Proceedings of the U.S. Geological Survey (USGS) Sediment Workshop, February 4-7, 1997

Background

Figure 1. Status of U.S. Geological Survey bridge scour projects.

The U.S. Geological Survey (USGS) has worked in cooperation with the Federal Highway Administration (FHWA) and state highway departments for almost 50 years on research and data collection to design and maintain bridges that are resistant to the dynamic and often damaging processes of alluvial rivers (table 1). During this period, 37 USGS District offices have been involved in bridge scour projects (fig. 1). These projects have ranged from qualitative site assessments to detailed data collection with state-of-the-art instrumentation. The experience and innovation of USGS personnel in the collection and analysis of field data has put the USGS at the forefront of bridge scour research.

Table 1. Selected history of USGS projects associated with the impacts of streams on highways [USGS, U.S. Geological Survey; FHWA, Federal Highway Administration; DOT, Department of Transportation].

Year	Description of Activity
1950s	USGS begins providing bridge site reports on the hydraulics and hydrology of selected bridge sites to several State Departments of Transportation. This activity is ongoing in several states.
1964	The USGS began a study of scour at selected bridge sites in Alaska (Norman, 1975). FHWA funds bridge scour research by USGS personnel at Colorado State University's hydraulic laboratory.
1969	The USGS in cooperation with the FHWA, Mississippi DOT, Alabama DOT, and Louisiana DOT began a 5-year study on backwater and discharge (Schneider and others, 1977).
1978	USGS publishes report on countermeasures for hydraulic problems at bridges (Brice and Blodgett, 1978).
1981	USGS publishes report on study of flood characteristics of the Sacramento River near the Gianella Bridge (Blodgett, 1981).
1986	USGS publishes report on pilot study for collection of bridge-scour data (Jarret and Boyle,1986). USGS develops backwater model with routines to compute backwater from bridges over waterways (Shearman and others, 1986).
1987	FHWA funds the USGS to initiate the National Bridge Scour Program.
1988	A model agreement for studies with USGS is included in the FHWA technical advisory and interim procedures to evaluate scour at brides in this advisory. Tennessee DOT initiates study with USGS to assess bridge-scour problems across the State. This is the first Level I - Assessment.
1989	USGS publishes first report on the use of geophysical techniques for assessing scour. This opened a new marketing area for geophysical equipment manufacturers (Gorin and Haeni, 1989).
1989-90	State DOT's fund the USGS in eight States to collect real-time scour data at selected bridges.
1990-93	State DOT's fund the USGS in 10 States to complete Level I - Assessments (modeled after the 1988 study initiated in Tennessee). State DOT's fund the USGS in 11 states to make hydrologic, hydraulic, and scour analyses at selected bridges for the 100-year and 500-year floods.
1991	FHWA funds the USGS to develop instrumentation for detailed scour measurements and for quick scour measurement to determine bridge safety.
1993	FHWA funds the USGS to collect detailed scour data utilizing state-of-the-art instrumentation.
1993-94	USGS demonstrates data-collection equipment and techniques to many State DOT's.
1994	First USGS national scour report receives Director's approval.
1996	USGS national bridge scour report published (landers and Mueller, 1996). The USGS national bridge scour data-management system is released for distribution (Landers and other, 1996).

Historically, most of the research on scour at bridges has been conducted in the laboratory with little or no validation by field data. The lack of field data reflects the difficulty associated with collecting hydraulic and sediment transport data during floods in sufficient detail to study scour processes and to improve design and evaluation methods. The USGS, in cooperation with the FHWA and state highway departments have collected over 400 limited-detail measurements of local pier scour (Landers and Mueller, 1996; Landers and others, 1996).

Limited-detail measurements of scour consist of cross-sections and water velocities collected from the bridge deck during flood conditions. These real-time measurements are supplemented with bridge geometry and bed-material data, usually collected during low-flow conditions. The limited-detail measurements have been used to evaluate published pier-scour prediction equations. Scour observed in the laboratory appears to be consistently deeper than scour observed in the field. The limited-detail measurements have indicated potential differences between processes observed in the laboratory and those observed in the field (Landers and Mueller, 1996; Mueller, 1996). Evaluation of contraction and abutment scour typically requires detailed hydraulic and channel geometry data collected in a reach extending upstream of to downstream of the hydraulic influence of the bridge. Limited-detail data are good for evaluating local pier scour but lack the spatial coverage necessary for a good evaluation of contraction and abutment scour.

New Instrumentation and Data-Collection Technology

The USGS has developed equipment and techniques for collecting detailed data sets needed to characterize and study the processes associated with scour at bridges. A complete detailed data set should include three-dimensional velocity measurements, channel bathymetry, bed-material load, bed-material samples, water-surface elevation, water-surface slope, water temperature, and discharge. Although the methods for accurately measuring bed-material load are still lacking, new technology, improvements in existing technology, and application of instruments used in hydrographic surveying and oceanographic research have made the collection of the bathymetric and hydraulic data feasible. Portable scour-measuring systems consist of the following four components: (1) a method to measure the horizontal position of the data collected, (2) the instrument(s) for making streambed-elevation and water-velocity measurements, (3) a deployment system, and (4) a data-storage device.

