High alert from High Sierra

21 February 2011

A new flood warning network has been installed at Fairfax County dams in the US

In the past, residents of Fairfax County, Virginia had very little warning about sudden water level changes of the high hazard PL-566 dams located throughout the county. In June 2007, in conjunction with the Northern Virginia Regional Hazard Mitigation Plan, the county developed a floodplain management plan. The overall goal was to protect life and property by achieving and maintaining a high level of preparedness for extreme rainfall events.

Officials from the Stormwater Planning Division (SWPD) in the Department of Public Works sought to establish an automated flood warning system for the New Alexandria, Belle View, and Huntington areas and seven high-hazard dam sites. In March 2008, High Sierra Electronics (HSE) was awarded a contract to supply, install and maintain an ALERT flood warning network.

ALERT is an acronym for Automated Local Evaluation in Real Time, which is a method of using remote sensors in the field to transmit environmental data to a central computer in real time. This standard was developed by the National Weather Service and has been used by the National Weather Service, US Geological Survey, US Army Corps of Engineers, US Bureau of Land Management and numerous state and local agencies. ALERT systems have become a standard in real time environmental data collection because of their accuracy, reliability and low cost.

Initially, a total of nine HSE packaged rain gauge stations equipped with water level sensors and one repeater station (to extend the signal range) were installed in July 2008. The system’s remote gauges are battery powered with a solar panel recharging system. Data is transmitted by VHF radio using HSE’s Model 3206 ALERT data transmitter and interpreted as measurements at the county’s base station.

The base station, which consists of radio receiving equipment and a computer running DEC Data Systems DataWise software, collects the coded signals and processes them into meaningful hydro-meteorological information. When alarm conditions are met, DataWise sends notification to appropriate personnel via text message and e-mail.

In addition to regular timed reports, the rain gauges report each millimeter of rainfall to the base station. The water level sensors report any change in level based on a user selectable setting as small as one tenth of a foot. The county has set alarm criteria in DataWise for rainfall levels, water levels, and the rate of water level rise at the sites.

When alarm conditions are met, the base station software sends notification to emergency-response personnel. The system also retrieves and archives data from nearby tidal gauges, US Geological Survey stream gauges, and the Lake Barcroft discharge gauge, rain and level sensors. The county uses the DataWise forecasting component, which uses local precipitation estimates to generate predicted hydrographs at desired locations.

Shortly after HSE completed installation, tropical storm Hanna descended on the area in September 2008. The system was used to monitor rainfall and flood levels during the storm and provided useful, accurate results. Real time data was recorded and alarms initiated, as most of the sites reported rainfall over eight inches.

“Hannah came through and dropped more than a 100-year rainfall in some areas”, said Don Lacquement, engineer with SWPD. “This storm was the first opportunity we had to see the High Sierra Electronics’ equipment function during a significant event. Their system, together with the data collection software, was a tremendous asset in responding to this large storm”.

The second phase of the project, initiated in 2010, involved HSE installing instrumentation at the 11 remaining state-regulated dams operated by the county. Installation was recently completed, which included a combination of rain gauges and water level monitoring stations. Future plans include HSE adding automatic monitoring capability at 14 of the county’s piezometers. Water level stations will be installed in 2011, so real-time data can be transmitted to the base station and incorporated into the floodplain management programme. This data will assist in the development of long-term statistics for water resource planning.

Sue Swenor, Marketing Manager, High Sierra Electronics, 155 Spring Hill Drive, Suite 106, Grass Valley, CA 95945, US. www.highsierraelectronics.com

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