Illinois Urban Manual
PRACTICE STANDARD
ROCK OUTLET PROTECTION
(no.) CODE 910
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(Source: USDA - Natural Resources Conservation Service - Illinois)
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DEFINITION
A section of rock protection placed at the outlet end of culverts, conduits,
or channels.
PURPOSE
The purposes of this practice are to prevent scour erosion at stormwater
outlets, to protect the outlet structure, and to minimize the potential for
downstream erosion by reducing the velocity and energy of concentrated
stormwater flows. The practice also reduces the effects of turbidity and
sedimentation downstream.
CONDITIONS WHERE PRACTICE APPLIES
This practice applies where discharge velocities and energies at the outlets
of culverts, conduits, or channels are sufficient to erode the receiving channel
or area. This applies to:
- Culvert outlets of all types.
- Pipe conduits from all sediment basins, dry and wet basin detention storm
water ponds.
- New channels constructed as outlets for culverts and conduits.
- Where outflows from conduits or channels do not exceed 10 fps.
The design of structurally lined aprons at the outlets of pipes and paved
channels sections applies to the immediate area or reach below the pipe or
channel and does not apply to continuous rock linings of channels or streams.
The design of rock outlet protection depends entirely on the location. Pipe
outlets at the top of cuts or on slopes steeper than 10 percent cannot be
protected by rock aprons or riprap sections due to re-concentration of flows and
high velocities encountered after the flow leaves the apron.
CRITERIA
Tailwater depth - Depth of tailwater immediately below the pipe outlet must
be determined for the design capacity of the pipe. Manning's Equation may be
used to determine tailwater depth. If the tailwater depth is less than half the
diameter of the outlet pipe and the receiving stream is wide enough to accept
divergence of the flow, it shall be classified as a Minimum Tailwater Condition.
If the tailwater depth is greater than half the pipe diameter and the receiving
stream will continue to confine the flow, it shall be classified as a Maximum
Tailwater Condition. Pipes which outlet onto a flat area with no defined channel
will be assumed to have a Minimum Tailwater Condition.
Apron length - Apron length (La) shall be determined from
Table 2 according to the appropriate tailwater
condition and velocity out of the conduits.
Apron width - When the pipe discharges directly into a well-defined channel,
the apron shall extend across the channel bottom and up the channel banks to an
elevation one foot above the maximum tailwater depth or to the top of the bank,
whichever is less.
If the pipe discharges onto a flat area with no defined channels, the
following criteria will be followed. Apron width will be 3 times the pipe
diameter at the upstream location. The downstream width will be the pipe
diameter plus the apron length for pipes with minimum tailwater conditions and
the pipe diameter plus 0.4 times the apron length for pipes flowing under
maximum tailwater conditions.
Bottom grade - The outlet protection apron shall be constructed with no slope
(0.0% grade) along its length. There shall be no overfall at the end of the
apron. The elevation of the downstream end of the apron shall be equal to the
elevation of the receiving channel or adjacent ground.
Alignment - Outlet protection apron shall be located so that there are no
bends in the horizontal alignment.
Materials - Outlet protection may be done using rock riprap, concrete block
or gabions.
Thickness - The maximum thickness of the riprap layer shall be 1.5 times the
maximum stone diameter for d50 of 15 inches or less; and 1.2 times the maximum
stone size for d50 greater than 15 inches. Table 1
lists some examples.
Stone quality - Stone for riprap shall consist of field stone or rough unhewn
quarry stone. The stone shall be hard and angular and a quality that will not
disintegrate on exposure to water or weathering. The specific gravity of the
individual stones shall be at least 2.5.
The median size stone for riprap shall be determined from
Tables 1 and 2
for the material specified for the tailwater condition present. The placement of
the riprap shall conform to construction specification
61 LOOSE ROCK RIPRAP.
Recycled concrete equivalent may be used provided it has a density of at
least 150 pounds per cubic foot, and does not have any exposed steel or
reinforcing bars.
Filter Fabric - In all cases, filter fabric shall be placed between the
riprap and the underlying soil to protect soil movement into and through the
riprap. The material must meet or exceed requirements specified in material
specification 592 GEOTEXTILE,
Table 1 or 2, class I, II, or III.
