Abstract Description: In May and June of 2024, the Rapidan Dam drainage basin in south central Minnesota experienced precipitation resulting in the fifth wettest month on record and the second largest flooding event in the hydropower project's 114 years of operation. The flooding mobilized an accumulation of interlocked woody debris masses that broke loose from the upstream boater safety cable and highway bridge piers that blocked spillway bays reducing discharge capacity. The blockage caused rising reservoir water to overtop the crest and flow around low areas at the left abutment. These flanking flows concentrated around obstructions and knick-points increasing the localized hydraulic energy and turbulence causing scour of weak fill and geologic materials composing the abutment leading to uncontrolled release of the reservoir and accumulated sediment. The breach enlarged vertically and laterally, fully capturing the Blue Earth River and bypassing around the left side of the still standing and intact concrete Ambursen slab and buttress dam. During the flooding and dam breach the project owner and operator implemented the project Emergency Action Plan (EAP). Fortunately, the failure did not result in any loss of life. However, consequences incurred included: o Undermining and loss of a residential structure o Demolition of the historic Rapidan Dam general store o Loss of transformers, substation, and County maintenance storage shed o Damaged the stepped concrete overtopping protection just downstream of the powerhouse o Loss of functionality and benefits of the dam (i.e. hydropower, recreation) o Erosion and transport of impounded reservoir sediment flushed through the breach and deposited along the downstream riparian corridor o Regressive scour caused undermining and destabilization of the upstream bridge piers jeopardizing its structural integrity ultimately resulting in closure, demolition, and replacement of the structure o Post-failure mitigation and restoration of the river system Following the failure the Federal Energy Regulatory Commission assembled a technical Independent Forensic Team (IFT) composed of dam safety professionals from the Army Corps of Engineers and Bureau of Reclamation. The mission of the IFT was to evaluate the project background, site conditions, operations, performance, past dam safety evaluations and modifications; evaluate the sequence, physics, and root cause(s) of the failure; assess the decisions made and actions taken by responsible parties; and develop lessons learned from the failure to inform the dam safety community and improve the FERC dam safety program. This presentation summarizes the IFT review findings and lessons learned related to debris management, planning, or system changes; early failure mode development recognition; intervention methodologies and preparation; operational communication during ownership transfer; and importance of experience with dam management responsibilities, safety incidents, and emergency response. IFT Report: https://www.ferc.gov/dam-safety-and-inspections

Abstract Description: Demopolis Lock and Dam is sited below the confluence of the Black Warrior and Tombigbee rivers at Demopolis, Alabama. The lock provides navigation service for commercial navigation and recreation interests in Alabama's Black Warrior river basin and, through the Tombigbee River's Tenn-Tom Waterway, serves users from multiple states and federal navigation systems. The project was designed and constructed by the Mobile District, US Army Corps of Engineers (USACE) and was put into service in 1954.

At 5:45 a.m. January 16, 2024, a large block of the lock's concrete upstream miter sill broke away, leading to an uncontrolled release through the lock and its open downstream miter gates, which are not designed to be closed under flowing conditions. This type of breach was unprecedented in USACE, as were the response and repair measures required to arrest breach flow and return the lock to service. Breach flow was arrested by tug-assisted closure of the lower miter gates. The lock was repaired, during the flood season, by rebuilding and enlarging the lock miter sill using underwater concrete forming and placement methods. Navigation service was disrupted through many inland waterways for four months. Causal mechanisms were postulated by examination of design, operation, and construction records; concrete testing and examination; and Potential Failure Modes Analysis. Contributory factors identified included the age and use of the lock; accidents and operational events resulting in adverse loading conditions on the sill; design factors, most notably the upper miter gates' vertically-framed arrangement and the absence of steel in the sill to resist tensile loading and crack propagation; and construction factors owing to the arrangement of unbonded block joints. Advance intervention in this case was not likely, as the presence of cracking was hidden and not expressed visually. Lock owners with vertically-framed miter gates should assess the design, construction, and condition of their miter sills to identify miter sill breach risk. This paper summarizes the incident, response, damage assessment, repair, and incident investigation, concluding with recommendations.

Two consecutive multi-day storm systems impacted the Hawaiian Islands during a 14-day period from 03/11/26 - 03/25/26, resulting in adverse flood conditions in several locations, including Wahiawa Dam on the Island of Oahu. The flood loading resulted in three distinct rapid-rises in reservoir levels that triggered two rounds of emergency notifications warning of a possible dam failure.

