Hydrodynamic effects on refugia of cobble-bar vegetation established along river bend.

Akihiko Nakayama, Ichiro Fujita, Kayo Asami, Takeshi Kawatani

Friday 3 july 2015

12:33 - 12:36h at North America (level 0)

Themes: (T) Hydro-environment, (ST) Ecohydraulics and ecohydrology, Poster pitches

Parallel session: Poster pitches: 15F. Environment - Ecohydraulic

In order to find ways of conserving cobble bars found along many natural rivers in Japan that have become habitats of rare animals and plants including sparse herbaceous vegetation, we have conducted numerical flow simulation study along with field surveys of a river in Western Japan. We know from our previous studies that this cobble-bar plant community is maintained by frequent flushing of fine sediment by small floods, but there must be areas, which we call refugia, where plants are not totally washed away by large floods. The habitats are found along reaches with rapidly changing plan, cross section and bed slope with sharp bends, expansions and contractions, most of which are difficult to be reflected by established one-dimensional flow analysis, so we have employed an advanced three dimensional flow simulation method. We focus on the mid reach of Ibogawa River in Hyogo Prefecture, Japan, where the cobble-bar plant community exists. The average bed slope in this reach is 1/200 to 1/400 and the average annual maximum discharge is 760 m3/s, but a large discharge as much as 1600.m3/s occurred in 2009 which destroyed most of the habitats but not the refugia. The Large-Eddy Simulation (LES) of the flood flows of varying flow rate was conducted in the 0.5km x 0.5km region enclosing the refugia treating the bed fixed. Distributions of flow velocity, water level and bed shear stresses were obtained for a variety of flow rate focusing on the severity of impact on the plant community of flood disturbances. The LES analysis shows details of the flow characteristics during flood events. Most of the plant community area goes underwater during medium floods and is denuded by fluvial disturbances, but it can later be re-colonized by new seedlings. Furthermore, during the large floods, the flat area near the downstream edge of the cobble-bar is found to experience only mild bed shear. This area corresponds to the location where the plant population survived floods and it plays the critical role as sources of re-colonization after disturbances. Hence the flow analyses indicate that the form of the bend and bar is a strong factor for the condition of the refugia.