Seiche

The case is of water sploshing in a tub. The simplest motion to some approximation is the one in which water surface remains flat (Assume no turbulence). A similar phenomenon occurs in lakes and is called a Seiche. Imagine a lake of rectangular cross section with a Length $L$ , depth $h$ and width $b$ . Assume the Kinetic Energy is due to horizontal motion of water only (As vertical amplitude is insignificant) and the potential energy is due to displacements above the horizontal. Find the time period (in minutes) of small oscillations of the water. Note that in a Seiche the water surface resembles that of an incline (If the displacement of the extreme right end above horizontal be $z<<h$ then that of the left end is $-z$ ). There is no piling up of water anywhere. Compute this for Lake Geneva $h=150m, L=70km$