Coastal Change Hazards: Hurricanes and Extreme Storms

Storm-Impact Scale

Storm-Induced Water Levels

Estimates of hurricane-induced water levels, R_low and R_high, are necessary for predicting the potential coastal change during an approaching hurricane. R_high is maximum water-level elevation expected during a hurricane and includes the astronomical tide, storm surge, and wave runup. R_low is an effective still-water level during a storm and is composed of the astronomical tide, storm surge and wave setup.

Figure 1. Modeled, maximum surge elevations, as simulated by the NOAA SLOSH model, for a category 3 hurricane making landfall in the Pamlico Sound basin of North Carolina at a forward speed of 15 m/s. The values shown, representing the 'maximum envelope of water,' are obtained by running several similar hypothetical storms onshore along parallel tracks (shown by black arrows). [larger version]

Storm surge is the temporary rise in water level due primarily to winds and pressure within a hurricane. In order to use the storm-impact scaling model in a predictive model, the elevation of surge for the five hurricane categories is modeled by the National Hurricane Center using the NOAA SLOSH (Sea, Lake and Overland Surges from Hurricanes) model, a real-time forecast model for hurricane-induced water levels for the Gulf and Atlantic coasts (Figure 1). The numerical model is based on linearized, depth-integrated equations of motion and continuity (Jarvinen and Lawrence, 1985). Changes in maximum surge elevations are forced by time-varying wind-stress and pressure gradient forces which depend on the hurricane's location, minimum pressure, and size measured from the eyewall out to the location of maximum winds (Jarvinen and Lawrence, 1985). The SLOSH model does not incorporate astronomical tides, wave runup or setup; however, astronomical tides are included in the final results (Houston et al., 1999). While the results are location specific, accounting for local water depths, proximity to bays and river, etc., the results are accurate to ± 20% of the calculated value (2004). Error in SLOSH values can also arise from differences between the parametric wind models which force SLOSH and the hurricane's actual wind field (Houston et al., 1999).

References

NOAA, 2004. Hurricane Preparedness. National Centers for Environmental Prediction, National Hurricane Center, http://www.nhc.noaa.gov/HAW2/english/surge/slosh.shtml.

Houston, S.H., Shaffer, W.A., Powell, M.D. and Chen, J., 1999. Comparisons of HRD and SLOSH surface wind fields in hurricanes: Implications for storm surge modeling. Weather and Forecasting, 14: 671-686.

Jarvinen, B.R. and Lawrence, M.B., 1985. An evaluation of the SLOSH storm surge model. Bulletin of the American Meteorological Society, 66(11): 1408-1411.

Stockdon, H.F., R.A. Holman, P.A. Howd, and A.H. Sallenger, 2006. Empirical parameterization of setup, swash, and runup, Coastal Engineering, 53(7), pp. 573-588.

Wave runup (R(t)) is the time-varying fluctuation of water-level elevation at the shoreline due to wave breaking (Figure 2). Wave setup (η), the time-averaged water level, is the super-elevation of still water at the shoreline, again due to wave breaking. The magnitude of both runup and setup are related to offshore wave period, wave height (H), and foreshore beach slope (β). The elevation of wave runup and setup are calculated from modeled offshore wave conditions using field-data-based empirical parameterizations (Stockdon, et al., 2006). During storm conditions, the wave runup and setup can double the elevation of water levels at the coast beyond that due to storm surge alone.

Figure 2: Wave runup (R) is the time-varying elevation of water level at the shoreline, measured here in reference to the still water level (SWL). The schematic shows wave runup (R), and its components, time-averaged wave setup (η) and time-varying swash (dashed lines), as a function of wave height (H) and beach slope (β). [larger version]

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