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Post-landslide event response report and data

From August 16th to 20th 2022, an atmospheric river bringing moisture down from the tropics led to a sustained five-day period of heavy rainfall for much of New Zealand. In particular, prolonged and heavy rain fell over Northland, Taranaki, Wellington and the Marlborough and Tasman regions. Taranaki Maunga and some areas at the top of the South Island experienced up to 1.2 m of rain, and large areas in the Nelson and Tasman districts experienced rainfall totals with annual return intervals of >250 years (https://hirds.niwa.co.nz/). NIWA reported that this was the strongest atmospheric river for August since records began in 1959 (https://niwa.co.nz/news/). A GeoNet landslide rapid response (LRR) for this event was activated on the 19th of August, as several of the criteria for activating a rapid response were met. The GeoNet LRR for this event included: (1) August 19 – September 7: desktop study and formal collection of all available landslide reports for later incorporation into a national landslide database. Local Territorial Authorities were contacted to collate available local knowledge on the impacts of the event. This information was used to determine whether an escalation was warranted, and, if so, what the appropriate actions would involve. Following these activities, the main areas of concern were: Northland, Taranaki, Wellington, Marlborough, Nelson/Tasman and the West Coast; (2) August 19–24: aerial and field reconnaissance in the Nelson/Tasman region, reported on by Massey et al. (2022); (3) September 7: aerial reconnaissance of Marlborough Sounds, reported on by Rosser and Townsend (forthcoming 2022). This report covers the New-Zealand-wide landslide impacts of the August 16–20 rainfall event

During 16–19 August 2022, an extreme rainfall event affected much of New Zealand. The National Institute of Water & Atmospheric Research (NIWA) refer to this event as the “strongest August atmospheric river (AR) on record” and classified it as a 1-in-120-year rain event for Nelson. The National Emergency Management Agency (NEMA) classified the event as ‘N1’, which elicits a minor national-level response. The rain event triggered many hundreds of landslides and flooding across the Nelson Tasman and Marlborough regions. In Nelson Tasman, NEMA reported that up to 570 properties were damaged and many roads, along with state highways SH 6 and SH 63, were closed, mainly from landslides and flooding. The main water supply pipeline to Nelson also failed, as it was hit by a landslide. The GNS Science Landslide Duty Officer, in consultation with the Engineering Geology Team and others at GNS Science, activated a landslide rapid response (LRR) for this event on 19 August 2022 under the GeoNet programme, as several of the criteria for activating a GeoNet LRR were met. These criteria were: direct damage of greater than NZ$1 million, economic losses of greater than NZ$10 million, threats to public health (such as contaminated water supplies) and significant research interest. The purpose of this LRR is to ensure that appropriate advice is available to maximise public safety, as well as to acquire and collect reliable and consistent landslide information and data. The aim of this report is to document the activities and results from the LRR carried out for the Nelson Tasman region during the LRR period (19/08/2022 to 2/09/2022). The GeoNet LRR for Nelson Tasman comprised the following steps: 19–21 August: Collate information on landslides reported in the media and on social media; 22–23 August: Carry out helicopter reconnaissance of the main areas affected; and 23–24 August: Carry out a walkover (field reconnaissance) of the Tāhunanui landslide in Nelson. The data presented in this report was made available to Nelson City Council on the day of collection to support them, and their representative contractors, in responding to this event. It is intended that additional data collection will continue through the recovery phase of the event. This perishable data acquired in the field will help train rainfall-induced landslide forecast models being developed by GNS Science and NIWA to forecast the location and severity of landslides in future rain events across all of Aotearoa. (The authors)

Rock avalanches are low frequency natural hazards that can alter landscape morphology, and constraining the timing, volume and emplacement dynamics of pre-historic rock avalanches is crucial for understanding the hazards posed by these events. Here we perform cosmogenic nuclide dating, topographic reconstruction and runout modeling of the Molveno rock avalanche, located north of Lake Garda in the Province of Trento, Italy. The unique morphology of the deposits, which features numerous large scarps and prominent lineaments, have previously led researchers to interpret the Molveno rock avalanche as being the result of multiple events. Our results show that the Molveno rock avalanche had a volume of approximately 600 Mm3, and failed from a prominent niche located on Monte Soran. 36Cl cosmogenic nuclide dating results shows that the deposits were emplaced as a single event approximately 4.8 ± 0.5 ka, and suggests that the unique deposit morphology is due to the emplacement processes acting during and soon after failure. Numerical runout modeling shows that this morphology could have resulted from a combination of runup and extensional spreading of the debris along the complex valley floor topography. The ages we determined for this event are coincident with the nearby Marocca Principale rock avalanche (5.3 ± 0.9 ka), which may suggest a common trigger. Our results have important implications for interpreting the morphology of rock avalanche deposits, and contribute to the evolving understanding of rock avalanche processes in the Alps., Frontiers in Earth Science, 8, ISSN:2296-6463

Geophysical Research Abstracts, 21, ISSN:1029-7006, ISSN:1607-7962

International audience; The understanding of the physical processes controlling water fluxes from hillslope aquifers to surface waters is critically needed for predicting stream discharge and establishing sustainable water resources management strategies. Two major hypotheses are currently being debated: the first considers that climate, stream network morphology and topography have a first-order control on groundwater flow and stream recession. Thus the prediction of stream baseflow can be approximated by considering idealized homogeneous groundwater reservoirs with effective properties. The second hypothesis suggests that the structural heterogeneity of the landscape (lithological variations, geological discontinuities, spatial distribution of topographic and tectonic stresses) might significantly control the partitioning of water within the landscape, and in turn impact how streamflow recession behaves.In this contribution we provide a comprehensive analysis of stream discharge and geomorphological databases across various geological domains distributed throughout the globe in order to identify the main factors that might influence recession constants at baseflow. A novel computational approach for single recession analysis revealed that recession constants might strongly deviate from what is predicted from idealized theories, with power law recession constants ranging from 1.5 to more than 10. A correlation analysis allows us to identify the control of key features of the landscape on groundwater storage and streamflow recession constants, with depth to bedrock, lithology and maturity - i.e. degree of weathering and fracturing - of the landscape being the main drivers. We tested these observations through a numerical modelling framework which allowed us to quantify the variability of streamflow recession behaviors raising from the drainage of hillslopes with different structural and geometrical properties. We identify the respective roles of hydraulic conductivity contrasts between the upper and lower compartments, the depth of the shallower compartment and the steepness of the slope on streamflow recession parameters. We finally discuss the implications of these findings in establishing future strategies for the improvement of groundwater storage and stream discharge predictions.

Geophysical Research Abstracts, 20, ISSN:1029-7006, ISSN:1607-7962

Geophysical Research Abstracts, 20, ISSN:1029-7006, ISSN:1607-7962

Geophysical Research Abstracts, 20, ISSN:1029-7006, ISSN:1607-7962

Geophysical Research Abstracts, 20, ISSN:1029-7006, ISSN:1607-7962

Moisture content alteration in rock masses has been observed to cause deformation sometimes followed by structural instability and catastrophic failure. While this deformation is typically interpreted to result from poroelastic effect, accompanied changes in elastic properties have been reported from \textit{in situ} observations. These elastic changes have been observed to disturb elastic wave propagation in the rock mass. However, characterization of moisture-induced elastic variations of brittle rocks using wave propagation theory, despite their central role in the geohazard, geotechnical and rock-physics community, has not been universally understood due to limited experimental data. In this study, laboratory investigation on well-characterized granite provides new insights into the role of the water availability on the material elastic variations. The main objectives of this work are three-fold: (a) to quantify the characteristics of piezoelectric (PZT) transmitters and receivers for ultrasonic measurements; (b) to study acousto-mechanical responses of the intact granite to moisture uptake; (c) to extend the acousto-mechanical analysis to the granite, containing a portion of the imposed fracture, in response to moisture uptake. A methodology is developed to integrate a FEM-based tool for simulating elastic wave propagation with laboratory experiments of the ball drop. This methodology aims to absolutely calibrate the PZT receiver for its instrumental response that helps interpret messages carried by radiated elastic waves. The FEM-based tool is first validated against the generalized ray theory and is then extended down to 1 kHz using physical-based boundary conditions. Ball drops are performed with a broad range of diameters; then an algorithm of spectrum cropping and linking is developed to derive the instrumental response from 1 kHz to 1 MHz from a single receiver to an array of PZT receivers. Further we characterize the generated force by a PZT transmitter actuated by high voltage pulses (HVPs). Magnitude of the peak force is quantified using wave propagation theory and calibrated PZT receivers: $\sim$ 0.011 Newton/Voltage. These quantitative characteristics of the PZT transmitter and receiver allow the possibility of using propagated elastic waves to examine the elastic variation of the studied host material through long-term ultrasonic monitoring. Different from experimental and theoretical studies centered on macropores, moisture-induced elastic changes is investigated in Herrnholz granite, a microcracked nanopore-dominated medium (80 \% of the surface area exhibits pore diameters below 10 nm). This rock has a fine-grained structure with a mean crystal size of 0.23 mm and shows nearly an isotropic and homogeneous P-wave velocity structure. We study time-lapse acousto-mechanical responses of a freestanding intact specimen resulting from a gradual wetting process of 98 hours using ultrasonic and digital imaging techniques. Linkages between ultrasonic attributes and adsorption-induced stress/strain are established for the conditions of a moving wetting front. Gassmann theory, previously validated in channel-like nanoporous media, breaks down in predicting the P-wave velocity increase of studied granite. However, squirt flow -- a theory recognized to characterize wave velocity increase and attenuation in microcracked macropore-dominated media at pore scale -- also accounts for the observed increase of P-wave velocity in microcracked nanopore-dominated media. The transmitted amplitude changes in direct P waves are explained and predicted using the elastic wave propagation within P-wave first Fresnel zone and reflection/refraction on the wetting front. The study is further extended from elastic variations of the intact specimen to fracture stiffness changes in a companion fractured specimen subject to the moisture uptake over 270 hours. Transmitted amplitude variations of direct P waves are measured across the single natural fracture using ultrasonic pulse transmission; in parallel, the real-time moisture profile is tracked in the host rock using digital imaging correlation (DIC). We observe significant discrepancies in the elastic amplitude changes between the fractured specimen and its companion intact specimen, which allowed to infer the contributions from the wetting fracture. Adsorption-induced deformation in the fractured specimen could degrade the transmitted amplitude by one more order of magnitude than that in the intact case. A 2D numerical model is built for the elastic wave propagation in the wetting fractured specimen. Moisture-dependent fracture stiffness that minimizes the difference between simulation results and experimental measurements is estimated using an inverse modeling technique. The derived fracture stiffness degrades logarithmically as the wetting front migrates from the fracture's mouth to its tip. This thesis first time contributes an acousto-mechanical dataset of moisture-induced elastic variations over days to weeks of intact and fractured nanopore-dominated rocks. Reported elastic variations are accurately examined using wave propagation theory from conventional rock physics. Key findings are fundamental to characterize changes in elastic properties of the near-surface and underground rock mass using \textit{in situ} active geophysical monitoring.