Geomorphological evolution of a sand spit beach with groin field exposed to winter sea ice (Gulf of St. Lawrence, Canada)

Abstract:

Beaches reply to seasonal variations of hydrodynamic conditions by adopting an accreting beach profile during summer and an erosive one during the winter stormy season; however, the situation is more complex in cold climate where the ice foot attached to the upper foreshore and the sea ice reduce sediment movement for several months. The Hope Town sand spit (Gulf of St. Lawrence, Eastern Canada) was slowly eroding since the 1970's, endangering the main coastal highway crossing the spit. In 2012, the Ministry of Transportation of Québec has constructed a groin field and undertook a beach nourishment to stabilize the spit. In their landward segment, the five groins are wooden-made while they are composed of large boulders in the seaward part, to resist to sea ice, present from December to March.

Our objectives are 1- to evaluate the morphological response of the sand spit beach after the construction of the groins by means of seasonal topographic surveys and grain size sampling; 2- to assess the historical coastline retreat rates using aerial photographs from 1963 to today.

Topographic surveys since fall 2017 show that during summer, the sediment tends to accumulate on the upper beach and at the last groin near the sand spit end, which highlights the efficiency of the structure to intercept longshore drift from northeast. However, the lower part of the foreshore is eroding. The water and its sedimentary load tend to bypasses the groin crest during fall storm surges and spills out to the lagoon inlet, contributing to the erosion of the upper beach. There is still no clear erosional pattern detected over the three heterogeneous winters. Finally, the sediment grain size follows the seasonal hydrodynamic variations as expected: finer at the end of summer in August, while it is particularly coarser in October and in May at the end of winter.

The historical coastline analysis highlights as the most erosive periods the 20 years after the dismantlement of a jetty in the late 1960’s and the 2005-2007 period, possibly associated with more storm events. The 2007-2016 period shows an accretive trend due to groin construction and beach nourishment in 2012.

These results show that if these composite groins resist to local coastal conditions with winter sea ice, they seem only partially limiting coastal erosion, possibly because of limited sediment supply.