Hydraulics Testing of Wilson Bridge Designs
The development process for construction of the new Woodrow Wilson Bridge across the Potomac River near Washington, D.C., is receiving a great deal of public scrutiny. The new bridge will be built immediately south of the existing Woodrow Wilson Bridge, which is an essential element of the I-495 beltway around the nation's capital.
The proposed design has two parallel six-lane bridges to replace the existing single six-lane bridge and, like
the existing bridge, incorporates a drawbridge for ship traffic. The existing bridge will be used during the construction period.
This bridge project has been a major news item for several reasons beyond the usual public interest. First, it is well-known locally that the existing Wilson Bridge is in need of significant repair, in part because it has been subjected to much heavier use than was anticipated when it was opened in 1961. Second, the bridge has become a notorious bottleneck because often it simply cannot meet traffic demand, particularly during peak-traffic rush hours. The bridge is the southernmost of seven bridges crossing the Potomac River in metropolitan Washington, D.C., and there are no bridges downstream (south) for more than 80 kilometers. Traffic on the East Coast's major north-south interstate highway passes over the bridge and around the eastern side of Washington.
The project is a complex, coordinated effort of federal, state, and metropolitan organizations. The new bridge will be located in two states, Maryland, Virginia, and the District of Columbia. The highway agencies of these governments are heavily involved in the project. Because the Wilson Bridge is the only federally owned bridge in the Interstate Highway System, the design and construction also entails a stronger-than-usual partnership of federal and state agencies.
Local citizen and historic preservation groups have opposed the new 12-lane bridge, which they believed
would encroach upon and diminish the unique character of historic Old Town Alexandria, Va. They sued to stop construction, but on Dec. 17, 1999, a federal appeals court, reversing an April 1999 U.S. District Court ruling, determined that the $1.9 billion project fully complied with all environmental and historic preservation laws and, therefore, it could proceed according to schedule.
Construction is anticipated to begin in late 2000, and the project, which also includes the reconstruction of four major interchanges, should be fully completed in 2006.
Bridge Scour Is a Major Design Concern
Hydraulic engineers in the United States are keenly aware of the problems associated with bridge scour because more bridge failures in this country are due to bridge scour than all other causes combined. Bridge scour is the erosion of stream bed material around bridge foundations. The Federal Highway Administration's Hydraulic Engineering Circular No. 18 (HEC-18), "Evaluating Scour at Bridges," has the best available guidelines for estimating bridge scour; however, the proposed Wilson Bridge has a number of features that are not adequately covered by existing guidelines, and that made it a special challenge.
First, the new bridge will have fewer but much wider piers than the existing bridge. Scour at extremely wide piers has been an area of needed research for several years.
Second, the piers are designed to have exposed pile foundations to be capped near the water surface. Scour at complex piers is the topic of a new procedure that is being prepared for the next version of HEC-18.
Third, soil borings indicated that there are some relatively erosion-resistant layers below the stream bed in the vicinity of the proposed new bridge. An erodibility index method for determining the limiting scour depths in resistant soil layers was the topic of a technical paper that was presented at the annual meeting of the Transportation Research Board held in Washington, D.C., in January 2000. Moreover, time-dependent scour in cohesive soil layers is the subject of an ongoing National Cooperative Highway Research Program (NCHRP) project being conducted at Texas A&M University.
Finally, the bascule piers that support the drawbridges will be protected from vessel impact by very large
dolphins and a fender system. There are no guidelines to determine how these vessel-impact structures would affect scour at the piers they are intended to protect. Physical models in a hydraulic flume seemed the only way to evaluate these structures.
FHWA Provides Technical Support and Coordinates With Experts
Researchers at FHWA's Hydraulics Laboratory, located at the Turner-Fairbank Highway Research Center (TFHRC) in McLean, Va., and hydraulic specialists from FHWA's headquarters and two of the recently formed FHWA resource centers saw this bridge as a unique opportunity to provide technical support and to implement the latest technology for predicting scour depths to be used for the design of foundations (piers) for the new bridge.
The Potomac River is an approximately 1.5-kilometer-wide tidal estuary at the bridge site. There will be 18 piers for each of the two parallel structures. The piers will be subject to both flood flow (down river) and tidal surge flow (upriver).
Preliminary estimates, using existing guidelines, indicated that scour could be more than 24 meters deep for some of the piers without even considering the effects of the dolphins that are proposed to protect the bascule piers from vessels. Bridge scour is a major problem to be reckoned with in the design of this bridge.
An interdisciplinary design team was established to resolve the scour issue. Coordination of the team activities was carried out by the general engineering consultant, Potomac Crossing Consultants, and by the Maryland State Highway Agency (MDSHA). Researchers from the FHWA Hydraulics Laboratory and technical experts from FHWA headquarters and from the Eastern Resource Center in Baltimore were part of this team along with engineers from Parsons Transportation Group (PTG), which was responsible for the bridge design and for preparation of the scour report for the project; Rumel Klapper and Kahl (RK&K), responsible for water surface profile modeling; Mueser Rutledge Consulting Engineers (MRCE), responsible for geotechnical design of the foundations; and Hardesy and Hanover Consulting Engineers, responsible for instrumenting the existing piers to monitor scour.
