• TABLE OF CONTENTS
HIDE
 Front Cover
 Title Page
 Introduction and Experimental...
 Results
 Conclusions






Group Title: UFL/COEL (University of Florida. Coastal and Oceanographic Engineering Laboratory) ; 94/026
Title: Pier scour model tests for SR 688
CITATION THUMBNAILS PAGE IMAGE ZOOMABLE
Full Citation
STANDARD VIEW MARC VIEW
Permanent Link: http://ufdc.ufl.edu/UF00085086/00001
 Material Information
Title: Pier scour model tests for SR 688
Series Title: UFL/COEL (University of Florida. Coastal and Oceanographic Engineering Laboratory) ; 94/026
Physical Description: Book
Creator: Sheppard, D. Max
Publisher: Department of Coastal and Oceanographic Engineering, University of Florida
Place of Publication: Gainesville, FL
Publication Date: 1994
 Subjects
Subject: Scour (Hydraulic engineering)
 Notes
General Note: This publication is being made available as part of the report series written by the faculty, staff, and students of the Coastal and Oceanographic Program of the Department of Civil and Coastal Engineering.
 Record Information
Bibliographic ID: UF00085086
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.

Table of Contents
    Front Cover
        Front Cover
    Title Page
        Title Page
    Introduction and Experimental plan...
        Page 1
    Results
        Page 2
        Page 3
        Page 4
    Conclusions
        Page 5
        Page 6
        Page 4
        Page 7
        Page 8
Full Text





UFL/COEL-94/026


PIER SCOUR MODEL TESTS FOR SR 688 INTRACOASTAL
WATERWAY (ULMERTON ROAD) BASCULE BRIDGE
PINELLAS COUNTY, FLORIDA





FINAL REPORT


By


D. M. Sheppard


December 1994



Submitted to:

Parsons Brinkerhoff Corporation
and
Florida Department of Transportation
District 7
















FINAL REPORT
PIER SCOUR MODEL TESTS
FOR
SR 688 INTRACOASTAL WATERWAY
(ULMERTON ROAD)
BASCULE BRIDGE
PINELLAS COUNTY, FLORIDA









SUBMITTED TO:

PARSONS BRINKERHOFF CORPORATION
AND
FLORIDA DEPARTMENT OF TRANSPORTATION
DISTRICT 7





SUBMITTED BY:

D. M. SHEPPARD
COASTAL AND OCEANOGRAPHIC ENGINEERING DEPARTMENT
336 WEIL HALL
UNIVERSITY OF FLORIDA
GAINESVILLE, FLORIDA 32611




DECEMBER 1994
REVISED JANUARY 1995









PIER SCOUR MODEL TESTS
FOR SR 688 INTRACOASTAL WATERWAY
(ULMERTON ROAD) BASCULE BRIDGE
PINELLAS COUNTY, FLORIDA

INTRODUCTION:

Estimating local scour depths for complex multiple pile bridge piers under storm
conditions is difficult. The procedures and equations in HEC-18 are known to be somewhat
conservative to account for the uncertainty in the data on which the equations are based. The
author of this proposal, along with his graduate students, are presently investigating ways of
improving local scour prediction for these types of structures. As part of this investigation a
number of specific bridge piers have been successfully modeled in the Civil Engineering flume at
the University of Florida. One of the objectives of the current research is to provide DOT
engineers with improved scour prediction equations for these complex structures. The work
reported on here involves local sediment scour modeling of two piers that are in close proximity
to each other. The piers in question are on the existing and proposed bridges over the intracoastal
waterway on State Road 688 (Ulmerton Road Bridge) in Pinellas County, Florida. The proposed
new bridge will be constructed just north of the existing bridge.

OBJECTIVE:

The objective of this proposed work was to determine the maximum scour depths (scour
depths that will occur at transition from clear water to live bed conditions) for two bridge piers in
close proximity to each other. There was concern that the closeness of the piers might result in
enhanced scour depths.

EXPERIMENTAL PLAN AND PROCEDURES:

Physical model studies to determine the maximum local sediment scour depths (those that
occur at transition from clearwater to live bed conditions) for existing and proposed piers on the
SR 688 Bascule Bridge over the intracoastal waterway were conducted. Geometric scaling was
used. to size the model, water depth, scour depth, etc. The model to prototype length scale used
was 1:40.

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 38.8 ft3/sec (1100
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.

Schematic drawings of the test setup are presented in Figures 1 and 2. The model piers
(one of the existing and one of the proposed) represent those piers most likely to experience the









greatest local scour depths during a design storm event. Note that these drawings are not to
scale.

The following tasks were performed:
Task 1 A. Two pier models, one for the existing bridge and one for the proposed bridge were
designed and constructed.
B. The models were placed in the flume, the test area compacted and leveled, and the
instruments calibrated (see Figure 1).
C. The 26 hour local scour test was conducted
D. Post experiment measurements were made.
E. The data was reduced and analyzed.

Task 2 The two models were relocated in the flume to simulate flow from the opposite direction
and Tasks 1B-1E were repeated (see Figure 2).

Task 3 A preliminary letter report was written and submitted on November 11, 1994.

Task 4 This final report was completed and submitted on December 12, 1994.

RESULTS:

The environmental parameters and the test results for the two tests are presented in
Tables 1-5. Table 1 gives the conditions for Test 1 where the flow was to the southwest
(proposed piers are upstream of the existing piers). Table 2 gives the conditions for Test 2 where
the flow is to the northeast (proposed piers are downstream of the existing piers).


Table 1. Environmental Parameters for Test 1.
Flow is toward the Southwest
Bridge: State Road 688 (Ulmerton Road) Bascule Bridge WPI 7117136.
General: The proposed new bascule bridge is in close proximity to the existing bascule bridge.
Model tests conducted here are for one of the existing bascule piers and one of the
proposed bascule piers as shown in Figure 1. The piers chosen are the ones most
susceptible to local scour. In Test 1 the flow is from the Northwest and toward the
Southwest, i.e. the proposed bridge is upstream of the existing bridge.
Scale: Water Water Depth Average Computed Critical Skew Duration:
Depth: Temperature: Velocity (U): Depth Average Angle:
Velocity (Uc):
1 :40 7.15" 24.5 30.0C 0.77 ft/sec 0.91 ft/sec 0* 26 hours









Table 2. Environmental Parameters for Test 2.
Flow toward the Northeast
Bridge: State Road 688 (Ulmerton Road) Bascule Bridge WPI 7117136.
General: The proposed new bascule bridge is in close proximity to the existing bascule bridge.
Model tests conducted here are for one of the existing bascule piers and one of the
proposed bascule piers as shown in Figure 2. The piers chosen are the ones most
susceptible to local scour. In Test 2 the flow is from the Southwest toward the
Northeast, i.e. the proposed bridge is downstream of the existing bridge.
Scale: Water Water Depth Average Computed Critical Skew Duration:
Depth: Temperature: Velocity (U): Depth Average Angle:
Velocity (Uc):
1 :40 7.15" 24.0 29.0C 0.77 ft/sec 0.91 ft/sec 0 26 hours


Table 3 gives the results of Test 1 and Table 4 the results of Test 2.

Table 3. Scour Results for Test 1.
Flow is toward the Southwest
Maximum Measured Scour Depth at Existing Pier:
The existing pier was located downstream of the proposed pier in this
test. The scour depth for the existing pier varied throughout this test as
the sediment removed from the upstream pier was deposited in the
vicinity of the existing pier.
Maximum Measured Scour Depth at 26 hours: 0.0 in.
Equilibrium Scour Depth: 0.0 in.
The maturity of the scour after 26 hours at a point downstream of the
leading structure (i.e. how close to equilibrium the scour depth is after
26 hours model time) is not known. This, however, is not important in
this case since the duration of the design storms for this location is such
that equilibrium will not be reached for this point in the structure.

Maximum Measured Scour Depth at Proposed Pier: 6.5 in
Equilibrium Scour Depth at Proposed Pier: 7.7 in.
There is a correction for the test velocity being slightly lower than
critical (i.e. U/U. = 0.85) and for the scour only being 90% of
equilibrium after 26 hours.









Table 4. Scour Results for Test 2.
Flow is toward the Northeast
Maximum Measured Scour Depth at Existing Pier: 4.0 in
The existing pier was upstream of the proposed pier in this test and
experienced the maximum scour depth (for both structures) at the end of
the 26 hour test.
Equilibrium Scour Depth: 4.75 in


Maximum Measured Scour Depth at Proposed Pier: 3.4 in
(See discussion for downstream pier in Table 3)
Equilibrium Scour Depth at Proposed Pier: 4.04 in
The proposed pier was downstream of the existing pier in this test and
thus the scour hole would be much less mature (than the leading
structure) at the end of 26 hours.


Table 5 gives the maximum, equilibrium local scour depths for the prototype piers.

