I overheard an interesting conversation last night in a restaurant (I admit I cannot verify what was said so it's definitely rumour only).The gist of it was that dams like this were built with side spill outlets controlled by valves. These are meant to be used to control the maximum water level, to prevent over-topping of the wall but unfortunately these valves could no longer be opened at this dam, because the mechanism has rusted solid over many years.