EP 1110-2-9
31 Jul 94
discharge-frequency relationships through available/
relationship. Roughly size a "minimum facility" interior
applicable regression equations at key locations, to use in
flood control system for each. With the economist, per-
later comparisons.
form risk and uncertainty studies to establish the claim-
able level of protection (risk-based) and average annual
benefits resulting for each.
cal frequency storm data from the National Oceanic and
C-9. Levee and Channel Alternative Evaluation
Atmospheric Administration HYDRO 35, and National
Weather Service Technical Publications 40 and 49.
a. For two sizes of channel, reestablish stage-
Develop the Standard Project Storm. Develop rainfall
pattern for each storm, including precipitation depth-area
frequency relationships for each of three levee sizes (six
adjustments. Develop corresponding hydrograph for each
alternatives). Evaluate the discharge and stage uncertain-
hypothetical event throughout the basin using the cali-
ties for with-project conditions. Roughly size a "mini-
brated hydrologic model of Paragraph C-5.
mum facility" interior system for each alternative, if
necessary. With the economist, perform a risk-based
analysis to determine project benefits and claimable level
c. If judged appropriate, further calibrate model to
of protection for each alternative. Perform qualitative
reproduce the peak discharge calculated from the statisti-
sediment analyses for channel modifications to roughly
cal analysis at the gage site. Emphasize the 2-year
determine dredging frequency for channel maintenance.
through 10-year event, since the data record is short.
After economic analysis to tentatively establish the NED
Make adjustments to loss rates and unit hydrograph coef-
plan (levee height) from among the nine alternatives,
ficients for rarer events, as judged reasonable. Compare
design top of levee grade for controlled overtopping.
results to statistical and regression-derived peak discharge
frequency relationships; further adjust coefficients as
b. If a channel modification is included in the NED
considered reasonable.
plan, perform sensitivity tests to determine the importance
of channel maintenance assumptions and costs on the
d. Using the results of steps a., b., and c., above,
NED plan. If a more conservative sedimentation analysis
adopt a discharge-frequency relationship at each needed
results in significant cost increases, possibly invalidating
location. Develop probability distribution of discharge
the NED plan, additional sediment analyses will be
uncertainty for use in risk-based analyses.
required in feasibility. Hydrologic engineering work for a
quantitative sediment analysis is not included in this esti-
e. Determine corresponding water surface profiles
mate. Adjust final levee grade for any sediment effects.
and inundated areas for selected frequencies at required
locations. Furnish data to planning and economics.
c. As necessary, furnish hydrologic information, as it
becomes available, to other study team members: stage-
f. Adopt stage-discharge relationship at each required
duration and frequency to environmental, data for Envi-
location for damage computations. At the gage site with
ronmental Assessment Report, etc.
15 years of data, determine deviations about the adopted
stage-discharge relationship. Further evaluate through
d. Nonstructural analysis of emergency procedures in
sensitivity studies. Develop probability distribution of
the event of levee overtopping--evacuation and flood
stage uncertainty for risk-based analysis.
warning.
C-7. Future Without-Project Analysis
C-10. Residual Flooding and Interior Flood
Control
Determine future stage-discharge relationships, based on
future watershed changes affecting the hydraulics. If
necessary, adjust discharge-frequency and stage-discharge
risk/uncertainty relationships. Furnish data to economics.
damages with the NED project. Evaluate higher levels of
interior flooding protection compared to the "minimum
C-8. Levee Alternative Evaluations
facility." Interior flood control measures are distinguished
from minimum facilities in that these additional measures
For the preliminary levee alignment, develop revised
require net benefits and minimum facilities do not require
discharge-frequency and stage-frequency relationships for
incremental economic justification, only cost-effective
each of the three different levee heights. If judged neces-
design.
sary, determine revised stage-discharge risk/uncertainty
C-2
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