Repair or Replace? Considerations for Dam Rehabilitation

Abstract Only - Major rehabilitation efforts are often required to bring dams into compliance with current regulations. Often times the dam owner or engineer is challenged to decide between the repair or replacement of a deficient structure, or part of the structure, based on costs, known deficiencies, site constraints, functionality, risk considerations, etc. Repairs typically involve salvaging the existing structure while minimizing impacts to the overall site. Conversely, replacement projects include removal and replacement of deficient structure, which typically have larger site impacts. This paper will showcase dam rehabilitation projects and present the factors and considerations used to answer the ultimate question… to repair or to replace?
Project examples to be discussed:
- A concrete overlay and post-tensioned rock anchors were used to repair an ogee spillway with stability and freeboard deficiencies. The cost-savings were used by the owner to fund additional rehabilitation projects.
- For a dam with significant and numerous spillway and earthen embankment deficiencies, the decision was made to replace the spillway and completely reconstruct the embankment to current standards. The owner also requested to improve the functionality of the lake by raising the pool elevation and raising the embankment crest.
- A deteriorated concrete spillway was replaced with a labyrinth spillway because of current site constraints (limited footprint). The spillway was constructed off-line while the normal pool was controlled by the existing spillway. In this manner the functionality of the lake was preserved throughout construction.
- An emergency repair on an undermined section of a run-of-river concrete gravity dam required one-third of the dam to be reconstructed. The remaining portions of the dam continued to be active to preserve the city’s water supply source.
- Subsurface investigations revealed artesian conditions at the toe of a concrete spillway. Based on risk considerations, a design was developed to repair the existing structure by adding concrete mass to meet stability requirements.
- Initial investigations performed on a concrete riser revealed the need to repair or replace the structure. In addition, erosion protection was required on the grass-lined emergency spillway. Because the reservoir had limited functionality and posed downstream risks upon failure, the dam was ultimately breached and decommissioned.