Nature-based solutions in coastal environments: the Sustainable Urban Drainage approach for beach erosion management in dune barrier systems

Dune barrier systems have historically been essential landscapes for sun-and-beach tourism, playing a key role in coastal economies worldwide. However, tourism development has led to significant land use and land cover changes that have adversely affected these environments. The establishment of impervious urban surfaces, replacing the sandy substrate, has drastically reduced water infiltration. The increased urban runoff has promoted waterlogging during coastal storms. Traditional storm drain systems, constructed to transport excess water through beach areas, have contributed to beach erosion by promoting sedimentary imbalances. Coastal erosion directly impacts the morphodynamics of these environments but also led to the degradation of the biotic communities of the dune, beach, and intertidal sectors. In addition, urban runoff discharges reduce coastal aquifer recharge rates, compromising the main freshwater supply for many coastal communities living on barrier environments.

Faced with this problem, Nature-Based Solutions (NBS) represent an alternative to traditional engineering solutions through an integrated approach with the environment. In particular, Sustainable Urban Drainage Systems (SUDS) project non-structural interventions to functionally reproduce the natural hydrological cycle of the intervened site. Its operational scale often targets urban micro-basins, taking into account an infiltration-retention double behaviour. The aim of this work was to assess the impact of urban runoff on beach erosion and propose and evaluate various NBS-SUDS as management strategies. As study case, Villa Gesell municipality was proposed. It was located on the Easter Sandy Barrier of the Buenos Aires province, Argentina, representing a highly anthropized coastline with the second most-demanded beach tourist destination of the country. Erosion impacts were assessed by monitoring in situ beach data between 2022 and 2024. UAV aerophotogrametric imagery and high-resolution DGPS control points were processed using the structure-for-motion approach. Beach digital elevation models were obtained to estimate sedimentary imbalances and to identify environmentally sensitive areas related to urban storm rain discharges. Finally, the introduction of NBS-SUDS strategies was modelled by the Storm Water Management Model (SWMM) using climatic, topographic and land use/land cover data as input.

The results of this study showed a concentration of more than 25 storm drains in a linear waterfront of 5 km. Each discharge point can lead to the development of beach scarps representing the 50 % of beach width and sedimentary losses up to 230 m³ for a single storm event. Different SUDS strategies as permeable pavements, infiltration pits and ditches, retention basins, French drains, green ditches, retention ponds and natural lagoons or wetlands, were proposed. These tools were modelled modeled to have a synergistic effect to respond to a specific problem. Moreover, the implementation of SUDS could lead to the recovery of coastal environments through the reintroduction of native vegetation species. This work represents a paradigm shift in beach management, where erosion has historically been conceived and managed from the sea and not from cities. It is hoped that this work will serve as a direct contribution to the development of public management policies for the regeneration of coastal environments in the face of the predicted effects of Climate Change.