EP 1110-2-9
31 Jul 94
(5) If streamflow data are insufficient to develop ana-
(2) Storm drainage requirements of the community
lytical frequency curves, use the following procedure:
(storm sewer design frequency, on-site detention, etc.).
(a) Obtain frequency curves from similar nearby
(3) Other considerations and information.
gaged basins.
b. Select future years in which to determine project
(b) Develop frequency curves at locations of interest
hydrology.
from previous regional studies (USGS, Corps of Engi-
neers, State, etc.).
(1) At start of project operation (existing conditions
may be appropriate).
(c) Determine frequency hydrographs for each event
from hydrologic model and develop a corresponding fre-
(2) At some year during the project life (often the
quency curve at the locations of interest throughout the
same year as whatever land use planning information is
basin.
available).
(d) Plot all the frequency curves (including those
c. Adjust model hydrology parameters for all subareas
using other methods if available) and, based on engineer-
affected by future land use changes.
ing judgement, adopt a frequency curve. The adopted
curve may not be any of the developed curves, but simply
(1) Unit hydrograph coefficients, usually reflecting
the best estimate based on the available data.
decreased time-to-peak and decreased storage.
(e) Calibrate the hydrologic model of each frequency
(2) Loss rate coefficients, usually reflecting increased
event to the adopted frequency curve. The frequency
imperviousness and decreasing infiltration characteristics.
curve at other locations may be determined from the
calibrated model results, assuming consistent peak flow
(3) Routing coefficients, usually reflecting decreased
frequencies.
travel times and storage capabilities.
(6) Quantify the uncertainty in the discharge-
d. Operate the hydrology model and determine addi-
frequency relationship at all locations where damage
tional discharge-frequency relationships throughout the
computations will be performed. As appropriate, use gage
watershed that represent future, without-project conditions.
data, regression equations, calibrated models to determine
equivalent length of record.
e. Evaluate the need to adjust uncertainty parameters
of stage-discharge and discharge-frequency relationships,
(7) Determine corresponding water surface elevations
compared to existing conditions.
and profiles for selected frequencies from the rating
curves developed by the water surface profile evaluations.
D-8. Alternative Evaluations
D-7. Future Without-Project Analysis
For the alternatives jointly developed with the members of
the interdisciplinary planning team, modify the hydrologic
Where hydrologic and/or hydraulic conditions are
and/or hydraulic models to develop the effects of each
expected to significantly change over the project life,
alternative (individually and in combination) on flood
these changes must be incorporated into the hydrologic
levels. Alternatives can be either structural (reservoirs,
engineering analysis. Urbanization effects on watershed
levees, channelization, diversions, pumping, etc.) or non-
runoff are the usual future conditions analyzed.
structural (flood forecasting and warning, structure raising
or relocation, floodproofing, etc.). Considerable less
a. From future land use planning information
hydrologic engineering effort is necessary for modeling
obtained during the preliminary investigation phase, iden-
non-structural alternatives compared to structural.
tify areas of future urbanization or intensification of exist-
ing urbanization.
a. Procedure.
(1) Consider duplicating existing and future without-
(1) Types of land use (residential, commercial, indus-
hydrologic engineering models for individual analysis of
trial, etc.).
D-5
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