Erosive bar migration using density and diameter scaled sediment

Abstract

Mobile bed scaling (for scales smaller than 1:1) does not allow for ideal similarity between all nondimensional parameters. This leads to an exaggerated presence of one process over another or the development of scale effects. The complexity of the surf zone makes it especially difficult to separate and quantify the effect of sediment scaling on each individual process. As a result, the impact of sediment scaling on the holistic morphodynamic process is poorly understood. Profiles were obtained at TU Delft (scale 1:10) following the testing regime used during the SANDS project. This set made use of density and diameter scaled sediment (?s = 1200 kg/m3 and d50 = 0.52 mm); scaled according to the bed load model of Henriquez, Reniers et al. (2008). The set used for analysis here includes the Hannover (prototype scale 1:1) and Deltares (Froude scale 1:6) profiles obtained during SANDS. The present work looks to extend what is currently known about the impact of sediment scaling on nearshore transport processes. Three main objectives are identified: 1) Showing profile development and inferred transport rates, compare nondimensional parameters across the nearshore and surf zone; 2) Identify how spatial variance of dominate hydrodynamic and sediment transport regimes across the surf zone are influenced by the subsequent scaling; 3) Explain morphological differences due to scaling by observing the small scale transport process. Analysis of the profiles showed a marked degree of geometrical similarity between all three facilities. More specifically, the development of a bar-trough system and subsequent offshore migration. The transport rates were, however, divergent. Compared to Hannover, the transport rates at Deltares were low where conversely TU Delft was high. Using ideal scaling theories as a basis, the cross shore distribution of the nondimensional parameters at TU Delft suggested that both sediment mobility and near bed suspension were similar with prototype. However, due to a large fall velocity, the Dean number was underestimated. From the use of Froude scaling at Deltares, the nondimensional parameters suggested that neither near bed nor suspended load transport regimes were reproduced correctly. This can mostly be attributed to the rippled bed state and reduced Shields value. A simple depth and time averaged model was also used to decompose the sediment concentrations. These concentrations were used to check consistency of the nondimensional parameter agreement with the manifestation of the physical process. Inconsistent with the nondimensional results, TU Delft showed that near bed sediment mobility and suspension was high. It is believed this led to the exaggerated sediment fluxes. Concentrations at Deltares suggest the rippled bed introduced a thinned boundary layer which caused decreased sediment mobility and the decreased sediment fluxes.