The FEH (1999) presents a rainfall depth-duration-frequency (DDF) method. This replaces the Location, M5-60, and ratio r values which are described in the Flood Studies Report (FSR), and used in the Wallingford Procedure. The FEH retains the FSR rainfall profile and Areal Reduction Factor procedures.

The FEH analysis considered durations of 1, 2, 6 and 12 hours and 1, 2, 4 and 8 days, and return periods from 2 to 1000 years. It includes a CD-ROM with the 6 parameters defined on 1km grid covering the UK.

The formulae used to obtain rainfall depths from the 1 km point DDF values are:

lnR = (cy + d1) lnD + ey + f for D = <= 12 hours

lnR = lnR12 + (cy + d2) (lnD – ln12) for 12 = D = <= 48 hours

lnR = lnR48 + (cy + d3) (lnD – ln48) for D = > 48 hours

Where:

y = -ln(-ln(1-1/T))

R = rainfall depth (mm)

D = duration (hours)

T = return period (years)

And c, d1, d2, d3, e, f are parameters

And d1, d2, and d3 are for the duration ranges <12, 12-48, and >48 hours respectively.

The Langbein formula is applied to return periods between one year and less than five years.

1-1/TAM = exp(1-1/TPOT)

Where:

T = return period

When creating a new FEH storm event group, default return periods and durations are used.

The following return periods are used (Years): 1.58, 5, 10, 20, 25, 30, 50, 75 and 100. The user can also add additional return periods. However, you cannot remove any return periods.

The following durations are used (Hours): ½, 1, 2, 3, 4, 6, 8, 10, 12, 16

The following durations are used (Days): 1, 1.5, 2, 3, 4, 5, 6, 7, 8

The depth is calculated given the parameters and then converted to intensity in mm/h and then converted and stored in in/h.

Notes Tab

The notes tab provides a way for the user to write notes about the storm event group.

Library Tab

This tab displays information about the storm data definition that is currently highlighted in the list pane. If the storm data definition is derived from an engineering library, the synchronization details can be found here. If the storm data definition was created manually for this model, the synchronization details will display the message Orphan (local), indicating that the storm data definition was not derived from a library entry.