The hydraulic spillway or spillway is a hydraulic structure intended to promote the free or controlled passage of water in surface runoff, with the spillway being exclusively for drainage and not for measurement. There are various types depending on the form and use made of them, sometimes in a controlled manner and other times as a safety measure in case of storms in dams.

Spillway as a dam element

It has several purposes, among which the following stand out:

In a dam "Dam (hydraulic)") the part of the structure that allows the evacuation of water is called weir, either on a regular basis or to control the level of the water reservoir.

Generally, the waters close to the free surface of the reservoir are discharged, as opposed to the bottom discharge, which allows the controlled exit of water from the deep strata of the reservoir.

Weir as a channel element

Weirs are used in conjunction with floodgates to maintain a navigable river or to provide the level necessary for navigation. In this case, the spillway is built significantly longer than the width of the river, forming a "U" or making diagonals, perpendicular to the passage. Since the spillway is the part where the water overflows, a grand and majestic spillway allows a greater amount of water to pass through with a small increase in the depth of the spill. This is done in order to minimize fluctuations in the upstream river level.[1]​.

Weirs allow hydrologists a simple method to measure discharge in water flows. Knowing the geometry of the upper area of ​​the spillway and the water level above the spillway, it is known that the liquid goes from a slow to a fast regime, and above the thick-walled spillway, the water adopts the critical draft.

Landfills are widely used in rivers to maintain the water level and be used as lakes, navigation and recreation areas. Hydraulic mills usually use dams to raise the water level and take advantage of the rise to move the turbines.

Because a weir increases the oxygen content of the water passing over the crest, it can have a beneficial effect on the local ecology of the river. A dam artificially reduces the speed of water, which can increase sedimentation processes upstream; and an increase in downstream erosion capacity. The dam where the spillway is located, by creating a gap, represents a barrier for migratory fish, which cannot jump between levels.

See Thin-walled weir.

Landfills can be classified in several ways: