Methodology development and assessment of scour envelope curves for SCDOT bridges in the Charleston coastal environment Active
The U.S. Geological Survey, in cooperation with the South Carolina Department of Transportation (SCDOT) and Federal Highway Administration (FHWA), has evaluated clear-water and live-bed pier, contraction, and abutment scour associated with over 200 bridges throughout South Carolina for many years, predominantly at non-tidally influenced sites. These evaluations resulted in the development of envelope curves as tools for assessing the potential for scour at inland bridges in South Carolina. Based on the findings from these studies, similarities can be seen between riverine and tidally-influenced site geometry. However, the bi-directional flows that occur at tidally-affected and tidally-controlled bridges compounded with the unique geological characteristics associated with the tidal environment may limit the application of the empirically derived South Carolina bridge pier-scour envelopes for tidal sites. Therefore, there is a need to collect bridge pier-scour data at tidally-influenced sites and to establish a means of representing the pier scour observed by analyzing predicted pier scour.
Relevance and Impact
This study supports the following natural hazards strategic action under the long-term mission goals of the USGS: "Enhance understanding of the linkages among natural hazards, the environment, climate, and society, and the ways by which climate variability and change influence the frequency and intensity of natural-hazard events" (U.S. Geological Survey, 2007) and the USGS Water Science goal to, "anticipate and respond to water-related emergencies and conflicts" (Evenson and others, 2013). The study also addresses one of the priority actions in the USGS Water Science Strategy to, "expand and enhance water-resource monitoring networks" (Evenson and others, 2013). The study will be beneficial to the SCDOT as it could establish a reproducible and consistent guideline for measuring coastal scour processes. If successful, the envelope curves that are developed will be useful to bridge design engineers for new bridges near the coast and for the maintenance and risk assessment of existing bridges identified as having scour predictions exceeding design depths.
Objectives
- Develop methods for field measurements of historic (sub-bottom) and current (bed surface) bridge scour.
- Estimate the range of hydraulic parameters that likely produced the historical and current scour at selected sites where gage and/or model data currently exist.
- Evaluate the performance of current methods for predicting tidal scour at selected sites as well as exploring modifications for prediction improvement.
- Where possible, confirm and(or) extend the currently used envelope curves using the collected scour data.
- Develop a strategy for collecting and analyzing bridge scour along the South Carolina coastline to define envelope curves or the limits thereof, if possible, that could be used to assess the range of scour depth for given field conditions such as tidally-influenced/affected/controlled rivers, bays, and estuaries.
Methodology Development
Due to the process of infilling, pier-scouring associated with flooding typically is no longer quantifiable by means of a rod or echosounder. By profiling the sub-surface of the river bottom, the extent of the scouring can be seen as differences in material composition or density, identifying the contrast between infill and the scoured river bottom. Ground penetrating radar (GPR) and continuous seismic reflection profiler (CSRP) will be used to test the accuracy and efficiency in collecting scour data in a coastal environment. The latter instrumentation is proposed due to the attenuation that occurs in saline water when using a GPR. An echosounder will collect bathymetry data alongside the GPR and CSRP as a means of validation and for use in processing. Simultaneously, two GPS units accompanied by a motion reference unit will be mounted to these devices to provide corrections in spatial processing and for mapping purposes. The profiling equipment will follow transects established upstream and downstream of the bridge at distances of 1, 25, 100, 150, 200 feet or beyond, if necessary, as well as longitudinal tracks along the left, middle and right of the channel and left and right side of piers. Where possible, cores will be taken in the pier-scour holes and used in the validation/correction of the GPR and CSRP sub-surface profiles.
Because of the tidal influences and site-specific characteristics, flow measurements will be taken at the flood and ebb tide. Data collected at these times will be used in calibration/validation of modeled data and to determine if redundancies are encountered, leading to measurements taken only at the most informative tide(s).
Sediment samples will be collected upstream and downstream of the bridge to determine a geometric mean grain size for the study site and to determine any upstream to downstream fining. Additionally, site characteristics such as river width, pier geometry, approach flow depth, approach flow velocity and angle of attack will be recorded. Most of these characteristics are necessary in the use of pier-scour prediction models.
Detailed Analysis
The methodology developed will be applied to each site survey providing a basis for consistency in field measurements. Data will be collected at approximately 20 tidal sites in the Charleston Harbor. If limiting factors arise at a site, another site will be selected for surveying if possible or the limitation will be reflected in the documentation. To avoid skewing the observed pier-scour depth an analysis will be conducted to determine if a distinction can be made between the associated forms of scour. When the associated forms of scour can clearly be dissociated, the pier-scour depth will be corrected for, and the other contributing forms of scour will be provided to SCDOT for review against existing envelope curves. When a distinction cannot be made, or is subjective, the pier-scour depth will be included as is in the prediction performance methods along with the corrected pier-scour depths.
Determination of hydraulic characteristics in tidal waterways will likely require the assistance of a coastal engineer or hydraulic modeler. This technical assistance will be utilized when assessing the tidal range effecting a bridge crossing, local geography and channel constrictions, susceptibility to impacts of storm surge and wave action from hurricane events, and unsteady flow modelling in 2-D or 3-D. These assessments will be used to estimate the flow depth and velocities that formed the historic scour hole measured in the field. Analysis will be performed to determine the applicability of current pier-scour prediction equations such as the CSU equation and FDOT’s methodology. Relationships among the field collected parameters will be tested to determine if envelope curves similar to those developed for SCDOT in previously published reports (Benedict and others, 2006; Benedict and Caldwell, 2012, 2016; Benedict and others, 2018) are valid within tidally-influenced areas.
