Flow Retardance by Channel Weeds and Their Control

by John C. Stephens,
R. D. Blackburn,
D. E. Seaman,
L. W. Weldon,

Serial Information: Journal of the Irrigation and Drainage Division, 1963, Vol. 89, Issue 2, Pg. 31-56

Document Type: Journal Paper


Water weeds raise Manning's n values in proportion to density of growth according to weed-type, as controlled by channel geometry, area, and flow. Large channels (with or without weeds) Having hydraulic radii above 20, customarily have n values between 0.024 and 0.031. In moderate-size channels of the southeast, having cross-sectional areas between 100 sq ft and 2,000 sq ft, n values are less than 0.036 with light-weed growth; between 0.036 and 0.052 with moderate growth; and exceed 0.052 with heavy growth. Submersed and emersed weeds increase n values exponentially as the product of velocity and the hydraulic radius declines below a VR threshold value of approximately 20 ft. Aquatic weeds can be controlled by mechanical or chemical methods. Mechanical methods, although widely accepted, usually give control for only a short time and may induce spreading. Herbicides are more effective in many cases than other measures, and their use is rapidly increasing as better products and application techniques become available.

Subject Headings: Channel flow | Flow control | Vegetation | Hydraulics | Density currents | Geometrics | Water flow | Cross sections

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