Development of a Guidance Manual for Assessing Scour Using the South Carolina Bridge-Scour Envelope Curves Completed

Development of a Guidance Manual for Assessing Scour Using the South Carolina Bridge-Scour Envelope Curves

PROBLEM STATEMENT

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Bridge-scour equations presented in HEC-18 (Richardson and Davis, 2001) reflect the current state-of-the practice for predicting scour at bridges. While these laboratory-derived equations provide an important resource for assessing scour potential, there is a measure of uncertainty when applying them to field conditions. The uncertainty and limitations are acknowledged by laboratory researchers (Breusers and others, 1977; Ettema and others, 1998; Melville and Coleman, 2000) and confirmed in field investigations (Mueller and Wagner, 2005; Benedict and others, 2006; Benedict and Caldwell, 2006; and Benedict and Caldwell, 2009).

Because of this uncertainty, HEC-18 (Richardson and Davis, 2001) recommends that engineers evaluate the computed scour depths obtained from the laboratory equations and modify them if they appear unreasonable. Perhaps the best way to evaluate the reasonableness of predicted scour is by comparing it to field measurements of historic scour. Such field data show scour depths resulting from high flows, and provide a reference for evaluating predicted scour. Realizing the value of historic scour measurements, the U.S. Geological Survey (USGS), in cooperation with the South Carolina Department of Transportation (SCDOT) conducted a series of three field investigations (Benedict, 2003; Benedict and Caldwell, 2006; Benedict and Caldwell, 2009) with the goal of collecting historic scour measurements in order to better understand regional trends of scour within South Carolina. The studies collected historic scour data at approximately 200 riverine bridges (figure 1) including measurements of clear-water abutment, contraction, and pier scour, as well as live-bed contraction and pier scour. These investigations have provided valuable insights for regional scour trends and have yielded bridge-scour envelope curves for assessing all components of scour at riverine bridges in South Carolina (figure 2). The application and limitations of these envelope curves are documented in 4 reports. Each report addresses different components of bridge scour and there is a need to develop an integrated procedure for applying the envelope curves to help assess scour potential at riverine bridges in South Carolina.

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OBJECTIVES

The primary objective of this project is to develop an integrated procedure for assessing scour potential at riverine bridges in South Carolina utilizing the regional bridge-scour envelope curves developed in the three previous field investigations (Benedict, 2003; Benedict and Caldwell, 2006; Benedict and Caldwell, 2009; and Benedict and Caldwell, 2012; figure 3).

APPROACH

To accomplish the objective of this project the following steps will be taken:

1. Literature review: A review of selected publications that are currently used for guidance in scour prediction will be reviewed to assist in developing an integrated procedure for applying the bridge-scour envelope curves.

2. Verification of the South Carolina bridge-scour envelope curves: Other field data identified in a second literature review will be used to verify and, if necessary, modify the South Carolina bridge-scour envelope curves.

3. Development of an integrated procedure for applying the envelope curves: Utilizing the previous field investigations in SC, along with findings from items 1 and 2, develop an integrated procedure for assessing scour potential at bridges in South Carolina using the South Carolina bridge-scour envelope curves.

4. Development of the 500-year flow envelope-curve coefficient to assess scour potential at this flow magnitude

5. Development of a spreadsheet to automate the integrated procedure: To assist in applying the integrated procedure for assessing scour with the South Carolina bridge-scour envelope curves, a spreadsheet application will be developed to automate selected components of the integrated procedure.

6. Development of a site-comparison database by merging the 3 existing databases: The 3 databases from the previous investigations will be combined to create a single database organized in a format that will allow site comparisons.

7. Documentation of the integrated procedure in a guidance manual: The integrated procedure for using the South Carolina bridge-scour envelope curves will be documented in a Scientific Investigations Report (SIR) that provides practical guidance for applying the procedure.

8. Training Class: The USGS will provide a training class for SCDOT personnel regarding the application of the guidance manual.

BENEFITS

This project will benefit the SCDOT by

(1) evaluating the South Carolina regional bridge-scour envelope curves with other sources of field data to verify and if necessary, modify the envelope curves;

(2) developing an integrated procedure for utilizing the South Carolina regional bridge-scour envelopes to assess scour potential at riverine bridges in South Carolina; and

(3) developing a spreadsheet that will automate the computational components of the integrated procedure to help evaluate scour potential at a given bridge in a consistent and quick manner.

While the scour assessing procedures developed in this investigation will be directly applicable to South Carolina, they may be applicable to streams in other states where stream characteristics are similar to South Carolina.

PUBLICATIONS

It is anticipated that the findings of this investigation will provide guidance to the practitioner for assessing scour potential in South Carolina. The findings will be documented in a USGS Scientific Investigations Report. While the report will be directed to the technical audience in South Carolina, it will also benefit those outside of South Carolina who have an interest in understanding bridge scour in the field setting. Additionally, several journal articles will be developed documenting the verification of the South Carolina bridge-scour envelope curves.

REFERENCES

Benedict, S.T., Caldwell, A.W., Clark, J., Young, R.S., 2017, Online Database for Clear-water Abutment, Clear-water Pier/Contraction and Live Bed Pier/Contraction Scour.

Benedict, S.T., 2003, Clear-water abutment and contraction scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99: U.S. Geological Survey Water Resources Investigation Report 03-4064, 137 p.

Benedict, S.T., and Caldwell, A.W., 2006, Development and evaluation of clear-water pier and contraction scour envelope curves in the Coastal Plain and Piedmont of South Carolina, U.S. Geological Survey Scientific Investigations Report 2005-5289, 98 p.

Benedict, S.T., and Caldwell, A.W., 2009, Development and evaluation of live-bed pier and contraction scour envelope curves in the Coastal Plain and Piedmont provinces of South Carolina, U.S. Geological Survey Scientific Investigations Report 2009-5099, 108 p.

Benedict, S.T., and Caldwell, A.W., 2012, Modification of selected South Carolina bridge-scour envelope curves: U.S. Geological Survey Scientific Investigations Report 2012-5029, 37 p.

Benedict, S.T., Deshpande, N., Aziz, N.M., and Conrads, P.A., 2006, Trends of abutment-scour prediction equations applied to 144 field sites in South Carolina, Open-File Report 2003–295, 150 p.

Breusers, H.N., Nicollet, G., and Shen, H. W., 1977, Local scour around cylindrical piers, Journal of Hydraulic Research, International Association of Hydraulic Engineering and Research, v. 15, no. 3, p. 211-252.

Ettema, R., Melville, B.W., and Barkdoll, B., 1998, Scale effect in pier scour experiments, Journal of Hydraulic Engineering, American Society of Civil Engineering, v. 124 no. 6, p. 639-642.

Melville, B.W. and Coleman, S.E., 2000, Bridge Scour, Highlands, Colorado, Water Resources Publications, LLC., 550 p.

Mueller, D.S., and Wagner, C.R., 2005, Field observations and evaluations of streambed scour at bridges, Federal Highway Administration, Publication FHWA-RD-03-052, 122 p.

Richardson, E. V., and Davis, S. R., 2001, Evaluating scour at bridges: Federal Highway Administration Hydraulic Engineering Circular No. 18: Federal Highway Administration Publication No. FHWA -IP-90-017, 131 p.