4. Study Methods 



The barge was usually moored at the diving station the day before the station was to be 

 occupied. Generally, current velocities were read over the crest of large-scale bed forms 

 (sand waves), thus the barge was moored with the boom over a crest. Figure 7 shows the 

 barge at an intertidal sand wave station. A 3-meter tide staff was placed near the barge for 

 easy reading. (Figure 7.) At intertidal locations two staffs were used, one intertidal and one 

 subtidal. Relative elevations of each were obtained by transit. 



Four persons generally made up a diving station crew: 2 divers, a diver tender and a 

 person to record velocity, depth, and temperature measurements. Tide gauge readings were 

 taken every 7.5 minutes, and velocity, depth and temperature readings every 15 minutes. 

 The divers also obtained velocity measurements from floats, and recorded depth over bed 

 form crests with a simple scale. 



m. BED FORM CLASSIFICATION 



In this study, bed forms are classified by spacing: ripples, spacings less than 60 

 centimeters; megaripples, spacings from 60 centimeters to 6 meters; and sand waves, 

 spacings greater than 6 meters. Figure 9 illustrates megaripples with superimposed ripples; 

 Figure 10 shows sand waves with a spacing of about 14 meters. Klein (1970), in a study of 

 Bay of Fundy intertidal bed forms, groups megaripples and small sand waves together as 

 dunes and has a separate category for large sand waves. Allen (1968) makes no distinction 

 between megaripples and sand waves, classifying both as large-scale ripples. 



Glossaries by Allen (1972), and the U.S. Naval Oceanographic Office (1966), define 

 megaripples and sand waves as the same bed form. A task force report on bed forms in 

 alluvial channels, prepared for the Hydraulics Division, American Society of Civil Engineers 

 (1966), uses the term sand wave to describe several bed form types, and megaripple is not 

 included in their nomenclature. These bed form classifications are compared to the 

 classification in this study, (see Table.) 



As well as a spacing difference, megaripples are morphologically distinct from sand 

 waves. Megaripples are characterized by sinuous to highly cuspate crests, usually with 

 well-developed scour pits in front of the crests. Some megaripples have straight crests and 

 scour pits occur at intervals along the crest. Megaripples also have a small spacing-to-height 

 ratio in comparison with sand waves. 



Sand waves have straight to sinuous crests; scour pits are absent or at best poorly 

 developed. Sand waves, which have a large spacing-to-height ratio, are termed 

 two-dimensional bed forms by some workers, whereas the megaripples are termed 

 three-dimensional bed forms. 



Heights and spacings of bed forms were measured on a 50-by 50-meter grid system for 

 most of the intertidal parts of flood- and ebb-tidal deltas, and on two lines of measurement 

 in subtidal flood channels. The results of the field measurements are plotted in Figure 11. 

 Megaripples (solid circles) show a good grouping at spacings less than 6 meters, while sand 

 waves (solid squares) show a wider spread of spacings. Sand wave spacings tend to 

 concentrate at 11 to 16 meters. Megarij)ples appear to increase in height with increase in 

 spacing, although there is much scatter. Sand waves show no height-spacing trend, nor do 

 megaripples and sand waves when considered together. These two morphologically different 

 bed forms, megaripples and sand waves, thus show a distinct spacing difference. 



10 