Gabions - shall be made of hexagonal triple twist mesh with heavily
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 sizes. The mats shall be a minimum of 9 inches thick.
The area on which the gabion is to be installed shall be graded as shown on
the drawings. Foundation conditions shall be the same as for placing rock
riprap. Where required, a cut off may be needed to prevent undermining of the
main gabion structure. Gabions shall meet the requirements for installation as
shown in construction specification 64 WIRE MESH GABIONS.
CONSIDERATIONS
The outlets of channels, conduits and other structures are points of high
erosion potential, because they frequently carry flows at velocities that exceed
the allowable limit for the area downstream. To prevent scour and undermining,
an outlet stabilization structure is needed to absorb the impact of the flow and
reduce the velocity to non-erosive levels. A riprap-lined apron is the most
commonly used practice for this purpose because of its relatively low cost and
ease of installation. The riprap apron should be extended downstream until
stable conditions are reached even though this may exceed the length calculated
for design velocity control.
Riprap-stilling basins or plunge pools reduce flow velocity rapidly. They
should be considered in lieu of aprons where overfalls exit at the ends of pipes
or where high flows would require excessive apron length. Consider other energy
dissipaters such as concrete impact basins or paved outlet structures when
conduits are flowing more than 10 fps. These will require a special design and
table 2 cannot be used.
PLANS AND SPECIFICATIONS
Plans and specification for installing rock outlet protection shall be in
keeping with this standard and will describe the requirements for applying the
practice to achieve its intended purpose. At a minimum include the following:
- Location where the practice will be installed.
- Dimensions of the practice.
- Plan view, profile and cross section of each channel reach between the
storm drain outlet under consideration and the existing publicly maintained
system or the natural stream channel receiving the discharge flow.
- Rock size.
- Rock thickness.
- Fabric specifications.
All plans shall include the installation, inspection, and maintenance
schedules with the responsible party clearly identified.
Standard drawing IL-610 PIPE OUTLET TO FLAT AREA or
IL-611 PIPE OUTLET TO
CHANNEL may be used as the plan sheet.
OPERATION AND MAINTENANCE
Inspect riprap 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.
NRCS IL
August 1994
TABLE 1
ROCK RIPRAP SIZES AND THICKNESS
IDOT Gradation Number |
d50 (in.) |
dmax (in.) |
Minimum Blanket Thickness (in.) |
RR-3 1/ |
5 |
10 |
15 |
RR-4 |
9 |
14 |
20 |
RR-5 |
12 |
19 |
28 |
RR-6 |
15 |
22 |
32 |
RR-7 |
18 |
27 |
32 |
TABLE 2
Minimum IDOT Rock Sizes and Apron Length for Maximum and Minimum Tailwater
Conditions
Culvert
Dia. (in.) |
Minimum Tailwater |
Maximum Tailwater |
5 fps 1/ |
10 fps 1/ |
5 fps 1/ |
10 fps 1/ |
Rock
Gradation |
La (ft.) |
Rock
Gradation |
La (ft.) |
Rock
Gradation |
La (ft.) |
Rock
Gradation |
La (ft.) |
12 |
No. 3 |
10 |
No. 3 |
12 |
No. 3 |
12 |
No. 3 |
15 |
18 |
No. 3 |
14 |
No. 4 |
16 |
No. 3 |
12 |
No. 3 |
16 |
24 |
No. 3 |
16 |
No. 4 |
20 |
No. 3 |
14 |
No. 4 |
17 |
30 |
No. 3 |
18 |
No. 4 |
22 |
No. 3 |
16 |
No. 4 |
20 |
36 |
No. 4 |
20 |
No. 5 |
24 |
No. 3 |
16 |
No. 4 |
22 |
48 |
No. 4 |
24 |
No. 6 |
28 |
No. 4 |
20 |
No. 4 |
24 |
60 |
No. 5 |
32 |
No. 6 |
36 |
No. 4 |
22 |
No. 5 |
26 |
72 |
No. 6 |
40 |
No. 6 |
44 |
No. 5 |
24 |
No. 5 |
29 |
96 |
No. 7 |
50 |
No. 7 |
54 |
No. 5 |
26 |
No. 5 |
32 |
1/ Maximum conduit velocity fps.
Rock Gradation source: Illinois Department of Transportation, 1997.
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