new paper out
The 2015 Gorkha earthquake in Nepal caused severe losses in the hydropower sector. The country temporarily lost ~20% of its hydropower capacity, and >30 hydropower projects were damaged. In our paper that was just published in Geophysical Research Letters, we show that the projects hit hardest were those that were affected by earthquake‐triggered landslides. These projects are located along very steep rivers with towering sidewalls that are prone to become unstable during strong seismic ground shaking. A statistical classification based on a topographic metric that expresses river steepness and earthquake ground acceleration is able to approximately predict hydropower damage during future earthquakes, based on successful testing of past cases. Thus, our model enables us to estimate earthquake damages to hydropower projects in other parts of the Himalayas. We find that >10% of the Himalayan drainage network may be unsuitable for hydropower infrastructure given high probabilities of high earthquake damages.
Of course, we conducted the analysis primarily using TopoToolbox. A few functions that we used and partly developed for the purpose of our analysis are
Schwanghart, W., Ryan, M., Korup, O., 2018. Topographic and seismic constraints on the vulnerability of Himalayan hydropower. Geophysical Research Letters, in press. [DOI: 10.1029/2018GL079173]
see also Nature News article by Jane Qiu, 2018. Landslides pose threat to Himalayan hydropower dream. [DOI: 10.1038/d41586-018-06212-8]
Our paper on uncertainty quantification and smoothing of longitudinal river profiles has now been published in Earth Surface Dynamics. The paper describes a new approach to hydrologically correct and smooth profiles using quantile regression. Smoothing is based on curvature regularization with the constraint that elevation must decrease with increasing flow distance. We thus refer to this technique as constrained regularized smoothing (CRS). We compare CRS-derived profiles to profiles obtained from the common methods of filling and carving, and show that CRS outperforms these methods in terms of accuracy and precision.
Check out the new TopoToolbox functions that accompany the paper:
Schwanghart, W., Scherler, D., 2017. Bumps in river profiles: uncertainty assessment and smoothing using quantile regression techniques. Earth Surface Dynamics, 5, 821-839. [DOI: 10.5194/esurf-5-821-2017]
The Himalayan history is rich with a sequence of destructive earthquakes. In the last century, ground-shaking, collapsing houses, and landslides in the wake of earthquakes killed tens of thousand of people, wreaking havoc to the Himalayan nations. The 2015 Gorkha Earthquake was the latest in a series of severe earthquakes to hit Nepal.
Seismic hazard analysis in the Himalayas is based on few instrumental records and a paleoseismic record extending back ~1000 years. Paleoseismology largely relies on rupture histories derived from fault trenches, written accounts, and liquefaction features. Other records derived from e.g. lake sediments are scarce.
In a now published paper in Quaternary Science Reviews, Amelie Stolle et al. documents our research in the Pokhara Valley in Nepal. The valley was massively and repeatedly aggraded by several cubic kilometers of debris in the wake of medieval earthquakes in the region. The paper extends on our 2016 paper in Science, offering new radiocarbon dates and detailing the sedimentology of the infills. Based on our findings, we argue that valley fills in the Himalayas may offer substantial additional evidence for past earthquakes subsidy to the current portfolio of paleoseismological records.
Stolle, A., Bernhardt, A., Schwanghart, W., Hoelzmann, P., Adhikari, B.R., Fort, M., Korup, O., 2017. Catastrophic valley fills record large Himalayan earthquakes, Pokhara, Nepal. Quaternary Science Reviews, 177, 88-103. [DOI: 10.1016/j.quascirev.2017.10.015]. << free link to paper until December 26, 2017 >>
Today, I came back from an excellent workshop (organized by Darrel Maddy) in Spain focussing on the late Quaternary development of the Bergantes catchment. Located in an extremely beautiful landscape, this catchment features numerous fluvial terraces that were extensively studied and dated by Mark Macklin and Paul Brewer together with four PhD students between 2005 and 2009. A solid chronology together with high resolution terrain and climate data provide the benchmark data against we will test numerical landscape evolution models (LEM).
Assessing the capabilities of LEMs to reconstruct real landscapes, however, involves several challenges among which high parametrization is a severe one. Thus, in order to get a grip on the uncertainty and sensitivity of LEMs, Chris Skinner from the University of Hull led a study in which we assessed the parameter space of CAESAR-Lisflood and its effects on several output metrics derived from hundreds of simulations.
