Experimental study of the flow in and above very flexible vegetation

Optical measurements were performed in an open-channel tilting flume filled with vegetation surrogates in regimes of strong deformation at high values of the Cauchy number, Ca, in [1720;22766]. Multi-plane telecentric 2D particle image velocimetry (PIV) measurements gave access to flow components both above and inside the vegetation. We show that the canopy layer separates into two distinct sub-layers. In the lower part, the wake sub-layer, located below the maximum vegetation density height hν, vegetation behaves as a simply reconfigured canopy, hν decreasing with Ca following laws found for flexible canopies. Above hν, a flapping sub-layer appears, whose upper boundary hc scales with the water depth H. There, vegetation band excursions homogenize the mean velocity field, erasing all dispersive terms, and profiles of the double-averaged longitudinal velocity follow a log law, as in sparse vegetation canopies, with an inflection point in the velocity profiles found just above hν.