A unique piece of engineering heritage lies along the banks of the Grand Canal near Sallins, County Kildare. It is a complex circular overflow device constructed from stonework. According to The Inland Waterways Association of Ireland (link below) it is the only example of this type in Ireland. It is probably 200 years old.
It is particularly interesting from an engineering prospective, as whenever it is mentioned in publications, it is usually described as unnecessarily complex and over-engineered. One reference I remember, but that I haven’t found a link for yet, even described its purpose as a mystery!
See link to one such ‘over engineered’ reference here, click here.
The design of this overflow was driven by a need to reduce erosion of the small stream to which it discharges to. But its design also allows it to be ‘turned on and off’, by the use of sluice valves. So it is a relatively ‘intelligent’ canal overflow, when compared with the common ‘dumb’ types. These are usually simply low walls with streams running behind to catch the spill over water.
As the overflow is completely overgrown, I have done a sketch of it to show its layout. However I couldn’t access all areas, so some detail may well be missing. The sketch I have drawn is below, double click to enlarge and inspect:
How it works: Looking at the sketch, it can be seen that water enters the overflow on the left, shown in section A-A, through a tunnel from the canal. The flow is regulated by a sluice valve. The water fills the outer basin until it is full and then pours over the lower wall into the inner basin. The water flows from the inner basin though a tunnel to the stream at the rear. For added protection there is a high level spillway to deal with any blockage to the inner basin outflow tunnel. Presumably there is a second drainage tunnel to empty the outer basin for maintenance etc, but I could not locate it.
This then is the design that has been referred to as being over engineered. But I am not so sure. The reason for the double basin arrangement is clear as it is to remove power from the incoming water thus preventing damage to the stream behind. However I think it has the further advantage that once the outer basin is full, the water pressure difference between the canal and outer basin is significantly reduced. This then takes pressure off the sluice valve and makes it easier to operate, but more importantly easier to close.
While it may be the only example of its type in Ireland, there are canal overflows in England which are similar in design, although much smaller, see photograph of two below. It is worth noting how well maintained the English overflows are compared with their Irish counterpart.
So while there is doubt as to whether the overflow is over-engineered, it is certainly overgrown. If it was cleared out and exposed properly, people could see it clearly, admire it and maybe even debate its merits…..
Update. Why was the overflow located here? See comments section below for a discussion of the potential reasons for the overflow at this particular location.