Ripple migration and sand transport under quasi-orthogonal combined flows on the scotian shelf

Bed state, ripple migration and bedform transport of fine sand (D-50 = 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 migra tion. 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 exp ression Theta(cb)' = [Theta(w)' + Theta(c)'] = 0.04. The value is 20% less than the threshold criterion defined by GRANT and MADSEN (1979) for wave-do minant flows and extends it to the current dominant flow conditions when fl ows are orthogonal to each other. In addition, a threshold for saltation/su spension under combined flows is defined by the expression Theta(cb)' = [Th eta(w)' + 1.5 Theta(c)] = 0.17. This threshold marks the onset of saltation and grain bypassing of ripple lee faces and approximates closely the break off Shields parameter (Theta(br)' = 0.16) of GRANT and MADSEN (1982). Bedfo rm 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 r ange 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 S OULSBY (1983).