31 Mar 95
Figure 3-5. HEC-IFH data entry menu
(1) PRECIP module.
recording and non-recording rain gauge data. See the PRECIP
(a) This module contains continuous rainfall (normally
user manual (USACE 1989) for more information.
historic records) and/or historical storm records and
(e) Hypothetical frequency storm depth-frequency-duration
subbasin rainfall records.
relationships are normally developed from standard rainfall
depth-frequency-duration information published by the National
(b) Rainfall data for recording and nonrecording rain gauges
Weather Service. These data are entered into HEC-IFH as
generally can be obtained from the National Weather Service
illustrated in Figure 3-8. HEC-IFH uses this information to
(NWS) publications or CD's. Figure 3-6 shows what data can
compute rainfall distributions for up to seven storms ranging
be obtained from the National Climatic Data Center. Estimates
from 50 percent to 0.2 percent exceedance frequency.
of rainfall data may also be acquired from newspaper articles
Figure 3-9 illustrates a rainfall hyetograph for a hypothetical
that describe flooding after a large storm event and from rain
gauges placed by local citizens, drainage districts, public works
departments, and college or university science departments.
(f) HEC-IFH allows the user to compute a standard project
storm (SPS) using the same computation method utilized in the
(c) Rainfall data can be entered into HEC-IFH manually, or
HEC-1 computer program. The SPS is normally used to
imported from an existing HEC-DSS database. HEC-IFH
generate a large event to evaluate how the system would
checks imported values for missing data and either replaces
perform if the event occurs. Figure 3-10 illustrates a typical
them with zeros, or terminates the procedure.
SPS precipitation distribution.
recommended to correct missing values using external utilities
before importing them to HEC-IFH. One-year, one-month, or
(g) After the rainfall records are adjusted and verified,
one-day hyetograph plots can be generated from the rainfall
weightings are assigned to each gauge so that a composite
rainfall record is developed for each subbasin. The weightings
for CSA and HEA, respectively.
are based on conventional methods as described in Section 3.2.2
of the HEC-IFH user's manual.
(d) HEC's PRECIP program is a useful tool for developing