Will water waves flow faster through a wide or narrow opening?
It depends on character of waves. For usual gravity waves like those produced by wind, width of opening should not represent any difference (except maybe for very narrow openings where viscosity and surface tension are important). In case of tidal waves, as the wave enters narrow space it no longer travels as quickly as before, so it begins to ‘pile-up’; the wave-front becomes steeper and taller, and there is less distance between crests.
