ETL 1110-2-367
31 Mar 95
line-of-protection and the minimum facility in place will not
be worse than what would be expected in the future without
the project, upgrade the selected minimum facility accord-
Exterior stage from historical period-of-record
ingly.
discharge hydrographs. Typically, discharge hydrographs are
more readily available than stage hydrographs. If discharge
c.
Develop without-project condition stage-frequency
hydrographs are employed, a rating curve is incorporated which
relationship with the minimum facility in place. After the
is used to convert flow to stage at the index locations. The
stages are transferred to primary and secondary outlet location
minimum facility is selected, it is evaluated using continuous
as described above, if required.
simulation analysis and general rain hypothetical event
analysis. The results of the analysis can be used to test the
Exterior stage from computed period-of-record
effectiveness of the minimum facility gravity outlet by assessing
discharge. If recorded stages or flow are not available,
local runoff flooding that occurs during blocked conditions.
discharge hydrographs can be computed from rainfall-runoff
Results of the analysis establish the base plan or without-project
analysis. Flow is converted to stage and stage transferred to the
condition stage-frequency relationships for evaluating additional
outlet locations as described above, if required.
interior flood damage reduction measures as described in
Section C-1-6.
San Pablo Bay impact on exterior stage for CSA. If it
is determined that tidal fluctuations in the San Pablo Bay
(1) Define Continuous Simulation Analysis (CSA) plan
influence the stages at the interior area outlet locations, a family
using HEC-IFH that incorporates period-of-record interior area
of rating curves for each interior outlet that gives Napa River
rainfall, existing condition runoff characteristics, existing
stage based on Napa River flow and stage in San Pablo Bay is
interior ponding area, and the selected minimum facility,
required. These relationships are developed by determining
seepage, and period-of-record exterior stages at the interior area
water surface profiles for various stages in the bay. Analysis
outlet.
period San Pablo Bay stages are also required and could be
obtained from historical data or generated based on known tidal
(a) Define PRECIP module for CSA. Historical, period-
cycles. These data are used by HEC-IFH to determine the
of-record rainfall data for representative recording and non-
appropriate exterior stage at the gravity outlet for each time
recording gauges are used. The data are generally retrieved
period in the analysis.
from NWS magnetic tapes or from available CD ROM and
stored in an HEC-DSS file where they can be imported directly
(d) Define seepage (AUXFLOW module) data for seepage
into HEC-IFH. Gauge weightings are specified for determining
inflow from the Napa River to the interior ponding area, if
basin average precipitation.
appropriate. A relationship between differential head (the
exterior stage minus the interior stage) and seepage inflow is
(b) Define rainfall, runoff, pond, and outlet parameters.
defined and incorporated. No seepage occurs when the interior
Existing condition rainfall-runoff and routing (RUNOFF
stage is equal to or greater than the exterior stage. Data are
module) parameters, ponding area characteristics (POND
developed based on field measurements or empirical
module), and the minimum facility are defined for CSA in the
information.
same manner as previously described for HEA.
(2) Exercise HEC-IFH using the developed CSA data
(c) Define exterior stage (EXSTAGE module) data for
modules and specify either a partial duration or annual series
CSA. Historical, period-of-record discharge, or stage hydro-
frequency analysis. Results will include a graphical fit interior
graphs for main river gauges are obtained from available
stage-frequency relationship.
electronic media and stored in an HEC-DSS file for direct
importing to HEC-IFH. Napa River period-of-record stage
(3) Examine the periods of local flooding (Napa River
hydrographs at each interior outlet location are determined by
below pre-project flood level) and determine the extent of
one of the following methods, each of which can be
local flooding caused by blocked gravity outlet conditions.
accomplished using HEC-IFH.
If flooding resulting from this condition is considered worse
than pre-project local flooding, the minimum facility may
Exterior stage from historical, period-of-record stage
require the addition of a pump to alleviate induced flooding.
hydrographs. Typically, the gauge data (index location) will
In this case, pumping capacity would need to be evaluated
need to be transferred to interior area outlets (primary and
using the CSA plan data. See Section C-1-6 for evaluating
secondary) locations by incorporating transfer functions that
pumping capacity.)
relate index stage to primary and secondary outlet locations.
These transfer relationships are developed from water surface
(4) Define a new general rain HEA plan using HEC-IFH
profiles and are used by HEC-IFH to determine the exterior
that incorporates precipitation depth-duration-frequency data for
stage at the outlets for each time period during pond routing
C-7
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