• TABLE OF CONTENTS
HIDE
 Front Cover
 Title Page
 Report documentation page
 Disclaimer
 Table of Contents
 List of Tables
 List of Figures
 Introduction and body
 Part A: Hydrodynamic study
 Part B: Scour study
 Part C: Overall results and...
 References
 Acknowledgement
 Appendix A: Hydrodynamic study
 Appendix B: Scour study
 Appendix C: Analysis of hydrodynamic...
 Appendix D: Data






Group Title: UFL/COEL (University of Florida. Coastal and Oceanographic Engineering Laboratory) ; 93/009
Title: Pier sediment scour tests for Merrill-Barber Bridge
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Permanent Link: http://ufdc.ufl.edu/UF00079960/00001
 Material Information
Title: Pier sediment scour tests for Merrill-Barber Bridge
Series Title: UFL/COEL (University of Florida. Coastal and Oceanographic Engineering Laboratory) ; 93/009
Physical Description: Book
Creator: Sheppard, Donald Max
Publisher: Coastal and Oceanographic Engineering Department, University of Florida
Publication Date: 1993
 Subjects
Subject: Merrill P. Barber Bridge (Fla.)
Scour at bridges
 Notes
Funding: 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: UF00079960
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
        Front Cover
    Report documentation page
        Unnumbered ( 3 )
    Disclaimer
        Unnumbered ( 4 )
    Table of Contents
        Page i
        Page ii
    List of Tables
        Page iii
    List of Figures
        Page iv
        Page v
        Page vi
        Page vii
    Introduction and body
        Page 1
        Page 2
    Part A: Hydrodynamic study
        Page 2
        Page 3
        Page 4
        Page 5
    Part B: Scour study
        Page 6
        Page 7
        Page 5
    Part C: Overall results and conclusions
        Page 8
        Page 7
        Page 9
        Page 10
        Page 11
        Page 12
        Page 13
        Page 14
        Page 15
        Page 16
        Page 17
        Page 18
        Page 19
        Page 20
        Page 21
        Page 22
        Page 23
        Page 24
        Page 25
        Page 26
        Page 27
        Page 28
        Page 29
        Page 30
        Page 31
    References
        Page 32
    Acknowledgement
        Page 33
    Appendix A: Hydrodynamic study
        A
        A-1
        A-2
        A-3
        A-4
        A-5
        A-6
        A-7
        A-8
        A-9
        A-10
        A-11
        A-12
        A-13
        A-14
        A-15
        A-16
        A-17
        A-18
        A-19
        A-20
        A-21
    Appendix B: Scour study
        B
        B-1
        B-2
        B-3
        B-4
        B-5
        B-6
        B-7
        B-8
        B-9
        B-10
        B-11
        B-12
        B-13
        B-14
        B-15
        B-16
        B-17
        B-18
        B-19
        B-20
        B-21
        B-22
    Appendix C: Analysis of hydrodynamic and scour results
        B-23
        C-1
        C-2
        C-3
        C-4
        C-5
        C-6
        C-7
        C-8
        C-9
        C-10
        C-11
        C-12
        C-13
        C-14
        C-15
        C-16
        C-17
        C-18
        C-19
        C-20
        C-21
        C-22
    Appendix D: Data
        D
        D-1
        D-2
        D-3
        D-4
        D-5
        D-6
        D-7
        D-8
        D-9
        D-10
        D-11
        D-12
        D-13
        D-14
        D-15
        D-16
        D-17
        D-18
        D-19
        D-20
        D-21
Full Text




UFL/COEL-93/009


PIER SEDIMENT SCOUR TESTS FOR MERRILL-BARBER BRIDGE






FINAL REPORT





D. Max Sheppard

and

Maximo Ramos III


October 1993












FINAL REPORT


PIER SEDIMENT SCOUR TESTS FOR
MERRILL-BARBER BRIDGE


















D. MAX SHEPPARD

AND

MAXIMO RAMOS III


OCTOBER 1993


I







Technical Report Documentation Page
1. Repoer No. 2. Gove..lim-. AcceO.u. No. 3. H4tcip.en's Catalog No.


4. Tatl end Subetlh e 5. Report Dole
October 1993
Pier Sediment Scour Tests for Merrill-Barber Bridge Oct r
6. Perlorming Ogenitalon Code

8. Periormeng Orgonization Report No.
7. Authol ) 49104511246:
D. Max Sheppard, Maximo Ramos
9. Per(oming Ogan, etion Name and Addrir 10. Work Unit No. (TRAIS)
Coastal and Oceanographic Engineering Department
University of Florida I. Contract of Glot No.
Gainesville, FL 32611-2083
13. Type of Report ind Period Covered
12. Sponsoring Agency oame and Address Final Report
Florida Department of Transportation,District IV
780 SW 24th Street
Ft. Lauderdale, Florida 33315-2696 14. Sonrin Aaecy C
88030-3514
15. Supplementory Notes



16. Abstract
The objectives of this study were 1) to determine the maximum structure-induced
local sediment scour depths for the proposed bridge piers for the Merrill Barber
Bridge over Indian River on State Road 60 in Indian River County, Florida and 2) to
determine the feasibility of predicting equilibrium local scour depths near complex
multiple pile bridge piers from bottom shear stresses on the prescoured bed. A
series of hydrodynamic tests were conducted in a laboratory flume (100 ft long x 8 ft
wide x 2 ft deep) where flow velocities near model piers were measured with a two
component constant temperature anemometer at a height of 3 mm above the bed. Bottom
shear stresses were then estimated from the flow measurements. The piers (which are
1/15 scale models of proposed Merrill Barber Bridge piers) consisted of thirtysix
square piles (3 columns of 12) and a pile cap that was positioned at different
elevations above the bottom. Two different pile cap shapes were also considered. A
simple relationship between the prescoured bottom shear stress and the equilibrium
local scour depth was postulated.
Sediment scour tests were then conducted in the same flume with the same
models. The average duration of these tests was 28 hours. Scour depths were
measured periodically throughout these tests using an acoustic transponder. The
scour measurements were used 1) to establish the maximum scour depths for the Merrill
Barber Bridge piers and 2) to calibrate and test the scour-shear stress relationship.
Even though the range of conditions tested was somewhat limited, the approach appears
promising and should be pursued further. A number of interesting findings were made
regarding the rate at which scour occurs in these complex structures.

17. Key Words 18. Distribution Statement
Bridge.Pier Scour, Local Scour, No restrictions. This document is avail-
Multiple Pile, Pile Caps, Footings able to the public



19. Security ClIssif. (o1 tis report) 20. Security Classif. (of this pogo) 21. Mo. of Pogs 22. Price
134
Unclassified Unclassified


Form DOT F 1700.7 (8-72)


Reproduction of completed page authorized










DISCLAIMER

"The opinions, findings and conclusions expressed in this publication are those of the
authors and not necessarily those of the Florida Department of Transportation or the
U.S. Department of Transportation. "
"Prepared in cooperation with the State of Florida Department of Transportation and
the U.S. Department of Transportation."














TABLE OF CONTENTS


INTRODUCTION ............................................


BODY


PART A:

Hydrodyna
Obj
Exp
Res

PART B:


Scour Stud
Obj
Exp
Res

PART C:

Overall Re
Mer
Use
Gen

REFEREN

APPENDED

HYDROD'
A.1
A.2
A.3


mic Study ............... .........................
ectives of the Hydrodynamic Study ........................
erimental Plan and Procedures ..........................
ults of Hydrodynamic Experiments ........................


.................................................

ectives of the Scour Study .............................
erimental Plan and Procedures ..........................
ults of Scour Experiments .............................




suits and Conclusions ................................
-rill-Barber Bridge Piers ...............................
of Prescoured Bottom Shear Stress to Estimate Local Scour .........
eral Comments ....................................

CES .... ........ ... ... .. ......... ... ... .. ... .

X A ..... ...... .. ... ...... .. .... .... ...... ....

YNAMIC STUDY ..................................
Background .................. ..................
Experimental Procedures ............................
Data Acquisition .................................


A.3.1 X Sensor Probe


... 32

... 2
. 3
. 3
. 4







. 7

. 7

. 7
. 7
. 8
. 8

32

. .A34

.. A1
.. A1
. .A2
.A4
.. A4
A4


A.3.2 Temperature Probe ............................ A4
A.3.3 Temperature Correction ......................... A5
A.3.4 Vortex Shedding from Probe Holder ................. A5
A.4 Flow Visualization Measurements ....................... A6


I








APPENDIX B ..............................................B15


SCOUR STUDY ..................
B.1 Background ............
B.2 Instrumentation and Calibration
B.2.1 Depth Readings ....
B.2.2 Position Readings .
B.3 Data Acquisition .........
B.4 Data Reduction ..........
B.5 Test Procedures .........
B.6 Time Rate of Scour .......


APPENDIX C ............:.................................C23

ANALYSIS OF HYDRODYNAMIC AND SCOUR RESULTS ............... C1
C.1 Mathematical Model .................................. C1

APPENDIX D ............................................. D1

DATA .................................................. D1


.......











LIST OF TABLES


Sequence of Hydrodynamic Tests ...............
Sequence of Scour Tests ....................
Calibration Tests .........................
Maximum Scour Depths ....................


. 10
. 11
. 11
. 12


Table 1.
Table 2.
Table 3.
Table 4.

Table A.1
Table A.2
Table A.3
Table A.4

Table A.5

Table A.6

Table A.7


Table B.1

Table C.1

Table D. 1
Table D.2
Table D.3
Table D.4
Table D.5

Table D.6

Table D.7


Some formulas for bed load and total load (Sleath, 1984) ..........

Values of x and y at grid points of hydrodynamic study ...........
Results of hydrodynamic test H-1 .........................
Results of hydrodynamic test H-2 .........................
Results of hydrodynamic test H-3 .........................
Predicted and measured bottom elevations for scour test S-1
(no pile cap structure). ................................
Predicted and measured bottom elevations for scour test S-2
(700 pile cap structure top position).........................
Predicted and measured bottom elevations for scour test S-3
(70 pile cap structure bottom position). ................... .


C4

D1
D3
D5
D7

D9

D15

D21


Constant temperature anemometer technical data. ................ A7
X sensor probe technical data. ........................... A8
Temperature probe technical data. ...... ............... .. A8
Variation of mean velocity.with time at the same location
(x = -4.92 ft; y = 0.269 ft and z = 0.12 in). .... ......... .. A9
Variation of mean velocity across the tank in front of and away from the
influence of the structure (x = -4.92 ft and z = 0.41 ft) ............ A9
X sensor probe cosine response probe vertical
(x = -4.92 ft; y = 1.46 ft and z = 0.12 in). ................... A10
X sensor probe cosine response probe horizontal
(x = -4.92 ft; y = 0.692 ft and z = 0.12 in). ................. A10

Echo sounder technical data. ........ .................. B6












LIST OF FIGURES


Figure 1 Definition sketch of pile cap. ............................ 13
Figure 2 Predicted bottom elevation (row 3, x = 1.31 ft) (see Figure
A.3 for definition of axes). ............................. 14
Figure 3 Predicted bottom elevation (row 5, x = 3.28 ft) (see Figure A.3
for definition of axes). ................................ 15
Figure 4 Predicted bottom elevation (row 7, x = 5.25 ft) (see Figure A.3
for definition of axes). ................................ 16
Figure 5 Predicted bottom elevation (row 3, x = 1.31.ft) (see Figure A.3.
for definition of axes) ............ .................... .17
Figure 6 Predicted bottom elevation (row 5, x = 3.28 ft) (see Figure A.3
for definition of axes). ................................. 18
Figure 7 Predicted bottom elevation (row 7, x = 5.25 ft) (see Figure A.3
for definition of axes). ................................ 19
Figure 8 Predicted bottom elevation (row 3, x = 1.31 ft) (see Figure A.3
for definition of axes). ................................ 20
Figure 9 Predicted bottom elevation (row 5, x = 3.28 ft) (see Figure A.3
for definition of axes). ................................ 21


Figure 10 Predicted bottom elevation (row 7, x
for definition of axes). ........
Figure 11 Predicted bottom elevation (row 3, x
for definition of axes). ........
Figure 12 Predicted bottom elevation (row 5, x
for definition of axes). ........
Figure 13 Predicted bottom elevation (row 7, x
for definition of axes). ........
Figure 14 Predicted bottom elevation (row 3, x
for definition of axes). ........
Figure 15 Predicted bottom elevation (row 5, x
for definition of axes). ........
Figure 16 Predicted bottom elevation (row 7, x
for definition of axes). ........
Figure 17 Predicted bottom elevation (row 3, x
for definition of axes). ........
Figure 18 Predicted bottom elevation (row 5, x
for definition of axes). ........
Figure 19 Predicted bottom elevation (row 7, x
for definition of axes). ........


