Image analysis to measure sorting and stratification applied to sand-gravel experiments

Doctoral thesis English OPEN
Orrú, C. (2016)

The main objective of this project is to develop new measuring techniques for providing detailed data on sediment sorting suitable for sand-gravel laboratory experiments. Such data will be of aid in obtaining new insights on sorting mechanisms and improving prediction capabilities of morphodynamic models. Two measuring techniques have been developed. The first technique is aimed at measuring the size stratification of a sand-gravel deposit through combining image analysis and a sediment removal technique. The image analysis technique is based on color segmentation of areal images of a bed surface composed of colored sediment. After taking images a thin layer of sediment is removed, and a new series of images is taken of the newly exposed part of the deposit. The resulting size stratification data is characterized by a high spatial resolution. Using the technique one can avoid time consuming sieve analysis, and the data is processed using an automatic and fast image processing algorithm. The second technique is aimed at measuring the spatial and temporal changes in the bed surface texture during flow in a sand-gravel laboratory experiment. The above image analysis technique is used to process images of the bed surface taken during flow. A floating device, which is moved over the entire observation section, enables taking images of the bed surface. Two degradational laboratory experiments were conducted. In the first experiment the initial bed was composed of a bimodal mixture of gravel and sand and it was characterized by a stepwise downstream fining pattern and a downstream sand reach. Only the sand fraction was mobile. A static armor formed over the bimodal reach due to a lack of sediment supply and conditions of partial transport. The armoring over the bimodal upstream reach resulted in an abrupt spatial transition in mean grain size of the bed surface and in bed slope between the upstream reach and the downstream sand reach. The difference in the bed slope and so the flow depth between the two reaches led to the formation of an M1 backwater curve over the bimodal reach. The streamwise increase of the flow depth and the associated decrease of the sediment mobility resulted in downstream fining over the upstream reach. Despite the steady water discharge the morphodynamic equilibrium state was characterized by the presence of a backwater curve. This was because partial transport conditions prevented the bimodal reach from establishing normal flow conditions. In this experiment partial transport conditions controlled the temporal changes of the considered sand-gravel reach. The laboratory experiment was reproduced using a numerical morphodynamic sandgravel model. The numerical model was based on the Hirano active layer model and the backwater equation. The detailed data on spatial and temporal changes of the bed surface texture measured using image analysis were of aid in calibrating the model, in particular in tuning the hiding coefficient in the sediment transport relation. We found a good agreement between measured and predicted data. In the second experiment we investigated the stability of an armor under high flow conditions. The armor was created under a low flow rate and a lack of sediment supply, using a trimodal mixture composed of sand and gravel. The increased water discharge induced a breakup of the armor, which reformed under continued peak flow conditions. The breakup occurred through the dislodgement of the coarse surface particles which then exposed the finer substrate. Consequently, the bed surface locally became finer, which allowed for some degradation. Yet quickly a new armor formed that was coarser than the initial one. The armor breakup corresponded to a sudden and local increase of the sediment transport rate that gradually decreased during armor reformation.
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    JOURNAL ARTICLES • Orrú C., Chavarrías V., Uijttewaal W. S. J., and Blom A., Image analysis for measuring the size stratification in sand-gravel laboratory experiments, Earth Surface Dynamics 2, 1, 217-232 (2014).

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