Context: Urbanization increases runoff, necessitating engineered channels to prevent flooding.

Application: Natural meandering streams in cities are often replaced by straight, concrete-lined channels (e.g., the Los Angeles River). Concrete has a much lower Manning's $n$ roughness coefficient than natural earth, allowing the channel to transport water much faster for the same cross-section and slope. While highly efficient hydraulically, this subcritical-to-supercritical acceleration increases the risk of severe erosion downstream if the energy is not dissipated properly.

Context: Culverts must safely pass stream flows under roads without causing backwater flooding.

Application: A culvert is an open channel that can sometimes flow full as a pipe. Engineers design culverts primarily for inlet control (where capacity is limited by the entrance geometry) or outlet control (limited by barrel friction and tailwater). Determining whether the flow inside the culvert will be subcritical or supercritical is vital. If supercritical, a hydraulic jump might form inside or just downstream of the culvert, requiring energy dissipators like a stilling basin to protect the roadway embankment from scour.