Part 2. RISK ASSESSMENT 7. DAM/CANAL FAILURE 7.1 GENERAL BACKGROUND 7.1.1 Causes of Dam Failure Dam failures in the United States typically occur in one of four ways: • Overtopping of the primary dam structure, which accounts for 34 percent of all dam failures, can occur due to inadequate spillway design, settlement of the dam crest, blockage of spillways, and other factors. • Foundation defects due to differential settlement, slides, slope instability, uplift pressures, and foundation seepage can also cause dam failure. These account for 30 percent of all dam failures. • Failure due to piping and seepage accounts for 20 percent of all failures. These are caused by internal erosion due to piping and seepage, erosion along hydraulic structures such as spillways, erosion due to animal burrows, and cracks in the dam structure. • Failure due to problems with conduits and valves, typically caused by the piping of embankment material into conduits through joints or cracks, constitutes 10 percent of all failures. The remaining 6 percent of dam failures are due to miscellaneous causes. Many are secondary results of other disasters, such as earthquakes, landslides, storms, snowmelt, equipment malfunction, structural damage, and sabotage. The most likely disaster-related causes of dam failure in Ada County are earthquakes, excessive rainfall and landslides. Poor construction, lack of maintenance and repair, and deficient operational procedures are preventable or correctable through regular inspections. Terrorism and vandalism are concerns that all operators of public facilities plan for; these threats are under continuous review by public safety agencies. 7.1.2 Irrigation Canals Much of the arid land of Southwest Idaho was developed through reclamation projects of the early 1900s. These projects included dams to collect water and provide flood control and canals to deliver water to agricultural areas. Many canals crisscross the state, but they are not generally perceived as flood hazards. New development has encroached on the canals and the areas around them. Numerous housing developments in Ada County lie below large-capacity canals. This proximity creates risk to life, safety and property. Because of widespread ownership issues (private canals, irrigation districts, etc.) data for canal failure events is not readily obtainable. The Silver Jackets technical advisory group has expressed strong interest in monitoring this issue and the Idaho Office of Emergency Management anticipates further discussions regarding this hazard. With a water delivery system that includes over 400 miles of canals (see Figure 7-1), Ada County and the Boise area have the highest urban canal density in the United States. These canals are generally well-maintained by their owners/operators because it is their livelihood. However, these facilities can convey flows as high as 2,800 cubic feet per second (cfs), and they have not been evaluated according to engineering standards. The assessment of risk associated with canals is limited in this plan. Canal owners/operators were invited to participate in this plan update process, but chose not to at this time. Future updates should continue to seek participation from these entities to better understand the risk posed by these facilities. 7-1 Gem County Ada County Figure 7-1. Canal System Legend BoiseRiver STAR EAGLE Canals GARDEN Water Bodies CITY Boise Count MERIDIAN ¨¦§184 BOISE y Canyon County ¨¦§84 KUNA Elmore County Owyhee County Base Map Data Sources: Ada County, U.S. Geological Survey . 0361.5 Miles Ada County does not warrant the accuracy, reliability or timeliness of any information on this map, and shall not be held liable for losses caused by using this information. 2017 Ada County Multi-Hazard Mitigation Plan: Volume 1—Countywide Elements Dam/Canal Failure 7.1.3 Regulatory Oversight The potential for catastrophic flooding due to dam failures led to passage of the National Dam Safety Act (Public Law 92-367), which requires a periodic engineering analysis of every major dam in the country. The goal of this FEMA-monitored effort is to identify and mitigate the risk of dam failure so as to protect the lives and property of the public. Idaho Department of Water Resources Dam Safety Program The Dam Safety Program of Idaho’s Department of Water Resources monitors dams at the state level. The Department currently regulates nearly 600 water storage dams and more than 20 mine tailings impoundment structures throughout the state. The program regulates dams greater than or equal to 10 feet in height or reservoirs greater than or equal to 50 acre-feet in storage capacity. Each dam inspected by IDWR has a classification for size and risk: • Large—40 feet high or more or with a storage capacity of more than 4,000 acre feet of water. 104 dams are currently listed as large. • Intermediate—More than 20 but less than 40 feet high or with a storage capacity of 100 to 4,000 acre feet of water. 198 dams are currently listed as intermediate. • Small—20 feet high or less and a storage capacity of less than 100 acre feet of water. 