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Field Observations and Evaluations of Streambed Scour At Bridges

CHAPTER 3: DESCRIPTION OF FIELD METHODS

GENERAL

The type of data collected at a bridge is determined by the scour processes being studied and the equipment's capability to collect the desired data. The type of scour being studied (pier, abutment, or contraction) determines which variables need to be measured and the spatial distribution of these data. For local pier scour, measurement of the approach velocity, bed material properties, and cross-sectional data to define the reference surface and maximum depth of scour may be sufficient. For abutment and contraction scour, hydraulic conditions in the approach section are important. Unfortunately, the site configuration may limit the data that can be collected accurately and safely. Floodplains commonly are covered by trees and other woody vegetation, which makes measurement of the velocity and flow distribution virtually impossible. Debris and ice accumulations may prevent measuring streambed elevations in key areas. On small streams, the duration of the flood may not allow sufficient time to collect all of the desired data; all desired data are rarely collected at a site.

The USGS has defined two classes of field data: limited-detailed and detailed.⁽²¹⁾ In general, limited-detailed data can be collected from the bridge deck, and the spatial extent of the data is restricted to the upstream and downstream edges of the bridge. Detailed data require the deployment of equipment from a boat to collect data over an area extending beyond the hydraulic effect of the bridge. Most available data are from limited-detail data collection efforts sponsored by State highway departments. The FHWA-sponsored national bridge scour studies are responsible for most of the detailed data sets that have been collected.

A brief summary of these types of data is presented here. For a more detailed discussion of limited-detail data collection see Landers and Mueller,⁽²¹⁾ Jarrett and Boyle,⁽³⁵⁾ and Mueller and Landers.⁽³⁶⁾ A comprehensive presentation of detailed data collection and the necessary equipment can be found in Landers and Mueller,⁽²¹⁾ Mueller and Landers,⁽³⁶⁾ and Mueller.⁽⁵⁾

LIMITED-DETAIL DATA

Limited-detail data primarily are collected to evaluate published equations, investigate the relations between local scour and explanatory variables, and develop envelope curves for the maximum observed scour. Limited-detail data sets should include the following data:⁽²¹⁾

• Water discharge.
• Water-surface elevations at the bridge.
• Cross sections, approximately one bridge width upstream and downstream of the bridge (it is desirable to measure this during the flood, but low-water approach and exit sections are usually acceptable).
• Approach flow velocity for each pier location.
• Bed material samples (it is desirable to collect these during a flood, but low-water samples are usually acceptable).
• Notes on debris accumulations, surface currents, roughness, and vegetation.
• Photographs of the bridge and stream reaches upstream and downstream.
• Water temperature.
• Bridge and pier geometry.
• Soil boring logs for the bridge crossing.

Rantz et al.⁽³⁷⁾ and Landers and Mueller⁽²¹⁾ describe equipment and procedures for discharge and approach velocity measurements. Bed material samples can be collected using the equipment and procedures described in Landers and Mueller, Mueller and Landers, Edwards and Glysson, Ashmore et al., Yuzyk, and the International Organization for Standardization (1992). (See references 21, 36, 38, 39, 40, and 41.)

DETAILED DATA

Detailed data sets are similar to limited-detail data except the data are denser and their spatial extent broader. Ideally, detailed data sets include real-time measurements of hydraulic and channel-geometry data at several times during the flood hydrograph. Data are collected both upstream and downstream in an area that extends just beyond the hydraulic effect of the bridge. The extent of the hydraulic effect of the bridge both upstream and downstream depends on the degree of contraction, the size and configuration of the floodplains and approach embankments, and the slope of the stream. The goal is to collect data at least 10 bridge widths upstream and downstream, but this may be adjusted in the field as site conditions dictate. Detailed data sets allow distinction between local, contraction, and general scour occurring at the highway crossing, and are needed to advance the understanding of complex bridge scour processes. Detailed data sets should include the following:

• Water-discharge hydrograph.
• Water-surface elevation hydrograph.
• Water-surface slope.
• Detailed channel-geometry data at and near the bridge.
• Channel geometry in the river reach upstream and downstream of the bridge.
• Flow velocities (magnitude and direction) in the entire study reach.
• Bed material samples.
• Suspended-load and bed-load measurements (if possible).
• Notes on surface currents, channel roughness, and vegetation.
• Approximate measurements of debris piles present.
• Photographs of the bridge and stream reaches upstream and downstream.
• Water temperature.
• Bridge and pier geometry.
• Soil boring logs for the bridge crossing.

The procedures and equipment for making discharge measurements and collecting suspended-load and bed material samples are the same as those used for the limited-detail measurements. The spatial extent of the bathymetric and velocity data is much greater for detailed data sets than for limited-detail data sets; therefore, instruments must be deployed from the water surface rather than from the bridge deck. It is also highly desirable to measure the direction of flow in addition to the velocity magnitude.

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