The allowable headwater depth is the depth of ponded water permitted at the entrance to a culvert. Allowable headwater depths are generally classified as either physical controls or arbitrary controls.
Physical headwater controls are topographic features that should be protected against periodic inundation. They include the roadway pavement and upstream productive property or structures. Additionally, high points between adjacent watersheds should typically be used as physical headwater controls. The use of a watershed break elevation as a headwater control will protect against the unnecessary diversion of runoff from a watershed to an adjacent watershed.
Arbitrary headwater controls are typically used to ensure the efficient operation of the culvert while protecting the roadway embankment from erosion and subsequent washout. The control may be a function of either the design flood or the base flood. Typical limits include a percentage of the barrel diameter or rise such as 1.2D or 120 percent of the barrel diameter or rise, or a permitted depth of ponding above the barrel such as D + 2 ft. Greater depths of ponding for the base flood are typically permitted. Large-span structures usually have more restrictive limitations.
