A precipiton‐based approach to model hydro‐sedimentary hazards induced by large sediment supplies in alluvial fans

Abstract

International audienceMountain ranges are frequently subjected to mass wasting events triggered by storms or earthquakes and supply large volumes of sediment into river networks. Besides altering river dynamics, large sediment deliveries to alluvial fans are known to cause hydro-sedimentary hazards such as flooding and river avulsion. Here we explore how the sediment supply history affects hydro-sedimentary river and fan hazards, and how well can it be predicted given the uncertainties on boundary conditions. We use the 2D morphodynamic model Eros with a new 2D hydrodynamic model driven by a sequence of flood, a sediment entrainment/transport/deposition model and a bank erosion law. We first evaluate the model against a natural case: the 1999 Mount Adams rock avalanche and subsequent avulsion on the Poerua river fan (West Coast, New Zealand). By adjusting for the unknown sediment supply history, Eros predicts the evolution of the alluvial riverbed during the first post-landslide stages within 30 cm. The model is subsequently used to infer how the sediment supply volume and rate control the fan aggradation patterns and associated hazards. Our results show that the total injected volume controls the overall levels of aggradation, but supply rates have a major control on the location of preferential deposition, avulsion and increased flooding risk. Fan re-incision following exhaustion of the landslide-derived sediment supply leads to sediment transfer and deposition downstream and poses similar, but delayed, hydro-sedimentary hazards. Our results demonstrate that 2D morphodynamics models are able to capture the full range of hazards occurring in alluvial fans including river avulsion aggradation and floods. However, only ensemble simulations accounting for uncertainties in boundary conditions (e.g., discharge history, initial topography, grain size) as well as model realization (e.g., non-linearities in hydro-sedimentary processes) can be used to produce probabilistic hazards maps relevant for decision making

A New Efficient Method to Solve the Stream Power Law Model Taking Into Account Sediment Deposition

“…During a simulation, the landscape evolves until it reaches an equilibrium topography (when the elevation in the mountain range and the slope in the foreland become stable; Figure ). In our simulations, river channels continuously migrate laterally in the foreland (Figure b; see the animation Movie S3 on water discharge in the supporting information), similar to observations made in laboratory experiments (Van Dijk et al, ) and other numerical simulations (Croissant et al, ; Pepin et al, ). By considering the axial topography (Figure a; the corresponding animation Movie S2 is presented in the supporting information), we can indeed observe that the foreland steady‐state configuration is a dynamic steady state with the continuous formation and destruction of river banks.…”

Landslides control the spatial and temporal variation of channel width in southern Taiwan: Implications for landscape evolution and cascading hazards in steep, tectonically active landscapes

“…While the landslides themselves can be quite catastrophic (Tsou et al, 2011), the propagating impacts of the landslide material through the landscape can cause significant, long-lived hazards (Nguyen et al, 2013). For example, aggradation in downstream riverbeds following the event can change flood inundation patterns and frequency (Stover and Montgomery, 2001;Chen and Petley, 2005;Lane et al, 2007;Robinson and Davies, 2013;Croissant et al, 2017a) as well as influence channel bank mobility (Lisle, 1982;Lane et al, 2007). Regions with mountainous topography and high populations, such as Taiwan, are particularly vulnerable to cascading hazards associated with regional landsliding initiated by earthquakes and extreme precipitation.…”

Morphodynamic research challenges for braided river environments: Lessons from the iconic case of New Zealand

“…Kasai et al, 2005; Page et al, 2007; Tunnicliffe et al, 2018), tectonic forcing (earthquakes, uplift; e.g. Croissant et al, 2017), glacial sediment production, as well as volcanic eruptions (Campbell, 1973; Manville et al, 2005). Aggradation may be driven either by oversupply from upstream, or by rising base‐level downstream, e.g.…”

“…An emerging question in New Zealand river management has been how best to deal with river aggradation from these very active headwater catchments. A common engineering approach has been to confine the river, on the premise that more efficient transport could be achieved via a steepened gradient and deeper single-thread channel, but this does not always work, and may introduce a number of additional, compounding issues (Croissant et al, 2017). The river will have a greatly reduced capacity to adjust laterally (channel switching), which may seem desirable in the short term, but in cases where there is continued build-up of material, additional hazards can be present (e.g.…”