When the head losses in a culvert are critical, the PEO may consider the use of a hydraulically improved inlet. Contact the RHE for guidance when considering using a hydraulically improved inlet. These inlets provide side transitions as well as top and bottom transitions that have been carefully designed to maximize the culvert capacity with the minimum amount of headwater; however, the design and form construction costs can become quite high for hydraulically improved inlets. For this reason, their use is not encouraged in routine culvert design. It is usually less expensive to simply increase the culvert diameter by one or two sizes to achieve the same or greater benefit.
Certain circumstances may justify the use of an improved inlet. When complete replacement of the culvert is too costly, an existing inlet-controlled culvert may have its capacity increased by an improved inlet. Improved inlets may also be justified in new construction when the length of the new culvert is long (more than 500 feet) and the headwater is controlled by inlet conditions. Improved inlets may have some slight advantage for barrel- or outlet-controlled culverts, but usually not enough to justify the additional construction costs. If the PEO believes that a site might be suitable for an improved inlet, the RHE shall be contacted. Also, HDS-5 contains a significant amount of information related to the design of improved inlets.
