Outstanding New Short Span Bridge 2003
Designer - W.E.C. Engineers, Inc.
Owner - PA DOT District 2 - 0
Contractor - Francis J. Palo, Inc.
The bridge is located on S.R. 1011 Sec. A01 over Knapp Creek in McKean County. Designers at W.E.C. Engineers paid close attention and provided careful planning towards an overall design that was sensitive to aesthetics and environmental implications.
Some of the immediate challenges posed were to widen the roadway while providing adequate stream protection and to restore the heavily eroded streambank. Also, the alignment of the stream had to be revised to ensure that impacted wetlands were avoided and to eliminate local flooding as well as to maintain stream profile, the overall hydraulic opening was increased.
The 126-ft span was ideal for a prestressed concrete I-girder design, which not only was economically efficient but also minimized future maintenance costs. The bridge also highlights stub abutments that provide an aesthetically pleasing view of the bridge and surrounding area.
The approved design developed from the need to protect the roadway embankment and bridge structure from the erosive forces produced by Knapp Creek. The stream meander has moved against the embankment and began eroding the toe of slope. Rock had been dumped to protect the embankment, but was not successful. Field observations revealed that with each subsequent storm, additional erosion from the stream bank and embankment was occurring. Because a portion of the bend rested against the embankment, remediation was necessary to avoid possible failure of the roadway or significant future maintenance requirements and costs.
To ensure long-term protection of the embankment and structure, the Pennsylvania Department of Environmental Protection (DEP) was contacted to discuss possible options. Realignment of the stream by shortcutting the bend was not considered feasible. Therefore, shifting of the stream’s thalweg away from the embankment and re-establishment of streambank vegetation was necessary.
The outside bend of the streambank exhibited moderate to high erosion rates with slopes exceeding 30 percent, and a cover of woody vegetation was existent. The point bar opposite the unstable cut bank is covered with annual vegetation and low woody vegetation and a large tree is present that probably stabilizes the point bar.
A deep pool formed adjacent to the concave bank by the higher velocities. Because velocities are lower on the inside of bends, sediments are deposited in this region, forming point bars. In addition, the centrifugal force in the bend causes a transverse water surface slope and helicoidal flow with a bottom velocity away from the outer bank toward the point bar.
Hydraulic problems occur at alluvial stream crossings at or near bends because bends are naturally unstable and the shifting of the thalweg which can result in unanticipated scour because of changes in flow direction and velocities. Non-uniform velocity distributions also cause scour of the bed and bank at the outside of the bend and deposition in the inside of the bend
There are two basic categories of protection measures used to stabilize streambanks. The measures consist of those that work by reducing the force of water against the streambank and those that increase their resistance to erosive forces. W.E.C. strove to combine both methodologies to provide a comprehensive design that will not only protect the nearby roadway but also provide ample advantages to the stream.
The selection of an appropriate countermeasure for a specific bank erosion problem is dependent on factors such as the erosion mechanism, stream characteristics, construction and maintenance requirements, potential for vandalism, and costs. Stream characteristics that influence the selection of countermeasures include: channel width; bank height, configuration, and material; vegetative cover; channel configuration; sediment transport condition; bend radii; channel velocities and flow depth; ice and debris; and floodplains.
Due to the high quality of the stream, W.E.C. did not want to use conventional practices such as lining the streambank with rock, as is typical of many roadway projects. The goal was to stabilize the existing streambank, and protect it from future erosion problems. Initially, W.E.C. diagnosed why the streambank was eroding so severely, then we prescribed a stabilization method to alleviate the problem.
Our findings showed that the stream was severely incised, and had no available floodplain to release the streams energy. There was also no riparian zone bordering this section of stream channel. In order to alleviate the existing problems, the design called for the streambanks to be excavated and establish a 3:1 slope.
The non-structural design includes the planting of native species that would be beneficial to the local wildlife population, as well as provide stabilization to the streambank. The vegetation established will provide many benefits to the local fish and wildlife populations, as well as increase the stream banks resistance to erosive forces. The vegetation near the channel provides shade to help maintain suitable water temperatures to fish, provide habitat for wildlife and contribute to the overall aesthetic quality of the bridge design.
After the slope was excavated, the banks were planted with sandbar willows, which were native to the area and found to thrive along stream corridors. Coconut coir logs were installed along the toe of the new slope up to the stream forming elevation in order to provide protection prior to the establishment of the vegetation. The stream forming elevation is very similar to the 2-yr storm flood elevations that were established during the H&H study. Coconut coir logs and streambank plantings were used in conjunction with the rock spurs to reduce the force of the water against the streambank, and to increase the resistance to the erosive forces of the water.
The structural design incorporated permeable spurs because they have been successfully used as a countermeasure to control the location of the channel in meandering streams and reduce streambank velocities. Spurs can be used in any type of channel material if they are designed correctly. Spurs project from the bank into the channel to alter flow direction, induce deposition, and/or reduce flow velocity. Permeable spurs cause deposition of bed material in transport and are better suited for use in regime and some threshold channels than in rigid channel conditions.
The following geomorphic factors were used to determine the best application of streambank protection for the project:
1. The stream width is forty-two feet.
2. The flow is perennial.
3. The bed material is silt loam.
4. The valley setting is low relief valley.
5. There is a wide floodplain.
6. No natural levees.
7. The streambed is incised.
8. The floodplain is alluvial.
9. The stream is highly meandering.
10. The stream is not braided or anabranched.
11. The point bar is narrow and long.
12. Road embankment failure caused by undercutting.
13. Observation of older mappings indicates that the stream is meandering.
14. There is evidence of bed load deposition downstream of the bridge.
15. A survey of the bridge in the late 1980s indicates that the scour hole located near the abutments is similar to existing conditions.
Based on the above site conditions, the spurs are designed with sufficient length to protect the toe of slope from the beginning of the bend to the abutment. Following the FHWA document “Stream Stability at Highway Structures,” HEC 20, 2nd edition, the length of the spurs was determined to be 8.5 feet long with varying angles to deflect the thalweg away from the streambank and roadway embankment. Scour at the terminus of the spurs was calculated and additional launching material was provided to protect the base of the spur. The profile of the spur begins behind the top of bank and drops quickly to be six inches above the base flow. The profile deflects the flow away from the streambank, encourages deposition of bed load between the spurs, and protects it from damage caused by floating debris.