Consider this, the TGD stands 175 m (574 ft) high and has a volume of 39.3 km3 (31,900,000 acre⋅ft). That is a LOT of potential energy. The spillways are currently dumping 116,000 m3/s. Even that amount is causing big problems downstream.
I remember reading, years ago, that if the Aswan High dam burst the wall of water would be 20 ft high when it reached Cairo. The AHD only stands 111 m (364 ft) high but Lake Nasser holds about three and a half times the water. I don't know what the wave height would be when it reaches Shanghai but it would be significant. There is also the point that much of the Yangtze flows through mountains. This will accelerate the velocity of the water and concentrate it because it can't spread out.
It won't just be the water that will be coming downstream. That wall of will scour EVERYTHING from its banks and deliver it into its lower reaches and the East China sea.
It will cause MASSIVE problems in the East China Sea and beyond. That debris will be a huge hazard to navigation. This means that ships transiting the area will have to greatly reduce speed if they transit it at all. This will affect the port of Shanghai and points north. This is where most of China's petroleum imports are unloaded.
A TGD break would take out most if not all of the bridges below the dam disrupting N/S transportation. Road/rail traffic loss will be significant but not the only loss. Bridges carry a lot more than traffic. They also carry electrical, communications, and pipeline traffic. While watching the videos of the air war just before Desert Storm kicked off did you ever wonder why we were bombing so many bridges? Now you know.
WWG1WGA
Garde la Foi, mes amis! Nous nous sommes les sauveurs de la République! Maintenant et Toujours!
(Keep the Faith, my friends! We are the saviors of the Republic! Now and Forever!)
LonePalm, le Républicain du verre cassé (The Broken Glass Republican)