FLOOD06972

• County: Statewide
• Basin: Statewide
• Title: Method of Estimate Effects of Flow-Induced Vegetation Changes on Channel Conveyances of Streams in Central Arizona
• Date: 1/1/1998
• Prepared For: Central Arizona
• Prepared By: USGS
• Floodplain - Doc Type: Educational/Technical/Reference Information

INTRODUCTION

In 1991, the U.S. Geological Survey (USGS),
in cooperation with the Flood Control District of
Maricopa County, began a 6-year study of
flow-induced vegetation changes and the resultant
effect on channel conveyances of streams in central
Arizona. Because accurate calculation of channel
conveyance is critical for open-channel hydraulic
studies, a major objective of this study was to
develop a relation to quantifY the effects of
flow-induced vegetation changes on channel
conveyances and computed water-surface
elevations. To accomplish this objective,
considerable field work was performed at selected
sites to col1ect the data that were needed to
accurately characterize streamflow, vegetation, and
channel conditions.

Computations of channel conveyance for
open-channel flow require an evaluation of channel
roughness. Channel roughness reflects the
channel's resistance to flow and usually is
expressed by a roughness coefficient. Manning's
roughness coefficient, n, represents the composite
effect of a variety of flow-resistance factors that
include bed material, channel shape, and
vegetation characteristics (Cowan, 1956). Cowan
(1956) indicated that channel vegetation can have
the greatest potential effect on the total roughness
coefficient selected for a reach, Thomsen and
Hjalmarson (1991) describe the major effect of
vegetation on total roughness for streams in
semiarid to arid climates typical of the
southwestern United States. In these types of
environments and in a period of only a few years,
vegetation may grow to ful1 maturity throughout
the main channel of natural and manmade streams,
which results in large increases in estimates of n
(Aldridge and Garrett, 1973; Thomsen and
Hjalmarson, 1991). Although the vegetation may
appear substantial, peak flows that are powerful
enough to layover or remove vegetation often
occur during moderate to large flooding in central
Arizona (Burkham, 1976; Phil1ips and Hjalmarson,
1994), The flattened or removed vegetation
markedly decreases preflow estimates of n, This
decrease in n increases peak-flow channel
conveyances and effectively lowers peak-flow

water-surface elevation of the flow compared to
preflow predictions.
Although past investigations have presented
information useful for assessing the effects of peak
flow on roughness characteristics of grasses in
manmade channels (Ree and Palmer, 1949; Soil
Conservation Service, 1954; Kouwen and Li,
1980; Temple, 1980; and Kouwen, 1992), almost
no guidelines are available to evaluate the effects
of flow on larger vegetation types such as brush
and small trees that commonly grow throughout
the main channel of streams in the southwestern
United States (House and I>earthree, 1995,
p. 3068). The ability of flows to substantial1y alter
vegetation characteristics and the lack of adequate
guidelines to assess these changes can result in
uncertainties and erroneous channel-conveyance
calculations, These uncertainties and errors could
result in poor management of waterways, poor
estimates of peak discharge, and improper design
of bridges, culverts, road grades, and other
water-related structures. The Flood Control
District of Maricopa County funded this study in
order to decrease the amount of uncertainty
associated with estimating peak-flow vegetation
conditions for channel-conveyance computations.

Purpose and Scope

The purpose of this report is to provide
information and new methodologies to engineers,
hydrologists, planners, and researchers so that
better assessments can be made to determine
variations of channel conveyance that result from
changes in vegetation characteristics. Data
col1ected for 26 peak flows at 19 sites in a 6-year
period in central Arizona were used to assess
flow-induced changes in vegetation conditions
(fig. I). A semiempirical relation that describes
flow-induced changes in vegetation is presented as
well as the subsequent effect of these changes on
flow resistance. The changes in water-surface
elevations for estimated preflow- and
postflow-vegetation conditions were computed and
presented for each of the study sites. The purpose
of presenting this information is to illustrate
potential errors that can accompany incorrect
assessments of n values caused by flow-induced
vegetation changes, An example case illustrates

2 Method to Estimate Effects ot Flow-Induced Vegetation Changes on Channel Conveyances 01 Streams in Central Arizona