= 5.25 ft)


(see Figure A.3


= 1.31 ft) (see Figure A.3
= 3.28 ft) (see Figure A.3
= 5.25 ft) (see Figure A.3
= 1.31 ft) (see Figure A.3

= 3.28 ft) (see Figure A.3
= 1.31 ft) (see Figure A.3

= 3.28 ft) (see Figure A.3


= 5.25 ft) (see Figure A.3


= 1.31 ft) (see Figure A.3
= 3.28 ft) (see Figure A.3

= 5.25 ft) (see Figure A.3
= 5.25 ft) (see Figure A.3


....... 22

....... 23

....... 24

....... 25

....... 26


. 27


....... 28

....... 29

....... 30









Figure A.1

Figure A.2

Figure A.3
Figure A.4
Figure A.5

Figure A.6
Figure A.7
Figure A.8

Figure A.9

Figure A.10

Figure A. 11

Figure A. 12

Figure A.13

Figure A. 14

Figure A. 15

Figure A. 16


Figure B. 1

Figure B.2

Figure B.3

Figure B.4

Figure B.5

Figure B.6
Figure B.7
Figure B.8
Figure B.9


Shear stress profiles in front of and away from the influence of
the structure at x = -4.92 ft and y = 0.364 ft. ................ All
Velocity profile corresponding to shear stress profiles in Figure
A.1. ............ ............................ All
Grid points for hydrodynamic study (1:1 scale). ............... .A12
Grid points for hydrodynamic study (distorted 1:4.5 scale) ......... A12
Overall set-up of experimental apparatus and instrumentation for
hydrodynamic study. ................................ A13
Calibration curve for the temperature probe. ................... A14
Calibration curve for the x sensor probe. ................... A14
Mean velocity in front of structure vs. time at x = 4.92 ft and
z = 0.12 in (test H-). ................................ A15
Turbulent energy/unit mass in front of structure vs. time at
x = -4.92 ft and z = 0.12 in (test H-l). ................ A15
Unfiltered U and V velocity components for test H-1 at grid
point B2. ........................................ A16
Filtered U and V velocity components for test H-1 at grid point
B2. ........................................... A 17
Power density spectra of U velocity components in Figures A. 10
and A.11 ......................................... A18
Power density spectra of V velocity components in Figures A. 10
and A.11. ..................................... A 19
Mean velocity vs number of data points sampled (x = 0.335 ft;
y = 0.364 ft and z = 0.12 in) ............ ... ........ A20
Variation of mean velocity parallel to the 700 pile cap (top
position) structure at y = 0.364 ft and z = 0.12 in ............. .A20
Variation of mean velocity normal to the 700 pile cap (top
position) structure at x = 0.335 ft and z = 0.12 in. ............. A21


Test area of flume showing no pile cap structure and central
region near structure. ................................
Sediment size distribution for Category 1 sand
(away from structure). ...............................
Sediment size distribution for Category 2 sand
(anchoring layer). ..................................
Sediment size distribution for Category 3 sand
(around structure). ..................................
Overall set-up of experimental apparatus and instrumentation for
scour study................ .......................
Experimental set-up for scour study. ........................
Calibration curve for the echo sounder output signal .............
Unfiltered output signals after completion of test S-1. ............
Filtered output signals after completion of test S-1. ..............


B7

B7

B8

B8

B9
B10
B10
Bll
B12


I









Figure B. 10

Figure B. 11
Figure B. 12

Figure B. 13

Figure B.14
Figure B.15

Figure B. 16
Figure B. 17

Figure B.18
Figure B.19
Figure B.20


Figure C. 1


Figure C.2


Figure C.3


Figure C.4


Figure C.5


Figure C.6


Figure C.7


Figure C.8


Power density spectra of echo sounder output (Channel CHOO)
in Figures B.8 and B.9. ................... ........... B13
Modified calibration curve for echo sounder angle .............. B14
Power density spectra of y position potentiometer (Channel
CH03) in Figures B.8 and B.9. ........................... B15
Definition sketch showing measurements needed for conversion
of raw data to x-y coordinates. .......................... B16
Data points before start of scour test S-1 (No pile cap structure). ..... B17
Data points after completion of scour test S-1 (No pile cap
structure). ...................................... B17
Time rate of scour of piles 1, 2 and 6 of no pile cap structure. ...... B18
Time rate of scour of piles 1, 2 and 6 of 700 pile cap
(top position) structure. ............................ B19
Time rate of scour of pile 1. ........................... B20
Time rate of scour of pile 2. ............................ B21
Time rate of scour of pile 6. ............ ................ B22


Comparison of measured bottom elevation, predicted equilibrium
bottom elevation and dimensionless shear stress for the no pile
cap structure (row 2 : x = 0.33 ft). ........................ C5
Comparison of measured bottom elevation, predicted equilibrium
bottom elevation and dimensionless shear stress for the no pile
cap structure (row 3 : x = 1.31 ft). ........................ C6
Comparison of measured bottom elevation, predicted equilibrium
bottom elevation and dimensionless shear stress for the no pile
cap structure (row 4 : x = 2.30 ft). ................ ......... C7
Comparison of measured bottom elevation, predicted equilibrium
bottom elevation and dimensionless shear stress for the no pile
cap structure (row 5 : x = 3.28 ft). ........................ C8
Comparison of measured bottom elevation, predicted equilibrium
bottom elevation and dimensionless shear stress for the no pile
cap structure (row 6 : x = 4.27 ft) ....................... C9
Comparison of measured bottom elevation, predicted equilibrium
bottom elevation and dimensionless shear stress for the no pile
cap structure (row 7 : x = 5.25 ft)........................ C10
Comparison of measured bottom elevation, predicted equilibrium
bottom elevation and dimensionless shear stress for 700 pile cap
(top) structure (row 2 : x = 0.33 ft). ....................... C11
Comparison of measured bottom elevation, estimated and
predicted equilibrium bottom elevations and dimensionless shear
stress for the 700 pile cap (top) structure (row 3 : x = 1.31 ft). ....... C12


I









Figure C.9


Figure C. 10


Figure C. 11


Figure C.12


Figure C.13

Figure C.14

Figure C.15

Figure C. 16

Figure C. 17

Figure C.18

Figure C.19
Figure C.20

Figure C.21


Figure C.22
Figure C.23


Comparison of measured bottom elevation, predicted equilibrium
bottom elevation and dimensionless shear stress for the 700 pile
cap (top) structure (row 4 : x = 2.30 ft). ...................
Comparison of measured bottom elevation, predicted equilibrium
bottom elevation and dimensionless shear stress for the 700 pile
cap (top) structure (row 5 : x = 3.28 ft) ...................
Comparison of measured bottom elevation, predicted equilibrium
bottom elevation and dimensionless shear stress for the 700 pile
cap (top) structure (row 6 : x = 4.27 ft) ...................
Comparison of measured bottom elevation, predicted equilibrium
bottom elevation and dimensionless shear stress for the 700 pile
cap (top) structure (row 7 : x = 5.25 ft) ...................
Contours of dimensionless shear stress, rd, around no pile cap
structure. ........................................
Contours of dimensionless shear stress, rd, around 700 pile cap
(top position) structure. ............ ..................
Contours of measured bottom elevation x 102 (ft) around no pile


C13


C14


C15


C16

C17

C17


cap structure after completion of scour test. ................. C18
Contours of predicted equilibrium bottom elevation x 102 (ft)
around no pile cap structure. .............. ........... C18
Contours of measured bottom elevation x 102 (ft) around 700
pile cap (top position) structure after completion of scour test ....... C19
Contours of predicted equilibrium bottom elevation x 102 (ft)
around 700 pile cap (top position) structure. .................. C19
700 pile cap (top position) structure before the scour test. .......... C20
700 pile cap (top position) structure after completion of the
scour test ....................................... C20
Comparison of measured bottom elevation, predicted equilibrium
bottom elevation and dimensionless shear stress for 700 pile cap
(bottom) structure (row 3 : x = 1.31 ft). ................... C21
700 pile cap (bottom position) structure before the scour test. ....... C22
700 pile cap (bottom position) structure after completion of the
scour test. ....................................... C22









PIER SEDIMENT SCOUR TESTS FOR
MERRILL-BARBER BRIDGE



INTRODUCTION

Local sediment scour near complex, multiple pile structures is difficult to predict due
to the complexity of the flow in the vicinity of these structures. At present one must rely on
empirical equations based primarily on laboratory data for most structure-induced scour
predictions. The number of studies and the quantity of data available for multiple pile
structures is very limited.' There were four objectives of the study reported on here. The
primary' objective wvas. to obtain local pier scour data to.aid in the establishment of design
scour depths for Merrill-Barber Bridge piers. The second objective was to investigate the
possibility of inferring local scour depths and volumes from near bottom flow measurements
over the fixed, prescoured bed in the vicinity of the structure. A third objective was to
determine if modifications to the pile cap design (side wall slope) would have an impact on
the local scour depths. The fourth objective was to add to the database of scour data for
multiple pile structures with pile caps.

The experimental portion of this study was divided into two major categories, namely
that related to the hydrodynamicc tests" and that associated with the moveable bed or "scour
tests". Even though the same physical models and flume were used in both series of tests,
the objectives, procedures, instrumentation, etc. were different. Therefore, they will be
discussed separately. The specific objectives of each category of tests are outlined followed
by the overall philosophy and procedure used to achieve the results. This is followed by an
analysis of data from both test series and conclusions on the subjects stated in the objectives.
Throughout the report an attempt is made to present the main aspects of the analysis and
results in the body and leave the details of the experimental procedure, instrumentation used,
instrument calibration and the data for the Appendices.


BODY

The overall objectives of the research reported here were 1) to establish the maximum
pier sediment scour depths and patterns for proposed Merrill-Barber Bridge pier designs, 2)
to determine the effect of pile cap location and shape on the maximum pier sediment scour,
3) to examine the feasibility of predicting equilibrium sediment scour from estimates of
prescoured bottom shear stress made from near bottom flow measurements over the area
where scour is anticipated and 4) to obtain additional scour data for multiple pile structures.

All proposed pier designs for the Merrill-Barber Bridge consisted of three rows of
twelve square piles (for a total of thirty-six piles per pier) with a centerline to diameter ratio,
a/D, of three (see Figure 1). Also the overall dimensions of the pile caps tested were









constant for all tests. The quantities varied were, 1) the location of the caps relative to the
bottom (and water surface), 2) the pier side wall slope and in the hydrodynamic tests 3) the
pier orientation to the flow (i.e. the skew angle).


PART A:
Hydrodynamic Study

Fixed bed models have been effectively used in the study and analysis of sediment
transport for a number of years. The application has, for the most part however, been
restricted to general sediment transport and not structure-induced scour. Local, structure-
induced scour usually results in more significant changes in the bed over shorter distances
than general scour, thus it was not clear at the start of this project if such an approach would .
work. It is not unreasonable, however, to assume that, unless the structure-flow changes
drastically during the scour process, that the bottom will simply adjust (scour) until the
bottom shear stress is reduced to that of the area away from the structure. Although more
tests on different structures with different sediments and flow conditions are needed before
definite conclusions can be reached the procedure seems to work for the range of conditions
tested. A simple relationship between the near bottom flow parameters and the ultimate
scour in the vicinity of the structure has been hypothesized.

Another application for this approach that appears promising is in the selection and
placement of scour protection near complex pier structures. If scour protection in the form
of riprap, mats, etc. are to be used near a bridge pier, a hydrodynamic study such as the one
conducted here should be helpful in deciding the type and location of this protection.

Perhaps the best way to test the approach described above would be to directly
measure the bottom shear stress in the vicinity of a structure (for a given set of flow
conditions) and compare the results with those for a scour experiment for the structure
subjected to the same environmental conditions. There are flush mounted probes on the
market that measure bottom shear stress directly. These probes can be difficult to calibrate,
however, and present installation problems for flumes with limited access to the bottom.
Near bottom flow measurements (mean and fluctuating components of velocity) with a cross-
wire constant temperature probe are perhaps easier to make and calibration is relatively
straight forward if a variable speed instrument carriage is available. The drawback to this
approach is, of course, that the stress measured in the flow has to be extrapolated to the
bottom. For a variety of reasons (including insufficient time to construct and perfect a flush
mounted shear stress probe calibration facility) the approach involving flow measurements
just above the bottom was used in this study.

The configurations examined in the hydrodynamic tests are listed in Table 1.









constant for all tests. The quantities varied were, 1) the location of the caps relative to the
bottom (and water surface), 2) the pier side wall slope and in the hydrodynamic tests 3) the
pier orientation to the flow (i.e. the skew angle).


PART A:
Hydrodynamic Study

Fixed bed models have been effectively used in the study and analysis of sediment
transport for a number of years. The application has, for the most part however, been
restricted to general sediment transport and not structure-induced scour. Local, structure-
induced scour usually results in more significant changes in the bed over shorter distances
than general scour, thus it was not clear at the start of this project if such an approach would .
work. It is not unreasonable, however, to assume that, unless the structure-flow changes
drastically during the scour process, that the bottom will simply adjust (scour) until the
bottom shear stress is reduced to that of the area away from the structure. Although more
tests on different structures with different sediments and flow conditions are needed before
definite conclusions can be reached the procedure seems to work for the range of conditions
tested. A simple relationship between the near bottom flow parameters and the ultimate
scour in the vicinity of the structure has been hypothesized.

Another application for this approach that appears promising is in the selection and
placement of scour protection near complex pier structures. If scour protection in the form
of riprap, mats, etc. are to be used near a bridge pier, a hydrodynamic study such as the one
conducted here should be helpful in deciding the type and location of this protection.

Perhaps the best way to test the approach described above would be to directly
measure the bottom shear stress in the vicinity of a structure (for a given set of flow
conditions) and compare the results with those for a scour experiment for the structure
subjected to the same environmental conditions. There are flush mounted probes on the
market that measure bottom shear stress directly. These probes can be difficult to calibrate,
however, and present installation problems for flumes with limited access to the bottom.
Near bottom flow measurements (mean and fluctuating components of velocity) with a cross-
wire constant temperature probe are perhaps easier to make and calibration is relatively
straight forward if a variable speed instrument carriage is available. The drawback to this
approach is, of course, that the stress measured in the flow has to be extrapolated to the
bottom. For a variety of reasons (including insufficient time to construct and perfect a flush
mounted shear stress probe calibration facility) the approach involving flow measurements
just above the bottom was used in this study.

The configurations examined in the hydrodynamic tests are listed in Table 1.









