ETL 1110-2-367
31 Mar 95
C-1-6. Formulation and Comparison of Interior
general rain events occurring over the Napa River watershed as
Flood Damage Reduction Plans
well as the interior area. Exterior stages will be computed from
rainfall-runoff analysis and an appropriate stage-discharge rating
a. General. The objective of this task is to formulate a
for the Napa River at the interior area outlet. San Pablo Bay
set of flood damage reduction plans for each interior area. The
tidal effects on hypothetical exterior stages will be incorporated
condition with the line-of-protection and the selected minimum
using coincident frequency analysis, if required.
gravity outlet in place becomes the without-project condition for
evaluating additional features such as additional gravity outlets,
(a) Define a new precipitation data set (PRECIP module)
pumping stations, additional ponding area storage, and
using HEA by assembling general rain depth-duration-
nonstructural measures. The first step is to find the economic
frequency storm data for the 50-, 20-, 10-, 4-, 2-, 1-, 0.5-, and
optimal size and configuration for additional gravity outlet
0.2-percent chance exceedance events occurring over the local
capacity with the minimum facility in place. The second step is
interior areas as well as over the Napa River watershed.
to identify the economic optimal pump capacity, assuming that
the minimum facility and the optimal gravity outlets are in place.
(b) Define rainfall, runoff, pond, outlet, and seepage
The third step is to explore trade-offs of pumping capacity
parameters. Existing condition rainfall-runoff and routing
versus ponding area storage and would include evaluation of
(RUNOFF module) parameters, ponding area characteristics
nonstructural measures to increase nondamaging ponding area
(POND module), the minimum facility, and seepage are defined
storage. Finally, the conceptual feasibility of other flood damage
in the same manner as previously described.
reduction actions such as flood warning-preparedness and
institutional arrangements would be evaluated. The district and
(c) Exterior stages for each hypothetical event are
HEC will work closely together in the plan formulation and
computed discharge hydrographs and a specified rating. The
comparison process. The following paragraphs describe the
discharge hydrographs are computed from rainfall-runoff
procedures in more detail and show how both continuous
analysis as described above.
simulation and hypothetical event analyses can be applied.
(d) San Pablo Bay impact on exterior stage for general rain
b.
Determine economic optimal gravity outlet capacity.
HEA. If it is determined that tidal fluctuations in the San Pablo
Bay influence the stages at the interior area outlet locations, it
may be appropriate to develop a bay elevation-duration
(1)
Stage-frequency relationships.
relationship and use coincident frequency analysis to account for
the bay effect on the stage-frequency curve.
(a) Define new plans for evaluating gravity outlets using
data previously defined for the CSA with the minimum facility
(5) Exercise HEC-IFH using the developed HEA data
in place. Existing condition rainfall (PRECIP module), runoff
modules. Results will include a graphical fit interior stage-
and routing (RUNOFF module) parameters, ponding area
frequency relationship. This curve will help to determine if rare
characteristics (POND module), minimum facility (GRAVITY
combinations of events are being captured in the continuous
module), and seepage (AUXFLOW module) are the same as
simulation analysis and will help shape the final without-project
used for the CSA analysis of the selected minimum facility.
condition stage-frequency relationship.
(b) Assemble outlet characteristics for several standard
(6) Final stage-frequency relationships. Make appropriate
size outlets and develop composite rating curves for each using
adjustments to the CSA stage-frequency relationship based on
HEC-IFH.
the results of the without line-of-protection and with line-of-
protection and no outlet plans developed from local storm HEA
(c) Develop five or six gravity outlet configurations
and the results from the general rain HEA.
(modules) with one or more gravity outlets in addition to the
minimum facility outlet, each module representing an
(7) Future without-project condition stage-frequency
incremental increase in total outlet capacity.
relationships with the minimum facility in place. Repeat above
CSA and HEA incorporating expected future condition
(d) Exercise HEC-IFH, and using CSA, develop several
hydrologic parameters and develop future condition stage-
plans which incorporate the gravity outlet modules, described
frequency relationships.
above, and determine interior stage-frequency relationships for
each plan.
C-8
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