1.0 SUMMARY OF FINDINGS
Bridge Number 794004 is 2,130 ' long and 36 '-5 " wide, it has 55 spans( 1 movable and 54 fixed) and carries 2 lanes. The superstructure for the approach spans consist of simple span prestressed concrete girders with a concrete deck and the superstructure for the main span is a double leaf bascule structure. The bridge was built in 1959 over a tidal waterway, Halifax River / Intercoastal Waterway.
It is recommended that Bridge Number 794004 be rated for scour as a Scour Susceptible structure based on unfavorable pile embedment to unbraced length ratio estimate after scour, and erodible bed material.
The FDOT design storm surge hydrograph for New Smyrna Beach was used as the hydrologic input for the hydraulic model for both the 100-year and 500-year events. Attachment A includes the detailed Hydrologic Analysis.
The hydraulic analysis consisted of an unsteady-state simulation of the 100-year and 500-year storm surge event conditions using HEC-RAS. A model based on survey data, aerial photographs, Volusia County 1-ft Lidar and the NOAA Daytona Tsunami DEM( 2009) was developed to predict hydraulic conditions at the bridge crossing for the storm event discharge. HEC-RAS input / output are included in Attachment B Hydraulic Analysis.
The total scour anticipated at Bent 17( west bascule pier) is 18.6 ft during the 100-year storm surge event resulting in an estimated remaining embedment of 3.5 ft. post scour.( Per the Phase 1 report dated September 15, 2012, the estimated pile embedment at Bents 17 and 18 is 16.5 ft. This estimated embedment was assumed for the other interior pile bents.) Interior Bent 16 has an exposed pile length after scour of 36.3 feet, and a remaining embedment of 12.4 feet. This yields an estimated pile embedment to unbraced length ratio after the occurrence of the scour event below 1 / 2:1; where 11 / 2:1 ratio is a rule of thumb of stability. A Phase 3 Structural / Geotechnical Assessment is therefore recommended.
Summary of worst case scour conditions at Bents 16, 17 and 18:( Detailed scour calculations are included in |
Attachment C, Scour Analysis) |
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Event: 100-year |
Q100 = 42,340 cfs |
Vavg = 1.6 fps |
WSEL100 = approx 5.2 ft. |
Event: 500-year |
Q500 = 92,770 cfs |
Vavg = 2.1 fps |
WSEL500 = approx 9.6 ft. |
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Bent / |
Ground |
Current |
Contraction |
Local |
Aggradation / |
Total |
Scour |
Remaining |
Pier |
Elev. Embedment / |
Scour |
Scour |
Deggradation |
Scour |
Elev. |
Embedment / |
No. |
( ft.) |
Exposed( ft.) |
( ft.) |
( ft.) |
( ft.) |
( ft.) |
( ft.) |
Exposed( ft.) |
====================================================================================== |
16 |
-14.1 |
16.3 / 32.4 |
1.4 |
2.5 |
0.0 |
3.9 |
-18.0 |
12.4 / 36.3 |
17 |
-13.9 |
16.5 /* |
1.4 |
17.2 |
0.0 |
18.6 |
-32.5 |
3.5 / 13.0 |
18 |
-14.4 |
16.5 /* |
1.4 |
16.5 |
0.0 |
17.9 |
-32.3 |
3.7 / 12.9 |
====================================================================================== |
* Top of piles for bascule piers 17 and 18 are 5.6 feet and 5.1 feet below the existing streambed elevation, |
respectively. |
Recommendations: A Phase 3 Structural / Geotechnical Assessment is required given the un-favorable predicted remaining embedment to unbraced length ratio and near undermining of bascule pier piles. The recommended Vulnerability Rating is Tidal, Scour Susceptible, Medium Priority, Unknown Foundation, with Item 113 Code U.
Interim Plan of Action: a. Continue to watch for signs of movement or settlement or channel bed elevation changes during routine bridge inspections and following significant storm events / tidal surges.
b. Prepare contingency plans should the bridge require closure during severe storm events. 2.0 HYDROLOGIC ANALYSIS
Hydrologic analysis for Bridge Number 794004 was based on the FDOT specified storm surge at New Smyrna Beach for the Ponce Inlet. The 100-year discharge estimate for this bridge is Q 100 = 42,340 cfs based on the
unsteady flow hydraulic model, and corresponds to a water surface elevation of ± 5.2 feet NAVD at the bridge. The
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Supplementary Report Page SR-3