Hazard Mitigation R&D Series: Article 2—Scour, Flooding, and Inundation
The FHWA hydraulics R&D program conducts advanced and applied research to mitigate disaster risks by addressing three of the main hazards.
In recent history, flooding, coastal inundation, and scour of bridge piers and abutments have been among the leading causes of bridge failures in the United States. Recent examples of structures affected by flooding, inundation, or scour include the numerous bridges in New Orleans and along the Gulf Coast damaged by Hurricanes Katrina (2005) and Rita (2005), the damage to more than 2,400 bridge crossings during the 1993 Upper Mississippi River basin flooding, the 1994 failure of numerous bridges during Tropical Storm Alberto in central and southwest Georgia, and the 1987 failure of the I-90 bridge over the Schoharie Creek near Amsterdam, NY, which resulted in the loss of 10 lives and millions of dollars in bridge repair and replacement costs.
Given national costs due to scour-related damage, plus disruption to local economic activities from bridge closures, and the potential for devastating loss of life from floods and inundation, bridge foundations demand improved engineering analysis and design procedures to mitigate the consequences of natural disasters.
Researchers with the Federal Highway Administration (FHWA), under the Hydraulics Research program at the J. Sterling Jones Hydraulics Laboratory located at FHWA's Turner-Fairbank Highway Research Center (TFHRC), and their partners currently are conducting applied and exploratory advanced research to improve prediction of flooding-related damages and design guidance for mitigating impacts on bridges and other hydraulic structures.
Further, FHWA is collaborating with several laboratories and universities to help assure the program's success. For example, research partners at the Argonne National Laboratory's (ANL) Transportation Research and Analysis Computing Center (TRACC) in West Chicago, IL, and the Universities of Nebraska and Iowa are championing advanced engineering tools such as computational fluid dynamics to simulate extreme flood events and their interaction with bridge structures. Computational fluid dynamics uses numerical methods and algorithms to analyze and solve problems that involve fluid flows.
Past Research Contributions
The TFHRC hydraulics laboratory has been involved in a number of studies, including investigation of the Hatchie River Bridge collapse in Tennessee. Spans of the northbound U.S. Route 51 bridge over the Hatchie River collapsed on April 1, 1989. Five vehicles went into the river, and eight people were killed. To help determine the cause of the collapse, the National Transportation Safety Board (NTSB) asked FHWA to conduct hydraulic model studies of the two-column bent #70 with independent footings of the Hatchie River Bridge. Onsite investigation had established that failure of this bent probably triggered the collapse. The hydraulics laboratory tested a 1:20 scale model of the bent to determine how the maximum local pier scour might have occurred after the channel migrated to bent #70 and to obtain videotape shots of the local scour process for use as a visual aid in the NTSB public hearing conducted to gather evidence concerning the collapse.
Hydraulics Laboratory Facts
The J. Sterling Jones Hydraulics Laboratory features multiple hydraulic flumes of varying functions and sizes. Most prominent of these flumes is a 6-foot, ft (1.8-meter, m)-wide by 70-ft (21.3-m)-long tilting flume with a sediment recess for local scour modeling and a total pumping capacity of 15 cubic feet per second (ft3/s), 425 liters per second (L/sec), with variable-frequency drives capable of simulating in-flow hydrographs. Adjacent to the tilting flume is a 1.3-ft (0.4-m)-wide by 42-ft (12.8-m)-long force balance flume capable of measuring horizontal, vertical, and rotational forces on an object, a shear stress sensor at the base of the flume, 12 ultrasonic depth sensors used to measure water elevation at multiple locations along the tank, and a wave generator capable of inducing multiple wave patterns at varying heights and intensities. The facility also includes several particle image velocimetry (PIV) testing stations capable of recording 2-dimensional (2-D), 3-dimemsional (3-D), and 3-component (3-C) 3-D PIV data. The hydraulics laboratory also includes a 1.3-ft (0.4-m)-wide by 26.2-ft (8-m)-long tilting fish passage culvert flume to test low-flow entrance loss coefficients and to evaluate roughness coefficients for low flows. In addition, the lab contains an ex situ scour testing device to investigate hydraulic loading on cohesive soils samples. The soil samples are mounted on a shear and normal force balance to measure the soil erosion response.
