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Bridge Scour and Stream Instability Countermeasures: Experience, Selection, and Design Guidance-Third Edition
Design Guideline 1 Bendway Weirs Stream Barbs
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.
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).
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 (Smax) 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)
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.
When the channel radius of curvature is large (R > 5W) and S > L/tan(20°)
When the channel radius of curvature is small R < 5W and S < L/tan(20°)
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.
Supplemental information on the use of bendway weirs on tight bends (small radius of curvature) and complex meanders can be found in LaGrone (1996).
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.
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:
Smax > 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:
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.
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.
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.
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.