7910

• UCREFRP Catalog Number: 7910
• Author: Bovee, K. D. and R. T. Milhous.
• Title: Hydraulic Simulation In Instream Flow Studies
• USFW Year: 1978.
• USFW - Doc Type: Theory And Techniques, Instream Flow Information Paper No. 5.
• Copyright Material: NO

As discussed previously, Manning's n may vary from place to place in
the channel. However, both the energy slope and Manning's n also vary
with discharge. An example of this variation is illustrated by data from
Oak Creek in the Oregon Coast Range near Corvallis, Oregon. Figure 9
shows the variation in Manning's n over a wide range of discharges in Oak
Creek. The variation in energy slope is shown in Figure 10.
The practice of taking only one set of calibration measurements means
that the value of Manning's n and the energy slope are known with
certainty for only one flow. In other calculations, both variables may be
assumed constant, Manning's n alone held constant, or adjustments made to
Manning's n based on an estimate-of its value at other flows. However,
unless several sets of measurements are available, the true value of n is
not known with certainty for any but the calibration flow(s).
The relative importance of the energy slope and Manning's n in the
introduction of error can be seen by comparing the range of values in
Figures 7 and 8. The maximum range of the energy slope is from 0.009 to
0.012. When the square root is taken, this results in a range in S ? of
0.095 to 0.109, which would result in a maximum error in prediction% of
about 13% if the slope were assumed constant, and estimated at one of the
extremes of slope.
In contrast, the range of Manning's n is from about 0.075 to almost
0.5. If the range of flows of interest for Oak Creek were from 10 to 100
cfs, the variation in n would be from about 0.10 to 0.075, resulting in
maximum-potential error of about 133% if n is assumed.constant. If the
range of flows of interest were between 5 and 30 cfs, the range in n is
from about 0.15 to 0.075 and represents a maximum potential error of 200%
if n is assumed constant. Therefore, the reliability of the Manning
equation for making hydraulic predictions from one set of calibration
measurements is limited by the range of flows of interest and the extent
of extrapolation from the calibration flow.
LIMITATIONS TO THE USE OF THE DIRECT DETERMINATION APPROACH
Variations in Manning's n and energy slope result from the variable
nature of flow resistance and energy expenditure in natural channels. The
use of the direct determination approach directly or indirectly reflects
these variations. Therefore,. this technique eliminates many of the
problems associated with Manning's equation over a wide range of flows.
This approach is more time consuming than the use of the Manning
equation because more than one trip to the field is required. However,
proper study preplanning, and work scheduling can reduce travel and data
collection time, and allow available resources to be used quite
efficiently.
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