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TitlePhysical changes and evolution of Scots Bay Beach, Nova Scotia
DownloadDownloads
AuthorTaylor, R B; Shaw, J
SourceGeological Survey of Canada, Open File 8137, 2016, 53 pages, https://doi.org/10.4095/299475 (Open Access)
Image
Year2016
PublisherNatural Resources Canada
Documentopen file
Lang.English
Mediaon-line; digital
File formatpdf
ProvinceNova Scotia; Eastern offshore region
NTS21H/08SW
AreaScots Bay; Bay of Fundy
Lat/Long WENS -64.5000 -64.2500 45.4167 45.2500
Subjectsmarine geology; surficial geology/geomorphology; coastal studies; coastal environment; beach deposits; sea level changes; tidal environments; glacial deposits; depositional environment; erosion; lithology; sediment transport; sedimentation; Quaternary
Illustrationsphotographs; location maps; cross-sections; tables
ProgramGSC Atlantic Division, Director Office
Released2016 12 19
AbstractScots 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.
Summary(Plain Language Summary, not published)
Scots Bay is one of the largest bay head gravel barrier beaches within the inner Bay of Fundy. It is an established site for monitoring shoreline changes representative of a macro-tidal environment. The landward edge of the barrier has migrated 22 to 47 m landward (0.5 to 1.1m/year) since the 1940s and 12 to 18 m landward since 1984. Paleo-positions of the barrier beach were re-constructed using sediment cores and dated material collected at the site, rates of sea level rise, changing tide ranges and marsh growth documented at other marshes in the area. The beach in 3000 BP was a minimum of 150 m seaward and probably more than 300 m seaward of the present beach. By 2100 much of the marsh will be squeezed out and disappear as the barrier migrates landward (~90 m) and rebuilds against higher terrain. Extensive backshore flooding occurs during storms that coincide with higher high tide and sustained strong westerly winds. Tide gauge recordings of 8.5m or higher at Saint John N.B could be used to provide warning of upcoming flooding at Scots Bay N.S..
GEOSCAN ID299475