Determination of roughness coefficients for streams in west-central Florida

Abstract

Physical and hydraulic characteristics are presented for 10 west-central Florida stream reaches. These may be used as reference reaches for estimating Manning's roughness coefficient, n, in similar rivers and channels. Discharge in these reaches ranged from 1.8 to 3,010 cubic feet per second and water-surface slope ranged from 0.00002 to 0.00476 foot per foot. Sites were selected at or near U.S. Geological Survey streamflow gaging stations at which the stage-discharge relations were relatively stable. Crest-stage gages were installed at appropriate locations in each study reach in order to obtain water-surface elevations. Water-surface profiles were collected over a range of discharges at each of the 10 sites and the roughness coefficient was computed for each discharge. For the 104 recorded streamflow events used in this report, the computed roughness coefficients ranged from 0.021 to 0.218 and the mean was 0.072 . The median n value was 0.060; 70 percent of the time the n value was greater than 0.047 and 80 percent of the time the computed n was greater than 0.043. Bed material is so uniform at sites throughout west-central Florida that there is negligible variation in roughness as a result of variation in bed material. Streambank vegetation appears to be the major contributor to variations in roughness values for streams in west-central Florida. Channel roughness can be estimated quantitatively by evaluating the interaction of all roughness factors. Photographs of sites where n values have been computed can be used for comparison to estimate values at similar sites. Using a base n value of 0.040 and increasing n for factors of vegetation, channel obstructions, and other factors provides the best determination of n values for streams in west-central Florida.