Surge, Wave, and Tide Hydrodynamics (SWaTH) Network

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During large coastal storms, the storm surge and waves are the main cause of destruction and landscape change, transporting saline water, sediment, and debris inland. The USGS, in collaboration with stakeholders, has constructed a national Surge, Wave, and Tide Hydrodynamics (SWaTH) Network for the Atlantic, Eastern Pacific, and Central Pacific. SWaTH monitors and documents the height, extent, and timing of storm surge.

USGS technician on a bridge with instruments and laptop

USGS hydrographer Keith Lambert installs a rapid deployment gage at New Mill Creek at Chesapeake, VA, in advance of Hurricane Dorian. (Credit: Blake Dudding, USGS)

Scientific information, if reliably obtained and wisely applied, can strengthen our efforts to build resilient coastal communities before storms strike, and guide our response and recovery strategies. Past storms have shown that storm surge and waves are the primary drivers of coastal-community destruction and dramatic changes in the coastal and near-coastal environment. The energy of the surge and accompanying waves cause physical changes to the landscape. The landward extent of the surge (storm tide) transports saline water, sediment, and debris to constructed and ecologically sensitive environments that are, otherwise, rarely impacted by direct ocean waters. As we develop more effective coastal-management approaches, we have the opportunity to build on our experience and knowledge to prepare for and minimize risks from future storms. Documenting the height, extent, and timing of storm surge and understanding how overland storm tide and waves evolve and dissipate when they move across natural and man-made landscapes, is critical for improved storm-surge modeling. That in turn, will promote coastal resilience, facilitate better planning, and provide more effective early warning of storm-driven flooding.

 

Background

For decades, the USGS has provided critical information on near-shore storm hydrodynamics for decisions regarding emergency response and resource allocation before, during, and immediately after landfall of hurricanes and nor’easters. Much of this information was provided via a few real-time tide gages focused on near-shore waters that supplemented and extended the National Oceanographic and Atmospheric Administration’s (NOAA) National Ocean Service (NOS) tide-gage network. In addition, data from temporary sensors and high-water marks occasionally obtained by the USGS in cooperation with the Federal Emergency Management Agency (FEMA), the U.S. Army Corps of Engineers (USACE), and various state agencies, provided post-storm documentation of coastal floods; however, neither strategy provided sufficient, timely information for emergency operations or to facilitate improvements in storm-tide and wave modeling and prediction.

 

SWaTH

Following Hurricane Sandy, the USGS began construction of an overland Surge, Wave, and Tide Hydrodynamics (SWaTH) Network along the Northeastern Atlantic Coast from North Carolina to Maine. This network, developed collaboratively with local, State, Tribal, and Federal agency partners, features the integration of long-term NOS and USGS real-time tide gages; mobile, rapidly deployable, but temporary, real-time gages (RDGs), and mobile storm-tide sensors (STSs). A central strategy enabling the effective use of SWaTH is that most locations for the mobile RDGs and STSs have been pre-surveyed to NGVD 1988 datums and equipped with receiving brackets that permit rapid installation of instrumentation in the hours and days prior to a storm. Following Hurricane Joaquin, the SWaTH Network has expanded to include additional sites in the Atlantic (South Carolina to Texas, Puerto Rico), Eastern Pacific (Washington to California), and Central Pacific (Hawaiian Islands).

SWaTH consists of flood-hardened, real-time telemetered tide gages, real-time telemetered RDGs, and pre-identified locations for STSs. The STSs will be deployed in three distinct but integrated network configurations consisting of (1) a distributed array of stations representing the range of landscape types and infrastructure subject to surge and wave forces, (2) along transects from the coastline through the inland resource of concern (e.g. a wetland or coastal community), and (3) at existing tide and river monitoring stations where new data can be integrated with long-term records.

The transects will provide data for the analysis of wave height, frequency, and devolution as functions of distance inland. Associated meteorological data, supplemental high water marks, coastal river flow gages, coastal current monitoring, and other hydrologic data will provide ancillary information for interpreting changes in coastal hydrology and vulnerability of coastal ecosystems and communities in response to storm damage.

Installing rapid deployment gage near Myrtle Beach, SC

USGS scientists Tim Pojunas and Jym Chapman install a rapid deployment gage (RDG) at Withers Swash at South Ocean Blvd. near Myrtle Beach, SC. The RDG measures water-surface elevation and various meteorological parameters and transmits the data in real-time.

 

Improvements

Despite the utility of the SWaTH network, crucial gaps in network coverage, reporting frequency, and monitoring capabilities remain.  SWaTH density is not uniform and areas of the South Atlantic and Gulf Coasts are monitored relatively lightly. Additional SWaTH sites are needed to monitor these areas that are routinely impacted by surge events. The USGS is also expanding the network to include portions of the west coast.

The vast bulk of SWaTH monitoring is conducted using low cost, self-logging water-level pressure transducers that do not transmit real-time data. The USGS is exploring new, low-power, low-cost data transmission technologies to augment and eventually replace the self-logging instruments.

To date, SWaTH has focused primarily on monitoring water-levels and waves, yet there is a need to better document and model both surge currents (the movement of vast quantities of water by wind driven surface drag and tides) and the impacts of surge-induced saltwater intrusion. The USGS is actively seeking to identify and deploy mobile water-quality monitoring devices, particularly conductivity sensors, to document the distribution and persistence of saltwater and its toxicity to coastal stream, estuarian, and wetland biota.

 

The Flood Event Viewer

Delivering these data rapidly and in an easily accessible manner is critical to emergency agencies and local emergency responders. SWaTH data deliverables will include times series information for water elevation, wave height and frequency, and selected meteorological data. These data are stored in a nation-wide database and made available via the USGS Flood Event Viewer, a mapping application and direct web services. These services are structured to allow other agencies to directly use the data as it is collected and approved during, or shortly following, a flood event.

A screenshot of the USGS Hurricane Dorian Flood Event Viewer

Screenshot from the USGS Flood Event Viewer, taken Sept. 5, 2019, showing the data collecting during Hurricane Dorian.