WI: Coffer Dams on Yangtze collapse

Coffer dams were used to clear part of the Yangtze River to build the Three Gorges Dam. However they were extremely unstable and a flash flood could have collapsed them, sending a title wave racing towards Shanghai. So, lets say a flash flood breaks the Dams. What happens next?
 
Though the Yangtse isn't as infamous as the neighbouring Yellow for drastically re-drawing the map (*), it still has a nasty habit of bursting its banks and flooding vast areas...

My best guess would be a frantic scramble to evacuate several million people from the area at risk.
If it coincides with Spring melt or a typhoon, you might see the course change...

*) Rivers wriggle within their floodplains. IIRC, the Yellow considers the *entire* North Chinese Plain as such and, historically, has gone walk-about, causing mega-deaths...
http://www.waa.ox.ac.uk/XDB/tours/china8.asp
http://www.drben.net/ChinaReport/So...ver/Path_ot_Yellow_River-3-Lower_Reaches.html
Yes, it really was a tributary of the Yangtse on several occasions before, yet again, silting up that course and 'hanging a left'...
 
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