River bank erosion is one of the frequent but the most unpredictable disasters that occur every year in Bangladesh. In this paper, Landsat TM-5 and Landsat-8 imageries from 1989 and 2015 were used to detect changes of...River bank erosion is one of the frequent but the most unpredictable disasters that occur every year in Bangladesh. In this paper, Landsat TM-5 and Landsat-8 imageries from 1989 and 2015 were used to detect changes of present land use, river erosion and bar deposition in Chowhali Upazila, Sirajganj district of Bangladesh. This study reveals that human settlement, forest, seasonal crops and agriculture features decrease, while river coverage increases dramatically. About 1340 hectare areas have been eroded, while 630 hectares are deposited as channel bar in the study area over the last 26 years. Finally, an accuracy assessment is conducted between the test data and each land use feature. The overall classification accuracy was 97% and 98% in 1989 and 2015 respectively. Moreover, 98% accuracy is found in erosion while 97% is found in bar deposition areas.展开更多
The concentration of cosmogenic 10 Be in riverine sediments has been widely used as a proxy for catchment-wide denudation rate(CWDR). One of the key assumptions of this approach is that sediments originating from sub-...The concentration of cosmogenic 10 Be in riverine sediments has been widely used as a proxy for catchment-wide denudation rate(CWDR). One of the key assumptions of this approach is that sediments originating from sub-basins with different erosional histories are well mixed. A tragic debris flow occurred in the Seti River watershed, central Nepal,on May 5, 2012. This catastrophic debris flow was triggered by slope failure on the peak of Annapurna IV and resulted in many casualties in the lower Seti Khola. However, it provided an opportunity to test the assumption of equal mixing of sediments in an understudied rapidly eroding watershed. This study documents the CWDR of 10 Be to evaluate the extent of the influence of episodic erosional processes such as debris flow on the spatio-temporal redistribution of10 Be concentrations. Our data show that the debris flow caused little change in CWDR across the debris flow event. In addition to isotopic measurement, we calculated denudation rates by using the modeled concentrations in pre-and post-landslide sediments based on the local 10 Be production rate. The modeled result showed little change across the event,indicating that the debris flow in May 2012 played a minor role in sediment evacuation, despite the rapid erosion in the catchment. Our study concludes that although the 2012 event caused many casualties and severe damage, it was a low-magnitude, high frequency event.展开更多
River bank erosion is one of the major and unpredictable hazards worldwide including in Malaysia.Soil detachment at river banks is due to two processes:1)hydraulic erosion imposed by channel flow and 2)sub aerial eros...River bank erosion is one of the major and unpredictable hazards worldwide including in Malaysia.Soil detachment at river banks is due to two processes:1)hydraulic erosion imposed by channel flow and 2)sub aerial erosion due to the weakening and weathering of bank materials.This paper is focused on the second aspect of the erosion process which mainly depends on the combination of rainfall intensity and the ability of the soil to withstand the raindrop effects.The relative combination of sand,silt and clay in a soil is argued to have an impact on erosion resistance.In cohesive soil composition,sand forms the largest size ranging from 0.05 to 2 mm whereas silt is adequately moderate(ranging from 0.002 to 0.05 mm)and clay is the smallest of all three(less than 0.002 mm).With the knowledge that soil composition does indeed have an effect on erosion resistance,this paper will attempt to relate risk assessment index of river bank erosion specifically to soil composition.Thus,the objectives of this document are as follows;1)to produce risk assessment index for river bank erosion and 2)to carry out a case study for selected rivers in Malaysia pertaining to river bank assessment.The index is produced by inferring the previously developed scale on soil erodibility.Past researchers created the“ROM”scale(named after the researchers,Rolan and Mazidah)to assess degree of soil erodibility into five classes namely“critical”,“very high”,“high”,“medium”and“low”.Instead of using semi empirical formula from the“ROM”scale,a percentage of soil composition was inferred to produce risk assessment index.It was found that as the percentage of clay decreased,susceptibility index became higher and approached a critical level.Application of the newly developed index is verified by conducting a case study at the Langat River,Kajang,Malaysia.The soil composition was classified and form fitted into the index.It was found that the middle reach of the Langat river is susceptible to severe erosion due to low percentage of clay.This finding agreed well with the visual observation of these reaches as a large portion of gully type of erosion had been observed throughout the study.The establishment of risk assessment index which firmly indicates the relationship between soil composition and river bank erosion can be used as a tool in forecasting the risk levels.This formulation is well proven to assess river bank conditions and the associated critical shear stress is very much close with the previously published shear stress.展开更多
文摘River bank erosion is one of the frequent but the most unpredictable disasters that occur every year in Bangladesh. In this paper, Landsat TM-5 and Landsat-8 imageries from 1989 and 2015 were used to detect changes of present land use, river erosion and bar deposition in Chowhali Upazila, Sirajganj district of Bangladesh. This study reveals that human settlement, forest, seasonal crops and agriculture features decrease, while river coverage increases dramatically. About 1340 hectare areas have been eroded, while 630 hectares are deposited as channel bar in the study area over the last 26 years. Finally, an accuracy assessment is conducted between the test data and each land use feature. The overall classification accuracy was 97% and 98% in 1989 and 2015 respectively. Moreover, 98% accuracy is found in erosion while 97% is found in bar deposition areas.