Objectives of the Hydrodynamic Study


1. To obtain estimates of the bottom shear stress at points in the vicinity of the
pier where scour is anticipated.

2. To obtain a relationship between the bottom shear stress and the local scour at
that location.

Experimental Plan and Procedures

Two series of laboratory experiments were conducted on 1/15 scale models of
proposed Merrill-Barber bridge piers. The first series were hydrodynamic tests and included.
four different vertical positions of the pile cap, two different pile cap designs (see Figure 1)
and three different flow alignments. Near bottom flow measurements were made with a two-
component constant temperature anemometer over a grid that covered a portion of the
anticipated sediment scour area. The second series were sediment scour tests and included
three different pile cap locations and two different pile cap designs. The scour experiments
are described later in this report.

The flume in which these tests were conducted is 100 ft long x 8 ft wide x 2.5 ft deep
and has a flat, zero sloped bottom. The bottom in the central part of the flume is recessed
1.08 ft over an area 8 ft wide and 20 ft long as shown in the photograph in Figure B.1. This
recession was covered by five 4' x 8' steel plates for the hydrodynamic tests. A 100 hp
pump recirculates the water and the depth and flow rate were controlled by a V-notch shaped
weir at the entrance and a tail gate at the downstream end of the flume.

A comprehensive set of flow visualization tests were conducted first. These were
used to obtain a qualitative understanding of the flow field in the vicinity of the structure and
to determine the mean flow direction (and temporal variation in direction) at each grid point.
A relatively large variation in flow direction with time 300 was observed near the pier.
This is due to alternate vortex shedding from the piles and pile cap. The variation in flow
direction necessitated a rotation of the cross probe to a horizontal position from the normal
vertical position. The consequences of this rotation with regard to shear stress measurement
are discussed in detail in Appendix A. A grid was laid out along the side of the pier as
shown in Figures A.3 and A.4. Near bottom flow measurements were made at each grid
point with the probe directed in the mean flow direction. A small cutoff switch attached to
the probe support insured that the center of the cross wire probe was located 0.12 in (3 mm)
from the bottom at every grid point. The duration of each measurement was 2.7 minutes and
was based on the largest period of variation of the flow. The flow field for the grided area
was determined for each of the structure configurations given in Table 1.









Results of Hydrodynamic Experiments

The near bottom shear stress in a turbulent flow such as that considered here is
composed of two components, i.e.
T = TV + Tr (1)

where
T = total shear stress

T7 = A U = viscous component of shear stress

S. t "- ap'u ,.2 ,2 ) turbulent component of shear stress

Note that the turbulent component of the bottom shear stress is expressed in terms of the
horizontal components of the turbulent energy, where "a" is an experimentally determined
constant. For a detailed discussion of the reasons and rationale for such a formulation see
Appendix A. Estimated shear stress data for three of the structure configurations are given
in Tables D.2 D.4.

Most sediment transport theories assume that there is a relationship between the
transport and the bottom shear stress (or the excess shear stress above that required to put the
sediment in motion). As a first attempt to obtain a relationship between the prescoured
bottom shear stress and the equilibrium scour depth the following simple relationship was
used:
de f f(7)


=b o (2)


=b (rd 1)
where
de m equilibrium scour depth

b,c m empirically determined coefficients

7r total prescoured bottom shear stress

To total shear stress away from structure
Td
rd =
"o









The coefficients b and c were determined by using the shear stress measurements and
equilibrium scour depths estimated from the measurements made at the end of the 28 hour
tests. A least squares curve fit was used to determine coefficients. Scour depths versus time
plots at various locations from the leading edge indicate that the scour depths for the first two
rows of piles are near equilibrium. The maturity of the scour hole decreases significantly,
however, with the distance from leading edge of the structure. That is, the scour hole at
some distance from the upstream end of the pier cannot reach equilibrium until the sediment
being scoured around the piles upstream from that position has subsided. Thus, the time
required to reach an equilibrium scour depth will be shortest for the leading (upstream) edge
of the pier and longest for the trailing (downstream) edge. For the conditions considered in
this study (i.e. flow just below transition from clearwater to livebed conditions) the scour
depth at the upstream end of the pier was estimated to be 90.% of equilibrium after 28 hours
of steady flo.w. Percentages of equilibrium for the remainder of the structure' are difficult to
estimate. The rate.of scour plots shown in Figures B.16 B.20 are very informative.
Without this information it is difficult to judge the degree to which the scour hole has
progressed. As the rate of scour decreases the scour hole may appear to have reached an
equilibrium condition when in reality it is only a fraction of that value. More work on the
rates at which local scour occurs is needed.


PART B:
Scour Study

Scour experiments were also conducted as part of this study. Local scour
experiments on model structures are an accepted method for estimating scour near prototype
structures. Froude scaling laws have been used in the model specification and in estimating
the prototype scour depths from the laboratory measurements.

Objectives of the Scour Study

1. To obtain sediment scour measurements for the pier configurations proposed
for the Merrill-Barber Bridge.
2. To obtain local pier scour data that can be used to calibrate and test the scour
prediction equation developed as part of the hydrodynamic study.

Experimental Plan and Procedures

As stated above the same flume and the same 1/15 scale physical models used for the
hydrodynamic tests were used in the scour study. A sonar bottom scanning transponder was
used to monitor the scour in the vicinity of the pier periodically over the duration of the 28
hour tests. At the end of each test, mechanical (point gage) measurements of the scour hole
were made. Since the pump for the flume is not designed to transport sediment, a "sediment
trap" was built and installed at the downstream end of the tank to prevent sediment
transported out of the test area from reaching the pump.









The sediment used in the flume was divided into three categories according to grain
size distribution. The largest quantity (category 1) was as purchased and contained sizes
ranging from 0.2 mm to 0.8 mm (see Figure B.2). This sand was placed in regions of the
test area where scour was not anticipated. The second category contained grain diameters
ranging from 0.84 mm to 2.0 mm (see Figure B.3). This sand was used to cap the sand in
category one. The cap was approximately 2 inches (5 cm) in depth. The third category had
the size distribution shown in Figure B.4 and had diameters ranging from 0.42 mm to 0.84
mm. This sand occupied the central region of the test area. Figure B.1 shows the partition
used in separating the central region during the sand placement process. The intent was to
maintain a given sediment size distribution in the region of scour and to retard sediment
motion in the areas away from the structure.

Since one of the objectives of the study was to measure the maximum scour, the flow
rate and sediment size were selected so as to produce conditions as close to transition from
clear water to live bed as possible while at the same time maintaining the depth mean
velocity in the neighborhood of the 100 year design velocity (using Froude scaling). Froude
scaling was used to determine the water depth, which was maintained approximately constant
throughout both test series at approximately 1.25 ft (38 cm). The structure configurations
used in the scour tests were 1) no pile cap (i.e. pile cap above the water line), 2) rectangular
pile cap at the water surface, 3) rectangular pile cap resting on the bottom, 4) sloping (700
sides) pile cap at the water surface and 5) sloping (700 sides) pile cap resting on the bottom.
The initial flow rate was set just below the computed value for transition. The five tests
described above were then conducted at this flow rate.

After the scour tests on the pier structures described above were completed and the
scour depths found to be significantly less than predicted by the methods presented in HEC-
18 (1991), a decision was made to test a single cylinder under the same sediment, water
depth and flow conditions and at an increased flow rate. The purpose of these tests was to
determine if the conditions of the pier tests were close enough to transition (clear water to
live bed) that maximum scour was occurring. The first cylinder test (conducted at the depth
mean velocity used for the pier tests, 1.03 ft/sec) produced an ultimate scour depth ratio
(scour depth/cylinder diameter) of 1.68. The second cylinder test (conducted at a velocity of
1.22 ft/sec) produced an ultimate scour depth ratio of 1.91, some 14% larger than that of the
first test. The scour ratio of 1.91 matches the value obtained by Hannah (1978) for a single
(circular) cylindrical pile with a similar aspect ratio. This indicates that the flow rates used
for the pier tests were a bit low. Sand ripples formed away from the structure and sand was
transported down the flume for every test conducted indicating a live bed condition, at least
for a portion of the sand grains. However, more ripples formed and more sand was
transported during the second test with the cylinder indicating the conditions were closer to
transition for the D50 sediment. See Table 3 for the results of these tests as well as the
computed scour depths for these conditions using the HEC-18 equation. To see if the 14%
increase in scour depths found for the single cylinder could be applied to the multiple pile
structure, an additional test was made on the pier with no pile cap at the higher flow rate.
As with the cylinder the scour depth increased by approximately 14%. There was some


I









contraction scour during this test making these measurements not quite as precise as for the
other tests. It therefore seems appropriate to apply a 14% scour depth correction to the
mature scour depths near the leading edge of all five pier configurations.

Researchers have found that the rate of scour in model studies of approximately the
scale of these tests is such that 90% of the ultimate scour takes place in approximately 24
hours model time (see e.g. Hannah (1978)). The duration of the scour tests reported on here
are shown in Table 2 and ranged from 27.4 hours to 28.9 hours. For the purposes of
estimating ultimate scour depth, the conservative (in the sense of giving a larger scour depth)
assumption was made that the scour at the ends of these tests were produced in 24 hours.
The scour process was recorded by sonar measurements, video and point gage measurements
for all of the pier tests and by video and point gage measurements for the cylinder tests.

Results of Scour Experiments

The results of the scour experiments are presented in several ways. The scour depths
for each structure from point gage measurements at the end of each test are given in Table 4.
The scour depths for the model are given in feet and centimeters and for the prototype in feet
and meters.

As stated earlier, with the exception of the first two or three rows of piles, it is
difficult to know the maturity of the scour hole (i.e. how close the scour hole is to
equilibrium). For this reason, no attempt was made to estimate equilibrium scour depths
from the measured scour beyond the first few rows of piles. This estimated equilibrium data
was used to evaluate the coefficients in Equation 2. Contours of the measured scour are
shown in Figures C.15 and C.17. The fact that the scour hole is in various stages of
development from the front to the back of the pier should be kept in mind when viewing
these Figures. Measured profiles normal to the pier at selected values of x are shown in
Figures C.1 C.12.


PART C:
Overall Results and Conclusions

Merrill-Barber Bridge Piers

The equilibrium maximum sediment scour depths that occur at transition from
clearwater to live bed for the pier configurations considered in this study are given in
Table 4. The values presented in this Table are based primarily on results of the scour
experiments and represent the authors best estimates of scour depths for the pier designs
proposed for the Merrill-Barber Bridge. They take into consideration the fact that the scour
experiments were conducted at velocities slightly below transition (i.e. the measured depths
were increased by 14%) and the fact that at the upstream end of the pier the measured scour
depths are only 90% developed. A more detailed discussion of this subject is presented in









The coefficients b and c were determined by using the shear stress measurements and
equilibrium scour depths estimated from the measurements made at the end of the 28 hour
tests. A least squares curve fit was used to determine coefficients. Scour depths versus time
plots at various locations from the leading edge indicate that the scour depths for the first two
rows of piles are near equilibrium. The maturity of the scour hole decreases significantly,
however, with the distance from leading edge of the structure. That is, the scour hole at
some distance from the upstream end of the pier cannot reach equilibrium until the sediment
being scoured around the piles upstream from that position has subsided. Thus, the time
required to reach an equilibrium scour depth will be shortest for the leading (upstream) edge
of the pier and longest for the trailing (downstream) edge. For the conditions considered in
this study (i.e. flow just below transition from clearwater to livebed conditions) the scour
depth at the upstream end of the pier was estimated to be 90.% of equilibrium after 28 hours
of steady flo.w. Percentages of equilibrium for the remainder of the structure' are difficult to
estimate. The rate.of scour plots shown in Figures B.16 B.20 are very informative.
Without this information it is difficult to judge the degree to which the scour hole has
progressed. As the rate of scour decreases the scour hole may appear to have reached an
equilibrium condition when in reality it is only a fraction of that value. More work on the
rates at which local scour occurs is needed.


PART B:
Scour Study

Scour experiments were also conducted as part of this study. Local scour
experiments on model structures are an accepted method for estimating scour near prototype
structures. Froude scaling laws have been used in the model specification and in estimating
the prototype scour depths from the laboratory measurements.

Objectives of the Scour Study

1. To obtain sediment scour measurements for the pier configurations proposed
for the Merrill-Barber Bridge.
2. To obtain local pier scour data that can be used to calibrate and test the scour
prediction equation developed as part of the hydrodynamic study.

Experimental Plan and Procedures

As stated above the same flume and the same 1/15 scale physical models used for the
hydrodynamic tests were used in the scour study. A sonar bottom scanning transponder was
used to monitor the scour in the vicinity of the pier periodically over the duration of the 28
hour tests. At the end of each test, mechanical (point gage) measurements of the scour hole
were made. Since the pump for the flume is not designed to transport sediment, a "sediment
trap" was built and installed at the downstream end of the tank to prevent sediment
transported out of the test area from reaching the pump.









Appendix B. The results of the hydrodynamic study can provide predictions of equilibrium
scour depths for the area where bottom shear stresses are known. Selected bottom profiles
are presented in Figures 2 19. Note that the scour depths for skew angles greater than zero
were computed from the pre-scoured bottom shear stress measurements and scour
measurements do not exist for these cases. The values appear to be reasonable but they
should be used with caution.

Use of Prescoured Bottom Shear Stress to Estimate Local Scour

One of the objectives of this study was to determine if equilibrium local scour depths
could be predicted from prescoured bottom shear stress measurements. Even though
additional work is needed before definite conclusions regarding the feasibility can be made,
the results look very promising. The predictions made with'the simplistic model developed
as part of this study are reasonable and appear to fit the scour at those locations believed to
be close to equilibrium. The scheme also gives estimates of scour depths at locations that
are far from equilibrium that are consistent with the rate of scour plots for those areas. That
is, the predicted equilibrium scour depths are reasonable extrapolations of the measured
values at locations where the rate of scour plots indicate that the scour is far from
equilibrium.

