Experiment No. 2
RAINFALL-RUNOFF CHARACTERISTICS OF LONG DURATION SINGLE STORM RAINFALL
Object:-To study the rainfall-runoff characteristics of a long duration single storm rainfall.
Apparatus: Basic hydrology tank and stopwatch.
Theory: Hydrological cycle describes the complete movement of water between the atmosphere, the land surfaces and the water masses of the earth.
The Hydrologic cycle
Rain falling on a catchment area will make its way to the point of concentration where it will leave the catchment. When rain falls on the catchment the time taken for the water to reach the point of concentration will depend on the horizontal distance it has to travel and also on the velocity. The greatest time taken for rain falling on the catchment (the far corners) to reach the point of concentration is called the time of concentration.
A graphical record of river flow against time (usually from the start of the storm) is called a hydrograph.
The timing and intensity of the rainfall is shown by the block in the upper part of this diagram and if the rainfall persists at a constant rate for longer than the time of concentration tc of the catchment the runoff hydrograph will level off at a peak value on the catchment. Under these circumstances the recession curve part of the hydrograph is delayed until the rain stops.
A typical hydrograph
Experimental Setup/Methodology:
Set the slope of the catchment tank to a slope of about 1/200 (downhill towards the opening).
Switch on the pump and using the nozzle control valve set flow rate for the overhead spray nozzles as 14 lit/min. Leaving the valve position unaltered, switch off the pump and allow at least one hour to elapse for the sand bed to drain thoroughly. Allow rain to fall long enough to give a steady runoff value. Switch off pump and record the recession limb of the hydrograph. Use a stopwatch started (zero time) at commencement of rainfall and read weir discharge as frequently as necessary to show hydrograph form.
Observations:
Rainfall intensity of storm = ______ lit/min = ______ lit/sec;
Duration of storm = __________ sec; Time of concentration = _______________ sec;
Runoff step = 0.01 lit/sec;
S. No. | Discharge (lit/sec) | Cumulative Time (sec) | S. No. | Discharge (lit/sec) | Cumulative Time (sec) |
Graphs:
Histogram
The hydrograph should be properly shown as a block diagram (or histogram) as shown in below, because the discharge measurement data gives the volume passed from the apparatus between a series of specified equal times.
Hydrograph
Discharge values are calculated from a set of time and water column height readings in the way described above and they can now be plotted as individual points, which will result into a smooth curve called storm hydrograph as shown in below figure.
HistogramStorm Hydrograph
Results: