scispace - formally typeset

Proceedings ArticleDOI

Tidal Bridge Scour in a Coastal River Environment: Case Study

29 Oct 2010-pp 894-902

Abstract: As part of an innovative design/build project a data-rich hydraulics and scour analysis was performed for the proposed 5 kilometer long Washington Bypass Bridge over the Tar River in Washington, NC. This analysis featured: 1) a current and stage monitoring program, 2) historical aerial photograph analysis, 3) extensive long term bed elevation study, 4) debris scour evaluation, 5) variable skew angles due to spatial and temporal changes in flow characteristics, 6) complex pier analysis, and 7) use of a two-dimensional (2D) hydrodynamic model (TABS RMA-2) to evaluate riverine flooding and hurricane surge hydraulics causing extensive wetting-and-drying. The model was subjected to a comprehensive two-step model calibration/verification process. Low-flow conditions were calibrated and verified using project-collected Acoustic Doppler Current Profiling (ADCP) and tidal gage monitoring. Boat-mounted ADCP measurements collected by USGS during hurricane surge (Hurricane Dennis) and rain-induced flooding (Hurricane Floyd) were used for high-flow calibration and verification. This was a rare opportunity for a bridge designer to be able to evaluate scour using a sophisticated hydrodynamic model that was calibrated with field data collected during an event that represented the design conditions. Two-dimensional hydrodynamic modeling was used to simulate complex hydraulics of the project site located at the end of the 8,300 square kilometer watershed which is tidally influenced. Due to the size of the upstream drainage area and the proximity to the open ocean both rain-induced-flow and storm-surge scenarios were considered.
Topics: Bridge scour (61%), Hurricane Floyd (58%), Monitoring program (53%)

Summary (3 min read)

INTRODUCTION

  • AECOM was hired by Flat Iron -United Joint Venture to perform engineering services relating to the hydraulics and scour analysis for the Washington Bypass Bridge (BIN 353 -US17 over the Tar River) in Washington, NC .
  • Under this agreement, AECOM developed a 2-dimensional hydrodynamic model (RMA-2) to evaluate the flow depth and velocity for the 100-, and 500-year storm events.
  • The predicted velocity and depth information from these events were used to calculate scour depths at the bridge and assist in designing the bridge substructure units to withstand scour.
  • The analysis consisted of three subtasks.
  • The second subtask was obtaining various historical bathymetric surveys conducted at the US 17 Bridge crossing, located about 1.6 km downstream of the Washington Bypass Bridge, and calculating the vertical changes of the river bottom.

METHODOLOGY

  • In order to estimate the site-specific detailed hydrodynamic characteristics at the Washington Bypass Bridge it was necessary to construct a 2-dimensional hydrodynamic model.
  • Hydrodynamic modeling was accomplished using the Surface Water Modeling System (SMS) in conjunction with RMA-2.

Model Domain

  • The model domain was defined considering the area of interest, location of available data sources, and the limitation on computational resources.
  • The area of interest was confined to the vicinity of the bridge crossing.
  • In order to obtain an adequate solution at the area of interest, model boundaries were established distant from the area of interest and where USGS station locations were available as data sources.
  • The model domain was meshed using triangular and rectangular elements.
  • The approximate number of elements in the meshes used ranges from 9,000 to 23,000.

Calibration and Verification

  • The RMA-2 model is calibrated primarily with two (2) parameters: the Peelet Number (Pe) and the Manning Roughness Coefficient (n) , and was perfonned in two stages.
  • The channel calibration of the model was perfonned with the available data and the data sets collected during the project's monitoring period.
  • Input variables for the model included WSE from the Pamlico Gage and model elements account for storage in watershed (which were used beyond the Tranter's Creek and Grimesland boundaries).
  • Verification of the model was performed in two (2) ways .
  • Second, for high flow conditions USGS ADCP velocity transects data at the maximum velocity conditions during Hurricane Floyd at 4S meters upstream of US 17 were compared to model predictions.

IS

  • Once the model was calibrated and verified, two different scenarios and two different return intervals totaling four (4) flow cases were developed: a rain-inducedflow scenario and a storm-surge scenario, for 100-and SOO-year storms.

Rain Induced Flow Scenario

  • As seen during Hurricane Floyd, a hurricane causing significant rainfall over an already saturated ground can cause significant flooding in Tar/Pamlico watershed.
  • Another observation made during Floyd was the duration of the peak flow.
  • The duration is important for scour in cohesive sediment (such as this case), as equilibrium scour conditions require time to develop.
  • FEMA FIS (Reference 2) was used to determine peak flow values for the model boundaries.
  • Only high flows (i.e. >283 cms) in ebb direction were considered while generating the relationship.

Hurricane Surge Scenario

  • Upon establishment of the typical tide, design hurricane surges were developed using the Pooled Fund Study (Reference 5) that was coordinated by the SCDOT and Boundary Conditions for Bridge Scour Analysis (Reference 6) by the NCDOT.
  • The same data sources used to create the calibrated model (i.e. shoreline, bathymetry, topography, etc.) were also used to extend the model.
  • Due to the large floodplain-to-channel ratio in the area (>8) , most of the numerical model cells were dry except for a short duration during the entire simulation.
  • The deviation was observed to be 8 degrees during ebb times (i.e. rain-induced-flow scenario) and 20 degrees during flood times (flood periods produce peak velocity/depth combinations).
  • Figure 3 shows peak velocity magnitude and direction for the 100-year stormsurge scenario.

Scour Analysis

  • Scour evaluation and calculations were performed in accordance with the guidelines set forth in the most current editions of the Federal Highway Administration's Hydraulic Engineering Circular (HEC) HEC-20 3 rd Edition (Stream Stability at Highway Structures), HEC-18 4th Edition (Evaluating Scour at Bridges), and HEC-25 151 Edition (Tidal Hydrology, Hydraulics and Scour at Bridges).
  • The hydraulic data required for the scour analysis were extracted from the 2-D model, the soil data were supplied by Mactec Inc., and the preliminary bridge geometry was taken from AECOM' s preliminary bridge design plans.
  • All equation constants and coefficients used in the analysis were taken from literature.
  • The following four components of total scour were calculated: aggradation and degradation, general scour, local scour, and lateral stream migration .
  • Based on the results of the 2D modeling of peak flows caused by rainfall and peak flows due to storm surge hurricane surge conditions (landward flow) produced the worst-case design scour conditions at the proposed bridge location.

Analysis of Long-Term Bed Elevation Change

  • The analysis consisted of three subtasks.
  • The third subtask was a channel stability assessment performed by riverine geomorphology expert Prof.
  • Stanley Riggs of Eastern Carolina University, who has specific knowledge of the project site.
  • The historical aerial photograph analysis was performed by digitization of the shoreline in the vicinity of the proposed bridge crossing and then the subsequent observation of the resulting pattern and rate of the channel movement.
  • Considering the current rate of change, significant degradation is not expected.

Scour at Abutments

  • Per the preliminary bridge design plans, the abutments were determined to be outside of the flood elevation, and were not analyzed further for scour.

Computation of the Magnitude of Local Scour at Piers

  • Local scour at the proposed Washington Bypass Bridge was computed for hurricane storm surges with recurrence intervals of 100 years and 500 years using the CSU equation as presented in HEC-IS.
  • Bents on the overbank which lie outside the potential 30 meters of channel migration should be designed to withstand the smaller calculated scour depths in these areas .
  • .5,4.3) 4 (1.8, 5.5) Adequate data was a key in preparing a comprehensive scour analysis.
  • Especially, the importance of long-term ADCP data and how it can improve the confidence level of sophisticated hydrodynamic model simulations became apparent.
  • By limiting complex scour equations to simple scour equation, over estimation of scour at the extreme values could be minimized.

Did you find this useful? Give us your feedback

Content maybe subject to copyright    Report

Conference Paper, Published Version
Mahmutoglu, Serkan
Tidal Bridge Scour in a Coastal River Environment: Case
Study
Verfügbar unter/Available at: https://hdl.handle.net/20.500.11970/100220
Vorgeschlagene Zitierweise/Suggested citation:
Mahmutoglu, Serkan (2010): Tidal Bridge Scour in a Coastal River Environment: Case
Study. In: Burns, Susan E.; Bhatia, Shobha K.; Avila, Catherine M. C.; Hunt, Beatrice E.
(Hg.): Proceedings 5th International Conference on Scour and Erosion (ICSE-5), November
7-10, 2010, San Francisco, USA. Reston, Va.: American Society of Civil Engineers. S.
894-902.
Standardnutzungsbedingungen/Terms of Use:
Die Dokumente in HENRY stehen unter der Creative Commons Lizenz CC BY 4.0, sofern keine abweichenden
Nutzungsbedingungen getroffen wurden. Damit ist sowohl die kommerzielle Nutzung als auch das Teilen, die
Weiterbearbeitung und Speicherung erlaubt. Das Verwenden und das Bearbeiten stehen unter der Bedingung der
Namensnennung. Im Einzelfall kann eine restriktivere Lizenz gelten; dann gelten abweichend von den obigen
Nutzungsbedingungen die in der dort genannten Lizenz gewährten Nutzungsrechte.
Documents in HENRY are made available under the Creative Commons License CC BY 4.0, if no other license is
applicable. Under CC BY 4.0 commercial use and sharing, remixing, transforming, and building upon the material
of the work is permitted. In some cases a different, more restrictive license may apply; if applicable the terms of
the restrictive license will be binding.

