31 Mar 95
circular pipe that runs beneath the overflow ditch. This outfall
maintain the slopes and outlet invert elevations of the existing
location is the flow concentration point for Interior Area 5 and
outlets as close as possible. Required information was taken
was designated as the ponding area (see paragraph D-4d) and
from project drawings provided by the district and existing
storm sewer layouts provided by the City of Napa. Manual gate
the primary gravity outlet location for this interior area.
closure valves, as well as flap gates, will be included as part of
each new outlet. The minimum head differential required for
Additional existing outlets. There are three
gravity flow was specified as 0.5 ft. No special gate closure
additional existing outlets that cross the line-of-protection and
requirements were established. A typical layout of a drop inlet
are to be replaced with new gravity outlets with drop inlets.
box culvert at the primary location for Interior Area 5 is shown
They are all upstream from the primary gravity outlet and are
in Figure D-11.
designated and analyzed as secondary outlets for HEA and CSA.
These outlets are described below:
Location 5.1. One 24-in. pipe at Imperial Way.
Typical pumping station configuration and
Location 5.2. One 18-in. pipe at North Bay Drive
operation were determined through coordination with the
(to be replaced by a 24-in. drop inlet).
district. Criteria for number of pumps and pumping station
capacity were that each pumping station would have a total of
Location 5.3. One 30-in. pipe at Lincoln Avenue.
three pumps, each having two-thirds of the total designated
station capacity. Two of these pumps would be operated as
There are a few small outlets that convey a minor portion of
needed and one would be for backup in case one of the other
interior runoff from Interior Area 5 into Napa Creek from the
pumps went out of service. For example, a 300-cfs pumping
left bank (north side). These outlets will not be cut off by the
station would include three (200-cfs or 90,000-gpm) pumps,
project because they are upstream from the Napa River tieback
two of which would be operating for a maximum possible
levee where channel excavation is the only project feature. The
station capacity of 400 cfs. Pump head-capacity-efficiency
effects of these outlets were considered negligible in the analysis
relationships were determined from pump performance curves
of Interior Area 5.
provided by the district. Figure D-12 shows the relationships
for a 200-cfs (90,000-gpm) pump unit.
Pump on and off elevations were determined so that
locate outlet inverts and ditches that will be cut off by the line-
the pumps would come on to effectively reduce damaging stages
of-protection, bridges, hydraulic structures, and floodplain
and turn off when stages dropped below damaging levels.
channels and overbank areas. Several meetings were held with
However, pumps should not cycle on and off over very short
the Napa Public Works Department and Sacramento District to
periods of time. Pump on/off elevations were determined based
discuss existing and proposed storm conveyance systems and
on the "zero damage" elevation and rate of rise for specific
proposed interior features that would convey storm runoff
ponding areas for a specific interior area. Pump on/off
through the line-of-protection.
elevations may need adjusting depending on the final design
configuration of the pumping station. Preliminary on/off
elevations for the two operating pump units for a 100-cfs station
are shown in Figure D-13 and are based on a "zero damage"
The characteristics and configuration of typical new
elevation of 14.0 ft for Interior Area 5.
gravity outlets were defined to establish gravity outlet
parameters and to develop rating curves for the outlets. This
Auxiliary flow. Auxiliary flow includes auxiliary
information included culvert length, size, etc., invert elevations
inflow to the interior subbasin, diversions out of the system,
and slopes of existing storm sewers, culvert type (box or
seepage inflow from the exterior (Napa River) to the interior
circular, concrete or corrugated metal pipe, etc.), and entrance
area, and overflow out of the interior area. As indicated in
and exit configurations.
paragraph D-4f, the effect of the existing 54-in. and 72-in.
The typical outlet through the line-of-protection
pipes located upstream from the upper limits of the flood
was defined, after coordination and agreement with the study
protection project was represented by a diversion from the
manager, as a concrete box culvert with a grated drop inlet. The
upper subbasin in Interior Area 5. Specified diversions for
outlet inverts of the drop inlets are established by the existing
Area 5 are shown in Figure D-14. Seepage was not
storm sewer inverts entering the drop inlets. Lengths of the box
considered a factor because the inundation time for the
culverts were dependent on whether the line-of-protection
earthen embankments would be minimal and sheet-pile and
consisted of a setback levee, sheet-pile wall, or concrete flood
concrete floodwalls along the line-of-protection would be
wall at the outfall. Slopes of the box culverts were set to