Open Channel Flow: Drowning of Step Feature as Channel Discharge Increases



More about technical content and background details for this video can be found in the textbook, “Fundamentals of Open Channel Flow” by Glenn E. Moglen.

Link to book: https://www.amazon.com/Fundamentals-Open-Channel-Glenn-Moglen/dp/1466580062
Link to un-narrated video: https://www.youtube.com/watch?v=FL_FZCuJcUQ

Animation Details:
This is an animation from the book, “Fundamentals of Open Channel Flow”, by author Glenn E. Moglen, published by the CRC Press (2015). The animation shows the “drowning” of surface water features (an M1 curve upstream of the step and a hydraulic jump that migrates up the downstream side of the step until it disappears when the step no longer represents a choke). The main concept being conveyed is that a channel feature that has a profound effect on surface water profiles at low flows is “drowned” to the point of insignificance when the flow becomes large. Secondary concepts that can be observed are energy buildup (M1) in front of an obstruction and energy dissipation (hydraulic jump) downstream of an obstruction. The migration of the jump and the evolution of the profile in the vicinity of the obstruction are unique features that are illustrated as only possible in an animation. Three graphs appear below the figure. The lower-left graph shows the prescribed change in discharge from 2.5 ft3/s to 300 ft3/s. It is this increasing discharge that drives the drowning of the water surface profiles near the step. The lower-middle graph shows the excess energy [E0 – (Ec + delta_z)]. A curve is traced in red as the excess energy is negative (i.e. choke conditions exist). The curve shifts in color to black when the excess energy is positive and the choke conditions no longer exist. The lower-right graph shows the trace of two depths as they occur on the middle of the step. In dashed red is the critical flow depth. In solid blue is the actual flow depth. When choke conditions exist, these two traced curves coincide. When the excess energy is positive then the actual flow depth is greater than critical depth.

Conditions modeled: Q=2.5 to 300 ft3/s, S0=0.003 (mild for all discharges), Manning’s n=0.03, step height delta_z=0.5 feet, channel width=10 feet, channel length=1000 feet. The flume is rectangular in cross-section.

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