Estimates of roughness coefficients for selected natural stream channels with vegetated banks in New York

Abstract

Water-surface profiles were recorded and Manning's roughness coefficients computed for a range of discharges at 21 sites on unregulated streams in New York State, excluding Long Island. All sites are at or near U.S. Geological Survey streamflowgaging stations at which stage-to-discharge relations are relatively stable and overbank flow is absent or minimal. Crest-stage gages were used to record water-surface profiles. The channels included in the study have the following ranges in hydraulic characteristics: hydraulic radius, 0.91 to 13.4 feet; water-surface slope, 0.0003 to 0.014; and instantaneous or peak discharge, 77 to 51,700 cubic feet per second. The 84th percentile of the intermediate diameter of bed material ranges from 0.14 to 3.0 feet. Computed Manning's roughness coefficients (n values) range from 0.024 to 0.129. On channels with coarse-grained bed material, the relation between the computed n value and flow depth can be predicted from the energy gradient, relative smoothness (ratio of hydraulic radius or mean depth to a characteristic particle size of the bed material), stream-top width, and channel-vegetation density. The percentage of wetted perimeter that is vegetated can be used as an indicator of energy losses that are attributable to streambank vegetation. Bank vegetation generally has no measurable effect on the roughness coefficients of streams wider than 100 feet if less than 25 percent of the wetted perimeter is vegetated. For wide channels in which larger percentages of wetted perimeter are vegetated, bank vegetation appears to have a small additive effect on the roughness coefficient. On narrow channels (30 to 63 feet wide) in which the wetted perimeter is typically more than 25 percent vegetated, the magnitude of the energyloss effect of streambank vegetation depends on the season and on the type, density, and percent submergence of the vegetation. The presence of trees and brush on the banks of narrow channels increased the n value by as much as 0.005 in the nongrowing season and by an additional 0.002 to 0.012 during the growing season. This report discusses common methods of estimating Manning's roughness coefficients for stream channels, including use of published n-value data, comparison with photographs of channels for which n values have been computed, and n value equations. It also describes a procedure for evaluating flow-retarding factors of a channel and includes photographs and hydraulic data from the 21 channels studied.