交互砂州の発達過程に関する研究

Abstract

Alternating bars in rivers are very interesting phenomina and one of the most importantproblem in river mechanics. The hydraulic conditions of their formation and flowcharacteristics over them have been studied by many investigators and were clarifed inauthors' previous papers. But the process and mechanism of their development have notyet been made clear, neverthless which are expressed in several stability theories byusing basic equations in fluvial hydraulics with various kind of assumptions and are solvedby mathematical and numerical methods, because no confirmative investigation has beenconducted on them by means of detailed expriments. The elucidation of this process andmechanism not only varifies the validity of assumptions in stability theories, but also throwssome new light on the physical meanings of hydraulic parameters of bar formation andprediction of bar deformation under varing flow condition.Several experiments with detailed measurement of bar geometry and stream bed variationis carried out, changing channel width broadly in this study. It is suggested that thisprocess is devided into three stages on considering development of bar geometry and themechanism. These stages are as follows: 1st stage is the period in which bar edge hasbecome clear and bar length has almost developed, 2nd stage is the period in which barincreases in its height to a equilibrium value, and 3rd stage is a kind of equilibrium stateof bar geometry. Longitudinally averaged bed shows a Quosset hut-shape cross section inthe 2nd stage, the height of which increases corresponding to bar height, due to lateralsediment transport caused by meandering characteristic of flow over alternating bar. Onthese results of the experiments, the time during which bar has developed is proved tobe propotional to product of channel width (or bar length) and bar height devided by bedload per unit width.