Rock Avalanche Dams Research Articles

The Hapuku Rock Avalanche: Breaching and evolution of the landslide dam and outflow channel revealed using high spatiotemporal resolution datasets

Landslide dams have been recognised as significant components of multi-hazard cascading systems, linking slopes and rivers. Despite the potential for catastrophic consequences, landslide dam breaching and evolution remain under-researched and poorly understood, often due to the remoteness of large volume, valley-blocking landslides and the general lack of high resolution pre- and post-failure survey data. The Hapuku Rock Avalanche presents a unique opportunity to study landslide dam evolution and breaching timelines due to the accessibility of the site and the availability and resolution of pre- and post-failure remote sensing data. Field observations and mapping, sampling, geophysical surveying, and 27 remote sensing surveys from 2016 to 2022 have provided detailed data on the dam. The Hapuku landslide was the largest rock avalanche triggered by the 2016 Mw 7.8 Kaikōura earthquake sequence, occurring ∼9 km upstream of the main highway and rail corridor on the South Island of New Zealand. It dammed the Hapuku River, which rapidly formed a lake behind the 80 m-high deposit. Four major erosion events and three significant partial breach events, identified through observations and remote sensing data differencing, resulted in water outflow from the lake, significant erosion of the dam and deposition of sediment into the river. The partial breaches correspond with less than 1 in 10-year rainfall events in 2017 and 2018, and the first occurred 141 days after dam formation. Seepage and internal erosion of the dam were observed to be progressing upstream before the partial breaches, in which water overtopped the dam. The third partial breach event, 2 years after dam formation, was the most significant erosional event in the last 6 years. The dam has eroded episodically and more locally since 2018, and the degree of erosion appears to be decreasing with time, despite more intense storms. A small lake remains. The evolution of the Hapuku Rock Avalanche dam emphasises the complexity of dam and breaching evolution, which are often oversimplified.

Major geomorphic events and natural hazards during monsoonal precipitation 2018 in the Kali Gandaki Valley, Nepal Himalaya

Highest geomorphic activity in the central Nepal Himalayas is mainly driven by monsoonal precipitation. In contrast, the northern flanks of the Nepal Himalaya lay in the relative dry rain shadow of the mountain range.During the monsoon 2018, major floods and geomorphic events occurred in the Kali Gandaki (KG) valley impacting both the monsoon-affected and the dry parts of the catchments. We analyze the events and its triggers based on field observations, multiple satellite image interpretation, climatological analysis using Global Precipitation Measurement and MODIS snow cover data, hydrological analysis and media analysis.The hydro-meteorological triggers are complex. Exceptional precipitation in April and May 2018 occurred in the entire study area, followed by a rather dry period. Precipitation in August was exceptional in the northern part whereas below average in the South. We argue that dynamics of snow accumulation and delayed melting contributed significantly to flooding and increased geomorphic activity in the southern part in August whereas flooding in the northern part was mainly triggered by rainfalls. We thus define 2018 as an abnormal (pre-)monsoon year with less rainfall than average but being more catastrophic.Sediment dynamics in the study area are still controlled by the Dhampu rock avalanche dam and the braided river floodplain north of this knickpoint, where sediment pulses delivered from tributaries are rarely connected from the main river. During the monsoon floods 2018 sediment connectivity was given for most tributaries due to the steepness of the catchments.The study area is subject to major human impact. Mostly in the south, numerous hillslopes have been undercut by road construction, leading to higher geomorphic sensitivity. Severe landslides might thus be triggered in future even by less intense rainfall events.Magnitude and frequency of such abnormal (pre-)monsoon precipitation are highly relevant for sediment flux and natural hazards studies.

