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[HCG28-P06] Coastal transport process of pebble along the Seisho Coast, Kanagawa Prefecture, estimated from changes in lithological composition and form of pebble particles
Keywords:pebble, transport process, flatness, beach nourishment, Seisho Coast
Coastal erosion along the Seisho Coast, Kanagawa Prefecture has been one of the severe social problems on infrastructure. As a countermeasure to the coastal erosion of the Seisho Coast, artificial beach nourishment has been carried out since 1995. We estimate alongshore transport rate of pebble along the Seisho Coast using classical evaluation method of lithologic composition and flatness of pebble particles.
Gravel beach of the Seisho coast is maintained by sediment supply from the Sakawa River which is rich in tuff and diorite gravels derived from the Tanzawa Mountains (e.g., Aramaki and Suzuki, 1962). Based on a temporal change of coast line on old topographic maps, spatial change of lithological composition and flatness of pebble particles along the coast, coastal transport of pebble along the Seisho Coast is estimated as below.
(1) Temporal seaward shift of shoreline occurred during 1916 to 1933. It might have been caused by increase in sediment supply due to the Taisho Kanto Earthquake slope failures at the Tanzawa mountains on 1923.
(2) An abundance of flat tuff gravels at site C5 ca. 7 km east from the Sakawa river-mouth imply that pebbles may have been transported along the Seisho Coast from the river mouth since 1923–1933. Therefore, eastward average transport rate of 16-32 mm size pebbles is estimated as 70–80 m/y.
(3) Tuff gravels with low average flatness at site C6 may be brought directly from the lower reaches of the Sakawa River by artificial beach nourishment since 1995. Site C6 locating 0.6 km east from the beach nourishment site is near enough to the site. Whereas, tuff gravels having been brought the beach nourishment site do not reach site C7 2.0 km east from the site yet.
References
Aramaki, M. and Suzuki, T. (1962) Geographical Review, 35, 17-34.
Gravel beach of the Seisho coast is maintained by sediment supply from the Sakawa River which is rich in tuff and diorite gravels derived from the Tanzawa Mountains (e.g., Aramaki and Suzuki, 1962). Based on a temporal change of coast line on old topographic maps, spatial change of lithological composition and flatness of pebble particles along the coast, coastal transport of pebble along the Seisho Coast is estimated as below.
(1) Temporal seaward shift of shoreline occurred during 1916 to 1933. It might have been caused by increase in sediment supply due to the Taisho Kanto Earthquake slope failures at the Tanzawa mountains on 1923.
(2) An abundance of flat tuff gravels at site C5 ca. 7 km east from the Sakawa river-mouth imply that pebbles may have been transported along the Seisho Coast from the river mouth since 1923–1933. Therefore, eastward average transport rate of 16-32 mm size pebbles is estimated as 70–80 m/y.
(3) Tuff gravels with low average flatness at site C6 may be brought directly from the lower reaches of the Sakawa River by artificial beach nourishment since 1995. Site C6 locating 0.6 km east from the beach nourishment site is near enough to the site. Whereas, tuff gravels having been brought the beach nourishment site do not reach site C7 2.0 km east from the site yet.
References
Aramaki, M. and Suzuki, T. (1962) Geographical Review, 35, 17-34.