The interaction between bed-load transport and dune orientation

Bedforms play an important role in the sediment transport of a river. Because of their importance many experiments have been carried out to better understand bedforms. Most experiments concern transverse bedforms (with crests aligned perpendicular to the flow direction). However, bathymetry measurements show many instances of oblique dunes. Oblique bedforms induce flow and sediment transport in the transverse direction. Based on the flume experiments of Talmon [2009], Sieben & Talmon [2011] proposed formulae to determine the bed-load transport direction over oblique dunes. To apply these formulae the dune orientation has to be known. Therefore, Sieben & Talmon [2011] proposed to derive the dune angle by relating it to the relative dune migration rate along the dune crest. This was applied in the preliminary work of Weij [2012]. It was found that the current formulae give unrealistic results. The main objective of this thesis is to improve the prediction of dune orientation angle for large-scale modelling. The mechanisms involved in the formation of oblique dunes are investigated in a simplified environment. For the simplified environment we created a model based on the model created by Jerolmack & Mohrig [2005]. The model calculates the bed level change in one-dimensional slices, which are then coupled with transverse sediment transport to create a quasi 2D model. The results of show a dune orientation that eventually finds an equilibrium. Two explanations were given: (1) A gradient in transverse sediment transport can decrease or increase the dune migration rate, (2) Dune crests break up, and merge before reaching a larger angle. Based on these findings, the calculation of the dune angle is adapted in three ways: (1) The effect of transverse sediment transport on migration rate, (2) a reduced dune height for larger angles, (3) a limited dune orientation angle. The first measure is promising; it reduces the calculated dune orientation angles to more realistic values. However, the current implementation is problematic. The second measure is simple to implement and also reduces the dune orientation angle to more realistic values. However, the factor used to reduce the dune height has no physical basis. The third measure did not lead to satisfying results, but can be combined with the other measures. In the simplified environment cross-stream sediment transport was based on just the transverse slope. However, the flume experiments by Talmon [2009] show that sediment transport also depends largely on the changed flow pattern near the bed. We analysed the flume experiments carried out by Talmon [2009]. These experiments were carried out to quantify the transverse sediment transport over oblique dunes. Furthermore, we carried out detailed three-dimensional flow simulations. We used the detailed model of Nabi [2012] to carry out these flow simulations. Sieben & Talmon [2011] derived formulae for the mean sediment transport direction based on the experiments of Talmon [2009]. During our analysis of the flume experiments and three-dimensional flow simulations we found three main improvements of the formulae by Sieben & Talmon [2011]: (1) The length of the zones as proposed by Sieben & Talmon [2011] are inconsistent with the lengths seen in the flume experiments. (2) The relation between dune angle and crest parallel flow velocity appears to be exponential. (3) The near-bed flow direction on the stoss side of the dune (outside the transition zone) was assumed to be equal to the main flow direction. We found that there is a small but noticeable effect of the dune angle on the near-bed flow direction.