The Western Mail & South Wales News of 19 April 1929 had an article by M Franklin Thomas about “Ireland’s Big Engineering Scheme”. The article was illustrated with a map and three photographs were reproduced on the newspaper’s “picture page”. It had a couple of interesting points about the headrace and tailrace considered as navigations; I can’t recall seeing these points made elsewhere.
This [headrace] canal is level, but a flow of about 3½ miles per hour will be maintained owing to the water released through the turbines and navigation locks.
I don’t recall seeing a figure for the current. I presume that that is with three turbines running flat out.
There is an interesting account of the laying of the concrete apron that protects the banks against the “wave action set up by navigation, and the flow of the stream, wind, &c”. And the lock was to have an “ingenious arrangement by which the entering streams of water neutralise each other’s effect”.
Mr Thomas says
The tail race is one mile and a half long, cut from the solid blue limestone, and one of the most interesting points was the method adopted to permit barges to ascend the tail race against the enormous scour from the turbine discharges.
A special navigation channel is cut from the locks to a point some 200 yards below the outfall, and the bed of the tail race rises 20ft in the mile and a half, so that the depth of water will be 35ft at the outfall from the turbines and 15ft at the junction with the Shannon.
This will give a cushioning effect, and the rate of flow will be thus reduced to enable barges to navigate upstream. A bend is also provided where the special navigation channel joins the tail race and the rate of flow is estimated to be the same as that of the head race — 1.5 metres per second, or about 3½ miles per hour.
I would be glad to hear from anyone who can cast further light on this, and especially on whether the rising bed does have the intended effect.