Assessing the effectiveness of landslide slope stability by analysing structural mitigation measures and community risk perception

Rainfall-induced landslides seriously threaten hilly environments, leading local authorities to implement various mitigation measures to decrease disaster risk. However, there is a significant gap in the current literature regarding evaluating their effectiveness and the associated community risk perception. To address this gap, we used an interdisciplinary and innovative approach to analyse the slope stability of landslides, evaluate the effectiveness of existing structural mitigation measures, and assess the risk perception of those living in danger zones. Our case study focused on the Kutupalong Rohingya Camp (KRC) in Cox’s Bazar, Bangladesh, which is home to over one million Rohingya refugees from Myanmar. Although various structural and non-structural countermeasures were implemented in the KRC to mitigate the impact of landslides, many of them failed to prevent landslides from occurring. We utilised a variety of methods from the physical sciences, including the infinite slope, limit equilibrium (LEM), and finite element (FEM) approaches, to calculate the factor of safety (FoS) for specific slopes. Additionally, in the social sciences domain, we conducted a questionnaire survey of approximately 400 Rohingya participants to assess the community’s perception of the interventions and the degree of disaster risk. Our findings indicated that slopes with a gradient greater than 40° were unstable (FoS < 1), which was present throughout the entire KRC area. The effectiveness of the LEM and FEM methods was evaluated for four dominant slope angles (40°, 45°, 50°, and 55°) under varying loads (0, 50, and 100 kN/m2). The slopes were found to be stable for lower slope angles but unstable for higher slope angles (> 50°) and increased overburden loads (50–100 kN/m2). Different mitigation measures were tested on the identified unstable slopes to assess their effectiveness, but the results showed that the countermeasures only provided marginal protection against landslides. Survey results revealed that at least 70% of respondents believed that concrete retaining walls are more effective in reducing landslide occurrence compared to other measures. Additionally, about 60% of the respondents questioned the reliability of the existing structural mitigation measures. The study also found that the cohesion and friction angle of lower sandstone and the cohesion of upper soil layers are important factors to consider when designing and implementing slope protection countermeasures in the KRC area

Diurnal seismic ambient noise and seismic station performance characterization in the Bengal Basin, Bangladesh

Seismic ambient noise (SAN) energy can potentially blur regional and teleseismic arrivals as well as various microearthquakes at specific frequencies. Therefore, quantification of the SAN energy level in a region is required to optimize seismic station distribution for seismological investigations. Moreover, evaluation of station performance and noise source is possible from observation of SAN energy levels. The SAN energy distribution from seismic stations in the Bengal Basin (BB), Bangladesh has not yet been estimated. At the same time, this tectonically active and complex region is less studied using seismic methods. This study aims to quantify SAN energy and characterize its diurnal variation along with evaluating station performance at 11 seismic stations, which were temporarily installed in the deeper portion of the BB. Herein, the daily SAN energy level was determined within the period range of 0.02–30 s by estimating the power spectral density (PSD) of seismic data for 7 continuous days. SAN energy and its variation over time were observed using the probability density functions (PDFs) of PSDs and spectrograms, respectively. The sources of SAN energies at different period bands were also investigated by comparing the PSDs with daily variations in human activities, nearby noise sources, local meteorological factors (i.e., air temperature and precipitation), and sea level height. The insights from this study could be useful for the future deployment of seismic networks as well as seismological studies in the BB

AN ENGINEERING GEOLOGICAL ANATOMY OF THE PADMA RIVER BANK FAILURE AND EROSION, 2018: A CASE STUDY OF NARIA BANK SECTION, BANGLADESH

The Naria town of Bangladesh is developed on the right bank of the Padma River. The bank is an old natural levee of Meghna River. The Holocene-Recent geology of Naria is actively dominated by the fluvial processes of Ganges-Brahmaputra-Meghna River system where the deltaic sediments are characterized as unconsolidated fine sand and silt, covered by thin veneer of clayey silt and loam. The annual volume of water discharge and flow dynamics are dependent on the intensity of the rainfall, runoff and the length of dry winter. Excessive river bank erosion, channel avulsion, renewed submergence of floodplains, and formation of natural levees and channel-bars are due to natural geomorphological processes that impact the area by inevitable ground failures. The geological attributes of ground condition and drastic variations in water levels make the area extremely vulnerable to severe bank failures and erosion. A unique erosion phenomenon prevailing in this part of Bengal delta prompted this study. During Aug-Sept, 2018 a sudden complex attenuation of current, wave and vortex in the Padma water flow caused an extraordinary disaster and made more than 5000 people homeless overnight by devouring away houses including concrete buildings, factories and markets. It is observed that geologically the Padma River remained confined within a width of 5 miles striking NW-SE trend following the margins of older alluvium and Faridpur Trough. The river tends to a meandering pattern consisting of deep vertical trenches along the Naria curvature. The deep trenches form along right bank and render the ground increasingly more vulnerable to subaqueous slope failure due to presence of thick (~200 ft.) alternating cross-bedded silt and micaceous fine sand of very high dilatancy and low angle of friction. The present study identifies some application of technological advancement for developing real-time engineering geological mapping systems for monitoring and managing complex river bank erosion. Large scale 3D engineering geological map coupled with air-borne photogrammetric and radar inferrometry methods can be applied for real-time monitoring and prediction of differential settlements, subaqueous failures and ground movement. The point cloud maps developed using data from these systems can refine engineering geological maps for decision makers and improve the design of protective measure and sustainable engineering structures

Assessing rainfall-induced landslide risk in a humanitarian context: The Kutupalong Rohingya Camp in Cox's Bazar, Bangladesh

We have proposed a novel method for assessing landslide risk by applying cutting-edge geospatial technologies and combining hazard, exposure, and vulnerability components. The Kutupalong Rohingya Camp (KRC) in Cox's Bazar, Bangladesh, was taken as a case study area. The district was hosting over one million Rohingya refugees from Myanmar, fleeing genocide and crimes against humanity. Extensive vegetation removal, hill cutting, exceptionally dense camps, fragile shelters, unstable soils, and excess monsoon rainfall were linked to yearly landslides. At first, ten landslide factor maps were produced, and a detailed fieldwork-based landslide inventory was conducted. Next, we applied various statistical and knowledge-based approaches for landslide susceptibility modelling. The most accurate susceptibility map was utilised to produce landslide hazard maps for different rainfall return periods. Next, we used population and shelter density data for exposure mapping and integrated age and gender-based factors for social vulnerability analysis. The hazard, exposure, and vulnerability maps were combined to produce the landslide risk map. Finally, we calculated shelters and refugee populations at risk for different scenarios. Camp 10 had the highest risk value, and historical landslide events validated the results. This study is replicable in a similar context and can be scaled up. It provides authentic information on landslide disaster scenarios that should be adopted in the decision-making process to reduce future impacts of catastrophic hydrometeorological disasters in the KRC and surrounding host community areas