Avulsion Control Experiment using a Full-scale Levee at the Chiyoda Experimental Channel on the Tokachi River

 Photo 1 Site of the experiment
(click to enlarge)

 Photo 2 Control of the downstream
progress of avulsion by concrete armor
units (click to enlarge)

 Photo 3 Resumption of water flow after
removal of the concrete armor units.
(click to enlarge)

  Heavy rainfall disasters caused by typhoons and torrential rain have occurred many times in recent years, causing concerns over the occurrence of large-scale flood damage from inundation. Rivers have been consolidated, but levee breaches caused by overflow or the like still occur in large numbers, which has lead to the assumption that a levee breach has the potential to cause a catastrophic disaster in urban areas, where population and property concentrate.

  To investigate how avulsion occurs, the Civil Engineering Research Institute for Cold Region and the Ministry of Land, Infrastructure, Transport and Tourism have conducted studies using the Chiyoda Experimental Channel. The channel is one of the largest full-scale experimental channels in Japan (1300m long by 30m wide. It was installed on the Tokachi River, a Class A river in Makubetsu, Hokkaido, by the Hokkaido Regional Development Bureau for experiments and other studies. Still other studies on levee reinforcement have been conducted using full-scale models, as have studies on measures to control the inundation flow that results from the presence of riparian woods. Methods relating to these measures for the prevention of heavy rainfall disasters, however, have not yet been fully established.Therefore, in addition to measures for disaster prevention, such as levee maintenance, there is the need for improved controls to minimize damage in the case of levee breaches.

  For the purpose of mitigating flood damage from a levee breached by overflow or the like, the Civil Engineering Research Institute for Cold Region and the Hokkaido Regional Development Bureau have conducted experiments on a construction method that controls the progress of avulsion, using the full-scale levee at the Chiyoda Experimental Channel on the Tokachi River.

  In the experiment on avulsion control works, a group of blocks were installed on the slope of the levee (some of the blocks extended beyond the slope toe) 20m downstream from the point of artificially induced avulsion, as shown in Figure 1, and water was let flow from upstream. After the water was let flow, when the avulsion progressed downstream to the point of concrete armor unit (the avulsion control works), the concrete armor unit was let collapse with the failure of the levee.The downstream progress of avulsion was expected to be arrested by the collapsed concrete armor units which were expected to weaken the inundation flow from the river. The control effects and the like of the collapsed concrete armor units were examined.

  At approximately 180 minutes after the initiation of overflow, the avulsion reached the concrete armor units. After that, as shown in Figure 2, the avulsion widened to about 30m in length. The widening of the avulsion was arrested, and the stoppage of the downstream progress of the avulsion was confirmed. Then, to confirm the avulsion control effect of the concrete armor units, the blocks installed at the avulsion point were removed and water was let flow again. As a result, as shown in Figure 3, the avulsion began to progress downstreamward again, reaching about 50m in length, which confirmed that the concrete armor units had arrested the avulsion.

  The results of the experiment will be analyzed in detail to establish a more secure method of avulsion control.

(Contact: River Engineering Research Team, CERI)