UKC

A study of rockfall in the Grand Couloir was published recently, with lots of information on the different methods used to monitor rockfall and some recommendations for climbers. It's all available in English here: https://nhess.copernicus.org/articles/22/445/2022/nhess-22-445-2022.html

TL;DR: the main advice is to cross as early in the day as possible.

really interesting.  well done for posting this.  3.7 fatal accidents on average each year.

The video on this page is worth watching as well (cited in the article)

https://www.petzl.com/fondation/s/accidents-couloir-gouter?language=en_US

Sorry to sound grumpy and the study and your post are worthwhile to remind people of the danger, but isn't the TL;DR already well known and widely publicised before the study?

I've crossed the coulouir early (stayed at Tete rousse) while it was in shade and while past it and climbing up alongside the sun climbed in the sky and swung round warming it. It changed from Dr Jekyll to Mr Hyde in the space in 15 minutes or so. It was pretty clear cut difference. Anyone considering the route needs a fair warning of this

Yes, I think it is well known and some parts of the report seem blindingly obvious, but I thought it was interesting to read about the various monitoring tools and see the numbers to back up what most people are already aware of. And as they mention, there are still plenty of people crossing at the most dangerous times.

These data are being used to change the time of the first train from St Gervais to 6.30am as an experiment.  This will allow the fastest teams to cross the couloir at 9.30 (one of the safer times).  Unfortunately there seem to major problems further up on the Bosses ridge with a very large crevasse gradually forming (possibly becoming uncrossable). 

La Compagnie du Mont-Blanc (CMB) en charge du train rappelait qu’une expérimentation a déjà été menée avec un départ à 6h30. « Cela avait été un échec avec seulement 10 alpinistes par jour en moyenne », précisait alors Damien Girardier, directeur d'exploitation à la CMB, qui se disait tout de même « ouvert à une nouvelle expérimentation entre le 15 juin et 10 juillet ».

Le nouvel horaire matinal du train proposé par la Compagnie du Mont-Blanc permettra aux alpinistes de quitter le nid d’Aigle (...) aux alentours de 8h du matin », indique leur communiqué. « Les plus entraînés pourront franchir le couloir vers 9h30 au moment où la probabilité de chute de pierres est la plus faible de la journée. Les moins rapides seront passés avant midi, à l’heure où les déstabilisations rocheuses regagnent en intensité. »