In this contribution we investigate the hydrodynamic response in Alfacs Bay (Ebro Delta, NW Mediterranean Sea) to different anthropogenic modifications in freshwater flows and inner bay–open sea connections. The fresh water coming from rice field irrigation contains nutrients and pesticides and therefore affects in multiple ways the productivity and water quality of the bay. The application of a nested oceanographic circulation modelling suite within the bay provides objective information to solve water quality problems that are becoming more acute due to temperature and phytoplankton concentration peaks during the summer period when seawater may exceed 28 ^∘C, leading to high rates of mussel mortality and therefore a significant impact on the local economy. The effects of different management “solutions” (like a connection channel between the inner bay and open sea) are hydrodynamically modelled in order to diminish residence times (e-flushing time) and water temperatures. The modelling system, based on the Regional Ocean Modeling System (ROMS), consists of a set of nested domains using data from CMEMS-IBI for the initial and open boundary conditions (coarser domain). One full year (2014) of simulation is used to validate the results, showing low errors with sea surface temperature (SST) and good agreement with surface currents. Finally, a set of twin numerical experiments during the summer period (when the water temperature reaches 28 ^∘C) is used to analyse the effects of proposed nature-based interventions. Although these actions modify water temperature in the water column, the decrease in SST is not enough to avoid high temperatures during some days and prevent eventual mussel mortality during summer in the shallowest regions. However, the proposed management actions reveal their effectiveness in diminishing water residence times along the entire bay, thus preventing the inner areas from having poor water renewal and the corresponding ecological problems.

This article investigates water circulation in small-scale (~10 km²), shallow (less than 4 m) and micro-tidal estuaries. The research characterizes the hydrodynamic wind response in these domains using field data from Fangar Bay (Ebro Delta) jointly with three-dimensional numerical experiments in an idealized domain. During calm periods, field data in Fangar Bay show complex water circulation in the inner part of the estuary owing to its shallow depths and positive estuarine circulation in the mouth. Numerical experiments are conducted to investigate wind-induced water circulation due to laterally varying bathymetry. For intense up-bay wind conditions (wind intensities greater than 9 m·s⁻¹), an axially symmetric transverse structure occurs with outflow in the central channel axis and inflow in the lateral shallow areas. These numerical results explain the water circulation observed in Fangar Bay during strong wind episodes, highlighting the role of the bathymetry in a small-scale environment. During these episodes, the water column tends to homogenize rapidly in Fangar Bay, breaking the stratification and disrupting estuarine circulation, consistent with other observations in similar domains.

Estuaries and coastal bays are areas of large spatio-temporal variability in physical and biological variables due to environmental factors such as local wind, light availability, freshwater inputs or tides. This study focuses on the effect of strong wind events and freshwater peaks on short-term chlorophyll a (Chl a) concentration distribution in the small-scale and microtidal, Fangar Bay (Ebro Delta, northwestern Mediterranean). The hydrodynamics of this bay are primarily driven by local wind episodes modulated by stratification in the water column. Results based on field-campaign observations and Sentinel-2 images revealed that intense wind episodes from both NW (offshore) and NE-E (onshore) caused an increase in the concentration of surface Chl a. The mechanisms responsible were horizontal mixing and the bottom resuspension (also linked to the breakage of the stratification) that presumably resuspended Chl a containing biomass (i.e., micropyhtobentos) and/or incorporated nutrients into the water column. On the other hand, sea-breeze was not capable of breaking up the stratification, so the chlorophyll a concentration did not change significantly during these episodes. It was concluded that the mixing produced by the strong winds favoured an accumulation of Chl a concentration, while the stratification that causes a positive estuarine circulation reduced this accumulation. However, the spatial-temporal variability of the Chl a concentration in small-scale estuaries and coastal bays is quite complex due to the many factors involved and deserve further intensive field campaigns and additional numerical modelling efforts.

