Design of Manning's-roughness-coefficient study in New York State

Field-collected hydraulic data can be used to compute roughness coefficients for channels with different hydraulic characteristics. Suggested criteria for the design of a roughness-coefficient study include: (1) location of the study reach close to an established streamflow-monitoring site with a stable high-flow stage-to-discharge relation; (2) uniform hydraulic conditions within, and upstream and downstream from, the study reach; (3) selection of at least three cross sections within the study reach that reflect the average size and shape of the channel; (4) use of peak-stage recorders at each cross section to record the maximum elevation of the water surface for a given high flow; (5) measurement of streambed particle sizes and computation of characteristic diameters, such as d50 and d84; (6) record of major flow-retarding factors (obstructions, vegetation, or frequent changes in channel size or shape) within the study reach that could cause additional energy losses beyond those resulting from the bed and bank materials alone; (7) documentation of channel conditions with photographs; (8) computation of the n value through a computer program developed for this purpose; and (9) analysis of the data from each site to determine trends and changes in the n value with flow depth and to identify discrepancies in the computed n values resulting from measurement error or violations of the assumptions on which the computational method is based.