09:15 〜 09:30
[HGM04-02] 岩盤河川における基盤岩強度と河川勾配の関係：福島県阿武隈川流域河川での侵食モデル適用
キーワード：岩盤河川、岩石強度、粒径、被覆率
Bedrock river incision is one of the major processes controlling the development of mountain landforms. Understanding the factors that control the morphology of bedrock rivers could contribute to a better reconstruction of past tectonic activities. Among various factors, it is critical to clarify the relationship between the erosion rate and the strength of the bedrock, especially understanding the river erosional processes in tectonically active regions such as Japanese islands where bedrock with a wide diversity in lithologic strengths is adjacent to each other because of complicated geological structures. To explore methods for predicting the impact of bedrock strength on fluvial erosion, we applied two types of erosion models that consider bedrock strength and compared their reproducibility of the actual topographic features. The stream power and sediment flux-dependent models were examined here.
These models were applied to tributaries of the Abukuma River, Fukushima Prefecture, Northeastern Japan, which has been considered a geomorphologically steady-state region. The strength of the igneous and sedimentary rocks distributed in this area was measured by the rock strength tests for collected samples, and the gravel size of the riverbed was also measured using drone images. The river profiles in the surveyed area were estimated using digital elevation data from the Geospatial Information Authority of Japan. The survey results exhibited that, although the bedrock in this area has a significantly different strength, the river gradient hardly changes at the points where the bedrock type changes, implying that the difference in bedrock strength has almost no effect on the erosion rate. Some knickpoints are on the points where drainage area changes greatly. The sediment flux-dependent model reproduced these observational results adequately, while the stream power model failed to predict profiles fit to the observation. The results of the sediment flux-dependent model calculations exhibited that this nearly uniform erosion rate is caused by the effect of sediment covering the riverbed. The erosion in weak bedrock areas causes a slight decrease in riverbed gradient, but the reduced shear stress induces the cover of the riverbed by sediment in such areas. This sediment cover effect prevents the impact of gravel particles on the bedrock, resulting in a negative feedback effect that suppresses the erosion of weak rocks. This feedback mechanism produces profiles that are more consistent with observed topography.
Therefore, in natural rivers, sediment cover can obscure the topographic expression of variations in bedrock strength. It should be noted that the trend in the riverbed coverage by sediment observed in practice did not necessarily match the model's predictions, suggesting that there is room for further improvement in the sediment flux-dependent model.
These models were applied to tributaries of the Abukuma River, Fukushima Prefecture, Northeastern Japan, which has been considered a geomorphologically steady-state region. The strength of the igneous and sedimentary rocks distributed in this area was measured by the rock strength tests for collected samples, and the gravel size of the riverbed was also measured using drone images. The river profiles in the surveyed area were estimated using digital elevation data from the Geospatial Information Authority of Japan. The survey results exhibited that, although the bedrock in this area has a significantly different strength, the river gradient hardly changes at the points where the bedrock type changes, implying that the difference in bedrock strength has almost no effect on the erosion rate. Some knickpoints are on the points where drainage area changes greatly. The sediment flux-dependent model reproduced these observational results adequately, while the stream power model failed to predict profiles fit to the observation. The results of the sediment flux-dependent model calculations exhibited that this nearly uniform erosion rate is caused by the effect of sediment covering the riverbed. The erosion in weak bedrock areas causes a slight decrease in riverbed gradient, but the reduced shear stress induces the cover of the riverbed by sediment in such areas. This sediment cover effect prevents the impact of gravel particles on the bedrock, resulting in a negative feedback effect that suppresses the erosion of weak rocks. This feedback mechanism produces profiles that are more consistent with observed topography.
Therefore, in natural rivers, sediment cover can obscure the topographic expression of variations in bedrock strength. It should be noted that the trend in the riverbed coverage by sediment observed in practice did not necessarily match the model's predictions, suggesting that there is room for further improvement in the sediment flux-dependent model.