  Monami is the synchronous waving of a submerged seagrass bed in response to
unidirectional fluid flow. Here we develop a multiphase model for the dynamical
instabilities and flow-driven collective motions of buoyant, deformable
seagrass. We show that the impedance to flow due to the seagrass results in an
unstable velocity shear layer at the canopy interface, leading to a periodic
array of vortices that propagate downstream. Each passing vortex locally
weakens the along-stream velocity at the canopy top, reducing the drag and
allowing the deformed grass to straighten up just beneath it. This causes the
grass to oscillate periodically. Crucially, the maximal grass deflection is out
of phase with the vortices. A phase diagram for the onset of instability shows
its dependence on the fluid Reynolds number and an effective buoyancy
parameter. Less buoyant grass is more easily deformed by the flow and forms a
weaker shear layer, with smaller vortices and less material exchange across the
canopy top. While higher Reynolds number leads to stronger vortices and larger
waving amplitudes of the seagrass, waving is maximized at intermediate grass
buoyancy. All together, our theory and computations correct some misconceptions
in interpretation of the mechanism and provide a robust explanation consistent
with a number of experimental observations.
