STUDY OF MORPHODYNAMIC AND SEDIMENTOLOGICAL CHANGES IN THE OUALIDIA LAGOON (MOROCCO) USING BATHYMETRIC DATA: FIRST INVESTIGATIONS AFTER THE SEDIMENT TRAP DREDGING

Coastal lagoons are highly dynamic and physically complicated systems. They are environmentally productive and socio-economically valuable. Contemporary global development and management pressures require a better understanding of their dynamics and sustainability. The present study focuses on the problem of water confinement in the Oualidia lagoon (Atlantic coast of Morocco). This lagoon is characterized by an asymmetric tidal propagation, with a shorter duration of the flood (rising tide) than the ebb (falling tide). In the long term, this contributes to the reduction of depths and the confinement of water upstream. After extensive studies, a sediment trap was created in 2011 to trap the finest sediment in the upstream part of the lagoon. This study aims to analyze the morphodynamical and sedimentological changes in the lagoon of Oualidia, after the sediment trap dredging. For this purpose, bathymetric surveys covering 6 years between 2006 and 2012 were analyzed, providing sufficient data to identify the morphological changes that the lagoon has undergone during this period. The data analysis was followed by a study of the lagoon bed dynamics using profile lines extracted from the bathymetric data in a GIS environment. As a result, the findings partly show that over 6 years, an average height of +0.65 m was gained by the lagoon, while the average change in the eroded areas was estimated to be −0.42 m. In addition, the eroded area in the lagoon was estimated to be about 1,513,800 m2 with an erosion volume of 633,383 m3, while the accumulated area found was about 2,699,396 m2 with an accumulation volume of 1,765,866 m3. These changes can be related to the large input of marine sediment, mainly caused by tidal currents and waves, but also to the creation of a sediment trap in the upstream area of the lagoon.

Seasonal variation of oceanographic processes in Indus river estuary

Field investigations were conducted to study spatial and temporal (seasonal) variations in meteorological, hydrodynamic and hydrological variables in Indus River Estuary. The investigations were undertaken during wet, (moderate fluvial discharge), flood (highest fluvial discharge) and dry (zero fluvial discharge) seasons to obtain surface and near bed data during flood and ebb tides. Tides were semidiurnal, showing an asymmetric pattern with longer ebb tides and shorter flood tides. The hydrodynamic data revealed strong seasonal variation, the ebb velocities were significantly higher than flood current velocities during wet season, whereas a slight difference was found in current velocities during dry season, while the ebb phase lasted longer than flood during wet season; however no significant difference was observed during dry season. On the other hand during flood period the water currents were substantially higher and unidirectional related to the strong river flow. Turbidity values were considerably higher during flood season, than wet and dry seasons along the channel. However hydrological parameters such as temperature and dissolved Oxygen also revealed seasonal and spatial fluctuations, though they were within permissible range. The salinity distribution along the channel was related to the incoming river flow and tidal propagation. Higher salinity values were recorded in dry season, suggested that salinity variation at Estuary was due to salt intrusion from the North Arabian Sea, related to the absent of fluvial discharge form Indus River. Present study revealed substantial changes for hydrology and hydrodynamic conditions of the Indus River Estuary, related to the varying Indus River flow, as well as winds are another important atmospheric force in this region which enhanced the tidal forcing during southwest monsoon.

A generalized analytical solution of groundwater head response to dual tide in a multilayered island leaky aquifer system

The hydraulic properties of coastal aquifer systems are relevant to various hydrogeological, hydro-ecological and engineering problems. This study presents an analytical solution for predicting groundwater head fluctuations induced by dual-tide in multilayered island aquifer systems, consisting of an unconfined aquifer on the top and any number of leaky aquifers below. The solution was derived via the methods of matrix differential calculus and separation of variables. It is more general than any existing analytical solutions for the tidal pressure propagation since the new solution can consider multilayered aquifer systems along with the effects of leakage and aquifer length. Using this solution, we illustrated potential errors that may occur due to neglecting one or more vital factors affecting groundwater fluctuations. Besides, we articulated the groundwater response to the dual-tide in complex coastal aquifers. Considering that some thin semipermeable layers may be ignored in practical field investigation, we also demonstrated the effects due to simplification of aquifer layers. The results showed that with the increase in the number of overlapped leaky layers, the tidal propagation in the bottom part of multilayered aquifer system approaches that in a single confined aquifer with the same transmissivity and storage.

