Field experiment to restore a gravel bar and control growth of trees in the asahi river

The number of active gravel bars on the Asahi River in Okayama Prefecture (located in western Japan) has continuously decreased over the last 20 years, whereas stable vegetated areas have rapidly expanded on river bars during the same period. In a basic study to examine the re-establishment of gravel bars as a river restoration technique, an artificial gravel bar was experimentally created in the field. The effect of flood flows on the bar was then examined. Preliminary numerical simulations of the annual scale flood predicted that flooding strong enough to move stones would uniformly flow over the restored gravel bar and that the gravel bed could be maintained due to the increase of shear stress on the bed. However, monitoring of the restored bar showed that branches of Salicaceous species (willows) and seeds of Eragrostis curvula (weeping love-grass) quickly re-vegetated the downstream part of the test area. Additional numerical simulations, using the observed hydrograph, suggested that branches and seeds would be brought to the bar during the flood recession period.