ETL 1110-2-367
31 Mar 95
l.
Water surface profile data. Water surface profiles
damage reduction measures will assume that the adopted
minimum facility is in place and is described in paragraph
for with-project conditions were developed by the district
D-6.
using the HEC-2 program. These profiles were used to
determine rating curves for the Napa River at interior area
b.
Napa River flooding without line-of-protection.
outlet locations. The water surface profiles were also used
to determine exterior stage transfer relationships for
The source of serious flooding in the City of Napa is the
transferring the computed exterior stage at the primary outlet
Napa River and to a lesser extent Napa Creek. The
location to the secondary outlet locations. The rating curve
recommended flood damage reduction project protects the
for Napa River at the Area 5 primary location is shown in
city from flooding up to the 1-percent chance flood for both
Figure D-15.
the Napa River and Napa Creek. The basis for sizing the
minimum facility is to assure that flooding from local storm
m. Stage-damage relationships. Representative stage-
runoff, when the Napa River and Napa Creek are below
bank full, is not more frequent with the line-of-protection in
damage relationships for the interior areas at runoff
place than without the line-of-protection in place.
concentration points are required for economic analysis and
identification of interior plans that maximize net flood
c.
Local runoff flooding without line-of-protection.
damage reduction benefits. Economic analysis is not part of
this investigation; therefore, complete stage damage
Local flooding was evaluated without the line-of-protection in
relationships were not required. The elevation where
place, assuming the present storm sewer system in place, and
significant damage begins or "zero damage" was required in
Napa River and Creek below flood stage. Stage-frequency
order to establish the size of the minimum facility and to set
relationships for this condition were not developed due to lack
pump on/off elevations. These elevations were provided by
of data. Storm sewer system design criteria for the City of
the district.
Napa, for existing and new systems, were well-documented
and were used to establish the target condition for the
D-5. Without-project Conditions Analysis for
minimum outlet facility analysis. The first criterion used was
Minimum Facility Evaluation
that only minor street and gutter flooding should occur up to the
10-percent chance (10-year) flood event. Minor street and
a.
General. The without-project analysis involves
gutter flooding in this case is defined as not exceeding a depth
evaluation of conditions without and with the line-of-
that would result in flooding more than 10 ft from the street
protection in place. Degrees of flooding for these conditions
gutter. The second criterion was that no significant damage
are needed to select a minimum facility. The without-project
from flooding would occur in residential and commercial areas
condition used to formulate and evaluate the interior flood
from floods up to the 4-percent chance (25-year) flood event.
Figure D-15. Exterior rating curve for Interior Area 5
D-13
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