Wave Climate

The following section briefly describes the wave regime of Pu‘ukoholā Heiau NHS and Kaloko-Honokōhau NHP.

Pu‘ukoholā Heiau NHS and Kaloko-Honokōhau NHP are located on the Kohala and Kona coasts (respectively) of the Big Island of Hawai‘i. These coasts are primarily west facing coastlines, and receive large north and south Pacific swell during winter and summer months respectively. These large swells produce coastal impacts in the form of coastal erosion, overtopping and inundation. However the nature of these impacts is seasonal: they occur during high swell season, and are followed by calm conditions which favor recovery. Thus to understand the seasonal nature of the waves and coastal impacts, we must investigate the seasonal nature of the occurrence of large swell in Hawai‘i. The following is a description of the Hawaiian wave cycle and how it affects the coastal hazards at Pu‘ukoholā Heiau NHS and Kaloko-Honokōhau NHP.

Hawaiian swell regimes

The four dominant regimes responsible for large swells in Hawai‘i are: north Pacific swell, trade wind swell, south swell, and Kona storms. The regions of influence of these regimes, outlined by Moberly & Chamberlain (1964), are shown in Figure 2. A wave rose depicting annual swell heights and directions (Vitousek & Fletcher 2008) has been added to their original graphic. The average directional wave spectrum in Hawaiian waters is bimodal and dominated by the north Pacific and trade wind swell regimes (Aucan 2006). Although quite important to the complete Hawaiian wave climate, south swell and Kona storm regimes do not occur with comparative magnitude and frequency as north Pacific and trade wind swell regimes. The Hawai‘i buoy network from the National Oceanic and Atmospheric Administration (NOAA) National Data Buoy Center (NDBC), shown in Figure 2, provides data for understanding the local wave climate. Buoy reports are available via the World Wide Web at: http://www.ndbc.noaa.gov/maps/Hawaii.shtml.

Figure 2Hawai‘i dominant swell regimes after Moberly & Chamberlain (1964), and wave monitoring buoy locations. From Vitousek & Fletcher (2008).

Figure 2 . Hawai‘i dominant swell regimes after Moberly & Chamberlain (1964), and wave monitoring buoy locations.  From Vitousek & Fletcher (2008).

Inter-annual and decadal cycles including El Niño Southern Oscillation (ENSO) occurring approximately every three to four years (Goddard & Graham 1997), and Pacific Decadal Oscillation (PDO) occurring around 20-30 years (Mantua et al. 1997; Zhang et al. 1997), influence the variability of the Hawaiian wave climate. These large-scale oceanic and atmospheric phenomena are thought to control the magnitude and frequency of extreme swell events. For example, strong ENSO events are thought to result in larger and more frequent swell events (Seymour et al. 1984, Caldwell 1992, Inman and Jenkins 1997, Seymore 1998, Allan & Komar 2000, Wang & Swail 2001, Graham & Diaz 2001, Aucan 2006). Understanding the magnitude and frequency of extreme wave events is important as they may control processes such as coral development (Dollar & Tribble 1993, Rooney et al. 2004), sediment supply (Harney et al. 2000, Harney & Fletcher 2003) and beach morphology in Hawai‘i and abroad (Moberly & Chamberlain 1964, Ruggiero et al. 1997, Kaminsky et al. 1998, Storlazzi and Griggs 2000, Rooney & Fletcher 2005, Ruggiero et al. 2005).

