Allt a’Gheallaidh road culvert

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.

The view from above the culvert during the works. The bank on the left hand sid was reinforced with concrete as was the floor. Previously the culvert had a natural substrate base.

The view from above the culvert during the works. The bank on the left hand side was reinforced with concrete as was the bed of the culvert. Previously the culvert had a natural substrate base. A natural sediment base in a culvert is a more desirable situation than an artificial stream bed as there is much less chance of it becoming a barrier to fish passage. The council assured us that if the other culvert requires maintenance every effort will be made to maintain a natural riverbed throughout.

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.

The river bed downstream of the culvert has been dredged out over a distance of 100m

The river bed downstream of the culvert has been dredged out over a distance of 100m

The downstream side of the culvert before the riverbed was reinstated

The downstream side of the culvert before the riverbed was reinstated

Yesterday the new channel was opened up again and the dredged riverbed was reinstated.

Flow reinstated down the new concrete culvert

The reinstated riverbed downstream of the culverts. It looked quite good remarkably quickly afterwards.

The reinstated riverbed downstream of the culverts. It looked quite good remarkably quickly afterwards.

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.

A trout parr in the low water channel on the downstream end of the new culvert. It must have moved downstream from above the culvert otherwise it would have had to survive the excavator reinstating the riverbed below the culvert.

A trout parr in the low water channel on the downstream end of the new culvert. It must have moved downstream from above the culvert otherwise it would have had to survive the excavator reinstating the riverbed below the culvert.

Spey Fishery Board