Debris flow-slide initiation mechanisms in fill slopes, Wellington, New Zealand

AbstractAuthigenic chrysotile, stevensite, calcite, aragonite and pectolite have formed together in a Holocene ophiolitic debris flow in Southland, New Zealand. Mineral growth occurred about 4700-5700 years ago. The temperature of formation of these minerals is estimated from climatological data to be 5–10°C Surface water and groundwater δ180 is currently about –10‰, and was estimated to be about –9.5±1‰ during mineralization. Coexisting calcite (δ180 = +23‰) and aragonite (δl8O = +24‰) were in equilibrium with each other and with the groundwater at 5–10°C Stevensite δ180 is +14 to +16‰, chrysotile has δ180 = +5.5‰, and authigenic pectolite has δ180 near +10‰. Carbon isotope ratios for calcite and aragonite are strongly depleted (δ13C = –13 to –18) which suggests that dissolved CO2 had δ13C below -27. This isotopically light carbon probably resulted from a high organic component of carbon dissolved in the groundwater.

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Periglacial preconditioning of debris flows in the Southern Alps, New Zealand

10.26686/wgtn.17008210 ◽

2021 ◽

Author(s):

◽

Katrin Sattler

Keyword(s):

New Zealand ◽

Debris Flow ◽

Debris Flows ◽

Southern Alps ◽

Ice Growth ◽

Frost Weathering ◽

Flow Formation ◽

Flow Systems ◽

Spatially Distributed ◽

Flow Activity

The lower boundary of alpine permafrost extent is considered to be especially sensitive to climate change. Ice loss within permanently frozen debris and bedrock as a consequence of rising temperature is expected to increase the magnitude and frequency of potentially hazardous mass wasting processes such as debris flows. Previous research in this field has been generally limited by an insufficient understanding of the controls on debris flow formation. A particular area of uncertainty is the role of environmental preconditioning factors in the spatial and temporal distribution of debris flow initiation in high-alpine areas. This thesis aims to contribute by investigating the influence of permafrost and intensive frost weathering on debris flow activity in the New Zealand Southern Alps. By analysing a range of potential factors, this study explores whether debris flow systems subjected to periglacial influence are more active than systems outside of the periglacial domain. A comprehensive debris flow inventory was established for thirteen study areas in the Southern Alps. The inventory comprises 1534 debris flow systems and 404 regolith-supplying contribution areas. Analysis of historical aerial photographs, spanning six decades, identified 240 debris flow events. Frequency ratios and logistic regression models were used to explore the influence of preconditioning factors on the distribution of debris flows as well as their effect on sediment reaccumulation in supply-limited systems. The preconditioning factors considered included slope, aspect, altitude, lithology, Quaternary sediment presence, neo-tectonic uplift rates (as a proxy for bedrock fracturing), permafrost occurrence, and frost-weathering intensity. Topographic and geologic information was available in the form of published datasets or was derived from digital elevation models. The potential extent of contemporary permafrost in the Southern Alps was estimated based on the statistical evaluation of 280 rock glaciers in the Canterbury region. Statistical relationships between permafrost presence, mean annual air temperature, and potential incoming solar radiation were used to calculate the spatially distributed probability of permafrost occurrence. Spatially distributed frost-weathering intensities were estimated by calculating the number of annual freeze-thaw cycles as well as frost-cracking intensities, considering the competing frost-weathering hypotheses of volumetric ice expansion and segregation ice growth. Results suggest that the periglacial influence on debris flow activity is present at high altitudes where intense frost weathering enhances regolith production. Frost-induced debris production appears to be more efficient in sun-avert than sun-facing locations, supporting segregation ice growth as the dominant bedrock-weathering mechanism in alpine environments. No indication was found that permafrost within sediment reservoirs increases slope instability. Similarly, the presence of permanently frozen bedrock within the debris flow contribution areas does not appear to increase regolith production rates and hence debris flow activity. Catchment topography and the availability of unconsolidated Quaternary deposits appeared to be the cardinal non-periglacial controls on debris flow distribution. This thesis contributes towards a better understanding of the controls on debris flow formation by providing empirical evidence in support of the promoting effect of intense frost weathering on debris flow development. It further demonstrates the potential and limitations of debris flow inventories for identifying preconditioning debris flow controls. The informative value of regional-scale datasets was identified as a limitation in this research. Improvement in the spatial parameterisation of potential controls is needed in order to advance understanding of debris flow preconditioning factors.

