Ripple Migration and Sand Transport Under Quasi-Orthogonal Combined Flows on the Scotian Shelf

Abstract

Bed state, ripple migration and bedform transport of fine sand (D50 = 0.23 mm) were recorded with time-lapse photography for 12 days in 22 m of water on Sable Island Bank, the Scotian Shelf. Canada. Near-bed wave and steady flows were recorded and correlated with observed bed state and ripple migration. A well-defined threshold for the traction of fine sand under largely orthogonal combined flows is apparent in our data. It is defined by the expression (-)_cb' = [(-)_w' + (-)_c’] = 0.04. The value is 20% less than the threshold criterion defined by GRANT and MADSEN (1979) for wave-dominant flows and extends it to the current dominant flow conditions when flows are orthogonal to each other. In addition, a threshold for saltation/suspension under combined flows is defined by the expression (-)_cb’= [(-)_w’ + 1.5(-)_c] + 0.17. This threshold marks the onset of saltation and grain bypassing of ripple lee faces and approximates closely the breakoff Shields parameter (-)_br’ = 0.16) of GRANT and MADSEN (1982). Bedform transport measurements are applicable only below this point; termed the equilibrium range by GRANT and MADSEN (1982). The sediment transport model SEDTRANS matched the duration and magnitude of sand transport over a wide range of conditions of combined flow. This was achieved by using the method of GRANT and MADSEN (1979), but minimizing the friction factor (f_cw’) to 0.006. This value equals the pure unidirectional flow drag coefficient of SOULSBY (1983).