JOHNS PASS BRIDGE SCOUR ASSESSMENT MODEL

The Johns Pass Bridge connecting Treasure Island and Madeira Beach was constructed in 1971. As early as 1976, Florida Department of Transportation (FDOT) officials noted severe scour in the vicinity of the bridge. By 1984 the tips of three of the pilings which had been driven 20 ft below the original bed had become exposed, while others were within 1 to 2 ft of exposure. In attempts to stabilize the bridge pilings and the surrounding bed, the FDOT installed additional crutch pilings, fenders and rip-rap rubble. Subsequent monitoring has indicated that these improvements may have further aggravated the scour conditions at the bridge. As of 1987 the bed beneath the bridge was eroding at a rate of 12 to 18 in. per year. Of immediate concern is whether the high scour rate will continue to occur and whether this will further endanger the structural integrity of the bridge. The scope of this investigation was to conduct a hydrodynamic assessment of the current local scour deposition problems and to help answer the above questions. The analytical tool selected for this study was a two dimensional finite difference, explicit model, previously developed for inlet studies by the University of South Florida (USF) Center for Modeling Hydrologic and Aquatic Systems (CMHAS). Seven distinct structural alternatives were simulated under two seasonal conditions. The results indicate that with the present inlet bathymetry and bridge structures, continued high erosion is expected. The presence of pilings was demonstrated to be a primary factor in erosion at the bridge. One candidate for remediation or abatement of this problem is the replacement of present crutchbents with new ones installed to be structurally sound for bed depths considerably deeper than 50 ft. It is important to reiterate that Johns Pass Bridge suffers from contraction scour which results from an obstructed flow cross section. Any design of replacement crutchbents which would reduce the net effective surface area of the supports would be advantageous from a hydrodynamic and sediment dynamic standpoint. The partial or full armoring of the bridge alignment was also considered. These simulations indicate that the bridge alignment would probably remain stable during the short term, however significant scour could occur at the upstream and downstream armor-sand interfaces. Eventually this could lead to undermining which could migrate towards the bridge. Therefore to ensure bridge stability, frequent maintenance at the interfaces would be necessary. Examination of this structural concern is beyond the technical functionality and scope of this project. This option is the least favorable based upon the potential of increased erosion from secondary flows and turbulence.

  • Corporate Authors:

    University of South Florida, Tampa

    College of Engineering, 4202 East Fowler Avenue
    Tampa, FL  United States  33620

    Florida Department of Transportation

    Haydon Burns Building, 605 Suwanee Street
    Tallahassee, FL  United States  32301
  • Authors:
    • Vincent, M S
    • Ross, M A
  • Publication Date: 1992-7

Language

  • English

Media Info

  • Features: Appendices; Figures; References; Tables;
  • Pagination: 138 p.

Subject/Index Terms

Filing Info

  • Accession Number: 00626779
  • Record Type: Publication
  • Report/Paper Numbers: Proj No. 99700-7542-119, WPI 0510579 C-3534
  • Files: TRIS, STATEDOT
  • Created Date: Feb 9 1993 12:00AM