WP1 - Modelling of erosion in rivers, at the coast and in mass movements

It is crucial to obtain an overview of situations of practical interest where erosion is important and to try to characterize the type of erosion mechanism. Earlier work (both outside and inside NGI) has proposed models for many of these mechanisms, but many of these models are purely empirical, often not consistent with fundamental laws of mechanics and/or have not been properly validated.

The first task in this sub-project is therefore to produce a state-of-the-art report (or review paper) that 

• describes relevant settings where erosion is important
• attempts a classification of erosion mechanisms that can be applied to the settings identified previously
• critically reviews erosion models proposed in the literature
• provides recommendations for erosion mechanisms that should be studied in MERRIC and for modeling approaches that appear promising.

Further work in WP1 during 2017 will depend on the findings of the state-of-the-art report. Examples of studies that can be carried out in 2017, if prioritized, aim at extending earlier numerical and analytical results:

1. The analytical model for erosion by a gravity mass flow over a perfectly brittle bed in the infinite-slope approximation (Issler, 2014) is limited to moderate bed shear stresses. If the bed shear stress exceeds some threshold, an entire layer of bed material may be detached at once, but will only gradually be accelerated and mixed into the flow. An analytical solution to this problem would greatly extend the applicability of the model.
2. Plowing is an important entrainment mechanism in dense, relatively slow mass flows like wet-snow avalanches and presumably debris flows. The process can be observed directly, but a self-consistent description expressing the erosion rate in terms of the flow parameters and bed properties is still missing.
3. Shear and normal stresses that a gravity mass flow exerts on the bed may generate excess pore pressure by destroying the texture of the bed material. This will influence both the retarding shear stress and the entrainment of the disturbed bed material. Propagation of pore pressure may also lead to ejection of the uppermost layers of the bed right in front of the flow.
4. Parameters influencing the initiation of density currents under water and factors contributing to erosion by flowing water, both on land and under water.

Potential contents of WP1 in 2018 and 2019

Modeling of erosion processes in rivers, at the coast and in mass movements:

• Formulation of modelling principles with regard to flow, interaction with substrate and erosion
• Examine further erosion mechanisms
• Run detailed simulations and apply these results to propose simplified depth-averaged models.