Section 8: Design Rainfall Hyetograph Methods
Anchor: #i1027396Use of the Rainfall Hyetograph
A rainfall hyetograph is a graphical representation of the variation of rainfall depth or intensity with time. Rainfall-runoff hydrograph methods require a description of this variation. It is possible to use actual rain gauge data in rainfall-runoff models if the data are recorded using a small enough time period (such as 15-minute increments). Often such data are not readily available.
For design, the use of a single measured rainfall event without consideration of other events is not practical because storms vary considerably from event to event with no probability of occurrence established.
Anchor: #i1027411Storm Distributions
In the Rational method the intensity is considered to be uniform over the storm period. Unit hydrograph techniques, however, can account for variability of the intensity throughout a storm although the overall depth for a storm will be the same for a given duration for each method. Therefore, when using unit hydrograph techniques, determine a rainfall hyetograph or distribution. The NRCS Type II and II distributions are examples of standardized distributions that are available for use. These two distributions are typically described in either an incremental or accumulative rainfall format, usually in 15-minute increments. In addition, they are also considered to be dimensionless. That is, they represent a distribution of one inch of rainfall over a 24-hour period to which a design (frequency) rainfall depth can be applied. The distribution itself is arranged in a critical pattern with the maximum precipitation period occurring just before the midpoint of the storm.
The table below represents the NRCS 24-hour Type II and III distributions.
|
Time, t (hours) |
Fraction of 24-hour Rainfall |
|
|---|---|---|
|
|
Type II |
Type III |
|
0 |
0.000 |
0.000 |
|
2 |
0.022 |
0.020 |