Rahul Devrani | University of Delhi (original) (raw)

Papers by Rahul Devrani

Advances in Remote Sensing Technology and the Three Poles, Dec 10, 2022

Advances in Remote Sensing Technology and the Three Poles, Dec 10, 2022

<p>Himalayan rivers transport approximately 10<sup>3</sup&... more <p>Himalayan rivers transport approximately 10<sup>3</sup> Mt of sediment annually from their source in the steep topography of the High Himalaya to ocean basins. However, the journey from source to sink is not necessarily a smooth one: on the way, sediment can become trapped in montane storage systems, such as river valleys or floodplains. While sediment is stored in valleys, climate and erosional signals that we may wish to read from the final sedimentary record can be modified or even destroyed. We therefore need to understand the spatial distribution, volume and longevity of these valley fills. However, controls on Himalayan valley location and geometry are unknown, and sediment volume estimates are based on relatively untested assumptions of valley widening processes.</p> <p>In this work we use a new method of automatically detecting valley floors to extract 1,644,215 valley-floor width measurements across the Himalayan orogen. We use this dataset to explore the dominant controls on valley-floor morphology, and to test models of valley widening processes. We use random forest regression to estimate the importance of potential controlling variables, and find that channel steepness, a proxy for rock uplift, is a first-order control on valley-floor width. We also analyse a newly compiled dataset of 1,797 exhumation rates across the orogen and find that valley-floor width decreases as exhumation rate increases. We therefore suggest that valley-floor width is adjusted to long-term exhumation, controlled by tectonics, rather than being controlled by water discharge or bedrock erodibility. We also hypothesise that valley widening predominantly results from sediment deposition along low-gradient valley floors, controlled by the ratio of sediment to water discharge, rather than lateral bedrock erosion.</p>

Advances in Remote Sensing Technology and the Three Poles, Dec 10, 2022

Journal of the Geological Society of India

Himalayan rivers transport ≈ 103 Mt of sediment annually to ocean basins. River valleys are an im... more Himalayan rivers transport ≈ 103 Mt of sediment annually to ocean basins. River valleys are an important component of this routing system: while sediment is stored in valleys, signals of climate change and erosional patterns can be modified or even destroyed. Despite a critical need to understand the spatial distribution, volume and longevity of these valley fills, controls on valley location and geometry are unknown, and estimates of sediment volumes are based on never-tested assumptions of valley widening processes. Here we extract 1,644,215 valley-floor width measurements across the Himalaya to determine the dominant controls on valley-floor morphology for the first time, and to test underlying assumptions of sediment storage volumes. We use random forest regression to estimate the importance of potential controlling variables, and find that channel steepness, a proxy for rock uplift, is a first-order control on valley-floor width. We also analyse a novel dataset of 1,797 exhumat...

Tectonics, 2021

Understanding the processes and mechanisms responsible for exhumation are essential for determini... more Understanding the processes and mechanisms responsible for exhumation are essential for determining the evolution of landscapes. Our knowledge of these issues in continental convergent margins is still incomplete despite decades of research. Here, we present a record of exhumation rates obtained from thermochronology across the collision zone of the Himalayan‐Tibetan orogen, which has been carved by the dextral Karakoram fault (KF) along its western margin. Our thermochronology results across the KF suggest that transpression controlled exhumation from ∼10 to 8 Ma. Exhumation rates at the margin of the Tibetan plateau, in the SE Karakoram range, to the north of the KF are twice as high as the Ladakh range, to the south of the KF, since at least middle Miocene. We hypothesize that rapid exhumation of the Tibetan margin was in response to topographic uplift and subsequent erosion due to the convective removal of the lower lithosphere, triggered by the rollback of the subducting Indian lower crust during the Oligocene‐Miocene in an early phase. Our thermochronology results document an additional two‐fold increase in the exhumation rates of the Tibetan margin since the late Mio‐Pliocene. We propose that the underthrusting of the Indian Plate below Tibet during the late Mio‐Pliocene was responsible for this additional increase of exhumation rates of the Tibetan margin in the late phase. These exhumation rates are in good agreement with the topographic slope, relief, and channel steepness data of these regions. Thus, the coupling of tectonics and topography dictated the exhumation patterns since late Mio‐Pliocene.

