Hydraulic Design of Highway Culverts - DOT On-Line Publications

Skewed inlets slightly reduce the hydraulic performance of the culvert under inlet control
conditions. The differences are minor and are incorporated into the inlet control nomographs for
box culverts (Charts 11 and 12). As an illustration of the minor effects of inlet skew,
comparisons of flow capacity were made on a single barrel 1829 mm by 1829 mm (6 ft by 6 ft)
box culvert with various inlet skew angles operating in inlet control (Table 7).
Table 7a--Effect of Inlet Skew Angle on Flow Capacity of
1829 mm x 1829 mm Box Culvert. 1
(Flow in m 3 /s (ft 3 /s))
SKEW ANGLE, DEGREES
HEADWATER 0 o 15 o 30 o 45 o
0.91 m
1.83 m
2.74 m
2.40 m 3 /s
6.796 m 3 /s
11.214 m 3 /s
1 Values from Chart 11A, Appendix D
2.407 m 3 /s
6.626 m 3 / s
8.382 m 3 /s
161
2.322 m 3 /s
6.456 m 3 /s
11.044 m 3 /s
Table 7b--Effect of Inlet Skew Angle on Flow Capacity of
(6 ft by 6 ft) Box Culvert. 1
(Flow in ft 3 /s)
2.265 m 3 /s
6.286 m 3 /s
10.874 m 3 /s
SKEW ANGLE, DEGREES
HEADWATER 0 o 15 o 30 o 45 o
3 ft
6 ft
9 ft
85 ft 3 /s
240 ft 3 /s
396 ft 3 /s
1 Values from Chart 11B, Appendix D
85 ft 3 /s
234 ft 3 /s
296 ft 3 /s
82 ft 3 /s
228 ft 3 /s
390 ft 3 /s
80 ft 3 /s
222 ft 3 /s
384 ft 3 /s
Inlet skew should be avoided for culverts with tapered inlets and for multiple barrel culverts.
Structural design complications result when a tapered inlet is skewed (26). Both tapered inlets
and multiple barrel culverts perform better with the inlet face oriented normal to the barrel. The
interior walls of multiple barrel culverts may promote sedimentation and unequal flow in some
barrels when the inlet is skewed. The embankment fill should be warped to fit the culvert when
avoiding inlet skew (Figure VI-20).
3. Multiple Barrels. Multiple barrel culverts may be necessary due to certain site conditions,
stream characteristics, or economic considerations (Figure VI-21). Roadway profiles with low
fills often dictate the use of a series of small culverts. Multiple barrel culverts are also used in
wide, shallow channels to limit the flow constriction. To accommodate overbank flood flows,
relief culverts with inverts at the flood plain elevation are occasionally used. Multiple barrel box
culverts are more economical than a single wide span because the structural requirements for
the roof of the long span are costly.