The Sakonnet River Bridge carries Rhode Island Route 24 (RI 24) between Portsmouth and Tiverton, Rhode Island. RI 24 is a key link in the transportation system connecting Massachusetts, Rhode Island, and Aquidneck Island. The existing bridge is in a deteriorating condition and is structurally deficient. It does not meet requirements for seismic protection, shoulder width, or structural capacity. Accordingly, it will be replaced with a new structure located immediately south of the existing alignment.
Q and A
Question:
What was done in terms of validation?
Paul Gilbert,
PB Network Advisor, Seattle, Washington
Answer:
Three sources of data were used to calibrate the FESWMS model. The sources of velocity data used were two National Oceanic and Atmospheric Administration (NOAA) Physical Oceanographic Real-Time System (PORTS) gages, the U.S. coast Pilot 2, and a technical publication authored by Kim and Swanson. Each of these data sources provided information on tidal velocities for normal tidal cycles.
Question:
Were any alternative models available and, if so, why weren't they used? Also, can you tell a bit more about running the model and the resources needed?
Alan Knott,
PB Network International Advisor, Manchester, UK
Answer:
There are a handful of other models available with capabilities similar to FESMWS. We generally use this model because it is the model developed by the Federal Highway Administration. Additionally, PB has a history with the FESWMS program as its developer, David Froehlich, is a former PB employee. Running the FESMWS model requires extensive survey data within the river system. Detailed bathometric surveys are necessary within the project area and generally this data is supplemented with publicly available data from NOAA. |
The Sakonnet River is a wide tidal waterway that flows from Mount Hope Bay to the Long Island Sound and is interconnected with the Narragansett Bay. This complex hydraulic system contains several smaller bays, islands, shoals and passes, where tidal and riverine currents mix and large volumes of water are stored. Figure 1 shows the location of the bridge and the surrounding water bodies.
Complex Hydraulic Conditions Require a 2-D Flow Model
The hydraulic conditions at the new bridge location were judged to be too complex for a conventional one-dimensional hydraulic analysis. Hence, we used a two-dimensional finite element hydrodynamic model, FESWMS-FST2DH, which allowed us to perform detailed analysis of the system's hydraulic properties. This model was developed by David Froehlich, Ph.D., a former PB employee, for the Federal Highway Administration and has been used on several of our projects since the late 1990s.1
FESWMS-Flo2DH uses the finite element method to solve the two-dimensional, depth-averaged equations of continuity and momentum. The study area or "solution domain" is represented in FESWMS by a finite element network comprised of a series of interconnected quadrilateral or triangular elements. These elements, used to describe the study area, are assigned hydraulic parameters, such as Manning's roughness coefficient using property codes.
System-Wide and Localized 2-D Flow Models
Figure 1: Site Location Map.
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Figure 2: FESWMS Model Domain.
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Figure 3: Results of the FESWMS Hydrodynamic Modeling.
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System-Wide Model. The model needed to include the entire hydrodynamic system to capture its effects on the project site and to provide a valid representation of the two open boundaries to the Sakonnet River. The model included:
Sakonnet River
Mount Hope Bay
Narragansett Bay, including its multiple bays, islands, shoals and passes
Three inlets from the Long Island Sound
Portions of the Taunton River and Providence River.
In total, the domain of the hydrodynamic model covered nearly one-third of the state of Rhode Island. Figure 2 shows the model network in relation to the state.
The FESWMS hydraulic model was combined with riverine and tidal hydrologic models to produce a comprehensive view of the hydraulic environments produced by this system.
The riverine hydrologic models considered the flows and flooding caused by the watershed tributary to the system, which includes the majority of Rhode Island, parts of Connecticut and a large portion of Massachusetts.
The tidal hydrologic models considered currents created by tidal circulation and flooding caused by a large hurricane propagating up the coastline, making landfall at the project location, and producing a 3-m (9-foot storm) surge up the Sakonnet River and Narragansett Bay. (The storm used closely matched the famous September 1938 hurricane that ravaged much of southern New England.)
Localized Model. On a smaller scale, the location of the proposed bridge presented its own hydraulic challenges that could be properly handled only by a multi-dimensional hydraulic model. The replacement bridge is located immediately downstream of an old railway causeway that blocks more than half of the river channel. The hydraulic modeling and analysis showed that this constriction creates a "jet" of flow directed at one of the pier locations during storm events. Additionally, the piers sheltered from the jet of flow by the causeway are subject to swirling flows caused by resulting eddies. Figure 3 shows the results of the FESWMS-FST2DH modeling at the location of the new bridge.
Modeling Results Used for Scour Analysis
The scouring of bed material from around bridge foundations is generally believed to be the most common cause of bridge failures today. The scour depth is a function of flow depth and velocity at the bridge piers. Typically, maximum scour at bridges in tidal waters does not occur at the same time as peak water surface levels because water velocities drop to zero as water surfaces peak and the flow reverses. Results from the hydraulic analysis of the Narragansett Bay/Sakonnet River system showed that:
Peak scour potential occurs on the falling (ebb) storm surge at the pier south of the causeway opening because of the influence of the railroad causeway.
Peak scour potential at the other piers may occur on either the flood or ebb surge, depending on the strength of eddies that form in the sheltered areas south of the railroad causeway.
Scour potential at the bridge was evaluated using the guidelines and procedures presented in the Federal Highway Administration Hydraulic Engineering Circular No. 18: Evaluating Scour at Bridges, Fourth Edition (HEC-18). In the scour analysis, total potential scour was estimated as the sum of long-term trends, contraction scour, and local scour. Each of these three components were evaluated through in-depth analysis of the hydraulic results of the FESWMS modeling and the geotechnical and physical properties of the site.
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1 Since the late 1990s, PB has used this modeling technique to produce evaluations for several landmark bridges including, Connecticut Route 34 over Lake Zoar in Stevenson, Connecticut; I-95 over the West River in New Haven, Connecticut; New Jersey Route 72 over Manahawkin Bay in Long Beach Island, New Jersey; The Bonner Bridge in Outer Banks, North Carolina; The Cooper River Bridge in Charleston, South Carolina; and the Sikorsky Bridge in Stratford, Connecticut.
For articles on this modeling program and other hydraulic-system topics covered by David Froehlich, please see "PB Technotes," a regularly featured column in the Networking section of PB Network, issues No. 40 through No. 50.
Justin Lennon is a water resources engineer. During his two years with PB, he has worked on several projects on the east coast involving various types of hydrologic and hydraulic studies. His areas of interest include hydrologic and hydraulic modeling, stream restoration and stabilization design, and hydraulic design of in-stream structures. |
