Bridge Scour and Stream Instability Countermeasures: Experience, Selection, and Design Guidance-Third Edition

Design Guideline 1 Bendway Weirs Stream Barbs

1.1 INTRODUCTION

Bendway weirs, also referred to as stream barbs, bank barbs, and reverse sills, are low elevation stone sills used to improve lateral stream stability and flow alignment problems at river bends and highway crossings. Bendway weirs are used for improving inadequate navigation channel width at bends on large navigable rivers. They are used more often for bankline protection on streams and smaller rivers. The stream barb concept was first introduced in the Soil Conservation Service (now the Natural Resource Conservation Service, NRCS) by Reichmuth (1993) who has applied these rock structures in many streams in the western United States. The NRCS has recently published design guidance for streambarbs in their National Engineering Handbook (NRCS 2007).

The U.S. Army Corps of Engineers Waterways Experiment Station (WES) developed a physical model to investigate the bendway weir concept in 1988 (USACE 1988, Watson et al. 1996). Since then WES has conducted 11 physical model studies on the use of bendway weirs to improve deep and shallow-draft navigation, align currents through highway bridges, divert sediment, and protect docking facilities. WES has installed bendway weirs to protect eroding banklines on bends of Harland Creek near Tchula, Mississippi. The U.S. Army Corps of Engineers, Omaha District, has used bendway weirs on the Missouri River in eastern Montana. The Missouri River Division (MRD) Mead Hydraulic Laboratory has also conducted significant research and testing of underwater sills. Bendway weirs are a relatively new river training structure and research is providing useful information on their use and effectiveness.

1.2 DESIGN CONCEPT

Bendway weirs are similar in appearance to stone spurs, but have significant functional differences. Spurs are typically visible above the flow line and are designed so that flow is either diverted around the structure, or flow along the bank line is reduced as it passes through the structure. Bendway weirs are normally not visible, especially at stages above low water, and are intended to redirect flow by utilizing weir hydraulics over the structure. Flow passing over the bendway weir is redirected such that it flows perpendicular to the axis of the weir and is directed towards the channel centerline. Similar to stone spurs, bendway weirs reduce near bank velocities, reduce the concentration of currents on the outer bank, and can produce a better alignment of flow through the bend and downstream crossing. Experience with bendway weirs has indicated that the structures do not perform well in degrading or sediment deficient reaches.

Bendway weirs have been constructed from stone, tree trunks, and grout filled bags and tubes. Design guidance for bendway weirs has been provided by the U.S. Army Corps of Engineers, Omaha District, WES, and the NRCS. The following geometric design guidelines for stone bendway weirs reflect guidance provided by NRCS (2007), LaGrone (1996), Saele (1994), and Derrick (1994, 1996). The formulas provided by LaGrone were developed to consolidate many of the "rules of thumb" that currently exist in the field. The formulas are not based on exhaustive research, but appear to match well to current practices. Installation examples were provided by Colorado Department of Transportation (CDOT), Washington State Department of Transportation (WSDOT), and Tennessee Department of Transportation (TDOT).