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Stratigraphy, facies architecture and emplacement history of the c. 3.6 ka B.P. Ngatoro Formation on the eastern flanks of Egmont Volcano, western North Island, New Zealand

10.26686/wgtn.17009600.v1 ◽

2021 ◽

Author(s):

◽

Benjamin John Dixon

Keyword(s):

New Zealand ◽

Debris Flow ◽

Mass Flow ◽

Extended Phase ◽

Facies Architecture ◽

Eruptive Event ◽

Central Java ◽

Flow Deposit ◽

History Of ◽

Mass Flows

The Ngatoro Formation is an extensive volcaniclastic deposit distributed on the eastern lower flanks of Egmont Volcano, central North Island, New Zealand. Formally identified by Neall (1979) this deposit was initially attributed to an Egmont sourced water-supported mass flow event c. 3, 600 ¹⁴C years B.P. The Ngatoro Formation was subsequently described by Alloway (1989) as a single debris flow deposit closely associated with the deposition of the underlying Inglewood Tephra (c. 3,600 ¹⁴C yrs B.P) that had laterally transformed into a hyperconcentrated- to- flood flow deposit. Such water-supported mass flows have been well documented on volcanoes both within New Zealand (i.e. Mt Ruapehu) and elsewhere around the world (i.e. Mt Merapi, Central Java and Mt St Helens, Washington). This thesis comprises field mapping, stratigraphic descriptions, field and laboratory grain size and shape analysis, tephrochronology and palaeomagnetic analysis with the aim of refining the stratigraphy, facies architecture and emplacement history of the c. 3,600 ¹⁴C yrs B.P. Ngatoro Formation. This study has found that the Ngatoro Formation has a highly variable and complex emplacement history as evidenced by the rapid textural changes with increasing distance from the modern day Egmont summit. The Ngatoro Formation comprises two closely spaced mass flow events whose flow and emplacement characteristics have undergone both proximal to distal and axial to marginal transformations. On surfaces adjacent to the Manganui Valley on the deeply incised flanks of Egmont Volcano, the Ngatoro Formation is identified as overbank surge deposits whereas at the boundary of Egmont National Park it occurs as massive, pebble- to boulder-rich debris flow deposits. At intermediate to distal distances (17-23 km from the modern Egmont summit) the Ngatoro Formation occurs as a sequence of multiple coalescing dominantly sandy textured hyperconcentrated flow deposits. The lateral and longitudinal textural variability in the Ngatoro Formation reflects downstream transformation from gas-supported block-and-ash flows to water-supported debris flows, then subsequently to turbulent pebbly-sand dominated hyperconcentrated flows. Palaeomagnetic temperature estimates for the Ngatoro Formation at two sites (Vickers and Surrey Road Quarries, c. 10 km from the present day Egmont summit) indicate clast incorporation temperatures of c. 300°C and emplacement temperatures of c. 200°C. The elevated emplacement temperatures supported by the Ngatoro Formation’s coarse textured, monolithologic componentry suggest non-cohesive emplacement of block-and-ash flow debris generated by the sequential gravitational collapse of an effusive lava dome after the paroxysmal Inglewood eruptive event (c. 3,600 ¹⁴C yrs B.P.). The occurrence of a prominent intervening paleosol between these two events suggest that they are not part of the same eruptive phase but rather, the latter is a product of a previously unrecognised extended phase of the Inglewood eruptive event. This study recognises the potential for gravitational dome collapse, the generation of block-and-ash flows and their lateral transformation to water-support mass flows (debris, hyperconcentrated and stream flows) occurring in years to decades following from the main eruptive phase. This insight has implications with respect to the evaluation of post-eruptive hazards and risk.