<p>Rivers in the Himalaya have extensively been used as a tool to understan... more <p>Rivers in the Himalaya have extensively been used as a tool to understand the mechanism of valley aggradation and incision resulting due to precipitation variability and ongoing deformation. The suture zone tectonics control the landscape building along the upper Indus River, however, intensified monsoon encroaches on the region and modify the surface processes. The valley is filled with outwash fans and aggradation pulses at ~ 52 ka, ~ 28 ka and ~16 ka when the monsoon was strengthened. At the younger time, during ~13 to ~9 ka, the Indus valley has suffered an incision phase marked by exposure of bedrock near the Indus-Zanskar confluence.</p><p>The present study constrained the paleodischarge of the Indus River during periods of established river aggradation, incision and flood-time to understand climatic settings during the enhanced sediment-load modulated by the increased discharge. At the valley filling time (47–23 ka), clast geometric data of the largest imbricated clasts from the riverbed as well as the aggraded sequences were utilized to calculate discharges. Sand-silt couplets marked as slack water deposits (SWDs) of age 14–10 ka at Indus-Zanskar confluence were used to constrain the paleodischarges during net river incision. The catchment-scale paleodischarge derived from valley fill sequences and SWDs ranges from 834±47 to 4457±253 and 19030 to 47954 cumecs. Incision-time discharges were three to ten-fold higher than from the aggradation time observed that the aggradation in the Himalayan rivers occurred in transient time (33–21 ka and 17–14 ka) when the sediment load in the rivers increased just after the glaciation. Hence, the aggradation in the Indus River has occurred when the sediment to water ratio was higher and the river carrying capacity has reduced, subsequently, the incision was initiated when sediment to water ratio reduced and the river sediment carrying capacity increases during post-glacial climatically wet phase (early Holocene).</p>

<p>The high and low flood events in a braided river system cause perpetual ... more <p>The high and low flood events in a braided river system cause perpetual changes in the channel morphodynamics and make it difficult to understand a channel response.  Therefore, the management of braided channel morphodynamics becomes a challenging issue for flood mitigations and river restoration purposes. The lower Brahmaputra river basin has the world’s most extensive braided river system, and every year it confronts reoccurring monsoon driven flooding causing widespread flood inundation and changes in channel morphodynamics. Such adequate conditions promise a natural laboratory to understand dynamics channel morphodynamics changes with high and low flood events. The present work integrates the mutual effects of varying channel area, width and sinuosity, and sediment bar area along a braided channel reach in the selected reach in the Brahmaputra River in thirty events during high discharge months of 2018, 2019 and 2020. To observe detailed channel morphodynamics changes, we developed 100 grids with a width of ~6.25 km enclosing the selected reach. These grids are maintained stationary for each year and were used to extract the Brahmaputra River channel area (BRCA)  and Brahmaputra River sediment bar area (BRSBA) and average Brahmaputra River channel width (BRCW) for each grid. We developed a site-specific Google Earth Engine algorithm to delineate the channel and sediment bar in the selected reach to perform supervised classification on Sentinel-1 SAR GRD and Sentinel-2 level-1C.</p><p>The results show that grids upstream of the selected reach have a high BRCA, BRSBA and BRCW, and these grids are located around high clustered flood inundated hotspot regions.  We also found that the world's largest river island  (Majuli) is also located in this zone. In the present study, we also observe that if we consider BRCA, BRCW and BRSBA for the same event, the BRSBA has a high correlation with the BRCW compared to the BRCA. Further, we compared the impact of the sinuosity on BRCA, BRCW and BRSBA of regular and influential flood events. During influential events, the sinuosity has only a good impact on the BRCA, and during regular events, it has a higher impact on BRCA and BRSBA. We conclude that in the braided river system of the Brahmaputra River, the channel and bar area and channel width are highly correlated during flood events, and the channel sinuosity also controls channel and bar area and channel width during regular and influential flood events.</p>

EGU General Assembly Conference Abstracts, Apr 1, 2018

The length of the main trunk in a river basin is an important morphometric parameter and it depen... more The length of the main trunk in a river basin is an important morphometric parameter and it depends on the size of the drainage basin. The Ganga River Basin is one of the largest basins in the world with the Ganga River considered to be the main stem. Variable lengths of this river in the literature motivated us to study its exact length and also to test whether geomorphically it is the longest river in the basin. The results show that the maximum river length of 2758 km is attained when source is considered at the headstream of the Tons River. This length is more than any of the traditional lengths of the Ganga River in the literature. We propose to call the longest segment of the river in Ganga Basin as the Himalayan Foreland River.

