OK, so this is the short answer in “back of the envelope” style math. There some physics involved here and we are going to simplify things. The fundamental differance is between the force exerted by the “compressible” flow of gas (air) and the non “compressible” flow of water.
The physics are basically the mass multiplied by the velocity squared. So, let’s look at water, fresh water, moving at just 3 miles an hour, a slow walk. This calculator shows the force to 38.5 lbs of force per square foot. The BASIC (the formula we once used) for the force of hurricane speed winds are calculated from the Ensewiler Formula, P = 0.00256 x V2, where V = Wind Velocity in MPH and P = the Differential Pressure in Pounds per Square Foot (PSF). A wind of 110 miles an hour equates to a load of just 31 lbs per square foot. At 150 miles and hour the force of the wind is 57.6 lbs per square foot. This is a HUGE design load. But at just 5 miles per hour the load of the water is 105 lbs per square foot, nearly double the design wind load.
The above math is why we have “break away” walls for the first floor of new houses subject to flooding. The loads get to be too large to economically design for so we sacrifice the wall on the first floor to save what’s holding up the living area above it, the pilings.
