# Aveiro Canyon Head (Portugal) Submarine Slope Instability Assessment
## 1 Introduction  
Mass wasting events are the main dynamic process affecting marine sedimentary deposits [1; 2], and are recognized as one of the main marine geohazards that can result in destruction of seabed infrastructure, collapse of coastal areas into the sea and landslide-generated tsunamis [3; 4]. For economic development and marine resources growing exploration, the instabilities of the sea floor are of utmost interest. In fact, an increasing number of offshore engineering constructions require a stable substratum and the knowledge of the processes affecting the bottom sediments' stability, for risk and integrity assessments and the evaluation of maintenance costs. In natural sedimentary processes, the identification and mapping of submarine instabilities is important for the understanding of the downslope processes that characterize the particle transfer between the continental shelf and the deep ocean, through creeks, small valleys and submarine canyons, such as the Aveiro Canyon (Portugal) [5; 6; 7; 8]. Submarine landslides are important mechanisms/agents for modelling and transporting large amounts of sediment along the continental slope to the deep ocean [3; 9], leaving traces and marks in the submarine morphology and sedimentary deposits distribution. Many factors have been suggested as probable or possible causes to the initiation of submarine landslides [3], which can be divided into the following types [3; 10; 11]:1. Related to the geological characteristics of the landslide material [3]; * Movements in recent deposits of weak, under-consolidated and metastable soils in areas of rapid sedimentation (deltas or canyons) [3; 10; 11]; * Movements due to the failure of a week layer [3; 10; 11]. 2. Driven by transient external events [3]. * Movements in old, normally consolidated deposits, triggered by significant changes in sedimentation and erosional regimes, which may be caused by sea-level changes (as a driver of sedimentation or erosion changes) or migration of erosional channels [3; 10; 11]; * Movements in old deposits, caused by tectonic processes, such as earthquakes [3; 10; 11]. It is also noted that human activities can contribute to submarine landslide triggering, with works such as excavation, loading, or changes in the hydraulic regime which may cause an increase of erosion or deposition [3; 11]. Submarine mass movements (SMM) often result from the combination of more than one factor [3]. In detail, gravitational load, occurs when stresses in the seafloor soils equal or exceeds the available shear strength, causing sudden movement [12]. Creep-like movements also occur in cohesive soils, resulting in a slow and persistent movement over a long period of time [12]. Also, the seafloor is affected continuously by the dynamic action of currents, tides and surface waves. Depending on the sediment properties, the wave height and length, and the depth of the water column, larger waves displacement cause water pressure variations which can affect the stability of the sea bottom sediments [12]. These bottom water pressure variations caused by severe storm surges may be enough to cause ruptures in \"soft\" sediments up to depths of about 120 m [13]. In earthquake events, cyclic shear stresses caused by the propagation of seismic waves in the sedimentary layers can originate soil ruptures [12]. Slope movements can be classified in different types (e.g., landslides, flows, rock falls, among others [14]), regarding the mechanisms involved and with implications on the affected volume, triggering mechanism and their location. However, in terms of sediment transport volume driven by gravity-driven processes, only slides, debris flows and turbidity currents make a significant contribution to the particle transfer between coastal and deeper systems [3]. Some authors [3; 15; 16; 17] reported that occasionally, movements in cohesive sediments may evolve downslope from slide to debris flow, and from debris flow to turbidity current through gradually increasing disintegration and entrainment of water.