In another example of past research, the TFHRC laboratory conducted small-scale scour tests for the Woodrow Wilson Memorial Bridge replacement. The researchers tested 31 different model scenarios in the tilting flume and conducted 71 test runs with durations of 46 hours each. The scour evaluations were part of the process that led to design changes that saved millions of dollars. The savings resulted from reducing the predicted scour depths by an average of 15 to 20 feet (4.5 to 6 meters) for approximately 648 of the piles, using fewer but larger piles, and incorporating vertical piles instead of battered piles, which are more difficult and expensive to install, for the very deep foundations. Savings also resulted from reductions in equipment costs and the time needed for construction. Engineers gained a greater understanding of how pier protection systems for vessel impact, which are placed around the bridge's foundations to prevent collisions, affect the scouring process. After the researchers found that the proposed position of the pier protection system would double the amount of scour, the design was changed.
Applied Hydraulics Research
When a bridge crossing over a waterway is partially or entirely submerged during a flood, the deck might be subjected to significant hydrodynamic loading. In addition, the pressurized accelerated flow can create severe potential for scour because scouring the channel bed is one of the only ways for a river to dissipate energy and reach equilibrium when it is carrying a given discharge under pressurized flow.
To address these issues, the hydraulics research and development (R&D) program at TFHRC conducted two studies in fiscal years (FY) 2008 and 2009—one on hydrodynamic forces and the other on pressure flow scour—using small-scale physical experiments and computational fluid dynamics-based simulations.
The results of the first study are new charts for bridge designers to use when estimating hydrodynamic loading. The FHWA Hydraulic Design Series 1 (HDS-1), Hydraulics of Bridge Waterways (FHWA-EPD-86-101), will be updated to incorporate the new charts. One of those design charts, for example, compares experimental data with the simulation results for drag forces (drag coefficients), which act in a direction opposite to the oncoming flow velocity.
The outcome of the second study will be included in the new edition of FHWA's Hydraulic Engineering Circular No. 18 (HEC-18) Evaluating Scour at Bridges (FHWA NHI 01-001). The upcoming edition will include a new design chart for estimating pressure flow scour as a function of the inundation number. Over its lifespan, any given bridge will likely be inundated or submerged for a short period of time. The study analyzed the impact (that is, the scour) on the riverbed if a bridge is submerged or inundated under pressurized flow. This pressure flow scour has to be considered in the design of the bridge foundation and will affect the depth at which the footings are placed.
This study researched clear water scour, which is defined as scour of a riverbed when the water cannot transport sediment because the flow is too slow and therefore not powerful enough to suspend particles long enough to carry them downstream—a rare situation in the field. Most floods are sediment-laden flows where the water's capacity for sediment transport governs the amount of scour. During most storms, field flow velocities and stream power are extremely high, so upstream sediment from the riverbed moves rapidly downstream toward any infrastructure. This latter phenomenon is called live bed scour, as opposed to clear water scour. The TFHRC researchers analyzed clear water scour first to set a baseline and will address live bed scour in the research's second phase.
TRACC Computing Facility
The FHWA hydraulics laboratory's numerical modeling occurs at the Argonne National Laboratory's (ANL) Transportation Research and Analysis Computing Center (TRACC). TRACC is studying computational fluid dynamics-based simulation techniques. Engineers compare these simulations to tests conducted at the hydraulics laboratory.
The U.S. Department of Transportation (USDOT) and ANL established TRACC as a general purpose advanced computing and visualization facility available for use by the transportation community for a variety of applications. Staff from USDOT's Research and Innovative Technology Administration and FHWA identified specific initial applications and technologies that should have highest priority for research and development (R&D) and user support.
