Herschel Henderly
Herschel Henderly was interviewed by Katy Barber in Eugene, Oregon on 23 October 1999.
Since 1980, Herschel Henderly has tended Cottage Grove and Dorena dams from an office overlooking Cottage Grove Lake. Drawn to the job because of the beauty of the area, Henderly’s position demands that he be proficient in public relations, the enforcement of recreational regulations, and flood management. He is just as likely to be found on a tractor as at a desk which is why, he says, he has stayed.
They are [Dorena and Cottage Grove] earth fill embankment dams with concrete control structures. I think that’s pretty much the appropriate terminology.
The construction is determined by economics. It’s a lot cheaper to build an earth fill embankment with a clay core, what we call an impervious clay core, hopefully the clay is there to keep any water from seeping through. That’s why we like to call it impervious. Anyway, it’s a lot cheaper to build the earthen embankment than it is concrete and you only really need the concrete where you’re controlling the water discharge so, for example, at Cottage Grove, we have a couple thousand feet of earth embankment and about two hundred feet of spillway. But that’s the critical part.
Cottage Grove has three regulating outlets. Those are the tunnels through the bottom of the dam that control discharge and Dorena has five because it’s bigger and it need to be able to discharge more water. Physically, each of these is designed to have as many redundant systems as possible within reason. For example, on each of these regulating outlet, these tunnels, you have two gates to control the discharge. The upstream gate is always all the way up and we operate with the downstream gate. If the downstream gate needs maintenance or gets stuck or plugged or there’s any problem then we can always close the upstream gate to take care of the maintenance on the downstream gate. If we need to we can operate with the upstream gate. If everything goes to heck you can always close off the intake on most dams with steel plates lowered down from a crane. Not on these so well but on most dams you can.
We have commercial power to operate the gates with and a backup generator. Most gate operations are done by remote control from Lookout Point by an operator via computer and telephone lines. If there’s any problem with the telephone lines he can give us a call on the radio and we can go down and do it by hand. So there’s quite a bit of redundancy built in. There’s never been a case that either dam in 50 and 57 years they’ve been in operation where it was not possible to make a gate change when one was needed. And of course they’re designed that way because if things are going to go wrong of course it’s going to happen when you need it the worst. When there’s a storm and you’ve lost commercial power and there’s trouble getting through and the telephone line’s down and things like that.
Both dams have what we call uncontrolled spillways. That is, we have a concrete spillway to take any water that cannot pass through the regulating outlets with very high walls on each side so the lake has to get really high before it could possibly go over the earth fill embankment. The reason for this is that if water starts to spill over the earth fill embankment you are going to lose the dam very quickly. Erosion sets in and it will just cut right down through it like a knife through warm butter. You can lose the entire – I shouldn’t say the entire dam – you’ll cut all the way down to the bottom and lose the entire lake in just a matter of hours if that happens. So the design is such that under almost any possible realistic situation and that would be starting with the lake pretty near full because of repeated storm events, saturated soils, heavy snows in the hills followed by a heavy warm rain washing everything off at once and flowing downstream, you’re still going to be able to pass all the water coming down over the spillway through those concrete side walls.
