On Sept. 9, 2023, the storm Daniel, after striking Greece on the previous days (Sept. 5-6-7, 2023) with torrential rains (over 600 mm of rain in Volos), moved toward the Gulf of Sirte area and, as a result of favorable weather and sea conditions, made the transition to a Mediterranean hurricane (medicane). On Sept.10, Daniel made landfall along the eastern coast of Libya.
The ASCAT (Advanced Scatterometer)-C sensor measured around 19 UTC winds at 10 m around 80-85 km/h (Figure 1). These are remarkable values, given the satellite product’s spatial resolution of 25 km. ASCAT is operated by EUMETSAT and operates at a C-band frequency (5.255 GHz), which gives the instrument an operational observational capability in any weather situation and for any type of atmospheric structure from the mesoscale.
From the precipitation point of view, the IMERG satellite product (Integrated Multi-satellitE Retrievals for GPM, NASA) provides, at spatial resolution of about 10 km, remarkable values with daily cumulative, on September 10, up to 300 mm, near the Libyan coast.
Hydrological modeling showed a significant signal regarding the response of all basins in northwestern Cyrenaica as early as Saturday. The flow rates predicted by the modeling chains available on EFAS and GloFAS showed the rarity of the phenomena, estimating a return period well above the 1-in-20-year threshold, which can be defined as severe or locally even at the 1-in-100-year values and, therefore, extreme.
The intensity of the phenomena was also amplified by the characteristics of the area, covered in sparse vegetation cover and with very low retention capacity.
In this context, extreme streamflow formed in a very short time, typical of Mediterranean flash floods, of around 3 hours and most likely even less.
Figure 3 shows a synoptic picture of the severity of expected flow rates between Sunday and Monday in the sections modeled by EFAS (LISFLOOD hydrologic mod. fed by ECMWF’s IFS atmospheric forecast ensemble).
As can be seen in the next figure, the response of the hydrologic basins, mostly wadis (i.e., non-perennial streams), was estimated to be about 10 times higher than the values usually contained in the riverbeds with no flood protections in place (about 2 years).
It should also be noted that the assessment of possible impacts provided by EFAS, (approximately130,000 people potentially affected) was still subject to large uncertainties due to:
- the lack of calibration of the hydrological forecast chains
- the lack of water level observations in both real- and deferred-time
- the presence of weirs, largely clay-filled, in the headwater and median reaches of the basins
This last point is particularly relevant for the city of Derna, which appears to be the most affected. At present the collapse of two dams on Wadi Darnah, which runs through the city center in a south-north direction, is believed to have unleashed the flood waves that, according to official sources, devastated about a quarter of the urban area , killing thousands.
