31 Mar 95
(e) Define new plans, and using HEA, assemble general
ponding and little coincidence between interior runoff and
rain depth-duration-frequency storm data for the 50-, 20-, 10-,
high exterior stages), there would be little residual flood
4-, 2-, 1-, 0.5-, and 0.2-percent chance exceedance events
damages with the selected outlet in place. If gravity outlets
occurring over the local interior areas as well as over the Napa
are shown to be ineffective and residual damages are
River watershed, and determine the interior stage-frequency
significant, pumps may be justified. The same steps
relationships for each plan. The analysis is similar to that
described for evaluating additional gravity outlet capacity
described for the general rain HEA of the minimum facility but
are appropriate for identifying the economic optimal
will include analysis of several plans incorporating the
pumping capacity. Some differences in the analysis are
additional gravity outlet capacities defined in (c) above. The
relationships will help determine if rare combinations of events
are being captured in the continuous simulation analysis. These
Base condition. The base condition for evaluating
relationships will also help establish the upper end of the
pumping capacity is with the minimum facility and, most
graphical curve determined in (d) above.
likely, the economic optimal gravity outlet configuration in
place. Several plans are evaluated against the base plan,
(f) Define additional plans using HEA and local storm
each with an incremental increase in pumping capacity.
depth-duration-frequency data for the 50-, 20-, 10-, 4-, 2-, 1-,
0.5-, and 0.2-percent chance exceedance events occurring over
(3) Pump operation criteria. Pump on and off elevations
the interior area with unblocked conditions on the Napa River.
must be determined so that the pumps come on to effectively
Determine the interior stage-frequency relationships for each
reduce damaging stages and turn off when stages drop below
plan. This relationship will help to determine if rare
damaging levels. However, pumps should not cycle on and off
over very short periods of time. Therefore, "on" elevations are
simulation analysis and may help to shape the final stage-
usually set below flood stage and "off" elevations are usually set
1 to 2 ft below "on" elevations. "On" and "off" elevations can
also vary by season (monthly) if appropriate. Two or more
(g) After examining the results of the continuous and
pump units make up a pumping plant or station. Several units
that can be used for backup and which can be operated in phases
relationship for each gravity outlet plan.
to step up total capacity usually prove to be more effective than
a few large-capacity pumps.
(h) Develop future condition stage-frequency relationships
by repeating the described steps using expected future
(4) Type of events and analyses. CSA, general storm
hydrologic condition data, if appropriate.
HEA, and local storm HEA with blocked gravity conditions
would be performed to derive final existing and future condition
(2) Determine equivalent expected annual damages (EAD)
stage-frequency relationships, as described above, for the
for each gravity outlet plan.
gravity outlet plans.
d. Evaluation of increased storage capacity. It is
(a) The district will provide cost estimates of various sized
gravity outlets and stage-damage relationships by damage
prudent to investigate the trade-offs between pumping
category for existing and potential future conditions.
capacity and ponding area storage capacity. Pumps are
expensive and an increase in storage capacity will typically
(b) EAD for each plan will be determined using the
allow reduction in required pumping capacity. There are
developed stage-frequency relationships, the stage-damage
several measures that can be evaluated, including increasing
relationships, and HEC's EAD program.
the physical size of the ponding area and implementing
nonstructural actions that will reduce the damage for a given
(c) A plan comparison array including residual
equivalent EAD, expected annual inundation reduction
benefits, average annual costs, and net benefits will be
Increasing the size of ponding areas. The potential
developed identifying the economic optimal plan. This plan
for excavating larger ponding areas should be explored, if
will most likely become the base plan for evaluating
physically possible. The sensitivity of ponding area size
versus pumping capacity can be readily determined using
HEC-IFH. The plan with the identified economic optimal
Determine economic optimal pumping capacity.
gravity outlet and pumping station would be the base plan
for determining if excavation is feasible.
General. If the analysis for determining the
economic optimal gravity outlet indicates that gravity outlets
Nonstructural measures. Temporary evacuation,
are very effective (considerable peak runoff attenuation from
relocation, flood proofing, and other nonstructural measures