Use of a hydrodynamic model for the management of water renovation in a coastal system

Abstract

Abstract. 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.

Short-Term Response of Chlorophyll a Concentration Due to Intense Wind and Freshwater Peak Episodes in Estuaries: The Case of Fangar Bay (Ebro Delta)

“…After all these Chl a concentration peaks, a sharp decrease in Chl a concentration could be observed (values of around 2 mg•m −3 in the FANGAR-III campaign, and 4 mg•m −3 in the FANGAR-IV campaign). Finally, comparing both campaigns, autumn Chl a concentrations showed larger mean values in comparison to the summer; however, the peak values during NW wind episodes showed similar values (for instance, 6 For instance, values of Chl a concentration during E1 revealed larger values in comparison to precedent data. Discrepancies in the Chl a concentration during E2 were observed, likely associated with short episodes and less intensity in comparison to E1 or E3.…”

“…In this sense, the biological habitat and the ecosystem characteristics are highly influenced by physical and morphologic dynamics. Estuaries and coastal environments are characterised by complex hydrodynamics with high temporal variability in water circulation due to the joint effect of wind, tides, freshwater inflow, and other episodic events [1][2][3][4][5][6][7][8][9]. In a small-scale, shallow coastal bay (few meters water depth) such as Fangar Bay in the Ebro Delta (NW Mediterranean Sea), the concentration of chlorophyll a (Chl a) shows high variability on a seasonal scale rather than an inter-annual scale [10].…”

Wind-Driven Hydrodynamics in the Shallow, Micro-Tidal Estuary at the Fangar Bay (Ebro Delta, NW Mediterranean Sea)

“…The numerical details of ROMS are described extensively in [24]. This model has been used and validated in similar bays and estuaries, such as Alfacs Bay located south of the Ebro Delta (e.g., [13,26,27]). The domain used is inspired in Fangar Bay and consists in an idealized domain due to the difficulty of achieving good bathymetry.…”

How Agriculture, Connectivity and Water Management Can Affect Water Quality of a Mediterranean Coastal Wetland

“…The Mediterranean Spanish coastal systems seem to be severely impacted by agriculture and the use of the limited water resources, which has become especially complicated in relation to rice crops, for example, in the Albufera Natural Park and the Ebro Delta [89,90]. In the Doñana National Park, the loss of water quality was related in equal parts to industrial and urban pressures (WWTPs) and also to rice cultivation [80,91].…”