VI. BED FORM SEQUENCE 



This study of flow conditions and bed form morphology has led to the recognition of a 

 sequence of intertidal estuarine bed forms based on increasing /Zou) strength, with velocity 

 as the major contributing parameter. This sequence, shown in Figure 19, is a modification of 

 an earlier version (Boothroyd, 1969), differing mainly in terminology. The sequence may be 

 compared to that of Simons, Richardson, and Nordin (1965), shown at bottom of figure. 



The sequence begins with Unear ripples and goes to cuspate ripples, termed low- and 

 high-energy ripples by Harms (1969). The sequence continues to linear megaripples, which 

 may be transition bed forms. Increasing flow strength causes a change to cuspate 

 megaripples with well-developed scour pits and then to planed-off megaripples. Planed-off 

 megaripples are of two types: 1) short spacings analogous to washed-out dunes of Simons, 

 Richardson, and Nordin (1965); and 2) long spacings which plot near the 6-meter boundary 

 in Figure 11. Rhomboid megaripples, the last form in the sequence, show little sUpface 

 development and are essentially a plane bed form. These bed forms were discussed by Smith 

 (1971). Sand waves represent an end member on a separate branch at lower flow strengths. 



A plot of velocity versus log-depth (Figure 20), similar to those of Southard (1971), 

 delineates fields where each member of the sequence of bed forms occur. Since unsteady 

 flow conditions occur throughout the tidal cycle, bed form morphology is constanfly 

 changing and a given bed form may be stable only during a part of the tidal cycle. Diver 

 observation of bed form changes was used to establish field boundaries. Most measurements 

 in Figure 20 were for water depths greater than 20 centimeters ranging up to 300 

 centimeters. 



Vn. INTERTIDAL BED FORM ORIENTATION AND DISTRIBUTION PATTERN 



Complex intertidal and sub tidal topography controls bed form type and orientation on 

 tidal deltas. Figure 12 illustrates bed form distribution on the intertidal part of the Parker 

 estuary flood-tidal delta; Figure 21 gives bed form orientation on a part of the flood-tidal 

 delta. (Compare Figure 16 with Figure 8.) Megaripples occur on low-intertidal ebb shields 

 subject to liigh-velocity flood-and-ebb flow. Flood-oriented sand waves occur in areas 

 shielded from ebb flow, and transition bed forms occur in partially shielded areas high on 

 ebb shields and in shallow channels. 



Bed form orientations at low water show a strong bimodal pattern (Figure 21.) Sand 

 waves remain flood-oriented throughout the tidal cycle, while megaripples and transition 

 bed forms become alternately flood- and ebb-oriented. Figure 21 shows the tight 

 class-interval grouping by sand waves and the more diverse megaripple and transition bed 

 form pattern. 



Vni, SUBTIDAL FLOW CONDITIONS AND BED FORM ORIENTATION 



In subtidal channels, bed form type and orientation is controlled by complex intertidal 

 and subtidal topography. Figure 22 shows a longitudinal and a transverse bottom profile of 

 subtidal and intertidal topography and the nature of flood versus ebb channels, (see Figure 

 12 for profile locations). 



Tidal-current velocity curves for two subtidal stations on the flood-channel profile are 

 shown in Figure 23. The curves are similar in maximum flow velocity and flood asymmetry. 

 Their is also a similarity to the velocity curve of the flood-oriented intertidal sand waves. 

 (Figure 14.) Therefore, flood-oriented sand waves, shielded from ebb flow, are similar in 

 morphology, migration habit, and crossbed type whether in depths of 7 meters (MLW), less 

 than 2 meters (MLW), or intertidal. 



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