Dam Failures in Derna, Libya 25

[SSCon]

https://eos.org/thelandslideblog/the-failed-dams-in-wadi-derna-in-libya

 

[SSCon]

In case it was missed. I could find every other article from that edition of the journal but the relevant one. Maybe someone else's google fu will be better.

https://www.hidrotehnika.rs/en/libya/wadi-derna/

The dam break is a very complicated problem and it can’t be foreseen exactly. Many researchers have studied the problem of dam break by predicting the behavior and the propagation of a flood wave using different approximations. In the present research, the study of dam break was performed using a mathematical modeling as a tool to predict and simulate the dam break problem. Two major mathematical models were employed namely BREACH and FLDWAV for predicting such failure parameters as well as simulating the flow routing after dam break. Derna dam (northern east coast of Libya) was taken as a case study of an earthen dam. So, collected information about Dema city and Derna dam were included in the current work. Piping failure was considered the most predictable one for Dema dam. For the current study it was shown that dam break cause a potential danger towards Dema City, since flooding occurs in the city for all hypothetical scenarios. It was recommended to predict the routing of the flow after failure due to lowering the retained water surface elevation in the reservoir and due to the use of levees and increasing walls heights in the city area.

 

[1503-44]

Thanks both. I'm just gonna use those retention values and not worry any more. Seems to have converged, on 18 and 1.5M. If I need to get dam height, I'll put that much water in the lake valley profile and calculate a level at the dam location. But I might not get into it that far. I'll just go with those retention volumes for now.

Humm, yeah, that Derna link is circular.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[LittleInch]

I've not seen anywhere any video or eye witness about the movement of collapse of either dam so we don't know if it failed before overtopping or was just overtopped, and then promptly worn away very rapidly then water flow continued as per the storm surge water.

As said before, it looks to me like the past 20+ years resulted in virtually no water being stored for any appreciable period of time behind either dam so this could have lead to the the dam drying out in the heat and then becoming stressed when water arrive dint he space of a few hours or less up to its max capacity.

Or the initial period of rain filled them up and then they got hit by the peak hour or two of rain which did the damage.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

It seems like if it was a spillway/drain problem, things would have happened just a bit slower than they did. I saw one story where someone was out making a video of the dam filling up, like at midnight. No video posted. He went home, then at around 01:30 he heard a very loud noise and was swimming a few minutes after that. I'd say it was a quick failure of the dam itself. Drainage system failure and internal washout of the drain works would probably take longer than an hour or so. Just guessing of course. That's as much as I've been able to deduce about the time line of things. Also fits with that supposed 20m initial wave someone (was it you?) mentioned. The loud noise and nearly immediate flood also fits. Would a tunneling erosion through the drain come with a loud bang? It also seems like the flow would have come a bit more gradually. Dam burst fits better I think.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[dik]

My uncle Dennis was a geotekkie and was involved with the Portage Mountain Dam in BC. I wonder how any possible rainfall increases are addressed?

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[1503-44]

My logic leads to increasing bypass capacity. Adding an additional one on the other side?
Or as a temporary or operating work around, you might be able to reduce the level permanately, although you'd lose power gen capacity if it was hydroelectric. There are times when high rainfall accumulates not due to extreme events over several months, then when a storm is forecast, they will prerelease water, before the storm arrives, to make room for the expected catchment. I think that must be the typical solution. Problem is, like this one, if high intensities become frequent and can overflow an almost or completely empty dam, it becomes a white elephant. You'd always be running on near empty and even empty is a risk. Only choice would be increase bypass, or demo.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[Just Some Nerd]

In my home country Australia, we introduced an explicit factor in our wind code for cyclonic regions to account for the changing climate. Right now that factor is 1.0. But it was deliberately introduced acknowledging the currently quantifying increased risk.

Minor correction...the climate multiplier for wind speeds is already 1.05 in regions B2, C and D.

----------------------------------------------------------------------

Why yes, I do in fact have no idea what I'm talking about

 

[TugboatEng]

As populations grow, the supporting structures must also. This often means upwards. As structures grow upwards they get less protection from the boundary layer effect so it makes sense that the wind loading factor should also be increased.

 

[1503-44]

But (I presume) the factor is independent of height. It's a climate dependent factor, apparently set by region. Basic wind loads already have a height factor.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[LittleInch]

Dams which remain full of water and drain areas with a high silt load gradually fill up and hence the volume available to store water in the event of a large rainfall goes down relative to water height.

Most dams seem to be lifed for about 100 years at which point the volume of water they can store is about 10% if the volume it started with.

Even hydro dams basically just start to create a channel in the silt down to the hydro entrance but min water level will just keep going up.

