EP 1110-2-9
31 Jul 94
(3) Bridge computations--estimate how high the
(4) Adopt hydrologic and hydraulic model parameters
selected floods will reach on each bridge and select either:
for hypothetical flood event analysis.
(a) Normal bridge routine.
(5) Quantify uncertainty of the stage-discharge rela-
tionship at each site where damage analysis is to be
(b) Special bridge routine.
performed. As appropriate, use recorded gage data, com-
parison of profile to high-water marks, minimum devia-
(4) Develop cross sections above and below bridges
tion, and engineering judgement.
to model effective bridge flow (use artificial levees or
D-6. Frequency Analysis for Existing Land-Use
ineffective flow area options, as appropriate).
Conditions
The next phase of the analysis addresses how often spe-
based on hydrologic analyses of historic storms and plot
cific flood levels will occur at all required points in the
peak discharge versus river mile. Compute a series of
study watershed. The procedures include developing
water surface profiles for a range of discharges. Analysis
discharge-and stage-frequency relationships at stream
should start below study area so that profiles will con-
gages (when available) through statistical analysis using
verge to proper elevations at study limits. May want to
recorded peak discharges and at other required locations
try several starting elevations for the series of initial
using available hypothetical storm data.
discharges.
a. Statistical analysis. Using the procedures
c. Manual check. Manually check all large differ-
described in Bulletin 17B (Water Resources Council
ences in water surface elevations across the bridge, say,
1982), determine and plot analytical and graphical fre-
greater than 3 ft.
quency curves at each stream gage. Adopt stage/
discharge frequency relations at each gage. Regional
d. Results. The results are a series of rating curves at
relationships, regression analyses, and the results of hypo-
desired locations (and profiles) that may be used in subse-
thetical storm studies will be used to extend the records
quent analyses. Additional results are a set of storage
for rarer floods.
versus outflow data by reach which, along with an esti-
mate of hydrograph travel time, allow the development of
b. Hypothetical storms (HEC-1).
modified Puls data for the hydrologic model.
(1) Obtain hypothetical frequency storm data from the
D-5. Calibration of Models to Historic Events
National Oceanic and Atmospheric Administration
(NOAA) HYDRO 35, National Weather Service (NWS)
a. General. This study step concentrates on
Technical Publications (TP) 40 and 49, or from appro-
"de-bugging" the hydrologic and hydraulic models by
priate other sources. Where appropriate, develop the
recreating actual historic events, thereby gaining confi-
Standard Project and/or the Probable Maximum Storm.
dence that the models are reproducing the observed
hydrologic responses. This effort would continue from
(2) Develop a rainfall pattern for each storm. Include
the activities described in Paragraph D-3.
precipitation depth-area adjustments, where necessary.
b. Calibration procedure.
(3) Develop a corresponding hydrograph for each
hypothetical event throughout the basin using the cali-
(1) Check historic hydrographs against recorded data,
brated hydrologic model.
make adjustments to model parameters, and rerun the
model.
(4) If deemed necessary, calibrate model of each fre-
quency event to known frequency curves. Adjust loss
(2) If no stream gages exist, check discharges at
rates, base flow, etc. as required, while remaining within
rating curves developed from water surface profiles at
reasonable limits for each parameter. The peak flow fre-
high-water marks.
quency at each ungaged area is assumed to be consistent
with calibrated peak flow frequencies at gaged locations.
(3) Adjust models to correlate with high-water marks.
D-4
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