The length of the main trunk in a river basin is an important morphometric parameter and it depen... more The length of the main trunk in a river basin is an important morphometric parameter and it depends on the size of the drainage basin. The Ganga River Basin is one of the largest basins in the world with the Ganga River considered to be the main stem. Variable lengths of this river in the literature motivated us to study its exact length and also to test whether geomorphically it is the longest river in the basin. The results show that the maximum river length of 2758 km is attained when source is considered at the headstream of the Tons River. This length is more than any of the traditional lengths of the Ganga River in the literature. We propose to call the longest segment of the river in Ganga Basin as the Himalayan Foreland River.

Episodes, 2012

As the discharge increases and sediment supply decreases, the rivers incise forming terraces. Whe... more As the discharge increases and sediment supply decreases, the rivers incise forming terraces. When considered over a long period, the sediment storage in the valleys become a source for sediment supply and may contribute a significant amount of the total sediment budget. An estimate made by mapping the area of the terraces and multiplying it with their thickness suggests ~ 4800 million tonnes of sediments is trapped between Pipalkoti and Devprayag in the Alaknanda valley (Fig. 1a). These terraces continuously get eroded by various processes such as gullying, undercutting by rivers and collapsing under the influence of gravity contributing to the adjacent rivers. Therefore, apart from the rate of erosion of the bedrock, it is essential to calculate the rates of erosion of such valley fill deposits and quantify the amount of sediment supplied by them to the river systems. In this study, an attempt has been made to determine the rate of erosion by calculating the volume of sediment removed from ã 17 ka terrace located in NW Lesser Himalaya. River terraces (referred to as terraces from here onwards) form when a river becomes unstable and incises its bed (e.g. Schumm et al., 1987; Bridgland, 2000; Wegmann and Pazzaglia, 2009). Thus, terraces record the time of instability of the channel, making it an excellent geomorphic marker. Accordingly, terraces are commonly studied to identify tectonic and climatic events causing changes in the river dynamics in the geological past (e.g. Wegmann and Pazzaglia, 2009). Study area A requirement of this study was a terrace of known age with least interference from external processes such as anthropogenic activity or sediment supply on terrace surface by local stream, so that sediment volume of the terrace is only altered due to natural erosional processes. Thus, a terrace located in the Srinagar (Garhwal) region along the left bank of the Alaknanda River was chosen for the study (Figs. 1a,b) which fulfilled the above requirement. A village named Swit is located on top of the terrace under investigation, hence from here onwards this terrace is referred to as the Swit Terrace. It is located 5 km ENE from the main Srinagar city (Figs. 1a,b). The elevation of the Swit terrace is ~ 655 m asl (above sea level) and covers an area of 0.6 km 2 and the elevation of its top from the river bed is ~ 100 m. A small stream originating from the adjacent valley walls, and locally called Swit nala, flows into the Alaknanda river. The local streams are seasonal and are fed mainly by the monsoon. The study area receives an average annual rainfall between 1000 mm to 1500 mm (Sati et al., 2007) with maximum rainfall occurring between mid-June to mid-September. Method A geomorphic map of the study area prepared by previous worker Depositional river terraces in tectonically active regions are used to determine the relative roles of tectonic and climatic changes in the landscape evolution of an area. Apart from providing evidences of tectonic and climatic shifts, these terraces over a long period act as a source of sediment, which is eroded from them. A terrace located in the Lesser Himalaya of known age and a small drainage network developed over it (indicating it to be the only source of erosion), was studied to estimate the rate of erosion. The rate of erosion was calculated by comparing the pre-erosion topography of the terrace modelled in this study by the present day topography of the terrace. Results show that these stored sediments in the form of a depositional terrace are being eroded at a rate of 190 tonnes/year. The results suggest that the rate at which these sediments are eroded is considerable and contributes significantly to the total sediment budget of the river.

