4PM Tuesday 9/12/2023 – A Libyan flood in Derna (also spelled Darnah) killed thousands and authorities say more than 10,000 are missing.

At this hour, exact numbers are hard to come by as large areas are still cut off from rescue workers and reporters. But to put those estimates in perspective, Harvey killed 36 in Harris County.

The Cause: Months of Rain Fall in Day

Torrential rains from Mediterranean storm Daniel burst two dams, wiped out four bridges and flooded large areas of Derna. The flash floods swept homes, bodies and vehicles into the sea while knocking out communications in large parts of the area.

Daniel dumped months of rain in one day on a large swath of northern Libya.

Libya is normally one of the driest countries in the world. The Sahara dominates most of the country’s climate. But the Mediterranean also influences rainfall along the northern coast.

Wikipedia says the annual rainfall in Derna is around 11 inches. But CNN reported that in Derna, eight months worth of rain fell in one day.

In Al Bayda, about 50 miles west, Libya’s National Meteorological Centre said that 16 inches of rain fell between 10 Sep 8am and 11 Sep 8am, a new rainfall record.

Weather Spark shows that average precipitation in Derna during September amounts to less than an inch. And even in the wettest years, Derna receives at most 4 inches per month.

Weather Spark also indicates that Derna receives rainfall on less than one day in an average September.

Terrain Funneled Floodwater Toward City

Derna lies along the coast of the Mediterranean Sea, at the end of a long, narrow, natural valley, called a wadi, which is dry for much of the year.

As the city was inundated by Daniel, which made landfall in Libya on Sunday night, the wadi funneled rushing water into the center of the city. Riverbanks swelled, bridges were washed out and two dams farther up the wadi burst, adding their waters to the deluge.

Tropical-like storms rarely happen in the Mediterranean, but they do happen occasionally. And when they do, meteorologists call them “Medicanes,” short for Mediterranean hurricanes.

World and Social Media Focus on Flood

YouTube is awash in videos from the Derna flood.

NBC reported that a quarter of the city of 125,000 people was destroyed.

This CBS story shows pictures of the incredible destruction. This flood didn’t invade homes slowly, but leave them standing. It pummeled homes and whole neighborhoods.

Google “flood Derna Libya” to find hundreds of links and the latest news. Stories are being updated almost hourly.

Parallels With Harvey

Perusing many of those stories revealed several parallels with Hurricane Harvey which struck the Houston area in 2017.

Rainfall Records

Both areas encountered record rainfalls. Harvey dumped far more rain than Daniel. But then Daniel’s rainfall fell over a much drier area where infrastructure was designed to handle smaller amounts. Still, both were records, and both overwhelmed local infrastructure with unexpectedly high volumes.

Dam Concerns

Both Houston and Derna lay downstream from dams. During Harvey, the SJRA cited fear of failure of the Lake Conroe Dam as the reason for its massive release. In Derna, the dams just failed.

Lack of Warning

During Harvey, downstream residents received no warning that Lake Conroe would open its massive floodgates. Many stepped out of bed in the middle of the night to find themselves in knee deep water. CNN reported that there was no warning that the Derna dams could fail and no order to evacuate. Many who tried to evacuate found themselves cut off by collapsed bridges, washed out roads, and rising water, just as in Houston.

Lack of Maintenance

Aljazeera indicated that lack of maintenance may have played a role in the Derna disaster. The dams that failed hadn’t been maintained for years, even though severe erosion had been reported in scientific journals for more than a decade. Likewise, deferred maintenance played a role in flooding the Lake Houston Area during Harvey. During the drought in 2011, the water level in Lake Houston fell so low that the City of Houston could have removed sediment with dump trucks instead of dredges. But Mayor Anise Parker refused to do so. The accumulated sediment later wound up blocking the San Jacinto West Fork and contributing to the flooding of thousands of homes in the Humble/Kingwood area.

Enforcement of Building Codes

In Libya, political turmoil since 2011 reportedly distracted government from the mundane task of enforcing building codes. As a consequence, many hastily constructed buildings went up without adequate safeguards or supervision, and were swept away during the flood. Sound familiar?

Learning from Disasters

We should learn from the past and others’ mistakes. I wonder how many other communities have been surprised by:

• Record rainfalls
• Dam failures or unexpectedly high dam releases
• Warning systems that failed and left people with no time or way to evacuate
• Impacts of deferred maintenance
• Lax enforcement of building codes

I hope the world learns the lessons of the Derna disaster so that we can compare them to other floods and get smarter about mitigation measures that make a difference.

Posted by Bob Rehak on 9/12/2023

2205 Days since Hurricane Harvey