Study on Hydraulic and Hydrological Invariance: Analysis and Assessment Across Different Regions of Italy

Urbanization alters the natural hydrological balance by increasing impervious surfaces, which reduces infiltration and amplifies peak runoff. To address these impacts, several Italian regional regulations introduced the concept of hydraulic and hydrologic invariance, ensuring that post-development runoff conditions remain comparable to pre-development ones. This study aimed to evaluate how the application of a single regulatory framework—the Lombardy Regional Regulation (R.R. 19 April 2019, n. 8)—affects the design of detention volumes when applied to different climatic contexts across Italy. The research focused on understanding how variations in rainfall characteristics influence the sizing of stormwater detention systems under a uniform set of design criteria, in order to assess the robustness and adaptability of the regulation beyond its regional scope. The research was based on a reference case study representing a typical commercial area composed of a shopping center, parking areas, and a green zone. Six cities were selected according to the Köppen climate classification and rainfall data from the FOCA database, covering a wide range of climatic conditions. The methodology followed the hydrological procedure prescribed by the regulation, applying the most critical parameters at each step to represent the most conservative design scenario. The results showed that, despite the large variability in rainfall intensity and distribution among the selected cities, the calculated detention volumes differed across locations but ultimately remained below the minimum regulatory threshold. As a result, all designs were governed by the minimum storage requirement established by the Lombardy Regulation, leading to a single, uniform solution consisting of a detention structure for temporary storage (and also the possible integration of infiltration systems or other innovative sustainable approaches) and controlled discharge to the sewer network. This outcome highlights that the normative thresholds tend to dominate over local climatic variability, ensuring hydraulic safety under all tested scenarios but potentially leading to over-dimensioned solutions in regions with lower rainfall intensity.