Determining sedimentation rates by accounting for past compaction in the Mekong Delta as input for 3D delta evolution modelling

<p>The Vietnamese Mekong River Delta has been formed by the deposition of soft, fine-grained sediments during the last thousands of years. Natural compaction of these unconsolidated deposits over time and with increased overburden load is one of the main drivers of subsidence in this delta. High rates of natural compaction may have a considerable increased flood vulnerability of the lowly elevated delta plain and ultimately result in permanent inundation.</p><p>Following the loading history of accumulating sediments during the Holocene delta evolution, it is possible to estimate delta-wide present-day natural compaction rates. The ultimate goal of this study is to provide reliable input data on Holocene sedimentation rate throughout the Mekong Delta for a novel 3D numerical model to simulate delta formation and its dynamic evolution during the late Holocene. In order to achieve this, it is fundamental to first take into account previous compaction that already happened to the sediments in the past to estimate the original sedimentation rate of Holocene sediments.</p><p>We employed a 1D decompaction module to compute the original, uncompacted thickness of Holocene delta sequences from lithological borelogs to estimate the amount of virgin sediment that has been deposited in time. The original thickness of Holocene sediments was determined after investigating geomechanical properties of Holocene deposits and decompaction of lithological boreholes spread over the delta. To determine the sedimentation rate for the borelogs with missing dating information, the age was estimated by using a linear distance interpolation of age isochrones starting from a limited number of boreholes, where both stratigraphy and sediment ages are available.</p><p>As a final step, the estimated sedimentation rates from each of the borelogs are interpolated to arrive at delta-wide sedimentation rates and lithology during the Late Holocene. This provides the required input data for the 3D model to simulate natural consolidation during the delta evolution and accurately assess present natural compaction rates.</p>

Mathematical Reconstruction of Eroded Beach Ridges at the Ombrone River Delta

Several remotely sensed images, acquired by different sensors on satellite, airplane, and drone, were used to trace the beach ridges pattern present on the delta of the River Ombrone. A more detailed map of these morphologies, than those present in the literature, was obtained, especially at the delta apex, where beach ridges elevation in minor. Beach ridges crests, highlighted through image enhancement using ENVI 4.5 and a DTM based on LiDAR data, were then processed with ArcGIS 9.3 software. Starting from this map, a method to reconstruct beach ridges segments deleted by the transformations of the territory is proposed in this paper. The best crest-lines fitting functions were calculated through interpolation of their points with Curve Expert software, and further extrapolated to reconstruct the ridges morphology where human activity, riverbed migration, or coastal erosion eliminated them. This allowed to reconstruct the ridges pattern also offshore the present delta apex, where the shoreline retreated approximately 900 m in the last 150 years. Results can be further used to implement conceptual and numerical models of delta evolution.

Evolution of the Ofanto River delta from the ‘Little Ice Age’ to modern times: Implications of large-scale synoptic patterns

We reconstruct the evolution of the Ofanto River delta from the 17th century to the present using historical maps (1600–1850), official IGM topographic maps (1850–1980) and recent aerial photographs (2015), and we compare long-term morphological changes with the evolution of the delta of the Volturno River during the same time period. The aim of this study is to define the role of climatic (flood frequency, synoptic pressure patterns) and anthropogenic factors (deforestation, anthropogenic sediment subtraction of river sediment) in the evolution of the Ofanto delta. We analysed the importance of each factor on the evolution of the delta and compared them with the simultaneous behaviour of the Volturno delta to highlight the role of large-scale synoptic pressure patterns. We found that the main driver of different delta evolution phases is weather-climatic condition, while anthropogenic factors interacted with the delta evolution in different ways but did not control the first-order evolution. In particular, analysing the data on recent floods, we found that the most favourable situations for both rivers are omega-blocking, deep low-pressure trough and strong meridional circulation (mode Ω) which create Mediterranean low-pressure systems. Instead, a zonal circulation (mode W) can only cause floods on Volturno. Because the evolution of a delta is driven by the frequency of floods, and because we found that the frequency of floods is guided by synoptic patterns, a relationship can be established between delta evolution and synoptic patterns in the past. Consequently, past phases of the contemporary progradation of the Ofanto and Volturno deltas suggest the increasing frequency of mode Ω, while phases of simultaneous progradation of the Volturno delta and stability and/or retreat of the Ofanto delta are indicative of the increasing frequency of mode W. The only exception occurred during the last evolutionary phase (60 years), when anthropogenic sediment subtraction was prevalent.