Nature-based strategies for water renewal in micro-tidal coastal bays: application to the Ebre delta in the NW Mediterranean

The Fangar Bay, located at the northern lobe of the Ebro delta in the Spanish Mediterranean coast, supports an important aquaculture industry that must coexist with eco-tourism and water sports enterprises.These activities have experienced an important growth in the recent years, which, together with the increase in temperature and acidification associated to climate change have led to water quality degradation and mortality events affecting aquaculture production.The bay mouth is controlled by the morphodynamic evolution of a fronting sand spit subject to longshore sand transport gradient.The sheltered character of this bay result in limited wave and tidal flushing power, leading to limited water renovation and quality degradation.The low water fluxes and long renewal times result in eutrophication and anoxia episodes, aggravated by the effects of global warming that increases the duration of high temperature periods.The present growth of the spit is narrowing the shallow bay entrance, reducing even more the water circulation inside the bay, therefore jeopardizing the sustainability of current socio-economic activities within the bay domain.The objective of this work is to explore different Nature-based strategies to increase water renovation and quality within the bay, without compromising the overall morphodynamic sustainability of the delta and without generating a significant Carbon footprint. Two main alternatives are considered to increase the circulation inside the bay: combining irrigation discharges with favourable wind events and controlled appearance of breaching channels and overwash fluxes to promote exchanges between bay and coastal sea waters. The performance of these Nature-based solutions will be assessed by means of nested and coupled hydro-morphodynamic models validated with in-situ and Satellite data. The paper will end with some conclusions on the exportability of the proposed Nature-based solutions for this and other micro tidal environments.Preprin