Site Records

Glenmoriston: Surge Chamber

Since Glenmoriston has an extremely long submerged tailrace tunnel, it was necessary to build a downstream surge chamber, to control surges in the tailrace when machines are started and stopped. It was designed to handle a worst-case surge, which would occur when the machines trip, causing a strong initial downsurge, and then are brought back to full output ten minutes later - when the upsurge from the machines starting would collide with the now-returning wave of the initial downsurge.

The initial design had the surge shaft, which is around 50ft in diameter by 100ft high, and the tunnel labled as 'surge gallery' in the drawing below, running to the base of the surge chamber. The 'surge gallery' also doubled as an access tunnel to the base of the shaft, which facilitated construction of this area of the station. However, model tests revealed a problematic resonance between the surge shaft and the surge gallery. To prevent this resonance, an expansion gallery was added to the design, connecting the top of the surge shaft to the surge gallery, as the drawing below makes clear:

As can be seen from the above drawing, the surge gallery and expansion gallery unite in an alcove off the access tunnel. The following image shows the view from the access tunnel; this area was heavily modified during the 1999 refurbishment. Previously there had just been a metal gate here - the new door and concrete were installed in an attempt to reduce the seepage of radon gas into the station:

Close-up of the door. The illumination in this area is dreadful; apologies for the poor quality contrast-stretched images:

Through the door - light goes from bad to 'black cat in coal celler'; Image quality goes from dreadful to 'who are you trying to kid?'. If you use your imagination, you can make out the bare rock walls of the small chamber, the entrance to the lined tunnel of the surge gallery on the left, and the darker hole of the entrance to the unlined expansion gallery on the right.

If you followed the right-hand tunnel in the above picture, you would eventually come to this place - where the expansion gallery bells-out to full width at the head of the surge shaft. Another one from the Duncan Logan archives:

If you followed the left-hand tunnel, and the place was dewatered, you would end up here - the base of the surge shaft. Look at the drawing above to make sense of it: the photo was taken from the end of the tailrace. The two openings on the right with sloping roofs are the draft tubes. The opening on the left is the start of the surge gallery. All this is normally full of water, to a depth of probably 40-50ft. Another one from the Duncan Logan archives.

Words and images © 2005 Michael J. Ross