ETL 1110-2-367
31 Mar 95
This second criterion was interpreted that the interior stage
with the minimum facility in place becomes the target for
resulting from the 4-percent chance event should not exceed the
damage reduction of proposed additional interior flood damage
start of significant damage elevations determined by the district
reduction measures. As described previously, the minimum
office. Based on the past performance of the existing sewer
facility was sized to provide interior flooding relief so that
system and the overall reasonableness of the criteria, the storm
during low exterior stages (unblocked gravity outlet conditions)
sewer system design criteria were adopted for sizing the
the local interior area runoff will pass the design storm sewer
minimum facilities.
outflow without an increase in interior stages over natural or
without line-of-protection conditions.
d.
Assess future without-project conditions impacts.
b.
Selecting the minimum facility for Interior Area 5. A
Future conditions that could affect Napa River interior area local
runoff flooding were considered. Hydrologic and/or hydraulic
series of analyses of gravity outlet capacities and configurations
conditions are not expected to significantly change over the
using local storm hypothetical events analysis (HEA) and
project life and, therefore, no changes needed to be incorporated
assuming unblocked conditions were conducted using HEC-
into the hydrologic and hydraulic analysis. The interior areas
IFH. Physical characteristics of the gravity outlets were
described in paragraph D-3i. A new plan was defined for each
are fully urbanized, so future urbanization would have minimal
effect on watershed runoff.
Proposed and planned
gravity outlet capacity to be evaluated and the interior stage-
improvements in the existing storm sewer system, as described
frequency relationship was developed for each outlet. Plan
by the City of Napa, were evaluated and incorporated in the
components as defined in HEC-IFH for one of the plans
interior areas where appropriate. There were no planned
evaluated for Interior Area 5 are shown in Figure D-16. Stage-
changes to the existing storm sewer system in Interior Area 5.
frequency relationships of gravity outlets were compared to the
storm sewer design criteria described previously and the outlet
D-6. Minimum Facility Analysis
size that came closest to meeting the criteria was selected. For
Interior Area 5, the selected minimum facility was a double 5-
a.
General. The adopted minimum facility, sized
by 5.5-ft box culvert. The "zero damage" elevation is 14.0 ft,
according to the criteria described in paragraph D-5c, is a
and the 4-percent chance elevation based on the results of the
justified part of the line-of-protection. Stage-frequency relation-
HEA unblocked condition simulation is 13.55 ft. The stage-
ships for the condition with the minimum facility in place
frequency relationship with the minimum facility in place is
become the condition without the minimum facility in place for
shown in Figure D-17. The 10-percent chance stage is below
evaluating potential interior flood damage reduction measures
the criterion elevation for street flooding and therefore this
over and beyond the minimum facility. The residual damage
minimum facility is adequate.
Figure D-16. Plan components, minimum facility - HEA, unblocked
D-14
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