The media are reporting that water has now "entered the spillway" - i.e. that overtopping has begun. The report, which is from the Chinese Agency Xinhua, carries a rather strange quote from an official from NDMA:
"The next 12 hours could be crucial as the water flow would reach its maximum intensity," commented an official of the National Disaster Management Authority (NDMA), the organization mainly responsible for the overall rescue and damage control operation.
Maybe this is a misquote? The next twelve hours are crucial of course, but there is a good chance that a breach won't develop in this time. The water level will continue to rise until outflow (seepage + spillway flow) = inflow. As some of the water is being released, this may well take much more than 12 hours to develop. In addition, a breach will only start when erosion of the base occurs at the highest point of the saddle. This may well take time to occur.
So, I would like to make two points:
1. The development of a breach in the next 12 hours is very possible; the failure of a breach to develop does not mean that one won't occur. At Tangjiashan the breach took several days to start, but was then rapid;
2. It is also possible that an initial small flood will happen, but that this will then cease. For example, this could be the case if the sides of the channel collapse, blocking the flow. We must be clear that if there is a small flood that then declines then the situation is certainly not over. People must not go back into the danger zone until we know that the threat has passed.
Of course what we are hoping for now is slow erosion and the release of the water over the next few days. Such a scenario is certainly not beyond the bounds of possibility.
I will post again as more information becomes available, but don't be surprised if this takes a few days to develop.