Seabed disturbance and sediment mobility due to tidal current and waves on the continental shelves of Canada

Waves and tidal currents can interact to produce strong seabed shear stress and mobilization of sediments on continental shelves. Modelled wave and tidal current data for a 3-year period were used in a combined-flow sediment transport model to simulate the seabed shear stresses and the mobilization of uniform medium sand on the continental shelves of Canada. The modelling results are presented to establish the first national framework of seabed disturbance and sediment mobility on the continental shelves of Canada. Strong waves and tidal currents on the Canadian continental shelves produce mean bed shear velocity >5 cm·s⁻¹. Medium sand can be mobilized >50% of the time over many areas on the shelves. The mobilization by tidal currents occurs over 36% and by waves over 50% of the shelf area, demonstrating that mobilization of sediments is dominated by waves on the Canadian continental shelves. Combined shear stresses due to wave and tidal current interaction further increase sediment mobilization to over 68% of the shelf area. The spatial variation of the relative importance of wave and tidal disturbances allows classification of the continental shelves into six disturbance types. Innovative Seabed Disturbance (SDI) and Sediment Mobility (SMI) indices are proposed to quantify the seabed exposure to oceanographic processes and sediment mobilization, incorporating both the magnitude and frequency of these processes. The proposed SDI and SMI, together with the disturbance type classification, can be used as standard parameters to best quantify seabed disturbance and sediment mobility on other shelves of the world.

Sediment mobilization by bottom trawls: a model approach applied to the Dutch North Sea beam trawl fishery

Bottom trawls impact the seafloor and benthic ecosystem. One of the direct physical impacts is the mobilization of sediment in the wake of trawl gear components that are in contact with or are close to the seabed. The quantity of sediment mobilized is related to the hydrodynamic drag of the gear components and the type of sediment over which they are trawled. Here we present a methodology to estimate the sediment mobilization from hydrodynamic drag. The hydrodynamic drag of individual gear components is estimated using empirical measurements of similarly shaped objects, including cylinders, cubes, and nets. The method is applied to beam trawls used in the Dutch North Sea flatfish fishery and validated using measurements of beam trawl drag from the literature. Netting contributes most to the hydrodynamic drag of pulse trawls, while the tickler chains and chain mat comprise most of the hydrodynamic drag of conventional beam trawls. Taking account of the silt content of the areas trawled and the number of different beam trawl types used by the fleet, sediment mobilization is estimated as 9.2 and 5.3 kg m−2 for conventional 12 m beam and pulse trawls, respectively, and 4.2 and 4.3 kg m−2 for conventional 4.5 m beam and pulse trawls.

Vegetation changes and sediment transfers in the catchment of Lake Alaotra, Madagascar

<p>To understand the driving processes for changes in African ecosystems and related problems such as soil degradation, it is crucial to gain insight in the relative importance of human disturbance and climate change. Madagascar is known for its particularly high erosion rates in the central highlands, yet the role of human disturbance versus natural processes are not well understood and is a topic of ongoing debate. Recent studies have challenged the traditional view that the currently observed intense erosion processes and sediment fluxes in Madagascar are mainly driven by recent large-scale deforestation. However, at present almost no quantitative data is available to couple vegetation dynamics and sediment fluxes over time in Madagascar. This study aims to provide more insight in landscape changes (vegetation changes, sediment mobilization and deposition) in central Madagascar, and in the specific role of man and climate. The study focuses on the 1800 km² catchment of Lake Alaotra, located ca 200 km northeast of Antananarivo. Lake Alaotra is formed in a graben system in the highlands of Madagascar, and is the largest freshwater lake of the country (400 km²). A pollen record from the lake was used to reconstruct regional vegetation changes. Radiocarbon dates of extracted pollen provide a detailed chronostratigraphic framework. Augerings and radiocarbon dates from floodplains and marshes in the catchment were used to reconstruct the sediment deposition history. The pollen record and charcoal data shows the vegetation changes over the last 3000 years. The main observed shift in vegetation is a transition from a woodland/grassland mosaic towards an open grassland, starting ca 1850 years ago, which coincides with the onset of human activities. Data on floodplain sedimentation show an increase in accumulation rates in the last 600 years, from ca 1 mm yr<sup>-1</sup> to ca 30 mm yr<sup>-1</sup>, which can be linked to increased hillslope erosion rates during that time period. The sedimentation wave, however, does not reach Lake Alaotra nor the surrounding marshes as floodplains act as a buffer. Overall, this study provides a spatial and temporal integrated reconstruction of vegetation changes in the Lake Alaotra catchment and the link with sediment mobilization and deposition, thereby providing a better understanding of environmental changes in central Madagascar and its driving forces.</p>

