Design Storms
Drainage and Utilities design storms include:
Rational Design Storms
Design storms for use with the Rational method can be created with one of two methods.
- The I-D-F table method uses a table of duration versus intensity values to describe rainfall events of a particular frequency (return period).
- The e, b, d coefficients method uses a collection of three coefficients (e, b, d) to define a mathematical relationship between the rainfall intensity and the duration of the rainfall event for a given frequency.
Both methods yield the equivalent of a rainfall I-D-F curve, and therefore must be created for use in a particular geographic location.
Cumulative Rainfall Curve Storms
Hydrograph methods, such as the SCS Unit Hydrograph procedure, cannot use I-D-F curves for rainfall data (as used in the Rational method). Instead, complex hydrograph methods require time-based rainfall curves. Design storms for use with the hydrograph methods (e.g., SCS Unit Hydrograph) can be created with one of two methods: time-depth or synthetic.
Time-depthThe time-depth curve method uses a table of time versus rainfall depth values to describe the rainfall event. This method is typically used when gauged data from actual storm data is available.SyntheticThe synthetic curve method uses a table of time versus rainfall depth fraction values, a duration multiplier, and a total rainfall depth to describe the rainfall event. This arrangement is very flexible because the same rainfall event shape can be used for storms of various durations and total depth.