COBBLE OR
GRAVEL ARMORS
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| COBBLE OR
GRAVEL ARMORS
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1. CATEGORY
2.0 – Bank Armor and Protection
2. DESIGN STATUS
Level II
3. ALSO KNOWN AS
River Stone Blanket.
4. DESCRIPTION
Riprap Revetment is the most widely used form of streambank protection. Although properly designed riprap revetments can be quite effective, the color and angularity of quarried stone is not a natural component of stream corridors. Cobbles are natural stones larger than 6.5 cm (2.5 in.) in diameter that have been rounded by the abrasive action of flowing water, while gravel is material smaller than cobble, but larger than sand (larger than about 5 mm) (0.2 in). Rounded river cobble or gravel blanket presents a more natural appearance, and can be as effective as riprap.
5. PURPOSE
Cobble or Gravel Armor is used to protect a sloping bank against fluvial entrainment by flow in the stream or over the top of the bank. Use of natural materials with colors and shapes consistent with the local environment make the resulting structure visually unobtrusive.
6. PLANNING
Useful for Erosion Processes:
Toe erosion with upper bank failure Scour of middle and upper banks by currents Local scour Erosion of local lenses or layers of noncohesive sediment Erosion by overbank runoff General bed degradation Headcutting Piping Erosion by navigation waves Erosion by wind waves Erosion by ice and debris gouging General bank instability or susceptibility to mass slope failure
Spatial Application:
Instream Toe Midbank Top of Bank
Hydrologic / Geomorphic Setting
Resistive Redirective Continuous Discontinuous Outer Bend Inner Bend Incision Lateral Migration Aggradation Conditions Where Practice Applies:
HEC-23 (Lagasse et al., 1997) notes that riprap is useful for controlling lateral instability, but only finds secondary application in controlling vertical instability. Bank slope may be steep or flat but not vertical. Site conditions must allow for a 1V:3H slope.
Complexity:
Low.
Design Guidelines / Typical Drawings:
Granular materials must be carefully sized to avoid removal during flows that generate high shear stresses. The compilation of Shields constants presented by Buffington and Montgomery (1997) may prove helpful as it includes many values from natural streams (as opposed to laboratory flumes), and extensive information regarding the collection and derivation of each value of dimensionless critical shear stress. Gravel is typically too small for toe protection, but is often incorporated in mid- or upper-bank treatments (Henderson and Shields, 1984). Cobble or Gravel Armor is generally designed and constructed in a fashion similar to riprap. Cobbles should have hardness and resistance to degradation similar to good quality quarried riprap.
Design details for the toe and upstream and downstream ends of cobble revetment are similar to riprap revetment. These details are quite critical, as failures often originate at the endpoints or toe of the structure.
7. ENVIRONMENTAL CONSIDERATIONS / BENEFITS
Although properly designed riprap revetments can be quite effective, the
color and angularity of quarried stone is not a natural component of stream
corridors. Because cobble and gravel are naturally found in river environments,
they are less conspicuous and more aesthetically pleasing than riprap. Cobble
and gravel may produce some aquatic benefits by providing stable substrate
for certain types of benthic macroinvertebrates and spawning habitats for
some species of fish. However, gravel or cobble spawning habitat is often
degraded if the material is not periodically disturbed by flow to flush away
finer sediments that fill interstices in the coarse matrix. Since protection
measures must be stable at design flow, flushing of gravel or cobble may
not occur.
Adverse environmental effects could be caused by the disturbances associated with cobble or gravel mining. Since bank slope for cobble revetment must be more gradual than for riprap revetment, additional area must be sacrificed along the top bank, leading to loss of riparian habitats. On the other hand, volunteer vegetation establishes more readily on the more gradual slope, and such vegetation is not deleterious to revetment performance, although there are inspection, access, and conveyance issues in some cases (Shields, 1991). The effects of vegetation on flow capacity (conveyance) of river channels are discussed in the Special Topic: Management of Conveyance.
8. HYDRAULIC LOADING
Permissible shear and velocity for stone structures are related to the size
of rock used in construction. Other factors, such as the angularity of the
stone, the thickness of the layers of stone that comprise the structure, and
the angle at which the faces of the stone structure are constructed also come
into play. See comments regarding stone sizing in the section below on materials
and equipment.
9. COMBINATION OPPORTUNITIES
Cobble or Gravel Armor may be combined with vegetation on the upper bank
and various structures for toe protection (Henderson & Shields, 1984).
10. ADVANTAGES
Cobble and gravel are usually more natural looking than limestone riprap.
11. LIMITATIONS
Revetments do not provide bed erosion control, and do not provide protection
against geotechnical failures of bank slopes. Limited protection is provided
against subsurface erosion phenomena such as piping. Rounded cobbles do not
interlock as well as angular stones, and therefore the bank slope must be
decreased to no more than 1V:3H to achieve stone stability under design
flow conditions (Mifkovic and Petersen, 1975).
12. MATERIALS AND EQUIPMENT
Construction equipment is the same as is needed for building quarry stone
structures. Washing and grading equipment will be needed if the natural stone
must be obtained from local deposits. Detailed guidance for designing stone
structures for bed and bank stabilization structures is beyond the scope of
this guideline, and many approaches are available (see Special Topic: Designing
Stone Structures).
Use of riprap larger than 1 m (3 ft) in diameter is unusual, and in most cases,
impractical. Maynord (1995) in Escarameia (1998) presents equations for sizing
stone that produce sizes 30% larger for rounded stone than for angular riprap.
13. CONSTRUCTION / INSTALLATION
When Cobble Armor is constructed, banks must be graded to a stable slope
and geotextile or filter layers of finer granular materials may be required
if leaching of fine materials is a problem (see Special Topic: Geotextiles
and Root Penetration).
Local sources of river cobble are required for cost efficiency. If local sources are available, cobble may be cost-competitive with riprap. Typical costs for stone structures range from $20 to $40 per metric ton ($20 to $40 per U.S. ton), including costs for placement, but this neglects costs for site grading, filter layers, etc.
15. MAINTENANCE / MONITORING
HEC-23 (Lagasse et al., 1997) indicates that riprap revetment requires
a moderate commitment of resources for maintenance. Revetments must be
periodically inspected and stone must sometimes be replaced. If the underlying
bank experiences slope failure, regrading or sometimes installation of
measures to address subsurface flow must be applied. Some engineers recommend
periodic removal of vegetation growing on or through a revetment, but available
evidence suggests that vegetated revetments, although they are more difficult
to visually inspect, are just as durable as revetments maintained free
of vegetation (Shields, 1991).
16. COMMON REASONS / CIRCUMSTANCES FOR FAILURE
Cobble revetments are susceptible to the same failure mechanisms as riprap
revetments, such as toe scour, flanking, and removal of the underlying bank
material by leaching or slope instability. In addition, direct removal of individual
stones by flow, especially in turbulent settings that feature impinging flow,
is a hazard.
17. CASE STUDIES AND EXAMPLES
Cobble was used extensively for revetment along the Sacramento River prior
to about 1970, and about two-thirds of the revetments found along a 56 km (35
mile) reach in the late 1980s were made of cobble (Shields, 1991). The cobble
revetments were no more or less likely to sustain damage during a large flood
than riprap revetments. Many of the cobble revetments exhibited damage when
inspected in the late 1970s, but these damages appeared to be related to the
lack of adequate toe design, the cumulative scour since placement, and the
lack of interlocking between cobbles. Damage did not appear to be related
to the presence or absence of woody vegetation (Water and Engineering Technology,
1989).
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 Courtesy Santa Clara Valley Water District |
 Courtesy Santa Clara Valley Water District |
 Courtesy Santa Clara Valley Water District |
 The same site in April of 2004 |
18. RESEARCH OPPORTUNITIES
The value of Cobble Armor as habitat for fish and macroinvertebrates
relative to quarry stone riprap revetment has not been established.
19. REFERENCES
Buffington, J. M., & Montgomery, D. R. (1997). A systematic analysis
of eight decades of incipient motion studies, with special reference to
gravel-bedded rivers. Water Resources Research, 33(8), 1993-2029.
Escarameia, M. (1998) River and channel revetments. Thomas Telford,
Ltd., London.
Henderson, J. E., & Shields, F. D., Jr. (1984). Environmental
Features for Streambank Protection Projects. (Technical Report E-84-11),
US Army Engineer Waterways Experiment Station, Vicksburg, MS.
Lagasse, P. F., Byars, M. S., Zevenbergen, L. W. & Clopper, P. E. (1997). Bridge scour and stream instability countermeasures: Experience, selection and design guidance. (Hydraulic Engineering Circular No. 23, FHWA HI 97-030), Washington, D. C., pp. 1.3-1.11. (pdf)
Maynord, S. T. (1995). Corps riprap design guidance for channel protection.
In C. R. Thorne, S. R. Abt, F. B. J. Barends, S. T. Maynord, and K. W.
Pilarczyk. (eds.). River, coastal and shoreline protection: erosion
control using riprap and armourstone. John Wiley & Sons, Ltd.,
Chichester, U. K., 41-42.
Mifkovic, C. S., & Petersen, M. S. (1975). Environmental Aspects, Sacramento
Bank Protection. Journal of the Hydraulics Division, American
Society of Civil Engineers, Vol. 101, No. HY5, 543-554.
Shields, F. D., Jr., (1991). Woody Vegetation and Riprap Stability Along
the Sacramento River Mile 84.5 to 119. Water Resources Bulletin 27(3):
527-536. (pdf)
Water and Engineering Technology. (1989). Geomorphic analysis and bank
protection alternatives report for Sacramento River (RM 178-178) and Feather
River (RM 0-28). Prepared for the U.S. Army Corps of Engineers Sacramento
District, Sacramento California. Water and Engineering Technology, Inc.,
Fort Collins, Colorado.
