From the (guest) editors’ desk: Observations and lessons learned from recent storm events
Jon Miller, Bret Webb, and Irene Watts
Coastal windstorms create unsteady, unpredictable storm surges in a fluvial Maine estuary
Preston Spicer, Pascal Matte, Kimberly Huguenard, and Laura N. Rickard
Storm surges create coastal flooding that can be damaging to life and property. In estuaries with significant river influence (fluvial), it is possible for tides, storm surge, and river discharge to interact and enhance surges relative to the immediate coast. These tide-surge-river interactions were previously identified in a fluvial Maine estuary as higher frequency (>four cycles per day) oscillations to storm surge which were proposed to be incited by enhanced friction and resonance during certain windstorm events (Spicer et al. 2019). The relative contributions to tide-surge-river interaction from atmospheric forcing variables (wind, barometric pressure, and externally generated surge) remains unclear. This work seeks to decompose and analyze a recent windstorm surge event to better isolate the effects of atmospheric forcing on tidesurge- river interaction. Results show total storm surges in the fluvial estuary to be two times larger than at the estuary mouth because of tide-surge-river interaction. Analysis indicated at least 50% of the magnitude of tide-surge-river interactions are created by non-tidal forcing, in the form of wind, enhancing frictional energy in the estuary. The remaining tide-surge-river interaction is likely a result of changes in tidal wave propagation speed due to surge deepening the mean estuary water level.
A comparison of the temporal evolution of hydrodynamics and inlet morphology during Tropical Storm Fay (2020)
K.A. McPherran, S.M. Dohner, and A.C. Trembanis
The record-setting North Atlantic hurricane season of 2020 had 30 named storms and reinforced the need for high-resolution, small-scale data collected in the nearshore zone during storm events to characterize storm impacts on coastal settings. To address these needs, hydrodynamic and morphologic data were collected during the 2020 Atlantic hurricane season, capturing fair weather conditions and the passage of Tropical Storm Fay (July 2020) near Chincoteague Inlet, Virginia. A sector-scanning rotary sonar captured high-resolution imagery of bedform evolution and data were analyzed to relate the migration of bedforms to the concomitant hydrodynamic conditions during the storm event. During the peak of the storm on 10 July 2020, significant wave height and period in Chincoteague Inlet were 0.96 m at 9.6 s arriving from the SSW (201°). The ripple field evolved during the storm in a manner consistent with that found in Hay and Mudge (2005): irregular ripples (O 20 cm wavelength) dominated during fair weather conditions, which developed into a washed-out, flat bed state as the storm arrived. During the peak of the storm, lunate megaripples (O 1 m wavelength) formed and migrated shoreward. A substantial outflow of freshwater from Chincoteague Bay occurred for up to seven days post-storm, and sediment transported by this outflow could serve as a yet-unidentified sediment source for the rapid growth of southern Assateague Island. This outflow of freshwater dampened waves and hindered ripple field recovery for up to seven days post-storm. These extremeevent datasets are critical to inform coastal flood models and management decisions, as this work recognizes an increased risk of flooding for the town of Chincoteague from even the offshore passage of tropical storms.
Remote bed-level change and overwash observation with low-cost ultrasonic distance sensors
Ian R.B. Reeves, Evan B. Goldstein, Katherine A. Anarde, and Laura J. Moore
Few datasets exist of high-frequency, in situ measurements of storm overwash, an essential mechanism for the subaerial maintenance of barrier islands and spits. Here we describe a new sensor platform for measuring bed-level change and estimating overwash inundation depths. Our MeOw (Measuring Overwash) stations consist of two ultrasonic distance sensors, a microprocessor board, and a camera and are capable of withstanding the impacts of large storm events, can be left unattended to collect data for months to years, and are relatively inexpensive. With the exception of the camera, the MeOw stations are built with all open-source hardware and software. Herein we provide complete instructions for manufacturing the MeOw stations and present observations from a single MeOw station for a three-month (2019) deployment on a frequently overwashed section of Smith Island, VA. The MeOw stations captured three large storm events over the course of the deployment (Hurricane Dorian, Tropical Storm Melissa, and a November nor’easter), as well as several high-tide events. Based on our interpretation of the raw data, bed-level changes occurred throughout the deployment from both storm and non-storm overwash, but were particularly large during Tropical Storm Melissa where initial accretion of approximately 0.15 m was followed by 0.77 m of erosion over three days. The maximum overwash inundation depth occurred during the nor’easter and measured approximately 0.83 m. The variability in bed level over the course of our experiment highlights the importance of in situ high frequency bed-level measurements for constraining overwash inundation depths. MeOw stations are ideally suited for measuring storm overwash — or any process that necessitates tracking bed and water level elevations at high frequency during harsh conditions.
