Sunday, May 30, 2010
Water flowing through the spillway at Attabad
On May 29 water started flowing through the spillway at Attabad. The photo here, from Dave's Landslide blog, shows an early stage of the overtopping. The spillway is working well at these low discharges. There were some reports of landslides into the lake today, and estimates that these slides had increased the lake level by 2 feet, but I don't know if these have been verified. As of this a.m. the discharge through the spillway is estimated to be 4 cusecs (cubic meters per second).
The lake level is rising about 2.5 cm per hour, and inflow into the lake is estimated to be 71 cusecs. There is thus about a 7 cusec difference between inflow and outflow. This water is stored in the lake causing its level to rise. As it rises, the discharge through the spillway increases. One limiting case is that the spillway channel will stay at the current width, in which case the increase in discharge is directly proportional to the rate at which the lake level rises and increases the depth of the water flowing through the spillway channel.
At the other extreme, if the spillway erodes and widens, then the discharge will increase as both the area increases and the depth increases. Erosion at the bottom of the channel would increase the depth of the flow through the spillway even further. It is this later case that worries the landslide experts because the discharge could increase so rapidly under these conditions. Dave is updating his blog frequently, so tune in there rather than here!