MEASURING THE EFFECTS OF WINTER STORMS (2013-2014) ON BOULDER AND MEGAGRAVEL ACCUMULATIONS IN WESTERN IRELAND

Coastal boulder ridges accumulate above high tide, and are active only during extreme wave conditions. Some are found on cliff tops as much as 45 m above high water (AHW); others occur up to 250 m inland at the back of gently sloping platforms. They can include megagravel of order 100 t. These deposits record maximum wave intensities for their locations and hold the key to understanding inland transmission of fluid forces, but little is known about their dynamics. The winter of 2013-14 brought record-breaking storms and high seas (with wave heights up to 25 m measured in Irish waters), causing dramatic geomorphologic change at many coastal locations. Using a database of boulder-ridge photographs collected in recent field seasons, we were able to document changes in the accumulations, and measure movements of specific clasts at locations in western Ireland, including coastal Clare, the Belmullet peninsula, and the Aran Islands.

Cobbles and boulders were moved in abundance, but some of the megagravel was impressively large. Examples of masses include an isolated block of ~430 t transported ~5 m laterally on a supratidal platform; slabs >20 t ripped from bedrock and incorporated into a ridge at 11 m AHW and 90 m inland; and an 18.5 t block moved from ridge base to crest (translated 12 m inland and 4.5 m vertically) in a ridge located 130 x m inland and 17 m AHW.

Our data document reorganisation of existing boulder accumulations as well as creation of new clasts from ripped-up bedrock. Within the boulder ridges, clasts shifted upward and inland. Boulders that had been low down on the seaward face of a ridge in last year’s photos were found further up the face, or in some cases at the ridge crest. Other clasts moved from the ridge face over the crest and into the back-ridge area. Within the back ridge, clasts travelled further landward. These observations indicate that the ridges migrate progressively landward. Mass was also added to the deposits. Uplift and dismemberment of bedrock slabs subjacent to the ridge base was the most common source of new material, but in some cases bedrock blocks were quarried and transported several 10s of m to their final resting places

The changes documented in this study prove the role of storms in growth and migration of supratidal boulder ridges, and provide insight into the sediment dynamics of these enigmatic deposits.