Assessing tide correction under altimetry tracks: an innovative validation methodology using USV (Unmanned Surface Vehicle) in-situ measurements

<p>Satellite altimetry recently reached an unprecedented level of global coverage with 7 missions flying simultaneously. While altimeters have been originally designed for open ocean and have improved our understanding of the large-scale ocean dynamic, the exploitation of coastal altimetry data remains a challenge that mobilizes a large effort in the scientific community. The future SWOT mission will solve this issue and certainly revolutionize our view of the coastal waters by<strong> </strong>mapping SSH with an unprecedented resolution.</p><p>One challenging aspect of coastal altimetry is the lack of accuracy in some geophysical corrections, which are critical to derive accurate sea-surface height anomalies (SSHA) near the coast. Especially, uncertainties in ocean tides is still an issue for the exploitation of altimetry in nearshore regions. Global tide models are usually used in most altimetry products. Despite their considerable progress in the last decade, their accuracy tends to decrease near the coast (Lyard, F. et Al., 2020).</p><p>Difficulties encountered in modelling the coastal tide are mainly due to its non-linear behaviour caused by changes in depth, shoreline interactions or varying bottom drag as it propagates onto shallower waters. The distortion of tidal propagation can thus be represented as additional tidal waves, which reflect overtides compound tides. These interactions are numerous and a great number of constituents have to be considered in order to reproduce accurately the tidal signal in shallow regions. Consequently, efforts in developing regional modelling of coastal areas are encouraged, as well as the consideration of ocean/shelf/land as a modelling continuum, for the preparation and exploitation of the future SWOT mission (Ayoub, N. et Al., 2015).</p><p>Moreover, these shallow-water waves exhibit smaller wavelengths than major astronomical ones, and there is a critical need for observations with short space and time scales to appreciate their spatial variability. While tide models are classically validated against tide-gauges confined to the coast, new opportunities are emerging with the development of kinematic GNSS systems. Chupin et Al. (2020), have demonstrated the ability of the Cyclopée system (a combination of a GNSS antenna and an acoustic altimeter) mounted on an USV to map sea surface height in motion. At a fixed point, the Cyclopée system provides similar accuracy than the best tide-gauge systems (and is therefore a way to propagate tide gauges measurements under satellites tracks).</p><p>Through a methodology based on crossover measurements; we demonstrate in this study the potential of the USV PAMELi, developed at the University of La Rochelle, for assessing tide corrections under altimetry tracks, in the scope of future coastal altimetry applications (e.g. storm surge or wave setup). For this purpose, the Pertuis Charentais area (France) is addressed as a modelling case study with a new regional barotropic configuration based on SCHISM model (Zhang, J. et al., 2016). After being compared against coastal tide-gauges, our SCHISM model as well as other available global solutions are assessed though this methodology applied under the pass 216 of Sentinel-3A.</p>

A waterline method to derive intertidal bathymetry from multispectral satellite images and its application to hydrodynamic modelling

<p>Bathymetric data are a key parameter to assess shallow-water hydrodynamic processes. In-situ surveys provide high data quality; however, surveys are expensive and cover a limited spatial extent. To fill this gap, over recent years, the Satellite Derived Bathymetry (SDB) techniques have been developed. The present work aims to elaborate a technique to estimate bathymetric data from satellite images for intertidal zones. The method applied in this work is composed of 6 steps: (1) image querying and pre-processing is done through Google Earth Engine application (API) using Copernicus Sentinel 2A and B, product type 2A. (2) Identification of the intertidal zone for the study area by temporal variability of the Normalized Difference Water Index (NDWI). (3) Recognition of the waterline in each image by the use of an adaptive threshold technique; and assignment of the elevation for each detected waterline based on local observed tide heights. (4) Validation of the estimated bathymetry by comparison with LiDAR measurements. (5) Implementation of a SDB correction: numerical and/or statistical and, (6) assessment of the validity of SDB for hydrodynamic modelling. The SDB technique was applied to 4 different estuaries in New Zealand: Maketu, Ohiwa, Whitianga and Tauranga Harbour showing similar or better estimations in comparison to previous works using optical or synthetic aperture radar (SAR). For Tauranga Harbour, results from the statistical and dynamical corrections showed that the major error source is due to the image optical properties and environmental conditions when the image was acquired (35%). However, the tidal propagation can significantly decrease the SDB accuracy (13%). Finally, the use of the SDB in numerical simulations does not present huge differences in the predicted waterlevels in comparison to the use of survey bathymetry, showing that SDB could be potentially used for coastal flooding simulations.  </p>