Tidal Bridge Scour
in
a Coastal River Environment: Case Study
Serkan Mahmutoglu, P.E.
AECOM, 500
So
uthborough Drive South Portland, ME 04106 PH: (207) 775 -2800
ABSTRACT
As part
of
an
innovative design/build pro
je
ct a data-rich hydraulics and scour
analysis was performed for the proposed 5 kilometer long Washington Bypass Bridge
over the Tar River
in
Washington, NC. This analysis featured: I) a current and stage
monitoring program, 2) historical aerial photograph analysis, 3) extensive long term
bed elevation study, 4) debris scour evaluation,
S)
variable skew angles due to spati
al
and temporal changes
in
flow characteristics,
6)
complex pier analysis, and 7) use
of
a
two-dimensional (2D) hydrodynamic model
(TABS RMA-2)
to
evaluate riverine
flooding and hurricane surge hydraulics causing extensive wetting-and-drying.
The model was subjected to a comprehensive two-step model
calibration/verification process. Low-flow conditions were calibrated and verified
using project-collected Acoustic Doppler Current Profiling (ADCP) and tidal gage
monitoring. Boat-mounted ADCP measurements collected by USGS during hurricane
surge (Hurricane Dennis) and rain-induced flooding (Hurricane Floyd) were used for
high-flow calibration and verification. This was a rare opportunity for a bridge
designer
to
be
able to evaluate scour using a sophisticated hydrodynamic model that
was calibrated with field data collected during an event that represented the design
conditions.
Two-dimensional hydrodynamic modeling was used
to
simulate complex
hydraulics
of
the project site located at the end
of
the 8,300 square kilometer
watershed which
is
tidally influenced. Due to the size
of
the upstream drainage area
and the proximity to the open ocean both rain-induced-flow and storm-surge
scenarios were considered.
INTRODUCTION
AECOM was hired by Flat Iron - United Joint Venture
(FLUNN)
to perform
engineering services relating to the hydraulics and scour analysis for the Washington
Bypass Bridge (BIN
353 -US17 over the Tar River)
in
Washington, NC (Figure I) .
Under this agreement, AECOM developed a 2-dimensional hydrodynamic model
(RMA-2) to evaluate the flow depth and velocity for the
100-, and 500-year storm
events. The predicted velocity and depth information from these events were used to
calculate scour depths at the bridge and assist
in
designing the bridge substructure
units to withstand scour.
As part
of
this study, AECOM designed and oversaw the required data
collection performed by
Ocean Surveys, Inc. (OSI) which was used for the calibration
and verification
of
the hydrodynamic model. The data collection occurred during a
Spring Tide and involved three (3) tide gages, one
(I)
"upward looking" Acoustic
Doppler Current Profiler (ADCP), and ADCP transects
of
cross sections at and either
side
of
the bridge.
894

5,000
I""""'Ij
-
Feel
SCOUR
AND
EROSION
Figure
1.
Project location.
895
Legend
()
05
1 Gage
USGS Gage
Due to the size
of
the upstream drainage area
(8
,300 sq. km) and the proximity
to the open ocean (i.e. to Atlantic Ocean through Pamlico River and Pamlico Sound),
both rain-induced-flow and storm-surge scenarios were considered separately as
indicated by HEC-25
(1
st Edition, Section 2.8 Page 2.31).
Local pier scour was calculated for each
of
the 127 pile bents (piers) for 8
different conditions: 2 (rain/surge) x 2
(l00
-year/500-year) x 2 (debris/no-debris) = 8.
Upon evaluating the results, scour calculations were determined to be sensitive to
debris. Therefore, a debris accumulation potential evaluation
per
HEC-9 3
rd
Edition

