i work on hydro electric power stations and often see drawings showing the 50, 100, 500, 1000 year flood water levels at some structure such as the powerhouse o

i work on hydro electric power stations and often see drawings showing the 50, 100, 500, 1000 year flood water levels at some structure such as the powerhouse or intake. The civil engineers design the plant to withstand the 500 year flood. I've personally seen the 100 year flood level occur at about 7 different projects, and I only work on site at 1 project per year generally.