North Pacific swell

Hawai‘i, located in the middle of a large swell-generating basin, the north Pacific, receives large ocean swell from extra-tropical storms that track predominantly eastward from origins in the northwest Pacific. The north Pacific storminess reaches a peak in the boreal winter, as the Aleutian low intensifies and the north Pacific high moves southward. The strong winds associated with these storms produce large swell events, which can travel for thousands of miles until reaching the shores of Hawai‘i. In summer months, the north Pacific high moves northward and storms in the north Pacific become infrequent (Flament et al. 1996). Figure 3 shows the satellite-derived wave average heights over the north Pacific in the winter and summer. The average winter wave heights in the north Pacific are approximately ≥ 3 m while the summer wave heights are approximately ≤ 2 m. Figure 3 gives the average wave heights of the north Pacific, however the dynamic system typically involves individual storm events tracking eastward with wave heights of 5-10 m. These swell-producing storms occur during winter months with typical periods of 1-1.5 weeks (for 5-7 m swells), 2-3 weeks for (for 7-9 m swells) and one month (for swells 9 m or greater). Many north Pacific storms produce swells that do not reach Hawai‘i. Storms that originate in high latitudes and those that track to the northeast send swells to the Aleutians and the Pacific Northwest. Swells that originate from storms in lower latitudes, and those that track slightly to the southeast, reach Hawai‘i with the largest wave heights.

Figure 3. Satellite (JASON-1) derived average wave heights [m] over the North Pacific in the summer and winter.

Figure 3 . Satellite (JASON-1) derived average wave heights [m] over the North Pacific in the summer and winter.

Hawai‘i receives north Pacific swell with an annually recurring maximum deep-water significant wave height of 7.7 m (Vitousek & Fletcher 2008) with peak periods of 14-18 sec. However, the size and number of swell events in Hawai‘i each year is highly variable – varying by a factor of 2 (Caldwell 2005). The annual maximum wave height recorded from buoy 51001 ranges from about 6.8 m (in 1994, 1997, 2001) to 12.3 m (1988).

The seasonal cycle of north Pacific swell reaches a peak in winter and a trough in summer, with a daily average significant wave height around 4 m. Aucan (2006) depicted the monthly average directional spectra from buoy data at Waimea (buoy 51201) and Mokapu (buoy 51202) that showed the dominance of north Pacific swell out of the northwest in winter months, and relatively persistent energy out of the northeast associated with trade wind swell. Buoy locations can be seen via the World Wide Web at: http://www.ndbc.noaa.gov/maps/Hawaii.shtml.

Trade winds and trade-wind swell

Occurring about 75% of the year, the trade winds are northeasterly winds with an average speed of 15.7 mph and direction 73° with standard deviations (1σ) of 5 mph and 23°. In winter months, the north Pacific high flattens and moves closer to the islands decreasing trade wind persistence. Although the number of windy days in summer months increases, the mean trade-wind speed in summer and winter months remains relatively similar (Figure 4).

Figure 4. The number of days per season that the trade winds occur with a certain speed (data from Buoy 51001).  The days per season are shown in red for winter months and blue for summer months.  Notice the persistence of typical trade winds (~ 16 mph) during summer months.

Figure 4 . The number of days per season that the trade winds occur with a certain speed (data from Buoy 51001).  The days per season are shown in red for winter months and blue for summer months.  Notice the persistence of typical trade winds (~ 16 mph) during summer months.

The persistent trades generate limited fetch trade wind swell on northeast facing coasts. Choppy seas with average wave heights of 2 m (1σ = 0.5 m) and peak periods of 9 sec. (1σ = 2.5 sec.) from the northeast characterize trade wind swell in Hawai‘i. Although these represent nominal conditions, trade-wind swell can exceed 5 m in height and have periods of 15-20 sec.

Southern swell

Southern swell arriving in Hawai‘i is typically generated farther away than north Pacific swell. These swells are usually generated from storms south of the equator near Australia, New Zealand and as far as the Southern Ocean and propagate to Hawai‘i with little attenuation outside the storm-generated region (Snodgrass et al. 1966). South swell occur in summer months (Southern hemisphere winter months) and reach Hawai‘i with an annual significant wave height of 2.5-3 m and peak periods of 14-22 sec, which are slightly longer than north Pacific swell (Armstrong 1983, Vitousek & Fletcher 2008).

