Waves and Coastal Erosion
Glossary Terms
Hawai‘i is known for its high waves caused by distant storms in the northern and southern hemispheres and for its tropical cyclones passing in the vicinity. High waves can create hazardous conditions including debris overwash, flooding, erosion, high wave energy and turbulence in the nearshore zone, and strong currents. The largest waves that reach O‘ahu generally arrive in winter as a result of intense storm activity in the North and Northwest Pacific.
South-facing shorelines receive four to six foot swells during the summer months from distant storms in the Southern Hemisphere. Trade wind waves can be high, but have less energy than north and south swells. Trade wind swells have a greater easterly directional component, which enables them to refract around to south and southwest-facing shorelines producing wave heights of one to four feet. High waves from hurricanes most often hit the eastern shores as hurricanes approach the islands from the east, and south- and west-facing shorelines as the storm passes to the south and west. Hurricane-generated waves combined with storm surge and high tides can overwash coastal roads and properties, as they did along the Wai‘anae coast during Hurricane ‘Iniki in 1992.
|
1957 |
September 1 - 17; Hurricane Dell |
|
1957 |
November 30 - December 1; Hurricane Nina |
|
1959 |
August 4 - 7; Hurricane Dot |
|
1971 |
January 16; High Surf |
|
1982 |
November 23; Hurricane ‘Iwa |
|
1989 |
March 1 - 4; High Surf |
|
1989 |
June 18 - 20; Tropical Storm Dalilia, high surf |
|
1992 |
September 11; Hurricane ‘Iniki, high surf |
|
1997 |
September 23 - 25; Typhoon David, high surf |
|
1998 |
January 23 - 31; 15-20 foot NNW swell |
Source: Fletcher and others 2002.
A tsunami is a series of great waves most commonly caused by violent movement of the sea floor. It is characterized by speed (up to 590 mph), long wave length (up to 120 miles), long period between successive crests (varying from five minutes to a few hours, generally 10 to 60 minutes), and low height in the open ocean. However, on the coast, a tsunami can flood inland hundreds of feet or more and cause much damage and loss of life. The only tsunami-generated wave reported to inundate the Wai‘anae coast was 14 feet and occurred in 1946. For more information about coastal storms that affect Hawai‘i, please see the Ke a‘o Ulu: Winds and Storms section.
Coastal Erosion
Coastal erosion, or the loss of shoreline resulting from waves and other physical processes, is widespread and locally severe in Hawai‘i and other low-latitude areas. Typical erosion rates in Hawai‘i range from 15 to 30 centimeters (0.5 to one foot) per year (Hwang 1981, Sea Engineering Inc. 1988, Makai Ocean Engineering Inc. and Sea Engineering Inc.1991). Studies on O‘ahu (Fletcher and others 1997, Coyne and others 1996) have shown that nearly 24 percent of sandy shoreline, or 27.5 kilometers (17.1 miles) of an original 115 kilometers (71.6 miles), has been either significantly narrowed or lost since the 1940s. Nearly one-quarter of the Hawaiian Islands' beaches have been significantly degraded over the last half-century, and all shorelines have been affected to some degree. O‘ahu shorelines are by far the most studied; however, beach loss has been identified on the other islands as well, with Maui recording over eight kilometers of beach loss and 20 percent beach narrowing island-wide since 1949 (Fletcher and others 2003). These losses led Maui County in 2003 to change its construction setback, or distance from the shoreline where construction is allowed, from a fixed distance to one that reflects the hazards associated with coastal erosion.
Causes of Erosion and Beach Loss
Coastal erosion and beach loss in Hawai‘i are caused by numerous factors, most of which are poorly understood or poorly quantified. In general, these factors fall into three categories: (1) human impacts to sediment availability, (2) seasonal changes to waves and currents, and (3) long-term sea level rise.
Construction of shoreline protection structures limits coastal land loss, but does not alleviate beach loss. These structures may actually accelerate beach loss by prohibiting natural sediment exchange between the dune and beach systems. Obvious impacts to sand availability, such as beach mining and emplacement of structures that interrupt natural sediment transport pathways or prevent access to abutting sand deposits, remove sediment from the active littoral system.
Waves and currents in Hawai‘i and other island communities are subject to significant changes on a seasonal basis. Large waves on the north and west shores during winter months may temporarily erode beaches. In the summer months, large waves affect shorelines with southern exposures. Seasonal stormy episodes can remove sand that may not be wholly returned to the beach during fair-weather conditions.
It has been documented that global sea level is rising. Over the last century, sea level rose around the world at an average rate of approximately two millimeters per year. During the last decade, satellites have measured sea-level rise at a rate exceeding three millimeters a year. Local subsidence of islands can worsen the impact of rising sea level. In general, rising sea level leads to a natural landward migration of the shoreline (Coyne and others 1996).
Dramatic examples of coastal erosion, such as houses and roads falling into the sea, are rare in Hawai‘i, but the impact of erosion remains very serious. The signs of erosion are much more subtle, and typically start as a "temporary" hardening structure designed to mitigate an immediate problem. This can eventually result in a proliferation of structures along a stretch of coast. The natural ability of the sandy shoreline to respond to changes in wave climate is then lost.
