31 Mar 95
(2) If modified Puls routing criteria are to be determined
from water surface profile analyses, the entire section must be
event analysis (HEA).
used (for storage) with high "n" values in the noneffective flow
(b) Enter appropriate rainfall-runoff and routing para-
analytical calculation and/or comparison with "n" values
meters, if any, to define RUNOFF module.
determined analytically from other similar streams.
(2) Develop normal depth rating for the interior runoff
approach to the Napa River. Napa River is assumed to be low
floods evaluated will reach on each bridge and select either: (a)
and, therefore, there will be no backwater effect.
normal routine, or (b) special routine.
Stage-damage relationships. Representative stage-
and exercise HEC-IFH to compute interior runoff
hydrographs. The program computes the interior area runoff
damage relationships for the interior areas at runoff
and routes the runoff to the area outlet for each hypothetical
concentration points (proposed outlet locations) are required for
event. Peak flow is displayed for each hypothetical storm
identification of interior plans which maximize net flood damage
C-1-4. Without-project Conditions Analysis for
(4) Determine interior stage-frequency relationship. The
Minimum Facility Evaluation
peak flow for each hypothetical storm runoff event will be used
with the normal depth rating to determine the maximum interior
a. General. The without-project analysis involves
elevation for each event. The stage-frequency curve will be
determination of conditions both without the line-of-protection
and with the line-of-protection in place. Stage-frequency
d. Local runoff flooding, with line-of-protection and no
relationships for these conditions are needed to select a
outlets. This analysis assumes the line-of-protection is in place
minimum facility. The without-project condition used to
formulate and evaluate the interior flood damage reduction
and the local conveyance systems to the Napa River are blocked
measures will assume the minimum facility is in place and is
by the line-of-protection. It becomes the without line-of-
therefore described in Section C-1-5, "Minimum Facility
protection condition for the minimum facility analysis and
Analysis." The procedures described assume that HEC-IFH will
represents an upper bound for the stage-frequency relationship
be used to determine interior area local hypothetical storm event
with the minimum facility in place.
relationships including the 50-, 20-, 10-, 4-, 2-, 1-, 0.5-, and
0.2-percent chance exceedance interior runoff events are
b. Napa River flooding without line-of-protection. This
developed for each interior area. The analysis is the same as
information should be available from the line-of-protection
described for the without line-of-protection condition, except the
design analysis. It is used to determine Napa River flood
runoff will now pond behind the line-of-protection.
elevations over the interior areas and to compare the elevations
with those caused by local flooding when the Napa River is
(1) Enter appropriate elevation-area relationship and
below flood stage (see paragraph C-1-4c). A series of stage-
interior ditch rating, if required, to define the ponding area
frequency relationships for the 50-, 20-, 10-, 4-, 2-, 1-, 0.5-, and
adjacent to the line-of-protection POND module.
0.2-percent chance exceedance events should be developed and
provided for each interior area.
(2) Define a new plan using HEA and exercise HEC-IFH
to compute interior stage-frequency relationship. The program
Local runoff flooding without line-of-protection. This
computes the interior area runoff and routes the runoff to the
ponding area where it is stored assuming no outlet to the Napa
analysis is for local flooding without the line-of-protection in
River. The program displays the maximum interior elevation
place, assuming the present storm sewer system is in place and
for each hypothetical event and a graphical fit stage-frequency
the Napa River is at or below flood stage. It is the target
condition for the minimum outlet facility analysis. Stage-
frequency relationships including the 50-, 20-, 10-, 4-, 2-, 1-,
Assess future without-project conditions impacts.
0.5-, and 0.2-percent chance exceedance events are developed
for each interior area as described below.
Assess future conditions effects on Napa River interior area
local runoff flooding. The effect may well be minimal. Where
hydrologic and/or hydraulic conditions are expected to
hypothetical storm runoff hydrographs.
significantly change over the project life, these changes must be
incorporated into the H&H analysis. Urbanization effects on
Enter local hypothetical storm depth-duration-
watershed runoff are the usual future conditions analyzed. The