Abstract Description: Over the past 40 years numerous embankment dams and earth-cut spillways in the United States have been armored using Articulating Concrete Blocks (ACBs) to provide erosion protection. Several dams and spillways armored with ACBs have been overtopped and performed satisfactorily with overtopping flow depths and velocities exceeding 4 feet and 30 feet per second, respectively. Much has been learned about what works and what does not work. Of the ACB installations that have failed or experienced damage, the underlying issues have been attributed to one of several possible failure modes that were not understood or adequately addressed during the design.

This presentation will share information on several recent ACB embankment armoring projects completed in the Northeastern region of the United States. ACB spillway armoring failures will be discussed that have been attributed to one of several potential failure modes that may not have been understood or adequately addressed during design. State of the practice design features to address these potential failure modes will be presented. This information is important for engineers to consider during their designs, and for regulators reviewing ACB armoring designs, so that future failures and unnecessary damage resulting in costly maintenance can be prevented.

Abstract Description: The design of hydraulic jump stilling basins typically follows standardized procedures from Federal organizations (Reclamation, NRCS, USACE) who developed said procedures using smooth chute criteria. The increasing number of spillway designs with stepped spillways has highlighted the need to more broadly understand the interaction of stilling basins with upstream stepped chutes. Therefore, this study employed a systematic experimental campaign at the Utah Water Research Laboratory to expand the use of Reclamation Type II and St. Anthony Falls basins to moderate-sloped stepped spillways common to embankment dams. The basins were each designed for an incoming Froude number of 4 and were tested for a range of unit discharges below, at, and above the design discharge. Sensitivity of the hydraulic jump surface and pressures within the basin to the step height and tailwater were assessed by testing two steps heights and three ratios of conjugate depth (10% below, 10% above, and equal to conjugate depth). A qualitative parametric study to evaluate possible improvements to the St. Anthony Falls basin was also performed by modifying the placement and size of the baffle blocks. It was found that streamwise basin floor pressure profiles do not follow hydrostatic conditions with maximum values occurring at the basin entrance and near basin elements. Fluctuations of flow depths and pressures were greatest at these locations. Between the St. Anthony Falls basin baffle blocks, some pressures were below atmospheric pressure. Increasing the step height by a factor of two reduced the mean flow depths and basin floor pressures as much as 20%. Tailwaters less than conjugate depth often resulted in sweep out or the hydraulic jump toe occurring within the basin while tailwaters above conjugate depth improved jump stability and placed the hydraulic jump toe upstream of the basin. The St. Anthony Falls basin was more robust to tailwater sensitivity than the Type II basin. To contextualize the results, comparisons of hydraulic jump profiles were made to the more common Type III basin, among others. The study concluded that standardized design procedures for Type II and St. Anthony Falls basins are applicable to moderate-sloped stepped spillways and Froude numbers less than 4 while modifications to the St. Anthony Falls basin baffle blocks may improve basin performance.

Abstract Description: Overtopping of embankment dams is attributed to a large portion of historic dam failures. Changes in the understanding of extreme hydrologic events have resulted in a large number of existing embankment dams identified as hydrologically deficient, requiring remediation. The use of Roller-Compacted Concrete (RCC) and Soil Cement (SC) stepped overtopping protection has become a popular remediation alternative due to the energy dissipation provided by the steps and the economy of the construction methods and materials. The typical performance requirements of the cemented materials used for stepped overtopping protection alternatives have mostly been based on rules-of-thumb of unconfined compressive strength. Many projects have included a "sacrificial" outer edge of less competent materials that may not have the required compaction effort to meet strength requirements; but are not considered to be part of the critical section. Often the estimate of energy dissipation provided by the stepped geometry is based on idealized conditions and do not reflect the actual as-built condition nor consider the range of potentially eroded geometric conditions of the steps. This presentation explores the issue of non-idealized step geometry through a design case history for a large USACE Dam Safety Modification Project. This case history includes the construction of multiple prototype demonstration sections, the evaluation of erosion characteristics by means of a large-scale submerged jet test, and evaluation of the resulting impacts on energy dissipation by non-idealized step edge geometry.

Abstract Description: This presentation will describe the process, key tools and initial results associated with integrating risk analysis into the Wyoming State Engineer's Office (Wyoming SEO) dam safety program in 2026. The Wyoming SEO leveraged FEMA funds to initiate Semi Quantitative Risk Assessment (SQRA) risk workshops and prepare (SQRA) reports for several dams in the State of Wyoming. The presentation will demonstrate how SQRA guidelines used by Reclamation and the State of Colorado were used to develop customized templates and tools to facilitate a very efficient and cost effective SQRA process for dams in Wyoming.

