FLOOD09807

• County: Statewide
• Community: Nationwide
• Basin: Statewide
• Title: A Uniform Technique for Determining Flood Flow Frequencies
• Date: 12/1/1967
• Prepared By: Water Resources Council - Hydrology Committee
• Floodplain - Doc Type: Educational/Technical/Reference Information

magnitude of the changes. The effects of sharp bends, constrictions, and
side-to-side shifting of the low-water channel may extend downstream for
several hundred feet. The n value for a reach below these disturbances
may require adjustment, even though none of the roughness-producing
factors are apparent in the study reach. A maximum increase in n of
0.003 will result from the usual amount of channel curvature found in
designed channels and the reaches of natural channels used to compute
discharge (Benson and Dalrymple, 1967).

Obstructions (n3)

Obstructions--such as logs, stumps, boulders, debris, pilings, and
bridge piers--disturb the flow pattern in the channel and increase
roughness. The amount of increase depends on the shape of the obstruc-
tion; its size in relation to that of the cross section; and the number,
arrangement, and spacing of obstructions. The effect of obstructions on
the roughness coefficient is a function of the flow velocity. When the
flow velocity is high, an obstruction exerts a sphere of influence that
is much larger than the obstruction because the obstruction affects the
flow pattern for considerable distances on each side. The sphere of
influence for velocities that generally occur in channels that have
gentle to moderately steep slopes is about 3 to 5 times the width of the
obstruction. Several obstructions can create overlapping spheres of
influence and may cause considerable disturbance, even though the
obstructions may occupy only a small part of a channel cross section.
Chow (1959) assigned adjustment values to four degrees of obstruction
(table 2) .

Vegetation (n4)

The extent to which vegetation affects n depends on the depths of
flow, the percentage of the wetted perimeter covered by the vegetation,
the density of vegetation below the high-water line, the degree to which
the vegetation is flattened by high water, and the alinement of vegetation
relative to the flow. Ibws of vegetation that parallel the flow may have
less effect than rows of vegetation that are perpendicular to the flow.
The adjustment values given in table 2 apply to constructed channels that
are narrow in width. In wide channels having small depth-to-width ratios
and no vegetation on the bed, the effect of bank vegetation is small, and
the maximum adjustment is about 0.005. If the channel is relatively
narrow and has steep banks covered by dense vegetation that hangs over the
channel, the maximum adjustment is about 0.03. The larger adjustment
values given in table 2 apply only in places where vegetation covers most
of the channel.

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