A study of erosion and deposition in John's Pass and the development of a sediment scour/deposition model for the Florida Department of Transportation.
Undermining of bridge pier piles constructed in tidal inlets is a common problem. Scour mechanisms in tidal inlets are often combinations of local contraction and regional tidal inlet dynamics. A numerical model for quantitatively evaluating and assessing the scour and deposition magnitudes associated with contraction and inlet geomorphological changes was developed. The model used a two-dimensional, dynamic numerical hydraulic model coupled to a moveable bed sediment scour and deposition model developed specifically for this need. The model provides for subgrid features such as pier piles and limited inhomogeneity in sediment grain size. A simplistic representation for armoring associated with sediment sorting is also provided. Initial application of the model to John's Pass in West Central Florida was made to evaluate alternative remedial actions.
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 feet below the original bed into hard material believed to be limerock had become exposed, while others were within 1 to 2 feet 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 had 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 inches per year. A concern for the structural integrity of the bridge prompted the interest to develop a model to help predict the overall scour depth at the bridge. It was also desired that the model help in the evaluation of remedial actions to prevent further bridge scour.The scour-deposition investigation of the Johns Pass Bridge consisted of a hydrodynamic and sediment dynamic analysis of the inlet by applying the USF/CMHAS two-dimensional SCOUR model. To obtain useful information about these processes, seven distinct structural alternatives were simulated under two seasonal conditions. The results indicated that with the present inlet bathymetry and bridge structures, continued high erosion at Station 170 (scour hole 2) was expected. Scenarios which incorporated potential future depths of scour hole 2 of 50 feet, indicated that erosion may occur even below this depth. The presence of pilings was demonstrated to be a primary factor in erosion at the bridge. In this regard, in simulations which removed pilings, erosion was virtually eliminated, while simulations which added pilings frequently aggravated erosion.
One candidate for remediation or abatement of this problem was the replacement of present crutchbents with new ones installed to be structurally sound for bed depths considerably deeper than 50 feet. It was 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 in this investigation. These simulations indicate that the bridge alignment would probably remain stable during the short term, however significant scour would 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. This option was the least favorable based upon the potential of increased erosion from secondary flows and turbulence.Thank you for visiting USF-CMHAS. For questions or comments go to contact information. Last modified on Friday, September 17, 1999. Copyright, 1999 USF-CMHAS