Critical hydraulic gradients for seepage-induced failure of landslide dams

Landslide dams formed by rock avalanche processes usually fail by seepage erosion. This has been related to the complex sedimentological characteristics of rock avalanche dams which are mostly dominated by fragmented and pulverized materials. This paper presents a comprehensive experimental programme which evaluates the critical hydraulic and geometrical conditions for seepage-induced failure of landslide dams. The experiments were conducted in a flume tank specifically designed to monitor time-dependent transient changes in pore-water pressures within the unsaturated dam materials under steady-state seepage. Dam models of different geometries were built with either mixed or homogeneous materials. Two critical hydraulic gradients corresponding to the onset of seepage erosion initiation and collapse of the dam crest were determined for different upstream inflow rates, antecedent moisture contents, compactive efforts, grain size ranges, and dam geometries. Two major types of dam failure were identified: Type I and Type II. These were further subdivided into minor failure processes which include exfiltration, sapping, downstream toe bifurcation, and undermining of the downstream face. The critical hydraulic gradients for seepage erosion initiation varied from 0.042 to 0.147. Experiments conducted with the mixed materials indicate that the critical hydraulic gradients for collapse of the dam crest increased with an increase in uniformity coefficient. The deformation behaviour of the dams was significantly influenced by particle density, pore geometry, hydraulic conductivity, and the amount of gravel and pebbles present in the materials. The results indicate that the critical seepage velocity for failure of the dams decreased with an increase in downstream slope angle, but increased with an increase in pore geometry, dam height, dam crest width, upstream inflow rate, and antecedent moisture content.

Post-rock-avalanche dam outburst flood sedimentation in Ram Creek, Southern Alps, New Zealand

Rock avalanches are common in mountainous regions that are tectonically active. They are capable of forming natural dams of uncertain persistence that have significant impacts on the river system over wide spatial scales and possibly over geological time scales. Here we combine field data and digital elevation model (DEM) analysis to show the response of Ram Creek, New Zealand, to 28years of sediment dispersion following the 1968 emplacement of a co-seismic, rock-avalanche dam that breached catastrophically in 1981. The results show a system that has not attained equilibrium, being unable to move the quantity of dam-derived sediments, and will likely not attain equilibrium before the next major sediment input; it is in a state of persistent disturbance where localised reworking dominates. Erosion in Ram Creek is focussed on lateral bevelling and bedrock gorge widening rather than vertical incision to keep pace with tectonic uplift. Importantly for studies of tectonic geomorphology, this widening — which if sustained will form a strath terrace — does not represent a period of reduced uplift. Stream metrics (concavity and steepness) are unable to differentiate the identified rock-avalanche-induced knickpoint from tectonic and lithological knickpoints.

Holocene versus modern catchment erosion rates at 300 MW Baspa II hydroelectric power plant (India, NW Himalaya)

300MW Baspa II is India’s largest private hydroelectric facility, located at the Baspa River which is an important left-hand tributary to the Sutlej River in the NW Himalaya (India). In this valley the Sangla palaeo-lake has been dammed around 8200yr BP behind a rock-avalanche dam and Baspa II is located exactly on top of this palaeo-lake. This special location represents a very rare possibility to evaluate the short-term, river load and hydrological parameters measured during the planning and operational stages of Baspa II with the long-term parameters gained from the palaeo-lake sediments from the catchment. Sedimentological and geomorphological investigations of the lacustrine sediments have been used to reconstruct environmental changes during >2500years of its existence. The Mid-Holocene erosion rates of the Baspa catchment estimated from the volume and duration of deposition of the exposed lake sediments are at 0.7–1.0mmyr−1, almost identical with the modern erosion rates calculated from river gauge data from Baspa II. Several charcoal layers and charcoal pieces from the uppermost palaeo-lake levels around 5000calyr BP might be related to woodland clearance and they possibly represent one of the oldest evidences for human presence in the Baspa Valley during Neolithic time.

