Articles | Volume 4, issue 2
Research article
09 Jun 2016
Research article |  | 09 Jun 2016

Armor breakup and reformation in a degradational laboratory experiment

Clara Orrú, Astrid Blom, and Wim S. J. Uijttewaal

Abstract. Armor breakup and reformation was studied in a laboratory experiment using a trimodal mixture composed of a 1 mm sand fraction and two gravel fractions (6 and 10 mm). The initial bed was characterized by a stepwise downstream fining pattern (trimodal reach) and a downstream sand reach, and the experiment was conducted under conditions without sediment supply. In the initial stage of the experiment an armor formed over the trimodal reach. The formation of the armor under partial transport conditions led to an abrupt spatial transition in the bed slope and in the mean grain size of the bed surface, as such showing similar results to a previous laboratory experiment conducted with a bimodal mixture. The focus of the current analysis is to study the mechanisms of armor breakup. After an increase in flow rate the armor broke up and a new coarser armor quickly formed. The breakup initially induced a bed surface fining due to the exposure of the finer substrate, which was accompanied by a sudden increase in the sediment transport rate, followed by the formation of an armor that was coarser than the initial one. The reformation of the armor was enabled by the supply of coarse material from the upstream degrading reach and the presence of gravel in the original substrate sediment. Here armor breakup and reformation enabled slope adjustment such that the new steady state was closer to normal flow conditions.

Short summary
Our study focuses on armor breakup and reformation in a laboratory experiment under conditions with limited sediment supply. We developed a novel technique that allows for measurement of the temporal and spatial changes of the bed surface texture during flow. The armor, which was formed under base flow conditions, broke up under a higher flow rate, which led to a fining of the bed surface and local degradation. Under continued peak flow a new armor formed that was coarser than the initial one.