ETL 1110-2-367
31 Mar 95
Appendix D
about 60,000. Basin land use consists mainly of vineyards in the
valley area north of the City of Napa and limited mixed use in
Case Study for Analysis of Interior Flood
the marshland or reclaimed tidal land south of the city.
Damage Reduction Measures,
Napa River, Napa, California
D-3. Description of the Proposed Flood Damage
Reduction Project
D-1.
Introduction
a.
Napa River and Napa Creek.
The current
recommended plan for the City of Napa provides for protection
a.
This case study presents part of the hydrologic
against the 1-percent chance event from the Napa River and
engineering analysis results of interior flood damage reduction
Napa Creek. The proposed plan consists of channel excavation,
measures for the City of Napa, CA. It was conducted by the
sheet-pile walls, concrete floodwalls, setback earthen levees, a
Hydrologic Engineering Center (HEC) for the Sacramento
bypass channel, and related environmental mitigation measures.
District Corps of Engineers. The objective of the hydrologic
b.
Interior area measures. The interior flood damage
engineering analysis was to determine the minimum outlet
facility associated with the proposed line-of-protection, the
reduction measures will consist of replacing approximately 21
stage-frequency relationships for the without-project conditions,
existing storm sewers in 8 interior areas with minimum gravity
and the stage-frequency relationships for a range of gravity
outlets through the Napa River line-of-protection. Additional
outlet and pumping station sizes and configurations for the
outlet capacity by gravity or pumps will be provided where
interior areas.
economically justified. The proposed improvements for Napa
Creek consist of channel excavation only and, therefore, will not
b.
This case study presents the results of applying the
include interior measures. The case study presented here will
HEC-IFH program for evaluation of one of the several interior
describe the analysis of interior measures for one of the areas.
areas involved in the overall investigation. The case study
D-4.
Interior Area Data and Information
includes a description of the study area, the Napa River
Assembly
proposed flood damage reduction project, interior area data
and information, without-project conditions analysis for
a.
General.
minimum facility analysis, minimum facility analysis, and
stage-frequency for interior flood damage reduction plans.
The Sacramento District was responsible for developing data
(1)
Hydrologic data and other information required for
for the without-project conditions, including stage-damage
the analysis of the interior area were assembled. They include
relationships, cost estimates of the flood damage reduction
data for both the interior and exterior (Napa River) areas. The
measures, and other data required to perform the economic
information is applicable for any analytical method, but was
analyses of each plan. The design requirements for
specifically targeted for application of HEC-IFH. Appropriate
conveyance systems, inlet and outlet works, and the
information was assembled to permit analyses using continuous
economic analyses of project components are beyond the
simulation analysis (CSA) with period-of-record historical data
scope of the case study presented herein.
and hypothetical event analysis (HEA) with synthetic storm
event data.
D-2.
Description of the Study Area
(2)
CSA is attractive because it preserves the
a.
The Napa River basin is located about 50 miles north
relationship between Napa River stages at interior outlet
of San Francisco, CA. The basin is about 50 miles long on a
locations and interior area runoff. A drawback of CSA is the
north-south axis, varies between 5 and 10 miles in width, and
difficulty of defining rare flood events when only a relatively
has a drainage area of about 426 sq miles (see Figure D-1). The
short period-of-record is available as is the case for the Napa
north, east, and west limits of the basin are formed by portions
area. Therefore, HEA was adopted for this study to define the
of the north coast mountain range. The southern limit is
full range of flood events. The stage-frequency relationships
bounded by San Pablo Bay.
from HEA and CSA were compared to help substantiate the
reasonableness of the HEA results. Hydrologic data and other
b.
The Napa River originates near Mount St. Helena and
required information are described as an analyst would assemble
empties into the Mare Island strait that flows into the tidal
and enter the data into HEC-IFH. Data sets and module
marshland and sloughs of San Pablo Bay. The City of Napa is
information are shown by including representative program
located in the lower third of the basin and has a population of
screens as figures where appropriate.
D-1
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