Estuarine Dynamics and Ecosystems

The Ganges-Brahmaputra-Meghna (GBM) delta is one of the most dynamic tide dominated delta in the world. The flow generated in the upper catchment of the GBM basins is passing through the estuarine systems of the GBM delta. The particular feature of these estuarine systems is characterized by extreme flow concentration in one single estuary, the Lower Meghna Estuary. Due to highly dynamic nature and a huge flow volume, it is difficult to measure the discharge in the Lower Meghna estuary. Till now, there is no measuring station and no reliable estimate of total flow volume which is passing through the Lower Meghna estuary. In this study, a 1D dynamic model (HEC RAS unsteady) is applied to compute the discharge in the Lower Meghna estuary. In the schematized model network, all the major rivers of the delta are included that contribute flow to the Lower Meghna estuary. The model is validated using measured water level available at the Lower Meghna estuary. The validated model is used to generate the temporal variation of discharge and the flow volume passing through the Lower Meghna estuary for different flooding scenario.

Rosetta promontory is located on the Mediterranean Sea where the Rosetta branch of Nile meets the sea. Shoreline erosion and the river mouth sedimentation at Rosetta promontory were magnified after construction of Aswan High Dam (AHD) in 1964 that dramatically decreased both water discharge, and sediment transport to the sea. Many attempts to solve the erosion, and sedimentation problem were performed. Hard protection works have been implemented since 1989 such as seawalls on the tip of the promontory and several groins along the eastern and western shores of the promontory. Additionally continuous dredging works were implemented within the river mouth. However, the shoreline along Rosetta Promontory is still unstable and these measures have a limited effect on the promontory stability where it transfers the problems to the adjacent areas.
Two techniques of coastal nourishment are proposed in this study to avoid further degradation of the Rosetta coastal zone and to minimize erosion problems. These techniques include near shore and beach nourishment.
A calibrated/verified Coastal Modeling System (CMS) was used to address the effect of the nourishment on the morphology of the hot spot areas (erosion and accretion).　Results of different scenarios were discussed in detail. Beach nourishment on the western headland of the promontory will modify the stability of the shoreline specially the western side of the promontory as well as decrease the siltation problem inside the river mouth.

The detrended correspondence analysis (DCA) applied to the relative abundance of ostracoda species and physicalchemical,
sedimentological and geochemical parameters allowed to identified four sectors in the estuary of the Arade
River: Sector I - located between the mouth of estuary and Portimão City and is characterized by the largest number
of species of ostracoda, higher salinity and pH, sandy sediment and higher protein concentration. This sector has the
largest marine influence with high phytoplankton productivity; Sector II - located in the vicinity of Parchal City, it is
characterized by the highest value of diversity H' and the occurrence of the species Leptocythere lacertosa, Herpetocypris
helenae and Leptocythere porcellanea. In this sector was identified the highest concentration of sulfur and fine sediments;
Sector III - located between Silves and Parchal cities, has the highest concentration of organic carbon, fine sediment and
biopolymers (carbohydrates, lipids and proteins). In this sector were identified only the dominant species in the estuary
(Loxoconcha elliptica, Cytherois fischeri e Leptocythere lacertosa); Section IV - located in the Silves City, this sector
had the highest lipid content and the silt fraction of the whole estuary. The sectors identified in Arade River Estuary,
this is characterized as the most confined where the effluents from Silves City are deposited. The DCA analysis showed
that the Loxoconcha ellipitica is the most characteristic species of this sector, and it can be considered with a confined
environments bioindicator.