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Damming News From Washington State
Trax3001BBS writes "A 65-foot (20-meter) crack has been found in Wanapum Dam, one of the major dams along the Columbia River in southern Washington. Water levels are being lowered to both reduce water pressure and give the inspectors access to the area. 'Earlier this week, an engineer noticed a slight irregular "bowing" above the spillway gates near where cars can drive across the dam. When divers finally took a look under water they found a 2-inch-wide crack that stretched for 65 feet along the base of one of the dam's spillway piers.' The article goes on to say, 'Even if the dam doesn't fail, the significance of the damage is likely to require extensive repairs and that, too, could impact the entire Columbia River system. "All these dams coordinate to generate energy on a regional scope," Stedwick said. "If Wanapum is impacted, that has impacts on dams upstream as well as below." Upstream dams would be required to handle more water; there's only one lower dam (Priest Rapids). After that is the last free flowing section of the Columbia river. I've taken walks along that section, and I've seen it deviate (higher or lower) by amazing amount of water, so it can handle the changing flow rate. Making this situation more complex, a large group of people would like that particular dam removed, as well as the one above and below it (think of the fish!). On top of that, after the Priest Rapids dam (downstream from Wanapum Dam) is the Hanford Nuclear Reservation, once a site for Plutonium production. Either of these issues could generate a ton of attention. Personally, I'd like to give the engineer that noticed a slight irregular 'bowing' my congratulations."
Water doesn't "soften" concrete. H2O is molecularly bound into its structure and is a necessary part of maintaining the strength of concrete. Water invading earthen dams, on the other hand, is a more serious problem.
I am becoming gerund, destroyer of verbs.
It has nothing to do with the moisture. It has to do with pressure. A dam represents a boundary layer between high pressure (behind the dam) and low pressure (air in front of the dam. An intact dam's structure distributes those stresses evenly throughout its structure, and transfers them into the mountains/hills at the sides of the dam. A crack shifts the stresses which would've been borne by the cracked section to the uncracked sections, and particularly at the corners of the cracks. The high stresses at the corners cause the crack to grow. Eventually the crack grows large enough that the stresses are more than the uncracked section can withstand, and the dam fails.
That's what the engineer noticed. The dam was bulging because the uncracked section was holding back so much more stress than its design load that it physically deformed. 65 feet is a damn big crack (no pun intended). That engineer deserves a ticker tape parade in his honor.