Jean-Baptiste Charles Joseph Bélanger (4 April 1790 – 8 May 1874) was a French applied mathematician who worked in the areas of hydraulics and hydrodynamics. He was a professor at the École Centrale des Arts et Manufactures, École Polytechnique and École des Ponts et Chaussées in France. In hydraulic engineering, he is often credited improperly for the application of the momentum principle to a hydraulic jump in a rectangular open channel in 1828. His true contribution in 1828 was the development of the backwater equation for gradually varied flows in open channels () and the application of the momentum principle to the hydraulic jump flow in 1838. () Life of Bélanger Born in Valenciennes on 4 April 1790, Bélanger was the son of Charles Antoine Aimé Joseph Bélanger, master locksmith, and of Jeanne Françoise Joseph. He studied in Paris at the École Polytechnique and later at the École des Ponts et Chaussées. As Ingénieur du Corps des Ponts et Chaussées, he started his engineering career in 1816 at La Réole. From 1821, he moved to work on the Somme navigation canal and after 1826 on the Ardennes navigation canal. It was during these two missions that he studied specifically the hydraulics of gradually varied open channel flows. He later became a lecturer at the École Centrale des Arts et Manufactures between 1838 and 1864, at the École des Ponts et Chaussées from 1841 to 1855, and at the École Polytechnique from 1851 to 1860. At the École Centrale, one of his students was Gustave Eiffel (1832–1923) who built the Eiffel tower and engraved Bélanger's name around the first floor together with the names of 71 other scientists. Jean-Baptiste Bélanger retired in 1864 and died on 8 May 1874 at Neuilly-sur-Seine where his body was buried. Contribution to open channel hydraulics During his professional career, his 1828 treatise was a major contribution to modern open channel hydraulics. The work was focused on the study of gradually varied open channel flows. The originality of essay was the successful development of the backwater equation for steady, one-dimensional gradually varied flows in an open channel, together with the introduction of the step method, distance calculated from depth, and the concept of critical flow conditions. In 1828, Bélanger understood the rapidly varied nature of the jump flow, but he applied incorrectly the Bernoulli principle to the hydraulic jump. The correct application of momentum considerations to