This study has now been accepted for discussion in the journal Geoscientific Model Development and can be accessed here.
No, this post is not about my favorite interpretation of this song by Senor Coconut. Its about our latest discussion paper submitted to ESURF in which Dirk and I introduce a novel way to smooth river profiles. We show that DEMs in valley bottoms are characterized by errors with positively-skewed distributions and are often biased to higher values. These errors are more pronounced in high topographic relief, thus limiting our ability to interpret profiles. We also assess uncertainties of profiles derived from different globally available DEMs and find that the ALOS World 3D 30 m (AW3D30) DEM outperforms other DEMs in terms of precision and accuracy.
The manuscript comes with a set of functions that are now part of TopoToolbox and that I will cover in more detail in the following weeks. Here is a quick overview:
STREAMobj/smooth STREAMobj/quantcarve STREAMobj/crs STREAMobj/crslin STREAMobj/crsapp
Have fun exploring these new functions. I will be back here with more information soon.
Turbidites sporadically deliver large amounts of sediment from shelf areas to deep marine depocenters. These submarine mass movements are well recognized in marine sediment cores, yet their formation, triggers and controls are less well constrained. In a now published paper in Earth and Planetary Science Letters, we have analyzed seven marine sediment cores in three study sites along the Chile convergent margin. The sites span a pronounced on-shore climatic gradient from arid in the North to humid in the South.
Sediments in the three sites provide a detailed record of turbidite deposition over the last glacial-deglacial cycle from ~20 ka to present. All sites reveal a steep decline of turbidite frequency and thickness during deglaciation, a temporal pattern that has commonly attributed to sea-level rise and inundation of shelf areas. Our data suggest, however, that sea-level rise is not the most dominant control. Rather, turbidite deposition ceases simultaneously with pronounced climatic change on-shore predating significant changes in sea-level. Warming and changes in precipitation have likely altered terrestrial erosion and sediment transport systems. Analysis of the on-shore geomorphological situation suggests that sediment connectivity played an important role although its control differs regionally. While highly connected systems along the steep gradients in the northern part of our study site have rapidly conveyed the erosional signal of aridification, retreating piedmont glaciers in the southern part left numerous proglacial lakes that act as sediment traps. These sediment traps shut down coarse sediment transfer to the marine realm.
Our analysis shows that turbidites can be reliable recorders of onshore climatic change. The exact role of the effects of the sediment transport system, however, may strongly differ while producing similar depositional patterns offshore, and it is challenging to invert these from the sedimentary record alone. Understanding the terrestrial sediment transport system on millennial time scales is thus of vital importance for the interpretation of sediment records of climate variability.
Bernhardt A, Schwanghart W, Hebbeln D, Stuut J-BW, Strecker MR. 2017. Immediate propagation of deglacial environmental change to deep-marine turbidite systems along the Chile convergent margin. Earth and Planetary Science Letters 473 : 190–204. [DOI: 10.1016/j.epsl.2017.05.017]. **** Free pdf download link active until August 16, 2017 ****
On 24. May 2015, a landslide near the village Baisari, Myagdi District Nepal, dammed the Kali Gandaki. 15 hours later, the dam breached and released a flood wave. Fortunately, people downstream were warned of the imminent flood and no casualties were reported.
In a recent paper by Jeremy Bricker et al. (2017), we hindcasted the event using numerous 1D and 2D hydrodynamic models. Moreover, we used raw and smoothed topographies of the valley floor to investigate the effect of DEM uncertainties on flood wave propagation modelling. We show that using unsmoothed valley thalwegs result in delayed modelled flood wave arrival times that may be critical for the effectiveness early warning systems. A 2D model produced results most in line with field observations.
One of the most striking aspects of our study is the use of crowd-sourced video material available on youtube. We used video material recorded at two bridges crossing the Kali Gandaki to estimate flow depth and speed. If hydrological gauges are unavailable or destroyed, these videos provide an important source of information to assess the magnitude of these extreme events.
Bricker, J.D., Schwanghart, W., Adhikari, B.R., Moriguchi, S., Roeber, V., Giri, S. (2017): Performance of models for flash flood warning and hazard assessment: the 2015 Kali Gandaki landslide breach in Nepal. Mountain Research and Development, 37, 5-15. [DOI: 10.1659/MRD-JOURNAL-D-16-00043.1]