[cc438]

The threat posed by small dams and their short designed life expectancy was demonstrated in dramatic fashion earlier this year. The historic flooding that impacted South Carolina's midlands earlier this year was driven by a cascade of dam failures.

Everyone saw the pictures of Columbia, SC but that flooding was mostly due to "normal" causes. The city's drainage system was overwhelmed by rainfall that exceeded historic levels by several orders of magnitude (higher than even a 1,000 year storm) and the Broad river consequently overflowing its banks. However, that wasn't the only place impacted by the storm. South Carolina has an enormous number of small earthen dams, built on private land with the land owner responsible for maintenance.

Obviously, these dams were not built to handle such a storm but even if they were, the lack of oversight led to a lack of maintenance, worsening their chances of surviving the initial floods. As upstream dams broke, downstream dams were subjected to massive surges, causing a cascading series of failures, greatly increasing the amount of damage caused by the storm.

There are 10,000 to 20,000 unregulated small dams in the state according to the state. They pose an enormous risk as the flooding clearly demonstrated but SC isn't alone in sharing this burden.

[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.