896
SCOUR
AND EROSION
(Debris Control Structures - Evaluation and Countenneasures) was perfonned for
each bridge component. Contraction scour caused by the proposed bridge, which
is
scour due
to
contraction
of
the flow's conveyance area caused by the bridge structure
and its approaches, was also calculated.
In addition to local and contraction scour, long-tenn bed elevation change was
analyzed. The analysis consisted
of
three subtasks. The first subtask utilized historical
aerial photographs
of
the area to digitize the shoreline and then to observe the
evolution
of
the shoreline changes
in
time. The second subtask was obtaining various
historical bathymetric surveys conducted at the
US
17
Bridge crossing, located about
1.6
km
downstream
of
the Washington Bypass Bridge, and calculating the vertical
changes
of
the river bottom. The third subtask was the channel stability assessment
perfonned by geomorphologist
Prof. Stanley Riggs
of
Eastern Carolina University,
who
is
an expert on the riverine geomorphology for this site.
METHODOLOGY
In
order
to
estimate the site-specific detailed hydrodynamic characteristics at
the Washington Bypass Bridge it was necessary to construct a 2-dimensional
hydrodynamic model. Hydrodynamic modeling was accomplished using the Surface
Water Modeling System (SMS)
in
conjunction with RMA-2.
Model
Domain
The model domain was defined considering the area
of
interest, location
of
available data sources, and the limitation on computational resources. The area
of
interest was confined to the vicinity
of
the bridge crossing.
In
order to obtain an
adequate solution at the area
of
interest, model boundaries were established distant
from the area
of
interest and where USGS station locations were available
as
data
sources.
For the stonn surge scenario the downstream model boundary needed to be
moved further downstream (35
km
downstream
of
Pamlico USGS gage) to utilize
available data stations.
The model domain was meshed using triangular and rectangular elements.
The approximate number
of
elements
in
the meshes used ranges from 9,000 to
23,000.
Calibration
and
Verification
The RMA-2 model
is
calibrated primarily with two (2) parameters: the Peelet
Number (Pe) and the Manning Roughness Coefficient (n) , and was perfonned
in
two
stages.
In
the first stage, the detailed infonnation collected by project survey team
was used to calibrate only the model cells representing the channel. Calibration with
this detailed data was limited to the channel because during the data collection period,
water was confined to the channel.
In
the second stage, an extreme event was used to
calibrate the overbank areas
(i
.
e.
wetlands and other floodplain areas) while they are
exposed
to
flow. The data records collected at the US
17
crossing by USGS during
Hurricane Dennis
in
1999 were used for this purpose.
The channel calibration
of
the model was perfonned with the available data
and the data sets collected during the project's monitoring period. The point
in
-situ

SCOUR
AND
EROSION
897
ADCP and the project's Tide Gage
No.1
(TG I) were used to compare the actual
data with the model results.
Overbank calibration
of
the model was performed using data collected by the
USGS at the Pamlico Gage. Input variables for the model included WSE from the
Pamlico Gage and model elements account for storage
in
watershed (which were used
beyond the Tranter's Creek and Grimesland boundaries). Calibrated channel
properties were kept the same and the overbank depth varying Manning's n roughness
was used for calibration.
Verification
of
the model was performed
in
two (2) ways . First, for low flow
conditions flow measurements at
Pamlico gage were compared to model results.
Second, for high flow conditions USGS ADCP velocity transects data at the
maximum velocity conditions during Hurricane Floyd at
4S
meters upstream
of
US
17
were compared to model predictions.
Calibrated model prediction
of
Hurricane Dennis compared
to
flow
measurements taken at the
USGS Pamlico Gage
is
shown
in
Figure
2.
3398
2832
2265
I 1699
:;:
o
u::
1133
566
....
.
....
+.
IS
' 0
"
'I
im~'(h
r
)
Figure 2. Calibrated model simulation
of
Hurricane Dennis.
RESULTS
..
IS
Once the model was calibrated and verified, two different scenarios and two
different return intervals totaling four (4) flow cases were developed: a rain-induced-
flow scenario and a storm-surge scenario, for
100- and
SOO
-year storms.
Rain Induced Flow Scenario
As seen during Hurricane Floyd, a hurricane causing significant rainfall over
an already saturated ground can cause significant flooding in
Tar/Pamlico watershed.
Another observation made during Floyd was the duration
of
the peak flow. Unlike the
tidal surge, which has a peak that passes within a few hours (i.e. dynamic), Hurricane

Citations
More filters

01 Jan 1999
Abstract: Stream instability is characterized by lateral or vertical instability. Such instability has led to the failure of bridges and loss of life. Lateral instability is caused by lateral migration of the stream or river, while vertical instability can be caused by a combination of contraction scour, long term aggradation or degradation, and local scour. Countermeasures for stream instability and scour are discussed for either existing or new bridges and are recommended for use by the Federal Highways Administration (FHWA).

42 citations


References
More filters




01 Jan 1999
Abstract: Stream instability is characterized by lateral or vertical instability. Such instability has led to the failure of bridges and loss of life. Lateral instability is caused by lateral migration of the stream or river, while vertical instability can be caused by a combination of contraction scour, long term aggradation or degradation, and local scour. Countermeasures for stream instability and scour are discussed for either existing or new bridges and are recommended for use by the Federal Highways Administration (FHWA).

42 citations


OtherDOI
Abstract: .........................................................................................................................................................

22 citations


Performance
Metrics
No. of citations received by the Paper in previous years
YearCitations
19991