Entrance losses are the loss of hydraulic energy that occurs as water flows from an upstream condition, such as a channel, basin, or open area, into a closed conduit or structure, such as a pipe or culvert. In stormwater management and hydraulics, entrance losses represent a reduction in total head caused by turbulence, flow contraction, and changes in velocity and flow direction at the point of entry.
These losses occur because water must adjust from relatively unconfined or differently shaped flow conditions into the confined geometry of a pipe or structure. As flow enters the conduit, it often accelerates, contracts, and becomes more turbulent, dissipating energy in the process. The magnitude of entrance losses depends on factors such as the shape and alignment of the entrance, the roughness of the surfaces, the degree of flow contraction, and whether the entrance is sharp-edged or smoothly transitioned.
Well-designed inlet structures, such as flared end sections or properly constructed headwalls, can reduce entrance losses by providing a smoother transition for flow into the pipe. In contrast, abrupt or poorly aligned entrances increase turbulence and energy dissipation, reducing hydraulic efficiency and potentially limiting the capacity of the system.
Entrance losses are typically accounted for in hydraulic design using an entrance loss coefficient, which is applied to the velocity head of the flow to estimate the energy loss. Accurately estimating these losses is important for ensuring that culverts, storm drains, and other conveyance systems perform as intended under design flow conditions.
Overall, entrance losses are a key consideration in the design and analysis of stormwater infrastructure, influencing flow capacity, upstream water levels, and the overall efficiency of conveyance systems.