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Cyclone-triggered debris flow hazard on Takaka Hill, Tasman District, South Island, New Zealand

Lecture Notes in Civil Engineering - Geotechnics for Sustainable Infrastructure Development ◽

10.1007/978-981-15-2184-3_116 ◽

2019 ◽

pp. 893-900

Author(s):

Christine Prasad ◽

Clark H. Fenton

Keyword(s):

New Zealand ◽

Debris Flow ◽

Debris Flow Hazard

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The geomorphic impact of large landslides: A case-study of the actively moving Alpine Gardens Landslide, Fox Glacier Valley, West Coast, New Zealand.

10.5194/egusphere-egu2020-12411 ◽

2020 ◽

Author(s):

Saskia de Vilder ◽

Chris Massey ◽

Garth Archibald ◽

Regine Morgenstern

Keyword(s):

New Zealand ◽

Debris Flow ◽

West Coast ◽

Debris Flows ◽

Sediment Flux ◽

Valley Floor ◽

Aerial Imagery ◽

Glacier Valley ◽

Flow Activity

Large landslides can result in significant geomorphic impacts to fluvial systems, via increased sediment input and subsequent changes to channel behaviour. We present a case-study of the actively moving&#160; &#820;65 M m&#179; Alpine Gardens Landslide in the Fox Glacier Valley, West Coast, New Zealand, to analyse the ongoing geomorphic impacts within the valley floor. Debris flows, sourced from the toe of the landslide, travel down Mill&#8217;s Creek and deposit sediment on the debris fan at its confluence with the Fox River. This debris flow activity and associated changes in sediment flux and fluvial behaviour have resulted in re-occurring damage to, and current closure of roads and tracks within the Fox Glacier Valley floor, impacting access to the Westland Tai Poutini National Park, the Fox Glacier, associated tourism, and the Fox Glacier township economy.Initial movement of the Alpine Gardens landslide was detected in 2015, with aerial imagery analysis between March 2017 and June 2018 indicating that the landslide may be accelerating. This acceleration may potentially result in increased debris flow activity within the landslide complex and sediment flux into the Fox River. To monitor and understand the controls on movement rate, we installed a continuous GPS monitoring station along with rainfall gauges on the landslide in February 2019. On average, the landslide moves at a rate of 0.12 m/day &#177; 0.13 m/day, however this rate of movement of the landslide is closely correlated to and fluctuates with rainfall. Significant accelerations of 0.5 m/day have occurred after heavy rainfall, with these rainfall events also resulting in large debris flows.We document and investigate the geomorphic impact of the Alpine Gardens landslide on the Mill&#8217;s Creek debris fan and Fox Glacier Valley floor via terrestrial laser scanning, airborne LiDAR, UAV surveys and aerial imagery. From this, we derive a time-series of nine surface change models to document the sediment flux within the Alpine Gardens Landslide and Mill&#8217;s Creek debris fan complex. Our initial results reveal that between March 2017 and June 2019, approximately 14.7 M m&#179; was eroded from the landslide, of which 3.7 M m&#179; was deposited directly on the debris fan. A further 9.6 M m&#179; has been transported downstream into the fluvial system. Upstream aggradation has also occurred, with 1.1 M m&#179; deposited in the river valley immediately upstream of the debris fan between June 2018 and June 2019. Continued monitoring of the Alpine Gardens Landslide and volumetric changes of the landslide complex allows us to understand the controls on the movement and sediment flux within the landslide and the geomorphic impact of large actively moving landslides on the valley floor, particularly within alpine and glacial environments.&#160;

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