For design thickness of ductile iron culvert pipe based on different types of laying conditions, see ANSI/ASTM A716.
Culverts generally should not be designed to flow full or with the headwater submerging the entrance more often than once every 25 years. On minor, lightly traveled roads, overtopping of the roadway once in a few years may be of no consequence if the embankment is protected. For heavily traveled roads and on railroads, the size of the culvert opening should be such that, while submerging the entrance may occur on rare occasions, overtopping of the roadway will never occur. There is an economic relationship that must be evaluated between 1) providing a large culvert opening; 2) repairing the roadway in case of overtopping; 3) maintenance of culvert and channel; 4) interference with heavy traffic; and 5) safety.
There are various ways to determine the required size of a culvert:
1) Inspection of the old structure at the site, or of structures up and downstream.
2) Use of an empirical (Talbot) formula to determine directly the size of opening required.
3) Use of the Jarvis-Myers formula to ascertain the amount of water reaching the culvert, then a second formula to determine the size of culvert required to carry this amount of water.
4) For culverts using pipes, nomographs have been developed based on Manning's formula.
Determination of the culvert size depends on the flow generated by the upstream watersheds. Various methodologies and modifications thereof have been developed to determine the amount of runoff from any particular rainstorm.
The amount of water a culvert will carry is controlled by several things: slope of the flow line and streambed above and below, elevation of the headwater (at inlet), type of inlet, roughness of culvert interior, and height of the backwater.
If a culvert were the same size as the channel above and below, the design problem would be simple. However, a culvert is usually a constriction in the channel, and during flood flow the water will pond upstream from the culvert. If the culvert has a free discharge and sufficient slope, there will be a drop down of the water surface at the inlet; the velocity increases but the culvert does not run full. The minimum slope of the culvert that allows the maximum discharge of water is the so-called "critical slope."
Flared types of inlets are more efficient hydraulically than are straight headwalls, square edge entrances, and entrances where the culvert projects out into the inlet pond. In other words, a flared or "rounded" entrance admits more water into a culvert with a free outlet and increases its capacity.
A drop inlet may be used where the culvert is to serve a soil-savings dam, or where the difference in elevation at inlet and outlet justifies some protection against high outlet velocities. Drop-inlet culverts normally require considerably larger sizes than are shown by the Talbot formula. The vertical or sloping drop-inlet is usually made the same size as the flatter portion of the culvert.
A single culvert opening is generally the most satisfactory because it offers the least obstruction to flow and to drift. However, if using a single opening causes undesirable ponding of the flood plain upstream, the solution may be a low multiple opening. Problems can arise when the space between the roadway and the streambed is constrained because of other design requirements or because of existing utility lines that might not be easily or cheaply relocated. Under these circumstances culverts, small multiple pipe barrels or small box openings may have to be used. These type structures, though, are easily blocked by debris, are subject to siltation difficulties, and often require frequent maintenance.
The ideal grade line for a culvert is one that produces neither silting nor excessive velocities and scour, one that gives the shortest length, and one that makes replacement simplest.
Velocities as great as 10 ft per second cause destructive scour downstream and to the culvert structure itself unless protected. Safe streambed velocities depend upon streambed materials and may range from 1.5 fps for colloidal fine sands and non-colloidal clays to 5.0 fps for cobbles. Allowable velocities for different soils and rock types are listed in numerous engineering texts. This information can also be obtained from the Soil Conservation Service.
A slope of 1 to 2 percent is advisable to give a gradient equal to or greater than the critical slope, provided the velocity is permissible. In general, a minimum slope of 0.5 ft in 100 ft is recommended to avoid sedimentation.
Precast concrete box culverts are supplied by CON/SPAN[R] Bridge Systems. Gratings for culvert drains are provided by IKG Borden, IKG Industries.