Due to time and cost constraints flow measurements adjacent to the piles were not
made in this study. A special constant temperature probe support is needed for these
measurements and it was not available. As a result the shear stress for these areas had to be
extrapolated from the values along the side of the pier. The values of shear stress needed to
predict the equilibrium scour depths obtained from the scour measurements fall within the
range of values measured by other investigators (Carstens and Sharma (1975)). A series of
tests are needed where shear stress is measured much closer to the piles. In addition,
structure shapes other than the ones considered here need to be tested to determine the
universality of the coefficients obtained in this study.

General Comments

The results of a number of researchers including the authors of this report (see
Sheppard et al. (1992)) indicate that the maximum local structure-induced scour occurs just
prior to transition from clear water to live bed scour. As velocities increase in the live bed
regime sand waves can form and propagate through the test area. The combination of local
scour and sand wave trough depth will produce a second local maximum scour depth which
may or may not exceed the transition maximum, depending on, among other things, the
sediment size (see Melville (1984)). The scour measured in these tests is limited to
"structure-induced local scour" and does not contain sand waves of any significant size. The
presence of sand waves (coupled with local scour) could thus produce a scour hole deeper
than the values obtained in these tests. Sand wave heights can be estimated using expressions
given in the literature (see e.g. Raudkivi and Witte (1990), van Rijn (1984)).









contraction scour during this test making these measurements not quite as precise as for the
other tests. It therefore seems appropriate to apply a 14% scour depth correction to the
mature scour depths near the leading edge of all five pier configurations.

Researchers have found that the rate of scour in model studies of approximately the
scale of these tests is such that 90% of the ultimate scour takes place in approximately 24
hours model time (see e.g. Hannah (1978)). The duration of the scour tests reported on here
are shown in Table 2 and ranged from 27.4 hours to 28.9 hours. For the purposes of
estimating ultimate scour depth, the conservative (in the sense of giving a larger scour depth)
assumption was made that the scour at the ends of these tests were produced in 24 hours.
The scour process was recorded by sonar measurements, video and point gage measurements
for all of the pier tests and by video and point gage measurements for the cylinder tests.

Results of Scour Experiments

The results of the scour experiments are presented in several ways. The scour depths
for each structure from point gage measurements at the end of each test are given in Table 4.
The scour depths for the model are given in feet and centimeters and for the prototype in feet
and meters.

As stated earlier, with the exception of the first two or three rows of piles, it is
difficult to know the maturity of the scour hole (i.e. how close the scour hole is to
equilibrium). For this reason, no attempt was made to estimate equilibrium scour depths
from the measured scour beyond the first few rows of piles. This estimated equilibrium data
was used to evaluate the coefficients in Equation 2. Contours of the measured scour are
shown in Figures C.15 and C.17. The fact that the scour hole is in various stages of
development from the front to the back of the pier should be kept in mind when viewing
these Figures. Measured profiles normal to the pier at selected values of x are shown in
Figures C.1 C.12.


PART C:
Overall Results and Conclusions

Merrill-Barber Bridge Piers

The equilibrium maximum sediment scour depths that occur at transition from
clearwater to live bed for the pier configurations considered in this study are given in
Table 4. The values presented in this Table are based primarily on results of the scour
experiments and represent the authors best estimates of scour depths for the pier designs
proposed for the Merrill-Barber Bridge. They take into consideration the fact that the scour
experiments were conducted at velocities slightly below transition (i.e. the measured depths
were increased by 14%) and the fact that at the upstream end of the pier the measured scour
depths are only 90% developed. A more detailed discussion of this subject is presented in









The results of the hydrodynamic tests and the rate of scour plots indicate that the time
required to reach equilibrium scour depths increases with distance from the leading edge of
the pier. This is an important finding that helps interpret the results of this and other
studies. The rate of scour for the front rows of piles seems to be similar to that of a single
pile structure. The time required to reach equilibrium scour depths at piles downstream from
the leading edge can be significant.

Flow misalignments (i.e. skew angles) of 7.50 and 150 were examined in the
hydrodynamic tests. Scour predictions based on bottom shear stress for these cases do not
have scour data to back them up but they seem reasonable.

Reversing flows such as: might occur during a hurricane could scour the other end of
the structure and perhaps fill or partially fill in the original scour hole. Even if the scour
hole is filled by the reverse flow the load bearing properties of this newly deposited sediment
are questionable and should be disregarded until tests can be conducted to prove otherwise.

The sloping (700) pile cap does not significantly alter the maximum scour depth in
any of the positions tested. The reduction in scour is only 4.6% for the pile cap at the
bottom and 3.4% for the pile cap at the surface. This result is somewhat surprising since the
dominant scour mechanism for these bluff bodies appears to be the "horseshoe vortex"
generated on the leading face of the structure.

The scour tests were performed for two positions of the pile cap; resting on the
bottom and located at the water surface (i.e. the top of the pile cap just above the water
surface). Three pile cap positions (bottom, mid depth and surface) were investigated in the
hydrodynamic tests. At the end of the 28 hour scour tests the scour depths were greater for
the pile cap at the surface than at the bottom. This, most likely, will be true for equilibrium
scour depths as well but it could be that the scour hole is much less developed for the case
when the pile cap is at the bottom. Unfortunately the hydrodynamic (shear stress) test results
are not helpful for this situation due to their inability to predict scour when the structure-flow
configuration changes dramatically as a result of the scour. Longer duration scour tests are
needed to resolve this question.

The presence of a number of piles and a massive pile cap such as that investigated
here has two somewhat opposing effects on the sediment scour potential. The turbulence
intensity (and presumably the bottom shear stress) increases with the number of bluff
structural elements (piles) but as the number of piles increase the flow velocity within the
structure is retarded thereby reducing the flow through the structure and the bottom shear
stress. It appears that the extent and volume of scour must be a function of the number of
in-line piles as well as the size, shape, spacing, etc. Thus, for a given pile shape, size and
spacing there could be a maximum extent to the scour with regard to the number of piles in
the in-line direction. The pier designs considered in these tests seems to have exceeded the
point of maximum extent of scour in that the predicted scour depths for the downstream piles
is significantly less than for those at the upstream end.









Rate of scour is obviously an important aspect of local structure-induced sediment
scour in the coastal environment. Using Froude scaling laws, 28 hours in the flume
corresponds to approximately 108 hours or 4.5 days for the prototype. Froude scaling for
rate of scour is probably not appropriate since prototype sediment and water are being used
in the flume studies. Nevertheless it is obvious that a significant amount of time is required
for the entire scour hole to reach equilibrium. The rate of structure-induced scour problem
is presently under investigation by the author. Data from this and other similar studies will
hopefully lead to a better understanding of the mechanisms involved and ultimately to
predictive models.



Table 1. Sequence of Hydr6dynamiic Tests


Test Number Pile Cap Type Pile Cap Position Flow Skew Angle
1 No Pile Cap NA 00
2 700 Pile Cap Top 00
3 700 Pile Cap Mid 00
4 700 Pile Cap Bottom 00
5 90 Pile Cap Top 00
6 900 Pile Cap Mid 00
7 No Pile Cap NA 7.50
8 700 Pile Cap Top 7.50
9 700 Pile Cap Mid 7.50
10 No Pile Cap NA 150
11 90 Pile Cap Top 150
12 900 Pile Cap Mid 150










Table 2. Sequence


Table 3. Calibration Tests


Test Structure Velocity Flow Depth Duration of Test
Number (ft/sec) (ft) (hours)
1 No Pile Cap 1.03 1.28 28.3
2 900 Pile Cap 1.03 1.28 27.4
(Top Position)
3 900 Pile Cap 1.03. 1.28 28.9
(Bottom Position) .. ** _
4 700 Pile Cap 1.03 1.28 28.6
(Top Position)
5 700 Pile Cap 1.03 1.28 28.1
(Bottom Position)
6 4 inch Cylinder 1.03 1.28 7.1
7 4 inch Cylinder 1.22 1.31 7.0
8 No Pile Cap 1.22 1.31 24.7


Test Duration (hr) Equilibrium HEC-18
No. Structure Water Depth Velocity Scour Depth Scour Depth
(ft) (ft/sec) Scour Depth (ft) (ft) (ft)
4in Dia. 7.1
6 Cylinder 1.28 1.03 0.56 0.49
__0.446
4in Dia. 7.0
7 Cylinder 1.31 1.22 -0.64 0.525
0.509


of Scour Tests











Table 4. Maximum Scour Depths


Model Prototype
Corrected
Scour Depth1 Equilibrium Maximum Equilibrium Maximum Corrected Maximum
Scour Depth Scour Depth Scour Depth
Structure ft cm ft cm ft cm ft m
No Pile Cap 0.54 16.6 0.60 18.4 0.71 21.7 10.59 3.3
900 Pile Cap 0.76 23.2 0.84 25.8 0.95 29.0 14.25 4.4
(Top Position)
900 Pile Cap 0.67 20.5 0.74 22.8 0.85 26.0 12.75 3.9
(Bottom Position)
700 Pile Cap 0.73 22.4 0.81 24.9 0.92 28.1 13.75 4.2
(Top Position)
700 Pile Cap 0.64 19.6 0.71 21.8 0.82 25.0 12.25 3.8
(Bottom Position)

1Scour depths are measured from the initial bottom and are positive down.




















S(positive up from th
prescouredbottom)l





/


/ -2"x2" Piles 01.51 -
1 2 3 4 5 6 7 8 9 10 11 12
,f ---- -- \ ------ --- ~--- --- 7


3 0 0 El El 0
a a 0 0 0 9 [
0 M [M M [M 0


PLAN VIEW


Pile Cap e
Rectangular 900
Sloping 70


0.37'
0.37'


- 6.23' >
SIDE VIEW


AEND


END VIEW


Figure 1 Definition sketch of pile cap.


Flow c
1.3
1.73' B

A


0) E


El El 0


______ 2J____________















-No
*-*-... 70
--- 90


pile cap 0 deg skew to flow
deg pile cap (top position) 0 deg skew to flow
deg pile cap (top position)- 0 deg skew to flow


I I I I I I I I I I I I I I I
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
y (ft)


2.0


Figure 2 Predicted bottom elevation (row 3, x = 1.31 ft)
(see Figure A.3 for definition of axes).


20

10

0



-20-

-30-
-o-

-40-

-50-

-60-

-70-

-80-

-90-


... g


0




o





0
o


_inn
IJ-


0- I
0.0


I














No pile cap -
*-* 70 deg pile cap
- -- 90 deg pile cap


30 -


0 deg skew to flow
top position) 0 deg skew to flow
top position) 0 deg skew to flow


20-

10-

0

-10-

-20-

-30-

-40-


I I I I I I I I I 1 I
0.2 0.4 0.6 0.8 1.0 1.2
y (ft)


1.4 1.6 1.8 2.0
1.4 1.6 1.8 2.0


Figure 3 Predicted bottom elevation (row 5, x = 3.28 ft)
(see Figure A.3 for definition of axes).


-50-

-60-

-70-

-80-

-90-

-100-
0.0














pile cap 0 deg skew to flow
deg pile cap (top position) 0 deg skew to flow
deg pile cap (top position) 0 deg skew to flow


0.2 0.4 0.6
0.2 0.4 0.6


0.8 1.0 1.2
y (ft)


. I
1.4


1.6
1.6


1.8 2.0
1.8 2.0


Figure 4 Predicted bottom elevation (row 7, x = 5.25 ft)
(see Figure A.3 for definition of axes).


S No
*-*** 70
--- 90


20-

10-

0-


-10-

-20-

-30-

-40-

-50-

-60-

-70-

-80-

-90-

-100
0.0


- -------I -------













S No pile cap -
-***70 deg pile cap
- 90 deg pile cap


0 deg skew to flow
(mid position) 0 deg skew to flow
mid position) 0 deg skew to flow


Figure 5 Predicted bottom elevation (row 3, x = 1.31 ft)
(see Figure A.3 for definition of axes).














- No pile cap 0 deg skew to flow
*-*** 70 deg pile cap (mid position) 0 deg skew to flow
- 90 deg pile cap (mid position) 0 deg skew to flow


30 1


20-
l-

10-

0-

-10-

-20-

-30-

-40-

-50-

-60-

-70-

-80-

-90-


_inn


0.0


I I I I j I jI 1
0.2 0.4 0.6 0.8 1.0
y (ft)


1.2 1.4 1.6 I 1.8
1.2 1.4 1.6 1.8


Figure 6 Predicted bottom elevation (row 5, x = 3.28 ft)
(see Figure A.3 for definition of axes).


-I


2.0














pile cap 0 deg skew to flow
deg pile cap (mid position) 0 deg skew to flow
deg pile cap (mid position) 0 deg skew to flow


0.2 0.4 0.6I
0.2 0.4 0.6


I I I i I I
0.8 1.0 1.2 1.4
y (ft)


I I


1.6 1.8


2.0


Figure 7 Predicted bottom elevation (row 7, x = 5.25 ft)
(see Figure A.3 for definition of axes).


-- No
-*-70
--- 90


30-

20-

10-

0


-10-

-20-

-30-

-40-

-50-

-60-

-70-

-80-

-90-


* -


-1nn


0
0.0
0.0













-No
*-*-*--** 70
------ 70
- 70


pile
deg
deg
deg


cap
pile
pile
pile


- 0 deg skew to flow
cap (top position 0 deg skew to flow
cap (mid position) 0 deg skew to flow
cap (bottom position) 0 deg skew to flow


20

10

0-

-10-


-20-

-30-

-40-

-50-

-60-

-70-

-80-

-90-

-100-
0.0


S I I I I I I I I I I I I I i
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
y (ft)


Figure 8 Predicted bottom elevation (row 3, x = 1.31 ft)
(see Figure A.3 for definition of axes).













