EP 1110-2-9
31 Jul 94
(4) Derived from reach hydraulics (Muskingum -
(c) Significant changes in soil type.
Cunge).
(d) Other.
e. Reservoir routing (if reservoirs are present). This
(4) Routing reaches.
type of routing must be performed where storage has a
significant effect on reach outflow values, with reservoirs
(5) Location of existing physical works (reservoirs,
being the most notable example. However, one must also
diversions, etc.) and potential location of alternate flood
apply these techniques where physical features warrant,
reduction measures to be studied.
such as roads crossing a floodplain on a high fill, espe-
cially where culverts are used to pass the flow
c. Subarea rainfall-runoff analysis of historic events.
downstream.
(1) Subarea rainfall.
(1) Develop surface area-capacity data (elevation-
surface area-storage relationships).
(a) Average subarea rainfall from isohyetal maps or
weighting of total gage precipitation.
(2) Develop storage-outflow functions based on outlet
works characteristics.
(b) Temporal distribution from weighing in accor-
dance with information from nearby recording rain gages.
f. Runoff hydrographs. Using the subdivided rain-
fall-runoff model, including existing projects and the
(2) Average subarea loss rates.
routing information of Section D-3 above, generate runoff
hydrographs for previously selected runoff events at
(a) From adopted values of parameter calibration.
desired locations. Final calibration of the hydrologic
model is described in Section D-5.
(b) From previous studies of similar basins in the
region.
D-4. Hydraulic Studies
(c) Others.
These studies are used to determine water surface profiles,
economic damage reaches, and modified Puls channel
(3) Unit Hydrograph Parameters.
routing criteria (if used). This example assumes that an
evaluation was previously made that a steady flow-rigid
(a) From relationships based on calibration results at
boundary water surface profile analysis is appropriate.
stream gages (Section II) and physical basin
characteristics.
a. Prepare water surface profile data.
(b) From previous regional study relationships of unit
(1) Cross sections (tabulate data for each section).
hydrograph parameters and physical basin characteristics.
(a) Make cross sections perpendicular to flow.
(c) From similar gaged or known basins.
(b) Each cross section should be typical of the reach
(d) From judgment, if no data are available.
from half the distance to the next section both upstream
and downstream of the current locations.
d. Channel routing characteristics.
(c) Develop effective flow areas. If modified Puls
(1) Modified Puls from water surface profile compu-
routing criteria are to be determined from water surface
tations (Hydrologic Engineering Center (HEC-2)).
profile analyses, the entire section must be used (for stor-
age) with high "n" values in the non-effective flow areas.
(2)
Optimized from stream gage data (HEC-1).
(2) Refine "n" values from field reconnaissance and
(3) Adopted parameters from previous studies, expe-
from analytical calculation and/or comparison with "n"
rience, etc.
values determined analytically from similar streams.
D-3
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