A survey of storm-induced seaward-transport features observed during the 2019 and 2020 hurricane seasons
Jin-Si R. Over, Jenna A. Brown, Christopher R. Sherwood, Christie Hegermiller, Phillipe A. Wernette, Andrew C. Ritchie, and Jonathan A. Warrick
Hurricanes are known to play a critical role in reshaping coastlines, but often only impacts on the open ocean coast are considered, ignoring seaward-directed forces and responses. The identification of subaerial evidence for storm-induced seaward transport is a critical step towards understanding its impact on coastal resiliency. The visual features, found in the National Oceanic and Atmospheric Administration, National Geodetic Survey Emergency Response Imagery (ERI) collected after recent hurricanes on the U.S. East Atlantic and Gulf of Mexico coasts, include scours and channelized erosion, but also deposition on the shoreface or in the nearshore as deltas and fans of various sizes. We catalog all available ERI and describe recently formed features found on the North Core Banks, North Carolina, after Hurricane Dorian (2019); the Carolina coasts after Hurricane Isaias (2020); the Isles Dernieres, Louisiana, after Hurricane Zeta (2020); and the southwest coast of Louisiana, after Hurricanes Laura and Delta (2020). Hundreds of features were identified over nearly 200 km of coastline with the density of features exceeding 20 per km in some areas. Individual features range in size from 5 m to 500 m in the alongshore, with similar dimensions in the cross-shore direction, including the formation or reactivation of outlets. The extensive occurrence of these storm-induced return-flow and seawardflow morphologic features demonstrates that their role in coastal evolution and resilience may be more prominent than previously thought. Based on these observations we propose clarifying terms for return- and seaward-flow features to distinguish them from more frequently documented landward-flow features and advocate for their inclusion in coastal change hazards classification schemes and coastal evolution morphodynamic models.
Coastal Observations: CDIP observations of recent extreme wave conditions on U.S. coasts
James Behrens, Ross Timmerman, Eric Terrill, Sophia Merrifield, and Robert E. Jensen
The Coastal Data Information Program (CDIP) maintains wave gauge stations for continuous coverage, with precision instruments and dedicated telemetry and dissemination infrastructure. Decades of this persistent, quality-controlled wave monitoring effort has provided the data required to generate metrics for wave climate at coastal locations across the United States and identify and characterize extreme wave events. During the extremely active 2020 North Atlantic hurricane season, the CDIP East Coast array recorded significantly elevated wave conditions generated by no fewer than 15 named storms. In California, meanwhile, long-term monitoring stations have measured new all-time maximum wave heights during recent storm events. Complete quality-controlled directional spectra and displacement data sets, as well as sea surface temperature and surface current data from the wave buoys, are publicly available at http://cdip.ucsd.edu.
