Stone Weirs

1. CATEGORY

1.0 – River Training

2. DESIGN STATUS

Level II

3. ALSO KNOWN AS

Chevron weirs, Reichmuth weirs, artificial riffles, rock ramps, low-head drop structures. See Techniques: Cross Vanes and Newbury Rock Riffles for similar techniques.

4. DESCRIPTION

The term "weir" is used here to refer to any structure spanning the stream that produces a drop in the water surface elevation. These structures are frequently made of angular quarried stone, but logs, sheet piling, concrete, boulders and masonry are also quite common.

5. PURPOSE

Well-constructed weirs can prevent or retard channel bed erosion and upstream progression of knickzones and headcuts, as well as providing pool habitats for aquatic biota. Weirs or grade control structures are often intended to raise or elevate the bottom of incised channels, with the ultimate goal of elevating a dropping water table. Small weirs or "check dams" are sometimes used to control erosion of gullies and small, ephemeral channels in order to limit sediment movement downstream.

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

Hydrologic / Geomorphic Setting

	Resistive
	Redirective
	Continuous
	Discontinuous
	Outer Bend
	Inner Bend
	Incision
	Lateral Migration
	Aggradation

Conditions Where Practice Applies:

Stream habitats degraded by erosion or sedimentation often lack stable pool habitats. Stone weirs address this deficit by creating downstream scour holes or backwater zones upstream (Shields and Hoover, 1991; Shields et al., 1995 and 1998). Generally, pools associated with downstream scour holes are more reliable since sedimentation often eliminates upstream pools. In addition, very well-designed weirs can prevent headward-progressing bed erosion, which is extremely detrimental to upstream habitats and downstream reaches impacted by sedimentation. Weirs with crests that make an angle with the bank may also be used to combat erosion on the outside of meander bends by modifying current patterns in a fashion similar to vanes or bendway weirs.

Complexity:

Moderate.

Design Guidelines / Typical Drawings:

Design criteria for low-head stone weirs are available from a number of sources. Perhaps foremost among these are works by Rice et al. (1996) and Robinson et al. (1998), who present design equations based on tests of steep, rock-lined channels that may be adapted for weir design by treating the downstream face of the weir as the steep chute. A spreadsheet containing these equations is available, as well as documentation. Johnson et al. (2001) provide criteria for weirs that resemble the letter "W" in plan placed upstream of bridge piers to control local scour. However, these criteria are based on a limited number of scale model tests and should be applied with greatest care. Traditional stone weirs have backwater effects, and as such, produce a hydrologic anomaly to the stream system. Therefore, stone weirs usually require maintenance if they are to continue to provide erosion control and habitat benefits.

Weirs placed on beds of sand or finer material are often undermined or flanked by erosion. Undermining may be addressed by excavating a key trench into the stream bed such that two-thirds of the stone (by volume) is placed beneath the existing bed elevation. Additional measures, such as driving sheet piling into the bed underneath the weir or lining the key trench with impervious geotextile, are sometimes used to ensure that structures are not undermined. The depth and width of downstream scour holes is sometimes controlled by pre-forming a basin downstream from the weir crest and lining it with stone. A series of carefully-spaced weirs can protect one another from progressive bed erosion, but if the weirs are too close together, aquatic habitats become more uniform as riffles are drowned by backwaters. Brookes and Shields (1996) provide guidance for planning and designing instream structures for aquatic habitat enhancement, and offer the following guidelines for stone weirs:

Stone weirs are more likely to be successful on streams with widths less than 9 m (30 ft) and bed slopes between 0.005 and 0.20.
Best placed in straight reaches.
The bed and banks should be stable.
The stone weir should be keyed in securely to both banks (see Special Topic: The Key to Stability is the Key).
Spacing between successive weirs should be at least 5 to 7 channel widths.
Height should be low enough to allow fish passage.
Weirs should be placed in reaches initially devoid of pool habitats.

