How can we model subsurface stormflow at the catchment scale if we cannot measure it?

Subsurface stormflow (SSF) can be a dominant run‐off generation process in humid mountainous catchments (e.g., Bachmair & Weiler, 2011; Blume & van Meerveld, 2015; Chifflard, Didszun, & Zepp, 2008). Generally, SSF develops in structured soils where bedrock or a less permeable soil layer is overlaid by a more permeable soil layer and vertically percolating water is deflected, at least partially, in a lateral downslope direction due to the slope inclination. SSF can also occur when groundwater levels rise into more permeable soil layers and water flows laterally through the more permeable layers to the stream (“transmissivity feedback mechanism”; Bishop, Grip, & O'Neill, 1990). The different existing terms for SSF in the hydrological literature such as shallow subsurface run‐off, interflow, lateral flow, or soil water flow reflects the different underlying process concepts developed in various experimental studies in different environments by using different experimental approaches at different spatial and temporal scales (Weiler, McDonnell, Tromp‐van Meerveld, & Uchida, 2005). Intersite comparisons and the extraction of general rules for SSF generation and its controlling factors are still lacking, which hampers the development of appropriate approaches for modelling SSF. But appropriate prediction of SSF is essential due to its clear influence on run‐off generation at the catchment scale (e.g., Chifflard et al., 2010; Zillgens, Merz, Kirnbauer, & Tilch, 2005), on the formation of floods (e.g., Markart et al., 2013, 2015) and on the transport of nutrients or pollutants from the hillslopes into surface water bodies (Zhao, Tang, Zhao, Wang, & Tang, 2013). However, a precise simulation of SSF in models requires an accurate process understanding including, knowledge about water pathways, residence times, magnitude of water fluxes, or the spatial origin of SSF within a given catchment because such factors determine the transport of subsurface water and solutes to the stream. But due to its occurrence in the subsurface and its spatial and temporal variability, determining and quantifying the processes generating SSF is a challenging task as they cannot be observed directly. Therefore, it is logical to ask whether we can really model SSF correctly if we cannot measure it well enough on the scale of interest (Figure 1). This commentary reflects critically on whether current experimental concepts and modelling approaches are sufficient to predict the contribution of SSF to the run‐off at the catchment scale. This applies in particular to the underlying processes, controlling factors, modelling approaches, research gaps, and innovative strategies to trace SSF across different scales

Description of failure mechanism in placed riprap on steep slope with unsupported toe using smartstone probes

This article is aimed at investigating the influence of toe support conditions on stability aspects of placed ripraps on steep slopes exposed to overtopping flows. All past experimental model studies investigating placed riprap stability under overtopping conditions have been conducted with ripraps constrained at the toe section. However, ripraps constructed on the downstream slopes of rockfill dams are generally not provided with any form of toe support. Hence, it is of importance from stability and economical standpoints to understand the failure mechanism in placed ripraps with realistic toe support conditions. This article presents findings from experimental overtopping tests conducted on model placed ripraps unsupported at the toe section. Employing Smartstone probes, a new technology in stone movement monitoring, laser measurement techniques and Particle Image Velocimetry (PIV) techniques, detailed description of failure mechanism in placed ripraps under overtopping conditions is presented within this study. Study findings demonstrate sliding as the underlying failure mechanism in placed ripraps with unsupported toes. Further, comparison of experimental results with past findings revealed that placed ripraps with unrestrained toes experience a fivefold reduction in stability, characterized by the critical overtopping magnitude as compared with placed ripraps provided with fixed toe supports. Furthermore, toe support conditions were found to have no effects on either the failure mechanism nor the overall stability of dumped ripraps. Further research is recommended to arrive at well-defined methodologies for design and construction of toe supports for placed ripraps