Search

The fluvial geomorphology in tectonically active (particularly rapid uplift) regions often undergoes continuous change. The rapid uplift is coincident with high erosion rates; consequently, incised valleys are formed. Mass flows (for example, avalanches, landslides, and debris flows) in incised valleys can markedly influence fluvial processes and even reshape valley geomorphology. However, these processes and long-term evolution corresponding to mass flows require further clarification. Field campaigns were carried out in the region near the Yigong Tsangpo and Palong Tsangpo Rivers (hereafter the Yigong and Palong Rivers), the two largest tributaries of the lower Yarlung Tsangpo River, to examine the feedback between fluvial processes and mass flows. Remote sensing images from recent decades were used to compare the channel morphology before and after typical mass flows (particularly catastrophic ones). The morphology of the lower Yigong River has evidently been impacted by landslides, while that of the Palong River has mainly been shaped by glacial processes and debris flows. At present, the morphology of the latter consists of alternating sections of gorges and wide valleys, with a staircase-like longitudinal profile. The gorge sections exhibit single and deeply incised channels with a high-gradient channel bed and terraces. In contrast, the wide valley sections consist of lakes, braided or anabranching channels, gentle bed gradients, and thick alluvial deposits. Debris flows occur more frequently in gullies in the reaches of the gorge sections and rarely in gullies along the wide valley sections. The occurrence of mass flow events has resulted in an imbalance of the previous (quasi-)equilibrium in the river morphology; however, this has triggered negative feedback that is driving the transient river morphology to a new state of (quasi-)equilibrium. • Feedback between fluvial processes and mass flows in rapid-uplifting southeast Tibet is studied. • Field campaigns and satellite images of typical reaches were combined in the study. • Lotus root-like planform and staircase-like longitudinal profile form due to mass flows. • Negative feedback exists pushing unstable river morphology to a new (quasi-)equilibrium state. • 'Stairs (dams)' more likely form spatially from upstream to downstream by mass flows. [ABSTRACT FROM AUTHOR]

• Even though check dams are full, they still control the debris flows. • The new model can quantitatively evaluate debris flow mitigation effect. • The check dams and afforestation play different roles in different stages. Check dams and afforestation are two common measures for debris-flow mitigation; however, there is a lack of methods allowing quantitative evaluation of check dam and afforestation effects on debris flow motion and deposition. Laogan Gully is an active debris-flow system with a large landslide on the right hillslope, currently representing the main source of debris flows in the catchment. Several debris flows have occurred since 1985 which led to the construction of check dams and the plantation of silver wattle (Acacia dealbata) trees between 1991 and 1993. These measures significantly reduced the volume of debris flows reaching downstream portions of the system. At the same time, large amounts of sediment were trapped on the gully bed and thereby increased the base level of the torrent, which again tended to stabilize the large landslide on the right hillslope. This paper analyzes sedimentation of debris-flow material on the gully bed and on the fan using 12 trial pits behind the check dams and 6 incised geological sections on the fan. The timing and extent of debris flow motion and deposition was reconstructed using scars and growth suppression events in the annual growth ring series of 306 Acacia trees trapped in the newly formed deposits. The gravitational potential energy of the sediment of the large landslide is calculated according to the change in the erosion base level behind the check dams. On the basis of these results, we constructed a semi-empirical vegetation coverage–landslide energy–debris flow volume model to quantitatively evaluate the effects of the check dams and afforestation on debris-flow mitigation. The model exhibited predictive capabilities for estimating the volume of debris flow generated by the reactivation of landslides. Check dams are found to play a major role in debris-flow mitigation in the early stages, whereas the Acacia trees play an increasingly important role once the check dams are filled. The partial resurgence of the landslide caused a sudden increase in the gravitational potential energy of the sediment source, which in turn affected the effectiveness of the debris-flow mitigation measures. [ABSTRACT FROM AUTHOR]

• Debris flows near the dam and barrier lake complicated river sedimentation rate. • Vegetation growth disturbance was consistent well with river sedimentation rate. • Trees recovery time lagged behind debris flow event 1–3 years in glacial valley. In incised mountain valleys, debris flows often cause damage to vegetation, especially when debris flows form fans or block receiving rivers. In such contexts, riparian trees may be partially buried in aggrading fans or temporary lakes. The Guxiang gully is the largest debris flow system along the Palongzangbo River on the Tibetan Plateau and at least 13 debris flows (volume > 104 m3) have occurred since 1953. Four of these debris flows (in 1953, 1965, 1975 and 2005) dammed the river forming a barrier lake which is now more than 25 km in length. The repeated damming by debris flows has increased the dam height and thereby affected the sedimentation rates in the barrier lake. The frequent debris flow activity has prevented vegetation recovery on the fan, while sedimentation in the barrier lake has allowed vegetation to colonize the deltas. This study investigated the effects of frequent debris flow damming on sedimentation rates as well as on revegetation and tree disturbance along the river. We documented the landscape evolution and measured the sedimentation thickness and grain size of deposited material at different locations of the barrier lake using cores. We also collected and analyzed growth-ring records from disturbed trees on the fan and the deltas of the barrier lake. We found that frequent debris flows had created a stepped river morphology and greatly increased sedimentation rates in the lake, affecting vegetation on the fan and behind the dam. [ABSTRACT FROM AUTHOR]

Debris flows are a common type of geological hazard. They move down slopes at rapid speeds, causing severe damage to buildings and humans. It is of great significance to study the development and movement characteristics of debris flows. In this study, we developed a numerical method of simulating the movement of debris flows. The phase fraction was used to represent the contents of the debris flow components. The Herschel–Bulkley–Papanastasiou model was used to simulate the movement of the fine particle–water mixture in the debris flow, and the pressure-dependent Coulomb viscoplastic model was used to describe the movement of the gravel-size particles. The interface between the debris flow and the air was obtained using the volume of fluid method. The above models were implanted in OpenFOAM. Finally, indoor experimental data were used to verify the numerical method developed in this study, and the experimental and simulation results were found to be in good agreement. The proposed method was then applied to debris flow prediction in the Aiwa Watershed, Beijing. The homogeneous flow model developed in this study can significantly reduce the number of calculations required and can be used for the three-dimensional simulation of large-scale debris flows. [ABSTRACT FROM AUTHOR]

• The excess topography Z E is the dominant control parameter in debris-flow gullies. • The sediment supply during debris flow events mainly comes from bank collapse. • Debris flow gullies of intermediate size have the highest Z E and are the most active. An investigation on 152 gullies along the Daheba River in the Tongde sedimentary basin was performed. Debris flows develop in gullies with an excess topography Z E , which represents the sediment availability, above a critical threshold value. Debris flows in the Daheba watershed are supply-unlimited, i.e sediment is abundantly available from the steep erodible gully banks. Debris flows consist of a head and a body. The body propagates faster than the head and constantly supplies it with sediment. The body and head propagate in an intermittent way through the transient storage of sediment on the riverbed and its subsequent remobilization. Although the main sediment supply is provided by bank collapse, debris-flow events also incise the gully bed. The growth and incision of debris-flow gullies in supply-unlimited watersheds is mainly controlled by the frequency of occurrence of debris flows, which is closely related to Z E. With growth of the gully drainage area, Z E and the debris-flow frequency initially increase, until they reach maximum values in gullies with a drainage area of intermediate size, which are assumed to be the morphologically most active gullies. With further growth of the gully drainage area, Z E and the debris-flow frequency decrease, which opposes the development of debris flows and leads to a more stable gully morphology. [ABSTRACT FROM AUTHOR]