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

20. Determine the no value by equation 9, using the adjustment
factors selected in step 19. The n4' value is the adjustment factor
for vegetation not accounted for by the vegetation-density method.

21. The vegetation density of the sampling area is determined using
equation 11 by measuring the cross-sectional area occupied by the trees
and undergrowth in the sampling area. An estimate of the depth of flow
on the flood plain is necessary to determine vegetation density and the n
value. By measuring 2 or 3 sampling areas in a subsection, a more
representative value for vegetation density can be determined.

22. The n value for the flood-plain subsection is determined by
using equation 6 or 7, depending on which method has been chosen. If the
quantitative method is being used, the n value for each subarea of the
flood plain is computed, using equation 7 and vegetation-density and
boundary-roughness values for each subarea.

23. Compare the study reach with f*1otograj:hs of other flood plains
in this report and in other references to determine if the final values
of n obtained in step 22 appear to be reasonable.

Examples of procedures for determining n values

A sketch of a hypothetical channel and flood plain is shown in
figure 23 and procedures for determining n values are outlined in table
4. 'Ihe channel and flood plain together are divided into three separate
reaches (A, B, C) and each reach has a cross section (1, 2, 3). The
shape of each cross section is shown in figure 23.

In section 1, the flow is confined to the channel. The channel is
composed of firm soil and no subdivision of the channel is necessary.
Steps 1 through 13 in "Procedures for Assigning n Values" are used in the
computation of n for this section. These steps apply only to channel
conditions.

I
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I

Flow in section 2 is also confined to the channel, which is composed
of three distinct parallel bands of (1) bedrock, (2) sand, (3) gravel and
cobbles. The n value for each segment is determined and a composite n
for the channel is computed by weighting each segment n value by its
wetted perimeter. Again, steps 1 through 13 are used in the computation
of n for this section.

The flow in section 3 is channel and flood-plain flow. The cross
section is divided into three subsections. Subsection 1 is flood-plain
flow through woods, subsection 2 is channel flow, and subsection 3 is
flood-plain flow through a cotton field.

In subsection 1, the flood plain is made up of dense woods having
little undergrowth. The procedure using the vegetation density of the
woods is used to determine the n value for the flood plain. The
vegetation density is determined from a representative sample area of the

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