Transition bed forms (Fig. 11) are concentrated around a 6-nieter spacing. They occur in 

 intertidal areas at margins of sand wave fields and at junctures between sand wave and 

 megaripples zones. Transition bed forms resemble dwarfed sand waves, with straight crests, 

 unable to fuUy develop in the hydrologic regime in which they formed. The spacing 

 differences of all three types may be explained by differences in hydrodynamic conditions 

 governing the formation and migration of the bed form. 



IV. FLOW CONDITIONS 



Current stations were occupied for a complete tidal cycle at over 60 intertidal and 

 subtidal locations in both estuaries. Figure 12 shows the location of 16 stations near the 

 Parker estuary flood-tidal delta complex. A segment of this flood-tidal delta can be used to 

 illustrate the complex relationship of flow conditions to bed forms and topography. This 

 segment, outlined in Figure 12, is shown in an aerial oblique view in Figure 13. 



Megaripples, sand waves and a transition form (small ebb -oriented sand waves), were 

 continuously monitored by divers at three intertidal diving stations. (Figure 13.) Depth and 

 velocity measurements were recorded at 15-minute intervals. Velocity measurements were 

 made over the crest of the bed form 30 centimeters below the surface. Repeated 

 measurements show equal velocity values from the surface to 20 centimeters above the bed 

 (limit of the ducted-rotor current meter). 



Average velocity curves (Fig. 14) show that megaripples have a high maximum flow 

 velocity (103 centimeters per second) and little or no velocity asymmetry; sand waves have 

 a lower maximum flow velocity (78 centimeters per second) and large velocity asymmetry; 

 and transition forms a lower maximum velocity (64 centimeters per second) and little 

 velocity asymmetry. 



Velocity asymmetry in New England estuaries is discussed by Hayes, Anan, and Bozeman 

 (1969), on differences between sand flats and channels. The sand wave curve (Fig. 14) is a 

 typical sand-flat curve but the megaripple curve, also from a sand-flat station, does not 

 exhibit typical velocity asymmetry. This is due to its location on an ebb shield which will be 

 discussed later. 



V. BED FORM MIGRATION 



Figure 15 illustrates velocity— bed form migration differences between megaripples and 

 sand wave stations. Ripple migration begins at about 30 centimeters per second and 

 megaripple migration at about 60 centimeters per second. Megaripples migrate during flood 

 and then reverse and migrate in an ebb direction for an approximately equal timespan. 

 Average megaripple migration rate is about 120 centimeter per hour (Fig. 16) and total ebb 

 distance migrated was 450 centimeters. Significant migration occurs during falling velocities 

 and water depths as the bed form emerges at the end of the tidal cycle. 



Sand wave migration occurs only during a small part of the flood-tidal cycle and not at 

 aU on the ebb. (Figure 15.) Shpface migration begins at about the same velocity as 

 megaripple -sUpf ace migration, but flow over megaripples reaches a higher maximum velocity 

 for a longer timespan. Intertidal sand wave migration measured for 3 months at stations on 

 the Parker flood-tidal delta is shown in Figure 17. The stations are plotted in Figure 18. 

 Migration during neap tides was 5 to 10 centimeters per tidal cycle while migration during 

 spring tides (full moon) was 40 centimeters per tidal cycle. Hence, megaripple migration 

 occurs at a rate 10 to 50 times greater than sand wave migration. 



14 