1.3 DESIGN GUIDELINES

1. HEIGHT - The height of the weirs, H, is determined by analyzing various depths of flow at the project site (Refer to Figures 1.1 and 1.2). The bendway weir height should be between 30 to 50% of the depth at the mean annual high water level. The height of the structure should also be below the normal or seasonal mean water level and should be equal to or above the mean low water level. The weir must be of adequate height to intercept a large enough percentage of the flow to produce the desired results. For applications relating to improved navigation width, the weir must be at an elevation low enough to allow normal river traffic to pass over the weir unimpeded.
2. ANGLE - The angle of projection, θ, between the bendway weir axis and the upstream bankline tangent typically ranges from 60 to 80 degrees. Experience has indicated that it is easier to measure this angle from the chord between two weirs in the field rather than using the bankline tangent. The chord is drawn from the points of intersection with the weirs and the bankline (Figure 1.1). The angle of projection is determined by the location of the weir in the bend and the angle at which the flow lines approach the structure. Ideally, the angle should be such that the high-flow streamline angle of attack is not greater than 30 degrees and the low-flow streamline angle of attack is not less than 15 degrees to the normal of the weir centerline of the first several weirs. If the angle of flow approaching the upstream weirs is close to head-on, then the weir will be ineffective and act as a flow divider and bank scalloping can result. If the angle of flow approaching the upstream weirs is too large then the weir will not be able to effectively redirect the flow to the desired flow path. Ideally, the angle should be such that the perpendicular line from the midpoint of an upstream weir points to the midpoint of the following downstream weir. All other factors being equal, smaller projection angles, θ, would need to be applied to bends with smaller radii of curvature to meet this criteria and vice versa. Experiments by Derrick (1994) resulted in a weir angle, θ, of 60 degrees being the most effective for the desired results in a physical model of a reach on the Mississippi River. Observations by LaGrone (1996), indicate that the angle, θ, of the upstream face of the structure is most important in redirecting flows. The upstream face should be a well defined straight line at a consistent angle.
3. CROSS SECTION - The transverse slope along the centerline of the weir is intended to be flat or nearly flat and should be no steeper than 1V:5H. The flat weir section normally transitions into the bank on a slope of 1V:1.5H to 1V:2H. The structure height at the bankline should equal the height of the maximum design high water. This level is designed using sound engineering judgment. The key must be high enough to prevent flow from flanking the structure. The bendway weir should also be keyed into the stream bed a minimum depth approximately equal to the D₁₀₀ size, but also below the anticipated long-term degradation and contraction scour depth.
4. LENGTH - The bendway weir length (L) should be long enough to cross the stream thalweg; however, should not exceed 1/3 the mean channel width (W). A weir length greater than 1/3 of the width of the channel can alter the channel patterns which can impact the opposite bankline. Weirs designed for bank protection need not exceed 1/4 the channel width. A length of 1.5 to 2 times the distance from the bank to the thalweg has proven satisfactory on some bank stabilization projects. The length of the weir will affect the spacing between the weirs.

Figure 1.1. Bendway weir typical plan view.

Figure 1.2. Bendway weir typical cross section.

5. LOCATION - Ideally, a short weir should be placed a distance (S) upstream from the location where the midstream tangent flow line (midstream flow line located at the start of the curve) intersects the bankline (PI). Additional bendway weirs are then located based on the site conditions and sound engineering judgment. Typically, the weirs are evenly spaced a distance (S) apart (Figure 1.1).
6. SPACING - Bendway weir spacing is influence by several site conditions. The following guidance formulas are based on a cursory review of the tests completed by WES on bendway weirs and on tests completed by MRD on underwater sills. Based on the review, bendway weirs should be spaced similarly to hardpoints and spurs. Weir spacing is dependent on the streamflow leaving the weir and its intersection with the downstream structure or bank. Weir spacing (S) is influenced by the length of the weir (L), and the ratios of weir length to channel width (W) and channel radius of curvature (R) to channel width. Spacing can be computed based on the following guidance formulas (USACE 1988, LaGrone 1996):

(1.2)

The spacing selected should fall within the range established by Equations 1.2 and 1.3, depending on bendway geometry and flow alignment. The spacing should not exceed the maximum established by Equation 1.4. Maximum Spacing (S_max) is based on the intersection of the tangent flow line with the bankline assuming a simple curve. The maximum spacing is not recommended, but is a reference for designers. In situations where some erosion between weirs can be tolerated, the spacing may be set between the recommended and the maximum.⁽⁴⁾

(1.4)

Results from the spacing formulas should be investigated to determine if the weir spacing, length, and angle would redirect the flow to the desired location. Streamlines entering and exiting the weirs should be analyzed and drawn in planform.

