31 Mar 95
(d) Overflow. Overflows occur when an interior ponding
date is the end of the last computation interval in the analysis.
area exceeds the available storage, which causes flows to spill
For example, hourly values for the month of October 1990
into an adjacent interior area. HEC-IFH assumes that the flow
would have a beginning date of 01OCT1990/0100 and an
leaving the original interior area does not return to that area.
ending date of 31OCT1990/2400. If the analysis of October
The overflow is defined by specifying a pond elevation-overflow
1990 consisted of daily instead of hourly values, the starting date
would be 01OCT1990/2400 (the end of the first day), and the
ending date is not changed.
(2) The specified beginning and ending date should be
a. Plan development. The interior analysis may be
consistent with the starting and ending periods of time series
performed after the input data entry is completed. The analysis
used as input for the calculations. After the dates are specified,
defines a plan that consists of a unique combination of modular
inflow time series used in the plan. If any of these time series
facilities. Figure 3-21 shows the data screen used to specify the
start after the beginning date of the interior analysis, or end
various data modules that comprise the plan.
before the ending date of the analysis, the interior analysis will
proceed using zero (0) for all missing values. If so, a message
A study typically will have different plans. The first plan may
is written to the error warning message file.
describe a minimum gravity outlet, a second plan may include
Computation time interval.
additional gravity outlet capacity, and a third plan may include
a pumping station. Each plan is given a unique plan ID. The
plan ID is used to identify the plan results.
(1) General. The computation time interval, shown in
Figure 3-21, is the time-step for all subbasin runoff, channel
routing, and pond routing computations for the interior analysis.
This value must be between 5 min and 24 hr. Choosing an
(1) The "Beginning Date for Analysis" and "Ending Date for
appropriate time interval is important. If the primary interior
Analysis" are entered as shown in Figure 3-21. The standard
problem is providing facilities to handle the volume of water
HEC-DSS format for time series data is used. The beginning
reaching the line-of-protection (such as a large ponding area in
date is the end of the first computation interval, and the ending
an agricultural area), a long computational time interval of up to
Figure 3-21. Plan specification screen