transports 15m^/hr/m of sediment of shelf length. Net transport of sedi- 

 ment is offshore. These data suggest that a single severe storm may be 

 more effective in transporting sediment than several small storms. 



Komar, Neudeck, and Kulm (1972) discuss the production of orbitals 

 by surface waves, which in turn create ripples, and rework shelf sedi- 

 ments. Table 2 shows relationships between depth of rippling and a vari- 

 ety of surface wave conditions (after Komar, Neudeck and Kulm (1972)). 



Table 2 



Relationships Between Depth of Rippling and a Variety of Surface 



Wave Conditions (after Komar, Neudeck, and Kulm (1972)) 



Surface Wave 

 Conditions 



Wave Period 

 (sec) 



Significant 

 Wave Heigfit 

 (m) 



Deptfi of 

 Rippling (m) 



Orbital 

 Diameter (cm) 



Ripple 



Wavelength 



(cm) 



Average Summer 

 Waves 



12 



2.13 



85 



38.2 



12.6 



Average Winter 

 Waves 



12 



3.05 



99 



38.2 



12.6 



Large Storm 

 Conditions 



12 



9.14 



138 



38.2 



12.6 



Long-Period 

 Storm Waves 



15 



9.14 



204 



47.7 



10.3 



Symmetrical (wave-generated) oscillatory shore-parallel ripple marks 

 (see section in Chapter 4 titled "Examples of Inner Shelf Sedimentary Fea- 

 tures" for additional information on ripple symmetry) exist on the Oregon 

 continental shelf out to water depths of -204 m, while asymmetrical rip- 

 ples are rare. Symmetrical ripples are covered by bottom orbital veloci- 

 ties (as calculated by the Airy wave theory) as well as unidirectional 

 currents while asymmetrical ripples are believed to be produced by inter- 

 nal waves (15- to 30-min period), as they are more similar to unidirec- 

 tional currents. It is believed that upwelling currents could not have 

 formed ripples (Komar, Neudeck, and Kulm 1972). 



Larsen et al. (1981) determined that at the -100-m depth on the Wash- 

 ington shelf, for sediment sizes 0.03-0.07 mm, a bottom oscillating cur- 

 rent of 13 cm/sec is needed to suspend sediments. These types of currents 

 and waves are common during winter storms in Washington, where 

 100-cm/sec velocities associated with 15-sec waves have been measured. 

 Draper (1967) calculated that fine sand on the shelf edge of Britain would 

 be moved at a depth of 183 m 20 percent of the year. Sternberg and 

 Larsen (1976) found that relatively frequent grain motion occurs at the 

 -75-m depth on the Washington shelf. 



In addition, computations of bed-load transport by Madsen and Grant 

 (1976) have shown that for conditions with 1.5-m, 13-sec waves, bed load 

 was entrained to a depth of -16 m. 



34 



Chapter 3 Evidence of Cross-Shore Sediment Transport 



