After some involvement in the 1830 Revolution, Belgrand started his studies at the Ecole des Ponts et Chaussées in 1831. At the time, geology was little taught and the soil was only investigated with a view to verifying its strength as a support for foundations. Hydrology was ignored completely. However, after witnessing how a flood had devastated the construction works for a small bridge in Burgundy, Belgrand discovered and studied the links between the geological composition of the soil and the groundwater regime. In 1846 Arago asked him to read his paper "Hydrological Studies in the Upper Part of the Seine Basin" (Etudes hydrologiques dans la partie supérieure du bassin de la Seine), in front of the Acedemy of Sciences, and this established his scientifc reputation.
In 1849, while working as a junior engineer in the Yonne Département, he published a description of the distribution of water in Avallon. In 1851 he published a geological map of the Avallon district and began the series of hydrological studies that he continued to work on for the next twenty-seven years.
Haussmann, the Prefect of the Seine Dépaartement, called upon Belgrand to look into the possibilities of improving the capital’s drinking water supply, appointing him Chief Engineer of the department responsible for navigation between Paris and Rouen. As Director of Water and Drains for Paris, in 1867 he directed the construction of a large network of main sewers that considerably improved the city’s drainage, which until then had resembled a vast cesspit.
To meet the city’s water supply needs and to improve the security of supplies, he built the highly durable Vanne aqueduct and Montsouris reservoirs.
During his period of office the daily reception capaicity of the Paris drains increased from 87,000 to 400,000 cubic metres.
For a twelve year period he also directed the Seine Basin hydrometry department. During this time he set up a flood warning system that served as a model for those established later in the other major French river basins.
Appointed to the Academy of Sciences in 1861 Belgrand was awarded the Legion of Honor for having continued to operate the city’s water and drainage system during the Siege of Paris, in spite of the difficulties.
In addition to a series of papers on the geology of the Paris basin, he published three major works, the beginning of a historical account of subterranean works in Paris: Etude préliminaire sur le régime des eaux dans le bassin de la Seine (1873), Les Aqueducs romains (1875) and Les anciennes eaux. A fourth vollume, les Eaux nouvelles was published in 1882 by one of his colleagues. A fifth, Les Egouts, followed later and described the major projects managed by Belgrand in order to provide Paris with a water supply and drainage system.