Virtual risk workshops were utilized that allowed subject matter experts, Wyoming SEO staff, and the dam owners from across the state to efficiently participate in SQRA workshops. The risk team utilizes the recently developed Wyoming Probable Maximum Precipitation Study tool and USGS tools to update hydrologic and seismic loadings for the dams. Consequences were also evaluated using available tools from other federal dam safety programs. This initial effort really helped the owners and the Wyoming Safety of Dams engineers better understand the overall risks and key risk driving potential failure modes at these high hazard dams.

Abstract Description: The State of Kansas Division of Water Resources recently funded a study to develop an efficient and cost-effective process to evaluate dam risks in Kansas. The Risk Informed Decision Making (RIDM) process developed for DWR combines methodologies from NRCS' Failure-Risk Indexing and USFWS' screening level risk and consequence matrix. These methodologies were then modified to be specific to Kansas and to work for a variety of dams with a variety of available design and construction data. The process was then pilot tested on 60 dams. This process was utilized to aid the State in defining and designating "unsafe" dams. The goal of the RIDM tool was to develop a procedure that would take approximately 12 hours to evaluate the risk associated with a dam. This presentation will cover the RIDM process developed, results of the pilot-testing, and how the State of Kansas may use RIDM in the future.

Abstract Description: For decades, dam safety in Colorado has been predominately regulated by comparing observations and analytical results to established standards. During that time, the responsibilities of regulators have increased, and the number of regulatory personnel has decreased. To help leverage the limited capacity of the regulators and improve the regulatory framework, the Colorado Dam Safety Program launched an initiative in 2021 to perform semi-quantitative risk assessments (SQRA) of all high hazard dams in the State. The State developed a method to perform streamlined SQRAs called the Comprehensive Dam Safety Evaluation (CDSE). The typical results of a CDSE include: a compiled summary of the dam's construction and performance history, identification of high-risk issues, prioritization of issues to help inform the regulator's yearly inspection, and recommendations for monitoring and mitigating risks. Another common result of a CDSE is that owners become more educated and motivated to proactively manage dam safety risks. Five years after launching the CDSE initiative, this presentation evaluates the program and considers its effectiveness in supporting dam safety regulation with limited resources. The presentation provides an overview of the program and its goals, a description of the CDSE process and how it compares to other SQRAs, a statistical review of the results from the CDSEs completed to date in Colorado, case histories, post-CDSE feedback from participants, and lessons observed from performing CDSEs in Nebraska and North Dakota.

Abstract Description: Indian Lake Dam is a cyclopean concrete and masonry dam constructed circa 1898. During rehabilitation to address stability, seepage, and general upgrades, a dive inspection into the gatehouse revealed extensive damage. Although minor repairs were always expected, the condition was much worse than anticipated and long-term performance of the structure was in question. The gatehouse is roughly 40 ft below water and the structure's issues complicated the repair solution. The challenges included 5-foot-diameter low level outlet conduits that had extensive bypass leakage making access dangerous/difficult, masonry construction with severe mortar loss that rendered the structural capacity extremely limited, and some portions of the support walls that were completely inaccessible making them very difficult to address. Design Lessons Learned: The design team developed plans to repair the structure using a three-sided reinforced concrete facing constructed with tremie concrete & divers. An early design modification enacted at the request of the contractor was to use precast concrete panels in lieu of conventional formwork which had several benefits. First, the thimbles for exterior sluice gates could be cast into the precast allowing the divers to more easily align the thimble with the existing opening. The exterior layer of reinforcing could be cast within the panel and formwork for the corners and vertical joints between the panel could be easily placed against the panels. The design team worked through the details which included room for lap lengths at vertical panel joints, room for standard hooks at the corners, and tolerance to allow for dimensional variations in the bedrock and existing dam. Construction Lessons Learned: Construction progressed through the harsh northern Adirondack, NY climate when the lake level was seasonably lowered. Even with the drawdown, the divers still had 30+- feet of water depth to contend with. How reinforcing detailing, tolerances, and interference challenges were overcome and other general lessons learned will be covered. What worked for specifics on tremie placement methods, placing reinforcement underwater, inspection, & verification of installation will be shared. Examples of how water management, diver safety, and other ancillary impacts will be included. This is a story of engineers, contractors, owners and regulators meeting the challenges presented by a complex problem.