storms. Synthesis and generalisation of a large number of actual storms is used to derive the former. The latter are events which have occurred in the past, and which may have well documented impacts on the drainage system. However, it is the usual practice in urban stormwater drainage to use synthetic design storms. Design storm duration is an important parameter that defines the rainfall depth or intensity for a given frequency, and therefore affects the resulting runoff peak and volume.

Current practice is to select the design storm duration as equal to or longer than the time of concentration for the catchment (or some minimum value when the time of concentration is short). Intense rainfalls of short durations usually occur within longer-duration storms rather than as isolated events. It is common practice to compute discharge for several design storms with different durations, and then base the design on the “critical” storm which produces the maximum discharge. However the “critical” storm duration determined in this way may not be the most critical for storage design.

Recommended practice for catchment containing storage is to compute the design flood hydrograph for several storms with different durations equal to or longer than the time of concentration for the catchment, and to use the one which produces the most severe effect on the pond size and discharge for design.

Rainfall Intensity-Duration-Frequency (IDF) Relationships

The total storm rainfall depth at a point, for a given rainfall duration and ARI, is a function of the local climate. Rainfall depths can be further processed and converted into rainfall intensities (intensity = depth/duration), which are then presented in IDF curves. Such curves are particularly useful in stormwater drainage system design because many computational procedures require rainfall input in the form of average rainfall intensity.

The three variables, frequency, intensity and duration, are all related to each other. The data are normally presented as curves displaying two of the variables, such as intensity and duration, for a range of frequencies.

 These curves shall be developed by the IMD based on the automatically recorded rainfall data for a long period. The IDF curve is then used for determining the design intensity of rainfall for the duration of rainfall which is equal to the time of concentration. This design intensity of rainfall is used for the computation of storm runoff.