Presenter: Justin Birchler (U.S. Geological Survey)

Description:
The Total Water Level & Coastal Change Forecast (TWL&CC) provides coastal communities with advance notice of potential elevated coastal water levels and coastal changes on sandy beaches that may threaten safety, infrastructure, or resources. The continuously operating model is a collaboration between USGS and NOAA and provides hourly information for select regions of the U.S. Gulf and Atlantic coastlines. The objective of this work is to assess the skill of the forecasts during a period of elevated total water level associated with the landfall of Hurricane Isaias in southern North Carolina in August 2020. Water level and wave observations were collected throughout the storm at three locations near Wrightsville Beach and provided information to assess forecast skill; a wave buoy at 16 m water depth, a tide gage located at the seaward end of Johnnie Mercers Pier at 1 m water depth, and a USGS Short-Term Network pressure transducer deployed at the landward end of the pier at an elevation of 2 m. In addition to observations, the non-hydrostatic phase-resolving model, Simulating WAves till SHore (SWASH), forced with hourly wave energy spectra during the peak of Hurricane Isaias, was used to complement the observations by computing the nearshore wave height, setup and runup. Preliminary model-data comparisons using a simple regression method suggest good agreement of storm-induced water levels between the TWL&CC Forecast and the wave buoy (r²=0.95), the tide gage (r²=0.80), the SWASH model (r²=0.70) and USGS sensors (r²=0.55). The TWL&CC Forecast quantifies potential storm effects and tests our understanding of the drivers of coastal change. It is valuable to do post-storm skill assessments utilizing measured or post-storm data when available to learn how the forecast model behaves during extreme conditions and to improve the model to increase forecast skill going forward.