Offshore wind is proposed as an energy source to upwell nutrient-rich deep water to the ocean photic layers. A spar-buoy wind turbine with a rigid tube about 300 m long is proposed as a pipe to drive deep water up to the surface. The minimum energy required to uplift the water is the potential energy difference between surface waters inside and outside the pipe, which depends on the background density profile. The corresponding surface jump or hydraulic head, h, calculated for several analytical and experimental density profiles, is of the order of 10 cm. If the complete turbine power (of the order of several MW) is used for raising the water (assuming a 100% pump efficiency), in a frictionless flow, very large water volumes, of the order of thousands of m3 s-1, will be transported to the photic layers. In a more realistic case, taking into account pipe friction in wide pipes, of the order of 10 m radius, and a power delivered to the fluid of 1 MW, the volume transport is still very large, about 500 m3 s-1. However, such a large amount of dense water could sink fast to aphotic layers due to vertical static instability (the fountain effect), ruining the enhancement of primary production. Hence, some ways to increase the turbulent entrainment and avoid the fountain effect are proposed. From the energetic viewpoint, artificial upwelling using offshore wind energy is a promising way to fertilize large open sea regions. This mariculture application is, however, severely subjected to atmosphere and ocean climatology, as well as to ecological dynamics. The general problem is multidisciplinary, and some important physical, engineering and ecological questions need to be seriously addressed to improve our confidence in the approach presented here.

F.-Pedrera Balsells, M.; Mestres, M.; Fernández, M.; Cerralbo, P.; Espino, M.; Grifoll, M., and Sanchez-Arcilla, A., 2020. Assessing nature based solutions for managing coastal bays. In: Malvárez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 1083–1087. Coconut Creek (Florida), ISSN 0749-0208.This work investigates the hydrodynamic response of two small and shallow bays located in the Ebro Delta, in order to solve the problems affecting mussel farms, such as the high temperatures reached in the summer period (>28 °C), causing the death of these bivalves, and the limited water renewal that causes anoxia within the bay. Nature Based Solutions are described focusing on two aspects: one, modifying the flow of these drainage channels, increasing the arrival of freshwater to the bays; and two, opening a gap in the sand bars so that there is a flow of sea water in and out through the innermost zone, which is where both bays have greater problems of water renewal. The impact of those solutions simulated with the Regional Oceans Modeling System (ROMS), consist of a set of nested domains that use CMEMS-IBI data for the initial and open boundary conditions (coarser domain). To validate the numerical model, data from in situ campaigns have been used for one full year (2014) for Alfacs Bay, and for the summer period (July-August 2017) for Fangar Bay. In addition, the Sea Surface Temperature (SST) is also studied as an indicator to reduce the mortality problems of mussels, but the results obtained do not reduce this variable. What the results do show is a decrease in residence time in both bays for both experiments, with the increase in flow being more viable than the increase in sand bar breakage.

The Ebro Delta (EU Natura 2000 site and Natural Park and Biosphere Reserve (UNESCO)) is a complex area that faces a set of challenges mainly due to anthropogenic pressure and climate change related risks. The two Ebro embayments (i.e. Fangar and Alfacs) are natural protected areas of value for the survival of many species, such as the critically endangered Pinna nobilis. Likewise, these bays are two of the most productive aquaculture areas along the NW Mediterranean Sea. The e3HOPE Service, led by NOW Systems (Nologin), provides a coastal operational forecast for the Ebro Delta. The system is based on a coastal downscaling of the Copernicus Marine Mediterranean regional solution, generating daily forecasts at 350 m resolution for the whole Ebro Delta and products at even higher resolution (around 70 m) for the Fangar Bay. A high-resolution ROMS model application, developed and tested by the LIM/UPC and integrated into the NOW operational framework, contributes to a seamless modelling in the area, filling the gap between the existing Copernicus regional forecast products and the local needs for a higher resolution model prediction. The e3HOPE coastal service is completed with a tailor-made visualization service layer co-designed with IRTA. The new NAUI web service deployed by NOW for the Delta and Fangar Bay is a highly customizable tool that allows visualization of both met-ocean observations (including additional local monitoring routinely performed at specific fixed CTD stations) and the e3HOPE model forecasts at selected locations. This new NAUI deployment at the Ebro Delta certainly eases, and enhances, the IRTA uses of (new and existing) operational forecasting capabilities, making coastal data more actionable and transferred into fit-for-purpose applications for management, supporting sustainable aquaculture activities and biodiversity conservation. This contribution is mainly focused on the skill of the current e3HOPE model release, being the model solution validated along 1 year with available in-situ observations (for temperature, salinity, sea level and surface and bottom water currents). Some insights into the operational forecast products delivered by the e3HOPE service, as well as into the planned service evolution (for 2024) is provided. Next e3HOPE operational release will include: (i) upgrade of the model system (to a wave-current coupled system, using COAWST), (ii) improvement of the fresh water forcing (substituting climatological approaches by predicted data for the Ebro river, and some local channels discharges), and (iii) improvement in the initialization (using CTD data), in the Fangar Bay domain.