FLOOD07520

• County: Statewide
• Basin: Statewide
• Title: Roughness Coefficients for Densely Vegetated Flood Plains
• Date: 1/1/1987
• Prepared By: USGS
• Floodplain - Doc Type: Floodplain Report/Masterplan

Rouqhness Cnn~entration

Tseng and others (1974) conducted experiments to determine channel
resi5tance coefficients from artificial roughness elements representative of
densely vegetated flood plains.

The energy losses of the flow in densely vegetated flood plains are due.
to bed roughness, bank roughness and the resistance of bushes, plants, and
trees in the flood plain.

By experimental analysis using a flume, Tseng attempted to achieve
various levels of channel resistance. This resistance was related to
statistical representations of spacing parameters where roughness elements
are spaced randomly as well as in a regular spacing.

In turbulent flow, channel resistance is composed of many types of
resistance. In a steady state, nonuniform-flow situation, Tseng showed that
the total resistance force could be expressed as

f

= (Cs + Cf + Cw) -

pV2
2

MX

(13)

where Cs = loss coefficient due to surface resistance
Cf = loss coefficient due to form drag,
Cw = loss coefficient due to surface waves,
p = fluid density, in slugs per cubic foot;
V == mean velocity in direction of flow, in feet per second;
B width of flume, in feet; and
AX = length of channel reach, in feet.

The Cw is difficult to define; therefore it was incorporated into surface
resistance and for.m resistance, so that the equation becomes

where

and

where

f

= (Cs + Cf)

PV2
BAX
2

(14)

Cs

fp
48'

(15)

Cf

C.NweY

BAX

(16)

f
p
C.
N

Darcy-Weisbach resistance coefficient,
wetted perimeter, in feet,
= drag coefficient for each roughness element,

number of elements in the flume area B6X,
width of element perpendicular to flow, in feet, and
depth of flow, in feet.

We
y

6