The TRACC components include high-performance computing, visualization, and networking systems. To take advantage of ANL's extensive experience in the acquisition and operation of similar user facilities, the system components were installed in dedicated facilities at the DuPage National Technology Park near the DuPage County Airport in Illinois. The TRACC computational cluster is a 512-core, customized system that comprises 128 compute nodes, each with two dual-core central processing units (CPUs) and 4 gigabytes of random access memory (RAM); a storage system consisting of 240 terabytes of shared redundant array of independent disks (RAID) storage that is expandable to 750 terabytes; a high-bandwidth, low-latency network for internode computations; and a high-bandwidth management network. The center also offers scientific visualization capabilities through the TRACC facilities at the same location. The scientific visualization facilities meet the needs for visualization of multidimensional data via a high-performance graphics cluster linked with a 15-panel liquid crystal display (LCD)-tiled display and a portal optimized for visual simulation and high-speed broadband connectivity.
Four New Studies for 2010
The hydraulics R&D program's team plans four new research studies to be initiated at the TFHRC laboratory in FY10 and completed in subsequent years.
The first study is a continuation of the FY08 and FY09 completed research on pressure flow scour, which addressed only clear water scour. The proposed study will extend current pressure flow scour predictions into the live bed scour range, utilizing a new flume at TFHRC that allows high-speed live bed scour tests.
This research also will study the influence of bridge piers on pressure flow. The current practice assumes a linear superimposition of general, contraction, and local pier scour. The latter is caused by vortices formed at the base of piers due to the pileup of water on their upstream surfaces and subsequent acceleration of flow. But these assumptions might not be applicable because of nonlinear interactions between local pier scour and general scour under pressure flow conditions. Hence, this research will improve understanding of the nonlinear effects on local pier scour under pressure flow conditions.
The second study will address the initial erosion process of cohesive soils. Scour of cohesive soils is a complex phenomenon that is not well understood. This study will apply various hydraulic loading conditions (flow velocities) to different cohesive soils and measure the erosion responses. The hydraulic loading is performed using an ex situ scour testing device that produces what are known as Couette flow conditions, using a moving belt in still water over riverbed cohesive soil samples. The gap between the samples and the belt is 0.59-inch (15-millimeters). Couette flow conditions refer to the laminar flow of a viscous fluid in the space between two parallel layers, one of which is moving relative to the other. The velocity distribution in the gap is approximately S-shaped. The riverbed cohesive soil samples are mounted on a force balance sensor that can measure horizontal (shear) forces and vertical (normal) forces during erosion.
The third study will analyze the effect of pier scour and pressure flow scour on coarse bed material. If riverbed material has more large sand particles than fine sand, pier scour will be reduced because large particles will withstand the hydraulic dislodging forces and therefore armor the scour hole. If the particle distribution consists of more fine sand than large particles, pier scour will be greater. This research will study the influence of sand particle distribution on bridge pier scour. The goal is to develop an improved scour reduction (adjustment) factor for coarse sand particle distribution by revisiting and improving the methodology suggested in HEC-18, a methodology that is difficult to apply.
The research study will consist of laboratory experiments, field data, and computational fluid dynamics simulations to derive a new adjustment factor. The influence of gradations of bed material, such as gravel, sand, and fine sediment, will be expressed in terms of an improved critical velocity equation and a more accurate gradation coefficient of the bed material, enabling the designer to compute the scour adjustment factor to obtain a more accurate overall scour estimate.
The fourth study for FY10 relates to analyzing the magnitude of buoyancy or uplift forces on culverts. Many culvert failures occur when a culvert is submerged with air trapped inside. The resulting buoyancy or uplift forces can be of such magnitude that the whole culvert system is pushed upwards and fails. This study will investigate culverts during subcritical flow (full flow conditions), with air trapped in the flow separation zone at the culvert inlet. The researchers will mount culvert models on a carefully designed force balance system (an array of force sensors—load cells—measuring the changes in forces) that will include tension coil springs with load cells supporting the culvert models. The researchers will use the change in spring tension to estimate the uplift forces. Computational fluid dynamics modeling will augment the culvert buoyancy experiments by converting the measured forces into uplift coefficients that the researchers then will use to develop design guidelines to be applied in the field.
