5:15 PM - 7:15 PM
[HGM04-P03] Changes in fluvial and hillslope morphology associated with increasing denudation rate
Keywords:Topographic relief, Denudation rate, Digital Terrain Model, Colluvial domain
Tectonics and climate determine the denudation rates and controls the landform of mountainous watersheds. Denudation rates and geomorphic quantities can be quantified by analyzing cosmogenic nuclides and Digital Terrain Models (DTMs), respectively. Building functional relationships between denudation rates and geomorphic quantities is important for discussing the geomorphic agents and related processes. It also enables to estimate the spatial distribution of denudation rates based on geomorphic quantities calculated from easily available DTMs. Topographic relief is one of the geomorphic quantities. Total landscape relief for unglaciated terrain can be divided into three components: fluvial, colluvial and hillslope relief. Fluvial relief is the elevation difference between the highest and lowest points of a channel section showing a concave longitudinal profile. Colluvial relief is the elevation difference at the section where erosion by debris flow is predominant and the channel slope is almost constant. Hillslope relief represents the height of a hillslope. Fluvial relief is empirically predicted to reach an upper limit to the increase in denudation rate, but it is not clear what the responses of colluvial and hillslope relief would be. Therefore, this study examines the relationships between denudation rates, the three components of relief, and related geomorphic quantities and confirms the consistency with the predictions.
We focused on mountainous watersheds in Japan where catchment-averaged denudation rates and high resolution DTMs are available. Topographic analysis was performed using LSDTopoTools (Mudd et al., 2023) and QGIS. In the topographic analysis, the DrEICH method (Clubb et al., 2014) was applied to identify channel heads. This method divides each watershed into a channel domain downstream of the channel head, and a hillslope domain upslope of it. For the channel domain, it was divided into fluvial and colluvial domains based on the slope-area plots. Firstly, for fluvial relief, it can be expressed as a function of the steepness index, ks, which previous studies have suggested reaches an upper limit at a certain denudation rate. Therefore, fluvial relief is also expected to reach an upper limit for increasing denudation rate. We plotted denudation rates against steepness indexes and fluvial relief calculated using DTMs and found both steepness indexes and fluvial relief reached an upper limit. Next, for the hillslope relief, it can be expressed as the product of the horizontal distance from the divide and the average gradient. Hillslope gradient reaches its threshold for an increase in the denudation rate, meaning an increase in the horizontal distance is necessary for the relief to increase once the hillslope gradient reaches its upper limit. Our analysis confirmed that hillslope relief increased with denudation rates. However, it was not possible to determine whether this relationship was a monotonic or logarithmic increase. Finally, colluvial relief showed an increase with denudation rate. Previous studies on drainage density have shown that total drainage density remains constant with increasing denudation rates, whereas colluvial drainage density increases with denudation rates. We therefore examined the ratio of colluvial domain in the channel domain and found that it increased with denudation rates. In drainage basins, the downstream extension of the colluvial domain with increasing denudation rate suggests that the influence of debris flow as a geomorphic process increases with the denudation rate. However, there are limited studies on the quantitative relationship between the debris flow process and denudation rate. Future work is therefore required to model the relationships between debris flow process, denudation rate and landform.
We focused on mountainous watersheds in Japan where catchment-averaged denudation rates and high resolution DTMs are available. Topographic analysis was performed using LSDTopoTools (Mudd et al., 2023) and QGIS. In the topographic analysis, the DrEICH method (Clubb et al., 2014) was applied to identify channel heads. This method divides each watershed into a channel domain downstream of the channel head, and a hillslope domain upslope of it. For the channel domain, it was divided into fluvial and colluvial domains based on the slope-area plots. Firstly, for fluvial relief, it can be expressed as a function of the steepness index, ks, which previous studies have suggested reaches an upper limit at a certain denudation rate. Therefore, fluvial relief is also expected to reach an upper limit for increasing denudation rate. We plotted denudation rates against steepness indexes and fluvial relief calculated using DTMs and found both steepness indexes and fluvial relief reached an upper limit. Next, for the hillslope relief, it can be expressed as the product of the horizontal distance from the divide and the average gradient. Hillslope gradient reaches its threshold for an increase in the denudation rate, meaning an increase in the horizontal distance is necessary for the relief to increase once the hillslope gradient reaches its upper limit. Our analysis confirmed that hillslope relief increased with denudation rates. However, it was not possible to determine whether this relationship was a monotonic or logarithmic increase. Finally, colluvial relief showed an increase with denudation rate. Previous studies on drainage density have shown that total drainage density remains constant with increasing denudation rates, whereas colluvial drainage density increases with denudation rates. We therefore examined the ratio of colluvial domain in the channel domain and found that it increased with denudation rates. In drainage basins, the downstream extension of the colluvial domain with increasing denudation rate suggests that the influence of debris flow as a geomorphic process increases with the denudation rate. However, there are limited studies on the quantitative relationship between the debris flow process and denudation rate. Future work is therefore required to model the relationships between debris flow process, denudation rate and landform.