No pile cap 0 deg skew to flow
-- 70 deg pile cap (top position) 0 deg skew to flow
....-- 70 deg pile cap mid position) 0 deg skew to flow
70 deg pile cap bottom position) 0 deg skew to flow


30

20-

10-

0

-10- -

-20-

-30

-40-

-50-

-60-

-70-

-80-

-90-


' I I I i I I
0.2 0.4 0.6 0.8 1.0
y (ft


1.2 1.4 1.6 I 1.8 2.0
1.2 1.4 1.6 1.8 2.0


Figure 9 Predicted bottom elevation (row 5, x = 3.28 ft)
(see Figure A.3 for definition of axes).


__in


0- I
0.0


I


I













- 0 deg skew to flow
cap top position 0 deg skew to flow
cap (mid position) 0 deg skew to flow
cap bottom position) 0 deg skew to flow


I I I I 1
0.6 0.8 1.0
y (ft)


I I I
1.2 1.4 1.6


1.8 2.0


Figure 10 Predicted bottom elevation (row 7, x = 5.25 ft)
(see Figure A.3 for definition of axes).


pile
deg
deg
deg


- No
*-* 70
----- 70
- 70


20-

10-

0


-10-

-20-

-30-

-40-

-50-

-60-

-70-

-80-

-90-


------------- ----------.-


In4


- I .UU-
0.0


0.2 0.4
0.2 0.4


i I














No pile cap 0 deg skew to flow
-** No pile cap 7.5 deg skew to flow
- No pile cap 15 deg skew to flow


.ou

20

10
0-

-10-

-20-

-30-

-40-

-50-

-60-

-70-

-80-

-90


-100--
0.0


I I I I I I I I I i
0.2 0.4 0.6 0.8 1.0
y (ft)


Figure 11 Predicted bottom elevation (row 3, x = 1.31 ft)
(see Figure A.3 for definition of axes).









23


1.2 1.4 1.6 1.8 2.0
1.2 1.4 1.6 1.8 2.0














-- No pile cop 0 deg skew to flow
*-** No pile cap 7.5 deg skew to flow
- No pile cap 15 deg skew to flow


30 1


20-
l-

10-

0-

-10-

-20-

-30-

-40-

-50-

-60-

-70-

-80-

-90-


0.2 0.4 0.6 0.8
0.2 0.4 0.6 0.8


I I I
1.0 1.2
y (ft)


-4


1.4 1.6 1.8


Figure 12 Predicted bottom elevation (row 5, x = 3.28 ft)
(see Figure A.3 for definition of axes).


_- 1 nn


- I 0.0
0.0


2.0














pile cap 0 deg skew to flow
pile cap 7.5 deg skew to flow
pile cap 15 deg skew to flow


I I I I I 1
0.6 0.8 1.0
y (ft)


I I I
1.2 1.4 1.6


Figure 13 Predicted bottom elevation (row 7, x = 5.25 ft)
(see Figure A.3 for definition of axes).


-No
- **** No
- No


20-

10-

0-

-10-

-20-

-30-

-40-


-50-

-60-

-70-

-80-

-90-


4


~"IIiI2I


1 nn


-I


--IuU
0.0
0.0


I .4
0.2 0.4


1.8 2.0
1.8 2.0














70 deg pile cap (top position) -
-*** 70 deg pile cap top position -
70 deg pile cap (top position -


30

20-

10-

0

-10-

-20-

-30-

-40- -

-50-

-60-

-70-

-80-

-90-


i 0nn


I1


0 deg skew to flow
7.5 deg skew to flow
15 deg skew to flow


0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
y (ft)




Figure 14 Predicted bottom elevation (row 3, x = 1.31 ft)
(see Figure A.3 for definition of axes).


-














deg pile cap (top position) 0 deg skew to flow
deg pile cap (top position 7.5 deg skew to flow
deg pile cap (top position 15 deg skew to flow


I I I I I I I I I I I
0.2 0.4 0.6 0.8 1.0 1.2 1.4
y (ft)


1.6 1.8 2.0
1.6 1.8 2.0


Figure 15 Predicted bottom elevation (row 5, x = 3.28 ft)
(see Figure A.3 for definition of axes).


70
--70
- -- 70


,JU

20-

10-

0

-10-

-20-

-30-

-40-

-50-

-60-

-70-

-80-

-90-


-1nn


- I


0.0


I












70 deg pile cap
- 70 deg pile cap
- -- 70 deg pile cap


10
t-


-10-
-lo

-20-

-30-

-40-

-50-

-60-

-70-

-80-

-90-


mid position) 0 deg skew to flow
mid position 7.5 deg skew to flow
mid position 15 deg skew to flow


0.2 0.4 0.6


I I I
0.8 1.0
y (ft)


1.2 1.4 1.6I 1.8
1.2 1.4 1.6 1.8


Figure 16 Predicted bottom elevation (row 7, x = 5.25 ft)
(see Figure A.3 for definition of axes).


4 ~. -


-Inn


-I-


0.0


2.0
2.0


-


c
c













S 70 deg pile cap mid
* 70 deg pile cap mid
--- 70 deg pile cap (mid


position 0 deg skew to flow
position) 7.5 deg skew to flow
position 15 deg skew to flow


y (ft)


Figure 17 Predicted bottom elevation (row 3, x = 1.31 ft)
(see Figure A.3 for definition of axes).














S 70 deg pile cap (mid position) deg skew to flow
70 deg pile cap mid position 7.5 deg skew to flow
S- 70 deg pile cap (mid position 15 deg skew to flow


30

20-

10-

0-
* o
o -10

S-20-




S-70-
0




-0-
-P

H-50-

-60-

o -70-

-80-

-90-

-100- I I I '
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
y (ft)




Figure 18 Predicted bottom elevation (row 5, x = 3.28 ft)
(see Figure A.3 for definition of axes).














70 deg pile cap (top position 0 deg skew to flow
* 70 deg pile cap top position -7.5 deg skew to flow
--- 70 deg pile cap (top position 15 deg skew to flow


30

20

10

0

-1C

-2C

-3C

-4C

-5C

-6C

-7C

-8C

-9C


I-


0.2 0.4 0.6 I
0.2 0.4 0.6


0.8 1.0 1.2
y (ft)


1.4


1.6


*


1.8 2.0


Figure 19 Predicted bottom elevation (row 7, x = 5.25 ft)
(see Figure A.3 for definition of axes).


_inn


0.0


-

-


r


c
r









REFERENCES


Brown, C.B. (1950), "Sediment Transportation." In Engineering Hydraulics, H. Rouse, Ed.
Wiley, New York.

Carstens, T. and H.R. Sharma (1975), "Local Scour Around Large Obstructions." Int.
Assoc. for Hydraul. Res., 16th Congr., Proc., V.2, Subj. B, 251-262.

Chang, F.M., D.B. Simons and E.V. Richardson (1967), "Total Bed-Material Discharge in
Alluvial Channels." Proc. 12th Cong. IAHR, Fort Collins, Colorado.

Du Boys, M.P. (1879), "le Rh6ne et les rivieres A lit affouillable." Mem. Doc. Ann. Ponts
Chauss6es. Ser.5, No.18, 141-195.

Garde, R.J. and M.L. Albertson (1958), Discussion of "The Total Sediment Load of
Streams." by E.M. Laursen, Proc. A.S.C.E.. J. Hydraulics Div., Vol.84, No.HY6,
1856-59-64.

Hannah, C.R. (1978) "Scour at Pile Groups." Master of Engineering Thesis at the
University of Canterbury, New Zealand, 92 p.

HEC-18, (1991) Hydraulic Engineering Circular No. 18, "Evaluating Scour at Bridges."
Pub. No. FHWA-IP-90-017.

Melville, B.W. (1984) "Live-Bed Scour at Bridge Piers." J. of Hydraulic Engineering,
V.110, No.9, 1234-1247.

O'Brien, M.P. and B.D. Rindlaub (1934), "The Transportation of Bed-Load by Streams."
Trans. A.G.U., Vol.15, 593-603.

Raudkivi, A.J. and H.H. Witte (1990), "Development of Bed Features." J. of Hydraulic
Engineering, V.116, No.9, 1063-1079.

Sheppard, D.M. and A. W. Niedoroda (1992) "Structure-Induced Sediment Scour Local
Scour Bounds & Assessment of Global Scour." Final report to Florida Department of
Natural Resources, PR 4910451123412 UPN No. 90100412, 84 pages.

Shields, A. (1936), "Anwendung der aehnlichkeitsmechanik und der turbulenz forschung auf
die geschiebebewegung." Mitt. Preuss. Versuchsanstalt Wasserbau Schiffbau. Berlin,
No.26.

van Rijn, L.C. (1984), "Sediment Transport, Part Im: Bed Forms and Alluvial Roughness."
J. of Hydraulic Engineering, V.110, No.12, 1733-1754.


1 /












ACKNOWLEDGEMENTS


The authors would like to thank the Florida Department of Transportation District IV
Office for supporting this research. The technical insight and input throughout this study by
Mr. Roberto Perez, P.E. and Mr. Rick Renna, P.E. with FDOT has been most helpful and
appreciated. Mr. Sterling Jones with the Federal Highway Administration (FHWA) also
provided helpful suggestions at various points in the project. A number of graduate students
headed by Maximo Ramos worked on different aspects of this study. They include Mike
Dombrowski, Kailash Krishnamurthy, Suresh Maddula and Andrade Prashant. Thanks to
Mr. Rick Renna, P.E. (FDOT) and Mr. Frank Balsamo, P.E. (Parsons Brinckerhoff Quade
and Douglas, Inc.) for their work on the contract administration. Also, thanks to Ms. Laura
Dickinson for her assistance with data reduction and word processing.




















APPENDIX A


HYDRODYNAMIC STUDY









HYDRODYNAMIC STUDY


A.1 Background

As stated in the introduction, the primary objective of this study was to determine if
local scour depths can be predicted from near-bottom flow measurements over the pre-
scoured bed near the structure. The hydrodynamic study with the fixed bed is discussed in
this appendix and details of the scour study are discussed in Appendix B.

Sediment transport is a complex process but it is generally believed that the movement
of sediment is related to the shear stress at the bed. For this reason, an attempt was made to
estimate the bottom shear stress from flow measurements at a fixed distance above the bed
0.12 in (3 mm). In a fully developed open channel flow, estimation of the bottom shear
stress is relatively straight forward. This is not the case for the complex "wake region" near
a multiple pile structure and certain assumptions regarding the stress distributions had to be
made. To further complicate the situation, the mean flow direction near the structure was
found to vary with time as a result of the vortices being alternately shed by the piles and pile
cap.

A two component Dantec 56C01 constant temperature anemometer was used for the
flow measurements (see Table A.1). The response of the Dantec R61 X sensor probe used
was such that the high frequency fluctuating components of the velocity could be measured
(see Table A.2). It was determined that when the X sensor probe was mounted such that the
filaments lie in parallel vertical planes, variations in the mean velocity direction in a
horizontal plane could not be resolved. That is, when the probe is oriented to measure
velocities in the x (horizontal) and z (vertical) directions, changes in the direction of the
horizontal velocity were not detected. This is to be expected since the probe is not designed
for this flow situation. This means that the turbulent component of the shear stress, pu'v',
could not be accurately measured near the structure where the horizontal component of
velocity undergoes fluctuations in direction as large as +300. Ordinarily, the in line
horizontal and the vertical velocities would be measured and the mean horizontal velocity, U,
and time average of the product u'w' computed in order to determine the total horizontal
shear stress

= puV (A.1)



To avoid the problem of the probe not being able to resolve variations in flow
direction, it was decided to rotate the probe 90 so that the filaments were in parallel,
horizontal planes. In this position, the probe measures the two horizontal components of
velocity, U and V. It was assumed that the turbulent shear stress is proportional to
p(u'2 + v'2). The total shear stress would then be










S= U + ap(2+ v2) (A.2)

where "a" is a constant to be determined from measurements in front of the structure at
x = -4.92 ft and z = 0.12 in (3 mm). The analysis resulted in a value of 0.10 for "a".

The value of "a" was computed using the expression:


a -U V
(u2 + v'2)

It was evaluated at a point in front of the structure at different heights above the bottom (z =
0.93, 4.15 and 7.38 in) and found to vary between 0.08 and 0.12. The value of a" ranged
from 0.04 to 0.09 next to the structure. The lower values of "a" next to the structure were
probably due to the inability of the X sensor probe to resolve horizontal variations in the
mean flow direction larger than +300. Based on the above, a value of 0.10 for "a" was
used throughout the flow field.

Figure A. 1 compares the shear stress profiles in front of and away from the influence
of the structure computed using Equations A.1 and A.2. The shear stress profiles are similar
and at z = 0.12 in (3 mm) both give the same value for 7t (4.59 x 10-3 lb/ft2). The velocity
profile corresponding to the shear stress profiles in Figure A.1 is shown in Figure A.2. A
dimensionless constant k, was also computed (k = 1.08) to relate the turbulent energy
obtained with the probe in the horizontal position to the turbulent energy with the probe in
the vertical position i.e.

U12 + V/2
k= U + 2 (A.3)
U/2 W/2


A grid was laid out over a portion of the area expected to scour. The region directly
in front of the structure was excluded due to the complexity of the flow direction resulting
from the horseshoe vortices. The grid is shown in Figures A.3 and A.4. In Figure A.3, the
x and y scales are equal. The y scale has been expanded 4.5 times in Figure A.4. This
distorted scale is used to present both hydrodynamic and scour results.

