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CRAMP: Wave Energy

Results of CRAMP Data Analysis

An analysis of the CRAMP data set was performed by Alan Friedlander and Eric Brown to determine the possible importance of wave impact on benthic sessile communities and reef fish communities. Since quantitative data on wave energy are not available for all sites, the first spatial analyses simply used northern wave exposure, southern wave exposure, sheltered southern exposure, sheltered northern exposure and fully sheltered or protected. North coasts are profoundly impacted by northern winter swell and southern coasts by summer southern swell. Results using this crude first approximation for wave energy are as follows:

Correspondence analysis of the benthic coverage data from all sites reveals that northern exposed reefs (N), northern protected or sheltered, (NP), southern exposed reefs (S) and southern protected or sheltered reefs (SP) and fully sheltered or protected reefs such as Kaneohe Bay (P) largely group into five corresponding clusters.

Correspondence analysis of the fish biomass data from all sites reveals that northern exposed reefs (N), northern protected or sheltered, (NP), southern exposed reefs (S) and southern protected or sheltered reefs (SP) and fully sheltered or protected reefs such as Kaneohe Bay (P) largely group into five corresponding clusters.

As shown above, wave energy is clearly one of the dominant natural forcing functions controlling the biological structure of Hawai‘i reef communities. Extreme wave events impact, abrade and fragment reefs coral communities along most of the Hawai‘i coastline. Wave height and direction are highly seasonal and the most destructive wave events occur on a decadal interval. The impact of a given wave regime on a given reef is difficult to describe accurately due to the complex manner that wave reflection, refraction and dissipation is influenced by the relative position of the islands to each other and island bathymetry.

The cost of measuring wave energy at the 30 CRAMP sites (2 depths at each site) precludes the possibility of this approach. As an alternative, CRAMP is using output from the data generated by the U. S. Naval Oceanographic WAM wave model to provide the second approximation of relative wave energy at each of our sites. The model nowcast is downloaded daily and values for wave height and direction added to the database. Accumulated data give us an ever improving picture of seasonal changes in wave patterns with mean, mode and maximum values encountered. Our experience with the WAM model is that it gives a fairly accurate representation of offshore wave conditions at each site. These data serve as a good proxy for wave energy. Actual wave energy at each site is a function of local bathymetry as well as offshore wave height and direction. Other models are being investigated in collaboration with Dr. Curt Storlazzii of the USGS cooperating group. These models will give a third approximation of wave energy at each site. Site to site variation in wave height and direction is considerable in the Hawaiian Islands. The example below of how North Pacific Swell is modified locally by the Hawaiian Islands shows the wide variation in impact of these waves within the archipelago.

This image generated by the Naval Oceanographic Wave Model (WAM) shows the effect of island topography on local wave impact from a large North Pacific Swell event. This pattern changes with intensity and direction of waves reaching Hawai‘i. Notice the large wave shadows created by the islands. Considerable variation in wave conditions at each of the CRAMP sites is observed on any given day. WAM data are being downloaded daily to prepare a database for each of the CRAMP monitoring sites.

General Summary of Wave Impact in Hawaiian Waters:

Hawai‘i is impacted by four major categories of waves. NE Trade Wind Waves are generated by the prevailing winds moving across a vast stretch of open water to the NE of Hawai‘i. These waves occur throughout the year but weaken in the winter months with weakening NE Trade Wind conditions. The North Pacific Swell is generated by powerful winter storms to the north of Hawai‘i. South Swell is generated by storms in the South Pacific during the southern hemisphere winter (summer in Hawai‘i). Kona Storm Waves are generated by storms to the SW of Hawai‘i and are more frequent in the winter months. The major characteristics of these waves are summarized in the table below. In addition Hurricane Waves can impact Hawai‘i during the late summer and fall.

Table 1: Waves influencing the main Hawaiian Islands (summarized from Moberly, 1987 and Patrick C. Caldwell National Oceanographic Data Center Honolulu, personal communication January 15, 2000).

Wave Type

Typical

Extreme

Direction

 

Height (ft)

Period (s)

Height (ft)

Period (s)

Mean

Range

NE Trade Wind Waves (Offshore, windward)

4-12

5-8

13-18

9-12

NE 45°

0° -90°

North Pacific Swell

8-15

10-17

16-25

18-25

NW 315°

282° -45°

South Swell

1-4

12-17

5-10

14-25

SSW 190°

236° -147°

Kona Storm Waves

3-5

8-10

6-10

11-14

SW 210°

258°-247°

Links to daily wave forecasts:

 

Last Update: 04/21/2008

By: Lea Hollingsworth

Hawai‘i  Coral Reef Assessment & Monitoring Program

Hawai‘i  Institute of Marine Biology

P.O. Box 1346

Kāne‘ohe, HI 96744

808-236-7440 phone

808-236-7443 fax

email: jokiel@hawaii.edu