Stone Weirs Typical Drawing

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.dgn

7. ENVIRONMENTAL CONSIDERATIONS / BENEFITS

Weirs made of stone provide valuable stable substrate for invertebrates, and cover and velocity shelter for smaller fish. Grade control structures sometimes trigger enough aggradation upstream to raise the channel bottom and thereby raise the adjoining water table, which can benefit a desiccated riparian zone. However, it is difficult to produce long-lasting improvements in stream ecosystems by adding structures when the causes of habitat quality degradation (e.g., watershed land use change, channelization, water quality degradation, hydrologic changes) are not addressed. Stone weirs provide benthic habitat, contribute to bed stability, create or maintains pool and rifle habitat for adult and rearing fish, and provide velocity refugia.

8. HYDRAULIC LOADING

Permissible shear and velocity for stone weirs 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. Hydraulic design criteria developed for stone revetments should be modified to allow for the higher levels of turbulence typical of channel-spanning structures. Channels with cross-sectional mean velocities > 3.5 m/s (11.6 ft/s) are usually poor candidates for stone weirs.

9. COMBINATION OPPORTUNITIES

Pole planting (see Technique: Willow Posts and Poles) may be used to create overhanging cover for pools up or downstream from stone weirs (Shields et al., 1995), or poles may be incorporated into the stone along the margin of the structure as it is built. Additional techniques that may be used to add woody cover on banks adjacent to stone weirs include Live Staking, Live brushlayering, Live Brush Mattresses, and Live Fascines. Banks downstream from weirs may also be protected using Vegetated Articulated Concrete Blocks, Vegetated Riprap, Soil and Grass Covered Riprap, Vegetated Gabion Mattress, or Cobble Armor. Boulder Clusters may be placed in weir pools to add aquatic habitat cover and complexity.

10. ADVANTAGES

Although stone weirs tend to be damaged more frequently than spur-type structures, they have greater potential influence on physical aquatic habitat. Stone weirs are natural-looking, and create a visual amenity as well as a habitat resource if well designed. The flexible nature of stone allows weirs to deform in response to slight changes in the adjacent channel boundary without failure. Often discontinuous structures like weirs produce superior environmental outcomes at lower cost than continuous measures like riprap revetment. If carefully designed, stone weirs are among the few techniques that are useful for stabilizing degrading beds in incising channels. However, designers should plan for future degradation, which will increase the drop height (head loss) over the weir unless downstream base level is positively controlled.

11. LIMITATIONS

Perhaps the most important limitation for stone weirs is the maximum drop height or head loss across the weir. In most cases, stone weirs should produce a change in the energy grade line greater than 0.6 m (2 ft), although very ruggedly designed weirs may approach 1.5 m (5 ft) drops. Stone weirs are generally poorly suited for extremely wide (> 50 m (165 ft)), steep (S > 0.2) or dynamic braided channels. For weirs to function properly in creating and maintaining downstream scour holes, velocity should not be below 0.25 m/s for extended periods. Unless banks adjacent to weirs are protected up to top bank, some additional land loss through bank retreat is likely to occur during and after construction before a stable bank configuration is reached.

12. MATERIALS AND EQUIPMENT

Stone for weirs should be well graded and properly sized. Detailed guidance for sizing stone 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.

13. CONSTRUCTION / INSTALLATION

A series of weirs should be constructed in an upstream to downstream sequence. This technique usually requires heavy equipment for excavation of the keys (tie-backs) and efficient hauling and placement of stone. Weirs can be constructed from within the stream, from roadways constructed along the lower section of the streambank, or from the top bank. The preferred method is from the point bar side of the stream (especially possible with ephemeral or intermittent streams), as this causes the least disturbance of existing bank vegetation. The least preferred is from the top of the bank, as it disturbs or destroys more bank vegetation and the machine operator's vision is limited.