Dr. George Annandale of Golder and Associates; Dr. Max Sheppard, University of Florida; Dr. Mufeed Odeh, U.S. Geological Survey's Biological Research Division (USGS-BRD) Research Laboratory in Turners Falls, Mass.; Dr. Jean Louis Briaud, Texas A&M University; Dr. Art Parola, University of Louisville, and Dr. Larry Arneson, FHWA Western Resource Center advised the team about various aspects of the scour evaluation.
Flume and Numerical Model Experiments
Researchers at the FHWA Hydraulics Lab have studied scour at complex piers around the country for several years, and they sponsored the development of a three-dimensional (3-D) sediment transport model through the National Highway Institute's Eisenhower Fellowship Program. The 3-D numerical model study also allows them to extrapolate physical model results from conditions that are not feasible to test in the flume. Dr. Xibing Dou developed this 3-D model, which is one of the first in the United States that can reproduce scour holes around proposed bridge piers that are similar to what is observed in flume experiments.
The Hydraulics Lab is equipped with a 2.4-meter- (8-foot-) wide flume for modeling scour around bridge piers. Due to the large size of the piers that had pile caps that ranged from 15 to 30 meters (50 to100 feet) wide, a relatively small scale of 1 to 100 had to be used for most of the models.
These small scale tests were originally intended as qualitative measures of the effects of design features, such as the dolphins. For example, the small-scale tests showed that three 13.7-meter (45-foot) dolphins proposed to protect the bascule piers from vessel impact would almost double the scour at the pier while an alternate fender ring concept for protecting the piers would actually reduce scour at the pier. There were many design changes as the scour estimates were factored into the geotechnical and structural calculations, and each one left a question about the scour effects. As a result, 27 small-scale experiments were conducted in the Hydraulics Lab.
Four large-scale experiments were conducted by Dr. Sheppard and Dr. Odeh in a much larger 6-meter- (20-
foot-) wide flume at the USGS-BRD laboratory in Turners Falls, Mass. The large-scale experiments are very expensive -- in the range of $20,000 to $40,000 each -- and time consuming; therefore, it was not feasible to test all the design changes at that scale. Small-scale tests could be conducted at the FHWA Hydraulics Lab for a fraction of the costs, but a few large-scale experiments were essential to evaluate scaling effects. These experiments showed that the small-scale results were unexpectedly close to the large-scale results when the same conditions were tested at both scales, and the team felt more confident using the small-scale results to predict scour at the piers.
The 3-D numerical model was applied by Dr. Dou. The model represented a full-scale simulation of the bridge scour. The 3-D results were generally lower than the small-scale model results extrapolated to full scale; therefore, the 3-D model generated further confidence in the small-scale results.
The grid generation and testing for the 3-D model were very time-consuming, so the 3-D model was not a feasible way to test all the design changes. Nevertheless, the model did produce reasonable results in a first-time full-scale application, and it shows promise for more efficient applications as automated grid modules are developed.
Other Special Studies
Dr. Steve Smith of URS Griener and Dr. Annandale developed a procedure for evaluating scour using an "erodibility index." This index can be applied to rock strata or cohesive bed materials to determine limiting scour depths when the soil strata are highly resistant to erosion. It is not a scour-prediction equation; rather, it is a systematic process of checking erosion resistance against erosion power as the scour hole deepens. Dr. Annandale met with soil exploration supervisors early enough to specify the appropriate soil tests that were needed to apply his method to this bridge.
Dr. Briaud is the principal investigator for a very important NCHRP study on scour in cohesive bed materials. He developed the prediction of "scour rate in cohesive soils" (SRICOS) procedure, which requires that a representative sample of each bed layer be tested in a special erosion function apparatus (EFA). Again by coordinating with the soil exploration supervisor, the design team was able to get the appropriate soil samples sent to his laboratory for testing.
All of the bridge scour-prediction methods and models are driven primarily by the local design velocities and
depths in the vicinity of the proposed structures in a river. Foundations for major bridges are typically designed for a 100-year-flood event and checked for stability for a 500-year-flood event.