Recognition and Occurrence of Different Sediment-Water Flows Triggered by High-Magnitude Hydrological Events in Mountain Catchments

<p>Sediment mobilization in small-medium size mountain catchments occurs by different flow types, categorized as debris-flows, hyperconcentrated-flows and water-flows, depending on the physical mechanisms governing flow rheology and particles interaction. During high-magnitude flow events, such transport mechanisms may take place concurrently in the same catchment at different sites of the channel network. One of the most important tasks in investigating dynamics of floods in these mountain catchments is to identify the transport mechanisms, since different flow types induce peculiar geomorphological hazards and dynamics. This work aims to improve criteria to recognize different flow types, with particular regard to hyperconcentrated-flows, and to analyze the transport mechanisms in mountain catchments in response to high-magnitude hydrological events.</p><p>Since direct monitoring of sediment mobilization during a flood is extremely difficult, a sound alternative is to consider the characteristics of the deposited material, which depend on the rheological proprieties of related flow. Through an extensive literature review, we identified the diagnostic criteria of the different flow types, grouping them in morphological and sedimentological evidences. A field-survey worksheet has been developed to ease the field-data collection and interpretation. The case-study selected for applying the survey procedure is the Tegnas catchment (Dolomites, Italy), a mountain basin draining an area of 51 km<sup>2</sup> affected by the Vaia Storm in October 2018, which induced large floods in several catchments of the Eastern Italian Alps. The deposits field-survey has been conducted in 35 sub-reaches of the Tegnas river and its major tributaries. In addition, we carried out detailed grain-size analyses, measured the angle of clasts-imbrication and collected samples for estimating the vegetal organic matter content through Loss-of-ignition procedure.</p><p>Field criteria allowed us to classify each sub-reach according to the deposits left after the event. Most of the steep tributaries have been interested by debris-flows, but also hyperconcentrated-flows have been recognized. Along the main stem, water-flow was the dominant process, although debris-flows and hyperconcentrated-flows deposits are documented where channel slope was very high (i.e. from 9 to 21%).   Hyperconcentrated-flow deposits occur also in the lower sub-reaches (i.e. channel slope from 0.3 to 6%), either at the confluence with debris-flow fed tributaries of where severe bank erosion occurred. We statistically analyzed data about clast imbrication angle (δ) and content of vegetal organic matter (OM<sub>LOI</sub>) obtaining significant results for both parameters. δ measured in debris-flow (50°-65°) and hyperconcentrated-flow deposits (45°-60°) is considerably higher than in water-flow sediments (30°-40°). Debris-flow and hyperconcentrated-flow deposits have higher OM<sub>LOI</sub> (2.5-5.5%) than water-flow deposits (1.5-3%). </p><p>The combination of field diagnostic-criteria and quantitative measure of additional parameters allows a reliable recognition of the flow types based on post-flood survey. Besides, this study allowed to point out that during high-magnitude floods the sediment mobilization in small-medium size catchments occurs through mechanisms that can be different from those expected for ordinary hydrological events using morphometric approaches. Solid material concentration or dilution (e.g. due to lacking of sediment sources or sediment disconnectivity) can explain the “unexpected” flow types during high magnitude events.</p>