Table 5. Predicted Local Scour Depths For Prototype Structures
Assuming Cohesionless. Erodable Sediment.
Maximum Equilibrium Local Existing Pier Proposed Pier
Scour Depths for Prototype
Piers:
Flow To The Southwest 0.0 ft 25.7 ft
Flow To The Northeast 15.8 ft 13.5 ft
Maximum 15.8 ft 25.7 ft


CONCLUSIONS:

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/Uo= 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.
4. The duration of the tests is such that the maximum scour depths reach approximately 90%
of their equilibrium depth (thereby minimizing extrapolation error).









5. Five hundred year water elevation predictions are used to determine water depths for the
tests. (In general, equilibrium scour depths increase with depth up to a ratio of water
depth to structure diameter of about four.)
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. And it takes much longer to reach equilibrium at locations within and on
the down stream end of a complex, multiple pile pier.)
7. For large, complex, multiple pile structures the variation in the scour depth near the
structure can be significant. Even though we record and report this variation we normally
make recommendations based on the maximum depth that occurs in the vicinity of the
structure.

The conservative nature of these tests should be kept in mind when using the results to establish
design scour criteria for the prototype 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. The duration of a scour
producing storm event is known to be important but the rate at which local scour occurs is not
well understood at this time and is a subject of current research by the author. It is most likely
that the existing bridge in question has experienced conditions that would produce significant
local scour if the sediment in the vicinity of the bridge were erodable. A time history of scour near
this bridge is probably not available but since the existing piers are gravity structures (i.e. they are
not supported by piles and are maintained in position simply by their weight), scour would have
been obvious as a result of pier settlement. The model tests predict a maximum scour depth for
the existing piers of 15.8 ft. The fact that scour has not been observed at this bridge site should
be taken into consideration in establishing the design scour depths for the proposed piers.

The major effect of the close proximity of the piers appears to be a sheltering of the
downstream pier. In both tests the downstream structure experienced less scour than it would
have were it a lone structure.

We were requested to make estimates of prototype local scour depths for the two main
piers for 100 year and 500 year water depths. These estimates along with the corresponding
water depths are presented in Table 6. Note that these estimates are based entirely on the model
results and do not take any of the factors discussed above into consideration.









Table 6. Estimated Local Scour Depths For Prototype Piers


Estimated Measured
Tested Piers 100 Year 100 Year 500 Year 500 Year
Water Depth Scour Depth Water Depth Scour Depth
Existing Pier 19.9 ft 14.2 ft 22.8 ft 15.4 ft
Proposed Pier 22.3 ft 24.5 ft
Other Piers
(across channel)estimated estimated
(across channel)
Existing Pier 11.9 ft 10.0 ft 14.8 ft 11.6 ft
Proposed Pier "19.0 ft 20.2 ft









Table 4. Scour Results for Test 2.
Flow is toward the Northeast
Maximum Measured Scour Depth at Existing Pier: 4.0 in
The existing pier was upstream of the proposed pier in this test and
experienced the maximum scour depth (for both structures) at the end of
the 26 hour test.
Equilibrium Scour Depth: 4.75 in


Maximum Measured Scour Depth at Proposed Pier: 3.4 in
(See discussion for downstream pier in Table 3)
Equilibrium Scour Depth at Proposed Pier: 4.04 in
The proposed pier was downstream of the existing pier in this test and
thus the scour hole would be much less mature (than the leading
structure) at the end of 26 hours.


Table 5 gives the maximum, equilibrium local scour depths for the prototype piers.

Table 5. Predicted Local Scour Depths For Prototype Structures
Assuming Cohesionless. Erodable Sediment.
Maximum Equilibrium Local Existing Pier Proposed Pier
Scour Depths for Prototype
Piers:
Flow To The Southwest 0.0 ft 25.7 ft
Flow To The Northeast 15.8 ft 13.5 ft
Maximum 15.8 ft 25.7 ft


CONCLUSIONS:

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/Uo= 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.
4. The duration of the tests is such that the maximum scour depths reach approximately 90%
of their equilibrium depth (thereby minimizing extrapolation error).









GLASS WALL


PROPOSED PIER


EXISTING PIER


14 7/8"


SIDE WALL


Figure 1. Plan view of models configured for test 1 (not to scale).


FLOW


14 3/8"








PROPOSED PIER EXISTING PIER


FLOW
DIRECTION
TEST 1


FLOW
DIRECTION
TEST 2


FLUME BOTTOM


Figure 2. Side view of models in test section (not to scale).




University of Florida Home Page
© 2004 - 2010 University of Florida George A. Smathers Libraries.
All rights reserved.

Acceptable Use, Copyright, and Disclaimer Statement
Last updated October 10, 2010 - - mvs