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 |
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* 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