Dam vulnerability: A rising threat to life & property

The Brazilian mining company Vale recently agreed to pay $7 billion to the state of Minas Gerais. This settlement did not include other pending charges against Vale tied to a horrific incident that killed 270 people and gave a real-life example of the massive consequences of a dam failure.

Dams, dikes and levees are an integral part of urban infrastructure in the United States and abroad. Dams are often used for electrical generation (hydropower), agricultural purposes, water storage and flood control. In addition, dams are also used to store hazardous materials which include oil products, mining byproducts and other waste substances. Many people may first visualize the Hoover Dam or other large concrete and steel structures that control the flow of rivers. However, most of the dams in the United States are earthen embankment dams, which simply consist of piled and compacted soil that is used to retain the water. While many dams are federally owned or built, over half of the dams within the United States are privately owned and most consist of smaller earthen embankments.

A little history

The importance of inspecting and maintaining one of our nation’s most critical infrastructures began in 1972 when Congress authorized the U.S. Army Corps of Engineers (USACE) to inventory dams through the National Dam Inspection Act. In 1975, the first National Inventory of Dams (NID) was published. The Water Resources Development Act of 1996 allocated funds for the USACE to continue to update the NID. During this time, the USACE began to work closely with the Federal Emergency Management Agency (FEMA) to further define and classify each dam throughout the United States.

In addition to the importance of maintaining dams, large dams are also considered critical infrastructure and are a priority for the Department of Homeland Security to protect not only against maintenance-related issues but also potential terrorist attacks. Following the September 11, 2001, attacks, the National Dam Safety and Security Act was passed to provide funding for inspection, maintenance and security of the dams throughout the United States. This act was re-authorized in the National Dam Safety Program in 2006 and more recently was re-authorized through 2023 in the Water Infrastructure Act of 2018.

Hazard potential classification system

Dams that meet the minimum criteria within the United States are classified by the NID based on data provided by federal and state government regulators. The data used to classify dams includes construction permits, building inspections and/or enforcement records, or other applicable specifications or records of a certain dam.

The purpose of the program was to prioritize each dam throughout the United States for maintenance and inspection. This classification system categorizes dams based on the potential loss of human life, economic loss, lifeline disruption and environmental impact in the event of a dam failure. The classification system includes low, significant and high-hazard potential. Here are the minimum criteria for dams to be a part of the NID.

Low hazard potential is when in a failure event there would be no probable loss of human life with low economic or environmental losses. Conversely, high-hazard potential would have a probable loss of human life with high economic or environmental losses.

The minimum criteria for dams to be a part of the NID are:

1. High-hazard potential classification — loss of human life is likely if the dam fails.
2. Significant-hazard potential classification — no probable loss of human life but can cause loss, environmental damage, disruption of lifeline facilities, or impact other concerns.
3. Equal or exceed 25 feet in height and exceed 15 acre-feet in storage.
4. Equal or exceed 50 acre-feet storage and exceed 6 feet in height.

Current conditions

The dams within the United States have an average age of 57 years. By 2025, 73% of dams in the NID will be over 50 years old. According to the American Society of Civil Engineers’ (ASCE) 2017 Infrastructure Report Card, the United States Dams were graded as a “D.” Due to population growth, slightly over 17% of the 91,457 dams within the United States are considered to be high-hazard potential dams and are cataloged in the NID. Coupled with a lack of federal or private funding, more than 2,000 of the high-hazard dams are considered to be deficient.

In addition to high-hazard potential dams, 11,354 dams are classified as significant hazards. Figure 1 is an image taken from the NID website (nid.usace.army.mil/), which shows the concentration of dams throughout the U.S.

The red icons indicated dams that have been classified as high-hazard potential, yellow as significant-hazard potential and green as low-hazard potential.

According to the NID, only 74% of high-hazard potential dams have Emergency Action Plans (EAPs). The purpose of EAPs is to identify potential hazards at a dam and pre-planned measures to respond to catastrophic events that could cause loss of life and property. An EAP also serves as a document that coordinates owners and emergency managers, identifying areas where there is a concern for public safety. Guidelines for EAPs can be found within FEMA’s Federal Guidelines for Emergency Action Planning for Dams (FEMA Publication No. P-64).

