31 Mar 95
analysis will derive a set of future condition stage-frequency
provide interior flood relief such that during low exterior stages
relationships for the conditions described in paragraphs C-1-4b,
(gravity conditions) the local interior area runoff will pass the
C-1-4c, and C-1-4d.
design storm sewer outflow without an increase in elevation
over natural or without line-of-protection conditions. Flood
stages with the minimum facility in place should not be
(1) Identify future development. From future land use
significantly higher than stages for less frequent flood events
planning information obtained during the preliminary
assuming it can be established that these less frequent flood
investigation phase, identify areas of future urbanization or
events have and will occur when the Napa River is below pre-
intensification of existing urbanization.
project flood stage.
Types of land use (residential, commercial, industrial,
b. Evaluate range of minimum facilities. The minimum
facility will normally include gravity outlets but may include
pumps if the coincidence of flooding between the interior and
(b) Storm drainage requirements of the community (storm
exterior is high. For example, the Napa River is high enough to
sewer design frequency, onsite detention, etc.)
block gravity outlets, but is below pre-project flood stage and
flooding occurs in the interior from local runoff. The sequence
Other considerations and information.
will be to evaluate a series of gravity outlets; then pumps, if
required. The physical characteristics of the gravity outlets
(2) Select future years in which to determine project
should be established prior to the analysis and refined as the
analysis proceeds. The analysis should be performed for the
(a) At start of project operation (existing conditions may
(1) Analyze series of gravity outlet capacities and
configurations using local storm hypothetical event analysis and
(b) At some year during the project life (often the same
assuming unblocked conditions. The analysis is the same as that
year as whatever land use planning information is available).
for the local flooding with the line-of-protection in place
(Section C-1-4), except gravity outlets through the line-of-
(3) Adjust model hydrology parameters for all areas
protection are incorporated.
affected by future land use changes.
(a) Define 3 or 4 gravity outlet configurations (modules)
(a) Unit hydrograph coefficients reflecting decreased time-
of increasing capacity. Outlet sizes should encompass the
to-peak and decreased storage.
largest storm sewer size or ditch capacity at the line-of-
(b) Loss rate coefficients reflecting increased impervious-
ness and soil characteristics changes.
(b) Define a new plan for each gravity outlet capacity to be
evaluated and, using local storm HEA, exercise HEC-IFH and
(c) Routing coefficients reflecting decreased travel times
determine the interior stage-frequency for each outlet.
through the watershed's hydraulic system.
(2) Compare stage-frequency relationships of gravity
(4) Operate hydrologic models, including HEC-IFH using
outlets with the storm sewer design event and with the local area
local storm HEA, and determine revised discharge-frequency
flooding stage-frequency relationships both with (no outlet) and
and/or stage-frequency relationships throughout the watershed
without the line-of-protection in place.
for future without-project conditions.
C-1-5. Minimum Facility Analysis -
(3) Select minimum facility. The minimum facility is
selected to assure that expected flooding and associated
damages from the local, interior area with the line-of-protection
in place are no worse than flooding from the local area (not
a. General. The minimum facility of the individual
including the Napa River) and associated damages were before
interior areas will be justified as part of the line-of-protection.
the line-of-protection was in place.
The stage-frequency relationships for the with-minimum-
facility-in-place condition becomes the without condition for
(4) Perform analysis for all interior areas and for
evaluating potential interior flood damage reduction measures.
expected future conditions. The expected future condition
The residual damage with the minimum facility in place is thus
hydrologic parameters are incorporated and the analysis is
the target for damage reduction of implemented interior flood
repeated using the selected minimum facility. If the selected
damage reduction measures. The minimum facility should
facility is not efficient to assure that local flooding with the