7. LENGTH OF KEY - Bendway weirs like all bankline protection structures should be keyed into the bankline to prevent flanking by the flow. Typically the key length (LK) is about half the length of the short weirs and about one fifth the length of the long weirs. Tests conducted by MRD found that lateral erosion between spurs on nearly straight reaches could be estimated by using a 20 degree angle of expansion (Figure 1.3). The following guidance formulas for LK were therefore developed. These formulas compute minimum LK which should be extended in critical locations. The need for a filter between the weir key and the bank material should also be determined. Guidelines for the selection, design, and specification of filter materials can be found in Holtz et al. (1995) and Design Guideline 16.

When the channel radius of curvature is large (R > 5W) and S > L/tan(20°)

(1.5)

Figure 1.3. Length of key for mild bends.

When the channel radius of curvature is small R < 5W and S < L/tan(20°)

(1.6)

NOTE: LK should not be less than 1.5 times the total bank height.

The NRCS guideline for length of key (LK) for short weirs or barbs (NRCS 2007, Saele 1994) is to key the barb into the bank a minimum distance of 8 ft (2.4 m) or not less than 1.5 times the bank height, which ever is greater.

8. TOP WIDTH - The top width of the weir may vary between 3 and 12 ft (1 m and 4 m), but should be no less than (2 to 3)*D₁₀₀. Weirs over 30 ft (9 m) in length will have to be built either from a barge or by driving equipment out on the structure during low flows. Structures built by driving equipment on the weir will need to be at least 10 to 15 ft (3 to 5 m) wide. Side slopes of the weirs can be set at the natural angle of repose of the construction material (1V:1.5H) or flatter.
9. NUMBER OF WEIRS - The smallest number of weirs necessary to accomplish the project purpose should be constructed. The length of the weirs and the spacing can be adjusted to meet this requirement. Typically, not less than three weirs are used together on unrevetted banks.
10. CONSTRUCTION - Construction of the bendway weirs are typically conducted during low flow periods for the affected river. Construction methods will vary depending on the size of the river. Construction on larger rivers may be conducted using a barge which would allow the rock to be placed without disturbing the bankline. For rivers where a barge is not available and where the bendway weir is longer than 30 ft (9 m), access will need to be made from the bank and equipment may need to be driven out on the weir as it is being constructed.

Supplemental information on the use of bendway weirs on tight bends (small radius of curvature) and complex meanders can be found in LaGrone (1996).

1.4 MATERIAL SPECIFICATIONS

1. Stone should be angular, and not more than 30% of the stone should have a length exceeding 2.5 its thickness.
2. No stone should be longer than 3.5 times its thickness.
3. Stone should be well graded but with only a limited amount of material less than half the median stone size. Since the stone will most often be placed in moving water, the smaller stone will be subject to displacement by the flow during installation.
4. Construction material should be quarry run stone or broken, clean concrete. High quality material is recommended for long-term performance.
5. Material sizing should be based on standard riprap sizing formulas for turbulent flow. Typically the size should be approximately 20% greater than that computed from nonturbulent riprap sizing formulas. The riprap D₅₀ typically ranges between 1 and 3 ft (300 mm and 910 mm) and should be in the 100 to 1,000 lb (45 kg to 450 kg) range. The D₁₀₀ rock size should be at least 3 times the calculated D₅₀ size. The minimum rock size should not be less than the D₁₀₀ of the streambed material.
6. Guidelines for the selection, design, and specification of filter materials can be found in Holtz et al. (1995) and Design Guideline 16.

1.5 BENDWAY WEIR DESIGN EXAMPLE

The following example illustrates the preliminary layout of bendway weirs for use in bank protection at a stream bend. The design uses guidelines provided in the previous sections.

Given

The stream width is 100 ft (30 m). The radius of the bend is 500 ft (152 m). The bank height is 10 ft (3 m), which is the mean annual high water level.

