View of the dune/beach system on the Miquelon-Langlade isthmus (looking north), between the Grand Barachois Lagoon (right) and the Atlantic Ocean
© BRGM
The need
The Regional Directorate for Land, Food and the Sea of Saint-Pierre et Miquelon (DTAM) has asked BRGM to provide information on the feasibility of artificially replenishing the sand along the western side of the Miquelon-Langlade isthmus in order to restore the balance of sediments at that location and create a safe buffer zone to mitigate the impact of storms and protect the base of the various structures. Before considering such work, the uncertainties as to the feasibility of such a replenishment operation had to be clarified. In particular, this involved confirming that the depths at the site are suitable for a replenishment operation, characterising the deposits on the foreshore, identifying potential sources of sediment supply at sea, estimating sediment flows along the western side of the isthmus and, finally, providing information to develop a potential replenishment strategy.
The results
Three activities carried out in parallel produced the following results:
- Sediment deposits on either side of the isthmus are limited and directly linked to the foreshore and the sedimentary dynamics of this coastline. These stocks do not therefore appear to be suitable for use. However, a potentially useful stock of sediments (in terms of thickness, estimated volumes and grain size) has been identified off the coast of Miquelon. Moreover, extraction activities at this location are unlikely to disrupt coastal dynamics in the shallows or on the coastline;
- Precise depth measurements in the area and the IOWAGA wave database were used to reliably estimate sediment transit along the western side of the isthmus. Sediment transit was assessed using the Kamphuis formula (2002) for the entire western side of the archipelago;
- Two replenishment configurations were modelled: (i) on the foreshore and (ii) on the beach. Their design was based on the recommendations of the CEREMA report (300,000 m3; 1.5 to 2 km long). For each configuration, 4 hydrodynamic real storm scenarios (with 1 to 10-year return periods) were modelled with XBeach. The eight models produced show significant sediment losses in the replenishment area. Nevertheless, the beach replenishment scenarios show lower losses (25 to 50%) than those simulating replenishment of the foreshore (up to 80%).
Digital Terrain Model (DTM) of the morphology and topography (m) of the substratum roof generated by interpreting seismic profiles (2022 acquisition, iXblue), and highlighting areas of sediment accumulation (velocity in the sediment: 1700 m/s) and bedrock outcrop (vertical exaggeration x5, grid spacing 1 km; produced using DELPH RoadMap)
© BRGM
Using the results
Low-impact solutions for protecting the coast from erosion and marine submersion, such as beach or foreshore replenishment, must be considered in an integrated management context. A one-off beach replenishment is a temporary measure that will not solve the problem once and for all. In the case of the Miquelon-Langlade isthmus, solutions should include dune management at Les Buttereaux, a sediment management plan and, more broadly, the implementation of a management strategy that takes into account the various practices throughout the isthmus.
The partners
- Direction des Territoires, de l'Alimentation et de la Mer de Saint-Pierre et Miquelon (DTAM, [Directorate for Land, Food and the Sea of Saint-Pierre et Miquelon])
