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Design and Construction of Bendway Weirs at US-51 over the Hatchie River

Design and Construction of Bendway Weirs at US-51 over the Hatchie River. Wesley Peck. Tennessee Department of Transportation Hydraulic Design Section. General Information. General Information. General Characteristics of the Hatchie River Mouth at Mississippi River

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Design and Construction of Bendway Weirs at US-51 over the Hatchie River

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  1. Design and Construction of Bendway Weirs at US-51 over the Hatchie River Wesley Peck Tennessee Department of Transportation Hydraulic Design Section

  2. General Information

  3. General Information • General Characteristics of the Hatchie River • Mouth at Mississippi River • Drains 2,600 square miles of west Tennessee and north Mississippi • Majority of drainage area is agricultural or undisturbed forested area • Only major river in west Tennessee undisturbed by man • Low channel gradient of approximately 0.001ft/ft • Class I Wild and Scenic River for state • Basin is home to two National Wildlife Refuges

  4. Historical Bridge Background The original bridge opened in 1936 was 4,201 feet long.

  5. Historical Bridge Background A bend of the river was straightened in order to build the new bridge.

  6. Historical Bridge Background The route was upgraded to four lane divided in 1974. A new 999 ft long bridge was added for southbound traffic.

  7. Historical Bridge Background The northbound bridge (built in 1936) collapsed into the river in 1989.

  8. Historical Bridge Background The bridge was replaced with a new structure which matched the one built in 1974.

  9. Problem A review by the National Transportation Safety Board (NTSB) determined the cause of the failure to be a combination of migration of the river bank and local scour.

  10. Problem The river bank moved to a point where bents that were previously on the floodplain were in the middle of the channel. Local scour due to a flood then further exposed the bent piles. The piles failed due to buckling and embedment caused by excessive exposure.

  11. Problem Analysis of Corps of Engineers cross-sections shows significant movement of channel bank since the first bridge was constructed. • 0.8 foot per year from 1934 to 1975 • 4.5 feet per year from 1975 to 1981 • 1.8 feet per year from 1981 to 1988 • No significant change since replacement of failed bridge

  12. Current Situation • The problem has shifted upstream due to riprap at the bridge. • Active erosion of outside bank of the 90° bend upstream of the bridge. • Active point bar formation on inside bank of upstream bend. • Large scour hole in channel at the upstream bend.

  13. Current Situation: Topographical Map

  14. Solution: Bendway Weirs Five bendway weirs were used due to their low profile and the ability to redirect flow in the channel. • The study area was modeled using two dimensional modeling software from FHWA and Brigham Young University. • Topography and bridge characteristics provided by field survey. • Manning’s values from field inspection and engineers judgement.

  15. Weir Design Characteristics: Hydrology

  16. Weir Design Characteristics: Location, Spacing, and Angle Design guidance in HEC-23. Modified based on information from Derrick, LaGrone, and Whittler. Also modified based on velocity vectors from FESWMS model. Angles vary from 60° in the center of the bend, to 80° at the edge of the weir field.

  17. Naming Conventions • Velocity vector • Angle of attack • Angle of departure • Degree of turning

  18. Model Results: Angle of Departure Angle of departure varies depending on where along the face of the weir it is measured. At all five weirs velocity is lower at the tip of the weir. The higher angles of attack result in higher angles of departure.

  19. Model Results: Angle of Departure Degree of turning is a measure of the efficiency of the weir. It is a measure of the difference between angle of attack and angle of departure. Lower velocities near the river bank mean that the weir is more efficient near the bank than at the tip.

  20. Model Limitations • FESWMS is a two dimensional model. This is a three dimensional situation. • Floodplain is not modeled.

  21. Project Summary & Current Status • System of five weirs built of riprap and rubble stone. • Permanent maintenance road. • Permanent cross-section markers for later reference.

  22. Summary & Conclusions • TDOT’s model is reasonably consistent with earlier work on bendway weirs. • Design guidance provided by LaGrone, and in Hydraulic Engineering Circular-23 is accurate within limitations of model. • Weir is more efficient nearer to the bank than at tip due to lower velocities.

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