I'm learning new things all the time about reservoir jargon. Today I was exposed for the first time to the "sediment layer" of a reservoir's water supply. Of course I've known that sediments settle and collect behind dams, sometimes to the extent that the volume of water in the reservoir shrinks over time. But more often, much of that sediment remains in suspension in the water, sinking to occupy the bottom level as what we'd recognize as watery mud.
The drought in Raleigh, North Carolina, has just about driven the region to begin providing water to residents from this sediment layer, according to the NewsObserver.
But once the muddy water has been tapped, there's other local source to draw from. Future planning relies on the return of the rains to refill reservoirs, after which the local and state governments will resolve to maintain higher standing levels to guard against future drought.
A federal study in the 1990s found that Falls Lake's bottom is filling more slowly than expected, leaving more room for water than first planned. In theory, the lake's bottom layer holds enough water to supply Raleigh and the seven other Wake County towns it serves for two to three months.
Under the proposal, the Army Corps of Engineers would sell up to about 6 1/2 billion gallons of the bottom water in four increments as needed. That's up to 87 days' worth at the lake's current draw-down rate, assuming no rain. That would come after Raleigh exhausts its regular supply, which by the most recent estimate on Tuesday stood at 113 days left.
Raleigh's City Council voted this week to ask the corps to let the city tap permanently into the surplus at the bottom. A permanent reallocation of half the lake's bottom layer would boost the city's supply by more than one-fourth -- until inevitable sediment accumulations erased it.
In a sense, the best-case scenarios are that drought periods in the future are no longer than a couple years at a time, an assumption that can no longer be considered reliable.