CHANGES IN FLOODFLOW CHARACTERISTICS OF A RECTIFIED CHANNEL CAUSED BY VEGETATION, JACKSON, MISSISSIPPI by K.V. Wilson Prepared by the U.S. Geological Survey in cooperation with the City of Jackson 1973 Journal of Research TABLE OF CONTENTS Abstract Floodflow Characteristics at Interstate Route 55 Computations of Manning's n Flood Profiles and Flow-line Profiles Summary References Cited FIGURES 1. Relation of stage to discharge, velocity, and area for an earthen canal constructed for Hanging Moss Creek at Jackson, Miss. 2. Vegetation in channel 450 feet upstream from Interstate Route 55, March 10, 1972 (n = 0.073). 3. Changes in Manning's n, 1964-72, and dates of photographs in figure 4. 4. The channel viewed upstream from Interstate Route 55.     A. March 25, 1971 (near crest). Discharge, 1,600 cfs; stage at downstream service, road, 276.8 feet.     B. March 29, 1971. Rod (left center) at crest of March 25.     C. September 9, 1971. Summer foliage.     D. March 10, 1972. Barren foliage; rod (upper left) at crest of March 2, 1972. 5. Flood of flow-line profiles on Hanging Moss Creek at Jackson, Miss. TABLES 1. Manning's n values. Hanging Moss Creek at Interstate Route 55 in Jackson, Miss., 1964-72 ABSTRACT Extreme changes in velocity, stage, and Manning's roughness coefficient, n, were observed during the first year after canalization of Hanging Moss Creek at Jackson, Miss. Additional changes were observed during the following 8 years. The channel, constructed during the summer of 1963, had a 50-foot-wide bottom, 2:1 side slopes, and 12-foot-depth. In March 1964, average velocities of 7.8 feet per second were measured at a 5 1/2-foot depth in the clean channel and Manning's n was computed to be 0.022. In October 1964, the average velocity was 3.2 fps at a 5 1/2-foot depth and Manning's n was 0.045. The channel was then lined with fairly thick vegetation consisting of small willows, weeks, and grass. In October 1970 (summer foliage existing), the average velocity was 2.0 fps at a 5 1/2-foot depth and Manning's n was 0.07. Willow trees, 8 to 10 feet high, then lined the channel. In March 1971 (barren foliage), Manning's n was increased to 0.07. These observations indicate that the commonly used values of Manning's n for channel rectification (0.02-0.03) are low and that the carrying capacity of earthern channels may be reduced 50 percent as a result of only 1 year's growth of vegetation and 70 percent as a result of 8 year's growth. The carrying capacity during summer foliage is approximately two-thirds the carrying capacity during barren winter foliage. LizardTech's Djvu plug-in is needed to view these reports. CLICK HERE TO VIEW THE ENTIRE REPORT For questions or comments, contact K. Van Wilson.