One-dimensional (1-D) hydraulic models, such as the U.S. Army Corps of Engineers HEC-RAS (Hydrologic Engineering Center River Analysis System), which is the model of choice for Federal Emergency Management Agency studies, are typically used to determine the velocities and flow depths for bridge scour evaluations. The HEC-RAS model was used for this bridge, but there were a number of problems that could not be addressed by a 1-D analysis, such as how the river would react to a 4.5-meter- (15-foot-) deep trench that will be used for construction in the shallow flow areas, how the flow would distribute across the full cross section when the main channel was on a bend, and the flow directions of currents as they would approach the bridge piers. Dr. Steve Chase from the FHWA Eastern Resource Center and Dr. Arneson augmented the 1-D analysis by applying the latest version of the 2-D SMS-FESWMS (Surface Modeling System -- Finite Elements Surface Water Modeling System) to the reach of the Potomac River in the vicinity of the Woodrow Wilson Bridge. The SMS-FESWMS model is a highly automated numerical model that can access digital elevation map (DEM) data to determine ground elevations at each computational node. The model efficiently produces a two-dimensional finite element network, which was used to refine estimates of local approach velocities and direction. The result was better estimates of local scour at the bridge piers. (See "FHWA Partners With Brigham Young University to Develop State-of-the-Art Hydraulic Modeling Environment" by Larry A. Arneson in Public Roads, November/December 1999, for a description of the SMS-FESWMS model.)
The Wilson Bridge presented a challenge for FHWA researchers and hydraulic engineers. At the same time, it provided an opportunity to demonstrate modeling techniques and new technologies that have not been widely used for bridge scour evaluations to a team of experienced engineers, who were able to assess the applicability of these techniques and technologies to a particular design situation.
First, there is no substitute for a good physical-model study when there are unusual features to be considered in design. Even the small-scale experiments gave a reasonable approximation of the expected scour. Large-scale tests are expensive and cumbersome to run; ideally, they should be scheduled after most of the design changes have been made. Large-scale tests must be carefully planned and scrutinized to gain maximum benefit from a significant investment of resources.
Two different methods were used for extrapolating model results to full scale. One was a geometric scaling procedure, which is the traditional technique of scaling the scour hole in proportion to the length ratios used to model the structure. The other is a University of Florida procedure proposed by Dr. Sheppard; this procedure uses the ratio of the structure width to sediment size as a scaling parameter. Dr. Sheppard has demonstrated that this parameter explains a lot of the discrepancies between measured field data and scour predictions from equations based on geometric scaling of laboratory data. Interestingly, the 3-D numerical model results agreed quite well with the results of the physical model with Dr. Sheppard's scaling parameter. The design team was intrigued by this procedure but opted not to use it because it yielded significantly lower scour estimates, thus less conservatism, for a very important structure.
The evaluations using soil analyses were very effective for determining scour elevations for several of the piers. The design team was not overly impressed when the soil layers were slightly more resistant to scour than stream power that was available to cause scour, but when the resistance was 10 to 15 times the available power, then there was a high confidence level for using the soil layer as a scour limit. As expected, the SRICOS method predicted shallower scour depths than the other procedures. These results gave greater confidence in identifying scour-limiting soil layers for several piers.
The SMS-FESWMS model is a very useful tool for evaluating the more complex flow patterns and for making sensitivity analyses for what will happen if the construction trench fills or does not fill prior to a major flood, which statistically could occur at any time during the life of the bridge. The modelers were able to use digital bathymetry based on National Oceanic and Atmospheric Administration (NOAA) data that had a datum of mean low water. The datum was adjusted to mean sea level and merged with the ground data obtained from the USGS digital elevation model. The HEC-RAS model was then downloaded and the elevations compared. The match was approximately 0.5 meters, which was considered to be acceptable. Even though the data did not match exactly with the HEC-RAS survey data, there was considerably more river data available with the digital bathymetry and the resulting 2-D model than the 10 river cross sections used in the HEC-RAS model.
The design team met in Baltimore on Jan. 5, 2000, to consider the various special studies that had been
conducted and to recommend scour elevations for the geotechnical engineers to use in the foundation design for each pier. It turned out that the depths of the bascule pier foundations are governed by the load requirements and would not be affected by the estimated scour depths. Foundation depths for piers on the opposite side of the flood plain in the vicinity of the secondary channel will be governed by scour resistant soil layers. Foundation depths for piers adjacent to the bascule piers will be governed by the physical model studies. Foundation depths for piers in the shallow flow median area will be governed by HEC-18 estimates with adjustments for wide pier scour-reduction coefficients.
At the time this article is being written, some foundation changes are still being discussed, and there have been suggestions that several more models may be tested in the FHWA Hydraulics Lab. Model studies are a small price to pay for the best enineering judgment that can be used to design a major bridge on our Interstate Highway System.
J. Sterling Jones is the laboratory manager of FHWA's Hydraulics Laboratory at the Turner-Fairbank Highway Research Center in McLean, Va. He is a registered professional engineer and has been with FHWA since 1971 as a hydraulic research engineer. He is a charter member of the American Society of Civil Engineers Task Force on Bridge Scour and is currently the Hydraulics Subcommittee chairman for the Transportation Research Board's Committee A2A03, which deals with transportation-related hydraulics, hydrology, and water quality.