FLOOD01296

• County: Statewide
• Title: Method to Estimate Effects of Flow-Induced Vegetation Changes on Channel Conveyances of Streams in Central Arizona
• Date: 9/16/1998
• Prepared For: Central Arizona
• Prepared By: USGS
• Floodplain - Doc Type: Educational/Technical/Reference Information

subsequently raise water-surface elevations in that
local area.

EFFECTS OF FLOW-INDUCED
VEGETATION CHANGES ON
CHANNEL CONVEYANCES

To determine the effect of flow-induced
vegetation changes on channel conveyance, the
overall effect of vegetation changes on Manning's
roughness coefficient (n) must be quantified.
Knowledge of the factors that exert the greatest
influence on n in natural and manmade channels is
needed to adequately describe and quantify total
energy losses.

Components of Manning's n

The general approach for estimating flow
resistance in stream channels is to first select a base
value of Manning's n for the bed material (nb;
Thomsen and Hjalmarson, 1991). The base n value
represents the size and shape of the grains of the
material along the wetted perimeter (Chow, 1959).
Cross-section irregularities, channel alignment,
vegetation, obstructions, and other factors that
increase roughness then are added to nb' The
following equation, first introduced by Cowan
(1956), is used to calculate the total composite n
for a channel

n = (nb+nl+nZ+n3+n4)m

where
nb = base n value for a straight uniform
channel;
n, = surface irregularities;
nz = variations in shape and size of the
channel;
n3 = obstructions;
n4 = vegetation; and
m = correction factor for meandering or
sinuosity of the channel.

Detailed explanations for each adjustment factor
can be found in Cowan (1956), Chow (1959),

Aldridge and Garrett (1973), Jarrett (1985),
Thomsen and Hjalmarson (1991), and Coon
(1995).

Base Value of Manning's n

Most sites in this study were selected for reach
and cross-sectional uniformity. Thus, the
composite n value is considered a function of only
nb and an adjustment for the vegetation
component, n4' The other components at most of
the sites were considered to have a negligible effect
on total roughness.
Although the composite n value was
considered verified at several sites (Phillips and
Ingersoll, 1998), most sites required n-value
estimation. A variety of techniques have been
presented in literature. that aid in estimating n
values. Reference to published tables and
photographs of verified n values is one method;
another is the use of equations that relate n values
to measurable channel and hydraulic components.
For sites having gravel-bed channels where the
n value was not verified, nb was primarily
estimated on the basis of a recently developed
equation for gravel-bed streams in central Arizona
(Phillips and Ingersoll, 1998). The equation relates
nb to flow depth (represented as hydraulic radius,
R) and the median size of the bed material and is
defined as

1/6
nb=O.0926R /1.46 + 2.231og(R/dso)' (8)

(7) The verified n value and not equation 8 was
used at sites where a roughness coefficient could be
back calculated from direct-current measurement
and Manning's equation. Verified n values were not
available for flows at the sand-dominated sites. The
base n value at these sites was estimated on the
basis of reference tables in Aldridge and Garrett
(1973), Thomsen and Hjalmarson (1991), and
Phillips and Ingersoll (1998).

Adjustment for Vegetation Component

If the flow forces are large enough to degrade
the channel substrate to the degree that vegetation
is removed before peak flow, the value of the
adjustment for vegetation (n4) approaches zero;
and therefore, the vegetation would not

20 Method to Estimate Effects of Flow-Induced Vegetation Changes on Channel Conveyances of Streams in Central Arizona