Chapter 17 - Coastal Processes and Tides

17.3 Storm Surges

Storm winds blowing over shallow, continental shelves pile water against the coast. The increase in sea level is known as a storm surge. Several processes are important:

1. Ekman transport by winds parallel to the coast transports water toward the coast causing a rise in sea level.
2. Winds blowing toward the coast push water directly toward the coast.
3. Wave run-up and other wave interactions transport water toward the coast adding to the first two processes.
4. Edge waves generated by the wind travel along the coast.
5. The low pressure inside the storm raises sea level by one centimeter for each millibar decrease in pressure through the inverted-barometer effect.
6. Finally, the storm surge adds to the tides, and high tides can change a relative weak surge into a much more dangerous one.

See Jelesnianski (1967, 1970) and §15.5 for a description of storm-surge models SPLASH and Sea, Lake, and Overland Surges from Hurricanes SLOSH used by the National Hurricane Center and §15.5.

To a crude first approximation, wind blowing over shallow water causes a slope in the sea surface proportional to wind stress.

(17.4)

where ζ is sea level, x is horizontal distance, H is water depth, T₀ is wind stress at the sea surface, ρ is water density; and g is gravitational acceleration.

If x = 100 km, U = 40 m/s, and H = 20 m, values typical of a hurricane offshore of the Texas Gulf Coast, then ζ = 2.7 Pa, and z = 1. 3 m at the shore. Figure 17.9 shows the frequency of surges at the Netherlands and a graphical method for estimating the probability of extreme events using the probability of weaker events.

Figure 17.9 Probality (per year) density distribution of vertical height of storm surges in the Netherlands. The distribution function is Rayleigh, and the probability of large surges can be estimated from extrapolating the observed probability of smaller, more common surges. From Wiegel (1964).

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