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Posted on 09/16/2017 5:47:46 PM PDT by NautiNurse
Hurricane Jose has been hanging around, waiting for attention in the wake of Hurricane Irma. Newcomer Maria threatens to impact the Caribbean Islands already devastated by Hurricane Irma, and brushed by Jose. Hurricane Jose threatens to brush or impact New England. Lee appears to be a fish storm at this time.
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The immediate outflow area from that dam is not heavily populated, per Google maps. It feeds into a canyon surrounded by public forests, but near the coast, two population centers, Quebradillas/Cacao, and Isabella, pinch in on it, so the key to this event will be depth of the outflow versus capacity of the canyon.
I haven’t found a heigth on the dam yet, looking.
USGS website for Gaujataca Dam:
https://pr.water.usgs.gov/public/rt/pr_lakes/lake_50010800.html
It holds back zero acrefeet at water elevation 557.4 ft. It holds back 34276 acre feet at elevation 646.0 feet above mean sea level. That indicates a head height of an instantly failing Dam of 88.6 ft.
A water level graph shows the level topping out at 646 ft so it’s full, it topped, and erosion from water spilling over the dam is most likely cutting a V-Trench, into the crest of the embankment.
That means it probably won’t fail all at once and you won’t see an 88 foot high wall of water roaring down that Canyon. A very rough estimate in cases like these historically, is around a half the head height, so up to 40 45 feet.
On the minus side, the outflow from that Waterway drops 650 ft in the 10 miles between the the dam itself and the coast. That will add a lot of energy to what will become a wall of water with a whole bunch of dead national forest trees, both recently standing, and down by the hurricane.
I saw one report that put the dam height at 120 feet. But the USGS numbers indicate actual spillway height, not the total height of the embankment so for now, the 88 foot figure is a safer bet.
I saw one report that put the dam height at 120 feet. But the USGS numbers indicate actual spillway height, not the total height of the embankment so for now, the 88 foot figure is a safer bet.
I’ve looked around quite a bit trying to learn just what authorities mean by “dam failure”. All I have learned is the failure is somewhere at the northern part of the dam. As yet they’ve not specified the nature of the problem. So is it a crack? Erosion underneath? Overtopping? Orrrrr???
The lake/reservoir holds some 11plus billion gallons and is over a square mile, closer to two..
The NWS twitter is keeping up but they have very little info themselves.
I hadn’t read all of your posts before I replied. I hoped it wouldn’t be that huge wall of water at once. It’s a nightmare. I read they’ve evacuated nearly all of Isabella but seemed to have added other areas.
Scroll down a little to the Twitter post from Weather Nation to see video of beginning dam failure. It’s starting slowly as you said, and you can see what’s probably goung to happen.
Looks as though a lot of the spillway has already been washed away.
Agreed, but the spillway is currently functioning properly, as of that video.
By the nature of the spillway, it’s an uncontrolled release, there’s no gates, they can’t stop it.
The heavy outflow has damaged the spillway and it’s headcutting, working its way up the embankment, but it has a ways to go and the embankment hasn’t failed yet.
The outflow is largely contained within the river channel...at least there’s not orders of magnitude difference.
The secret to longevity of earthen dams is zero penetration. Compacted soils will not withstand any surface flow. If the water gains access to compacted soil, without the “overshoot effect” from the remaining spillway liner, it will cut deeply and quick.
At that point, things could get dicey. That dam is wide and low, and it feeds a narrow canyon. You could see...amplification...of head heigth in those conditions, but it hadn’t happened when that video was shot. The rain will taper off, followed by reservoir inflow. It could hold. There’s actually an increased risk when the velocity of the outfliw begins to taper off.
At that point, the overshoot advantage goes away, and headcutting rates are likely to increase.
The dam management made the rght call to evacuate, and they may have time to pull it off. I didn’t see additional floimod control structures upstream, but they...could ..have options there. Same watershed loads happening there though.
News headlines are precisely correct. This dam is failing, it’s at risk, but it hasn’t failed yet, and there’s no certainty it will. Oroville in California was a LOT closer to a catastrophic release than this video indicates. A factor of ten to one...ish...assuming the video is current.
