MemberApril 29, 2021 at 9:47 pm
Well, all the scientists in the video agree that wind setdown (and the corresponding storm surge) occurs naturally and can displace a lot of water. We have seen this in a micro form when driving in the rain. The wind caused by our fast movement can drive the raindrops up and off the windscreen, but can also hold them there, trembling on the glass, when the forces of wind and gravity are balanced. Because of water’s natural viscosity, the drops tend to consolidate and ‘stand up’ a little (there must be a proper verb for this), the same force that stop-motion photography has shown causes falling raindrops to become perfectly spherical before they hit the ground.
Robert Carter (from 12:20 min in the video) is correct when he says that normally water that has been blown back by the wind is ‘flat’ on the windward side. Water that is blown away forms a ramp from 0 elevation at the front to whatever elevation the storm surge reaches at the back. But, if the water is being blown *up* a slight incline (too slight to prevent the water from rushing back) the water will keep ‘trying’ to run downhill towards the oncoming wind but will continue to be blown away. This will cause a rolling edge as the water runs downhill from underneath the displaced body of water, but is blown up and back from the action of the strong wind on the surface. Because of water’s natural viscosity (as above) this will cause a trembling edge to the water, just as we see with the raindrops being held in place on the windscreen of the vehicle. This is the ‘wall’ on the N side.
What about the ‘wall’ on the S side? Well, the water overtops the Great Bitter Lake for hours and is driven NW up the isthmus (the Hamsin wind is from the SE) until the point comes when no more water can be removed from the lake. At that point the water is still ‘trying’ to leave the lake, but gravity keeps pulling it back into the lake basin. So we get another rolling edge at the shoreline. This would probably be more choppy than ‘trembling’, but still a ‘wall’. The isthmus in this region just to the north side of the Great Bitter Lake opens wider to the NW than the N. Perfectly designed to hold all the water from the storm surge. The Egyptian soldiers had to run back to higher ground on the W side whence they came, but the bore-wave (the scientists didn’t talk about this) was coming from the NW. So yes, it is like running towards a closing door, hoping to get through before the wave hits them.