Seiche

A seiche is a standing oscillation of water in an enclosed or partly enclosed basin. A disturbance can displace water toward one end; restoring gravity and reflection at boundaries then support repeated oscillation. Wind and atmospheric-pressure changes can excite such motion in natural basins.^[ref]

For an ideal straight basin of length $L$, uniform shallow depth $h$ and two closed ends, no normal flow at the walls gives surface-displacement modes proportional to $\cos (m\pi x/L)$. The shallow-water dispersion relation then gives ${\omega }_m=m\pi \sqrt{gh}/L$ for $m=1,2,\dots$. The fundamental period is $T_1=2L/\sqrt{gh}$.

An illustrative model with $L=2\mathrm{m}$, $h=0.10\mathrm{m}$ and $g=9.81\mathrm{m}{\mathrm{s}}^{-2}$ gives $T_1\simeq 4.04\mathrm{s}$. A gently disturbed tank can test the period. Variable depth, open connections, friction and finite wave amplitude modify this ideal prediction.