A.2 Experimental Procedures

Prior to conducting the actual tests, the instrumentation had to be set up and
calibrated. Preliminary tests were also conducted in the flume to determine the optimum
location to place the model. It was determined that the flow over the V-notch weir had a
jetting effect such that the velocity across the tank was not uniform as the flow approached
the test area, so a fine mesh screen was constructed and placed directly downstream of the


I









weir. This significantly decreased the jetting effect. A floating wooden lattice was also
placed at the entrance of the flume to dampen the waves generated by .the weir and a plastic
wire mesh wave absorber was placed at the downstream tailgate to minimize wave reflection.
The anemometer measurements were sensitive to water temperature variations so a
temperature probe was placed near the anemometer probe as shown in Figure A.5. The
calibration curve for the temperature probe is shown in Figure A.6. The hydrodynamic tests
were conducted at water temperatures ranging from 25 to 29 OC.

The mean flow direction at each grid point had to be determined for each structure
configuration. The anemometer was then directed in the mean direction to minimize the
probe skew angle. Prior to conducting the hydrodynamic tests, the flow was started and a
vane was used to establish the mean flow direction at each grid point.

The hydrodynamic tests were conducted at night (starting around midnight) when the
air temperature was on the decline, to minimize the change in water temperature during the
test. Even with the decline in air temperature, the water temperature increased at a rate of
approximately 0.3 C/hour due to the pump and recirculation. Prior to starting the
experiment, the anemometer probe was towed at several velocities through the still water in
the flume to obtain a calibration curve for the velocity. A typical velocity versus probe
output voltage curve for the X sensor probe is shown in Figure A.7. After calibration, the
pump was started and the flow allowed to stabilize for approximately 45 minutes prior to
starting the flow measurements.

The probe was first moved in front and away from the influence of the structure and
the velocity measured at depths ranging from 0.12 to 7.38 inches above the bottom. This
was used to obtain the undisturbed velocity profile (Figure A.2). The carriage was then
moved along the structure and measurements were taken at the specified grid points. The
measurements were made at all grid points in one column prior to moving to the next
column, starting with points in front of the structure. The grid points are in rows and
columns with the rows having constant x values and the columns having constant y values.
Because the temperature in the flume increased at a rate of approximately 0.3 C/hour, the
velocity measurements in front of the structure were used to determine the variations in the
flow measurements due to changes in the velocity calibration. After completing the
measurements at the grid points, the velocities directly behind the structure were measured.

In order to compensate for the change in water temperature during the course of the
test, curves of the mean velocity and turbulent energy/unit mass in front of the structure
versus time were constructed as shown in Figures A.8 and A.9. Since the flow velocity did
not vary significantly across the tank, the mean velocity and turbulent energy/unit mass
curves could be applied to the entire test area. This allowed the mean velocity and turbulent
energy at each grid point to be non-dimensionalized by the mean velocity and turbulent
energy in front of the structure.









A.3 Data Acquisition


A Metrabyte 16F computer board and ACQWIRE data acquisition software were used
to digitize and record the analog signals from the X sensor and temperature probes. Scales
were placed along and across the flume and the readings were recorded manually. The
vertical position traversing mechanism was controlled by a lap top computer. In addition, a
cut off switch attached to the probe support was used for the lowest (0.12 in) position (see
Figure A.5). This switch was used to insure that the bottom velocity measurements were
made at the same fixed distance above the bed at all locations.

A.3.1 X Sensor Probe.

The probe has 2 perpendicular wires in parallel planes that sense the normal
components of the flow in the plane of the wires. The output voltages that are proportional
to the velocity components are processed by the ACQWIRE data acquisition software to
obtain U (velocity along the tank) and V (velocity across the tank) based on a velocity
calibration curve such as that shown in Figure A.7. Flow and temperature measurements
were sampled at a rate of 50 hertz. Due to the change in the average temperature of the
water in the flume, the X sensor probe had to be calibrated prior to each set of tests. The
two velocity components, U and V can be calculated from the sum and difference of the hot
film anemometer output voltages, Volti and Volt2, as follows:

U ( Voltl+Volt, ) (A.4)

and
V= ( Volt,-Volt, ). (A.5)


A typical unfiltered signal is shown in Figure A.10 and the same signal after filtering
is shown in Figure A.11. Plotted in Figures A.12 and A.13 are the power density spectra of
U and V for the time series shown in Figures A.10 and A. 11. The power density spectra of
the unfiltered U signal shows a large spike of energy at 10 hertz. This was caused by vortex
shedding from the probe holder. Although the noise at 10 hertz for the V signal was not as
large as for the U signal, a bandstop, fourth-order Butterworth filter at 10 hertz was applied
to both signals.

A.3.2 Temperature Probe

The temperature probe (see Table A.3) was attached downstream of the X sensor
probe to measure the instantaneous temperature fluctuations. Ideally, the temperature probe
would be located at the X sensor probe but since the temperature probe was rather large
(approximately 0.156 inches in diameter and 3.33 inches long), it was placed far enough









downstream so as not to disturb the flow at the X sensor probe. It was located
approximately one foot downstream of the X sensor probe. These water temperature
measurements were used in the ACQWIRE data acquisition software to apply a correction to
the velocity data after measurement.

A.3.3 Temperature Correction

The ACQWIRE data acquisition software applies a correction to the measured raw
voltages and is valid for small temperature changes such that the physical parameters of the
fluid can still be assumed to be constant. This temperature adjustment is shown below:



Vb = Vb (1- a-T (A.6)



where
Vbo = bridge voltage at ambient fluid temperature To,
Vb = bridge voltage at temperature T,
ao = temperature coefficient of resistance
(property of probe provided by manufacturer),
a = overheat ratio (Rh-RCo/RIo,
Rco = cold sensor resistance at To,
Rh = hot sensor resistance,
To = calibration fluid temperature (i.e. ambient temperature) and
T = fluid temperature during measurement.

A.3.4 Vortex Shedding from Probe Holder

Alternate vortex shedding in the wake region of the flow around the X sensor probe
support induced a response that influenced the flow measurements. The traversing
mechanism and probe support were more rigid in the x direction than the y direction thus the
response was greater in the y direction. The response is evident in the power density
spectrum of U shown in Figure A. 12. Note the spike in the spectrum at 10 hertz. If the
Strouhal Number, St, is assumed to be 0.2 for smooth cylinders, the vortex shedding
frequency, fy, can be computed as follows:
S, U (A.7)
d

If U = 0.98 ft/sec and
d = 0.02 ft,
then f, = 10 hertz.









This made it necessary to' filter the velocity measurements with a bandstop, fourth-
order Butterworth filter at 10 hertz. A pre and post filtered time series and energy density
spectrum are shown in Figures A.10 to A.13.

A.4 Flow Visualization Measurements

Several flow measurements were taken prior to the hydrodynamic study to determine
the different parameters that may affect the results. It was necessary to determine the
limitations of the X sensor probe and the characteristics of the flume. The results of these
tests are presented and discussed in this section.

1. Variation of mean velocity versus number of data points sampled

The sampling duration had to be longer than the lowest frequency component
of the flow. The sampling rate used was 50 hertz and a plot of mean velocity versus
number of data points sampled (i.e. the duration of the record) is shown in Figure
A. 14. Variations in the mean velocity appear to cease at about 8192 samples so this
duration record was selected for all tests.

2. Variation of mean velocity parallel and normal to the 70 pile cap (top position)
structure

The grid points chosen for the flow measurements were based on the results of
mean velocity measurements parallel and normal to the structure. The X sensor
probe was placed as close as possible to the 70 pile cap (top position) structure and
measurements taken at z = 0.12 in (3 mm) between pile 1 and 2, pile 2 and 3 pile 3
and 4 and so forth. Figure A. 15 shows the variation of the ratio of the mean velocity
next to the structure and the mean velocity in front of and away from the influence of
the structure, as the probe was moved along the structure. The results show that by
taking measurements between every other pile (e.g. pile 1 and 2, pile 3 and 4 etc.),
the velocity field along the structure can be mapped. Figure A. 16 shows the variation
of the mean velocity across the tank as the probe was moved away from the structure.
The probe was positioned at x = 0.335 ft and z = 0.12 in (3 mm) and the y position
of the probe was varied. The coordinates of the grid points used during the
hydrodynamic experiments are listed in Table D. 1.

3. Variation of mean velocity at the same location

The probe was positioned in front of and away from the influence of the
structure and three consecutive mean velocity measurements were taken at the same
location (x = -4.92 ft; y = 0.269 ft and z = 0.12 in). Table A.4 shows that the
mean velocity can fluctuate up to 2.5% even without the influence of the structure.









4. 'Variation of mean velocity across the tank


The probe was once again moved in front of the structure (x = -4.92 ft and
z = 0.420 ft) and flow measurements were taken as the probe was moved away from
the center of the tank. Table A.5 shows that the flow was fully developed across the
tank in the test area.

5. X sensor probe cosine response

Preliminary tests using a flow vane and streamers were conducted to determine
the changes in horizontal flow direction as a function of time near the structure. This
showed that the flow direction varied up to 300 from the mean. To determine if the
X sensor probe (filaments in the vertical direction) could resolve the horizontal flow
direction changes, the probe was placed in front of and away from the influence of
the structure and the mean velocity measured with the probe at various angles to the
flow. Column 2 in Table A.6 shows that the velocity component in the x direction,
U, measured by the probe increased as the probe was rotated away from the mean
flow direction. The correct cosine response of the X sensor probe is listed in column
3. Turning the probe on its side (filaments in the horizontal direction) improved the
ability of the probe to resolve changes in the horizontal flow direction. With the
probe in the horizontal position, the U and V velocity components can be measured.
Table A.7 lists the measured resultant velocity V U2 + V This value should, of
course, remain constant as the probe is rotated. The results show that the measured
velocity when the probe is at 150 to the flow is 1.3% less than the measured velocity
when the probe is at 00. This is within the range of fluctuations of the mean velocity
at the same location as shown in Table A.4

Table A.1 Constant temperature anemometer technical data.

Dantec 56C01 CTA Unit Serial No. 905B0121 688
Dantec 56C17 CTA Bridge Serial No. 905C0171 1447 and 1435

Top Resistance 200
Bridge Ratio 1 : 20
Sensor Resistance Range 3 30 0
Resistance Measuring 0.1 0/V
Sensitivity
Probe Cable Length 16.4 ft
Amplifier Shape Film
Temperature Range +5 to +40 C









Table A.2


X sensor probe technical data.


Dantec R61 Probe Serial No. 9055R0611

Wire 1 Wire 2
Sensor Resistance at 20 OC 6.96 0 6.10 0
Probe Lead Resistance 0.60 0 0.60 0
Coefficient of Resistance 0.39 %/oC 0.40 %/C
Overheat Ratio 0.045 0.045
Cable Resistance 0.14 0 0.04 0




Table A.3 Temperature probe technical data.

Analog Devices AC2626-J4 Temperature Probe
Serial No. 2461 9161

Accuracy 0.125 OC max @ 25 C
Temperature Range -55 oC to +140 OC
Voltage Supply +4 V to +30 V
Output 1 pA/K
Length 4.0 in
Diameter 0.187 in









Table A.4


Variation of mean velocity with time at the same location
(x = -4.92 ft; y = 0.269 ft and z = 0.12 in).


Test Number Mean Velocity (ft/sec)
1 0.781
2 0.764
3 0.761









Table A.5 Variation of mean velocity across the tank in front of and away
from the influence of the structure (x = -4.92 ft and
z = 0.41 ft).

Test Number Mean Velocity y (ft)
(ft/sec)
1 1.191 -0.344
2 1.220 0.312
3 1.217 1.04
4 1.198 1.95









Table A.6


X sensor probe cosine response probe vertical
(x = -4.92 ft; y = 1.46 ft and z = 0.12 in).


Flow Skew Angle Umasur UCoct
(ft/sec) (ft/sec)
0 0.754 0.754
7.5 0.760 0.748
15 0.766 0.728








Table A.7 X sensor probe cosine response probe horizontal
(x = -4.92 ft; y = 0.692 ft and z = 0.12 in).

Angle Velmcasured Velcor
(ft/sec) (ft/sec)
0 0.690 0.690
15 0.680 0.690
30 0.640 0.690


A10








1.0






o
0
0
m 0.6


aO
4 0.4



S0.2



on


- Probe Horizontal
*.*4. Probe Vertical





\
\



\
1
\1
\1
\1
\ \
\\b


0.1 0.2 0.3 0.4
Shear Stress (x 10**2 lbf/ft**2)


Shear stress profiles in front of and away from the influence of
the structure at x = -4.92 ft and y = 0.364 ft.


0.0 0.5


1.0 1.5
Velocity U (ft/sec)


Figure A.2


Velocity profile corresponding to shear stress
Figure A.1.


All


Figure A. 1


0.001


profiles in


. .


...


/ 3v
















foWW


1


2 3 4 5 6 7E




I E8


vIv T | --- -
-4.92 I
I OUTER EDGE OF PILES

I I
L---------------'



Figure A.3 Grid points for hydrodynamic study (1:1 scale).


1 2 3 4 5 6 7


4.92


E


A


x (ft) ---------- -- .-
I5.0.0-

PIE CAP



OUTER EDGE OF PILES


Figure A.4 Grid points for hydrodynamic study (distorted 1:4.5 scale).


A12


1.99


1.99


FLOW


I--I--II




II--"--------I -- I -- I -:1- II I



II--------------I -- I -- I -- II -- I -- I


0.36



7E
C
B
A











Vertical


Vertical
Traverse (z)







Probe Switch


X-Probe Thermocouple


Traverse Across The Tank (y)



Carriage


No Pile Cap Structure


Flow

z

tx


Flume Bottom


Figure A.5


Overall set-up of experimental apparatus and instrumentation for
hydrodynamic study.


