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Alpine flooding caused by once-in-a-century rainfall

Eylert Ellefsen
Archived blog post. This blog post has been transferred from our previous blogging platform. Links and images may not work as intended.

The thunderstorm Alex caused extreme flooding in the southern Alps this weekend as it hit France and the southern parts of the Alps:

We’ve all seen the devastation on TV as rivers flooded and villages were destroyed, prompting intense rescue operations. The Alpes-Maritime and Provence-Alps regions seem to be the hardest hit with reports of 400 – 600 mm precipitation over 24 hours from Friday to Saturday (2-3 October) when the storm peaked. Winds were also extreme, with some hourly prices dropping to zero over the weekend in the central western region on the back of exceptional wind power output in France and Germany.

The impact on the hydropower system is significant, with strongly increased energy inflows, run-river production and reservoir levels. In this blog post, we are offering preliminary details of how Alex is impacting the hydropower situation across the Alps.

Since it’s difficult to estimate how much water was spilt – i.e. water that did not reach any reservoirs – we will publish an update on the hydrological situation in the Alpine region in a week or two, when actual reservoir numbers are published.

Thunderstorm Alex

The thunderstorm Alex (also called “Brigitte” in Switzerland) hit France and Alpine region 2-3 October.  

The weather radar charts from Meteo France and Meteo Swiss show how the storm moved across France and the forecast hourly precipitation in the Alpine region, with close to 30-40 mm/hour in some areas.

Weather radar image captures.

In the charts below we show the observed precipitation across Switzerland for Friday 2 Oct and observed hourly river flows on some locations in Switzerland.

Observe that the so-called “Süd-seite” of the Swiss Alps (Ticino region ) was most severely hit, as was the Wallis area and Graubünden regions. In Ticino, around 400 mm of precipitation was measured over 24 hours, versus 150 mm in the Wallis area. This was the 2nd highest daily rainfall observed since 1900 for both locations!  

We observe that river flow in Switzerland surged between Friday and Saturday, wit average river-flows peaking at about 30 times their initial levels before easing off during Sunday and Monday.

River flows in Switzerland 1 to 5 October.

The impact on the hydropower system

The gross energy precipitation across the Alpine countries –meaning France, Switzerland, Italy and Austria – amounted to about 10 TWh accumulated for the 2nd and 3rd of October (Friday and Saturday). This is about 1 000 % of normal, but lots of water has clearly ended up outside power plants and reservoirs. Energy Quantified will do an updated blog in a week or two with the net precipitation and inflow data.

The price-effects of this “hydrology-shock” is so far difficult to measure as it needs to be quantified separately from the strong winds’ impact on spot prices across the CWE area (chiefly Germany and France). The hydrological impact on prices will mainly be reflected in higher reservoir and production levels during Q4, and limited in the short term.

Some impacts are already clear, however. In the charts below,  we see how run-river production increased during the weekend, but were still significantly below the maximum capacity of about 8,000 MW. This indicates that some river-plants were shut due to too strong flooding.

Run-of-river production in France late September and into October 2020 (MWh/h).
Inflow for Switzerland July to mid October 2020 (GWh).

Thunderstorm Alex brought with it around 10 TWh of precipitation across the Alps in two days, and while a lot of the water ended up outside the hydropower system, EQ has made the following estimates of core weekly hydrology numbers, quantified as the percentage of normal. These numbers refer to values before adjusting for water spillage.

Energy Quantified will publish an updated blog post on the impact form Alex in a week or two when more actual reservoir numbers are published and the hydropower situation is potentially stabilised.

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