Explanation:
Hydraulic Pressure-Control, On-Off Deluge Valve
FP-400Y-5DC
The BERMAD model 400Y-5DC is an elastomeric, hydraulic line pressure operated deluge valve, designed specifically for advanced fire protection systems and the latest industry standards. The 400Y-5DC is activated by a hydraulically operated relay valve, through which opening and closing of the valve can be controlled either with a remote hydraulic command or with a wet pilot line with closed fusible plugs. An integral pressure reducing pilot valve ensures a precise, stable, pre-set downstream water pressure. The optional valve position indicator can include a limit switch suitable for Fire & Gas monitoring systems. The 400Y-5DC is ideal for systems that combine a remote wet pilot line with a high pressure water supply.
The answer is 20 meters would be the water depth started to feel bottom. This is half of the wavelength. The reason behind this is the wave travels half or the partial of the wavelength underneath the surface. For that reason that is where the wave would be starting to feel the ground.
A. Units
the rest can be understand that he did not clarify
A discovery not the others