If you have a series of dams built along a valley, what will happen if the last one in the line fails suddenly and catastrophically?
You have a dam wall about halfway down a long narrow valley. It is made up like a sandwich. A layer of large shaped granite rocks connected by iron and metal bands, a internal layer- a mix of compacted rubble, and roman like concrete, and then a second layer of large rocks. The dam wall shape is designed like hoover dam with the curve against the water pressure.
After a few decades to centuries the wall starts to deteriorate. You are capable of 'dredging' up the collected silts to use on your farmlands but you don't have the technology level of the original dam builders so you can't 'properly' maintain it. So to protect your existing thriving farmlands and cities downvalley you build a second dam wall a couple hundred metres from the original wall before it deteriorates too badly. The original does not collapse or fail.
This new dam area is allowed to fill up and the water pressure on both sides of the original wall 'maybe' helps keeps it upright. To allow fish and water transfer between the two there could be alterations done to the top layer of the original wall like permanently open riverlocks (there could be a system to close these locks if necessary) or maybe just allow the wall to be completely submerged.
I have for a long time thought that they would have a way of moving water from one area to another. Diverting and managing water flow in and out of the dam/s to surrounding valleys. Using waterwheels, arcimedes screws, maybe even some sort of primitive suction action. Another couple of decades/centuries go by.
What happens to the original dam wall if the second dam wall fails suddenly?
Would the sudden loss of water pressure supporting the one side cause the original wall to fail at the same time? Especially if the original wall may be compromised internally by water seeping into the middle sandwich layer.
How do I get the original wall to hold?
Note I'm asking on worldbuilding as I'm not aware of any real life examples of dam's in series and dam failures like this.
EDIT I do believe I am actually thinking along the lines of the combination arch-gravity dam rather than just the arch dam like Hoover dam. This Question asked about a dam made up of blocks of stone rather than a continuous concrete setting seems to imply that a dam similar (but not exactly) will last at least for a century or two before failing due to the stresses.