PIER SCOUR MODEL TESTS FOR HATHAWAY BRIDGE PIER AND DOLPHIN SYSTEM IN PANAMA CITY, BAY COUNTY, FLORIDA (S.R. 30, U.S. 98)
FINAL REPORT By
D. M. Sheppard
Submitted to: Florida Department of Transportation District 3
PIER SCOUR MODEL TESTS FOR
HATHAWAY BRIDGE PIER AND DOLPHIN SYSTEM
IN PANAMA CITY BAY COUNTY, FLORIDA
(S.R.L 30, U.S. 98)
FLORIDA DEPARTMENT OF TRANSPORTATION
D. M. SHEPPARD
OCEANOGRAPHIC ENGINEERING DEPARTMENT
336 WELL HALL
UNIVERSITY OF FLORIDA GAINESVILLE, FLORIDA 32611
State Project No. UF Project No.
46020-1553 4910 4511 296 12 3110295
PIER SCOUR MODEL TESTS
HATHAWAY BRIDGE PIER AND DOLPHIN SYSTEM IN PANAMA CITY BAY COUNTY, FLORIDA (S.R. 30, U.S. 98)
Estimating local scour depths for complex multiple pile bridge piers under storm
conditions is difficult. The procedures and equations in BEC-18 are known to be somewhat conservative to account for the uncertainty in the data on which the equations are based. The FUWA requires that local scour depths for design conditions be either 1) computed using the equations and procedures in BEG-i18 or 2) determined by performing scale model flume tests. This report presents the results of a local scour flume test that was performed on a scale model of one of the existing Hathaway Bridge Piers with proposed dolphins at each end. The Hathaway Bridge is located in Panama City, (Bay County) Florida on S.R. 3 0, U.S. 98 over St. Andrew Bay.
The objective of this proposed work was to determine the maximum scour depths (scour depths that wifl occur at transition from clear water to live bed conditions) that will occur near the piers and dolphin system during a 1 in 100 year design storm event
EXPERIMENTAL PLAN AND PROCEDURES:
A physical model test to determine maximum local sediment scour depths for the most scour susceptible pier and dolphin system on the Hathaway Bridge was conducted. Geometric scaling was used to size the model, water depth, scour depth, etc. The model to prototype length scale used was 1:60. Drawings of the model are given in Figures 1 and 2, and photographs of the model and test setup are shown in Figures 3-6.
The tests were conducted in the 100 ft long x 8 ft wide x 2.5 ft deep flume in the
Hydraulics laboratory in the Civil Engineering Department at the University of Florida. This flume has a 100 hp pump and maximum discharge (with the existing weir) of 3 8. 8 ft3/sec (1 100 1/sec). This project has benefited from the fact that the flume was already configured for these types of experiments and since other similar tests will be conducted after these tests are completed there were no pre and post experiment costs. That is, there was no setup and breakdown costs associated with these tests.
Normally, we only conduct a 26 hour test to determine equilibrium local scour depths. However, since the dependency of the rate at which scour occurs on structure size is not well understood, we thought it prudent to test such a large structure for a longer period of time. This allowed for a more accurate extrapolation to equilibrium conditions. The test was stopped at 26 hours, measurements made and started again and run for a second 26 hours.
The following tasks were performed:
A. A model of the pier, dolphin, fender system was designed and constructed (see Figures
1 and 2).
B. The models were placed in the flume, the sediment in the test area compacted and
leveled, and the instruments calibrated (see Figures 3 5).
C. A 26 hour duration local scour test was conducted (Figure 6).
D. The flume was drained and the post experiment measurements were made.
E. The bed upstream of the structures was smoothed and leveled, and the flume filled once
F. A second 26 hour test was conducted (for a total of 52 hours).
G. The flume was drained and the post experiment measurements made.
H. The data from both parts of the test were reduced and analyzed.
Task 2. A preliminary letter report that included the test results and prototype design scour
depths was submitted to HDR Engineering, Inc. on March 16, 1995.
Task 3 This final report was completed and submitted on May 26, 1995.
The environmental parameters and test results for the two parts of the test are presented in Tables I and 2. The median sediment grain diameter (DO) used in both parts of the test was
0. 165 mrn and the sediment size standard deviation (a) was 1.29. The extrapolated scour depths correct the measured values for duration and U/Uc. That is, the equilibrium scour depth is predicted based on the scour depths at 26 and 52 hours and a correction is made for UIUc in the test being slightly less than 1. Table 3 gives the corresponding scour depth values for the prototype structures. Drawings of the model pier, dolphin, fender system are given in Figures I and 2. Figures 3 and 4 are photographs of the model prior to installation and Figure 5 shows the installation process. Figure 6 is a photograph of the local scour at the end of the first 26 hour test.