A13















a
S35.0


o


030.0


a

S25.0
Ei


Voltage Reading (volts)


Figure A.6 Calibration curve for the temperature probe.



3.0






...2.0-
O




0.I

> 1.0 -






0.0- I i i I i i 1 i
0.0 1.0 2.0 3.0 4.0 5.0
Voltage Reading (volts)

Figure A.7 Calibration curve for the x sensor probe.


A14











1.0



0.8



0.6



0.4-



0.2


i .i i I .i
0 1 2 3 4 5 6
Time (hours)


Figure A.8


Mean velocity in front of structure vs. time at x
z = 0.12 in (test H-1).


= 4.92 ft and


Figure A.9


2.0 -


41.0
a








0.0



0 1 2 3 4 5 6
Time (hours)
Turbulent energy/unit mass in front of structure vs. time at
x = -4.92 ft and z = 0.12 in (test H-l).


A15


** *


A

0
1)

Is-

















U (velocity along tank)


[sec]
chOOb

V (velocity across tank)


2 J 5 6 7 8 s 10 11 12 13 14


Figure A.10 Unfiltered U and V velocity components for test H-1 at grid
point B2.


A16


1.1

















U (velocity along tank)


[sec]
fchOOb
V (velocity across tank)


[sec]
fchOl b


11 12 1 1 14 15


Figure A.11 Filtered U and V
point B2.


velocity components for test H-1 at grid


A17























Power Density Spectrum of Unfiltered U (velocity along tank)


0.07


0.06


0.05


?o.-
0.04


0.03


0.02


0.01


0









0.07



0.06


0.05


0.04



0.03



0.02



0.01


2 4 6 8 10 12 14 16 18 20 22 24
[Hz]
pmfchOOb


Figure A.12


Power density spectra of U velocity components in Figures A. 10

and A.11.


A18


Power Density Spectrum of Filtered U (velocity along tank)


I





















Power Density Spectrum of Unfiltered V (velocity across tank)


0.045


0.04


0.035


0.03


S0.025
. n.

0.02


0.015


0.01


0.005



2 4 6 8 10 12 14 16 18 20 22 24
[HZ]
pmchOlb

Power Density Spectrum of Filtered V (velocity across tank)



0.045


0.04


0.035





0.025

0.02


0.015


0.01


0.005



0 2 4 6 5 10 12 14 16 18 20 22 24
[H]z
pmfchOlb






Figure A.13 Power density spectra of V velocity components in Figures A.10

and A.11.


A19
































-I


2000 4000 6000 8000
Number of Data Points Sampled


Mean velocity vs number of data points sampled (x = 0.335 ft;
y = 0.364 ft and z = 0.12 in).


2.0-




1.5-



1.0





a,

:


U.U
0.0


Figure A.15


1.0 2.0 3.0 4.0
x (ft)


Variation of mean velocity parallel to the 700 pile cap (top
position) structure at y = 0.364 ft and z = 0.12 in.


A20


1.20-





C, -
1.15










l 1.05 -
10-


1.00


Figure A.14


10000


5.0 6.0


I
































Z.U




S1.5




1.0

'-J


0.5




0.0o .
0.0 0.5 1.0 1.5 2.0
y (rt)


Figure A. 16 Variation of mean velocity normal to the 700 pile cap (top
position) structure at x = 0.335 ft and z = 0.12 in.


A21




























APPENDIX B



SCOUR STUDY


I










SCOUR STUDY


B.1 Background

Sediment scour experiments were performed in the same flume and with the same
model piers as the hydrodynamic study. The objectives of the scour study were: 1) to
provide scour data for calibrating and testing the scour prediction relationship developed as
part of the hydrodynamic study and 2) to provide laboratory scour data for multiple pile
structures with and without pile caps. Another important aspect of this study was the
measurement of the rates of scour.

The metal plates covering the recess in the flume were removed and the multiple pile
structure was centered in the test area. Sand was then placed around the structure (see
Figure B.1). Three different sediment sizes were used in the flume. The largest quantity of
sand (Category 1) was placed in the regions of the test area where no scour was anticipated.
This category had sediment sizes ranging from 0.2 to 0.8 mm (see Figure B.2). Category 2
consisted of sand that was used to anchor the sand in the first category and had a range of
sediment sizes from 0.84 to 2.00 mm (see Figure B.3). This minimized the amount of fine
sand transported down the flume and through the pump. The third category of sand was
placed in the region around the structure where scour was anticipated. This category had
sediment sizes ranging from 0.42 to 0.84 mm and a median grain size, d50, of 0.60 mm (see
Figure B.4).

Instrumentation with the ability to make in situ scour measurements (i.e.
measurements of the scour as it occurs) were developed. In the past, scour measurements
have been very laborious and time consuming. Recent developments in underwater acoustics
technology now allow techniques, formerly used only for bottom profiling at prototype
depths to be used in the laboratory. The main component of the system used was the Simrad
Mesotech 810 echo sounder (see Table B.1).

B.2 Instrumentation and Calibration

B.2.1 Depth Readings

The acoustic bed profiling system was developed, constructed and tested in the
Coastal Engineering Laboratory at the University of Florida. An acoustic pulse sent by the
echo sounder reflects off the bottom and the return pulse is sensed by the sounder. The time
required for the signal to travel to and from the bottom is measured and the distance
computed. Returns from particles in suspension also show up in the signal and must be dealt
with as noise. A schematic drawing of the system is shown in Figure B.5. The bowl which
sits just below the surface of the water when measurements are being made serves to
minimize the disturbance to the flow (see Figure B.6).


i









To allow the echo sounder to send acoustic signals to the bottom, a small window was
cut out of the bowl and a vinyl sheet glued over the slot. The acoustic reflection from the
vinyl sheet was minimal. The level of water in the bowl was such that the end of the
sounder head was always submerged. The echo sounder was mounted on a frame as shown
in Figure B.5 and a motor was used to rotate the head and thus the acoustic beam toward and
away from the structure. The entire depth measuring system was mounted on a carriage that
traversed the width of the flume.

The echo sounder operates at 2.25 Mhz. The 2.25 Mhz operating frequency was
chosen to give reasonable accuracies at short ranges, as well as to insure sufficient energy in
the reflection from the bed. A 10 1Jsec pulse is emitted at a 100 Hz rate. The returned pulse
is an analog time/amplitude history of the water column. The travel time of the signal is
proportional to the distance to a reflecting object. The amplitude of.the pulse at any point in
time is a measure of the number, size and density of reflecting particles in the water column.

The acoustic signal from the echo sounder was calibrated by positioning a flat metal
plate at different elevations below the sounder and the output recorded. A calibration curve
for the echo sounder is given in Figure B.7.

Noise in the acoustic data (Channel CHOO) came from reflections from the pile cap
and/or piles and suspended sediment. Since the main mechanism for sediment movement
was bed load transport, the noise caused by suspended sediment was minimal. A plot of the
raw echo sounder output is shown in Figure B.8. The output from the echo sounder was
filtered using a lowpass, sixth-order Butterworth filter with a cutoff frequency of 3 hertz. A
plot of the echo sounder output signal after filtering is shown in Figure B.9. Power spectra
for the unfiltered and filtered signals are shown in Figure B.10.

B.2.2 Position Readings

The angle the echo sounder head makes with the vertical was measured using a ten
turn precision potentiometer. A protractor mounted on the side of the unit was used in the
calibration process. The head was rotated to various angles and the output voltage from the
potentiometer recorded. Because of some play in the gears moving the sounder head, a lag
between the actual angle and the angle measured was observed. To compensate for this, a
lag was added or subtracted from the calibration curve depending on the direction the
sounder head was swinging. A plot of the modified calibration curve used is shown in
Figure B. 11 along with the actual readings during the calibration process. The modified
calibration curves are listed below:

for the swing from -70 to 00,

8 (degrees) = 39.60 11.16 V, (B.2)


I









for the swing from 00 to 320,
8 (degrees) = 39.102 11.16 V, (B.3)


for the swing from 320 to 00,
0 (degrees) = 42.60 11.16 V, (B.4)


and for the swing from 00 to -70,
6 (degrees) = 43.10 11.16 V. (B.5)


A small wheel in a track attached to a potentiometer was used to measure the position
across the flume and a similar arrangement was used to measure the position of the carriage
along the flume. Scales placed along and across the flume were used to calibrate the position
potentiometers. The start and ending points for each traverse was recorded in the log book.
A data reduction program was then used to transform the output from the echo sounder
signal and the position potentiometers to xyz coordinates.

B.3 Data Acquisition

A Data Translation DT2801 computer board and GLOBAL LAB data acquisition
software were used to digitize and record the analog signals from the echo sounder output
and the potentiometer readings at a sampling frequency of 40 hertz.

Channel CH01 recorded the output from the potentiometer measuring the position
along the flume (x direction) while Channel CH03 was used for the position of the carriage
across the flume (y direction). Both output signals were filtered using lowpass, sixth-order
Butterworth filters with a cutoff frequency of 0.5 hertz. The noise in Channel CH03 was
caused by sudden movements of the manually operated traverse across the flume. The
carriage was moved along the flume with a variable speed motor at a slow speed
(approximately 0.8 in/sec). The smooth motion along the flume minimized the noise in
Channel CH01. Figure B. 12 compares the power density spectra of Channel CH03 before
and after filtering. As mentioned before, the start and end points of the x and y traverses
were recorded in the log book.

The echo sounder angle signal was recorded on Channel CH02 and was filtered using
a lowpass, second-order Butterworth filter with a cutoff frequency of 20 hertz. The sounder
head was designed to swing from -90 to 350 but only data in the range from -70 to 320 was
used because of reflection and attenuation of the signal at the ends of the swing.









B.4 Data Reduction


A data reduction program was written to convert sediment scour data obtained from
the echo sounder and position potentiometers to bottom coordinates in rectangular coordinates
(x,y and z). The raw data consisted of: 1) voltage proportional to the distance from the echo
sounder head to the bottom, 2) voltage proportional to the position of the depth measuring
system along the tank (x position), 3) voltage proportional to the angle of inclination of the
sounder head (measured from a vertical line with the angle varying from -70 to 320) and 4)
voltage proportional to the position of the depth measuring system across the tank (y
position).

Figure B. 13 is a definition sketch of the measurements needed to convert the raw data
to x-y coordinates. The equations used are:

z = (R + R) cos 6 zo and
d = (R + Ro) sin 0,
where
R = distance from echo sounder face to bottom,
d = radial distance from echo sounder to bottom measurement,
zo = vertical distance from echo sounder pivot to original bottom,
Ro = distance from echo sounder pivot to its face and
0 = angle of echo sounder with vertical axis,
(-70 to +320).

After the data reduction process, the data was gridded using the surface fit program,
SURFER, which was used to interpolate and map the bottom. Figure B. 14 shows the data
points used to map out the bottom prior to the test while Figure B. 15 is a plot of the data
points after the test was completed. The number of data points used to map the bottom was
approximately 9,000.

B.5 Test Procedures

Prior to each scour test, the sand was compacted with a tamper, smoothed and
leveled. After the bottom was leveled, the flume was slowly filled with water to prevent
scour during the fill process. Before starting the flow, the bottom was mapped with the echo
sounder. This allowed the scour measurements to be compared with the original bottom.
After the pump was started, minor flow adjustments had to be made during the first 30
minutes of the test to obtain the desired velocities. Mapping of the bottom began a few
minutes after the pump was started and this continued throughout the duration of the test
(- 28 hours). The interval between the scans increased as the rate of scour decreased (from
- 10 minutes at the start to 90 minutes at the end). After the flow was stopped, the
bottom was mapped once more. The water was then drained from the flume and test area
and point gauge measurements of the bottom were made.









B.6 Time Rate of Scour

Since these experiments were conducted under clear water scour conditions, the scour
hole development could be observed visually. A soon as the flow was started, sediment
along the structure was put into motion. Due to the clear water scour conditions, there was
very little sediment being transported into the region surrounding the front row of piles, thus
the rate of erosion in this region is large. Further downstream in the structure, there was
initially (and for some time) more sediment being transported into the region than was being
scoured, thus accretion occurred. Accretion continued until the transport into the region fell
below the scour rate. The sediment transport processes were more rapid at first but
decreased with time making it very difficult to judge the "maturity" of the scour hole strictly
by observation.

Plots of scour depths versus time at different locations along the structure are very
informative (see Figures B.16 and B.17). From these figures, it is evident that the local
scour depths at rows 1 and 2 were approaching equilibrium after 28 hours. On the other
hand, a net scour at the piles in row 6 was just beginning 25 hours into the flow. Figures
B. 18 to B.20 compare the time rate of scour of piles 1, 2 and 6 for the no pile cap structure
and the 700 pile cap (top position) structure.









Table B.1 Echo sounder technical data.

Simrad Mesotech 810 Serial No. 9866
Transmitter
Transmitter 4 selectable pulse lengths and
2 selectable power levels
Operating Frequency 2.25 MHz
Output Power 200 Watts max
Beamwidth 0.8 degrees
Receiver
Maximum Range 8.53 ft
Repetition Rate 100 Hz
Assumed Sound Velocity 4838 ft/sec
Output Frequency 455 Hz
Miscellaneous

Temperature Operating: -10 to +40 C
Dimensions 3.5 in square x 8 in long









F ~- -~~t~i~hs~-~


- ~"~~ i !
i


~ "rr:' ,
/
-~~jl J~L


* aa ."~~ ~l


Figure B. 1


./W


Test area of flume showing no
region near structure.


