ETL 1110-2-367
31 Mar 95
Figure D-17. Stage-frequency for minimum facility - Interior Area 5 - HEA, unblocked
c.
Without-project condition stage-frequency relation-
(3)
Examination of CSA results for several historical
ship with the minimum facility in place.
events shows that interior and exterior flooding are typically
coincident, as illustrated in Figure D-19 for the February 1986
event. An exception to this was the January 1973 event,
(1)
After the minimum facility was selected, it was
where the 41-year record interior rainfall and resultant runoff
evaluated using general rain hypothetical event analysis
occurred while Napa River stages were very low (see Figure
(HEA). A new general rain HEA plan (Plan 5-2A) was
D-20). Timing of the peak interior runoff and the maximum
defined using precipitation depth-duration-frequency data for
exterior stage is critical in the Napa study due to the small
general rain events occurring over the Napa River watershed as
ponding area storage available. Due to this fact and the fact
well as the interior area. Exterior stages were computed from
that the historical CSA shows that the peak interior runoff can
imported hypothetical flood discharge hydrographs and a stage-
occur before, after, or simultaneous to the exterior peak stage,
discharge rating for the Napa River at the interior area outlet as
HEA stage-frequency relationships were adopted for the
previously described. The results of the analysis were used to
evaluation of interior features. HEA captures the critical
test the effectiveness of the minimum facility gravity outlet.
combinations of interior runoff and exterior stage that can
HEC-IFH assessed local runoff flooding that occurs during
occur, but are not always well-represented in the historical
blocked conditions (e.g., with general rain storms centered over
record. Figure D-21 shows a comparison of the stage-
the interior and exterior basin causing flooding on both the
frequency relationships for CSA and HEA. The differences
interior and exterior).
The resulting stage-frequency
in stage are minor considering a 2-ft difference in stage
relationship is shown in Figure D-18. Plan 5-1D is HEA with
(17.0 ft minus 15.0 ft) is equivalent to less than 0.25 in. of
unblocked exterior conditions and Plan 5-2A is HEA with
runoff from the interior area. The relatively good comparison
interior and exterior flooding conditions.
between the relationships helps prove the reasonableness of the
HEA-developed stage-frequency relationship. The HEA stage-
(2)
Continuous simulation analysis (CSA) was
frequency was adopted to establish the base plan or without-
performed using previously described period-of-record
project condition stage-frequency relationships for evaluating
composite rainfall. The purpose of evaluating CSA in addition
additional interior flood damage reduction measures as
to HEA is to compare the resultant stage-frequency
described in paragraph D-7.
relationships. CSA captures the relationship between interior
runoff and exterior stage, whereas HEA assumes interior and
exterior flooding are coincident.
D-15
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