Develop a preliminary layout for bendway weir placement for bank protection at the stream bend. The preliminary layout should include weir height, weir length, key length, and weir spacing. Assume the stone size will be established in the final design of the system.

Step 1: Determine the weir height.

Step 2: Determine the weir length.

Step 3: Determine the weir spacing.

Check against S = 4(L) = 4(25 ft) = 100 ft (30 m). Based on site conditions, use 100 ft (30 m).

Check against the maximum spacing, given by:

S_max > S, continue:

Step 4: Determine the key length.

Check for R > 5W and S > L/tan(20°)

Check against LK >= 1.5(Bank Height) = 1.5(10) = 15 ft (4.5 m)

LK must be set to 15 ft (4.5 m) because this value is greater than the value computed first.

Step 5: Preliminary Layout.

The preliminary layout for this stream bend as follows:

1.6 INSTALLATION EXAMPLES

Some illustrations of bendway weirs in use are shown in Figures 1.4 - 1.7. Figures 1.4 and 1.5 show short bendway weirs shortly after installation by CDOT on the Blue River near Silverthorne, Colorado in February 1997. These weirs were designed with weir lengths of 11.5 - 20 ft (3.5 - 6 m) at θ angles of 75° to the bankline tangent. The CDOT engineer indicated that adjustments in the field are equally as important and necessary as original design plans. It can be observed that the bendway weirs are being constructed at low flow conditions as discussed previously.

Figures 1.6 and 1.7 show bendway weirs installed by WSDOT on the Yakima River, Washington in 1994. Figure 1.6 shows the weirs at low flow conditions and Figure 1.7 shows the submerged weirs at normal to high flow conditions. Surface disturbances as flow passes over the weirs can be observed in Figure 1.7. These weirs were designed at θ angles of 50° to the bankline tangent to direct flow away from a critical pier at a bridge just downstream of this bend.

Figure 1.4. Bendway weirs installed on the Blue River near Silverthorne, Colorado (CDOT).

Figure 1.5. Bendway weirs installed on the Blue River near Silverthorne, Colorado (CDOT).

Figure 1.6. Bendway weirs on the Yakima River, Washington at low flow (WSDOT).

Figure 1.7. Submerged bendway weirs on the Yakima River, Washington at high flow (WSDOT).

1.7 CASE STUDY - BENDWAY WEIRS ON THE HATCHIE RIVER, TENNESSEE

On April 1, 1989 the north-bound bridge of U.S. Route 51 over the Hatchie River near Covington, Tennessee collapsed with the loss of eight lives. The flow was 8,620 cfs (244 m³/s) with a 2-year return period. However, the U.S. Geological Survey estimated that this 1989 flow was in the top 10 for overbank flow duration and the longest overbank flow duration since 1974 (Bryan 1989).

The foundation of the bridge consisted of pile bents on the floodplain and piers in the channel. The bents were supported on 20 ft (6.1 m) long timber piles embedded 1 ft (0.3 m) into concrete pile caps. The bottom of the pile caps for the floodplain bents was at an elevation 13 to 14 ft (4 to 4.3 m) higher than for the piers (Figure 1.8). The floodplain and river channel were erodible silt, sand, and clay. The north bound bridge was built in 1936 and spanned 4,000 ft (1,219 m) of the floodplain on 143 simple spans. The south bound bridge was built in 1974 and narrowed the bridge opening to 1,000 ft (305 m) on 13 spans.

The bridges spanned the Hatchie River on a meander bend. Bend migration to the north was well documented. From 1931 to 1975 the migration rate averaged 0.8 ft (0.24 m) per year; 1975 to 1981 (after the south bound bridge was built) was 4.5 ft (1.37 m) per year; and 1981 to 1989 was 1.9 ft (0.58 m) per year (Figure 1.8). The migration was such that in 1989 bent 70 was exposed to the flow. The combination of channel migration and local pier scour caused the bent to fail.

