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

Barnes (1967) catalogued verified n values for stable channels having
roughness coefficients ranging from 0.024 to 0.075. In addition to a
description of the cross section, bed material, and flow conditions during
the measurement, color f*1otograf*1s of the channels were provided.

A sand channel is defined as a channel in which the bed has an
unlimited supply of sand. By definition, sand ranges in grain size from
0.062 to 2 rom.

Resistance to flow varies greatly in sand channels because the bed
material moves easily and takes on different configurations or bed forms.
Bed form is a function of velocity of flow, grain size, bed shear, temper-
ature, and other variables. 'Ihe flows that produce the bed forms are
classified as lower-regime flow and upper-regime flow, according to the
relation between depth and discharge. The lower-regime flow occurs with
low discharges and the upper-regime flow with high discharges. An
unstable discontinuity in the depth-discharge relationship appears between
the two regimes and this is called a transitional zone. In lower-regime
flow, the bed may have a plane surface and no movement of sediment, or it
may be deformed and have small uniform waves or large irregular saw-
toothed waves formed by sediment moving downstream. 'Ihe smaller waves
are known as ripples, and the larger waves are known as dunes. In upper-
regime flow, the bed may have a plane surface and movement of sediment,
or it may have long, smooth sand waves in phase with the surface waves.
These waves are known as standing waves and antidunes. Bed forms on dry
beds are remnants of the bed forms that existed during receding flows and
may not represent bed forms present during flood stages.

'Ihe regime is governed by the
stream power, which is a measure of
is computed by the formula,

size of the bed materials and the
energy transfer. Stream power (SP)

SP = 62 RSwv

(4)

where: 62 = specific weight of water, in pounds per cubic foot;
R = hydraulic radius, in feet;
Sw = water-surface slope, in feet per foot;
and V = mean velocity, in feet per second.

The n value for a sand channel is assigned for upper-regime flow
using table 1, which shows the relation between median-grain size and the
n value. The flow regime is checked by computing the velocity and stream
power that correspond to the assigned n value. The computed stream power
is compared with the value that is necessary to cause upper-regime flow.
Figure 1, developed by Simons and Richardson (1966, fig. 28), may be used
for this purpose. If the computed stream power is not large enough to
produce upper-regime flow (an indication of lower-regime or transitional-
zone flow), a reliable value of n cannot be assigned. The evaluation of
n due to bed-form drag is complicated and different equations are needed
to describe bed forms. The total n value for lower- and transitional-
regime flows can vary greatly and depends on the bed forms present at a
particular time. Figure 2 illustrates how the total resistance in a
channel varies as bed forms progress from one type to another. The n

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