Agree Oroville was *this* close. But it seems the advantage they had was being able to get in there fairly soon and stop the potential catastrophe. Here, we have resources already strained, and it’s a question of how soon even emergency repairs can be made.
I didn’t see the time stamp on the video but it had to have been sometime around 2 pm. ( edst or ast, not sure). How much worse has it gotten in the last several hours I don’t know. I was interested to read the flash flood warning for the area is until 2 am. Not clear to me why they chose that time,but tthat’s it for npw.
Managers at Oroville re-opened the damaged spillway to take pressure off the emergency spillway, and they got away with it.
There’s nothing to be done at Guajataca. Inflow, outflow, and erosion are simply going to do as they will. They may have options upstream, but not locally, and the whole watershed is under pressure, limiting those options, if they exist.
The sun is pretty low in the video you linked. Eyeball squint estimate, 5-6 pm? Problem emerged at 2:30 pm edt, call it 3:30 local, as a guess.
Where the powerlines cross the spillway look to be remains of “dragon’s teeth.” Huge concrete blocks placed to break up and slow velocity of outflow.
A whole string of assumptions here...but...
In two hours, 30 to 40 percent of the horizontal spillway armor, above dragon’s teeth, washed away. Real problem potential, then, as of 8:30 pm EDT. Or,sooner, if an alternate channel develops. Or later, if increased flow velocity and overshoot slows head-cutting.
Variables yes. Options, few.
https://mobile.twitter.com/GSierraZorita?p=s
@GSierraZorita
20m
Police being evacuated from town of Isabela in the event of a collapse of the Guajataca reservoir. #MariaPR twitter.com/ricardorossell…
Well...this is the most recent info on Twitter. This info comes from Ricardo Rosello, Governor of PR.
Looks like your scenario is starting to play out.
Can’t see how much more misery this island can handle.
That's good to know... thank you.
The overhead image at Google maps shows two areas of potential concern on the downstream face of the Guajataca dam.
One near the right anchorage, one near the spillway. Both show potential percolation points thru the embankment and erosion channels, though not at alarming levels, if that’s what they are.
The video from last night, however, MAY be more troubling. For a very brief moment, the right abutment problem looks deeper and actively flowing. The area above the damaged part if the spillway looks to be a...fissure (not the first time this word’s been used), but I can’t tell if water is flowing there or not.
Active flow at either location would be a negative development of the highest concern.
0:18 in this video, upper left corner, embankment toe, right anchorage. What do you see there? Above and below the muddy pond that’s dry on Google maps? Is that an active flow? If it is, I can think of about one possible, historically likely, outcome to this situation.
https://www.youtube.com/watch?v=Z1Gt9R1hqoM&t=15s
Pretty amazing video. The trees....stripped.
So I THINK, though I may be disoriented, that around 00:40-00:50 in, is your top left corner at 00:18. If so, it’s not good as a secondary flow would seem to have begun.
Very tough to find updated info, though I read the governor said (last night) there’s significant damage and evacuation must continue.
Today there’s evac beginning near a lake called Toa Vaca.
Nothing new I’ve found either except a few quotes:
“...an engineer reported a ‘contained breach”..”
Breach means path way for flow, from upstream face to downstream face”, and an engineer would know that.
Governor today said “significant structural damage...” Last night said “fissure”.
CNN’s original article said “...a fissure...” and “...a crack...”
On Google and other aerials, you can see an intake structure of some kind above the area we are discussing, in the lake. But Google also shows that same area, on the downstream toe, looking like...an unaddressed but dry series of potholes. Not anywhere close to as “engineered” as that spillway. If this is an outlet structure, it’s pretty jank.
Speaking of Jank, (note capitalization)... NW of Ponce is the spanking new Portugeus dam, built by the ACE. Very posh.
ENE of Ponce, we have the...venerable...ahhh, dirtpile, holding back Lake Cerrillos. Sketch looking wier type emergency spillway, ala Oroville, no other options, and well...look at the footprint. It’s crooked. The transit must have been broken that day, and who-ever eyeballed it had a snootfull.
If you claim Toa Vaca in the pool, I get dibs on Cerrillos.
That looks like it turns before it gets to Chesapeake Bay where I have a bunch of grandkids. I like the outer banks and visited there recently. I know they got hit last year.
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