An Analysis of the Movement of Flood Waters by Aerial Photography

Abstract

The author from 1963 to 1966, in collaboration with the Ministry of Construction, made observations of the movement of flood waters in 13 main rivers in this country, using aerial photogrammetry. Our rivers are characterized by their swift currents and after a typhoon, when the water increases rapidly, the surface of these swift flood currents is covered with foam and there appears, when observed from an aircraft, a difference in hue in the muddiness caused by the macroturbulence. By shortening the time between exposures in the aerial photograph, we were able to measure the water current velocity over the entire surface, using the parallax method (Cameron effect) .Although the distribution of the current velocity over the entire surface at any given moment is extremely complicated, there could be observed a certain common pattern. Upon analysis of this pattern, we could determine a very close similarity to that pattern displayed by the distribution of surface current velocity observed in an experimental flume, where it is known that there are, along the longitudinal axis, parallel cycloidal flows, in which adjacent vortices have opposite directions of rotation.This analysis gives us cause to believe that it may now be possible by aerial photogrammetry not only to measure the current velocity of, but also to understand the structure of turbulence in the flood waters of our swift current rivers, which are otherwise unapproachable. Various basic problems which arise in connection with the measurement of turbulence by photogrammetry are also to be considered.