North America

PPL Montana plans permanent fix to Hebgen Dam flow control failure

Hydro operator PPL Montana called meetings Sept. 24-25 to brief the public on apparent stoplog failure at Hebgen Dam that is blamed for loss of flow control in Bay 4, quadrupling releases and dropping Hebgen’s storage reservoir eight feet.

Having re-established flow control using Hebgen’s intake structure control gates and temporary stoplogs, PPL Montana said it is working on a permanent solution at Hebgen, a storage dam in its 326.9-MW Missouri-Madison project (No. 2188). (HNN 9/8/08)

PPL Montana organized meetings Sept. 24 in West Yellowstone and Sept. 25 in Ennis to brief the public on the problem, discovered Aug. 31, and the activities that occurred since that time. The utility, which regained flow control through a temporary fix Sept. 21, also planned to describe its plans for permanent repair.

Hebgen Dam, an 85-foot-tall, 721-foot-long earthfill dam, regulates flow into the Madison River system. (HNN 6/19/08)

In the original event, flows from Hebgen Dam increased to 3,400 cubic feet per second from 850 cfs, dropping the level of the reservoir behind the dam by more than eight feet. In addition to taking temporary measures to regain flow control and reduce flow, the utility also released water from a reservoir below the dam and upstream of its 8-MW Madison project to guard against downstream flooding.

The cause of the original event is under investigation but missing stoplogs from Bay 4 in the Hebgen Dam intake structure were a factor, PPL Montana Regulatory Affairs Manager Mark Lambrecht said. PPL Montana said it plans to make the site safe for divers to inspect the intake structure to determine what happened. The utility then plans to replace the nine-foot timber stoplogs it used for the temporary solution with concrete ones.

The utility said it is controlling the flow of the Madison River using the intake structure’s control gates.

�Our plan is working and the river flow is down to about 2,200 cfs,� Pete Simonich, manager of generation assets for PPL Montana, said Sept. 21. �We are continuing to insert new stoplogs, and are maintaining flow with the control gates. We are starting to gradually reduce river flows to normal over the next three days to protect fish habitat.�

PPL Montana said instrument readings and federal inspectors determined the dam and the damaged intake structure remained safe and stable, and that the structures pose no risk to people downstream of Hebgen Dam.