Patterns and processes of beach and foredune geomorphic change along a Great Lakes shoreline: Insights from a year-long drone mapping study along Lake Michigan
Ethan J. Theuerkauf, C. Robin Mattheus, Katherine N. Braun, and Jenny Bueno
Coastal storms are an important driver of geomorphic change along Great Lakes shorelines. While there is abundant anecdotal evidence for storm impacts in the region, only a handful of studies over the last few decades have quantified them and addressed system morphodynamics. Annual to seasonal lake-level fluctuations and declining winter-ice covers also influence coastal response to storms, yet relationships between hydrodynamics and geomorphology are poorly constrained. Given this, the Great Lakes region lags behind marine coasts in terms of predictive modeling of future coastal change, which is a necessary tool for proactive coastal management. To help close this gap, we conducted a year-long study at a sandy beach-dune system along the western shore of Lake Michigan, evaluating storm impacts under conditions of extremely high water level and absent shorefast ice. Drone-derived beach and dune topography data were used to link geomorphic changes to specific environmental conditions. High water levels throughout the year of study facilitated erosion during relatively minor wave events, enhancing the vulnerability of the system to a large storm in January 2020. This event occurred with no shorefast ice present and anomalously high winter water levels, resulting in widespread erosion and overwash. This resulted in 20% of the total accretion and 66% of the erosion documented at the site over the entire year. Our study highlights the importance of both antecedent and present conditions in determining Great Lakes shoreline vulnerability to storm impacts.
2020 hurricane impact assessment for the northern Gulf of Mexico: Hurricane Sally and Hurricane Zeta
S. McGill, C. Sylvester, L. Dunkin, E. Eisemann, and J. Wozencraft
Regional-scale shoreline and beach volume changes are quantified using the Joint Airborne Lidar Bathymetry Technical Center of Expertise’s digital elevation model products in a change detection framework following the passage of the two landfalling hurricanes, Hurricanes Sally and Zeta, along the northern Gulf Coast in late fall 2020. Results derived from this work include elevation change raster products and a standard set of beach volume and shoreline change metrics. The rapid turn-around and delivery of data products to include volume and shoreline change assessments provide valuable information about the status of the coastline and identification of areas of significant erosion or other impacts, such as breaching near Perdido Key, FL, from Hurricane Sally’s impact. These advanced change detection products help inform sediment budget development and support decisions related to regional sediment management and coastal storm risk management.
Subaerial beach morphology change from multiple storms during the 2020 hurricane season
Tiffany Roberts Briggs, Nicholas Brown, and Michael S. Priddy
Frequent or consecutive storms impacting coastal areas can result in unexpected or variable impacts. This study evaluates spatiotemporal variability and cumulative impacts on the subaerial beach from four major tropical storms of varying intensity and proximity impacting the study area of Palm Beach County, Florida, during the 2020 Atlantic Basin Hurricane season. Impacts from Hurricanes Isaias, Laura, Sally, and Teddy were measured using Real-Time Kinematic Global Positioning System (RTK GPS) at 14 transects throughout the northern and southern portion of the county. Alongshore morphologic variability resulted from each storm, with some expected patterns of erosion and accretion with a few unexpected impacts. The first three storms caused swash or collision regime impacts on the Sallenger scale. Hurricane Teddy was the fourth storm to impact the study area, causing overwash at numerous locations. Whereas the first two storms of the season caused mostly erosion of the subaerial beach, the southeasterly approach of Hurricane Sally reversed the cumulative volume loss trend in the northern portion of the study area with accretion. Hurricane Teddy was the most distant storm but occurred at the highest tide and produced the largest waves and highest winds. The most variable patterns in erosion and accretion resulted from Hurricane Teddy, which also dominated the overall (or cumulative) volume and contour change. Further study is recommended for a multi-storm season that includes the subaqueous portion of the beach profile to elucidate trends of cross-shore and alongshore drivers of storm-induced morphology change.
