SAND-DUNE GEOMETRY OF LARGE RIVERS DURING FLOODS

The geometry of lower regime bed forms in several large sand-bed river s is investigated during average and flood conditions. The van Rijn me thod is revised because it generally underpredicts the dune height of most large rivers around the world. During floods in large sand-bed ri vers, upper-regime plane bed is not necessarily obtained when T = 25. Both parameters describing dune height and dune steepness do not decre ase as the transport-stage parameter Tincreases in the range 10 < T < 25. The analysis of bed-form data during large floods on the Meuse Riv er and the Rhine River branches indicates that both the dune height an d length generally increase with discharge while dune steepness remain s relatively constant. A reasonable approximation of the wavelength is lambda congruent to 6.5 h, where h is the flow depth. The dune height Delta varies as a function of the depth h and median grain size d(50) . Estimates can be obtained by Delta congruent to 2.5 h(0.7) d(50.)(0. 3)