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

The computation procedure is started by subdividing the cross-
section into a series of channel segments, as shown in Figure 5. Each
channel segment has the geometric properties of cross-sectional area
(a), hydraulic radius (ri), and each has a roughness coefficient (ni).
Thi following assumptions are made to continue the computation
procedure:
L The slope is the same for all channel segments.
2. There is no slope of the water surface normal to the direction
of flow.
3. Each channel segment is rectangular.
The mean velocity for each channel segment may be calculated from
the Manning equation as follows:
v = 1.486 ri 2/3 5? (?)
i ni
0
where,
v. = the mean velocity of the channel segment
ri = the hydraulic radius (a./w.) for the channel segment, based
on the stage as determined previously, and on the dimensions
of the segment.
5 = the slope, as previously described
n-i the roughness coefficient for the channel segment..
The calibration of this equation could be simplified considerably
by assuming that the roughness coefficient is the same for every channel
segment (i.e. nl = n2....= n.- = n ), where nods the roughness coeffi-
cient for the whole channel Is determined in the computation of the
stage-discharge relationship. The validity of this assumption depends
on the uniformity of the channel and channel materials, the roughness of
the banks, and so forth. In some situations, it will be apparent that
the assumption of constant roughness will not be true. In other cases,
there will be channel segments which will be out of the water at the
time the calibration measurements were made (for example, segment 8 in
Figure 5). Either situation may require an estimation of Manning's n
for a particular channel segment.
Where the roughness of a particular segment is unknown the relative
roughness can be specified. For instance, the segment with the lowest
roughness may have a specified value, n and the remainder have a
roughness related to n by a constant, ?i. The roughness of a particular
segment is then expressed as:
ni _ ci no
u
(12)
The value of c• may be estimated by comparison of the size of bed
materials in adjacent channel segments, by the following expression: