| SOIL AND GRASS COVERED RIPRAP
|
|
1. CATEGORY
2.0 – Bank Armor and Protection
2. DESIGN STATUS
Level II
3. ALSO KNOWN AS
Vegetated riprap, however the term vegetated riprap may also refer to revetments
allowed to revegetate naturally, or those planted with woody species in spaces
between stones.
4. DESCRIPTION
A layer of stone and/or boulder armoring that is vegetated using seeding or sodding methods, directly following construction. The goal of this method is to improve the aesthetic appearance of riprap, provide habitat for some aquatic animals, establish a medium to enable voluntary native plant establishment, and with the use of grass and forb roots, bind the revetment to the bank to reduce failures.
 |
 |
| Riprap naturally-vegetated with grasses and willows. Pend Oreille River, Usk, WA |
Non-vegetated riprap provides limited habitat benefits. Lower Mendenhall River, Juneau, AK |
5. PURPOSE
Soil and grass covered riprap combines the widely accepted, resistive, and continuous rock revetment techniques with aesthetically pleasing groundcover such as Sedges, Rushes, and Grasses. The riprap is intended to resist hydraulic forces when significant events occur, while the plants will provide some shade (reducing thermal pollution), will improve aesthetic value of the channel; and will create habitat for insects, soil invertebrates, and a few terrestrial animals. The soil-filled interstices that occur as a result of adding soil allow rooting to occur within the riprap, and may be critical to long-term survival of the vegetation and anchoring.
6. PLANNING
Useful for Erosion Processes:
Toe erosion with upper bank failure (Only useful if stream is intermittent, allowing establishment of terrestrial vegetation at bank toe) 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 lists the following river environments as suitable for ordinary riprap. Presumably, soil-covered riprap would also be effective under the same environmental conditions. However, soil covered riprap may not be suitable for conditions in which the soil would be highly susceptible to scour.
Characteristic
Riprap
Soil-covered Riprap
All
All
Avoid sites with impinging flows
1 to 2 m/sec (4 to 6 ft/sec), up to 2.5 m/sec (8 ft/sec) with establishment of grasses tolerant of inundation
All
All
2.5V:1H, slightly steep to flat
All
Moderate, may require irrigation Covering riprap with soil and grass or ground cover is feasible only if climatic conditions are conducive to the growth of vegetation cover or supplemental irrigation is practical. The practice has largely been confined to urban areas where aesthetics is a consideration, and where maintenance requirements are a major concern.
The designer should consider the time of year or season for soil fill and seeding and decide whether irrigation, either temporary or permanent, will be required. Provisions will also be necessary for future inspections and maintenance to ensure that the vegetation is not removed or damaged in the future.
Erosion by wind waves and/or navigation waves might have a greater tendency to wash the soil fill away, depending on the wave height and runup (see Special Topic: Revetments to Resist Wave Wash).
Complexity:
Moderate. An appropriate seed mix will be required.
Design Guidelines / Typical Drawings:
The granular material to be placed on the bank prior to riprap installation is critical for providing a gradual transition between the soil surface and the rock. The material may need to be applied as at least two layers of different sized particles, especially on banks made up of very fine soil particles (Escarameia, 1998).
Two configurations of soil being applied on riprap have been used: (1), an ordinary riprap blanket is covered with a layer of soil 30-60 cm (1-2 ft) thick from the top of the revetment down to base flow elevation, or (2), a crown cap of soil and plant material is placed over a riprap toe running along the base of a steep bank, effectively reducing bank angle. Soils used for fill should not be highly erosive.
A variety of methods may be used to establish plant materials including hydroseeding, seeding and mulching, sodding, and incorporation of willow cuttings or root stock in the fill materials (see Technique: Live Staking). When steep banks are re-graded to more gradual slopes and protected with riprap, sediment deposition may form berms that cover the riprap. Along the Sacramento River, berms have formed on revetments placed on a 1V:3H slope, but deposition on 1V:2H revetments has been slight. These berms support a wide range of naturally-occurring vegetation types depending upon maintenance practices (Fischer et al., 1991).
