Puget Sound Waves

Introduction

Most beaches in Puget Sound and Hood Canal are isolated from the Pacific Ocean by topography. As a result waves in the Sound are generated locally by wind blowing over the irregular channels and basins of the fjord system; they receive little to no swell from the eastern Pacific. Two consequences of this are that: (1) the waves are fetch-limited and low-energy compared to incident waves on the Pacific coast; and (2) the wave climate is tightly coupled to local wind patterns. In addition, vessel wakes may be a significant wave energy source, particularly in restricted channels, where large incident waves would not otherwise occur. There are few published wave records for Puget Sound. The records that do exist are of short duration and limited to only a handful of locations. So this discussion will rely mainly on inferring wave conditions from wind climate supplemented where possible with actual wave data.

Wind Patterns over the Pacific Northwest

The four weather patterns shown in Figure 1 represent about 60% to 70% of the weather experienced in the Pacific Northwest. Three of the patterns occur in the winter, the one in the lower right occurs during the summer.

The upper left-hand box represents probably the most typical winter configuration, with a low pressure region over the Gulf of Alaska and weather fronts sweeping across the northwest in an anti-cyclonic (counter clockwise) rotation. Winds from this pattern are southerly (that is, blow from the south to the north) and they can be strong at sea but the weather tends to break up as the fronts move inland.

The next pattern in the upper right are associated with deep low-pressure cells that move northeasterly from the central pacific. Sometimes these systems drop pressure rapidly and develop into extra-tropical cyclones---so called "bombs". They produce severe weather up and down the coast, occasionally with hurricane force winds over much of the coast. Most of the strongest storms in Puget Sound have been southerlies of this type including the 1962 Columbus day cyclone that marks the most powerful storm in the Northwest on record.

The next pattern on the lower-left occurs several times during the winter. A strong high-pressure system builds up behind the cascades and coastal ranges while the pacific northwest experiences stable, clear skys. Often freezing air is blasted through the mountain passes, particularly the Fraser River Valley in what are known as Gap winds when they blow out to see down the Straits of Juan de Fuca or Bora winds over the passes. This can lead to strong northerly winds blowing south across the Sound, at least one of these events in the nineties was very strong, resulting in a reversal of normal south-to-north sediment patterns in the south sound.

Towards the end of spring, a high-pressure system moves north up the west coast and settles offshore of Washington and Oregon. This system produces gentile northerlies across the main basin and occasionally strong westerlies down the straits of Juan de Fuca. Otherwise it protects the coast from disruptive weather events.

Storm Winds in Puget Sound

The pattern of strong winds during the winter and summer is shown in Figure 2. The black bars point towards the direction (the exact opposite of a flag in the wind) of winds blowing at least 10 m/s, the red bars are pointing towards winds of greater than 20 m/s. The length of the bar indicates the frequency of observations in this direction bin.

The pattern for winter on the left has both strong and severe winds blowing almost uniformly from the south and southwest as one would expect for weather blowing in from the coast up the Chehalis river valley. This is true even for the northern sections of the map.

Severe winds are less common during the summer but strong winds continue to be southerlies in the main basin, while northern Puget Sound is dominated by westerlies that blow down the straits. A convergence zone develops during the late spring and early summer where the winds converge on the triple junction area.

Wave Hindcasting with Numerical Models

Wave hindcasting is the technique of estimating past wave heights from atmospheric records (essentially the opposite of forecasting). Our goal is to collect historical weather data and run it through our newest numerical models so that we can recreate past wave events and build a wave climate map for Puget Sound.

We are using the SWAN wave model. SWAN (Simulating Waves Nearshore) is a third-generation wave model that computes random, short-crested wind-generated waves in coastal regions, including lakes and estuarine waters (Holthuijsen, Booij et al. 2003). The model is sophisticated, including a wide variety of wave propagation, generation and dissipation processes. SWAN does not model wave-induced currents (though these can be included if a suitable external hydrodynamic model can provide the information).

In this hindcast, winds blow up to 20 m/s moving south to north over Puget Sound, note the particularly strong winds in south sound and over Admiralty Inlet. Also, the tides during this run ebbed at better than 3 m/s in parts of Admiralty Inlet.

Waves in Hood Canal were relatively flat, while south sound got fairly large waves. The north part of the main basin is getting hammered with large waves up to 1.2 meters with periods of around 3 to 4 seconds. I was fascinated by the effect the strong tidal currents had on the wave heights in Admiralty inlet, especially the wave period which actually decreased while it was interacting with the currents. This effect is difficult to see from these images due to the colorramp which was designed to emphasize the low waves in Puget Sound.