~ 1 '
i:.


pile cap structure and central


100 -

90------- ---- --- --
90-- --

80

70------ -- --- ------
70------- -- ----

60

50

40----- ------

30 ------ -- ----

20 -


10----
0:-


Figure B.2


0.01


0.1 1
Sieve Diameter (mm.)


Sediment size distribution for Category 1 sand
(away from structure).


S


~" ~tsw
,,
t~s

































0.1 1
Sieve Diameter (mm.)


Sediment size distribution for
(anchoring layer).


Category 2 sand


100 --

90

80 ------ --------

70------ --

60------

50------ ---- ----

40------

30--

20 ---

10-----

0-- -- --- ---


0.1
Sieve Diameter


(mm.)


Sediment size distribution for Category 3 sand
(around structure).


1UU

90 ------ ---

80 -

70------

60------ -----

50----- ---- ---

40-- ---- ---- ---

30- -

20- -

10- -

0--


0.01


Figure B.3


Figure B.4

















Traverse Across The Tank


Flume Bottom


Overall set-up of experimental apparatus and instrumentation
for scour study.


Figure B.5








"j ,/,


II
9P. f


Figure B.6 Experimental set-up for scour study.


Figure B.7


1.5


B1.3

1.2



. 1.0

S0.9
o
0.8

. 0.7

0.6

0.5
0.0 0.5 1.0 1.5 2.0
Voltage Reading (volts)

Calibration curve for the echo sounder output signal.


B10


----
-------------



















Echo Sounder Output


2 WAA AAAAjWVA



0
0 0.1 0.2 0.5 0.4 0.5 0. 0.7 0.8 0.9 1 1.1 1.2
[minl
CHOO

Position Along Tank
0 -------------------------------------------------------------


0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2
[minj
CHOI

Echo Sounder Angle


0 0.1 0.2 0.3 0.4 0. 0 0. 0.8 0.9 1 1.1 1.2
[min]
CH02

Position Across Tank
0




.2

.3

-4


0 0.1 0.2 0.3 0.4 0.5 0.8 0.7 0.8 0.9 1 1.1 1.2
[mln]
CH03

Figure B.8 Unfiltered output signals after completion of test S-1.


Bl1


















Echo Sounder Output


4*




A2 ,AAAAAJAJAAAJAAAiAUUAAhjJ .



3 0.1 C.2 0.3 0.4 0.5 06 07 3 0.9 1 1.1 1.2
[mini
fchO0

Position Along Tank
0

4.









0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 3a 0.0 1 1.1 1.2
(mina
fchOl

Echo Sounder Angle













a 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0. 0. 1 1.1 1.2
[min]
fch02

Position Across Tank



-o

-8




--3


0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 3. 0.9 t 1.1 1.2
[mini
fch03

Figure B.9 Filtered output signals after completion of test S-1.


B12




















Power Density Spectrum of Unfiltered Sonar Output


[Hz]
pmchOO

Power Density Spectrum of Filtered Sonar Output


2.5
[Hz)
pmfchOO


3 3.5


Figure B. 10 Power density spectra of echo sounder output (Channel CHOO)

in Figures B.8 and B.9.


B13


016

0.24

0.22

0.2

0.18

0.16

0.14

0.12

0.1


0 0.5
































40.0


30.0



20.0



10.0



0.0


--U.U


i I i i I I I I il
.0 1.0 2.0 3.0 4.0 5
Voltage Reading (volts)


Figure B.11


Modified calibration curve for echo sounder angle.


B14


- -7.0 dog to 0.0 dog
**** 0.0 deg to 32.0 deg
*---- 32.0 deg to 0.0 deg
- 0.0 deg to -7.0 dog
***** Calibration Points

















Power Density Spectrum of Unfiltered Position Across Tank


0 0.1 0.2 0. 0.4 .5 0.6 0.7
[Hz]
pmch03

Power Density Spectrum of Filtered Position


0.8 0.9


Across Tank


[HZ]
pmfch03


Figure B.12


Power density spectra of y position potentiometer (Channel
CH03) in Figures B.8 and B.9.


B15


- -- -- --1--`


















Echo
Sounder


No Pile Cap Structure


z Scoured
t/ Bottom
x-i


The origin of the right hand coordinate system is located at the
original bottom on the front outer corner of pile C1.
Positive x is in the direction of flow, Positive z is upward and
Positive y is normal to the flow.


Figure B.13


Definition sketch showing measurements needed for conversion
of raw data to x-y coordinates.


B16










4.0




2.0




S0.0




-2.0




-A


-4.0 -2.0 0.0 2.0 4.0 6.0 8.0 10.0
x (ft)


Figure B.14 Data points before start of scour test S-1 (No pile cap structure).


2.0-




0.0 -




-2.0-


Figure B.15


-4.0 I ll I I I .
-4.0 -2.0 0.0 2.0 4.0 6.0 8.0 1
x (ft)

Data points after completion of scour test S-1 (NM
cap structure).


B17


OUTER EDGE OF PI.S


Mr
--}--


nTr w


o pile


0.0


I OM! OWr P=----
OUTER EDGE OF PILES










































Time (hours)


Figure B. 16


Time rate of scour of piles 1, 2 and 6 of no pile cap structure.


B18
















































Figure B.17


Time rate of scour of piles 1, 2 and 6 of 700 pile cap
(top position) structure.


B19





















50-




25-




0-




-25-


- 50 i i I i i i I i 1 1 | i i | i l i i I i i i I
0 5 10 15 20 25
Time (hours)





Figure B. 18 Time rate of scour of pile 1.









B20


70 deg Pile Cap (Top) Pile 1
-- No Pile Cap Pile 1








/ 1

^___________________




















S-'" NO rio e ;ap nie z

50

-2 4 0


, 2 5



2 -

-25-



2 -r
-5





0 5 10 15 20
Time (hours)





Figure B. 19 Time rate of scour of pile 2.
Figure B. 19 Time rate of scour of pile 2.


B21


I








































15 20
Time (hours)


Figure B.20


Time rate of scour of pile 6.


B22




















APPENDIX C


ANALYSIS OF HYDRODYNAMIC AND SCOUR RESULTS










ANALYSIS OF HYDRODYNAMIC AND SCOUR RESULTS


C.1 Mathematical Model

Preliminary tests indicated that the X sensor probe had to be rotated 900 (such that
the filaments were in parallel, horizontal planes) in order for the probe to resolve horizontal
variations in the flow direction (see Appendix A). In this position, the probe measured the
two horizontal components of velocity, U and V. It was assumed that the turbulent shear
stress is proportional to p(u'2 +v'2). The total shear stress would then be

t = pU + ap(u'2+ v'2) (C.1)


where "a" is a dimensionless constant to be determined from measurements in front of (and
not influenced by) the structure. The analysis resulted in a value of 0.10 for "a".

The dimensionless shear stress, rd, was then computed at each grid point where flow
measurements were made. The dimensionless shear stress is the ratio of the shear stress at
any point, r, and the shear stress of the undisturbed flow in front of the structure, (o, at z =
0.12 in. The surface fit program SURFER, was used to interpolate and map the
dimensionless shear stress over the study area. SURFER was also used to fit the scour data.
The grid laid out over a portion of the area expected to scour is shown if Figure A.4. With
the existing probe support, hydrodynamic measurements closer than 0.364 ft to the piles
could not be made due to the pile cap. This made it necessary to extrapolate the shear stress
from the closest grid point to the piles.

In attempting to obtain a functional relationship between the equilibrium scour depth
and the shear stress, the following guidelines were used. The relationship should be as
simple as possible and be directed at the specific problem at hand. That is, it should be
directed at equilibrium scour depths at the transition from clear water to live bed conditions
thus eliminating the need to introduce sediment properties, etc. The thought being that this
relationship could be extended to include a broader range of conditions at a later date. In
light of the above, the following form was selected:
d, = f () (C.2)

where
de = equilibrium scour depth,

f(r) =b= b( l (C.3)



C1










Td = (C.4)
'o

and b and c are empirical coefficients.

Since the increase in bottom shear stress caused by the different multiple pile
structures was the only parameter changing during the hydrodynamic experiments, the
coefficients b and c were determined by a least squares fit between the predicted and
estimated equilibrium scour depths at a section near the front of the pier. The coefficients
are considered constant for the range of conditions considered here. For a more general
formulation, the coefficients will undoubtedly be dependent on some of the quantities held
constant during these experiments. Various sediment transport equations have been
developed using the critical shear stress, Tc and/or bed shear stress, rs. The shear stresses in
these equations were raised to powers varying from 0.33 to 3 (see Table C.1).

The first step in evaluating the coefficients was to assign a value for c. Using the
dimensionless shear stress, Td, and the measured scour depth at grid point A3, a value for b
was computed. Grid point A3 (see Figure A.4) was selected because the time rate of scour
curves in Figures B. 16 and B. 17 indicate that this area is close to its equilibrium scour
depth. Grid point A2 was not chosen because rd is lower there than at A3. The reason being
that point A2 is far enough forward that the flow at that point is less influenced by the
structure. The values obtained for b and c are 0.152 and 0.5 ft respectively.

SURFER was then used to create a finer grid of the estimated shear stresses and the
corresponding scour depths at these grid points were computed using Equation C.2. One
constraint on the computed equilibrium scour depths was that the scour depth between two
adjacent grid points could not change more than that allowed by the sediment angle of
repose, 320 (Melville and Raudkivi, 1977). The predicted equilibrium scour depth is the
larger between the scour depth based on the dimensionless shear stress and the scour depth
which satisfies the sediment angle of repose.

To simplify the presentation of the results, bottom elevations instead of predicted and
measured scour depths are presented. Bottom elevation, z, and scour depth, de, are related
by z = -de. Figures C. 1 to C. 12 show rd, the measured bottom elevation after a 28 hour
scour test, and the predicted equilibrium bottom elevations for the no pile cap and the 700
pile cap (top position) structures. The estimated equilibrium bottom elevation shown in
Figure C.8 is the measured bottom elevation at 28 hours extrapolated to equilibrium and
adjusted for the velocity being below the transition between live bed and clear water
conditions. The extrapolation involved dividing the scour depths by 0.9 and multiplying by
1.14. The results indicate that the predicted equilibrium bottom elevations are closer to the
measured bottom elevations towards the front of the structure. Figures C. 13 and C. 14 are
contours of dimensionless shear stress around the no pile cap and 700 pile cap (top position)
structures. Figures C.15 and C.17 are the measured bottom elevations over the same area.









Figures C.16 and C.18 are contours' of the predicted equilibrium bottom elevations (for the
area where hydrodynamic flow measurements were made) based on the dimensionless shear
stresses in Figures C.13 and C.14. These figures show that the computed equilibrium
bottom elevations are deeper than the measured values throughout the area but the difference
is greater toward the back of the structure. This is one indication that the scour is much
closer to equilibrium near the front of the structure. Figures C.19 and C.20 show the 700
pile cap (top position) before and after the scour test.

Figure C.21 shows the results at row 3 (x = 1.31 ft) for the 700 pile cap (bottom
position) structure. The predicted equilibrium bottom elevations do not match the measured
bottom elevations for the cases where the pile cap was initially resting on the bottom (see
Figures C.22 and C.23). This however, is to be expected since for these cases the structure
shape near the bottom changes abruptly with the development of the scour hole. The scour
predictions are better suited for a structure with a pile cap that starts 0.42 ft above the
bottom and penetrates deep below the bottom. The scour test results for the cases where the
pile cap rests on the bottom should not be compared with the hydrodynamic test data for
these structures for the above stated reasons.

Flow measurements closer to the structure are clearly needed. As stated earlier, time
and cost constraints prevented the necessary modifications to the constant temperature probe
support to allow these measurements. Such measurements will be included in future studies.
One possible improvement would be to use flush mounted shear stress probes in the vicinity
of the structure where flows are highly unsteady and the shear stresses are large.























C3









Table C. 1 Some formulas for bed load and total load (Sleath, 1984).

Investigator Formula Comments
Du Boys (1879) Qs = Ar(o-rc) Bed Load
O'Brien and Qs = A(,-r)m Bed Load
Rindlaub (1934)
Shields (1936) Qs = AQS(70-T)/ Bed Load
((p,-p)gD)
Brown (1950) Qs = AD3a( / Bed Load
(ps-p)gd)
Garde and Qs = Q(D/d)f)) Total Load
Albertson (1958) (u*d/lv) _
Chang (1967) Qs = AfJU(-r) Total Load
















30

20

10

0

-10

-20

-30

-40

-50

-60

-70

-80

-90


-100--
0.0


Figure C. 1


I I II I I I I I I I
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.
y (ft)


Comparison of measured bottom elevation, predicted equilibrium
bottom elevation and dimensionless shear stress for the no pile
cap structure (row 2 : x = 0.33 ft).


--- measured bottom elevation
~- predicted equilibrium bottom elevation
S*-*-* dimensionless shear stress
-







-
-


-
















E 30
o

r 10
0
o- 0

I -10

0 -20
0
-30

S-40

S-50

-60

0 -70

-80

-90

-100
C








Figure C.2


1.0
y (ft)


Comparison of measured bottom elevation, predicted equilibrium
bottom elevation and dimensionless shear stress for the no pile
cap structure (row 3 : x = 1.31 ft).
















































S0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
y (ft)








Comparison of measured bottom elevation, predicted equilibrium
bottom elevation and dimensionless shear stress for the no pile
cap structure (row 4 : x = 2.30 ft).


-- measured bottom elevation
** predicted equilibrium bottom elevation
*-*-* dimensionless shear stress

S- -- -_ -
-







)-


30

20

10


-10

-20

-30

-40


-50-

-60-

-70-

-80-

-90-

-1io-
100-)


O.C


Figure C.3


-




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