Hydraulic Design of Highway Culverts - DOT On-Line Publications

Figure VI-30--Safety Grate Flush with Culvert Entrance
Safety grates promote debris buildup and the subsequent reduction of hydraulic performance.
Thorough analysis of this potential should be undertaken prior to the selection of this safety
alternative. Good design practice provides an open area between bars of 1.5 to 3.0 times the
area of the culvert entrance depending on the anticipated volume and size of debris. Bar grates
placed against the entrance of the culvert are unacceptable (Figure VI-30). Reference (47)
indicates that the head loss due to a bar grate can be estimated as follows.
H
2 ⎛ Vg
− V
= 1.
5⎜
⎜
⎝ 2g
⎞
⎟
⎠
g
2
u
(24)
Hg is the head loss due to the bar grate, m (ft)
Vg is the velocity between the bars, m/s (ft/s)
Vu is the approach velocity, m/s (ft/s)
g is acceleration of gravity 9.81 m/s 2 (32.2 ft/s 2 )
Another formula for the head loss in bar racks with vertical bars is found in reference (48) (49).
2
⎛ W ⎞⎛
V ⎞ u
Hg = K g ⎜ ⎟⎜
⎟ sinθ
g
⎝ X ⎠
⎜ 2g
⎟
(25)
⎝ ⎠
Kg is a dimensionless bar shape factor, equal to:
2.42 - sharp-edged rectangular bars
1.83 - rectangular bars with semi-circular upstream face
1.79 - circular bars
1.67 - rectangular bars with semi-circular upstream and downstream faces
w is the maximum cross-sectional width of the bars facing the flow, m (ft)
x is the minimum clear spacing between bars, m (ft)
θg is the angle of the grate with respect to the horizontal, degrees
170