[cjslep]

Accounting for failure frequency due to natural disasters occurs at the design (initial or retrofit) stage, not the maintenance stage. Should every mom-and-pop dam adhere to the once-in-ten-thousand-year-rule (eg: nuclear powerplants) or once-every-hundred-years (eg: steel framed commercial construction)? Cost effectiveness is also a huge concern.

Note that maintenance doesn't increase the expected failure frequency, it maintains it at the design spec. And I'm willing to bet that even with proper maintenance those small dams in SC would not have survived; they were way outside their designed specs.