EP 1110-2-9
31 Jul 94
the selected recorded runoff events for the basins above
(3) Determine initial estimate of "n" values for later
the stream gages.
use in water surface profile computations.
(a) Average subarea totals from isoheytal maps or
(4) Take photographs or slides of bridges, construc-
tion, hydraulic structures, and floodplain channels and
total gage precipitation weightings.
overbank areas at cross-section locations. Consider dictat-
ing notes to a hand-held tape recorder to get a complete
(b) Temporal distribution from weightings of nearby
and detailed record.
recording rain gages.
(6) Determine best estimate unit hydrograph and
c. Survey request. Write survey request for mapping
loss rate parameters for each event at each stream gage by
requirements and/or cross sections and high-water marks.
calibrating to recorded flood hydrographs.
D-3. Development of Basin Model (HEC-1)
(7) Make adjustments for better and more consistent
results between events at each stream gage. Adjustments
This phase of the analysis involves the selection of his-
are made to:
toric events to be evaluated, the development of runoff
parameters from gaged data (and/or regional data from
(a) Starting values of parameters and/or
previous studies) and correlating these data to ungaged
basins and the calibration of the basin model to historic
(b) Rainfall totals or patterns (different weightings
flood events. This step assumes that at least some record-
of rain gages).
ing stream gage data are in or near the study watershed.
(8) Hold constant the most stable parameters, or
a. Calibration of runoff parameters.
relationships between parameters, and resimulate rain-
fall/runoff process to estimate other parameters.
(1) Select historic events to be evaluated based on
available streamflow records, rainfall records, high-water
(9) Adopt final unit hydrograph and base flow
marks, etc.).
parameters for each gaged basin.
(2) From USGS rating curves and time-versus-stage
(10) Re-simulate with adopted parameters held con-
relationships for each event, develop discharge hydro-
stant to estimate loss rates.
graphs at each continuously recording stream gage. Esti-
mate peak discharge from flood crest gages.
(11) Use adopted parameters of unit hydrographs, loss
rates, and base flow to reconstitute other recorded events
(3) Develop physical basin characteristics (drainage
not used in the above calibration to test the correctness of
areas, slope, length, etc.) for basin above each stream
the adopted parameters and to "verify" the calibration
gage.
results.
(4) Select computation time interval ( t) for this and
b. Delineation of subareas. Subareas are delineated
subsequent analyses. The computation interval must:
at locations where hydrologic data are required and where
(a) Adequately define the peak discharge of hydro-
physical characteristics change significantly.
graphs at gages.
(1) Index locations where economic damage compu-
(b) Consider type of routing and reach travel times.
tations are to be performed.
(c) Have three to four points on the rising limb of the
(2) Stream gage locations.
unit hydrograph for the smallest subarea of interest.
(3) General topology of stream system.
(d) Consider types of alternatives and future
assessments.
(a) Major tributaries.
(b) Significant changes in land use.
(5) Using all appropriate rain gages (continuous and
daily), develop historic storm patterns that correspond to
D-2
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