FLOOD04403

• County: Statewide
• Basin: Statewide
• Title: Hydrologic Analysis of Ungaged Waterways with HEC-1
• Date: 4/1/1981
• Prepared For: US
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Educational/Technical/Reference Information

storm depends on the basin time of concentration, i,e" the travel time from
the upper portions of the basin to the point of interest farthest
downstream, The overall duration is dictated by the purpose of the estimate
and the importance of total runoff volume in the use of the design flood,

Rainfall frequency curves are derived from TP 40 (twiSr 1961), or other
appropriate source, for durations of 1, 3, 6, 12r and 24 hours, The values
are adjusted for annual series, since the TP 40 charts are for. partial
duration series, For a selected storm frequency, rainfall depths are
determined for each duration at desired time intervals and incremental depths
computed. If durations greater than 24 hours are needed TP 49 (NWS, 1964)
can be used. If the desired unit hydrograph interval is less than 30
minutes, the peak 3D-minute rainfall period is incremented using the
following:

a.

for the peak 15 roin use 0.72 of peak 30 min :rainfall,
for the peak 10 min use 0,57 of peak 30 min rainfall,
for the peak 5 min use 0,37 of peak 30 min rainfall,

b,

c.

The rainfall values from the maps represent pDint rainfall. However,
the average rainfall over a given area will be less than the maximum point
value in the area, Figure 4.2 gives the reduction in the point value for
various precipitation durations as a function of area size,

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Weather Bureau Technlcol Poper No. 40

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4.2, Adjustment of Rainfall Data for Area
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