You start asking about ‘High Impact’ from flood waters. Generally flood waters is not High Impact unless a dam collapses or you have an extreme flash flood. Usually if you are at a canal you will have fast water eroding the base of the wall or pushing the rubble masonry wall as the water goes through the canals.
To check if the design of the wall is sufficient see if any design calculations were filed with the local municipality or water agency. The calcs should identify the design water speed and height. If no calcs then you need an engineer to review the construction of the wall.
If you can’t afford an engineer first look at the base of the wall. Is there exposed dirt, is erosion already occurring? Look for photos from a few years ago to see if the wall has changed. Check the upstream ends of the walls on both sides. Are they flared out and do they extend below grade into the earth. If the rubble is just on the surface at the upstream then as the fast water rises it will undermine the wall and it will fail one piece at a time.
A random rubble masonry wall can survive a flood if correctly designed but the only way to know for sure is to have another engineer review the wall.
Usually a RR retaining wall is constructed in embankments (above the linings) of canal when the lateral force from the soil behind is considerably high due to various factors. During the floods, this force will exert more pressure. If the slope and base width of the wall is not designed properly, it will fail. Hence the design parameters should include this eventuality. (Max. Lateral pressure with a minimum factor of safety 1.50). Sometimes there is no need for cement mortar and the tiny gaps will be help full to act as weep holes.