Float-Method (Velocity-Area) Open-Channel Flow
A field estimate of open-channel flow (ditch, canal, outfall) with a float, a tape, and a stopwatch: Q = C x surface velocity x cross-sectional area. Time a float over a measured reach for the SURFACE velocity (distance/time), multiply by area (width x mean depth), and apply a float coefficient C to convert surface to the lower MEAN velocity -- C ~0.85 (0.8 rough/shallow to ~0.9 smooth/deep). A float running 20 ft in 10 s (2.0 ft/s) in a 4 ft x 1.5 ft channel gives 0.85 x 2.0 x 6 = 10.2 cfs (~4,580 gpm). Average several runs in the fastest thread, survey the real cross-section, use a straight uniform reach. A rough field gauging estimate, well below a metered measurement; a permit-compliance flow needs a calibrated device, and the metering standard and permit govern.
Formula and source
surface_velocity_fps = float_distance_ft / travel_time_s; cross_area_ft2 = channel_width_ft x mean_depth_ft; flow_cfs = float_coefficient x surface_velocity_fps x cross_area_ft2; flow_gpm = flow_cfs x 448.831.
Float (velocity-area) open-channel flow-gauging method (USGS / Standard Methods field practice), by name; a calibrated meter and the permit govern a compliance measurement.
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