From Wikipedia -
In hydrodynamics, the clapotis (from French: "lapping of water") is a non-breaking standing wave pattern, caused for example, by the reflection of a traveling surface wave train from a near vertical shoreline like a breakwater, seawall or steep cliff. The resulting clapotic wave does not travel horizontally, but has a fixed pattern of nodes and antinodes. These waves promote erosion at the toe of the wall, and can cause severe damage to shore structures. The term was coined in 1877 by French mathematician and physicist Joseph Valentin Boussinesq who called these waves ‘le clapotis’ meaning ‘standing waves’.
Even More Scientific..
|520 Bridge in Seattle.|
In the idealized case of "full clapotis" where a purely monotonic incoming wave is completely reflected normal to a solid vertical wall, the standing wave height is twice the height of the incoming waves at a distance of one half wavelength from the wall. In this case, the circular orbits of the water particles in the deep-water wave are converted to purely linear motion, with vertical velocities at the antinodes, and horizontal velocities at the nodes. The standing waves alternately rise and fall in a mirror image pattern, as kinetic energy is converted to potential energy, and vice versa.
Long Story Short -
Waves hit a wall and bounce back colliding with the incoming waves creating confused waves in all directions.
How does Clapotis affect paddlers?
The effect creates rough confused water which can be difficult to paddle in but is very good for building your skills in such conditions. Solutions? Use shorter strokes which act as a brace to keep you more stable. SUPs should in addition keep knees bent to let wave energy pass underneath.
Advanced paddlers find clapotis fun to paddle in. Scissor waves which are two waves colliding and sending energy upwards has been fun for kayakers for years. The effect can flip or throw a paddler vertical or even airborne.
Samples from Seattle's 520 Bridge on Lake Washington -