Advanced Hydraulics Research Program
FHWA's Exploratory Advanced Research (EAR) Program focuses on long-term, high-risk research with a high payoff potential. The program addresses underlying gaps faced by applied highway research programs, anticipates emerging issues with national implications, and reflects broad transportation industry goals. (To learn more about the EAR Program, visit www.fhwa.dot.gov/advancedresearch/.)
In August 2007, FHWA convened an international hydraulics research forum, gathering researchers and other stakeholders to stimulate advanced research in hydraulics and identify research priorities. The forum drew experts from universities, industry, and government transportation agencies, who reported on a broad spectrum of ongoing investigations and specific research needs. Discussion centered around three major areas of hydraulics research—coastal, inland, and environmental—and the need to establish communications, partnerships, and future directions. Two topics warranted special attention: advanced modeling capabilities (physical, numerical, and supercomputing) and the implications of climate change for the field.
According to Jorge E. Pagán-Ortiz, director of FHWA's Office of Infrastructure Research & Development, "The participants emphasized the importance of collaboration to maximize scarce funding and data-sharing to accelerate the progress of research. They recommended that annual or biennial hydraulics research forums be organized, a Web site be created for reporting and peer-reviewing hydraulics research, and a steering committee, working groups, and collaborative relationships be formed for planning and conducting research."
Among the high-risk topics with long-term potential discussed at the forum, participants called for research into applications of "smart materials," such as in integrated scour-monitoring systems. They also recommended development of hydrodynamically efficient bridge pier and deck systems, plus structures that can adapt optimally to flow conditions through the use of adaptive materials based on nanotechnology and biomimetics, which is the study and emulation of nature's models, systems, processes, and elements to solve engineering problems.
Forum participants identified two areas of high-risk, high-payoff research for the near term: development of smart particles for monitoring hydraulic hazards and scour countermeasures using advanced materials based on nanotechnology.
Two EAR-sponsored joint investigations are underway that address these study areas. Led by researchers at FHWA's hydraulics laboratory in collaboration with experts from the National Aeronautics and Space Administration's Jet Propulsion Laboratory, the first of the two joint studies is exploring new ways to measure and understand the complex flow fields and boundary pressure fields associated with bridge pier scour. This project is pursuing an integrated, flexible, sensing system that can measure changes in shear stress and pressure when a scour hole forms. Such a system would significantly aid small-scale experiments to address bridge scour problems by allowing the measurement of forces that occur during the scouring process. Measurement of those forces is necessary for the development of advanced scour countermeasure materials, which might have nanotechnology components.
The second joint project is working toward an advanced optical system to allow three-dimensional measurement of the entire instantaneous flow field around bridge pier models. The high-resolution, volumetric particle image system will be able to capture and quantify complex, unsteady flow fields in experimental bridge-scour research. Also led by the hydraulics laboratory, this joint study will enable hydraulic researchers to develop more precise models for predicting scour. This study will allow visualization of the complex, three-dimensional flow field around bridge piers during scouring. To predict the forces during the pier scouring process, the researchers will compare these flow fields with the computational fluid dynamics simulations performed at ANL.
Following the first international hydraulics research forum's recommendation for development of intelligent piers, the EAR Program convened a 1-day market research meeting in June 2009 to explore the potential of wireless, smart particle sensor networks for hydraulics research and monitoring. Such sensors would be capable of transmitting data on position, velocity, pressure, and other variables under a wide range of environmental conditions.
