BOULDER CLUSTERS

1. CATEGORY

Riparian buffer and stream corridor opportunities—instream habitat improvements

2. DESIGN STATUS

Level II

4. DESCRIPTION

Large boulders may be placed in various patterned clusters within the base flow channel of a perennial stream.

5. PURPOSE

Natural streams with beds coarser than gravel often feature large roughness elements like boulders that provide hiding cover and velocity shelters for fish and other aquatic organisms. If a constructed or modified channel lacks such features, adding boulder clusters may be an effective and simple way to improve aquatic habitat.

Conditions Where Practice Applies:

Boulder clusters should only be used in streams with beds of material coarser than medium gravel (~50 mm) and velocities that consistently exceed about 0.25 m/s (.8 ft/s).

Complexity:

Low.

Design Guidelines / Typical Drawings:

Use boulders only where the kind of habitat provided by boulder clusters (cover and general physical diversity) is limited. Avoid using more boulders than needed to attain habitat goals. Good results were obtained in Tenmile Creek, Colorado, a stream about 8-9 m (25-30 ft) wide with an average of 180 boulders/km (300/mi).

Boulders should occupy <10% of the flow area at bank-full flow (Fischenich & Seal, 1999), or less than one-fifth of the channel width (Barton & Cron, 1979).

Avoid placing boulders in low velocity regions like pools and slow runs. Velocity should exceed 1.2 m/s (4 ft/s) during events that fill the base flow channel.

Avoid placement of boulder groupings near the upper end of riffles.

Concentrate boulders in or near channel thalweg to ensure habitat availability during low flow, but place them well away from either bank. Avoid placements that will deflect flows toward erodible banks.

Boulder clusters may be configured to trap or pass large woody debris as desired. If trapping debris is desirable, boulders should be located downstream from source areas, should protrude above the water surface, and gaps between boulders should be narrower than the length of debris to be trapped. Conversely, more streamlined, lower configurations may be used if debris passage is desirable.

Boulders may be placed in entirely random configurations, in diamond-shaped clusters of four, or in such a way as to create step-pool sequences in steep channels.

Boulder Clusters Typical Drawing
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7. ENVIRONMENTAL CONSIDERATIONS / BENEFITS

Boulder clusters provide hiding cover and velocity shelters for fish through the turbulence found in their wakes. They also provide stony substrate for attachment-type macroinvertebrates. If bed material is fine enough for scour to occur, boulders also develop stable pool habitat and physical diversity associated with a range of depths, velocities, and bed material sizes. Boulder clusters can make a relocated or reconstructed channel look more natural and add visual interest to an otherwise uniform view. If desirable, boulder clusters may be configured to trap woody debris and provide additional cover benefits. Boulder clusters provide fish rearing habitat, and areas for adult fish.

8. HYDRAULIC LOADING

Boulders should be sized to withstand design velocities and shear stresses. Detailed analyses of the forces acting on a single boulder are straightforward, but accounting for forces on boulders placed in clusters, particularly in bends, is complex. Preliminary sizing may be based on stable boulders in hydraulically similar reaches. The Federal Highway Administration suggests that 0.6 m (2ft) diameter boulder weighing about 1000 pounds (454 kg) will resist movement in current velocities up to 3 m/s (10 ft/s), and a 1.2 m (4 ft) rock will be stable in velocities up to about 4 m/s (13 ft/s) . Similar suggestions are provided by Shields (1983), Barton and Cron (1979), and Johnson and Stypula (1993).

9. COMBINATION OPPORTUNITIES

Boulder clusters may be used to add aquatic habitat benefits to a wide range of channel bed and bank protection techniques.

10. ADVANTAGES

Boulder clusters are simple, natural-looking features that add visual diversity and habitat to degraded, uniform reaches. Consequences of failure are generally slight.

11. LIMITATIONS

Boulders are not recommended for use in sand bed or unstable streams.

12. MATERIALS AND EQUIPMENT

Irregular-shaped angular boulders of durable rock are preferred, although some installations have been done with gabions and other man-made materials. Gabions and similar devices are aesthetically inferior and eventually fail due to abrasion, corrosion or vandalism. Boulders may be placed using a large excavator with a thumb, or other heavy equipment.

13. CONSTRUCTION / INSTALLATION

Generally, operation from top bank is preferred over working in the channel. However, larger, wider channels may require equipment in the channel at low flow. Boulders should be placed using cranes or thumb-equipped excavators. They should not be dumped or rolled into the channel from top of bank.

14. COST

Cost varies widely with the availability of materials, proximity to the site, and site access. However, boulder clusters are usually one of the least costly types of aquatic habitat structures.

15. MAINTENANCE / MONITORING

Operation and maintenance requirements for boulder clusters are minimal. Clusters should be inspected annually or after major events to determine stability. Boulders that have moved short distances need not be relocated unless they are causing stability problems. More significant movement is indicative of design deficiencies, and harvesting and relocating boulders into zones of lower velocity should be considered (Fischenich and Seal, 1999).

16. COMMON REASONS / CIRCUMSTANCES FOR FAILURE

Boulders placed on sand or fine gravel beds often create such large scour holes that they bury themselves. Boulder clusters are poorly suited for stagnant or low-velocity reaches or for channels where normal flow depths or water quality is inadequate to support a fishery. Perhaps most important, boulder clusters should be used only where fish populations are habitat-limited.

17. CASE STUDIES AND EXAMPLES

Tenmile Creek, a Colorado trout stream, was relocated as part of a highway construction project in the 1970s. Boulder clusters (referred to as large rocks) were used in the relocated channel. Post project evaluations were quite positive.

Darby Creek

The Bartram Park/Darby Creek Stream Restoration Project improves 427 linear m (1,400 linear ft) of Darby Creek. The focal point of these improvements is the completion of a 34 x 30 x 3 m (110 x 100 x 9 ft) Double Cross Vane that corrects local channel instability. In addition, Boulder Clusters and Spur Dikes provide fish habitat in areas that have little or no flow variation or cover. And finally, over 244 linear m (800 linear ft) of newly planted riparian buffer provide filtration and habitat for fish and other wildlife along Darby Creek.

18. RESEARCH OPPORTUNITIES

Techniques for sizing boulders placed in complex groupings are needed. Better guidance is needed to assist designers in determining appropriate numbers of boulders to use in a given channel reach.

19. REFERENCES

Barton, J. R. & Cron, F. W. (1979). Restoration of fish habitat in relocated streams, FHWA-IF-79-3, U.S. Department of Transportation Federal Highway Administration, Washington, D. C.

Fischenich, C. and Seal, R. (1999). Boulder clusters. EMRRP Technical Notes Collection (ERDC TN-EMRRP-SR-11), U.S. Army Engineer Research and Development Center, Vicksburg, MS.

Johnson, A. W. and Stypula, J. M. (eds.) (1993). Guidelines for Bank Stabilization Projects in the Riverine Environments of King County. King County Department of Public Works, Surface Water Management Division, Seattle, Wash.

Shields, F. D. Jr. (1983). Design of habitat structures for open channels. Journal of Water Resources Planning and Management. 109(3):331-344.