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Rock Outlet Protection Fact Sheet

What is Rock Outlet Protection?

A pad or apron of heavy rock placed at the outlet end of culverts or chutes.

When is Rock Outlet Protection Used?

Rock Outlet Protection is installed where the energy at the outlets of culverts or chutes are sufficient to erode the receiving channel or area. This fact sheet does not apply to continuous rock linings of channels or streams. Pipes that dump water at the top of a slope, or down slopes steeper than 10 percent, or flow at rates greater than 10 feet per second require a site specific design that is beyond the scope of this fact sheet.

How is Rock Outlet Protection Installed?

Apron length: Apron length (La) shall be determined from Table 1.

Apron width: The apron width is based on the diameter of the discharge pipe, (D). The apron width will be 3D at the upstream end (Wu), and the downstream width (Wd) will be equal to (D + La). The apron shall extend across the channel bottom and up side slopes for a minimum height equal to the diameter of the pipe, (D).

Alignment: The apron shall be located so that there are no bends in the horizontal alignment. The apron should be level over its length, and the elevation of the downstream end of the apron must be the same as the elevation of the receiving channel or adjacent ground.

Thickness: The required thickness of a rock riprap apron is shown in Table 1.

Gabions: When a gabion mattress is used it shall be made of double twisted galvanized steel wire. Gabions shall be fabricated in such a manner that the sides, ends, and lid can be assembled at the construction site into mats of the specified dimensions. The mats shall be a minimum of 12 inches thick.

Materials: Outlet protection may be done using rock riprap or gabion mattresses to construct the apron. The rock shall consist of field stone or rough unhewn quarry stone. The stone shall be hard and angular and of a quality that will not disintegrate on exposure to water or weathering. Broken concrete may be used provided it does not have any exposed steel or reinforcing bars, and that it is broken into blocky pieces such that the largest dimension of each piece is no more than 3 times the smallest dimension. The required rock size is shown in Tables 1 and 2. In all cases a geotextile (filter fabric) shall be placed between the apron and the underlying soil to prevent soil movement into and through the riprap.

What Type of Maintenance Does Rock Outlet Protection Require?

Inspect rock outlet structures after heavy rains to see if any erosion around or below the riprap has taken place or if stones have been dislodged. Immediately make all needed repairs to prevent further damage. Remove any debris that has collected on the outlet pad.

FIGURE 1 - TYPICAL DETAIL FOR ROCK OUTLET PROTECTION BELOW A CULVERT

TABLE 1 - Rock Outlet Protection Apron Dimensions

Culvert Diameter
(inches)

d 50 Rock Size
(inches)

Apron Length, La (feet)

Upstream Width, Wu (feet)

Downstream Width, Wd (feet)

Thickness
(inches)

Rock Qty.
(tons)

12

6

12

3

13

18

15

18

9

16

4.5

18

24

20

21

9

18

5

20

24

35

24

9

20

6

22

24

60

30

9

22

7.5

24

24

75

36

12

24

9

27

30

120

42

18

26

10.5

30

36

180

48

18

28

12

32

36

215

 

TABLE 2 - Required Rock Gradation

Smallest dimension in inches

percent of

Gabion
Rock


6"d50


9"d50


12"d50


18"d50

rocks smaller
than size shown

8

12

15

21

30

100

6

9

12

18

24

50-70

4

6

9

12

18

35-50

3

2

3

4

6

2-10