Starting to wonder if figuring out hurricanes is nothing more than getting enough sleep and having the map at the right zoom level.
Around the same time as reports about the dam started coming out, we also got reports of significant structural damage at Nacogdoches and Lufkin, Texas.
On the map, those two cities and the dam (plus the towns of Livingston and Coldspring too) are all in a straight line. That points to some kind of a downburst type event, not unexpected in a hurricane which is in the process of coming ashore and collapsing.
Warm air always wants to rise and the process of condensation generates tremendous heat, which accelerates the ascent. But what goes up must come down, and in certain collapsing systems, a sort of unnatural equilibrium can occur, where the system is in stasis, but with tremendous potential energy, in the classic case of potential energy.
A huge mass of barely rising or falling wet air can be a time bomb waiting to happen, and when the right trigger comes along, it detonates.
A small pocket of descending air can accomodate more moisture as it warms adiabatically, and under the right conditions, it does, entraining moisture from the surrounding air. The process is essentially evaporation when viewed in terms of the overall energy budget, which removes that same tremendous quantity of heat and (again, under the right conditions) can initiate a cascade. The more that descends, the more that cools. The more that cools, the more that falls. It can be a very vicious cycle, I've seen marble headstones snapped clean off six inches above ground level across a fair sized cemetary resulting from a thunderstorm's collapse.
I think there's a good chance we had some sort of a downburst event along the line from Nacogdoches to Livingston, and that that downburst caught hold of a significant volume of already choppy lake water and shoved it up against the dam.
If all that's true, I predict you'll find wind damage at Cold Spring, southwest of the Lake, and possible surge damage on the southwest side of the lake as well. An aerial survey should also show a clearly defined damage path between Nacogdoches and Lufkin, and between Lufkin and the Lake.
In any event, the "wall" of water would have smacked into the dam at an oblique angle and at the very least, removed the riprap (chunks of old broken up concrete roadbed are sometimes used) facing. Odds are that there was some cratering on the upstream face as well, since the removal of tons of broken concrete or rock doesn't happen without affecting the underlying earthen fill. It may well have topped the dam and done some cratering on the other side, notched the dam's crest, transported some fill around, whatever it was that they first saw that raised the possibility in their minds that they could lose the whole dam.
You don't deliberately create a full scale flood without serious reason. The question then becomes how big a wave hit the dam. Enough mass, moving at a high enough velocity can raise questions as to the entire structure's integrity.
You can bet that they are watching the downstream face of the dam for any sign of seepage, which can take some time to manifest after any cracks or percolation are effected on the upstream face or in the core block structure. One tiny crack breaching the integrity of the core block (usually clay, sometimes concrete) can be all it takes, but full failure may take days to occur afterwards. Once water reaches the downstream face unnoticed, it's usually all over. It's very hard to stop water once a quantity of it starts flowing.
In the meantime, they get the water levels low as fast as they can. It allows them to see more of what has been damaged, and it reduces the pressure on the now questionable structure.
Unless you go looking for it (or the dam fails), you may not hear much more about this. Dam authorities like for the people downstream to regard their dams as monoliths, unfazed by wind or storm, always safe. There will be an assessment, and several reports released, but they might not be calling press conferences when they do.
This case may be an exception to that general policy, because once you roust the downstream residents out of their homes for an evacuation, there is also a need to reassure them that all has been inspected and pronounced safe.
We'll just have to watch the news and see what comes out.
Awesome post Jeffers.
Had Rita stalled in the Lufkin or Nacodoches area I believe Lake Livingston dam may have gave way.
The flood may have topped the dam that already had some sort of damage. Not good.
I wonder if they will drain the entire lake to check for leaks, and how bad the actual damage is/was.
The lake appears to be dropping about an inch per hour with the 80k cfs release.
Ps: Ive lived in tornado alley for 42 years and the worst wind ive seen was from a downburst.