South Carolina HurricaneWatch
South Carolina Hurricane Information
Preventing flood hazards, such as the hurricane induced storm surge, from becoming human disasters requires an understanding of the relative risks floods pose to specific communities and knowledge of the processes by which flood waters rise, converge, and abate. Historically, hurricane-induced storm tides have been documented through measurement of high-water marks left on structures or vegetation. However, these sources are not always reliable or accurate and do not provide quantitative information about the timing of the flooding, the sequencing of multiple paths by which the storm-surge waters arrived, or the magnitude of waves comprising floodwaters.
In response to these deficiencies, the U.S. Geological Survey – South Carolina Water Science Center (USGS-SCWSC), in cooperation with various municipal, State, and Federal cooperators, uses four monitoring approaches for measuring the timing, magnitude, and duration of hurricane storm-surge dynamics. Two approaches utilize a fixed network of gage locations and two approaches utilize the temporary deployment of water-level sensors.
Real-Time Monitoring Network
Real-time gage for Atlantic Intracoastal Waterway at Highway 9 near Nixon Crossroad, SC (station 02110777).
The USGS maintains a network of 42 real-time water-level and water-quality gages along the South Carolina coast. The gages record water level, specific conductance, temperature, and (or) dissolved oxygen at 15-minute intervals and are available on the web (sc.water.usgs.gov) with a maximum of a 1-hour time delay. The real-time network is maintained to monitor a large range of hydrologic conditions, from droughts to floods, and provide the data on the web for a broad base of stakeholders. In the event of a hurricane, the real-time network allows individuals to monitor the storm-surge data in near real time. Depending on the elevation of the gage house structure, the real-time gage may be at risk of being inundated by flood waters during extreme storm surge.
Real-Time Rapid Deployment Gages
Temporary real-time storm-surge gage attached to a structure near the U.S. 17 bridge on the Waccamaw River near Georgetown, SC.
During a hurricane there also may be a need to collect additional real-time storm-surge data to assist emergency management agencies to monitor storms from remote locations and make critical public safety decisions. The fixed real-time coastal network may not provide complete coverage of the coast and there may be locations where a temporarily deployed real-time gage would fill data gaps. The general approach for the real-time rapid deployment gage is a pre-fabricated “gage-in-a-box” that can be quickly attached to a bridge rail with little or no modification to the bridge structure. The design uses a radar water-level sensor housed in a small aluminum shelter along with telemetry equipment, solar panel, and antenna. It is feasible that a crew in South Carolina could deploy 3 or 4 temporary real-time gages in a day prior to a storm.
Hurricane Storm Surge Network
In addition to the real-time network, the USGS-SCWSC, in cooperation with the U.S. Army Corps of Engineers – Charleston District and the South Carolina Department of Transportation, established a hurricane storm-surge monitoring network for South Carolina. The network is designed for a cost effective and time-efficient monitoring of hurricane storm surge and is only activated in the event of an approaching hurricane. Currently (2011), the hurricane storm-surge network consists of 22 sites distributed along the South Carolina coast with an additional 17 sites to be installed in 2011. At each site, a bracket for a water-level sensor has been attached to a permanent structure, such as a bridge abutment or pier. In addition, some sites include a bracket for a barometric pressure sensor. The elevation of the water-level sensor bracket has been determined by differential leveling to a benchmark of known elevation. In the event of a storm, the sensors are attached to the bracket and the timing, duration, and magnitude of the storm surge is recorded. Upon retrieval of the sensors, the water-level data are easily adjusted to mean sea level because the bracket elevations were previously determined. The storm-surge data is then able to be quickly disseminated to interested agencies and coastal resource managers.
Temporary Deployment of Hurricane Storm Surge Sensors
Deployment of a temporary hurricane storm-surge sensor. Deployment includes a metal pipe containing storm surge sensor strapped to a power pole.
To augment the fixed location hurricane storm-surge and the real-time gaging networks, a temporary network of hurricane storm surge sensors can be deployed prior to a hurricane. As part of this monitoring approach, water-level and barometric-pressure sensors are deployed to areas of projected hurricane landfall. The addition of these sensors creates a concentrated network of storm-surge monitors which provide more spatially dense monitoring of hurricane storm surge. For example, temporary deployment of sensors in the projected hurricane path can be placed to monitor the escalation and attenuation of the hurricane surge along coastal rivers and across barrier islands, low-lying areas, and wetlands. After the passing of a storm, unlike the established hurricane storm-surge network, the locations of the temporarily deployed sensors must be surveyed to a known elevation. The USGS has successfully deployed temporary storm surge-networks for Hurricanes Rita (2005), Wilma (2005), Gustav (2008), Ike (2008), and Tropical Storm Ernesto (2006).
The 64 monitoring sites for the two fixed networks, with the addition of temporarily deployed sites, provide a cost-effective hurricane storm-surge monitoring network for South Carolina. The combined networks effectively leverage resources of over 10 cooperative funding agencies in South Carolina for monitoring hurricane storm surge. The data will be a valuable resource for emergency preparedness agencies to better protect public safety, for engineers to improve structural design of coastal infrastructures, and for oceanographers to evaluate and improve hurricane storm-surge computer models.