Advances in Remote Sensing Technology and the Three Poles, Dec 10, 2022

Advances in Remote Sensing Technology and the Three Poles, Dec 10, 2022

<p>Himalayan rivers transport approximately 10<sup>3</sup&... more <p>Himalayan rivers transport approximately 10<sup>3</sup> Mt of sediment annually from their source in the steep topography of the High Himalaya to ocean basins. However, the journey from source to sink is not necessarily a smooth one: on the way, sediment can become trapped in montane storage systems, such as river valleys or floodplains. While sediment is stored in valleys, climate and erosional signals that we may wish to read from the final sedimentary record can be modified or even destroyed. We therefore need to understand the spatial distribution, volume and longevity of these valley fills. However, controls on Himalayan valley location and geometry are unknown, and sediment volume estimates are based on relatively untested assumptions of valley widening processes.</p> <p>In this work we use a new method of automatically detecting valley floors to extract 1,644,215 valley-floor width measurements across the Himalayan orogen. We use this dataset to explore the dominant controls on valley-floor morphology, and to test models of valley widening processes. We use random forest regression to estimate the importance of potential controlling variables, and find that channel steepness, a proxy for rock uplift, is a first-order control on valley-floor width. We also analyse a newly compiled dataset of 1,797 exhumation rates across the orogen and find that valley-floor width decreases as exhumation rate increases. We therefore suggest that valley-floor width is adjusted to long-term exhumation, controlled by tectonics, rather than being controlled by water discharge or bedrock erodibility. We also hypothesise that valley widening predominantly results from sediment deposition along low-gradient valley floors, controlled by the ratio of sediment to water discharge, rather than lateral bedrock erosion.</p>

Advances in Remote Sensing Technology and the Three Poles, Dec 10, 2022

Journal of the Geological Society of India

Himalayan rivers transport ≈ 103 Mt of sediment annually to ocean basins. River valleys are an im... more Himalayan rivers transport ≈ 103 Mt of sediment annually to ocean basins. River valleys are an important component of this routing system: while sediment is stored in valleys, signals of climate change and erosional patterns can be modified or even destroyed. Despite a critical need to understand the spatial distribution, volume and longevity of these valley fills, controls on valley location and geometry are unknown, and estimates of sediment volumes are based on never-tested assumptions of valley widening processes. Here we extract 1,644,215 valley-floor width measurements across the Himalaya to determine the dominant controls on valley-floor morphology for the first time, and to test underlying assumptions of sediment storage volumes. We use random forest regression to estimate the importance of potential controlling variables, and find that channel steepness, a proxy for rock uplift, is a first-order control on valley-floor width. We also analyse a novel dataset of 1,797 exhumat...

Tectonics, 2021

Understanding the processes and mechanisms responsible for exhumation are essential for determini... more Understanding the processes and mechanisms responsible for exhumation are essential for determining the evolution of landscapes. Our knowledge of these issues in continental convergent margins is still incomplete despite decades of research. Here, we present a record of exhumation rates obtained from thermochronology across the collision zone of the Himalayan‐Tibetan orogen, which has been carved by the dextral Karakoram fault (KF) along its western margin. Our thermochronology results across the KF suggest that transpression controlled exhumation from ∼10 to 8 Ma. Exhumation rates at the margin of the Tibetan plateau, in the SE Karakoram range, to the north of the KF are twice as high as the Ladakh range, to the south of the KF, since at least middle Miocene. We hypothesize that rapid exhumation of the Tibetan margin was in response to topographic uplift and subsequent erosion due to the convective removal of the lower lithosphere, triggered by the rollback of the subducting Indian lower crust during the Oligocene‐Miocene in an early phase. Our thermochronology results document an additional two‐fold increase in the exhumation rates of the Tibetan margin since the late Mio‐Pliocene. We propose that the underthrusting of the Indian Plate below Tibet during the late Mio‐Pliocene was responsible for this additional increase of exhumation rates of the Tibetan margin in the late phase. These exhumation rates are in good agreement with the topographic slope, relief, and channel steepness data of these regions. Thus, the coupling of tectonics and topography dictated the exhumation patterns since late Mio‐Pliocene.

