Openness Ratio Spreadsheet is a 1-page legal document that was released by the United States Army Corps of Engineers on January 4, 2006 and used nation-wide.

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New England District, U.S. Army Corps of Engineers, Regulatory Division
● Openness Ratio (OR) is calculated by dividing a culvert's cross-sectional area by its length: OR = x-sec area ÷ length
● Along with other criteria, the MA PGP, General Condition 21, states that to qualify for the Category 1 (non-reporting):
1. New permanent stream crossings must have an OR ≥.25
OR =.25 =
(x-sec area)
or
OR =.25 = [(x-sec culvert area pre-embed) - embedded area]
culvert length
culvert length
2. Round culverts must be embedded at least 25%
● The chart below provides the diameter needed to meet the .25 OR for various culvert lengths, accounting for the
embedded area, when embedding 25%. All calculations must be done in meters. The conversion to feet is provided.
● The Corps uses the .25 OR as a guide for Category 2 projects reviews.
Derivation
Required Diameter for .25 OR & 25% Embedment
1. The MA PGP, Category 1 requires:
Required
Required
Length
* 25% culvert embedment (=.25 x culvert diameter)
Open Area
Diameter
2
2
* .25 openness ratio
(FT
)
(M
)
(FT)
(M)
(FT)
(M)
6
1.83
4.92
0.46
2.79
0.85
8
2.44
6.56
0.61
3.23
0.98
2. Embedded area calculated as follows:
10
3.05
8.20
0.76
3.61
1.10
12
3.66
9.84
0.91
3.95
1.21
14
4.27
11.48
1.07
4.27
1.30
16
4.88
13.12
1.22
4.56
1.39
18
5.49
14.76
1.37
4.84
1.48
20
6.10
16.40
1.52
5.10
1.56
22
6.71
18.04
1.68
5.35
1.63
24
7.32
19.68
1.83
5.59
1.70
26
7.93
21.32
1.98
5.82
1.77
28
8.54
22.96
2.13
6.04
1.84
30
9.15
24.60
2.29
6.25
1.91
32
9.76
26.24
2.44
6.45
1.97
34
10.37
27.88
2.59
6.65
2.03
36
10.98
29.52
2.74
6.85
2.09
Notes:
38
11.59
31.16
2.90
7.03
2.14
o
40
12.20
32.80
3.05
7.22
2.20
* a = 60
= 1.05 radians, if h =.25D
* A
= embedded area
42
12.80
34.44
3.20
7.39
2.25
1
44
13.41
36.08
3.35
7.57
2.31
46
14.02
37.72
3.51
7.74
2.36
3. OR =.25 = [(x-sec culvert area pre-embed) - embedded area]
48
14.63
39.36
3.66
7.90
2.41
culvert length
50
15.24
41.00
3.81
8.07
2.46
where:
2
* x-sec area =
D
/ 4
2
* embedded area for 25% embed = .62D
/4 = (A
)
1
* culvert length = L
Therefore:
2
2
=
.25
D
/4 - .62D
/4
L
or
1/2
D = .63L
1/4/2006
New England District, U.S. Army Corps of Engineers, Regulatory Division
● Openness Ratio (OR) is calculated by dividing a culvert's cross-sectional area by its length: OR = x-sec area ÷ length
● Along with other criteria, the MA PGP, General Condition 21, states that to qualify for the Category 1 (non-reporting):
1. New permanent stream crossings must have an OR ≥.25
OR =.25 =
(x-sec area)
or
OR =.25 = [(x-sec culvert area pre-embed) - embedded area]
culvert length
culvert length
2. Round culverts must be embedded at least 25%
● The chart below provides the diameter needed to meet the .25 OR for various culvert lengths, accounting for the
embedded area, when embedding 25%. All calculations must be done in meters. The conversion to feet is provided.
● The Corps uses the .25 OR as a guide for Category 2 projects reviews.
Derivation
Required Diameter for .25 OR & 25% Embedment
1. The MA PGP, Category 1 requires:
Required
Required
Length
* 25% culvert embedment (=.25 x culvert diameter)
Open Area
Diameter
2
2
* .25 openness ratio
(FT
)
(M
)
(FT)
(M)
(FT)
(M)
6
1.83
4.92
0.46
2.79
0.85
8
2.44
6.56
0.61
3.23
0.98
2. Embedded area calculated as follows:
10
3.05
8.20
0.76
3.61
1.10
12
3.66
9.84
0.91
3.95
1.21
14
4.27
11.48
1.07
4.27
1.30
16
4.88
13.12
1.22
4.56
1.39
18
5.49
14.76
1.37
4.84
1.48
20
6.10
16.40
1.52
5.10
1.56
22
6.71
18.04
1.68
5.35
1.63
24
7.32
19.68
1.83
5.59
1.70
26
7.93
21.32
1.98
5.82
1.77
28
8.54
22.96
2.13
6.04
1.84
30
9.15
24.60
2.29
6.25
1.91
32
9.76
26.24
2.44
6.45
1.97
34
10.37
27.88
2.59
6.65
2.03
36
10.98
29.52
2.74
6.85
2.09
Notes:
38
11.59
31.16
2.90
7.03
2.14
o
40
12.20
32.80
3.05
7.22
2.20
* a = 60
= 1.05 radians, if h =.25D
* A
= embedded area
42
12.80
34.44
3.20
7.39
2.25
1
44
13.41
36.08
3.35
7.57
2.31
46
14.02
37.72
3.51
7.74
2.36
3. OR =.25 = [(x-sec culvert area pre-embed) - embedded area]
48
14.63
39.36
3.66
7.90
2.41
culvert length
50
15.24
41.00
3.81
8.07
2.46
where:
2
* x-sec area =
D
/ 4
2
* embedded area for 25% embed = .62D
/4 = (A
)
1
* culvert length = L
Therefore:
2
2
=
.25
D
/4 - .62D
/4
L
or
1/2
D = .63L
1/4/2006