Why in the WORLD would you drain a reservoir . . .
- Thread starter jaym
- Start date
The short answer is because the BUR believes - based on experience gained during the massive flood of ‘83 - that the dam as built cannot be safely operated with the river outlet works at 3370’ MSL once the lake drops below minimum power pool at 3490’ MSL.
The risk is that cavatation will eventually destroy the concrete linings if they are used for very long, exposing the soft sandstone walls which will quicky erode to the point of total failure. As big a disaster as that would be now, in the past the risk was even higher, as the total failure of the Glen Canyon dam would have resulted in overtopping the Hoover Dam and its loss as well. Today, Lake Mead is low enough to contain all of Lake Powell if it should fail for any reason.
That just addresses the direct reason it might get drained, there are other reasons to drain it and just as many to keep it, but keeping it is contingent on keeping it above minimum power pool. Another winter like last year and that may not be possible. And if the Basin’s can’t agree on equitable reductions in use, one or two good winters won’t solve the basic problem of over allocation for the new normal climate anyway.
The scary part is even if the decision were made to drain Powell, there’s currently no feasible way to do it. The engineers didn’t put a drain at the bottom of the dam and installing one after the fact is neither cheap, nor easy (from both engineering and legal viewpoints)
The risk is that cavatation will eventually destroy the concrete linings if they are used for very long, exposing the soft sandstone walls which will quicky erode to the point of total failure. As big a disaster as that would be now, in the past the risk was even higher, as the total failure of the Glen Canyon dam would have resulted in overtopping the Hoover Dam and its loss as well. Today, Lake Mead is low enough to contain all of Lake Powell if it should fail for any reason.
That just addresses the direct reason it might get drained, there are other reasons to drain it and just as many to keep it, but keeping it is contingent on keeping it above minimum power pool. Another winter like last year and that may not be possible. And if the Basin’s can’t agree on equitable reductions in use, one or two good winters won’t solve the basic problem of over allocation for the new normal climate anyway.
The scary part is even if the decision were made to drain Powell, there’s currently no feasible way to do it. The engineers didn’t put a drain at the bottom of the dam and installing one after the fact is neither cheap, nor easy (from both engineering and legal viewpoints)
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And this question is for JFR and the other experts. Can the dam be operated as though 3490’ was in fact dead pool? IE, let water out through the turbine inlets without generating any power? Just let the turbines freewheel. Let outflow equal inflow as if the lake were at 3370’. That at least would preserve the river outlet works which could remain closed. But I don’t know if that’s even possible.
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Trix
Keeper of San Juan Secrets
Various discussions have explained that the turbines would very likely be damaged if opened below 3490. Apparently freewheeling is not an option. So, as I understand it, running turbines below 3490 is almost certain damage. Constant flow through outlet works may cause damage, although the tubes have been recently coated in hopes of preventing damage.
Also, apparently, running no water through the dam until it’s above 3490 is not an option; don’t remember reading that explanation. Dead humpback chubs and trout, of course.
Also, apparently, running no water through the dam until it’s above 3490 is not an option; don’t remember reading that explanation. Dead humpback chubs and trout, of course.
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My understanding was that the bottom of the turbine inlets IS 3490’ and it’s not possible to be “running the turbines below 3490’” because below that level the inlets would simply be dry. I have heard (on here) that the BUR is actually afraid to run the turbines with the lake below 3505’ due to the risk of cavatation damage to the turbines themselves as the water may “cone” into the inlets allowing lots of air into the incoming stream. This might prevent operating the dam as though 3490 were dead pool as I described above, I just don’t know if it’s unsafe (due to possible cavatation), or simply not possible to allow the turbines to freewheel anyway, or if freewheeling even reduces the cavatation risk in the first place assuming it was even possible.