The use of very short spurs or vanes, protruding 2 to 3 m (6.5 to 9.8 ft) into the channel, can promote sediment deposition and the occurrence of berms which can then be utilized for the natural recruitment of grasses, forbs, and woody shrubs.
Short rock spur at the toe of this revetted bank located below bridge abutment led to deposition which then allowed willow stakes to become established in the first year. 1996 Soquel Creek, Santa Cruz, CA Photo by J. McCullah
After more than 7 years the spurs and depositional berms have formed a stable substrate for vegetation. 2004 Soquel Creek, Santa Cruz, CA
7. ENVIRONMENTAL CONSIDERATIONS / BENEFITS
Environmental benefits of covering riprap with soil and grass or low ground cover are largely aesthetic, although thermal pollution is likely to be reduced. Even though access to the stream and therefore recreational benefits are improved, only a few aquatic or riparian habitat values are derived. Conversely, allowing larger brush and shrubs to volunteer on revetments (or planting them on soil covering over the rock) may hinder access but will improve riparian habitat.
8. HYDRAULIC LOADING
Soil-covered riprap with vegetative cover performs well in situations where
flow velocities in the vicinity of the bank do not exceed 1 to 2 m/sec (4 to
6 ft/sec). Critical velocities vary with the variety of vegetation used and
soil conditions.
9. COMBINATION OPPORTUNITIES
Often combined with structural protection of toe or vegetation on upper banks. This technique can be combined with Live Staking for the establishment of woody shrubs (willow, cottonwood, dogwood etc.). Use of barbs, vanes, etc. will often reduce velocities thereby encouraging deposition and reducing erosion of the soil infill.
10. ADVANTAGES
Soil and grass covered riprap provides the establishment of low growing vegetation
to improve the aesthetic appearance of conventional riprap. Thermal pollution
may also be reduced. Ground cover vegetation may provide the conditions required
to promote the establishment of native woody vegetation. Ground cover species
alone will provide desirable aesthetics without significantly reducing channel
capacity. Filling the voids within the riprap with soil, and therefore promoting
rooting, may bind the soil and create less droughty conditions. In addition,
roots may connect the soil cap to the heavy rock substrate, thus helping to
resist tractive forces. As the roots grow even further into the revetment (some
perennial grasses have been known to root up to 1.2 m (4 ft) in one year),
the soil cap, rock, granular material, and underlying soil substrate will be
bound.
11. LIMITATIONS
The establishment of grass on riprap may not be suited to banks subject to high velocities.
The granular material needed to provide suitable horticultural medium may be less stable and more prone to failure than the thick geotextiles used for flows with higher velocities and turbulence (Escarameia, 1998).
The establishment of vegetation within the channel has the potential to reduce channel capacity and conveyance in some situations.
12. MATERIALS AND EQUIPMENT
Obtain vegetation acclimated to inundation. Depending upon the local climate
and the scale of the project, it may be desirable to seed sedge and rush species
along the toe and middle section of the bank and to seed more drought tolerant
species along the top of the bank. A hydroseeder or soil/compost blower may
be required. Tackifier, mulch, compost, mycorrhizae, and fertilizer are among
the potential materials needed for seed establishment (see Technique: Vegetation
Alone).
13. CONSTRUCTION / INSTALLATION
Grade the bank to the desired slope and smooth the surface. If required, dig a toe trench for the keyway below where the riprap will be installed on the bank.
Install properly graded granular material first, and follow with rock that is sized for the flow velocities of the stream. Backfill any voids during construction with soil to allow for strong root to soil contact. Once the voids have been filled, cover the rock with 15-30 cm (6-12 in) of soil. The addition of compost to the soil prior to installation may greatly benefit vegetation establishment.