1. Range-azimuth tracking systems and Differential Global Positioning Systems (DGPS) are used to obtain positions of the boat and instruments to within 1 meter. Range-azimuth tracking systems are similar to total stations used for land surveying. The tracking system must be located where the operator can manually track a reflector mounted on the boat. During floods this can be a significant problem. Real-time, kinematic, DGPS allows for rapid collection of velocity and bathymetric data in open areas, but data collection near tree lines and bridges is hampered by loss of adequate satellite coverage caused by blockage of the sky by trees and bridge structure.

2. Acoustics are the primary means of collecting streambed-elevation and velocity data. A digital echo sounder with analog paper chart is the preferred instrument for making detailed bathymetric surveys. For inland waterway applications, an echo sounder operating at a frequency of 200 kHz, with three-degree transducer, and with a peak digitization algorithm generally works well. This instrument provides a good balance between the resolution of the acoustic signal and penetration of deep or sediment-laden waters. The Broadband Acoustic Doppler Current Profiler (BB-ADCP) allows three-dimensional velocity profiles and discharge to be measured in rivers and canals from a moving boat. The BB-ADCP measures water-velocity magnitude and direction by use of the Doppler shift associated with the reflection of acoustic energy off particles transported in the water column. The BB-ADCP uses this technique to compute a water-velocity component along each of the four beams. By use of trigonometric relations and the geometric arrangement of the beams, three-dimensional velocity vectors are computed for each depth cell. Thus, velocities can only be measured where the flow within a layer of the velocity profile is reasonably homogeneous through all of the beams. When the streambed is stable, the BB-ADCP uses the Doppler shift from the streambed to determine and compensate for the speed and direction of the boat. If the streambed is actively transporting sediment, DGPS can be used to compensate for the boat speed and direction.

3. Deployment of the instruments with a boat is necessary to obtain the spatial coverage required for detailed data sets. The use of manned boats during floods can be hazardous, in many situations. Development of an unmanned remote-control boat has eliminated the restrictions of collecting data from a manned boat. The remote-control boat is a 9-foot jon boat with an 8-horsepower outboard engine, modified to allow control with off-the-shelf recreational radio controls. The jon boat was modified to provide a wet well in the center for deploying instruments, the transom was reinforced, and additional floatation was added. Data collection from a manned boat is faster and more efficient, but when launching facilities or safety considerations prevent the use of a manned boat, the remote-control boat allows collection of data in areas that were previously inaccessible.

4. All data are radio linked to a field computer located either on shore or on the manned boat so that the position of the instrument and the data collected are recorded simultaneously.

5. Detailed data on scour at bridges have been collected using this system since 1993. The entire system and field personnel can be deployed within 24 hours to any location in the United States. To date detailed data have been collected at sites in California, Texas, Missouri, Illinois, Indiana, Iowa, and South Carolina. The remote-control boat has been used successfully during floods in South Carolina, Missouri, Illinois, and Indiana to collect data that would have been dangerous or impossible to collect using a manned boat. These data represent some of the most detailed measurements ever compiled of scour at bridges and the associated hydraulics. The data are currently being used to study scour processes and to evaluate the applicability of various numerical models.

Application to Other Sediment Studies

REFERENCES

Brice, J.C., and Blodgett, J.C., 1978, Countermeasures for hydraulic problems at bridges: Federal Highway Administration Report FHWA-RD-78-162.

Gorin, S. R., and Haeni, F.P., 1989, Use of surface-geophysical methods to assess riverbed scour at bridge piers: U.S. Geological Survey Water-Resources Investigations Report 88-4212, 33 p.

Jarret, R.D. and Boyle, J.M., Pilot study for collection of bridge-scour data: U.S. Geological Survey Water-Resources Investigations Report 85-4004, 46 p.

Jones, J.S., Alqalam, Kamel, Gratton, Buddy, and Summers, Brian, 1995, Effect of the 1994 Southeast flooding on the highway system in Georgia: Handout from presentation at the 1995 American Society of Civil Engineers Water Resources Engineering Conference, San Antonio, Tex.

Murillo, J.A., 1987, The scourge of scour: Civil Engineering, American Society of Civil Engineers, v. 57, no. 7, p. 66-69.

Landers, M.N., and Mueller, D.S., 1996, Channel scour at bridges in the United States: Federal Highway Administration Report FHWA-RD-95-184, 140 p.

Landers, M.N., Mueller, D.S., and Martin, G.R., 1996, Bridge-scour data management system user's manual: U.S. Geological Survey Open-File Report 95-754, 66 p.

Mueller, D.S., 1996, Local scour at bridge piers in nonuniform sediment under dynamic conditions: Fort Collins, Colorado State University, Ph.D. dissertation, 212 p.

Norman, V.W., 1975, Scour at selected sites in Alaska: U.S. Geological Survey Water-Resources Investigations Report 32-75, 160 p.

Rhodes, Jennifer and Trent, Roy, 1993, Economics of floods, scour, and bridge failures in Shen, H.W., Su, S.T., and Wen, Feng, eds., Hydraulic Engineering '93: New York, American Society of Civil Engineers, p. 928-933.

Shearman, J.O. Kirby, W.H., Schneider, V.R., and Flippo, H.N., 1986, Bridge waterways analysis model; research report: Federal Highway Administration Report FHWA-RD-86-108, 112 p.

Schneider, V.R., Board, J.W., Colson, B.E., Lee, F.N., and Druffel, L., 1977, Computations of backwater and discharge at width constrictions of heavily vegetated flood plains: U.S. Geological Survey Water-Resources Investigations Report 76-129, 64 p.