2 nd International Coastal Symposium, June 5-8, 2005, Höfn, Iceland WAVE SET UP, UNDERTOW & FREQUENCY DEPENDENT SEDIMENT FLUX ON A MULTI-BARRED LACUSTRINE SHOREFACE Brian Greenwood 1 , Allana Permanand-Schwartz 2 & Chris Houser 2,3 Professor 1 , Scarborough College Coastal Research Group, UTSC, Dept. Physical and Environmental Sciences, 1265 Military Trail, Scarborough, Ontario, M1C1A4, Canada. email: greenw@utsc.utoronto.ca Former Graduate Students 2 , Scarborough College Coastal Research Group, UTSC, 1265 Military Trail, Scarborough, Ontario, M1C1A4, Canada. Currently, Assistant Professor 3 , Dept. of Environmental Studies, University of West Florida, 11000 University Parkway, Pensacola, FL 32514, U.S.A. ABSTRACT Field measurements were made of the waves, currents (cross-shore and longshore) and the associated suspended sediment flux in a multi-barred, upper shoreface in southeastern Lake Huron using collocated electromagnetic flow meters and optical backscatter sensors. Measurements within the inner surf zone during a single storm event revealed: (a) dominant forcing for sediment flux was the mean cross-shore and the mean longshore current. The former was a direct result of wind and wave set up at the shoreline. (b) oscillatory transport was variable in terms of the frequency, direction and overall significance to the net flux of sediment. The major forcing was at infragravity frequencies, with little net transport at incident wave frequencies. (c) the longshore transport was dominated by the mean longshore current, but a significant sediment flux occurred a far infragravity frequencies (~0.001-0.003 Hz). The overall morphological response of the multi-barred profile was: (a) an enhancement of the innermost bar; (b) an offshore migration of both the inner and second bar crests, with an increase in the amplitude of the forms in both cases; (c) the beach face and step were eroded during the storm, loosing ~0.43 m 3 m -1 of sediment per unit shoreline width, causing significant recession (~3.5 m); (d) accretion on the crests and erosion in the troughs increased the relative relief and bar definition; however, this was associated with an overall erosion of the upper shoreface profile, which experienced a net loss of sediment volume of ~2.5 m 3 m -1 of shoreline width; and (e) accretion (up to ~ 0.70 m) occurred on the lakeward slope of the 3 rd bar in the outer surf zone, although the longshore current probably carried significant amounts away from the study area. INTRODUCTION The dynamics of multi-barred shorefaces are of considerable interest to coastal scientists because of the implied requirement of a cross-shore net flux of sediment that is spatially periodic in order to maintain the topographic highs (crests) and lows (troughs). Increasingly it is evident that sediment flux is not a simple coupling between the incident wave oscillatory currents and the bed and suspended load. For example, gradients in the mean water level across-shore (set-up) forced by wind stress and gradients in radiation stress under breaking waves (Svendsen, 1984 a, b; Greenwood and Osborne, 1990) induce near-bed return flows that are important not only hydrodynamically, but are also capable of considerable offshore sediment transport (Greenwood & Osborne, 1992; Masselink & Black, 1995). Secondary, low frequency oscillatory currents are also superimposed on the incident gravity waves and are capable of inducing transport both on- and off-shore (Osborne & Greenwood, 1992;