How to read the nomogram: The nomogram is integrating data forecasts for water level (using the FVCOM model) and wave height (using the Wave Watch 3 model). An empirical relationship exists between storm tide, waves and coastal flooding or splash-over damage (NWS coastal flood study). Knowing this relationship will allow us to predict when flooding and splash-over events (such as beach erosion) might occur based on forecast water level (tide height) and wave height data. Using these relationships, a series of lines can be drawn on the nomogram connecting the water level with forecast wave heights (figure 1).
(figure 1) The intersection of these lines creates distinct sections on the nomogram. When the nomgram is in motion, a real-time indicator moves through the 48 hour forecast as a series of red circles. The leading circle of the indicator represents the most recent reading, and the trailing circles are previous readings (in 1 hour increments). On a normal day, the indicator will simply move up and down the water level axis with the tide, and make minimal movement along the y-axis for wave height. (figure 2)
(figure 2) In a storm situation, the indicator will move further and further out along the x-axis (as the wave height increases), and further up the y-axis depending on where we are in the tide cycle, and how high the predicted water level is. For example, if the indicator moves further to the right along the Wave Height axis (figure 3), there is a likelihood that beach erosion could begin. The severity of the erosion depends on where in the sections the indicator moves.
(figure 3) If the indicator continues to move to the right, and crosses the yellow line “Splash Over Begins” (figure 4), it means the high water level and high tides have combined and could result in moderate to severe beach erosion.
(figure 4)
During the Patriot's Day Storm, there were four high-tide cycles in which the water level was near or above flood stage and the waves were greater than 20' in height. This combination caused the tremendous amounts of coastal damage seen during the storm. The Coastal Flooding and Erosion Forecast System actually predicted days in advance of the Patriot’s Day Storm that there was a high likelihood of this damage occurring. This capacity is extremely important to coastal property owners and emergency responders, resulting in dollars and lives saved.
For more information on the science behind this diagram: Comprehensive Study on Northern New England Coastal Flooding John Cannon, NOAA/National Weather Service Forecast Office, Gray, Maine A Glimpse of the Future Web - Forecasting storm damage on the Maine coast by Philip Bogden, John Cannon, Riley Young Morse, Ian Ogilvie, Brian Blanton and William Perrie, Journal of Ocean Technology 2008, volume 3, number 3
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