Usually, the keyways are excavated first and rock is dumped into the key. The rock is then formed into tie-backs (if needed) and finally the stone toe is constructed along a "smoothed" alignment, preferably with a uniform radius of curvature throughout the bend. In a multi-radius bend, smooth transitions between dissimilar radii are preferred.

14. COST

Costs for stone structures are generally directly proportional to the quantity of stone required. For example, costs range from $22 to $44 per metric ton ($20 to $40 per U. S. ton) of stone in place depending on the haul distance required and stream access. Stone quantities depend upon the size of the channel, weir crest angles, and bank and bed key-in distances.

15. MAINTENANCE / MONITORING

Stone weirs should be inspected annually an d after high flow events. Routine monitoring of fish and macroinvertebrate populations is always desirable. Maintenance usually involves removal of sediment deposits, vegetation, trash or woody debris trapped by the weir if they produce undesirable flow patterns and replacement of dislodged stone. In some cases weirs must be reconstructed or replaced with a larger number of weirs to prevent excessive drop heights.

16. COMMON REASONS / CIRCUMSTANCES FOR FAILURE

Stone weirs are susceptible to flanking during high flows, particularly if they are located in bends and if bank soils are highly erodible. When any type of grade control structure is placed in an actively degrading channel, one of the primary hazards is the advance of bed erosion into the structure from the downstream direction, increasing the drop height and triggering either undermining or removal of stone due to scour.

17. CASE STUDIES AND EXAMPLES

Shields et al. (1995 and 1998) describe stabilization and aquatic habitat rehabilitation in a small, incising stream with a sand and gravel bed using a series of stone weirs. Weirs were placed along the base flow channel at intervals equal to roughly six times the base flow channel width. Crests were 2 m wide, and crest elevations were 0.6 m (2 ft) higher than the existing stream bed, except for a central 1 m (3 ft) wide notch, for which the stone crest was at the bed elevation. Structure side slopes were equal to the angle of repose. Structures were keyed into the bed 0.3 m (1 ft). Stone size ranged from 0.2 to 450 kg (0.4 to 1000 lb) with 50 to 85% of stones weighing less than 36 kg (80 lb).

18. RESEARCH OPPORTUNITIES

Many weir design parameters (crest planform, width, and elevation; key-in details; protection of downstream stilling basin) are matters of professional judgment. An algorithm providing quantitative guidance based on project objectives and site conditions is needed.

19. REFERENCES

Brookes, A., Knight, S. S., & Shields, F. D., Jr. (1996). Habitat enhancement. Chapter 4 in A. Brookes, & F. D. Shields, Jr. (Eds.) River Channel Restoration. John Wiley and Sons, Chichester, U. K., 103-126.

Johnson, P. A., Hey, R. D., Brown, E. R., & Rosgen, D. L. (2002). Stream restoration in the vicinity of bridges. Journal of the American Water Resources Association 38(1):55-67.

Rice, C. E. Robinson, K. M., & Kadavy, K. C. (1996). Rock riprap for grade control. In Proceedings of the North American Water and Environment Congress, American Society of Civil Engineers, CD-ROM, ASCE, New York.

Robinson, K. M., Rice, C. E., & Kadavy, K. C. (1998). Design of rock chutes. Transactions of the American Society of Agricultural Engineers 41(3):621-626.

Shields, F. D., Jr., & Hoover, J. J. (1991). Effects of channel restabilization on habitat diversity, Twentymile Creek, Mississippi. Regulated Rivers: Research and Management 6(3):163-181. (pdf)

Shields, F. D., Jr., Knight, S. S., & Cooper, C. M. (1995). Incised stream physical habitat restoration with stone weirs. Regulated Rivers: Research and Management 10:181-198. (pdf)

Shields, F. D., Jr., Knight, S. S., & Cooper, C. M. (1998). Rehabilitation of aquatic habitats in warmwater streams damaged by channel incision in Mississippi. Hydrobiologia, 382, 63-86. (pdf)

	Instream
	Toe
	Midbank
	Top of Bank