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[ACG37-02] Changes in sediment production and discharge in mountain area due to climate change
★Invited Papers
Keywords:freezing and thawing weathering, sediment discharge, climate change, mountain area
Recently, heavy rainfall has been increasing due to climate change. This increases not only the flow discharge in the river channel, but also the amount of sediment that flows into urban areas downstream from mountain areas due to slope failure and riverbed erosion. Sediment runoff increases damage by reducing the channel conveyance capacity and overtopping the embankments. In addition, temperatures are rising, and snow cover is shrinking. This will change the weathering of bare slopes due to freezing and thawing (FT). FT weathering is a sediment producing process in which repeated freezing and thawing of water in the bedrock increases the pore space and destroys the structure. The sediment produced by FT weathering is finer than the sediment in the river channel. This can cause fine sediment to be transported downstream or reduce the strength of the bedrock, susceptible to slope failure. In this study, we predict changes in FT weathering and sediment discharge from mountainous areas due to climate change in the Pekerebetsu River in Hokkaido, Japan.
We used d4PDF as input data for climate change projections. This is the large ensemble climate simulation data. It includes present climate simulated from observed data such as sea surface temperature, and future climate simulated for a world in which the global average air temperature has risen by 4°C. We input temperature, solar radiation, snow depth, wind speed, and humidity for FT weathering, rainfall for sediment runoff. To predict FT weathering, we used the model that estimates soil temperature, ice content and the number of FT cycles by combining a heat balance equation for the ground surface and a heat conduction analysis. To predict sediment runoff, we used the one-dimensional sediment runoff model SiMHiS. In this model, sediment produced by slope failure is supplied to the adjacent river channel and discharged downstream. By inputting present climate and future climate data at 5 km resolution of d4PDF into these models, we predicted the changes in FT weathering and sediment runoff in a changing climate.
As for changes in FT weathering, we found that it increases or decreases depending on the combination of temperature rising and snow cover shrinking. If the temperature rises, the period when FT occurs actively shifts to the period when the temperature is about 0°C and the solar radiation is low. Thus, FT will decrease. However, if the snow cover decreases, the insulation effect of the snow cover is lost. Thus, FT will increase. As for changes in sediment runoff, we compared rainfall and sediment volume values for the same return period. The results showed that the rate of increase of rainfall is about 1.4 to 1.6 times while that of sediment is 2.9 to 5.5 times, indicating that the rate of increase in sediment is greater than that of rainfall. This is because slope failures occur more frequently. As sediment from the slope is supplied to the river channel and deposited, sediment that was previously difficult to move because it was hidden in large sediment gaps becomes easier to move as the gaps are filled. In addition, the sediment produced from the slope is finer than that in the channel where armoring occurs. When fine sediment is supplied to the river channel, the static angle of friction is reduced, making it easier for the larger sediment to move.
In this way, changes in FT weathering due to climate change depend on a combination of changes in temperature rise and snow cover shrinkage, and the rate of increase in sediment discharge is greater than the rate of increase in rainfall. FT weathering can re-destabilize a slope where slope failure has occurred, but this model does not consider this point. Therefore, it is an exciting challenge to develop a model which considers slope destabilization by FT weathering.
We used d4PDF as input data for climate change projections. This is the large ensemble climate simulation data. It includes present climate simulated from observed data such as sea surface temperature, and future climate simulated for a world in which the global average air temperature has risen by 4°C. We input temperature, solar radiation, snow depth, wind speed, and humidity for FT weathering, rainfall for sediment runoff. To predict FT weathering, we used the model that estimates soil temperature, ice content and the number of FT cycles by combining a heat balance equation for the ground surface and a heat conduction analysis. To predict sediment runoff, we used the one-dimensional sediment runoff model SiMHiS. In this model, sediment produced by slope failure is supplied to the adjacent river channel and discharged downstream. By inputting present climate and future climate data at 5 km resolution of d4PDF into these models, we predicted the changes in FT weathering and sediment runoff in a changing climate.
As for changes in FT weathering, we found that it increases or decreases depending on the combination of temperature rising and snow cover shrinking. If the temperature rises, the period when FT occurs actively shifts to the period when the temperature is about 0°C and the solar radiation is low. Thus, FT will decrease. However, if the snow cover decreases, the insulation effect of the snow cover is lost. Thus, FT will increase. As for changes in sediment runoff, we compared rainfall and sediment volume values for the same return period. The results showed that the rate of increase of rainfall is about 1.4 to 1.6 times while that of sediment is 2.9 to 5.5 times, indicating that the rate of increase in sediment is greater than that of rainfall. This is because slope failures occur more frequently. As sediment from the slope is supplied to the river channel and deposited, sediment that was previously difficult to move because it was hidden in large sediment gaps becomes easier to move as the gaps are filled. In addition, the sediment produced from the slope is finer than that in the channel where armoring occurs. When fine sediment is supplied to the river channel, the static angle of friction is reduced, making it easier for the larger sediment to move.
In this way, changes in FT weathering due to climate change depend on a combination of changes in temperature rise and snow cover shrinkage, and the rate of increase in sediment discharge is greater than the rate of increase in rainfall. FT weathering can re-destabilize a slope where slope failure has occurred, but this model does not consider this point. Therefore, it is an exciting challenge to develop a model which considers slope destabilization by FT weathering.