<p>Rivers in the Himalaya have extensively been used as a tool to understan... more <p>Rivers in the Himalaya have extensively been used as a tool to understand the mechanism of valley aggradation and incision resulting due to precipitation variability and ongoing deformation. The suture zone tectonics control the landscape building along the upper Indus River, however, intensified monsoon encroaches on the region and modify the surface processes. The valley is filled with outwash fans and aggradation pulses at ~ 52 ka, ~ 28 ka and ~16 ka when the monsoon was strengthened. At the younger time, during ~13 to ~9 ka, the Indus valley has suffered an incision phase marked by exposure of bedrock near the Indus-Zanskar confluence.</p><p>The present study constrained the paleodischarge of the Indus River during periods of established river aggradation, incision and flood-time to understand climatic settings during the enhanced sediment-load modulated by the increased discharge. At the valley filling time (47–23 ka), clast geometric data of the largest imbricated clasts from the riverbed as well as the aggraded sequences were utilized to calculate discharges. Sand-silt couplets marked as slack water deposits (SWDs) of age 14–10 ka at Indus-Zanskar confluence were used to constrain the paleodischarges during net river incision. The catchment-scale paleodischarge derived from valley fill sequences and SWDs ranges from 834±47 to 4457±253 and 19030 to 47954 cumecs. Incision-time discharges were three to ten-fold higher than from the aggradation time observed that the aggradation in the Himalayan rivers occurred in transient time (33–21 ka and 17–14 ka) when the sediment load in the rivers increased just after the glaciation. Hence, the aggradation in the Indus River has occurred when the sediment to water ratio was higher and the river carrying capacity has reduced, subsequently, the incision was initiated when sediment to water ratio reduced and the river sediment carrying capacity increases during post-glacial climatically wet phase (early Holocene).</p>

<p>The high and low flood events in a braided river system cause perpetual ... more <p>The high and low flood events in a braided river system cause perpetual changes in the channel morphodynamics and make it difficult to understand a channel response.  Therefore, the management of braided channel morphodynamics becomes a challenging issue for flood mitigations and river restoration purposes. The lower Brahmaputra river basin has the world’s most extensive braided river system, and every year it confronts reoccurring monsoon driven flooding causing widespread flood inundation and changes in channel morphodynamics. Such adequate conditions promise a natural laboratory to understand dynamics channel morphodynamics changes with high and low flood events. The present work integrates the mutual effects of varying channel area, width and sinuosity, and sediment bar area along a braided channel reach in the selected reach in the Brahmaputra River in thirty events during high discharge months of 2018, 2019 and 2020. To observe detailed channel morphodynamics changes, we developed 100 grids with a width of ~6.25 km enclosing the selected reach. These grids are maintained stationary for each year and were used to extract the Brahmaputra River channel area (BRCA)  and Brahmaputra River sediment bar area (BRSBA) and average Brahmaputra River channel width (BRCW) for each grid. We developed a site-specific Google Earth Engine algorithm to delineate the channel and sediment bar in the selected reach to perform supervised classification on Sentinel-1 SAR GRD and Sentinel-2 level-1C.</p><p>The results show that grids upstream of the selected reach have a high BRCA, BRSBA and BRCW, and these grids are located around high clustered flood inundated hotspot regions.  We also found that the world's largest river island  (Majuli) is also located in this zone. In the present study, we also observe that if we consider BRCA, BRCW and BRSBA for the same event, the BRSBA has a high correlation with the BRCW compared to the BRCA. Further, we compared the impact of the sinuosity on BRCA, BRCW and BRSBA of regular and influential flood events. During influential events, the sinuosity has only a good impact on the BRCA, and during regular events, it has a higher impact on BRCA and BRSBA. We conclude that in the braided river system of the Brahmaputra River, the channel and bar area and channel width are highly correlated during flood events, and the channel sinuosity also controls channel and bar area and channel width during regular and influential flood events.</p>

EGU General Assembly Conference Abstracts, Apr 1, 2018

The length of the main trunk in a river basin is an important morphometric parameter and it depen... more The length of the main trunk in a river basin is an important morphometric parameter and it depends on the size of the drainage basin. The Ganga River Basin is one of the largest basins in the world with the Ganga River considered to be the main stem. Variable lengths of this river in the literature motivated us to study its exact length and also to test whether geomorphically it is the longest river in the basin. The results show that the maximum river length of 2758 km is attained when source is considered at the headstream of the Tons River. This length is more than any of the traditional lengths of the Ganga River in the literature. We propose to call the longest segment of the river in Ganga Basin as the Himalayan Foreland River.