Kona storms

Giambelluca & Schroeder (1998) describe Kona storms as:
“low-pressure areas (cyclones) of subtropical origin that usually develop northwest of Hawai‘i in winter and move slowly eastward, accompanied by southerly winds from whose direction the storm derives its name, and by the clouds and rain that have made these storms synonymous with bad weather in Hawai‘i”.

Strong Kona storms generate wave heights of 3-4 m and periods of 8-11 sec., along with wind and rain, and can cause extensive damage to south and west facing shores (Rooney & Fletcher 2005). While minor Kona storms occur practically every year in Hawai‘i, major Kona storms producing strong winds, large wave heights and resulting shoreline change tend to occur every 5-10 years, during the 20-30 year negative PDO cycle (Rooney & Fletcher 2005). Consequently, Positive (warm) PDO, and El Niño phases tend to suppress Kona storm activity (Rooney & Fletcher 2005).

Maximum annual recurring wave heights in Hawai‘i

Although each wave regime (trade wind swell, north Pacific swell, south swell, and Kona storms) has its own underlying processes and mechanics, the sum of all of these regimes contribute to the wave heights and shoreline change in Hawai‘i. Thus evaluating extreme wave heights on a continuous scale around these islands is informative. Breaking waves at the shoreline are often composed of many swell sources from different storms and swell regimes. North Pacific (south) swell and trade wind swell are the most common sources of swell for north (south) facing shores. Thus the spectral approach to understanding swell and surf patterns, following Aucan (2006), is quite informative.

The maximum annually recurring significant wave heights and the largest 10% and 1% wave heights for various directions in 30° windows around Hawai‘i are given in Table 1 (Vitousek & Fletcher 2008). These annual wave heights are also depicted in Figure 2.

Table 1 . The observed maximum annually recurring significant wave heights(Hs) in meters and the largest 10% (H1/10) and 1% (H1/100)wave heights for various directions around Hawai‘i. GEV is the Generalized Extreme Value Analysis.


Wave Direction

Annual Hs (m) – GEV Model

Lower

Upper

Observed - Hs

H1/10

H1/100

0

30

5.9

7.4

9.8

30

60

6.0

7.6

10.0

60

90

5.1

6.5

8.5

90

120

4.3

5.5

7.2

120

150

2.8

3.5

4.6

150

180

3.0

3.8

5.0

180

210

2.4

3.0

3.9

210

240

1.6

2.0

2.7

240

270

1.5

1.9

2.5

270

300

3.7

4.7

6.2

300

330

5.9

7.5

9.9

330

360

5.8

7.4

9.7

 

Tides

The tides result from the varying gravitational attraction of the Earth to the Moon and Sun during orbit. Tides are composed as a sum of sinusoidal components that typically have their largest variability in diurnal (one cycle per day) and semi-diurnal (two cycles per day) frequencies. Large gravitational forces and maximum tides are also produced when the Earth, Moon, Sun system are aligned (referred to as syzygy). Conversely, minimal gravitational forces and tides result when the Earth, Moon, Sun systems are at right angles (referred to as quadrature). This alignment occurs on a monthly cycle as related to the moon phases in Hawai‘i, and periods when the tides are the largest (smallest) are referred to as spring (neap) cycles. The tide range in Hawai‘i is quite small compared with the rest the world, having a typical tide range [Mean Higher High Water (MHHW) – Mean Lower Low Water (MLLW)] of 0.58 m and a spring tide range around 1.0 m.

The astronomic tide typically represents the largest water level variability at a particular location. However other factors such as atmospheric pressure, wind setup, ENSO cycles, and oceanic disturbances can produce water level variability on the order of tens of centimeters. One important process influencing extreme sea level events in Hawai‘i is the occurrence of mesoscale eddies, which are large oceanic disturbances (a few hundred km in diameter), having elevated sea levels of around 15 cm (Firing & Merrifield 2004).

