supplement these studies, this paper investigates the weir techniques under the flood control system in the Edo era. In an attempt to consider this topic in a wider perspective, some irrigation weirs and some river separation projects are included in this investigation.
In those days, the weirs which divert river flow from main streams to diversion channels had been constructed at the mouth of the diversion channels. The structures of these weirs were of two types. One type allows flood to overflow pass the top of the weir gate. The other type has an opening at the center of the weir gate to keep the normal flow toward a diversion channel for use in irrigation and water transportation. During the Edo era, these weirs were built up of only a combination of natural materials, e. g., stones, soil, wood and plants. As such, these were, consequently, broken down by floods in a few years' time. Each Prefectural management, therefore, had to repeatedly repair its diversion weir to keep its flood control system using a diversion channel. Due to these technical weakpoints in the diversion weir during the Edo era, some big projects in the construction of diversion channels could not be started. These weak points had been solved by the development of modem weir technology by Japanese river engineers. The unfinished projects were then carried out using the modem technology.
This paper also clears some structural and constructional techniques of weirs under the irrigation and the river separation project used during the Edo era.
