Abstract
Barrages constructed over these last ten years were built using the New Design Standardsauthorized by the Ministry of Construction. Some of them have caused by deposition of sand andgravel on the sill of barrages during big floods.
The study of typical cases by analysing (1) surveying maps and/ or (2) aerial photograghs revealed such conclusions as follows:
(1) The New Design Standards enforced engineers to design and construct barrages with their sill elevation equal to that of the “planned” river bed, which is lower than natural one.
(2) In most observed cases, the dredged and mined reach of river bed around a barrage is not so sufficient that sand and gravel accumulate on the sill of the barrage during big flood.
(3) If deposition on a sill of a barrage consists mainly of sand, deposition under gates can be flushed by the operation of sluice gates during the falling stage of a flood.
(4) If deposition on a sill of a barrage consists of gravel, gravel is too heavy to be flushed. So some precaution against deposition is needed. The most likely technical coutermeasure is to dredge a sufficient reach length around the barrage. The most practically proposed dredging reach is about six times as long as the width of the river channel upstream of the barrage and two times the downstream.
