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COMPLETED PROJECT

Project Report:

SIR 2006-5108
Selection of Manning's Roughness Coefficient for Natural and Constructed Vegetated and Non-Vegetated Channels, and Vegetation Maintenance Plan Guidelines for Vegetated Channels in Central Arizona
by Jeff V. Phillips and Saeid Tadayon

Selection of Manning’s Roughness Coefficient for Natural and Constructed Vegetated and Non-Vegetated Channels, and Vegetation Maintenance Plan Guidelines for Vegetated Channels in Central Arizona

Project Number: CCC00
Project Chief:
Cooperating Agencies: Flood Control District of Maricopa County
Period of Project: COMPLETED

Flooding resulting from extremely dense vegetation in the channel that drains this urban area at Agua Fria River tributary near Youngtown, Arizona. A. Looking downstream from midchannel before the flow of December 10, 1991. B. Looking upstream from left bank during the flow of December 10, 1991. As shown, extremely dense vegetation in the channel resulted in significant backwater effects. Consequently, roads and nearby residential yards were flooded.

A. Vegetation in Rooney Wash, Oro Valley, Arizona (July 28, 2005) prior to thinning of main channel vegetation. B. Photograph from same location on August 3, 2005, following thinning of vegetation.

PROBLEM

Computation of flow in open channels requires evaluation of the channels resistance to flow, which is typically represented by a roughness parameter; such as Manning’s n. Roughness coefficients have been estimated and verified for selected channels throughout central Arizona and in other parts of Arizona (Aldridge and Garrett, 1973; Thomsen and Hjalmarson, 1991; Phillips and Ingersoll, 1998). Characteristics of natural channels and even some constructed channels and the factors that affect channel roughness vary greatly, however, and the combinations of these factors are numerous. Roughness coefficients in many cases cannot be determined with sufficient accuracy by formulas that use direct measurement of limited channel properties, such as the size of sediment on the bed. Other sources of channel roughness, such as vegetation and variations in channel shape, are difficult to quantify in natural and constructed urban channels with theoretical approaches. Selection of roughness coefficients for natural and constructed channels, therefore, is typically based on field judgment and skill that are acquired mainly through experience. The expertise necessary for proper selection of roughness coefficients can be obtained, in part, by examining characteristics of channels that have known or verified coefficients. The roughness coefficient can be verified by computations made by using data from streamflow measurements and from measurements of the physical features of the channel. Photographs of channel segments where n values have been verified can be used as a comparison standard to aid in assigning n values to similar channels (Barnes, 1967).

Furthermore, in the arid to semiarid Southwestern United States, one factor that retards flow and can have the greatest single impact on energy losses and resulting computed water-surface elevations is the vegetation occupying the channel bed, banks, and overflow areas. Vegetation characteristics for particular channel reaches may be larger than all other flow resistance elements (such as bed material size and shape, channel shape, surface irregularities) by a factor of three to four (Phillips and Ingersoll, 1998). Vegetation is a constantly changing factor as well; it can be laid over or removed during floodflows, or grow to substantial spatial densities and heights in just several years time (Phillips and others, 1998). Different species of vegetation also have different flexural strengths for a given size or height, which further complicates assessing flow effects on vegetation, and the subsequent effect of vegetation on flow energy losses. When vegetation either grows to significant height and densities, or is laidover and possibly removed during floodflows, roughness coefficients selected for flood-plain delineation (flood insurance studies, for example) years or decades earlier may no longer be valid. As a result, the computed water-surface elevations for the design discharge are no longer valid.

In past decades, these heavy growths of vegetation may have been modified or removed completely to allow for adequate conveyance of floodflows. However, as concerns have shifted emphasis in recent years toward preserving the riparian vegetation and underbrush that can provide habitat for many species of wildlife, as well as aesthetically pleasing areas for homeowners and businesses, a new approach or set of guidelines that integrates critical elements of flood mitigation and habitat and vegetation preservation is necessary to meet demands of all interested parties.

OBJECTIVES

The objectives include refinement and assessment of past and current techniques for estimating Manning’s n for natural and constructed channels with a significant and potentially evolving vegetation component. The final product will primarily consist of a new chapter for the Flood Control District of Maricopa County’s Drainage Design Manual as well as a USGS Scientific Investigations Report. The documents would begin with technical aspects of channel energy losses, move to selection of Manning’s n for simple channels, and then move on to plan guidelines for assessing vegetation conditions of various types, densities, and distributions.

The main objective is:

To develop new guidelines and methodologies for modification of channel vegetation while allowing the channel to convey the 100-year or other design discharge. The new methodologies presented will focus on various scenarios of vegetation conditions in natural as well as constructed channels. The new guidelines should allow adequate conveyance for flood control, as well as adequate vegetation for habitat preservation and aesthetically pleasing areas for homes and businesses in natural and constructed channels in Maricopa County.

APPROACH

Previous Manning’s n-value selection manuals funded by the FCDMC and other entities specifically related to channel resistance factors for the arid to semiarid southwest will be integrated into the planned documents. The documents will include a section on estimating Manning’s n for changing vegetation conditions, as well as vegetation growth and maintenance in constructed and natural channels adjacent to urbanized areas. This section will include a guide for vegetation modification when riparian vegetation may grow to substantial heights and densities.

RELEVANCE AND BENEFITS

The USGS in Arizona has historically produced guidelines used by the public and private sectors for estimation of Manning’s n for hydraulic computations. In addition to the new methodologies developed and adapted for central Arizona, this document will consist of a compilation of existing guidelines in a single comprehensive document. It is clear that an objective, science-based approach to develop procedures is needed that can be used to optimize the preservation of central Arizona’s riparian habitat and vegetated channel aesthetics for homeowners while ensuring that flood-conveyance channels are still able to mitigate flood damage. These guidelines would provide private sector managers and engineers; local, State, and Federal officials; developers; and conservationists with a new tool for estimating Manning’s n for decision making for this vital and emerging issue. This document also will be transferable to many other arid to semiarid locations in the Southwestern United States.

References

Aldridge, B.N., and Garrett, J.M., 1973; Roughness coefficients for stream channels in Arizona, U.S. Geological Survey Open-File Report, 87 p.

Barnes, H. H., Jr., 1967, Roughness characteristics of natural channels: U.S. Geological Survey Water-Supply Paper1849, 213 p.

Phillips, J.V., McDoniel, D., Capesius, J.P., and Asquith, W., 1998; Method to estimate effects of flow-induced vegetation changes on channel conveyances of streams in Central Arizona, U.S. Geological Survey Water-Resources Investigations Report 98-4040, 43 p.

Phillips, J.V., and Ingersoll, T.L., 1998; Verification of roughness coefficients forselected and natural channels in Arizona, U.S. Geological Survey Professional Paper 1584, 77 p.

Thomsen, B.W., and Hjalmarson, H.W., 1991; Estimated Manning’s roughness coefficients for stream channels and flood plains in Maricopa County, Arizona: Phoenix, Arizona, Flood Control District of Maricopa County, 126 p.