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Acoustic Doppler Flow Meters (ADFMs) are suitable to measure the flow discharge in culverts for the purpose of rating calibration and validation. However, best-measurement guidelines for ADFM placement to measure the flow through gated culverts are not available. Gates in culverts induce a complex flow field consisting of a jet below a hydraulic jump or a submerged eddy depending on whether the tailwater level results in free or full-pipe flow. The architecture and operational principles of ADFMs are suitable for measurements in well-developed flows; however, the distance from the gate where the flow becomes well developed depends upon the ratio of the gate-opening-area to the full-barrel-area (A G /A 0 ) and the gate Reynolds number (Re G ). Presently, neither a characterization of the distance from the gate where well-developed flow occurs, nor an assessment of biases in ADFM measurements of full-barrel, gated-culvert flows due to placement of the ADFM where the flow is only partly developed, are available in the literature. Tropea et al. (2007) reported that the reattachment length of a flow at a backward facing step occurs at a distance ~ 8 times the height of the step. Here we show that discharges measured in two gated culverts with an ADFM at distances (L) equal or larger than 8(D-G) from the gate (D=diameter or box-culvert height; G=gate opening) using as reference discharge measurements by Acoustic Doppler Current Profiler (ADCP) from a moving boat in the discharge canal, have biases of less than 6.4% (estimated as relative errors |Q ADFM -Q ADCP |/Q ADCP ). Moreover, our ADFM discharge measurements show biases between 0.4 and 6.4%, depending on the gate opening and culvert geometry. Additional analysis is underway to assess whether Re G strongly varies with the reattachment distance of the jet-induced eddy pocket and the non-dimensional distance from the gate, where flow becomes well developed L/(D-G).

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