Five research laboratories presented their ideas at the market research meeting, and a government review panel developed recommendations for moving forward. The panel consisted of experts from the U.S. Army Corps of Engineers, National Institute of Standards and Technology, U.S. Geological Survey, U.S. Naval Academy, and Oak Ridge National Laboratory. The panel recommended that a problem statement and a performance standard be developed as the basis for a full feasibility analysis. The panel members identified a number of challenges—concept validation, data accuracy, cost-benefit considerations, reproducibility, scope of application, and potential environmental impacts—and suggested that a research roadmap be developed as well. The panel also called for a followup meeting that would include a broader array of university and industry researchers.
Future Efforts and Expected Outcome
The applied FHWA hydraulics R&D program and its state-of-the-art laboratory will continue to conduct a variety of experiments pertaining to the characterization and potential impacts that flowing water can have on the performance of the Nation's surface transportation infrastructure. For example, the laboratory will continue to solve stream stability problems, as the TFHRC researchers did in the Bottomless Culvert Scour Study (FHWA-HRT-07-026), which improved an existing methodology for estimating scour in bottomless culverts and measures to reduce that scour. The lab also will continue to develop design standards for bridges that might become submerged in high-flood-risk areas, as well as contributing to design standards concerning scouring around bridge foundations and submerged decks. The laboratory also will continue to support practitioners and engineers with design guidance and tools, as in Effects of Inlet Geometry on Hydraulic Performance of Box Culverts (FHWA-HRT-06-138).
In addition, the advanced FHWA hydraulics R&D program will continue to work on promising, high-risk, high-payoff projects in transportation-related hydraulics research, guided by the multiyear strategic plan formulated as the result of the first international hydraulics research forum. The plan proposes to move away gradually from physical experiments and use more computational fluid dynamics modeling to develop design guidance for bridge designers. Successful collaboration between ANL-TRACC and the FHWA hydraulics R&D program is the first step in that direction.
Pagán-Ortiz predicts, "Future research will also lead to progress in many areas: development of rapid deployment smart sensing systems to capture hydraulic parameters during storm events, development of enhanced analytical and modeling capabilities [high-performance computing] for predicting hydraulic hazard effects and for assessing and screening stream stability and scour countermeasures, development of enhanced scour analysis capabilities and countermeasures for inland and coastal bridges, environmentally advanced hydraulic designs and designs that facilitate fish and wildlife passage through hydraulic structures, and development of prefabricated hydrodynamic bridge decks and bridge piers. Research also will focus on how climate change will potentially impact the hydrologic and hydraulic procedures used for designing new surface transportation structures and evaluating the performance and condition of existing structures."
Kornel Kerenyi is a hydraulic research program manager in the FHWA Office of Infrastructure R&D. He coordinates FHWA's hydraulic and hydrology research activities with State and local agencies, academia, and various partners and customers, and he manages the hydraulics laboratory. Kerenyi holds a doctorate in fluid mechanics and hydraulic steel structures from the Vienna University of Technology in Austria.
Junke Guo is an assistant professor in the department of civil engineering in the Peter Kiewit Institute at the University of Nebraska-Lincoln. He received his Ph.D. in fluid mechanics and hydraulics from Colorado State University. His research interests include turbulent boundary layer flows, open-channel turbulence and sediment transport, computational fluid dynamics, and environmental fluid mechanics.
Hydraulics of Bridge Waterways (HDS 1, March 1978), www.fhwa.dot.gov/engineering/hydraulics/pubs/hds1.pdf
Evaluating Scour at Bridges - Fourth Edition (HEC-18, May 2001), www.fhwa.dot.gov/engineering/hydraulics/library_arc.cfm?pub_number=17&id=37
Hydrodynamic Forces on Inundated Bridge Decks (FHWA-HRT-09-028), May 2009, www.fhwa.dot.gov/publications/research/infrastructure/structures/09028/index.cfm
Bridge Pressure Flow Scour for Clear Water Conditions (FHWA-HRT-09-041), October 2009, www.fhwa.dot.gov/publications/research/infrastructure/structures/09041/index.cfm