244 dams are currently listed as small. All statutory sized dams must be inspected by the IDWR no less than every five years. The frequency between individual dam inspections depends on such items as the project’s physical condition, method of construction, maintenance record, age, hazard rating, and size and storage capacity. Inspection reports prepared by the IDWR for non-federal dams are available through the state office in Boise (Idaho Dam Safety Web Site, 2011). U.S. Army Corps of Engineers Dam Safety Program The U.S. Army Corps of Engineers is responsible for safety inspections of some federal and non-federal dams in the United States that meet size and storage limitations specified in the National Dam Safety Act. The Corps has inventoried dams; surveyed each state and federal agency’s capabilities, practices and regulations regarding design, construction, operation and maintenance of the dams; and developed guidelines for inspection and evaluation of dam safety (U.S. Army Corps of Engineers, 1997). Federal Energy Regulatory Commission Dam Safety Program The Federal Energy Regulatory Commission (FERC) has the largest dam safety program in the United States. The FERC cooperates with a large number of federal and state agencies to ensure and promote dam safety and, more recently, homeland security. There are 3,036 dams that are part of regulated hydroelectric projects are in the FERC program. Two-thirds of these are more than 50 years old. As dams age, concern about their safety and integrity grows, so oversight and regular inspection are important. FERC staff inspects hydroelectric projects on an unscheduled basis to investigate the following: • Potential dam safety problems • Complaints about constructing and operating a project • Safety concerns related to natural disasters • Issues concerning compliance with the terms and conditions of a license. Every five years, an independent consulting engineer, approved by the FERC, must inspect and evaluate projects with dams higher than 32.8 feet, or with a total storage capacity of more than 2,000 acre-feet. 7-3 2017 Ada County Multi-Hazard Mitigation Plan: Volume 1—Countywide Elements Dam/Canal Failure FERC staff monitors and evaluates seismic research in geographic areas where there are concerns about seismic activity. This information is applied in investigating and performing structural analyses of hydroelectric projects in these areas. FERC staff also evaluates the effects of potential and actual large floods on the safety of dams. During and following floods, FERC staff visits dams and licensed projects, determines the extent of damage, if any, and directs any necessary studies or remedial measures the licensee must undertake. The FERC publication Engineering Guidelines for the Evaluation of Hydropower Projects guides the FERC engineering staff and licensees in evaluating dam safety. The publication is frequently revised to reflect current information and methodologies. The FERC requires licensees to prepare emergency action plans (EAPs) and conducts training sessions on how to develop and test these plans. The plans outline an early warning system if there is an actual or potential sudden release of water from a dam due to failure. The plans include operational procedures that may be used, such as reducing reservoir levels and reducing downstream flows, as well as procedures for notifying affected residents and agencies responsible for emergency management. These plans are frequently updated and tested to ensure that everyone knows what to do in emergency situations. U.S. Bureau of Reclamation The U.S. Bureau of Reclamation’s Dam Safety Program was officially implemented in 1978 with passage of the Reclamation Safety of Dams Act, Public Law 95-578. This act was amended in 1984 under Public Law 98-404, in 2000 under Public Law 106-377, in 2002 under Public Law 107-117, and in 2004 under Public Law 108-439 (Reclamation Safety of Dams Act, as amended). Program Development and administration of safety of dams activities is the responsibility of Reclamation’s Dam Safety Office located in Denver, Colorado. Dams must be operated and maintained in a safe manner, ensured through inspections for safety deficiencies, analyses utilizing current technologies and designs, and corrective actions if needed based on current engineering practices. In addition, future evaluations should include assessments of benefits foregone with the loss of a dam. For example, a failed dam can no longer provide needed fish and wildlife benefits. The primary emphasis of the Safety Evaluation of Existing Dams (SEED) program is to perform site evaluations and to identify potential safety deficiencies on Reclamation and other Interior bureaus’ dams. The basic objective is to quickly identify dams which pose an increased threat to the public, and to quickly complete the related analyses in order to expedite corrective action decisions and safeguard the public and associated resources.