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 material which includes oil products, mining byproducts, and other waste material. When you think of dams, you may automatically think of the Hoover Dam or other large concrete structures which control the flow of rivers. 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.
The importance of inspecting and maintaining one of our nation’s most critical infrastructures began in 1972 when congress authorized the U.S. Army Corp 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. Following 9-11, The National Dam Safety and Security Act was passed. The funding for inspection and maintenance of the dams throughout the U.S. was re-authorized in the National Dam Safety Program in 2006 and more recently in the Water Resources Development Act of 2018.
Dams which meet the minimum criteria within the U.S. 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. It should be noted that Alabama has no dam safety legislation or dam safety program. The purpose of the program was to prioritize each dam throughout the U.S. for maintenance and inspection. This classification system categorizes dams based on the potential loss of human life, economic loss, lifeline disruption, and environmental impact. The classification system includes low, significant, and high-hazard potential. Below are the minimum criteria for dams to be a part of the NID. Low-hazard potential is where in a failure event there would be no probable loss of human life, low economic, and/or environmental losses. Conversely, high-hazard potential would have probable loss of human life, high economic, and/or environmental losses.
Minimum criteria for dams to be a part of the NID:
The dams within the U.S. have an average age of 57 years. According to the American Society of Civil Engineers’ (ASCE) 2017 Infrastructure Report Card, U.S. Dams were graded as a “D.” Due to population growth, slightly over 17% of the 91,468 dams within the U.S. are considered to be high-hazard potential dams and that number is increasing every year. Coupled with 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, there are 11,354 dams which are classified as a significant hazard. Below is an image taken from the National Inventory of Dams (NID) website (nid.usace.army.mil/), which shows the concentration of dams throughout the United States.
The red icons indicated dams which 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 which could cause loss of life and property. An EAP also serves as a document which coordinates owners and emergency managers, identifying areas where there is 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).
The majority of dams throughout the U.S. are earthen dams. Earthen dams have become an economical construction design 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 are typically comprised of a homogenous soil embankment. A zoned embankment is similar but with an impermeable core and trench. Clay is often used as the core due to its impermeability, however concrete, sheet piles, or timber is 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.
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 seepage through the earthen embankment, which is known as “piping.” Both failures can be caused by a flood event. Above (adapted from NPDP, 2008b) is a diagram displaying the Cause of Dam Failures from 1975 to 2001.
In order to effectively assess the condition of a dam or levee after a catastrophic rainfall event a rapid visual assessment of a dam embankment would be imperative to identify potential hazards. Potential 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.
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 which could be associated with seepage or piping.
Above is a GPR transect with a highlighted anomaly which would be indicative of subsidence or another geologic hazard.
Geophysical surveys should be supplemented with soil borings and/or soil probes in order 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.
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. Data reviewed 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 to develop plans to mitigate the potential for future dam failures.
In summary, there is an increasing risk of dam failures which result in loss of life and property throughout the United States. As metropolitan areas continue to grow there is 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/costly consequences. It is imperative that an Emergency Action Plan is developed for each dam classified as a significant and high-hazard dam, and most importantly the public be educated on any nearby hazard. An onsite visual inspection by trained, unbiased professionals remains the most effective method to reduce the risk of dam failures.
We thank our colleagues W. Sharkey Bowers, PE and Jonathan Clarkson, PG who provided insight and expertise that greatly assisted this research.
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