The length of the main trunk in a river basin is an important morphometric parameter and it depen... more The length of the main trunk in a river basin is an important morphometric parameter and it depends on the size of the drainage basin. The Ganga River Basin is one of the largest basins in the world with the Ganga River considered to be the main stem. Variable lengths of this river in the literature motivated us to study its exact length and also to test whether geomorphically it is the longest river in the basin. The results show that the maximum river length of 2758 km is attained when source is considered at the headstream of the Tons River. This length is more than any of the traditional lengths of the Ganga River in the literature. We propose to call the longest segment of the river in Ganga Basin as the Himalayan Foreland River.

Episodes, 2012

As the discharge increases and sediment supply decreases, the rivers incise forming terraces. Whe... more As the discharge increases and sediment supply decreases, the rivers incise forming terraces. When considered over a long period, the sediment storage in the valleys become a source for sediment supply and may contribute a significant amount of the total sediment budget. An estimate made by mapping the area of the terraces and multiplying it with their thickness suggests ~ 4800 million tonnes of sediments is trapped between Pipalkoti and Devprayag in the Alaknanda valley (Fig. 1a). These terraces continuously get eroded by various processes such as gullying, undercutting by rivers and collapsing under the influence of gravity contributing to the adjacent rivers. Therefore, apart from the rate of erosion of the bedrock, it is essential to calculate the rates of erosion of such valley fill deposits and quantify the amount of sediment supplied by them to the river systems. In this study, an attempt has been made to determine the rate of erosion by calculating the volume of sediment removed from ã 17 ka terrace located in NW Lesser Himalaya. River terraces (referred to as terraces from here onwards) form when a river becomes unstable and incises its bed (e.g. Schumm et al., 1987; Bridgland, 2000; Wegmann and Pazzaglia, 2009). Thus, terraces record the time of instability of the channel, making it an excellent geomorphic marker. Accordingly, terraces are commonly studied to identify tectonic and climatic events causing changes in the river dynamics in the geological past (e.g. Wegmann and Pazzaglia, 2009). Study area A requirement of this study was a terrace of known age with least interference from external processes such as anthropogenic activity or sediment supply on terrace surface by local stream, so that sediment volume of the terrace is only altered due to natural erosional processes. Thus, a terrace located in the Srinagar (Garhwal) region along the left bank of the Alaknanda River was chosen for the study (Figs. 1a,b) which fulfilled the above requirement. A village named Swit is located on top of the terrace under investigation, hence from here onwards this terrace is referred to as the Swit Terrace. It is located 5 km ENE from the main Srinagar city (Figs. 1a,b). The elevation of the Swit terrace is ~ 655 m asl (above sea level) and covers an area of 0.6 km 2 and the elevation of its top from the river bed is ~ 100 m. A small stream originating from the adjacent valley walls, and locally called Swit nala, flows into the Alaknanda river. The local streams are seasonal and are fed mainly by the monsoon. The study area receives an average annual rainfall between 1000 mm to 1500 mm (Sati et al., 2007) with maximum rainfall occurring between mid-June to mid-September. Method A geomorphic map of the study area prepared by previous worker Depositional river terraces in tectonically active regions are used to determine the relative roles of tectonic and climatic changes in the landscape evolution of an area. Apart from providing evidences of tectonic and climatic shifts, these terraces over a long period act as a source of sediment, which is eroded from them. A terrace located in the Lesser Himalaya of known age and a small drainage network developed over it (indicating it to be the only source of erosion), was studied to estimate the rate of erosion. The rate of erosion was calculated by comparing the pre-erosion topography of the terrace modelled in this study by the present day topography of the terrace. Results show that these stored sediments in the form of a depositional terrace are being eroded at a rate of 190 tonnes/year. The results suggest that the rate at which these sediments are eroded is considerable and contributes significantly to the total sediment budget of the river.