Brown Bridge Dam Failure Report

The report on the Root Cause Analysis of the October 6, 2012, Failure Incident at Brown Bridge Dam – Temporary Dewatering Structure (TDS) is available in two parts.  The Executive Summary is highlighted below.

Brown Bridge Dam Failure Report plus Appendices A through C (8 MB PDF)

• Preface
• Executive Summary
• Authority
• Methodology
  • Project Description
  • About the Failure Event
  • Failure Modes Considered
  • Summary of Field Data Analyses
  • Probable Failure Mode
• Conclusions
• References
• Acknowledgements
• Appendix A Post-Failure Field Interviews
• Appendix B Drawings and Figures
• Appendix C 2012 Soil Boring Data

Brown Bridge Dam Failure Appendices D through I (7 MB PDF)

• Appendix D Historic Soil Borings and Construction Plans
• Appendix E Detailed Seepage Analysis
• Appendix F Post Failure Survey Data
• Appendix G Headwater and Tailwater Readings
• Appendix H TDS Sheetpile Data
• Appendix I TDS Failure Investigation Daily Logs

Executive Summary

The Brown Bridge Dam, formerly located on the Boardman River in Grand Traverse County, Michigan, was owned by the City of Traverse City and originally constructed in 1921 for the purpose of generating hydroelectricity. The dam was operated by Traverse City Light & Power (TCL&P) under lease from the City. It generated electricity continuously until its decommissioning in November 2006. Until the time of its removal, the Brown Bridge Dam consisted of an approximately 1600-foot long earthen embankment and a combined powerhouse/spillway structure. The dam had a structural height of approximately 46 feet. At normal pool elevation, the surface area of the Brown Bridge Pond was approximately 190 acres, and the storage volume was approximately 1,900 acre-feet.

In 2012, the City of Traverse City was granted a permit from EGLE's Water Resources Division to remove the Brown Bridge Dam and restore a natural river channel through the former Brown Bridge Pond impoundment. The project included a steel sheetpile walled diversion channel and drawdown structure, referred to as the TDS, to be constructed adjacent to the existing spillway structure.

On the morning of October 6, 2012, at first loading of the TDS water control structure, a boil was noticed downstream of the concrete slab lining the bottom of the channel. Flow from the boil increased quickly, ultimately resulting in failure of the TDS and collapse of the earthen embankment section located between the TDS and the Brown Bridge Dam spillway structure. The ensuing uncontrolled release of impounded water resulted in significant flooding along the Boardman River.

A series of post-failure investigations and subsequent geotechnical analyses found that the most likely failure mode of the TDS was internal erosion of the foundation material from underneath the water control structure within the TDS. The hydraulic loading that the control section was subjected to on October 6, 2012, resulted in an unstable subsurface soil condition, which led to erosion of the foundation soils, release of water underneath of the control structure, collapse of the earthen embankment adjacent to the south TDS wall, and ultimately the uncontrolled release of water from the Brown Bridge Pond.