Figure 1-3. Hydrography of a brief rainfall
The hydrography illustrates the relationship between the duration and intensity of the runoff in a 15-minute
rainstorm. Intensity is the rate at which the water is running in the drainage structure.
The bar graph of the intensity and duration of the rainstorm has been superimposed on the hydrograph. It
illustrates that the peak runoff lags behind the peak flow. This lag is caused by factors affecting loss of water
during rainfall and the size of the delineated drainage area. The time from the beginning of the storm until the
point of peak runoff is called the time of concentration (TOC).
The rainfall intensity used in the Rational Method is a measurement of the rate of precipitation expected to fall on
your watershed area. This rainfall rate is based on historical records of rainfall rates compiled for a specific
location. The rainfall intensity is the depth of water falling per unit of tie, and it is expressed in inches per hour
(in/hr). The following factors affect the rainfall intensity:
Design Storm. Drainage systems are built
r a period
of time. The longer
the larger and
expensive the design becomes. However, with the longer design comes greater protection. The time period that
you design your culvert to last is called the design storm or the return period of the storm.
Design Storm Frequency. The rainfall intensity
on storm frequency. This basically means that you will
design a culvert based on the probable occurrence of a storm that will