11:15 〜 11:30
[HCG20-03] クレバススプレイ地形の発達過程の実験的検討
キーワード：河川、氾濫、アナログ実験
Crevasse splay landforms are lobate landforms formed on flood plains when river floods breach their banks. Crevasse splays are one of the primary fluvial architectural elements and provide a record of past flood disasters, so their geomorphological characteristics and sedimentary facies have long been studied. However, few studies have quantitatively investigated the mode of the spatiotemporal development of crevasse splay landforms. This study investigated the developmental mode of crevasse splays using the flume experiments.
The flume experiment was conducted in a facility simulating the 2019 Chikuma River flood inundation at a 1/200 scale. An opening gate was made in the side wall of the channel (8 cm wide, 2 m long) through which the sediment-containing river flow flooded onto an approximately horizontal acrylic plate (1.5 m long, 1.0 m wide). The sediment consisted of two-grain sizes of approximately 0.2 mm and 0.09 mm. Dimensional analysis shows that these particles corresponded to the sediments equivalent of gravel and suspended sand in a real-scale river. Variously varying flood flow conditions revealed the following discoveries:
1. crevasse splay deposits develop in two separate regions: proximal and distal splays, corresponding to bedload and suspended load sediment transport. These two regions are often observed at the actual field scale, and it is possible to identify their palaeo-topography from the sedimentary structure.
2. Crevasse splays initially exhibited a developmental pattern extending downstream of the main river channel but developed into symmetrical geometry in the upstream and downstream directions over time. This is because the river mainstream strongly influences the flow direction of the initial inundation flow. In contrast, channel bifurcation occurred when a sediment pile developed sufficiently on the splay. The asymmetry of the shape of the crevasse splay would be an index of the developmental stage of the splay.
3. in the interior of the proximal splay sediments, the downstream direction was slightly coarser-grained. This process of grain-size segregation was as follows. The original flood flow was a sheet flow without a channel, resulting in coarse-grained sediments being spread over a wide area. In contrast, later, when the crevasse channel developed, the coarse-grained sediments became locally unevenly distributed.
These findings will provide an essential basis for interpreting the actual crevasse splay depositional facies. In the presentation, the relationship between the diversity of the crevasse splay and the hydrological conditions will also be discussed based on the results of numerical experiments.
The flume experiment was conducted in a facility simulating the 2019 Chikuma River flood inundation at a 1/200 scale. An opening gate was made in the side wall of the channel (8 cm wide, 2 m long) through which the sediment-containing river flow flooded onto an approximately horizontal acrylic plate (1.5 m long, 1.0 m wide). The sediment consisted of two-grain sizes of approximately 0.2 mm and 0.09 mm. Dimensional analysis shows that these particles corresponded to the sediments equivalent of gravel and suspended sand in a real-scale river. Variously varying flood flow conditions revealed the following discoveries:
1. crevasse splay deposits develop in two separate regions: proximal and distal splays, corresponding to bedload and suspended load sediment transport. These two regions are often observed at the actual field scale, and it is possible to identify their palaeo-topography from the sedimentary structure.
2. Crevasse splays initially exhibited a developmental pattern extending downstream of the main river channel but developed into symmetrical geometry in the upstream and downstream directions over time. This is because the river mainstream strongly influences the flow direction of the initial inundation flow. In contrast, channel bifurcation occurred when a sediment pile developed sufficiently on the splay. The asymmetry of the shape of the crevasse splay would be an index of the developmental stage of the splay.
3. in the interior of the proximal splay sediments, the downstream direction was slightly coarser-grained. This process of grain-size segregation was as follows. The original flood flow was a sheet flow without a channel, resulting in coarse-grained sediments being spread over a wide area. In contrast, later, when the crevasse channel developed, the coarse-grained sediments became locally unevenly distributed.
These findings will provide an essential basis for interpreting the actual crevasse splay depositional facies. In the presentation, the relationship between the diversity of the crevasse splay and the hydrological conditions will also be discussed based on the results of numerical experiments.