Upon completion of the project, all scripts, data and findings will be released in a publically available USGS publication product(s).
The U.S. Geological Survey, in cooperation with the South Carolina Department of Transportation (SCDOT) and Federal Highway Administration (FHWA), has evaluated clear-water and live-bed pier, contraction, and abutment scour associated with over 200 bridges throughout South Carolina for many years, predominantly at non-tidally influenced sites. These evaluations resulted in the development of envelope curves as tools for assessing the potential for scour at inland bridges in South Carolina. Based on the findings from these studies, similarities can be seen between riverine and tidally-influenced site geometry. However, the bi-directional flows that occur at tidally-affected and tidally-controlled bridges compounded with the unique geological characteristics associated with the tidal environment may limit the application of the empirically derived South Carolina bridge pier-scour envelopes for tidal sites. Therefore, there is a need to collect bridge pier-scour data at tidally-influenced sites and to establish a means of representing the pier scour observed by analyzing predicted pier scour.
Relevance and Impact
This study supports the following natural hazards strategic action under the long-term mission goals of the USGS: "Enhance understanding of the linkages among natural hazards, the environment, climate, and society, and the ways by which climate variability and change influence the frequency and intensity of natural-hazard events" (U.S. Geological Survey, 2007) and the USGS Water Science goal to, "anticipate and respond to water-related emergencies and conflicts" (Evenson and others, 2013). The study also addresses one of the priority actions in the USGS Water Science Strategy to, "expand and enhance water-resource monitoring networks" (Evenson and others, 2013). The study will be beneficial to the SCDOT as it could establish a reproducible and consistent guideline for measuring coastal scour processes. If successful, the envelope curves that are developed will be useful to bridge design engineers for new bridges near the coast and for the maintenance and risk assessment of existing bridges identified as having scour predictions exceeding design depths.
Objectives
- Develop methods for field measurements of historic (sub-bottom) and current (bed surface) bridge scour.
- Estimate the range of hydraulic parameters that likely produced the historical and current scour at selected sites where gage and/or model data currently exist.
- Evaluate the performance of current methods for predicting tidal scour at selected sites as well as exploring modifications for prediction improvement.
- Where possible, confirm and(or) extend the currently used envelope curves using the collected scour data.
- Develop a strategy for collecting and analyzing bridge scour along the South Carolina coastline to define envelope curves or the limits thereof, if possible, that could be used to assess the range of scour depth for given field conditions such as tidally-influenced/affected/controlled rivers, bays, and estuaries.
Methodology Development
Due to the process of infilling, pier-scouring associated with flooding typically is no longer quantifiable by means of a rod or echosounder. By profiling the sub-surface of the river bottom, the extent of the scouring can be seen as differences in material composition or density, identifying the contrast between infill and the scoured river bottom. Ground penetrating radar (GPR) and continuous seismic reflection profiler (CSRP) will be used to test the accuracy and efficiency in collecting scour data in a coastal environment. The latter instrumentation is proposed due to the attenuation that occurs in saline water when using a GPR. An echosounder will collect bathymetry data alongside the GPR and CSRP as a means of validation and for use in processing. Simultaneously, two GPS units accompanied by a motion reference unit will be mounted to these devices to provide corrections in spatial processing and for mapping purposes. The profiling equipment will follow transects established upstream and downstream of the bridge at distances of 1, 25, 100, 150, 200 feet or beyond, if necessary, as well as longitudinal tracks along the left, middle and right of the channel and left and right side of piers. Where possible, cores will be taken in the pier-scour holes and used in the validation/correction of the GPR and CSRP sub-surface profiles.
Because of the tidal influences and site-specific characteristics, flow measurements will be taken at the flood and ebb tide. Data collected at these times will be used in calibration/validation of modeled data and to determine if redundancies are encountered, leading to measurements taken only at the most informative tide(s).
Sediment samples will be collected upstream and downstream of the bridge to determine a geometric mean grain size for the study site and to determine any upstream to downstream fining. Additionally, site characteristics such as river width, pier geometry, approach flow depth, approach flow velocity and angle of attack will be recorded. Most of these characteristics are necessary in the use of pier-scour prediction models.
Detailed Analysis
The methodology developed will be applied to each site survey providing a basis for consistency in field measurements. Data will be collected at approximately 20 tidal sites in the Charleston Harbor. If limiting factors arise at a site, another site will be selected for surveying if possible or the limitation will be reflected in the documentation. To avoid skewing the observed pier-scour depth an analysis will be conducted to determine if a distinction can be made between the associated forms of scour. When the associated forms of scour can clearly be dissociated, the pier-scour depth will be corrected for, and the other contributing forms of scour will be provided to SCDOT for review against existing envelope curves. When a distinction cannot be made, or is subjective, the pier-scour depth will be included as is in the prediction performance methods along with the corrected pier-scour depths.
Determination of hydraulic characteristics in tidal waterways will likely require the assistance of a coastal engineer or hydraulic modeler. This technical assistance will be utilized when assessing the tidal range effecting a bridge crossing, local geography and channel constrictions, susceptibility to impacts of storm surge and wave action from hurricane events, and unsteady flow modelling in 2-D or 3-D. These assessments will be used to estimate the flow depth and velocities that formed the historic scour hole measured in the field. Analysis will be performed to determine the applicability of current pier-scour prediction equations such as the CSU equation and FDOT’s methodology. Relationships among the field collected parameters will be tested to determine if envelope curves similar to those developed for SCDOT in previously published reports (Benedict and others, 2006; Benedict and Caldwell, 2012, 2016; Benedict and others, 2018) are valid within tidally-influenced areas.
Upon completion of the project, all scripts, data and findings will be released in a publically available USGS publication product(s).