A crucial ingredient for any LEM is a fluvial erosion component regulating the way in which rivers transport sediment and incise into bedrock. The past 20 years have seen the development of a plethora of landscape evolution models (LEMs), enabling studies of the interactions among climate, tectonics and erosion. Numerical models are excellent tools to study relationships between processes regulating Earth surface dynamics and their interdependencies over various temporal and spatial scales ( Tucker and Hancock, 2010). Increased insight into the spatial distribution of landslides has resulted in improved landslide susceptibility assessments ( Guzzetti et al., 2006), but processes regulating landslide rate assessments ( Broeckx et al., 2020) and landslide-derived sediment dynamics remain less well understood ( Hovius et al., 2011 Croissant et al., 2017, 2019 Zhang et al., 2019 Broeckx et al., 2020). Unraveling the dynamic interplay between landslides and fluvial processes is key to understanding long-term landscape evolution and the associated sediment dynamics in mountainous terrain ( Egholm et al., 2013). In turn, hillslope failure through mass wasting chokes the rivers with sediment and prevents further bedrock incision until landslide-derived sediment has been evacuated from the system ( Larsen and Montgomery, 2012 Ouimet et al., 2007 Korup et al., 2010 Shobe et al., 2016 Glade et al., 2019). Through sediment evacuation and bedrock incision, fluvial incision lowers the base level for surrounding hillslopes, triggering hillslope failures. Fluvial channels in mountainous catchments play a dual role: they simultaneously incise into the bedrock and act as conveyor belts to carry eroded sediment out of the mountain range towards the ocean ( Milliman and Meade, 1983). Nonetheless, long-term landscape evolution in non-glaciated settings is mainly controlled by the interplay between tectonic uplift and fluvial dynamics ( Whipple and Tucker, 1999 Wobus et al., 2006). Landsliding is a highly effective erosional mechanism that dominates sediment mobilization rates in moderate-to-steep topographic settings ( Hovius et al., 1997 Ouimet et al., 2007 Broeckx et al., 2020). With HyLands we provide a new tool to understand both the long- and short-term coupling between stochastic hillslope processes, river incision and source-to-sink sediment dynamics. Finally, we illustrate the relevance of explicitly simulating landsliding and sediment dynamics over longer timescales for landscape evolution in general and river dynamics in particular. Second, we apply the model to a portion of the Namche Barwa massif in eastern Tibet and compare simulated and observed landslide magnitude–frequency and area–volume scaling relationships. We first illustrate the functionality of HyLands to capture river dynamics ranging from detachment-limited to transport-limited conditions. We describe and evaluate the HyLands 1.0 model using analytical solutions and observations. Erosion and sediment production by landsliding are calculated using a Mohr–Coulomb stability analysis, while landslide-derived sediment is routed and deposited using a multiple-flow-direction, nonlinear deposition method. Therefore, rivers can dynamically transition from detachment-limited to transport-limited and from bedrock to bedrock–alluvial to fully alluviated states. Fluvial sediment transport and bedrock incision are calculated using the recently developed Stream Power with Alluvium Conservation and Entrainment (SPACE) model. The hybrid nature of the model lies in its capacity to simulate both erosion and deposition at any place in the landscape due to fluvial bedrock incision, sediment transport, and rapid, stochastic mass wasting through landsliding. Here, we present HyLands, a hybrid landscape evolution model integrated within the TopoToolbox Landscape Evolution Model (TTLEM) framework. While many models have been used to study the dynamic interplay between tectonics, erosion and climate, the role of interactions between landslide-derived sediment and river incision has received much less attention. They enable evaluation of a range of hypotheses at varying temporal and spatial scales. Numerical landscape evolution models (LEMs) are well suited to study the interactions among these surface processes. Sediment delivery and its interaction with river incision therefore control the pace of landscape evolution and mediate relationships among tectonics, climate and erosion. Sediment dynamics are known to influence landscape evolution through interactions among landslide sediment delivery, fluvial transport and river incision into bedrock. Rivers then act as conveyor belts, evacuating landslide-derived sediment. Landslides are the main source of sediment in most mountain ranges.
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