ETL 1110-2-367
31 Mar 95
f.
Gravity outlets. Determine typical gravity outlet
(2) Determine loss rate methods and values. These
include monthly rates for continuous record analysis and event
information and operation criteria.
variables for hypothetical event analyses.
(1) Determine appropriate gravity outlet locations based
(3) Determine base flow. Continuous simulation analysis
on local conveyance systems, storm sewer system layouts and
can incorporate monthly rates, and hypothetical event analysis
invert elevations, and ponding area locations.
can incorporate an initial rate and recession variables.
(2) Define typical gravity outlet data: lengths from levee
(4) Determine streamflow routing method and para-
or floodwall dimensions, etc.; inverts/slope from storm sewer
meters.
and ponding area elevations; box or circular; concrete or MP,
etc.; entrance and exit configurations.
d.
Interior ponding area data.
(3) Define gravity outlet operation criteria: head
(1) Develop elevation-area relationships for each ponding
differential for closing, any gate closure requirements.
area adjacent to line-of-protection. (User should specify 15-20
points to define the relationship.) HEC-IFH will automatically
(4) Develop cost estimates for various gravity outlet types
generate the storage values. The minimum value should be at
and sizes.
or below the lowest invert elevation to be analyzed for that
g. Pumping stations. Determine typical pumping station
ponding area. The maximum value should be above the highest
stage anticipated in the analysis. (The program will not
data and operation criteria.
extrapolate above or below these maximum or minimum
elevations.)
(1) Define criteria for number of pumps including base
flow pump, back-up units, etc.
(2) If applicable, develop the discharge-elevation
relationship for the ditch that connects the ponding area to the
(2) Define pump characteristics: requirements for on/off
gravity outlet and/or pump. (Required only if the ponding area
elevation determination (may vary monthly in HEC-IFH); head-
is not adjacent to the outlets at the line-of-protection.)
capacity-efficiency relationships.
e.
Exterior stage data. These data must include
(3)
Develop cost estimates for various pumping capacities.
continuous stage hydrographs considering the historic patterns
h. Auxiliary flow data. Auxiliary flow includes auxiliary
of Napa River discharge values coincident with any tidal effects
on the exterior stages at the outlet locations of each interior area
inflow to the interior subbasin, diversions out of the system,
to be studied. The hypothetical storm analysis would likely
seepage inflow from the exterior (Napa River) to the interior
involve analysis assuming storms centered over both the interior
area, and overflow out of the interior area.
area and Napa River drainage basin. There is no apparent
straightforward manner to account for tidal effects with the
(1) Determine head-versus-seepage relationships for each
hypothetical approach, although a coincidence weighting
interior area.
method, based on percent time (probability) of the stages of the
San Pablo Bay associated with a series of hypothetical flood
(2) Determine diversions and diversion rates out of the
events occurring for each stage, may be appropriate.
system, and auxiliary inflow hydrographs, if appropriate.
(1) Obtain the period of record for elevations of the San
(3) Determine overflow potential and, if required, the pond
Pablo Bay at the mouth of the Napa River. The time interval
elevation-overflow discharge relationship.
must be sufficiently small to capture tidal effects (6-hr stages.)
i.
Water surface profile data. Water surface profile
(2) Obtain the period of record of the discharge values of
analyses are used to determine water surface elevations and
the Napa River at appropriate gauge locations. Determine if
rating relationships for the Napa River (and perhaps major
adjustments to the discharge values are required for the outlet
conveyance channels to the interior outlets), flood damage
locations of each interior area to be analyzed.
reaches, and modified Puls channel routing criteria.
(3) Develop a family of rating curves at the outlet locations
(1) Cross sections (tabulate data from each section). Make
based on various San Pablo Bay elevations and Napa River
cross sections perpendicular to flow. Sections should be typical
discharges. The analysis requires running a series of water
of reaches upstream and downstream of cross section. Develop
surface profiles for various bay elevations.
effective flow areas.
C-4
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