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TitrePhysical changes and evolution of Scots Bay Beach, Nova Scotia
TéléchargerTéléchargements
AuteurTaylor, R B; Shaw, J
SourceCommission géologique du Canada, Dossier public 8137, 2016, 53 pages, https://doi.org/10.4095/299475 (Accès ouvert)
Image
Année2016
ÉditeurRessources naturelles Canada
Documentdossier public
Lang.anglais
DOIhttps://doi.org/10.4095/299475
Mediaen ligne; numérique
Formatspdf
ProvinceNouvelle-Écosse; Région extracotière de l'est
SNRC21H/08SW
Lat/Long OENS -64.5000 -64.2500 45.4167 45.2500
Sujetsétudes côtières; milieu côtièr; dépôts cotiers; changements du niveau de la mer; milieux de marée; dépôts glaciaires; milieu sédimentaire; érosion; lithologie; transport des sediments; sedimentation; géologie marine; géologie des dépôts meubles/géomorphologie; Quaternaire
Illustrationsphotographs; location maps; cross-sections; tables
ProgrammeBureau du directeur, Division de la CGC atlantique
Diffusé2016 12 19
Résumé(disponible en anglais seulement)
Scots Bay Beach is one of the largest bay head gravel barrier beaches within the inner Bay of Fundy that are exposed to waves generated along the length of the bay. It is a 60 m wide transgressive barrier with a limited coarse sediment supply. Less than 3 m of coarse sediment overlie buried marsh clay which is often exposed along the mid-lower beach face. The steep sloping beach face is fronted by an 800 m wide, intertidal flat. The beach is backed by wave washover and/or dune deposits and a 100 to 200 m wide salt marsh. The marsh is drained by tidal creeks which enter the sea through an outlet near the centre of the beach.
Terrestial seismic reflection and refraction surveys, vibracoring and marsh boreholes reveal a complex coastal stratigraphy. Landward of the salt marsh about 18 m of lower velocity sediment interpreted as glaciofluvial or raised beach deposits overlie bedrock. Thickest marsh deposits cored were 8.8 m, but core penetration was limited because of abundant driftwood buried in the marsh. The intertidal flats, within 200 m of the beach face, consisted of wave reworked sands, granules and pebbles over a wedge of salt marsh clay and an intermediate velocity deposit interpreted as a diamicton. It extends to bedrock which slopes steeply westward to a depth of 19 m beneath the tidal flat. Barrier beach positions at 1000 years (1ka), 2000 and 3000 years BP (radiocarbon years (14C years) before present where present is 1950) were reconstructed using paleo-marsh elevations, sea level curves and a radiocarbon date of 3160 ± 50 BP from a vibracore sample collected within 150 m of the present beach face. The beach at 1 ka BP was no closer than 115 m from the present beach face and the 3 ka beach was a minimum of 150 m seaward and probably more than 300 m seaward of the present beach face (using a beach migration rate of 0.1 m /a). Scattered boulders and a thin bed of well-rounded polished granules and fine pebbles over truncated marsh deposits observed beneath the intertidal flat may represent a sedimentary facies of a transgressive macro-tidal barrier beach.
The barrier beach can be divided into seven distinct cross-shore zones between the intertidal flats and the back barrier marsh. Driftwood accumulation is an important factor controlling beach crest sedimentation and stability, particularly along the central and northern barrier where backshore dunes are absent. Rates of landward beach migration between 1945 and 1987 were documented by comparing repetitive air photography. The landward edge of the barrier migrated 22 to 47 m during the 42 years; a rate of 0.5 to 1.1 m/a, most along the central part of the barrier and least toward its north and south ends. Since 1984 shore migration was documented using repetitive cross-shore surveys at three locations. The landward edge of the northern barrier migrated 12 m and the beach crest at three sites has migrated 12 to 18 m during the 24 years since 1984, an average rate of 0.5 to 0.8 m/a. Rates of landward beach migration have been similar since the 1940s and higher than the suggested paleo-rates. Accelerated migration may have been triggered by the loss of sediment reserves by beach mining and the loss of wharf structures between the late 1940s and late 1950s. The beach crest can experience short term building phases and remain fairly stable for several years, e.g. 1986-1991. The beach crest is pushed landward episodically, only when westerly storms coincide with higher high tides e.g. Feb. 1976 and Oct. 2015, resulting in sheet wave overwash of the entire barrier. Storm wave run-up can exceed the elevation of the present beach crest of 7.6 m and the maximum flood level can reach 6.4 m (Geodetic Datum) along the back edge of the marsh. It was observed that flooding of Scots Bay Beach and the access road to the beach occurred after water levels reached 8.5 m or higher (Chart Datum) at Saint John, N. B., e.g. Oct. 30, 2015. Therefore, in the future, during periods of sustained strong westerly winds, water levels recorded at the tide gauge in Saint John, New Brunswick could provide a warning of potential flooding at Scots Bay Beach. Based upon rates of landward beach migration since 1984, much of the marsh, particularly behind the northern barrier, will be squeezed out and disappear by 2100, as the beach migrates and builds against the higher backshore.
Résumé(Résumé en langage clair et simple, non publié)
La baie Scots présente l'un des plus grand cordons littoraux de gravier à la tête d'une baie, dans la partie intérieure de la baie de Fundy. Il s'agit d'un lieu établi permettant de surveiller les changements du littoral représentatifs dans un milieu macrotidal. La bordure du côté terre du cordon a migré de 22 à 47 m vers les terres (0,5 à 1,1 /année) depuis les années 1940, et de 12 à 18 m du côté terre depuis 1984. Les paléo-positions du cordon littoral ont été reconstituées d'après des carottes de sédiments et des matériaux prélevés sur le site dont la datation a été établie, les taux d'élévation du niveau de la mer, l'évolution des amplitudes des marées et la croissance des marais documentée à d'autres sites marécageux de la région. La plage à 3000 ans BP était à un minimum de 150 m côté mer et probablement à plus de 300 m côté mer que la plage actuelle. D'ici 2100, une grande partie du marais sera repoussée et disparaîtra au fur et à mesure que le cordon littoral migrera vers les terres (-90 m) et se reformera contre des terrains plus élevés. De grandes inondations des hautes plages se produisent pendant des tempêtes qui coïncident avec des pleines mers supérieures et à de forts vents d'ouest soutenus. Des enregistrements de marégraphes de 8,5 m ou plus à Saint-Jean (N.-B.) pourraient servir à émettre des avertissements des inondations à venir à la baie Scots (N.-É.).
GEOSCAN ID299475