* Corresponding author. Tel.: 44-1703-596-660. E-mail address: c.amos@soc.soton.ac.uk (C.L. Amos). Continental Shelf Research 20 (2000) 1291}1315 Corrasion of a remoulded cohesive bed by saltating littorinid shells C.L. Amos*, T.F. Sutherland, D. Cloutier, S. Patterson Department of Ocean and Earth Sciences, Southampton Oceanography Centre, Empress Dock, Southampton, S014 3ZH, UK Martec Ltd. 1888 Brunswick Street, Halifax, Nova Scotia, Canada ISMER, Universite du Quebec a Rimouski, Rimouski, Quebec, Canada Estuarine Research Centre, Acadia University, Wolfville Nova Scotia, Canada Received 10 May 1999; received in revised form 26 October 1999; accepted 25 November 1999 Abstract Corrasion of a standard cohesive bed due to saltating gastropod shells of the species Littorina has been examined in a laboratory mini #ume. The purpose of the study was to examine the e!ects of shell size and number on bed erosion rate. The movement of shells by #ows explain why intertidal, glacial clays in the Bay of Fundy (which are covered in places with Littorinid shells) su!er erosion because bed erosion rate increased up to 20-fold with the introduction of a single shell to the #ume (27 m). The standard bed was made of potters clay which had an erosion threshold of 0.19 Pa and a #uid-induced erosion rate E  "0.072(;  !;  ) gm s, where ;  is the azimuthal current speed at height y"0.10 m. Shells of seven di!ering sizes (d  ) were used to de"ne the process of erosion by shell impacts. The threshold for shell motion (;  ) was linearly related to shell size in the form: ;  "9.1710 d  !0.22 m s. Motion began by intermittent rolling, followed by con- tinuous rolling and then by saltation. The shell speed in saltation was 68% that of ;  , thus 32% of the horizontal shell momentum was transmitted to the bed. The length/height ratio of saltations was 6.3, and was constant for all sizes, and the mean saltation frequency was 1.7 s. The shell erosion rate (E  ) increased with shell diameter for both the rolling and saltating phases. During the rolling phase, E  was up to 5 times greater than E  at the same current speed. During saltation, E  was up to 20 times greater than E  at the same current speed. The e!ect of shell number (1}7) was examined for the 7}10 mm size class. During rolling, E  increased linearly with shell number. For the saltating phase, E  increased in an asymptotic fashion, suggesting that groups of saltating shells a!ect the erosion process di!erently than single shells. The ballistic momentum #ux (¹) of saltating shells is highly dependent on the area of impact (A  ), which in the case of the littorinids, is along the rounded outer lip of the aperture (measured 0278-4343/00/$ - see front matter  2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 8 - 4 3 4 3 ( 0 0 ) 0 0 0 2 4 - 8