Dam construction and vulnerability

The majority of dams throughout the U.S. are earthen dams, which have become an economical construction design option to control the flow of water or impound hazardous material. There are various types of earthen dams, which include simple embankments and zoned embankments. Simple embankments typically comprise a homogenous soil embankment. A zoned embankment is similar but with a core and trench. Clay is often used as the core due to its impermeability; however, concrete, sheet piles or timber are also used to provide a water barrier.

Impermeable cores are used to impede the flow of groundwater through dams, which could cause seepage and piping failures. Conversely, some dam designs provide a high-permeability core which consists of a section of poorly-graded sand or gravel wrapped in a filter fabric that allows water to flow through the dam in a controlled manner. The filter fabric allows water to flow into the permeable core but precludes soil particles from moving through the embankment, which can cause the dam to fail in a mechanism known as “soil piping”.

As previously seen during Hurricane Katrina and Hurricane Harvey, dams and levees can fail from heavy rainfall events which exceed the designed storage and discharge capacities of the associated water bodies. According to the National Performance of Dams Program (NPDP), the primary cause of dam failures is due to flooding or overtopping, which is followed by water and soil movement through the earthen embankment, via soil piping, as previously discussed. Both failures can be caused by a flood event. Figure 2 highlights the cause of dam failures from 1975 to 2001.

Visual assessment

To effectively assess the condition of a dam or levee after a catastrophic rainfall event, a rapid visual assessment of a dam embankment is imperative to identify potential hazards. These hazards would include scour and erosion, slope failures and subsidence features. Scour, erosion and slope failures are most common with a rapid increase of flow through a water body. While scour is a visible surface condition, subsidence features may be associated with a subsurface issue such as piping. Piping occurs through cracks, voids or weak zones in a dam embankment. Soil particles are eroded or washed through these weak zones, creating a path for water to flow.

These rapid visual assessments can best be performed by manual inspections of the site supplemented by small UAVs or drone surveys. Early identification of dam hazards would assist in developing plans to mitigate the potential risk of failure; or, in the event of a failure, determining cause and origin.

Geophysical and geotechnical assessments

As part of a thorough geotechnical assessment, a geophysical survey should be completed. Geophysical tools such as Ground Penetrating Radar (GPR) and Electrical Resistivity Imaging (ERI) are widely used to detect subsurface anomalies that could be associated with seepage or piping.

Geophysical surveys should be supplemented with soil borings or soil probes to develop an accurate soil profile in the areas of interest. Soil borings, such as the Standard Penetration Test (SPT) boring, or soil probes such as the Cone Penetrometer Test (CPT), can identify loose soil zones within dams, which would be indicative of subsidence, slope failure or piping through the dam.

Evaluation methods

Upon completion of the site visit, geotechnical engineers and geophysicists perform a comprehensive forensic assessment by compiling and reviewing the data gathered during the site inspection. This includes soil surveys and testing, visual images captured during the site assessment (on the ground or by a rapid UAV or drone survey), and historic documents related to the dam. The forensic evaluation is performed to assess the existing condition of the dam and classify the hazard potential, identify hazards that may exist, determine the cause and origin of distress, and develop plans to mitigate the potential for future dam failures.

There is an increasing risk of dam failures that will result in the loss of life and property throughout the United States. As metropolitan areas continue to grow, there is a greater need for stormwater control and a reliable source of clean drinking water, which dams can provide. However, much like roads and bridges, if dams are not properly maintained they can be susceptible to failure, which can have catastrophic and costly consequences.

Sharkey Bowers, P.E. (wbowers@jsheld.com) is a licensed professional engineer with concentrated experience in forensic, geotechnical, and structural engineering. He has authored hundreds of technical reports providing professional engineering opinions regarding the cause, origin and duration of damage to properties. Special thanks to William Bowers, P.E., who provided insight and expertise that greatly assisted this research. Reprinted with permission from J.S. Held.

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