Answer to your 1st question:
Well, It totally depends on the design of retaining wall! Usually, flood water has not a high impact unless and until the dam collapses.
Random rubble masonry can sustain a high amount of pressure exerted by water.
The strength of the mortar used for joining of rubble stone is in between 5 to 7.5 MPa only.
Answer to your 2nd question:
When we talk about the design of retaining wall then it should be accurately done as per the local water agency and local municipality.
All the calculations should be done with respect to the design speed of water and height of retaining wall.
First, check the extreme ends of the Wall on both sides. Look at them carefully enrich weather Pyar flattered out or not. Check that the extended below the grid into the earth or not.
Whenever the rubble is just on the surface of the upstream side then definitely a retaining structure will fail at that time.