A new dam failure warning system has been designed to provide early alerts at Tuttle Creek dam in Kansas, writes James B Hummert
Long before Tuttle Creek lake and dam were built, the Manhattan, Kansas area suffered the disastrous effects of flooding due to excessive precipitation and overflow of the Big Blue and Kansas rivers in the summer of 1951. Although the probability of an earthquake-induced failure is low, studies by the Kansas City District of the US Army Corps of Engineers (USACE) indicate that the dam could fail in the event of a strong earthquake. It is estimated that such a failure could result in flood waters as deep as 5m in downtown Manhattan, affecting 13,000 or more people with a potential loss of 400 lives.
At the request of the downstream community and as a preemptive measure in addressing the seismic vulnerability of the dam, USACE retained URS to design, build, and maintain a Dam Failure Warning System (DFWS) to provide additional safety to occupants of the evacuation zone, including residents, workers, students, and visitors. The DFWS is a temporary measure that will remain in effect while USACE proceeds with a multi-year construction project to strengthen the foundation of the dam so it can withstand the maximum credible earthquake (MCE) that can be expected in the area, approximately 6.6 magnitude.
The Tuttle Creek Dam Failure Warning System is the first application of a comprehensive integration of new and old techniques to monitor structural integrity of a dam and automatically link these devices with a downstream warning system. The wide area networks also connect all stakeholders, including the emergency first responders, with USACE by providing custom built dam safety status indicators so that all decision makers are kept up-to-date with any safety related developments. The DFWS is made up of the following components and subsystems:
•Automated geotechnical instruments installed at the dam to measure seismic shaking, detect embankment/foundation deformation, and monitor changes in foundation pore pressures.
•Remote controlled video cameras to provide visual inspection of the dam following an earthquake.
•An automated data acquisition system (ADAS) to retrieve geotechnical instrument data and transmit alarm annunciation.
•Dam Safety Status Indicators (DSSIs), which are sophisticated alert notification units custom-built for first responders at USACE and county Emergency Operation Centers.
•A web (intranet) portal which provides remote access to instrument data, dam safety status, recent earthquake information, video camera images, and lake level data.
•A seismically hardened Critical Systems Building (CSB) which integrates the ADAS with the DSSIs and computers at remote locations via a private wide-area frame-relay network and backup satellite network. The CSB is designed to withstand damage from an earthquake in Manhattan, Kansas, and has an emergency backup power source in the form of a propane-fired generator.
•A six-siren warning system with six 4500W solid-state, tone- and voice-capable sirens located in the evacuation zone which provide a secondary community benefit of tornado siren coverage in areas that previously had no coverage.
•Indoor tone alert units at facilities that require special evacuation attention, such as schools, day care centers, and facilities for the elderly and handicapped.
•An evacuation plan for the nearby downstream communities and a community outreach program to educate the public.
Sounding the alert
All components are designed to function under earthquake conditions and are integrated to transmit real-time data from the dam to multiple remote locations. If an earthquake threatens the stability of the dam, the sirens will broadcast an evacuation tone and voice message instructing downstream residents, schools, and businesses to evacuate.
Trigger-levels of seismic shaking and dam deterioration have been defined to alert first responders and to activate the siren warning system. If the Strong Motion Accelerograph (SMA) units detect ground shaking corresponding to a significant earthquake (greater than a 4.5 magnitude), an autodialer will call key personnel and play a pre-recorded message detailing the conditions detected at the dam.
In addition, the DSSI units will provide remote locations with the information on the status of the dam using coloured indicator lights to represent different safety conditions. If the SMA units detect ground shaking corresponding to severe earthquake (greater than 5.7 magnitude) and damage to the dam is detected, the DSSI units will also display a countdown timer with a delay of between 30 minutes and two hours before automatic activation of the warning sirens. The delay provides time for USACE to assess the dam and stop automatic activations, if necessary, or initiate manual activation of the siren warning system.
Other innovative technologies used in the warning system included the use of ISO-Base seismic isolation platforms to protect the computer equipment racks in the CSB from seismic shaking; the use of Instant Messaging as a backup to phone and radio communication; the installation of linear sets of solar-powered ‘runway’ lights (named Embankment Alignment Indicators (EAIs) by URS) along the crest and toe of the dam to facilitate the visual detection of deformation under low-light conditions; the use of a Time Domain Reflectometry (TDR) cable together with automated inclinometers to measure potential post-earthquake displacement in the downstream toe area; and a series of nested loop cables extending along the dam crest to detect potential post-earthquake subsidence or collapse in the crest region of the dam.
Sustainable design techniques include the use of solar energy panels at the siren tower locations, communication towers, and remote measurement unit locations. Solar energy is also used to power the embankment alignment indicator lights. In addition, a propane-fired generator provides emergency power generation. The propane-fired generator is more reliable and quieter than a traditional diesel-powered generator. It is an approved alternate clean fuel and one of the cleanest burning of all alternative fuels.
The design, construction, and integration of a variety of complex components and subsystems presented significant challenges to the design team. The DSSI units were designed and built from scratch by URS electrical engineers and technicians. These units as well as the ADAS, video surveillance system, siren warning system, computers, and wide area networks were all tested independently prior to integration. Integration and scenario simulation testing were subsequently performed to ensure that the components and subsystems worked together to meet the functional design requirements.
Radio frequency communication and grounding and lightning protection also required special technical analysis. The local topography necessitated the installation of a repeater tower for successful radio frequency communication between the Critical Systems Building at the dam and the Riley County Emergency Operation Center.
The Tuttle Creek Dam Failure Warning System constitutes an important advancement in the practice of dam safety monitoring and early warning systems. This technology can also be used to provide monitoring and early warning for levee systems which, in the aftermath of Katrina, might prove to be a valuable instrument for the prevention of future disasters.
James B. Hummert, P.E., Vice President, URS Corporation, St. Louis, Missouri, US