ETL 1110-2-367
31 Mar 95
e.
storm water collector/conveyance systems.
relationship with Napa area field office counterparts such as
(2) Delineate interior subbasins for each area considering
Director of Public Works, City Engineer, and other local, state,
locations needed for stage-frequency relationships and effects of
and federal agency staff people. These people can be key
the storm sewer system.
contacts throughout the study. Other field activities are
described below.
(3) Select computation time interval ()t) for this and
subsequent analyses. The peak discharge of hydrographs at
(1) Interview local agencies, and residents along the
gauges, normally three to four points on the rising limb of the
stream, review newspaper files, etc., for historic flood data (high
unit hydrograph, must be defined adequately. Routing reach
water marks, frequency of road overtopping, direction of flow,
travel times should also be considered, as should the location
land use changes, stream changes, etc.). Document names,
and types of flood damage reduction measures to be analyzed.
locations, and other data for future reference.
The importance of using a small time interval is dependent on
the size of the available ponding area and the associated flow
(2)
Finalize cross-section locations/mapping require-
attenuation at the outlets.
ments.
b. Rainfall data. This activity includes the assembly of
(3) Determine initial estimate of Manning's "n" values for
later use in water surface profile computations.
historical storm records and hypothetical frequency event
data.
(4) Take photographs or slides of outlet inverts and ditches
that will be cut off by the line-of-protection, bridges,
(1) Obtain and verify historic rainfall records of nearby
construction, hydraulic structures, and floodplain channels and
recording and nonrecording rain gauges. Determine weighting
overbank areas at cross-section locations.
of gauges for each interior subbasin.
f.
Survey request. Write survey request for mapping
(2) Develop hypothetical frequency storm depth-
frequency-duration relationships for general rain and local
requirements and/or cross sections and high water marks for
storms.
Napa River and interior area conveyance systems.
C-1-3. Data/Information Assembly
(3)
Determine the characteristics of the SPS.
a. General. Data/information assembly is required for
c.
Runoff and channel routing data. Interior runoff
the analysis of the interior area. It includes data for both the
hydrographs may be computed using HEC-IFH or imported
interior and exterior (Napa River) areas. The information is
from an external HEC-DSS file generated by a different
applicable for any analytical method, but is specifically targeted
program. For example, the HEC-1 program may be used to
for application of the HEC-IFH computer program, and assumes
perform the runoff and channel routing of a complex system
that the analyses will be conducted using both a continuous
(more than two subbasins). Externally determined hypothetical
record and hypothetical event approach. An assessment of
or period-of-record runoff hydrographs may be imported into
HEC-IFH as an appropriate model should be made as early as
HEC-IFH and used in the computations.
possible.
(1) Determine interior subbasin drainage areas, unit
The continuous record analysis is the most straightforward
hydrograph methods, and variables.
approach because of the tidal effects on Napa River stages at
NOTE: HEC-IFH does not use kinematic waves, but
interior outlet locations and the need to investigate the
coincidence of exterior stages on gravity outlet flows and
HEC-1 can be used to compute hypothetical runoff
pumping discharges. Potential problems with the continuous
hydrographs using kinematic waves and imported into
analysis approach are lack of data and poor definition of the
HEC-IFH. The use of the kinematic wave approach is
interior runoff system. The hypothetical event analysis would
not possible for the continuous record analysis unless the
enable some refinement of the interior runoff system, but
runoff sequences are generated by another program
presents problems with the tidal effects and coincident interior
(other than HEC-1) and imported to HEC-IFH. An
and exterior storm analyses.
alternative would be to use a HEC-1 model with
kinematic wave and 1-in. of runoff to generate unit
(1) Define interior areas to be studied. Consideration must
hydrographs for each interior area. These unit
be given to alignment of the line-of-protection, minimum facility
hydrographs could be entered into HEC-IFH and used
requirements, runoff topology, topography of local ponding
for hypothetical event and/or continuous simulation
areas, present storm sewer systems and potential for additional
analysis.
C-3
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