300 MW Baspa II — India's largest private hydroelectric facility on top of a rock avalanche-dammed palaeo-lake (NW Himalaya): Regional geology, tectonic setting and seismicity

This study aims for the characterization of the geological setting of 300MW Baspa II, India's largest private hydroelectric facility which was built on top of a relict rock avalanche dammed palaeo-lake (Baspa Valley, NW Himalaya). Geologically, the hydroelectric installation is located in the Higher Himalayan Crystalline, just above the active Karcham Normal Fault, which is reactivating the Early Miocene Main Central Thrust, one of the principal Himalayan faults. The area is seismically active and mass-movements are common. At ca. 8200yr BP the Baspa River was blocked behind a 142×106m3 rock avalanche dam which created a ca. 260m deep palaeo-lake. The whole palaeo-lake was completely filled with sediments in about 3100years, making the Sangla palaeo-lake to a very rare example of a mass-movement dam with very long duration.The hydroelectric installation was built with its intake situated directly on top of the mass-movement dammed palaeo-lake of Sangla, utilizing the convex knick point in the river profile to increase the head for Baspa II for ca. 125m, compared to the reconstructed longitudinal profile of the river prior to the mass-movement, which amounts for about 18% of its design head.At least 5 levels of soft-sediment deformation have been recorded in the exposed part of the lacustrine sediments of Sangla palaeo-lake, including brecciated laminae, overturned laminae, folds, faults and deformation bands, separated by undeformed deposits. They are interpreted as seismites, indicating at least 5 earthquakes within 2495±297years strong enough to cause liquefaction. These observations extend the local seismicity record considerably into the past and indicate more and possibly stronger seismic events than might be expected from the instrumental measurements.

Formation and failure of the Tsatichhu landslide dam, Bhutan

At 00:30 (local time) on the 10th September 2003 a joint and foliation defined wedge of material with an estimated volume of 7–12×106 m3 slid into the narrow Tsatichhu River Valley, in Jarrey Geog, Lhuentse, eastern Bhutan. The Tsatichhu River, a north–easterly flowing tributary of the Kurichuu River, was completely blocked by the landslide. During its movement, the landslide transitioned into a rock avalanche that travelled 580 m across the valley before colliding with the opposite valley wall. The flow then moved down valley, travelling a total distance of some 700 m. The rock avalanche was accompanied by an intense wind blast that caused substantial damage to the heavily forested valley slopes. The resulting geomorphologically-typical rock-avalanche dam deposit created a dam that impounded a water volume of 4–7×106 m3 at lake full level. This lake was released by catastrophic collapse of the landslide, which occurred at 16:20 (local time) on 10th July 2004, after reported smaller failures of the saturated downstream face. The dam failure released a flood wave that had a peak discharge of 5900 m3 s−1 at the Kurichhu Hydropower Plant 35 km downstream.

Effects of the internal structure of a rock–avalanche dam on the drainage mechanism of its impoundment, Flims sturzstrom and Ilanz paleo-lake, Swiss Alps

The Holocene sturzstrom of Flims, Grisons, Switzerland mobilized 12 km 3 of Jurassic limestone making it one of the largest events of this kind on the earth. The material dammed a 10-km-long reach of the upper Rhine River Valley with thicknesses up to 400 m. A 40-km-long natural lake was formed upstream. The body of the avalanche deposits displays a well-preserved internal fabric. Five major sedimentary facies were identified related to the confinement of the deposits in a narrow valley that restricts the spreading of transported material. In the deepest parts of the avalanche deposit, a stratified facies attests that the movement was mainly non-turbulent, whereas a granular facies appears in the lateral and upper parts. The relative position of these facies within the natural dam has played an important role in the way the water flow eroded the dam crest and emptied the lake as suggested by the present valley slope morphology. Ledges corresponding to the levels of the different facies on north and south slopes of the valley correspond to steps in the narrowing of the incision. The outburst of the dam, if it occurred, only affected the upper part of the deposits that corresponds to the poorly cohesive granular facies. One hundred kilometres downstream, in Lake Konstanz (Bodensee) lacustrine sediments, the presence of two layers, contrasting with the normal grey varved deposits and separated by six laminites, are contemporaneous with the mass movement. These layers, which could correspond to the outburst of the dam, point to not one, but two major overflooding events.