Morphometric analysis for flood flow formation feature identification (on example of Ulaanbaatar agglomeration)

Water flows with significant flow rate feature a high destructive force and can lead to catastrophic consequences. Fluvial processes caused by uneven distribution of rain precipitation over the area pose risks to the developed inland foothill territories. The purpose of this study is to carry out a quantitative morphometric analysis of the territory in order to identify the formation features of flood flows. The analysis and ranking of catchment basins are performed using a basin approach. On the basis of SRTM images and the use of stock cartographic material in the GIS program the authors have built specialized electronic maps that allow to obtain quantitative parameters reflecting the morphometry of the basins under analysis including basin geometry, drainage network and terrain relief. On example of the Ulaanbaatar agglomeration territory it is shown how initial morphometric parameters of basins and watercourses (length, width, area, perimeter, erosion dissection, drainage network density, terrain relief coefficient, Melton coefficient, etc.) form the features of flood flow. For developed territories, the initial data on the catchment basin morphometry constitute the basis for compilation of specialized maps to be used in planning and construction. The combination of morphometric indicators on the territory of the Ulaanbaatar agglomeration indicates that there is possibility of large flood formation and development of dangerous mudstone flows in some catchment basins.

Comparison of Open Source DEM’s for Morphometric Analysis of Micro Watersheds: A Case Study from the Midlands of Kerala

Morphometric analysis with the help of remote sensing and GIS is now widely used to prioritise micro watersheds for planning interventions for soil and water conservation. DEM is the main digital data used to perform the morphometric analysis. There are different types of DEMs available to perform morphometric analysis using GIS techniques. But, no authentic information is available on the degree of accuracy levels of these DEMs to quantitatively determine various morphometric parameters for the watersheds in Kerala, with typical undulating and sloping terrain features. Hence, this research has been initiated to evaluate the quality of three popular DEMs viz. SRTM(Shuttle Radar Topography Mission), CARTOSAT(Cartography and satellite) and ASTER(Advanced spaceborne thermal emission and reflection Radiometer), each with 30 m spatial resolution. Two small sub watershed of Bharathapuzha river basin have been chosen for the study which are lying near to Valanchery town in Malappuram district. More than 21 morphometric parameters including drainage network, basin geometry, basin texture and basin relief characteristics were computed using these three types of DEMs and the outputs compared with google earth map. The results shows that the SRTM 30m DEM is characterized by higher accuracy compared to CARTOSAT and ASTER and has got better matching with google earth map data sources.

Kathmandu Basin as a local modulator of seismic waves: 2D modelling of nonlinear site response under obliquely incident waves

The 2015 Mw 7.8 Gorkha, Nepal earthquake is the largest event to have struck the capital city of Kathmandu in recent times. One of its surprising features was the frequency content of the recorded ground motion, exhibiting a notable amplification at low frequencies (< 2 Hz) and a contrasting depletion at higher frequencies. The latter has been partially attributed to the damper behaviour of the Kathmandu basin. While such weak high-frequency ground motion helped avoiding severe damage in the city, the catastrophic outcomes of earlier earthquakes in the region attest to a contrasting role of the Kathmandu basin as a broadband amplifier, in addition to possible source effects. Given the possibility of future strong events in the region, our main objective is to elucidate the seismic behaviour of the Kathmandu basin by focusing on site effects. We numerically model 2D P-SV wave propagation in a broad frequency band (up to 10 Hz), incorporating the most recent data for the Kathmandu basin geometry, soil stratigraphy and geotechnical soil properties, and accounting for the non-linear effect of multi-dimensional soil plasticity on wave propagation. We find that: 1) the Kathmandu basin generally amplifies low frequency ground motion (< 2 Hz); 2) waves with large incidence angles relative to vertical can dramatically amplify the high frequency ground motion with respect to bedrock despite the damping effect of soil nonlinearity; 3) the spatial distribution of peak ground motion amplitudes along the basin is highly sensitive to soil nonlinearity and wave incidence (angle and direction), favoring larger values near the basin edges located closer to the source, as observed during the 2015 event. Our modelling approach and findings can support the ongoing resilience practices in Nepal and can guide future seismic hazard assessment studies for other sites that feature similar complexities in basin geometry, soil stratigraphy and dynamic soil behaviour.

Long-period variability in ice-dammed glacier outburst floods due to evolving catchment geometry

We combine a glacier outburst flood model with a glacier flow model to investigate decadal to centennial variations in outburst floods originating from ice-dammed marginal basins. Marginal basins form due to retreat and detachment of tributary glaciers, a process that often results in remnant ice being left behind. The remnant ice, which can act like an ice shelf or break apart into a pack of icebergs, limits the basin storage capacity but also exerts pressure on the underlying water and promotes drainage. We find that during glacier retreat there is a strong, nearly linear relationship between flood water volume and peak discharge for individual basins, despite large changes in glacier and remnant ice volumes that are expected to impact flood hydrographs. Consequently, peak discharge increases over time as long as there is remnant ice remaining in a basin, the peak discharge begins to decrease once a basin becomes ice free, and similar size outburst floods can occur for very different glacier volumes. We also find that the temporal variability in outburst flood magnitude depends on how the floods initiate. Basins that connect to the subglacial hydrological system only after reaching flotation yield greater long-term variability in outburst floods than basins that are continuously connected to the subglacial hydrological system (and therefore release floods that initiate before reaching flotation). Our results highlight the importance of improving our understanding of both changes in basin geometry and outburst flood initiation mechanisms in order to better assess outburst flood hazards and impacts on landscape and ecosystem evolution.

Student Zone

A student training program, Engineering Seismology and Seismic Microzonation for Seismic Site Effects Assessment, was held 18–22 January 2020 in Lalitpur, Nepal. It was created through the collaboration of Thammasat University and Tribhuvan University, with support from Geoscientists Without Borders® (GWB). The goal of the program was to connect students with modern geophysical instrumentation and software through training. It specifically advanced theoretical and hands-on field-based knowledge pertaining to geotechnical earthquake engineering aspects and applications. The training served as part of a broader GWB project, Seismic Site Effects Study in Nepal, encompassing basin geometry, site characteristics, and the study of seismic site effects through microtremor measurements in Kathmandu Valley.

Two-Channel System Dynamics of the Outer Weser Estuary—A Modeling Study

In this paper, we unravel the mechanisms responsible for the development of the two-channel system in the Outer Weser Estuary. A process-based morphodynamic model is built based on a flat-bed approach using simplified boundary conditions and accelerated morphological development. The results are analyzed in two steps: first, by checking for morphodynamic equilibrium in the simulations and second, by applying a newly developed method that interprets simulations based on categorization of the two-channel system and cross-sectional correlation analysis. All simulations reach a morphodynamic equilibrium and develop two channels that vary considerably over time and between the simulations. Variations can be found in the location and depth of the two channels, the development of the dominant channel over time and the alteration in the dominance pattern. The conclusions are that the development of the two-channel system is mainly caused by the tides and the basin geometry. Furthermore, it is shown that the alternation pattern and period are dependent on the dominance of the tides compared to the influence of river discharge.