Figure 1.8. Documented channel migration of the Hatchie River, Tennessee.

The National Transportation Safety Board (NTSB 1990) investigated the failure and gave as probable cause "....the northward migration of the main river channel which the Tennessee Department of Transportation failed to evaluate and correct. Contributing to the severity of the accident was the lack of redundancy in the design of the bridge spans."

After the failure of the Hatchie River bridge, TDOT experienced additional instability on the north bank of the river, upstream from the replacement bridge. The solution was to design and install bendway weirs along the north bank (Peck 1999). A field of five bendway weirs was designed to halt the bank erosion. Design parameters were estimated using guidance from HEC-23 (First Edition). As part of the design process, a 2-dimensional hydraulic model was utilized. The model provided flow field data to refine and verify the bendway weir design. Construction was initiated and completed in the Fall of 1999. Figures 1.9 and 1.10 show the installed countermeasures at low flow.

Figure 1.9. Bendway weirs on northbank of Hatchie River looking upstream (TDOT).

Figure 1.10. Close up bendway weir on Hatchie River (TDOT).

1.8 REFERENCES

Bryan, B.S., 1989, "Channel Evolution of the Hatchie River near the U.S. Highway 51 Crossing in Lauderdale and Tipton Counties, West Tennessee," USGS Open-File Report 89-598, Nashville, TN.

Derrick, D.L., 1994, "Design and Development of Bendway Weirs for the Dogtooth Bend Reach, Mississippi River, Hydraulic Model Investigation," Technical Report HL-94-10, WES, Vicksburg, MS.

Derrick, D.L., 1996, "The Bendway Weir: An Instream Erosion Control and Habitat Improvement Structure for the 1990's," Proceedings of Conference XXVII, International Erosion Control Association, 2/27/1996 - 3/1/1996, Seattle, WA.

Derrick, D.L., "Bendway Weirs Redirect Flow to Protect Highway Bridge Abutments," U.S. Army Corps of Engineers Waterways Experiment Station, Vicksburg, MS, undated document.

Holtz, D.H., Christopher, B.R., and Berg, R.R., 1995, "Geosynthetic Design and Construction Guidelines," National Highway Institute, Publication No. FHWA HI-95-038, Federal Highway Administration, Washington D.C., May.

LaGrone, D.L., 1996, "Bendway Weir General Guidance Memorandum," U.S. Army Corps of Engineers, Omaha District, Omaha, NE, revised from 1995.

NTSB, 1990, "Collapse of the Northbound U.S. Route 51 Bridge Spans over the Hatchie River near Covington, Tennessee," April 1, 1989, NTSB/HAR-90/01, National Transportation Safety Board, Washington, D.C.

National Resources Conservation Service, 2007, "NRCS National Engineering Handbook, Part 654 - Stream Restoration Design," 210-VI-NEH, Washington, D.C.

Peck, W.W., 1999, "Two-Dimensional Analysis of Bendway Weirs at US-51 Over the Hatchie River," Proceedings, ASCE International Water Resource Engineering Conference, Session BS-2, August 8-12, Seattle, WA.

Reichmuth, D.R., 1993, "Living with Fluvial Systems," Workshop notes February 23 - 25, 1993, Portland, OR.

Saele, L.M., 1994, "Guidelines for the Design of Stream Barbs," Stream bank Protection & Restoration Conference, 9/22/1994 - 24/1994, SCS-WNTC, Portland, OR.

U.S.Army Corps Engineers, 1988, "Bendway Weir Theory, Development, and Design," USACE Waterways Experiment Station Fact Sheet, Vicksburg, MS.

Watson, C.C., Gessler, D., Abt, S.R., Thornton, C.I., and Kozinski, P., 1996, "Demonstration Erosion Control Monitoring Sites, 1995 Evaluation," Annual Report DACW39-92-K-0003, Colorado State University, Fort Collins, CO.