Modeling Wetland Rebuilding After Dyke Failure in Highly Anthropized River Deltas: A Case Study from the Po River Delta, Italy

Abstract

Nature-based solutions in coastal ecosystems offer more efficient and sustainable strategies to cope with climate changes and anthropogenic modifications compared to traditional hard engineering measures. In river deltas, managed realignment through levee breaching has become an increasingly common approach for restoring coastal wetlands and reestablishing natural depositional dynamics on previously reclaimed deltaic plains.

This study focuses on the formation of new wetlands in the Po River Delta (PRD) following dyke failures over the last 3 decades. During this period, portions of reclaimed land in the delta's most seaward sector were abandoned, flooded, and progressively transformed into vegetated wetlands.

Our study area, known as "Batteria" Island, is located in the PRD northeastern portion. Previously used as agricultural land dedicated to rice cultivation, the area was partially abandoned following significant subsidence and a series of large floods from the Po River during the 1970s. These floods caused widespread inundation by seawater and induced soil salinization. Subsequently, the area, left flooded, was utilized both as a hunting reserve and a fish farm before being permanently abandoned between the 1980s-1990s. The lack of maintenance led to the failure of several artificial dykes, ranging in height from approximately 1 to 3 meters, allowing river waters to inundate previously reclaimed, low-lying deltaic land. One of these dyke breaches, which occurred in 1999, resulted in the formation of approximately 30 hectares of new wetlands in less than 20 years.

In this study, we utilized a depth-averaged, coupled hydro-morphodynamic and sediment transport model to simulate wetland formation at Batteria Island. The numerical model was applied to an unstructured grid representing the entire PRD and was forced by 30 years of mean high-water levels and peak river discharge at the downstream and upstream boundaries, respectively. The model also incorporated a steady subsidence rate of 2 cm/year, derived from empirical data.

The model successfully reproduced wetland formation following dyke breaching, aligning with observations from aerial photos and bathymetric surveys. Consequently, we applied the same model to simulate dyke breaches at different locations within the PRD to evaluate the feasibility of using managed realignment to create new wetland areas of significant socio-economic and ecosystem value.

Our study highlights the inherent ability of highly anthropized river delta systems—characterized by extensive reclaimed land—to retain sediment and build new land when dykes are removed, whether naturally or artificially. This process enables these systems to recover a more natural, dynamic state, characterized by rapid and widespread wetland formation. Such a transformation enhances resilience to projected relative sea-level rise in the near future.

Keywords: Nature-based solutions, Burcio lagoon, Levee Breaching, Morphodynamic model, 2DEF, Shallow water area