Turbulent flow and drag over fixed two‐ and three‐dimensional dunes

Detailed measurements of turbulent flow were obtained over a fixed flat bed, two‐dimensional (2‐D) dunes and four types of three‐dimensional (3‐D) dune morphologies including (1) full width saddles, (2) full width lobes, (3) sinuous crests, and (4) irregular shaped crests. The time‐averaged turbulence structure over the fixed flat bed was dynamically similar to flow over a flat bed with active sediment transport. The flow field over 2‐D dunes conforms with previous observations of flow over mobile and fixed bed forms. Bed form three dimensionality significantly altered the flow field observed over the 2‐D dunes. Lobe‐shaped dune crest lines, curved crests with topographic highs that bowed downstream, enhanced the level of turbulence producing a better defined wake structure and more vigorous mixing in the separation cell than observed over 2‐D dunes. Saddle‐shaped dune crest lines, curved crests with topographic highs that bowed upstream, diminished the level of turbulence suppressing a well‐defined wake structure and mixing in the flow separation cell. Flow over sinuous‐crested bed forms with lobes and saddles was dynamically similar to flow over full width bed forms. Development of in‐line paired saddles and lobes in the downstream direction appears to be controlled by near‐bed velocity gradients, which were larger over saddles and smaller over lobes. These gradients control local boundary shear stress patterns that should promote scour and high transport rates over saddles and deposition on lobes. Two‐dimensional and 3‐D bed forms with the same height and length offered different levels of flow resistance. The flow observations support the idea that saddles decrease flow resistance and lobes increase flow resistance relative to a 2‐D dune. Bed form crests composed of irregularly spaced lobes and saddles suppressed turbulent flow structure development and reduced drag by 20% below levels for 2‐D or sinuous crested dunes. The results suggest that bed form crest shape needs to be accounted for in flow resistance calculations and that a single drag coefficient is inadequate where the bed can evolve through 2‐D, 3‐D, and irregular 3‐D bed form states.