Use of Rock in Erosion Control
Table of Contents
- Types of Erosion Control Systems Requiring Rocks
- Sizing Rocks or Stones
- Shape of the Rocks and Stones
- Gradation of Stones
- Determination of the Median Stone Diameter (D50)
- Placement of the Stone Riprap
- Thickness of a Layer of Stone Riprap
- Filter Media
- Maintenance Requirements
Rock and stone are used as components in many erosion control systems.
They normally absorb the erosive energy in the water and dissipate this
energy to the base below.
Types of Erosion Control Systems Requiring Rocks
- Rock Chute. This structure uses stones to drop a medium volume
of water from one elevation to another. It is usually used in conjunction
with a grassed waterway or open ditch.
- Watercourse Lining. This lining protects the sides and bottom of the
watercourse to prevent erosion or allow for a higher flow. Also, steeper
sideslopes and narrower channels can be considered. Slumping prevention,
due to side seepage, is another use of a watercourse lining.
Figure 1. Stone used as a watercourse lining providing
- Gabion Baskets. Stones are used as the filler for the baskets. The
gabion wire confines the rock which prevents rock movement and allows
for higher velocities of water. Also, steeper slopes can be used which
may be necessary in some projects.
Figure 2. Stones used as filler in gabion baskets.
- Low Level Crossings. Rocks or stones often are used to prevent erosion
from vehicle access through the bottom of a watercourse.
- High Stress Area Protection. Rocks or stones are used to prevent erosion
in specific locations such as under tile drainage outlets, around ditch
bank comers, beside drop structure inlets or on emergency spillways.
- Full protection occurs right after installation. No establishment
period is required as is the case for vegetative linings.
- Stone has some ability to heal itself. If a stone is moved
because of excessive erosive energy, often replacement stones
will roll in from a higher elevation.
- Rocks or stones have the ability to shift or float with base
or foundation movements. This eliminates any voids which may cause
increased stress on the system.
- Stone may be the most economically viable structure, especially
if properly sized and shaped stones are available locally.
- If the rocks or stones have to be transported from a long distance,
the cost can become prohibitive.
- Sometimes it is hard to specify and obtain a specific type
- Movement of rocks or stones from truck access point to the
project can be expensive and could cause compaction especially
under wet conditions.
- High drops, greater than 1.5 meters (5 feet), usually can't
be handled by a rock system.
Sizing Rocks or Stones
- Diameter. The most common method is to grade rocks or stones according
to diameter. Many Ontario Quarries have stone available in gradients
ranging from 50 mm (2") in size to 250 mm (10"). Larger
stone (often called armour rock) normally is available by special
order. Smaller sized stone is available, however, its use in erosion
projects is very limited.
- Weight. Weight is sometimes used in grading or specifying stone
size. However, in many cases, the relationship between weight
and diameter is required to design and complete a project. Table
1 gives an approximate conversion.
Table 1. Relationship between Diameter and Weight
for Angular Shaped Rocks
- rock density of 2400 kg/m3 (150 lb/ft3)
- rock is cubic in shape
- dimension is approximately 10% less than diameter specified
Shape of the Rocks and Stones
Most rocks or stones used in erosion projects are angular or jagged in
shape. This is important to ensure that the stones will "knit"
themselves together. This "knitting" action will tend to tie
the rocks together as a system. If a high erosive force is encountered
on one component (i.e. rock) the force will be distributed by this action
to the whole system.
Rounded stones commonly found on most farm fields will not be effective
in most erosion projects. If they are used, special provisions will be
required to ensure that they work effectively.
Broken concrete can substitute for rocks or stones provided that they
have an angular shape and a size relatively similar to the rock size required.
This concrete must be resistant to weathering, frost action and water
action. Additional debris such as reinforcing rods should be removed or
accounted for in the design.
The rocks, stones or concrete should not have a breadth or thickness
less than one-third of its length.
Gradation of Stones
A dense mass of well graded randomly placed angular shaped stones has
been proven to be much superior to a mass of large stones with correspondingly
large voids. In other words it is important that a range of stone sizes
be used in a project. This will fill the voids and ensure that the stones
will "knit" themselves together.
To specify a gradation one first has to select a D50 which stands for
the median stone diameter. In other words 50% of the total weight of stone
will have a size greater than the D50 and 50% of the total weight will
have a size smaller than the D50. This D50 is usually used to order the
stone size (see previous section on stone size).
Table 2 shows the approximate ratios of size required to give a good
Table 2. Gradation of Rock and Stone
|Size of Stone
||Percent of total weight smaller than the given size
K is the specified D50 size
From Ministry of Transport and Communications article on Rock RipRap 1978
The above table states that, for example, if a D50 rock = 200 mm (8")
or 1.6 kg (3.5 lb) is required, 10% of the rocks will weigh 1.6 kg (3.5
lb) or less, 50% of the rocks should weigh 16 kg (35 lb) or less, 80%
of the rocks should weigh 32 kg (75 lb) or less and all the rocks should
weigh less than 48 kg (105 lb). One must always remember that this is
only an ideal gradation. In actual practice it is impossible to obtain
this exact gradation, therefore, one can only strive to get as close as
Determination of the Median Stone Diameter (D50)
Other publications, available from the Ontario Ministry of Agriculture,
Food and Rural Affairs, have charts relating stone size to the waterflow
Placement of the Stone Riprap
Studies by the United States Corps of Engineers have shown that hand
placed riprap was not any better than an equivalent thickness of dumped
and shaped riprap. Therefore, placement methods should depend on the size
of job, access of equipment and availability of necessary labour.
Thickness of a Layer of Stone Riprap
Usually the thickness of a riprap layer should be 1.5-2.0 times the diameter
of the largest stone size but not less than 0.5m (18"). If the individual
design specifies a different thickness it should be followed.
A filter medium is normally required under the stone riprap to prevent
losses of the base material caused by surface or ground water flow through
the riprap. This medium can either be a granular filter layer or a synthetic
Maintenance of stone or rock structures is required to ensure a long life
expectancy. Steps should be as follows.
- Inspect the structure frequently especially after a large storm.
Check to see if any movement of the rock has taken place. The
void created by the displaced or shifted rock may have been filled
by rocks rolling in from a higher elevation. If this has occurred
the rock bank at the top of the structure will have to be refilled
periodically. Also, one must watch for voids in the rocks which
have not been refilled. If these occur regularly changes should
be made to the structure before complete failure occurs.
- Watch for foundation or base movement. This is usually caused
by a poorly placed or damaged filter material. Often another filter
cutoff will have to be installed to prevent water from tracking
under the filter material.
- Take care of other erosion and/or drainage systems above and
below this structure. Failure of these may cause failure of the
rock structure due to undermining, siltation or increased flow.
Qualified erosion control contractors and consulting engineers are available
for assistance in the design and construction of erosion control structures.
Conservation Authorities may offer technical assistance and construction
supervision in some areas of the province. Other erosion control factsheets
and information are available from the Ontario Ministry of Agriculture,
Food and Rural Affairs.
All erosion control projects must comply with existing legislation, i.e.
Lakes and Rivers Improvement Act, Fisheries Act, etc.