Coincidence of waves and tides

Coincidence of large swell and high tide events can cause severe coastal flooding and overtopping in Hawai‘i, whereas swell events occurring on low tides or neap cycles can be less severe (Caldwell et al. 2008). Using joint probabilities of wave and tide distributions, Caldwell et al. (2008) found the number of hours a particular combination of surf height and tide level are expected to be exceeded. We will employ a similar approach to estimating the overtopping frequency and severity for the parks.

Runup and inundation

We are most interested in the recurrence of high surf events because these events control many natural beach processes like rip current formation, erosion, and reef growth. Additionally the high surf events pose significant risk to coastal communities and ocean users in the form of overtopping and coastal flooding due to large runup events, property damage and drowning and ocean safety concerns. Wave runup is the maximum vertical height of the wave on a beach, and is influenced by the wave swash and setup. Coastal events such as tsunamis and hurricanes pose the greatest potential hazards in terms of the magnitude of flooding, property damage and loss of life; however they are rare (occurring with return periods of several decades) compared with high surf events, which occur several times per year. Many sources contribute to the maximum water level on a beach, including tide, wave setup, wave runup and other sources of water level variability (mesoscale eddies, sea-level rise). Coincidence of large swell and tide events can cause severe coastal flooding and overtopping in Hawai‘i (Caldwell et al. 2008).

Sea-level rise

Sea-level rise is a significant coastal hazard. If we consider sea-level rise as a coastal hazard alone, then low-lying coastal lands will be at greatest risk to sea-level impacts in the form of passive flooding. The time horizons for such impacts are often distant, relative to the rate of sea-level rise and the elevation of structures at risk. However considering sea-level rise as a coastal hazard interacting with large wave and tide events, we see that potential impacts due to sea-level rise (in the form of increased overtopping frequency associated erosion and shoreline change) appear on a much shorter time horizon.

There is much debate over quantifying potential sea-level rise scenarios. The IPCC has estimated six sea-level rise scenarios, which predict a range of sea levels from 0.1 - 0.88 m by 2100 (based on data and various climate models). Rahmstorf (2007) estimates sea-level scenarios of 0.5 - 1.4 m by 2100 (based on a fit of global temperature to sea-level and the projection of IPCC temperature predictions). Church & White (2006) found global sea-level to rise almost 20 cm between 1870 and 2004 based on data from tide gauges, and estimated 0.28 – 0.34 m of sea-level rise by 2100 based on a constant acceleration rate of 0.013 mm/yr2 from the historical data. Beckley et al. (2007), using satellite altimetry, found global sea-level rise rates increased from ~2.75 mm/yr (during 1993-2000) to ~3.75 mm/yr (during 2000-2007).

If we consider Hawai‘i as an isolated region in terms of global sea level and examine its unique sea-level history we see that sea-level rise ranges from ~1.4 mm/yr to ~3.8 mm/yr (Figure 5).

Figure 5. Sea-level history [mm] in Hawai‘i as observed from several tide gauges.

Figure 5 . Sea-level history [mm] in Hawai‘i as observed from several tide gauges.

The sea-level rise rates for the Big Island (and islands close to it) are larger than the rest of the islands due to island subsidence. The tide gauge at Kawaihae Harbor, near Pu‘ukoholā Heiau NHS, on the Big Island has reported the largest sea-level rise rate of 3.8 mm/yr. It is also the gauge with the shortest observation record. If we consider the Big Island to experience a sea-level rise rate that is the average of the Hilo and Kawaihae we find a rate of ~ 3.5 mm/yr. We have determined a hierarchy of sea-level rise scenarios based on rates found from Big Island tide gauges and global acceleration terms reported in the literature. The future sea-level predictions based on these scenarios are reported in Table 2.

Table 2 . Hierarchy of sea-level rise scenarios.

Mean Sea Level (MSL) increase [in m] relative to present (2008)

Scenario

Rate (mm/yr)

Acceleration (mm/yr2)

2025 (m)

2050 (m)

2100 (m)

Modest

3.5

0

0.06

0.15

0.32

Probable

4

0.013

0.07

0.19

0.48

Extreme

4

0.14

0.11

0.41

1.55