A parametric approach to delineating watersheds draining on linear infrastructures and assess their vulnerability to floods and landslides

Landslides are a growing threat to transport infrastructures, exacerbated by rapid urbanization, climate change, and artificially altered hydrological conditions. This study presents a systematic approach to delineating drainage basins potentially affecting linear communication networks, to enhance the resilience of such transport corridors. Knowledge of the drainage basins at intersection points with linear features provides essential data for assessing risks related to geohazards, such as rapid flow-like landslides, as well as flash floods. This example demonstrates the method on the road network of Italy, considering hydrological and geomorphological parameters calculated at 25 m spatial resolution. The software implementation of the methodology, developed within the open source environment GRASS GIS [1], is readily applicable to different areas, using similar input data.

This study used the European digital elevation model EU-DEM, and the official ANAS (the Italian agency for road management) vector graph, describing 29,500 km of roads. The method consists of the following steps, inspired by a similar application to the national railway network in Ref. [2]:

(i) We use the r.watershed hydrological model, based on a least-cost path method [3], to delineate a dense stream network with corresponding basins with minimum upslope contributing area of 25,000 m² at the stream initiation point.

(ii) Next, we considered a buffer of width d = 300 m on both sides of the road segments to select intersections between streams and roads; here, d is a parameter of the method.  This approach helps mitigating possible inaccuracies of input data, and includes situations where a stream segment flows on one hydrographic side of the main stream, and the road sits on opposite side. The procedure selects a conservative set of 66,018 intersection points.

(iii) Finally, we delineated watershed draining to each of the intersection points, using the software r.water.outlet [4], applied to each point with a data-parallel procedure. The result of this procedure is a polygonal vector layer containing all the watersheds associated to each interaction point.

Characterizing each watershed with morphometric indicators (area, slope, topographic wetness, and others) enables us to identify the road segments vulnerable to hydrological and geomorphological hazards, including flooding, erosion, and slope instability. This is in difference with the approach based on slope units [5], which are suited for slope-bound phenomena such as, for example, rockfalls [6], or for the determination of the likelihood of occurrence of landslide initiation points [7]. Watersheds of different sizes, relevant to phenomena with different reach distances and rapidity, can be selected in a parametric way. Preliminary results demonstrate the potential of the method to prioritize monitoring and maintenance of critical road segments.

References

[1] Neteler et al., Env. Mod. Softw. 31 (2012) https://doi.org/10.1016/j.envsoft.2011.11.014

[2] Marchesini et al., Eng. Geol. 332 (2024) https://doi.org/10.1016/j.enggeo.2024.107474

[3] Metz et al., Hydrol. Earth Syst. Sci. 15 (2011) https://doi.org/10.5194/hess-15-667-2011

[4] Ehlschlaeger, https://grass.osgeo.org/grass-stable/manuals/r.water.outlet.html

[5] Alvioli et al., Geomorphology 358 (2020) https://doi.org/10.1016/j.geomorph.2020.107124

[6] Alvioli et al., Eng. Geol. 293 (2021) https://doi.org/10.1016/j.enggeo.2021.106301

[7] Loche et al., Earth-Sci. Rev. 232 (2022) https://doi.org/10.1016/j.earscirev.2022.104125