Computer program NCALC user's manual; verification of Manning's roughness coefficient in channels

Abstract

Computations involving flow in open channels commonly require an evaluation of the roughness characteristics of the channel. The U.S. Geological Survey engages in a continuing effort to improve the understanding of flow resistance, usually in terms of Manning's roughness coefficient, n, in channels in the United States. Procedures for computing values of Manning's roughness coefficient from known discharge, water-surface profiles, and channel cross-sectional properties are presented, and have been programmed for automatic computation. General theory, procedures for onsite investigations and surveys, a description of the use of the computer program, an example problem, and additional channel-roughness-verification research needs are presented. INTRODUCTION Hydraulic computations involving flow in open channels commonly require an evaluation of the roughness characteristics of the channel. The selection of roughness characteristics for channels is subjective, even though extensive guidelines are available (Cowan, 1956; Chow, 1959; Aldridge and Garrett, 1973). The U.S. Geological Survey has made an attempt to improve the quantification of flow-resistance coefficients, and to provide predictive equations to aid in the selection of these coefficients, most commonly Manning's roughness coefficient, n. Several studies have been conducted to verify roughness coefficients for selected stream channels covering a range of flow and hydraulic conditions in the United States; flow-resistance verification is a continuing effort of the U. S. Geological Survey. Barnes (1967) presented verified n-value data for near-bankfull discharges, with color photographs and descriptive data for 50 stream channels throughout the United States. Limerinos (1970) verified n-value data for 11 streams in California for various depths of flow and developed a predictive equation for Manning's n as a function of relative smoothness. Aldridge and Garrett (1973) presented verified and onsite selected n-value data and guidelines for selecting n values for 35 streams in Arizona, with emphasis on sand-bed streams. Jarrett (1984) presented verified n-value data for 21 primarily highergradient streams (slopes greater than 0.002 ft/ft) in Colorado for varying depths of flow, and presented a predictive equation for Manning's n as a function of friction or water-surface slope and hydraulic radius. A computer program for computing values of Manning's roughness coefficient has been written. This computer program, NCALC, is used to compute the Manning coefficient n in an unsubdivided channel for a single event of measured or known relatively clear water. Input data are discharge, ground elevations and stationing to define individual cross sections, water-surface elevations at each cross section, and the distance downstream from a reference point to each cross section. This report discusses the theory of the Manning equation, gives detailed instructions for collecting and preparing the data, general instructions for submitting the input data for computer analysis, an example n-verification problem, and additional n-verification research needs. THEORY The Manning equation is used as the basis for computing the reach properties and verified n values in this report. The roughness coefficient term, n, appears in the general Manning equation for open-channel flow: