Cromarty Firth: Geomorphology and Coastal Processes

The distinctive landform of the Cromarty Firth was largely created during the last Ice Age when the deep glacial trough was gouged into the underlying sedimentary rocks. As sea levels rose post-glaciation, the rapid increase in levels outstripped the rise in land level, resulting in the flooding of the glacial trough.

Peripheral areas, for example those now occupied by Nigg and Udale Bays, were also flooded allowing sedimentation to take place. This sedimentation was fuelled by the large stores of sediment present in glacial and fluvio-glacial landforms, resulting in rapid sedimentation rates.

Continued sedimentation has resulted in the deposition of up to 60m of sediments at the head of the firth while deposits in Nigg Bay reach depths of over 9m. However, despite the large amount of sedimentation in the deep glacial trough at the head of the firth, the basic fjord morphology remains unaltered.

The subtidal channel extending from the River Conon reaches depths of over 50 m between the Sutors of Cromarty, while along its length depths of over 15m as far inland as Alness Point, 16.5km from the mouth are reached. It has an average width of 1-1.5km as far upstream as Foulis Point.

Since ca.3000 BC sea levels in the Cromarty have fallen as isostatic rise of the land has outstripped any changes in sea level. While its basic fjord morphology remains unaltered, three significant landforms have evolved since glaciation: the intertidal areas of Nigg Bay, Udale Bay and the delta of the River Conon.

Nigg Bay is an area of mudflats and salt marshes which lies in a shallow hanging valley adjoining the main glacial trough of the Firth. It is essentially an estuary system which is constrained at its outer edge by a steep drop-off into the fjord. Likewise, Udale Bay is an estuarine landform constrained by the large-scale glacial features. The transition from tidal mudflat and salt marsh to fresh water has been largely removed by reclamation. At the head of the Cromarty Firth, between Conon Bridge and Foulis Point and extending into Dingwall Bay, the delta of the River Conon is characterised by a transition from open mudflat to carr vegetation.

Each of these areas represents isolated landforms which are separated by the deep waters of the glacial trough. The low levels of sediments in the waters of the Firth have meant that each of these isolated inter-tidal areas are themselves restricted in their development and this in turn means that they are extremely sensitive to any changes in sea level or sediment supply in the Firth or its catchment.

The major anthropogenic impact on the physical processes in the firth has probably been that of reclamation. The largest area to be reclaimed is on the eastern shore of Nigg Bay, where approximately 93ha have been reclaimed for the oil rig fabrication yard and oil terminal. As well as the reclamation of intertidal mudflat areas, the construction also removed a small area of salt marsh near to Dunskeath Ness and a sand dune system extending from the foreshore inland towards Castlecraig. Smaller areas have also been reclaimed around Invergordon mainly for the use of the Port Authority, totalling 6ha. A 1.2km long causeway has also been built, extending from the disused airfield at Evanton across the intertidal mudflats of Alness Bay into the subtidal channel.

Coastal wetland areas at the head of Nigg Bay were subject to major drainage schemes prior to the onset of the present century, creating productive farming areas. This area was probably salt and brackish marsh merging into low pasture before drainage was undertaken.

In February 2003, the old sea wall at Nigg Bay was breached allowing a 25 ha field to flood and revert to intertidal habitat using a process known as coastal realignment. This is the first time that this approach to dealing with rising sea levels, flood risk and coastal habitat loss has been tried in Scotland.

The construction of the A9 road bridge between Ardullie Lodge and SW of Findon Mains involved the construction of embankments out into the firth to support the landward ends of the bridge, the southern projection being the longest at approximately 500m long. These can be expected to have affected the local pattern of tidal flows close to the shore of the firth. However, there is no available assessment of the magnitude or nature of such impacts.

A small amount of sediment is removed by the Port Authority as a result of maintenance dredging from the berths at Invergordon, the spoil being deposited in the deep between the Sutors.