There is currently a lot of discussion around the definition and meaning of a "Low Risk" hydro system. Whilst the definition of such a hydro installation is still being debated within officialdom we decided that it should at least describe an installation where no cast in situ concrete would be used, neither the banks nor the stream bed would be modified, any impoundment would probably be naturally occurring, a mechanism would exist to ensure that the hands off flow could be guaranteed and that there would be no material change to the geomorphology. In addition, it should be robust enough to withstand spate flows and be essentially self cleaning.
After much head scratching and playing with models in the sink, (and the bath) we came up with the first ever "Coffin Intake" which has been (temporarily) installed in a naturally occurring pool on the end of an existing old and redundant 6" steel penstock on an un-named Welsh hill farm.
The intake is 1.7m long, 700mm wide at the shoulder and 400mm wide at the foot, and 300mm deep. The sides and top are clad in 2.5mm thick perforated sheet with 3mm holes and an open area of 30%. The sloping sides are intended to ensure that there is a small but positive pressure on the perforated mesh sides as the water passes through the pool. The ratio of the mesh sides open area to the penstock is 17:1 ensuring that the velocity of the water passing through the mesh remains very low which in turn should prevent leaves from adhering to the sides as a result of negative pressure on the inside. The adjustable height legs have feet which are bolted to the rock slab below.
The unit has been in the pool and observed for several months now and so far it is proving its design concept very well.
Of course, there are those out there that might suggest that it has a dual function. Well, if one of those pesky inspectors should turn up on the farm unannounced then we could always invite them to inspect the inside of the intake at close quarters.