FLOOD08739

• County: Statewide
• Community: Statewide
• Basin: Statewide
• Title: Determination of Roughness Coefficients for Streams in Colorado
• Date: 1/1/1985
• Prepared For: CWCB
• Prepared By: CWCB
• Floodplain - Doc Type: Educational/Technical/Reference Information

Cross sections need to be located closely enough to accurately reflect
the hydraulic characteristics of a channel and its adjacent overbanks between
the cross sections. The sections need to extend across the entire flood plain
and need to be at right angles to the anticipated direction of flow in both
the main channel and overbanks. This may indicate the need for curved or
angled sections at some locations. The location of the sections needs to
ensure an accurate evaluation of energy losses between sections. For each
reach, the energy gradient, water-surface slope, and streambed slope need to
be as nearly parallel as possible. If any channel condition causes one of
these profiles not to be parallel, it is an indication that additional cross
sections are needed. A sufficient number of cross sections are needed to
accurately reflect flood-plain geometry.

Some general criteria for locating cross sections are listed below.
These criteria help satisfy the assumption of steady uniform flow in
individual subreaches.

1. The cross sections need to be located at major changes in bed
or water-surface profiles. If old flood profiles are available,
they can be used to locate the breaks in water-surface profiles.
2. The cross sections need to be placed at points of minimum and
maximum cross-sectional area, width, or depth. The number
of cross sections needs to be greater in expanding reaches
and in bends to minimize the relative degree of expansion between
cross sections and leave the individual subreaches more nearly
uniform.
3. The number of cross sections needs to be greater in reaches that
have moderate to severe changes in cross-section shape, even
though the total areas may differ only slightly from each other.
An example would be sections that change shape from just a main
channel to main channel with overbank flow.
4. The cross sections need to be located at abrupt changes in
roughness characteristics, for example, where the flood plain
is heavily vegetated in one subreach, but has been cleared and
cultivated in the adjacent subreach. The use of a cross section
twice, in close proximity and with different roughness values,
must suffice for the present to evaluate the frictional losses.
5. The cross sections need to be located at control sections if
critical or supercritical flow conditions exist. These controls
include natural and manmade weirs, check dams, rock walls, fences,
and severe obstructions.
6. The cross sections need to be located at tributaries where changes
in discharge are anticipated. The exact placement of the cross
sections varies, depending on the method of analysis and program
requirements.

Where abrupt channel changes occur, several cross sections need to be
established to accurately indicate the changes, regardless of the distance
between the sections. At bridges and other hydraulic structures, a sufficient
number of cross sections are needed to define the approach and outlet condi-
tions as well as the geometry of the structure, but these conditions are not
discussed in this report.

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