The USGS Water Science School
Monitoring Storm Tide and Flooding from Hurricane Sandy Along the Atlantic Coast of the United States, October 2012
USGS Open-File Report 2013–1043
The U.S. Geological Survey (USGS) deployed a temporary monitoring network of water-level and barometric pressure sensors at 224 locations along the Atlantic coast from Virginia to Maine to continuously record the timing, areal extent, and magnitude of hurricane storm tide and coastal flooding generated by Hurricane Sandy.
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, 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.
The South Atlantic Water Science Center (SAWSC) maintains a network of fixed location, real-time water-level and water-quality gages along the Georgia, North Carolina and South Carolina coast. The gages record water level, specific conductance, temperature, and (or) dissolved oxygen at 15-minute intervals and are available at the National Water Information System web site 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.
During a hurricane, there also may be a need to collect additional real-time storm-tide 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 could deploy 3 or 4 temporary real-time gages in a day prior to a storm.
To augment the fixed location real-time gaging networks and temporary rapid deployment gages, 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-tide monitors which provide more spatially dense monitoring of hurricane storm tide. For example, temporary deployment of sensors in the projected hurricane path can be placed to monitor the escalation and attenuation of the hurricane tide along coastal rivers and across barrier islands, low-lying areas, and wetlands.
There are two types of temporary storm surge sensors, the first type has a permanent bracket where the elevation has been previously surveyed and recorded. When a storm approaches the sensors can be quickly installed in the brackets and retrieved immediately after the storm. The second type can be deployed anywhere but require elevation surveys after the storm..
1. What does a storm-tide sensor look like? - It is a 1-1/2' aluminum or steel pipe strapped or bolted to a piling or other stable structure. The top will have a metal or PVC cap and the bottom will be open for the water to enter. The sensor housing protects a water-level pressure sensor inside. A unique USGS ID sticker will be on the outside. The sticker may be yellow or aluminum in color. If you find a sensor and have questions about it, please call the phone number on the sticker.
2. What type of data do the sensors collect? - Water-level and barometric pressure are recorded every 30 seconds for most sites. Sensors located on beaches record wave height every 2 seconds. The recording period lasts for 1 to 3 days depending on the magnitude of the storm and post-storm access to the sensor sites.
3. What is a storm-tide sensor deployment? - The USGS has developed a mobile network of rapidly deployable instruments with which to observe and document hurricane-induced storm-surge as they make landfall and interact with coastal features. From Maine to North Carolina, the USGS Surge, Wave, and Tide Hydrodynamics Networks, also called SWaTH, is a system of hundreds of pre-positioned, pre-surveyed brackets where storm-tide sensors can quickly be installed. With the SWaTH network, the USGS is able to improve its response time when monitoring coastal-storm tide and riverine flooding related to hurricanes and nor'easters. More information about the SWATH network can be found in this recently published report.
4. Why are you undertaking this work? - The work will enable USGS to compile data so that we can quantify storm-tide dynamics (wave heights, forces, speeds, and extent) for various storm conditions, topographies, ecologies, built environments, and land uses. This information will lead to better storm-tide models and more accurate flood forecasts, while informing decisions on designs of flood-protection infrastructure and future land use policies.
5. What is the nature of the work? - Storm-tide sensors (non-vented pressure transducers) are strapped to bridge piers, power and light poles, and other structures along the coast. Depending on the size of a storm and the potentially affected area, the effort can involve dozens of two-person teams deploying hundreds instruments 24 to 48 hours prior to a hurricane's landfall.
6. What are you going to do with the data? - Data are uploaded to the web as as a series of water level and water pressure measurements taken over time stage and pressure time series. We generate various graphics to create 3-D water-surface images, and depth and duration maps. Together they enable us to study surge flooding, including wave height, and moment by moment, visualize its interaction with the coastal features such as beaches, islands, estuaries, and streams. By tying these data together and with local topography, we can determine the rates at which flood waters transverse various water bodies and landforms, the major paths of penetration, their duration, and the height and frequency of waves that strike dunes and built infrastructure.
Data of this nature is quite rare and very valuable for determination of flood insurance maps, building codes, and for calibration of the hurricane inundation models. Accurate model forecasts are critical for community preparation of storm response and evacuation plans.
7. Are the surge data reported in real-time? - The surge data are not reported in real time but are logged on-site. They are processed and calibrated for barometric pressure, water density, and elevation data, and are then made available to the public. Real-time information is available from rapid deployed gauges that are installed at sites where we do not currently have permanent gauges. These rapid deployed gauges will augment a network of existing U.S. Geological Survey gauging stations already in place before the storm arrives.
8. What other kinds of data are needed? - There are several kinds of data that would complement this work and for which we seek collaborators. These include offshore water-level and wave-height data, wind speed and direction, inland water salinity, post-storm ecological assessments, and geological evaluations of beach and landform behavior, and engineering evaluations.
9. Who uses this information? - Our data is used by the Federal Emergency Management Agency, National Ocean and Atmospheric Administration, National Weather Service, and National Hurricane Center and the U.S. Army Engineer Research and Development Center, as well as state responders and emergency management officials.
10. Where can I learn more? - Recent efforts by the USGS to deploy these storm-tide sensors in response to Hurricane Maria can be read here. Reports on previous USGS storm surge documentation efforts as well as additional information about storm-tide sensors is available here.