The damage to infrastructure from the big spate on the 11th August was extensive including many road crossings. The road crossing on the Allt a’Gheallaidh (Pitchroy Burn) on the back road between Knockando and Grantown sustained some damage which required emergency repairs by Moray Council.
The Allt a’Gheallaidh (the “Carlsberg Burn” from the blog last year) is an important spawning burn with both salmon and sea trout spawning many miles upstream of the road. Maintaining fish passage was therefore of vital importance.
The road crossing consists of twin arched culverts constructed from corrugated steel and concrete. During the August spate the left bank culvert became partly blocked by trees and debris resulting in considerable damage to the bank and foundations with minor subsidence on the road surface. Consequently the council brought in contractors to shore up the banks and the foundations.
Downstream of the road the river channel had been recently dredged (not sure why, nor by whom) but it was reinstated once the concrete bed had been poured to strengthen the culvert.
Yesterday the new channel was opened up again and the dredged riverbed was reinstated.
Our main concern is that the new riverbed downstream of the culvert will be prone to settlement and erosion with the risk of a drop forming between concrete and the riverbed – a classic perched culvert scenario. We will watch how this situation develops after a few spates (that won’t be long judging by the rain falling outside the office window at present)
It is always amazing how quickly fish colonise new habitat. I was there about 2 hours after the introduction of flow down the concrete channel and already a couple trout parr had taken up resident in the low water channel.