基金supported by the College of Education,Korea University Grant in 2016
文摘The concentration of cosmogenic 10 Be in riverine sediments has been widely used as a proxy for catchment-wide denudation rate(CWDR). One of the key assumptions of this approach is that sediments originating from sub-basins with different erosional histories are well mixed. A tragic debris flow occurred in the Seti River watershed, central Nepal,on May 5, 2012. This catastrophic debris flow was triggered by slope failure on the peak of Annapurna IV and resulted in many casualties in the lower Seti Khola. However, it provided an opportunity to test the assumption of equal mixing of sediments in an understudied rapidly eroding watershed. This study documents the CWDR of 10 Be to evaluate the extent of the influence of episodic erosional processes such as debris flow on the spatio-temporal redistribution of10 Be concentrations. Our data show that the debris flow caused little change in CWDR across the debris flow event. In addition to isotopic measurement, we calculated denudation rates by using the modeled concentrations in pre-and post-landslide sediments based on the local 10 Be production rate. The modeled result showed little change across the event,indicating that the debris flow in May 2012 played a minor role in sediment evacuation, despite the rapid erosion in the catchment. Our study concludes that although the 2012 event caused many casualties and severe damage, it was a low-magnitude, high frequency event.
基金partially funded by Ministry of Higher Education Malaysia(MOHE)through Fundamental Research Grant(FRGS/1/2016/WAB05/IUKL/03/1).
文摘River bank erosion is one of the major and unpredictable hazards worldwide including in Malaysia.Soil detachment at river banks is due to two processes:1)hydraulic erosion imposed by channel flow and 2)sub aerial erosion due to the weakening and weathering of bank materials.This paper is focused on the second aspect of the erosion process which mainly depends on the combination of rainfall intensity and the ability of the soil to withstand the raindrop effects.The relative combination of sand,silt and clay in a soil is argued to have an impact on erosion resistance.In cohesive soil composition,sand forms the largest size ranging from 0.05 to 2 mm whereas silt is adequately moderate(ranging from 0.002 to 0.05 mm)and clay is the smallest of all three(less than 0.002 mm).With the knowledge that soil composition does indeed have an effect on erosion resistance,this paper will attempt to relate risk assessment index of river bank erosion specifically to soil composition.Thus,the objectives of this document are as follows;1)to produce risk assessment index for river bank erosion and 2)to carry out a case study for selected rivers in Malaysia pertaining to river bank assessment.The index is produced by inferring the previously developed scale on soil erodibility.Past researchers created the“ROM”scale(named after the researchers,Rolan and Mazidah)to assess degree of soil erodibility into five classes namely“critical”,“very high”,“high”,“medium”and“low”.Instead of using semi empirical formula from the“ROM”scale,a percentage of soil composition was inferred to produce risk assessment index.It was found that as the percentage of clay decreased,susceptibility index became higher and approached a critical level.Application of the newly developed index is verified by conducting a case study at the Langat River,Kajang,Malaysia.The soil composition was classified and form fitted into the index.It was found that the middle reach of the Langat river is susceptible to severe erosion due to low percentage of clay.This finding agreed well with the visual observation of these reaches as a large portion of gully type of erosion had been observed throughout the study.The establishment of risk assessment index which firmly indicates the relationship between soil composition and river bank erosion can be used as a tool in forecasting the risk levels.This formulation is well proven to assess river bank conditions and the associated critical shear stress is very much close with the previously published shear stress.