Factors controlling longshore variations of beach changes induced by Tropical Storm Eta (2020) along Pinellas County beaches, west-central Florida
Jun Cheng, Francesca Toledo Cossu, and Ping Wang
Tropical Storm Eta impacted the coast of west-central Florida from 11 November to 12 November 2020 and generated high waves over elevated water levels for over 20 hours. A total of 148 beach and nearshore profiles, spaced about 300 m (984 ft) apart, were surveyed one to two weeks before and one to eight days after the storm to examine the beach changes along four barrier islands, including Sand Key, Treasure Island, Long Key, and Mullet Key. The high storm waves superimposed on elevated water level reached the toe of dunes or seawalls and caused dune erosion and overwash at various places. Throughout most of the coast, the dune, dry beach, and nearshore area was eroded and most of the sediment was deposited on the seaward slope of the nearshore bar, resulting in a roughly conserved sand volume above closure depth. The longshore variation of beach-profile volume loss demonstrates an overall southward decreasing trend, mainly due to a southward decreasing nearshore wave height as controlled by offshore bathymetry and shoreline configurations. The Storm Erosion Index (SEI) developed by Miller and Livermont (2008) captured the longshore variation of beach-profile volume loss reasonably well. The longshore variation of breaking wave height is the dominant factor controlling the longshore changes of SEI and beach erosion. Temporal variation of water level also played a significant role, while beach berm elevation was a minor factor. Although wider beaches tended to experience more volume loss from TS Eta due to the availability of sediment, they were effective in protecting the back beach and dune area from erosion. On the other hand, smaller profile-volume loss from narrow beach did not necessarily relate to less dune/ structure damage. The opposite is often true. Accurate evaluation of a storm’s severity in terms of erosion potential would benefit beach management especially under the circumstance of increasing storm activities due to climate change.
Rapid-response observations on barrier islands along Cape Fear, North Carolina, during Hurricane Isaias
Ryan S. Mieras, Christopher S. O’Connor, and Joseph W. Long
Hurricane Isaias struck the Cape Fear Region of North Carolina around 23:00 EDT on 3 August 2020, making landfall at Ocean Isle Beach as a Category 1 storm with peak wind speeds of 80 mph. An array of nearshore Sofar Spotter wave buoys captured the wave field at two beaches off the coasts of Bald Head Island (south-facing and east-facing beaches) and Masonboro Island. Local variations in significant wave height and peak wave direction were observed along the Lower Cape Fear Region, due to large shoal features impacting the regional wave climate. A cross-shore transect of five pressure sensors was installed at the north end of Masonboro Island 2.5 days prior to landfall to measure storm surge, wave runup, and variation of gravity/ infragravity wave energy across the barrier island. The three fast-sampling wave gauges along the backshore became buried before Hurricane Isaias peak storm surge, and the two gauges on and behind the dune were never inundated. A low-cost (< $250) Storm Surge Observation Camera (SSOC) prototype captured storm surge and coastal erosion at Kure Beach, in conjunction with pre- and post-storm RTK GPS beach profile surveys. Kure Beach experienced more than 1.0 m of vertical erosion of the berm, while Masonboro Island experienced around 0.1 m of accretion across the backshore, despite nearly identical wave and wind forcing conditions at the two beaches separated by ~20 km. Pre-storm berm height and width (higher and wider at Kure Beach), as well as foreshore slope (steeper, 1:9, at Kure Beach), are likely factors influencing significant erosion at Kure Beach, while slight accretion was observed at Masonboro Island.
Forecasting oceanfront shoreline position to evaluate physical vulnerability for recreational and infrastructure resilience at Cape Hatteras National Seashore
Michael J. Flynn and David E. Hallac
The Cape Hatteras National Seashore (Seashore) is located along the Outer Banks of eastern North Carolina, and is renowned for its prominent historical landmarks and world-class recreation. Seashore managers maintain hundreds of assets that support visitor use. Additionally, and primary to the mission of the National Park Service (NPS), managers steward natural and cultural resources located on public and protected lands. The portfolio of assets managed by NPS within the Seashore carries a high level of risk due to its exposure to both coastal erosion and storm surge inundation. The impacts of Hurricane Dorian demonstrated the importance of examining the physical vulnerability of the entire portfolio managed by NPS within the Seashore. The purpose of this study was to 1) evaluate the functionality of the beta forecast tool available in the Digital Shoreline Analysis System (v 5.0); and 2) explore options for using the output to assess the potential physical vulnerability of NPS assets. The study determined that using the 10- and 20-year oceanfront shoreline position forecast provides decision makers with a first order screening tool that can be used to prioritize mitigation and adaptation strategies given the unpredictable nature of tropical and extra-tropical cyclones and uncertainty associated with sea level rise.