The morphodynamics of fluvial sand dunes in the River Rhine, near Mainz, Germany. I. Sedimentology and morphology

The dynamics of large isolated sand dunes moving across a gravel lag layer were studied in a supply-limited reach of the River Rhine, Germany. Bed sed iments, dune geometry, bedform migration rates and the internal structure o f dunes are considered in this paper. Hydrodynamic and sediment transport d ata are considered in a companion paper. The pebbles and cobbles (D-50 of 1 0 mm) of the flat lag layer are rarely entrained. Dunes consist of well-sor ted medium to coarse sand (D-50 of 0.9 mm). Small pebbles move over the dun es by 'overpassing', but there is a degree of size and shape selectivity. P opulations of ripples in sand (D-50 < 0.6 mm), and small and large dunes ar e separated by distinct breaks in the bedform length data in the regions of 0.7-1 m and 5-10 m. Ripples and small dunes may have sinuous crestlines bu t primarily exhibit two-dimensional planforms. In contrast, large dunes are primarily three-dimensional barchanoid forms. Ripples on the backs of smal l dunes rarely develop to maximum steepness. Small dunes may achieve an equ ilibrium geometry, either on the gravel bed or as secondary dunes within th e boundary layer on the stoss side of large dunes. Secondary dunes frequent ly develop a humpback profile as they migrate across the upper stoss slope of large dunes, diminishing in height but increasing in length as they trav erse the crestal region. However, secondary dunes more than 5 m in length a re rare. The dearth of equilibrium ripples and long secondary dunes is prob ably related to the limited excursion length available for bedform developm ent on the parent bedforms. Large dunes with lengths between 20 m and 100 m do not approach an equilibrium geometry. A depth limitation rather than a sediment supply limitation is the primary control on dune height; dunes rar ely exceed 1 m high in water depths of approximate to 4 m. Dune celerity in creases as a function of the mean flow velocity squared, but this general r elationship obscures more subtle morphodynamics. During rising river stage, dunes tend to grow in height owing to crestal accumulation, which slows do wnstream progression and steepens the dune form. During steady or falling s tage, an extended crestal platform develops in association with a rapid dow nstream migration of the lee side and a reduction in dune height. These dim inishing dunes actually increase in unit volume by a process of increased l eeside accumulation fed by secondary dunes moving past a stalled stoss toe. A six-stage model of dune growth and diminution is proposed to explain var iations in observed morphology. The model demonstrates how the development of an internal boundary layer and the interaction of the water surface with the crests of these bedload-dominated dunes can result in dunes characteri zed by gentle lee sides with weak flow separation. This finding is signific ant, as other studies of dunes in large rivers have attributed this morphol ogical response to a predominance of suspended load transport.