I’m measuring an open-top concrete channel (average cross-sectional dimension of flow 0.9m x 0.5m, height x width) and though it has some sediment at the bottom I tend to doubt your FF will work. The tight dimensions make me think the FF should be increased, but the fact that it is concrete (relatively smooth) tells me the opposite. Right now my best estimate is 60%. What do you think? My result is putting the flow at 66 Liters/second which I think is high.