Holdfast Training Services Trial Blog

Raystown

I was toying with a semi-technical blog, but then realised I hadn’t actually finished my “interesting things that USACE do that the Royal Engineers don’t do” mini-series.  So here goes, today I’ll be looking at the Raystown dam and reservoir.

I visited the site a couple of weeks ago.  This gave me the opportunity to tour the facilities, but seeing as it’s located 175 miles from Baltimore in the middle of deepest darkest Pennsylvania it also afforded a good excuse to do some walking and go camping for a night.  It was also the piece of critical national infrastructure that the cadets had to go and ‘recon’ so it made sense to do backbriefs etc on-location.

Anyway, a bit of background:  Construction began in 1968, and it was by most accounts quite unpopular at the time; the perceived wisdom being that all available funds should be used to support troops in Vietnam!  Construction was completed in 1972, and the accompanying recreation area was inaugurated a couple of years later in 1974.  The principal purpose of the dam was flood defence, with the creation of a recreation area a close second.  Power generation wasn’t even considered at this stage, however a 21MW hydroelectric plant has subsequently been added.  Interestingly this is the only part of the site (including wildlife management schemes) that isn’t directly administered by USACE.  Immediately following construction, the area was hit by tropical storm Agnes, which all but filled the reservoir only months after completion.  Had the dam not been built the Juniata (and subsequent valleys) would have experienced the worst flooding on record.

Raystown Earth Dam

Some facts and figures then:  The principal dam is 69m high, with a planned ‘constant’ water depth of 58m at the deepest point by the dam.  The dam itself is a classic earth (or embankment) dam with a clay core.  Material for the dam was harvested nearby, in an area which now serves as the emergency spillway should the reservoir overfill and other mitigation measures fail.  There is a secondary (much smaller) concrete dam which houses the main spillways and regulation gates.  The hydroelectric plant has its own water feed and under normal operation all water leaving the reservoir passes through the plant.  The spillway is only utilised when the volume of water in the reservoir becomes too great for the plant to handle on its own.  The lake is approximately 45km long, has a 180km long shoreline, covers an area of 34km², and has a flood capacity of around 306,000,000m³.  Interestingly there is no ‘Raystown’ nearby.  I asked about this, and the only answer I could get is that the project was named after a local trapper (presumably called Ray) of some fame from “the old days”!

Concrete Dam and Spillway

Some interesting engineering points came out of the facilities tour.  During construction 5 massive movable steel bulkheads were created and ingeniously stored within the dam.  The idea being that when required they could be hoisted out and dropped into position across the spillways allowing the main gates to be serviced.  However, once lowered into position the designers failed to take into account the ergonomics of the finished dam.  When it came to using them for the first time it became apparent that no crane could get close enough to lift them out because they are unable to deploy their spreaders sufficiently on the service road on the top of the dam.  The bulkheads have thus sat undisturbed, entombed in the dam since 1971!  Down at the bottom of the dam (below the lowest water level) there are a series of walls drains to reduce the pore-water pressures acting against the foot of the dam.  There are also a series of “bubblers” which are simple air pipes used to pump air out from the bottom of the spillway entrance in order to circulate water and to try and prevent or reduce damage caused by water freezing in and around the spillway gates.  There are three separate water inlets from three different depths within the reservoir.  This allows the operator to take different proportions of water from different depths in order to mix them and achieve the desired temperature before releasing water downstream; an important environmental consideration.

Emergency Spillway Tower & Bubblers and Wall-drains (within the concrete dam)

The reservoir also has a ‘plughole’, or in this case a 9m x 6m tunnel which runs from the bottom of the original valley out into the river beyond the dam.  It has never had to be utilised, but it is controlled from a giant tower which stands in the middle of the emergency spillway.  During the tour we were able to go down and look at the giant pistons which operate the ‘plug’.  Also of engineering interest is the emergency spillway.  This appear to be just a giant field, however it has been engineered to have specific properties.  The upper lip is bedrock which has been chiselled out of the hill in order to form a specific level below which the water will remain contained in the lake.  Onto this however an earth bund has been installed which is approximately 1m high.  This is made is made of ‘erodeable’ material which has been designed so that it will contain water up to a certain height, but that once breached it will be washed away emptying the lake down to the level of the bedrock lip.  Simple but clever engineering.

During my visit, I had to keep reminding myself that the site was completely designed, built and managed by USACE, and that this represents one of many similar projects across the country.  The reach and scope of the organisation is quite staggering, it really is a very different beast to our own military.

Other News

It looks like some of the QC contractors will be losing their jobs soon.  Apparently it’s not okay to just turn up at meetings, make promises then not do anything.  They have two weeks to turn it all around but it’s not looking good!  More to follow.  Finally, bacon covered doughnut anyone?………….