5:15 PM - 6:45 PM
[HGM03-P11] Origin and evolution of bedrock step-pool
Keywords:Step-pool, Bedrock, Mountain rivers
Stepped riverbeds in mountain rivers play a role in dissipating the energy of river water and stabilizing riverbanks. Step-pools are classified into three types based on the material forming step: boulders, log steps, and bedrock steps. Although the origin and evolution of boulder steps have been studied, few studies focused on bedrock steps. Previous studies have shown that there is no significant difference between boulder and bedrock steps in the morphological characteristics and their role in dissipating the energy of river water. However, the origin and evolution of boulder and bedrock steps are not necessarily the same because of their difference in step components. Therefore, in this study, we conducted field measurements of bedrock step-pools to consider whether the formation theory of boulder steps can be applied to bedrock steps and examined the evolution of bedrock steps.
We measured the step height, step length, and step rise, and recorded the location of each step-pool in two rivers: the Masuda River in Natori City, where tuff and tuff breccia are distributed, and the Uchihara River in Ishinomaki City, where slate and sandy shale are distributed. In Masuda River, the ratio of step height to step length (H/L) was about 0.27 up to a step length of 3–4 m. As the step length increased, with some steps showed a similar H/L value around of 0.27, we found steps having greater H/L like a waterfall and those having smaller H/L. Also, to calculate Froude number, we measured bankfull width and depth at about 100 m intervals and estimated flow velocity using Manning’s equation. bankfull width and depth to examine the origin of bedrock steps. Based on the Froude number and the observed step geometry, the keystone theory can apply to our data among the existing theories on boulder step formation. The keystone theory states that steps are formed when the small gravels are jammed upstream of a relatively immobile boulder, causing the longitudinal difference in erosion rates. Once the boulder is transported downstream, the jammed gravels will be transported, and the bedrock steps are exposed.
Based on the observed relationship between step height and length, we infer the evolution process of a bedrock step-pool. The constant H/L values observed for smaller steps suggests that step length and height increase at a nearly constant ratio at the initial stage of the evolution. In the next stage, the evolution of steps can be divided into 3 types: those that grow in the same manner as the initial stage, those that become like waterfalls, and those that have elongated-pool. Previous studies show that the difference in longitudinal erosion rates is important in the evolution of waterfalls. The same applies to bedrock steps, and these three types of evolutions can be explained by the longitudinal difference in erosion rates at the upstream of steps, steps, and pools. However, the origin and evolution of step-pools may be affected by the step and pool shape, so it is necessary to measure the transverse width of the step or the horizontal distance from the step crest to the deepest point of the pool.
We measured the step height, step length, and step rise, and recorded the location of each step-pool in two rivers: the Masuda River in Natori City, where tuff and tuff breccia are distributed, and the Uchihara River in Ishinomaki City, where slate and sandy shale are distributed. In Masuda River, the ratio of step height to step length (H/L) was about 0.27 up to a step length of 3–4 m. As the step length increased, with some steps showed a similar H/L value around of 0.27, we found steps having greater H/L like a waterfall and those having smaller H/L. Also, to calculate Froude number, we measured bankfull width and depth at about 100 m intervals and estimated flow velocity using Manning’s equation. bankfull width and depth to examine the origin of bedrock steps. Based on the Froude number and the observed step geometry, the keystone theory can apply to our data among the existing theories on boulder step formation. The keystone theory states that steps are formed when the small gravels are jammed upstream of a relatively immobile boulder, causing the longitudinal difference in erosion rates. Once the boulder is transported downstream, the jammed gravels will be transported, and the bedrock steps are exposed.
Based on the observed relationship between step height and length, we infer the evolution process of a bedrock step-pool. The constant H/L values observed for smaller steps suggests that step length and height increase at a nearly constant ratio at the initial stage of the evolution. In the next stage, the evolution of steps can be divided into 3 types: those that grow in the same manner as the initial stage, those that become like waterfalls, and those that have elongated-pool. Previous studies show that the difference in longitudinal erosion rates is important in the evolution of waterfalls. The same applies to bedrock steps, and these three types of evolutions can be explained by the longitudinal difference in erosion rates at the upstream of steps, steps, and pools. However, the origin and evolution of step-pools may be affected by the step and pool shape, so it is necessary to measure the transverse width of the step or the horizontal distance from the step crest to the deepest point of the pool.