These dams were essentially storm water holding "ponds". Only problem with that is no one seemed to think too much about what happened if the storm was bigger than the holding volume. So even if they both held ( say were made of concrete), the impact would still have been about as bad as the whole drained flow would simply overtop the dam and flow down like it would have in previous floods - the only saving grace being possible time to warn people and get an evacuation going - but difficult in the middle of the night in the middle of a hurricane in a country without a real functioning government.

Actual failure of the dams, especially the much larger one was a true disaster as it unleashed a HUGE wall of water onto the unsuspecting inhabitants, but very little would have stood up to that sort of rain fall. Only way around this sort of thing is to have good monitoring of the water levels and rain fall predictions, an effective alert system (sirens etc) and a population trained and able to move to higher ground. Some properties might be better to stay in place to avoid large numbers being caught in the open.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[hokie66]

And that, my friends is the story. Thanks, LittleInch for the summary.

 

[dik]

As far as the flooding goes, this sums it up pretty well.

"Climate change made the storm that devastated the Libyan city of Derna, killing thousands of people, up to 50 times more likely, experts say.

Up to 50% more rain had fallen as a result of human-caused greenhouse-gas emissions, climate scientists at the World Weather Attribution group found.

Years of conflict in the region compounded the vulnerability of people to flooding, the WWA report says.

And it turned the extreme weather into a full-scale humanitarian disaster.

The scientists used computer simulations to assess how much more likely such a storm was now compared with before the 1.1C of warming climate change has already brought.

But they cautioned a lack of data, particularly in Libya, meant considerable uncertainties in their findings."

https://www.bbc.com/news/science-environment-66854670

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Alistair_Heaton]

Just think about the one just filled in Ethiopia

 

[dik]

Is Egypt downstream...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[3DDave]

This disaster is an economic one, not engineering. There is a town called Valmeyer, Illiois that had enough money to move from the flood plain along the Mississippi to the bluffs above them. Problem solved, right? They were medium income and got FEMA money to do so. The same cannot happen in Libya.

I'm sure the other difficulty for evacuation would be that the streets would already be flooded and running fast from the storm. Possibly no power and no lights, few cars, so it would be people on foot running 5-10 miles in torrential rain, cold and threatening hypothermia, and the water already hiding obstacles and carrying debris. Few would have storm gear suitable for conditions. Where would they run to? Up steep slopes with water cascading down them?

I expect many of the dead were elderly or disabled.

There would need to be a huge facility for them to take shelter in. How would that be built and maintained?

The most telling part of the horror of the situation is lack of video recording the floodwaters sweeping through the city. Some before. Many after. None from the surrounding buildings that survived.

 

[AZPete]

1503-44,

Roll time back to 2020.

Using any information available, what do you design the dam capacity to be?

 

[1503-44]

Dont know just yet. I have the the wadi drainage area and the main channel mapped out in GEarth and just got the main channel,s elevation profile. I still have a few side channels to map and extract the slopes. I will build a Time of concentration model in Excel. Then I'll superimpose various rain intensities and their durations to calculate what the flow rates are at the dam locations and the safe flow rate that will not overflow the channel in the city. The difference of flow into the dam - safe flow out going into the channel through the city is the flow rate that should be stored in the dam.

The channel slope is 0.007, falling 482m in 70km. I don't think that is too bad a slope.
What makes this system bad, IMO, are

#1) lack of vegetation to soak up rain and mostly rock surface with high runoff coefficients and
#2) that the basin parallels the costal ridge and a lot of it is at 500m elevation.
#3 mostly very narrow channel that won't spill over and spread out into flat areas along the way down. What enters the channel isn't going anywhere except straight down the channel. The high head may keep water the flow moving fast even if the slope is not so great.

I live in a similar topography and when warm, wet air comes off the ocean and gets uplifted as it hits and climbs up the ridge, it usually starts raining first and quite heavily up on the ridge, right at that 500m elevation mark. That's about two km from me and here the sun is shining and you have no idea its raining cats & dogs up there until the cool air starts rolling down the slopes. 500m is just enough temperature drop to start squeezing rain out of saturated air. The entire basin probably took a good hit.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[1503-44]

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[TugboatEng]

Up to 50% more rain had fallen as a result of human-caused greenhouse-gas emissions, climate scientists at the World Weather Attribution group

At least they aren't hiding it anymore.

 

[TugboatEng]

As a follow-up, instead of trying to attribute this disaster to climate change so that funds can be misallocated, why not do something beneficial? Telling countries like Libya and Pakistan that we're going to reduce our GHG emissions isn't going to clear the silt from their flood control basins. Instead of trying to end the Western world as we know it, perhaps the UN should assemble some engineers to study flood control in historically flood prone areas with growing populations and poor political representation. That doesn't even mean building new dams, a simple survey could have warned Delma of the risks and encouraged residents to move to higher ground. We could build infrastructure, too. We'd probably get more bang for then buck in terms of lives saved building infrastructure in Africa and Pakistan than sending bombs to Ukraine.