[H-CG23] Earth surface processes related to deposition, erosion and sediment transport

[HCG23-P06] River incision models considering bedrock strength:
varification in tributaries of Abukuma River, Fukushima Prefecture, Northeastern Japan

*Yamanishi Nanako¹, Hajime Naruse¹ (1.Kyoto University Graduate school of science)

Keywords:grain distribution, stream power model, equilibrium state

In this study, two bedrock river incision models were applied to tributaries of Abukuma River to verify to what extent each model can reproduce changes in erosion rates due to differences in base rock strength. Bedrock river incision is one of the major processes controlling the development of mountain landforms. Several numerical models have been proposed to predict this fluvial erosion process. However, tectonic structures in plate subduction zones such as Japan are quite complicated, so that bedrocks showing wide diversity in lithologic strengths are adjacent in such regions. Therefore, it is necessary to consider the effect of bedrock strength in numerical models. Most existing incision models assume that the rock strength would be constant, and thus, those models have not been commonly applied in tectonically active zones such as Japan. The widely used river incision model is the stream power model, which assumes that the erosion rate is proportional to the energy loss rate of the water flow. This model lumps the physics of bedrock incision into very simple laws, so it was difficult to fit the effect of rock strength independently into the model. However, recently, some models that consider the rock strength using empirical correction coefficients have been proposed. On the other hand, models that consider the physical processes involved in river incision have also been developed. The sediment flux-dependent model assumes that saltating bed load gravels abrase bedrocks. This mechanistic model includes parameters such as grain size, rock strength, and sediment supply, and the physical properties of rocks measured in actual rivers can be used as model parameters. In this study, we applied the stream power model considering rock erodibility and sediment flux dependent model to tributaries of Abukuma River to verify incision models that can consider various rock strengths in a tectonically active zone, and calculate the topography change until a slope reaches equilibrium. The calculated results were compared with the actual topography to verify the differences in the reproducibility of each model. The results in the stream power model considering rock strength showed that rock strength influences river gradient at both non-equilibrium and equilibrium states. In contrast, in the sediment flux-dependent model, the effect of rock strength is more substantial in the non-equilibrium state, whereas it is nearly negligible in the equilibrium state. Furthermore, comparing the calculated results in an equilibrium between the two models shows that the sediment flux-dependent model results are closer to those of the actual topography. The results of this study provide a basis for developing river incision models that are more suitable for various rock strengths in tectonically active zones. In the future, we will examine and improve the parameters used in the literature in this study, such as roughness, and further case studies in erosion equilibrium and non-equilibrium rivers will enable us to develop an incision model more suited to actual rivers.