31 Mar 95
Figure 5-4. Plan comparison of stage-frequency relationships
(7) After examining the results of the CSA and HEA
relationships and the stage-damage relationships.
analyses, adopt a final stage-frequency relationship for each
gravity outlet plan.
(3) A plan comparison array including residual equivalent
expected annual damage (EAD), expected annual inundation
(8) If appropriate, develop future condition stage-
reduction benefits, average annual costs, and net benefits is
frequency relationships by repeating the above process using the
developed to identify the economically optimal plan. A similar
most likely and other (if required) future hydrologic conditions.
table is shown for pumping station sizing in the next section.
Other information on the flood hazard reduction, operation
d. Selection of gravity outlets. The following are steps
requirements, performance for a range of events and conditions,
environmental and other factors should be considered in
that may be used to determine the gravity outlet capacity at the
determining the recommended gravity outlet plan. This plan
primary location. Secondary outlet locations may use a less
should be the base plan for evaluating additional measures.
rigorous procedure if the locations are not critical.
Normally the economically optimum plan is chosen.
(1) The HEC-IFH results should be reviewed for reason-
Pumping station analysis overview. Pumping stations
ableness. The gravity outlet should be sized such that the
interior stage essentially follows the receding limb of the
may not be attractive if the gravity outlets are effective in
exterior stage hydrograph with consideration of the operating
reducing the flood damage and if there is little coincidence
head differential. HEC-IFH's output results can show this
between interior runoff and high exterior stages. Often,
however, additional gravity outlets are not justified and
significant residual damage exists. If most of the damage is
(2) An economic analysis is normally required for primary
from blocked conditions, pumps may be the only effective
outlet locations to determine the NED (USACE 1990a) gravity
means of evaluating interior flood waters. The same general
outlet size. The cost engineering team member provides cost
application steps for HEC-IFH used for additional gravity outlet
estimates of each gravity outlet HEC-IFH plan and the
capacity are appropriate for determining the economic optimal
economist will provide stage-damage relationships by damage
pumping capacity. Some differences and pumping station
category for existing and potential future conditions. The
analysis considerations are described in the following
expected annual damage for each plan is determined by the
study economist using the developed stage-frequency