Rip currents. Rip currents are also important in transporting sediment 

 in an offshore direction (Field and Roy 1984). Bowen and Inman (1969) 

 and Cook and Gorsline (1972) report that during the winter season, cross- 

 shore movement of sediment by rip currents is in an offshore direction. 

 Once transported offshore, sediment is confined by predominant seaward 

 oscillations caused by steep waves and strong winds. During summer, 

 long-period swells transport sediment landward to replenish the beach. 

 Cook and Gorsline (1972) also present a sediment transport system 

 whereby sediment is transported offshore outside of the breaker zone by 

 rip currents and general diffusion, and then onshore by wave action, 

 which separates silt and clay from sand. Sand is then moved alongshore 

 to depths dependent upon wave characteristics. Silt and clay are separated 

 in the sorting process and move out of the coastal drift system in 

 suspension. 



Reimnitz et al. (1976) used side-scan sonar to show seaward-trending 

 ripples out to depths of 30 m that are attributed to storm rip currents. 

 Cowell (1986) measured rip currents off headland-bounded beaches dur- 

 ing storms and measured velocities of greater than 1 m/sec extended to 

 hundreds of meters past the surf zone. However, Field and Roy (1984) be- 

 lieve that rip currents probably do not transport sand to a depth greater 

 than 45 m. 



Seymour (1983), in experiments at Santa Barbara, Torrey Pines, and 

 Virginia Beach (as part of the Nearshore Sediment Transport Study), also 

 documented rip currents as a mechanism of offshore sediment transport. 

 During periods of intense storm waves, Seymour (1983) documented the 

 formation of offshore bars, particularly at Santa Barbara. The formation 

 of these bars is attributed to excessive longshore sediment transport and 

 rip current outlets during these storms. 



Hyperpycnal plumes. Hyperpycnal plumes, or sediment/water flows 

 of dense concentration that plunge under flows of less dense concentration 

 associated with gravity flows (Bates 1953), may also result in seaward 

 transport where fine-grained sediments are present (no autosuspension is 

 needed). In studies by Wright et al. (1991), where bed slope was 0.6 deg, 

 suspended sediment concentrations were as high as 10 g/1, and underflows 

 were as thick as 2 m with downslope speeds of 10-40 cm/sec, Wright et al. 

 (1991) attributed this offshore-directed sediment flow to a rise of 0.6 m in 

 mean water level (during this particular storm) and a resultant strong 

 seaward-directed downwelling flow. 



Bar formation/migration. Osborne and Greenwood (submitted, 1992) 

 determined that cross-shore sediment transport at a non-barred inner shelf 

 in Nova Scotia and a barred inner shelf at Georgian Bay are similar and a 

 function of the following parameters: 



a. Local wind-forced low-frequency waves. 



Chapter 3 Evidence of Cross-Shore Sediment Transport 



29 



