FLOOD01888

• County: Statewide
• Title: River Hydraulics
• Date: 10/15/1993
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Educational/Technical/Reference Information

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reach. The latter type of projects may require less data
than the former. For example, an evaluation of the bed
material at and near the surface, through "grab samples"
or collection with hand augers, may be adequate. If the
material consists of fme sands, a detailed sediment study
may be required, possibly in the feasibility phase.

d. Data availability. Data are usually available from
the U.S. Geological Survey's (USGS) nationwide data
collection system. Corps' water data measurements
provide another source; in many parts of the United
States state agencies and water conservancy districts also
collect water data. If measured data are not available but
are required for the study, a data collection system is
necessary. Guidance on specifying and developing a
gaging system is available from the USGS (1977) with
additional information in ER 1110-2-1455. Definition of
the need for certain data and budgeting for its collection
should be included in the feasibility or reconnaissance
report cost estimates.

e. Accuracy of data. Results from numerical models
are routinely available 10 a precision of 0.01 foot, imply-
ing far more solution accuracy than that of the basic data.
The hydraulic engineer should be aware of the impact of
input data uncertainty relative to reliability of the compu-
tations. There are relatively few USGS discharge gages
having records rated as "excellent." This rating carries
an explanation that 95 percent of the daily discharge
values are within 5 percent of the "true" discharge (thus
5 percent are outside of that limit). "Good" records have
90 percent of the daily discharges within 10 percent. If
any specific discharge varies by 5 percent, the corre-
sponding stage could vary significantly depending on the
stream slope and geometry. Instantaneous peak dis-
charges presumably would be less accurate. Thus, a
potentially significant accuracy problem exists with the
basic data.

(1) Geometric data are more accurate than flow data;
however, some variation is still present, see U.S. Anny
Corps of Engineers (1989). If not located properly, cross
sections obtained by any technique may not be "represen-
tative" of the channel and floodplain reach for which
each section is used (see Appendix D). SignifICant errors
in water surface profile computations have occurred
when distances between cross sections were large.
Closer cross section spacings will improve the accuracy
of the proftle computations (i.e. the solution of the equa-
tions), but will not necessarily result in a better simula-
tion unless the sections are properly located 10 capture
the conveyance and storage in the reach. A more
detailed discussion of river geometry requirements is

EM 1110-2-1416
15 Oct 93

provided in Appendix D. The computer program
"Preliminary Analysis System for Water Surface Proftle
Computations (PAS)" is designed to assist with data
development for profile computations (U.S. Anny Corps
of Engineers 1988b).

(2) Sediment data have the most uncertainty, due
both 10 the difficulties in obtaining the measurements and
the incorporation of discharge and geometry measure-
ments in the calculation of sediment load. Sediment load
curves typically are the most important relationships in
sediment studies. This water discharge/sediment dis-
charge relationship should be sensitivity tested to evalu-
ate the consequences of an over- or under-estimate.

(3) Absolute statements as to the accuracy of fmal
hydraulic results should be tempered by an understanding
of the field data accuracy. The more accurate the fmal
hydraulics are required 10 be, the more accurate the data
collection must be. Sensitivity tests to evaluate possible
over- or under-estimates should be routinely made.

f. Hydraulic loss coefficients. Various energy loss
coeffICients are required for hydraulic studies. These
energy loss coefficients include channel and overbank
friction, expansion-contraction losses, bridge losses, and
miscellaneous losses.

(1) Manning's n. For the majority of hydraulic
studies, Manning's n is the most important of the hydrau-
lic loss coefficients (U.S. Anny Corps of Engineers
1986). The variation of water surface elevation along a
stream is largely a function of the boundary roughness
and the stream energy required to overcome friction
losses. Unfortunately, Manning's n can seldom be calcu-
lated directly with a great deal of accuracy. Gage
records offer the best source of information from which
10 calculate n for a reach of channel near a gage. These
calculations may identify an appropriate value of n for
the channel portion of the reach. Whether or not this
value is appropriate for other reaches of the study stream
is a decision for the hydraulic engineer. Determination
of overbank n values requires a detailed field inspection,
reference to observed flood profiles, use of appropriate
technical references, consultation with other hydraulic
engineers, and engineering judgment. For some streams,
n varies with the time of year. Studies on the Missouri
(U.S. Anny Corps of Engineers 1969) and Mississippi
Rivers have found that Manning's n is significantly less
in the winter than in warm weather for the same dis-
charge. If stages are 10 be predicted in the winter as well
as the summer, temperature effects must be addressed.
Similarly, many sand bed streams demonstrate a great

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