Seeding can be accomplished by hydroseeding (hydromulching), seed drilling, hand broadcasting, or mechanical broadcasting. Hydromulching sprays are mixtures of seed, mulch, and water. The mixtures may also include emulsifying/stabilizing agents that harden and provide temporary protection, and/or fertilizers that provide nutrients required for plant growth. Mixtures of seed and compost can be applied with a blower. It may be necessary to add erosion control fabric over the soil cap to avoid it washing away in heavy downpours or high flows. It may also be necessary to add a nonwoven geotextile under the soil layer on top of the rock if the rock must be maintained as free-draining. This might be true in the case of saturated banks that fail in sudden draw downs, or in a rotational landslide repair in the bank
Sod may be installed in place of seed. The sod must be rolled out parallel to the stream flow to minimize the potential for failure.14. COST
The initial costs of covering riprap with soil and establishing cover depend
on the cost of topsoil, fertilizer and seed, and the labor required to apply
them. This cost may be reduced if topsoil can be obtained from other parts
of the project. Maintenance costs may be more or less than ordinary riprap
revetment, which usually must be treated using herbicides or cutting to remove
volunteer vegetation. Visual inspection of vegetated riprap may be more difficult.
Conveyance issues may arise when riprap revetments are covered with soil
and seeded, but if vegetation is confined to grasses or other flexible species,
conveyance may be increased due to reduced surface roughness (Fischer et
al., 1991). Some workers suggest that large woody vegetation may create weak
zones in revetments or turbulence that leads to progressive failure during
high flows, but vegetation was found to have little impact on the durability
of revetments during high flows in a Sacramento River case study (Shields,
1991).
15. MAINTENANCE / MONITORING
Riprap should be visually inspected following any 1-year return interval or
greater flow, with focus on potential weak points, such as transitions between
undisturbed and treated areas. Soil on, above, and behind riprap may show collapse
or sinking, or loss of rock may be observed. Inspect riprap during low flows
annually, to ensure continued stability of the toe of the structure. Inspect
sod installations prior to successful rooting for torn or missing strips. Treat
bank, replace rock, and re-seed/sod when necessary.
16. COMMON REASONS / CIRCUMSTANCES FOR FAILURE
Flanking, overtopping or undermining of the revetment due to improperly installed
or insufficient keyways is one of the primary reasons for failure of riprap.
Improperly graded granular material can also cause undermining and failure
of the installation. Undersized stones can be carried away by strong currents,
and sections of the revetment may settle due to poorly consolidated substrate.
Vegetation may require irrigation if seeded or installed during the late spring
or summer, or in extremely droughty soils. Also, only particular grass species
can tolerate excess soil moisture when the banks are inundated.
17. CASE STUDIES AND EXAMPLES
In 1987, ongoing erosion problems in San Diego prompted the installation of vegetated riprap along the eroding banks of Tecalote Creek. Prior to installation, the banks were graded back, and granular fill was placed on the slope in lieu of filter fabric to allow root penetration into subsurface soils. A layer of riprap was applied, covered with soil, seeded, and compacted to 90%. This process was repeated until the riprap was level with the adjacent grade and was covered with a final layer of non-compacted soil and seed. A relatively large storm event occurred following installation and evidence of sediment deposition along the face of the structure indicated the soil used during construction was not being washed away. This project demonstrated that applying seed and soil with riprap could be effective in stabilizing riprap and increasing streambank habitat.
18. RESEARCH OPPORTUNITIES
Information Unavailable
19. REFERENCES
Escarameia, M. (1998) River and channel revetments. Thomas Telford, Ltd., London
Fischer, K. J., Harvey, M. D. and Pridal, D. B. 1991. Deposition on revetments
along the Sacramento River, CA. Fifth Federal Interagency Sedimentation Conference,
U.S. Subcommittee on Sedimentation, 2001. Proceedings of the Federal Interagency
Sedimentation Conferences, 1947 to 2001, Washington, D.C. Published on CD-ROM.,
pp. 4-102, 4-108
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.
