Integrating Ecological Connectivity and Flood Adaptation in Road–Stream Crossings: A Nature-Based Design Concept

Conventional culverts may disrupt longitudinal river continuity by modifying natural streambeds, restricting daylight, altering local flow conditions, and creating potential barriers to fish and other aquatic organisms. At the same time, road–stream crossings must provide sufficient resilience to accommodate increased water levels during flood events. This study presents a conceptual nature-based alternative to conventional enclosed culverts, based on a fish-friendly road crossing that combines natural-bed preservation, and passive flood-relief functions. The proposed system was examined through a conceptual design and functional assessment approach. The crossing consists of a bridge-like open-bottom structure that preserves the existing riverbed, an upper metal grating section, and lateral openings connected to auxiliary channels intended to accommodate excess water during elevated flow conditions. The design was qualitatively assessed according to its potential contribution to ecological connectivity, natural-channel continuity, flood resilience, multi-functionality, and compatibility with nature-based infrastructure principles. Particular attention was given to the interaction between ecological and engineering functions rather than to quantitative hydraulic performance. The conceptual assessment indicates that the proposed configuration can integrate several functions that are commonly treated separately in conventional crossing design. Preservation of the natural bed may support longitudinal habitat and sediment continuity, while daylight access may reduce the abrupt environmental transition associated with fully enclosed structures. The lateral relief system provides a passive mechanism intended to increase flood accommodation without replacing the central natural corridor. The concept therefore represents a hybrid configuration in which engineered components are used to maintain or enhance naturally functioning elements of the stream system. Important uncertainties remain regarding debris blockage, scour, structural safety, local hydraulic conditions, maintenance, and species-specific fish responses. The proposed crossing provides a conceptual framework for integrating ecological connectivity and flood adaptation into road infrastructure.