Table 1. Environmental Parameters.
Bridge: Hathaway Bridge (S.R. 30, U.S. 98) Panama City (Bay County), Florida. General: Large, 57 ft diameter, dolphins and fenders are to be constructed for the purpose of
protecting the primary bridge piers from ship impact. The purpose of these scale
model flume tests is to provide design local scour depths for these structures. The median sediment diameter, D50, and standard deviation, a, of the sediment used in
the flume tests were 0.165 mm and 1.29 respectively.
Part A First 26 hours
Water Water Depth .Average Computed Critical Flow Duration
Depth: Temperature: Velocity (U): Depth Average Skew of Test:
S _ale ___epth: Temperature__ Velo___ () Velocity (Uc): Angle:
1 : 60 10.8 in 26.90 C 0.90 ft/sec 0.94 fr/sec 00 26 hrs
Part B Second 26 hours
Water Water Depth Average Computed Critical Flow Duration
Scale: Depth: Temperature: Velocity (U): Depth Average Skew of Test: Depth:__ _____ Temperature:_ Velocity_ _: Velocity (Uc): Angle:
1 : 60 10.9 in 26.7 C 0.91 ft/sec 0.94 ft/sec 00 26 hrs
Table 2. Measured and Extrapolated Equilibrium Model Scour Results.
Part A First 26 hours.
Maximum Scour Depths at:
Upstream dolphin: Pier: Fender:
Measured: 4.93 in 2.66 in 5.62 in
Extrapolated*: 5.47 in 3.62 in 6.24 in
Part B Second 26 hours
Maximum Scour Depths at:
Upstream dolphin: Pier: Fender:
Measured: 5.69 in 3.08 in 6.50 in
Extrapolated*: 5.81 in 3.14 in 6.63 in
* Corrections made for duration of test.
Table 3. Predicted Local Scour Depths For Prototype Structures.
Maximum Equilibrium Local Scour Dolphin: Pier: Fender:
Depths for Prototype Piers and Dolphin: __ier ___ enderDolphin Systems: 29 ft 15.7 ft 33.2 ft
The extrapolated equilibrium scour depths based on the results of the 26 hour test are slightly less than those based on the 52 hour test results. The rate at which a scour hole approaches equilibrium depends on, among other things, the size of the structure but this relationship is not well understood at this time. Since the Hathaway pier-dolphin-fender system is the largest structure tested in this flume, we thought it would be prudent to extend the test an additional 26 hours. The rate at which scour occurs for this size structure is slightly less than anticipated and this accounts for the difference in the two projected equilibrium scour depths shown in Table 2 (one projection after 26 hours and one after 52 hours). The preliminary report submitted on March 16, 1995 was based on the 26 hour test results.
A conservative approach was taken in these model experiments in that the tests are conducted under conditions believed to produce maximum scour depths. These include the following:
1 The sediment in the vicinity of the piers is assumed to be 'cohesionless and erodable.
2. Tests were conducted near transition from clear water to live bed scour conditions (and the results extrapolated to transition, U/UJ, = 1).
3. The sediment grain size distribution used in the test area of the flume is very narrow with a relatively small standard deviation, a (1.29).
4. The 52 hour duration of this particular test is such that the maximum scour depths reached approximately 98% of their equilibrium depth (thereby minimizing extrapolation error).
5. Predicted one in one hundred year (storm return interval) water elevation was used to determine the water depth for these tests. (In general, equilibrium scour depths increase
with depth up to a ratio of water depth to structure diameter of about three.)
6. The scour producing event is assumed to be of sufficient duration that equilibrium scour depths are achieved. It is the author's opinion that the duration of most storm events in
Florida are not sufficient to create equilibrium scour depths, even on the leading edge of a
pier structure but certainly not in the interior or near the down stream end of these
complex multiple pile structures.
7. For large, complex, multiple pile structures the variation in the equilibrium scour depth near the structure can be significant. Once again, until this phenomena is better
understood and predictable, we recommend that the maximum scour depth be used
throughout the structure.
It is not necessary to have large velocities in order to produce local structure-induced scour near bridge piers. In fact, the maximum scour depths are thought to occur at transition velocities (transition from clear water to live bed scour conditions) which for most of Florida's erodable, cohesionless sediments are on the order of 1 to 2 ft/sec. For many environmental situations the duration of the scour producing storm event is believed to be the limiting factor for scour depth but until these processes are better understood, equilibrium scour depths will have to be used for design.
17.52 5.20 10.50
Figure 1. Drawing of model pier, dolphin, fender system. Dimensions in inches,
geometric scale 1:60.
Figure 2. Oblique view of model system.
0 Cl. 0
Figure 5. Photograph of model installation in test section of flume.
Figure 6. Photograph of model after a 26 hour test.