In deep ebb channels carrying a large volume of ebb-tidal flow, velocities are high, up to 

 200 centimeters per second, but the dominant bed forms are sand waves. (Figure 24.) Where 

 velocity asymmetry is low (Station A-3, Figure 24), the sand waves are nearly symmetrical 

 but where velocity asymmetry is high, the sand waves are ebb-oriented. (Station BR-N, 

 Figure 24). Diver observation confirms that when average velocity exceeds 80 centimeters 

 per second, megaripples are superimposed on the sand wave form. Planed-off megaripples 

 are common at these high velocities, including regressive ripples, and ripples migrating 

 transversely down troughs and across sand-wave shpfaces. 



Figures 25 through 28 summarize sand movement at Station 2, the symmetrical sand 

 wave station. Major sand movement is by megaripple migration. During the flood-tidal cycle, 

 megaripples migrate up to the sand wave crest and deposit sand on the avalanche shpface of 

 the sand wave. (Figures 25 and 26). During the ebb-tidal cycle, the flood-oriented sUpface of 

 the sand wave is modified by ebb megaripple migration up and over the slipface. (Figures 27 

 and 28.) Net migration of the sand wave slipface was 16 centimeters in the flood direction, 

 illustrating that tidal-current flow at this station is shghtly flood-asymmetric. 

 IX. SUMMARY OF BED FORM DISTRIBUTION 



The bed form distribution pattern of the Parker estuary flood-tidal delta shown in 

 Figure 29 is based on 35 kilometers of bottom profiles and diver observation. These data 

 show that megaripples occur on ebb shields, sand waves on large inclined areas seaward of 

 the shields, and transition bed forms in imperfectly shielded areas. Deepwater sand waves, 

 flood- or ebb-oriented, with superimposed megaripples, occur in channels around the 

 tidal-delta wedge. The same orientation pattern occurs in the Essex estuary. (Figure 30.) 



In the lower Parker estuary (Fig. 31) deepwater sand waves are the principal bed form 

 except where shallower depths lead to exclusively megaripple formation. 



X. CONCLUSIONS 



1. Bed forms are classified by spacing and not height. Ripples have spacing less than 60 

 centimeters; megaripples 60 centimeters to 6 meters; and sand waves greater than 6 meters. 



2. Average velocity curves show that megaripples are associated with a high maximum 

 flow velocity and little or no velocity asymmetry; sand waves have a lower maximum 

 velocity and high-velocity asymmetry. A third bed form type, transitional in spacing 

 between megaripples and sand waves, is associated with tidal-current flow of lower 

 maximum velocity than megaripples and Uttle velocity asymmetry. 



3. Megaripples migrate in both flood- and ebb-current directions; most intertidal sand 

 waves migrate in a flood direction and do not migrate during ebb flow. Sand wave and 

 megaripple slipface migration begins at about 60 centimeters per second, but flow over 

 megaripples reaches a higher maximum velocity for a longer timespan. Maximum sand wave 

 slipface migration is during fuU-moon spring tides (40 centimeters per tidal cycle) but is 10 

 to 50 times less than megaripple migration rates (450 centimeters per tidal cycle in either 

 flood or ebb direction), i 



4. A sequence of bed forms based on increasing flow strength, with velocity the most 

 important parameter, has been established. (Figure 14.) Velocity— depth plots may be used 

 to delineate fields where each member of the sequence occurs. 



28 