Erosion at Wai‘anae Coast Beaches
In an effort to establish baseline beach conditions, monitor seasonal beach fluctuations, and understand the dynamics of beach change in Hawai‘i, a program of beach and near-shore monitoring was initiated in 1994 on the islands of Maui and O‘ahu, called the Hawai‘i Coastal Erosion Study. Profiles on 42 O‘ahu and 37 Maui beaches were collected biannually (in the summer and winter) for five years in a joint effort by the U.S. Geological Survey and the University of Hawai‘i Department of Geology and Geophysics (Gibbs and others 2001). A beach profile is a cross-section of the beach taken perpendicular to a given beach contour. The profile is a single line that may extend over the backshore from a relatively fixed and stable point (for example the edge of a road, or a sea-wall), across the foreshore, and seaward underwater into the nearshore zone. By evaluating the changes in the profile over time, information on both long-term erosional or accretional trends as well as changes in seasonal beach volumes can be quantified.
Beach photos:
Yokahama | Mā‘ili | Mākaha | Pōka‘ī Bay | Nānākuli
Five beaches along the Wai‘anae coast of O‘ahu were studied: Yokohama Bay (also known as Keawa ulu), Mākaha, Pōka‘ī Bay, Mā‘ili, and Nānākuli. Similar to most beaches in Hawai‘i, beach behavior along the north-south oriented leeward coast of O‘ahu is driven by the complex interaction between beach orientation and exposure to the different waves and currents that result from seasonal changes in wave energy. In fact, of all the study areas on O‘ahu and Maui, seasonal changes in beach volume were the greatest among beaches along the Wai‘anae coast (Gibbs and others 2002). Additionally, high surf and hurricanes can also affect the coral reef structure off the Wai‘anae coast, which in turn alters local coastal processes, including sand movement and beach erosion. For more details on Wai‘anae coral reefs, please see the Marine Ecosystems: Coral Reefs section.
Beach volume is typically greatest during the summer months and lowest during the winter months. In general, beach sand moves southerly along the Wai‘anae coast during the winter due to the high waves associated with north swell, while during the summer, beach sand typically moves northerly in response to the reduction in north swell wave energy and an increase in energy from south swell and Kona storms.
Comparison of annual volume changes on the five beaches studied on the Wai‘anae coast show that beach behavior varies from north to south as exposure to this wave energy changes (see figure).
- To the north, Yokohama beach shows the most regular seasonal changes, where large volumes of sand erode from the beach each winter and a nearly equal volume is redeposited on the beach during the summer (see profile).
- Mākaha (see profile) and Mā‘ili (see profile) beaches show similar magnitudes of seasonal volume change; however, the seasonal variability is less regular than at Yokohama.
- Beaches within Pōka‘ī Bay are sheltered from much of the wave energy and expectedly show little change throughout the year (see profile).
- At Nānākuli, little seasonal variation but a longer-period rhythmic pattern of volume change was observed (see profile). As the southernmost beach studied along this section of coast, these patterns reflect the relatively low influence of north swell on this portion of the coast.
Overall, Yokohama, Mā‘ili, and Pōka‘ī Bay beaches accrete and erode in a regular pattern that changes little from year to year. This suggests the natural system is in equilibrium for this stretch of beaches. In contrast, Mākaha and Nānākuli show a more complex pattern of profile change, with sediment movement offshore where it may be lost from the system. This could be due to cross-shore sediment movement or a more variable offshore seafloor.
It is important to use caution when evaluating the beach profile data discussed here because it represents change over a single location on any given beach rather than the behavior of an entire beach system. The profiles were also collected only twice yearly and are therefore strongly influenced by oceanographic conditions immediately prior to data collection. A more comprehensive approach is to integrate the beach profile data with aerial photographic analysis to observe changes over a larger spatial scale. In addition, the time-scale of change is critical when evaluating beach erosion and coastal change. Long-term changes in relative sea level, the passage of hurricanes, seasonal waves, and beach and coastal management practices all contribute to coastal change. The combined effects of these are critical to understand and quantify when developing coastal management programs.
Assessment of Natural Hazards in the Hawai‘i Coastal Zone
The Hawai‘i Coastal Zone Management Program has identified the prevention and minimization of threats to life and property from episodic and chronic coastal hazards as a program priority.
The Atlas of Natural Hazards in the Hawai‘i Coastal Zone provides a detailed assessment and ranking of seven natural coastal hazards (Fletcher and others 2002). These hazards include: tsunamis, stream flooding, high waves, storms, erosion, sea level, and volcanic/seismic activity. The ranking is based on the historical trends and natural factors influencing site vulnerability and hazard intensity in the Hawaiian coastal zone. Wai‘anae's overall coastal hazard assessment ranks from moderate to low (three to four on a scale of one to seven) to high (six to seven) depending on the specific location and vulnerability to these seven coastal hazards.
References Cited
Coyne, M.A., R. Mullane, C.H. Fletcher, and B.M. Richmond. 1996. "Losing Oahu: Erosion on the Hawaiian Coast." Geotimes 41(12): 23-26.
Fletcher, C.H., E.E. Grossman, B.M. Richmond, and A.E. Gibbs. 2002. Atlas of Natural Hazards in the Hawaiian Coastal Zone. United States Geological Survey (USGS), University of Hawaii, State of Hawaii Office of Planning, and National Oceanic and Atmospheric Administration. http://pubs.usgs.gov/imap/i2761/
Fletcher, C., E. Grossman, B.M. Richmond, and A.E. Gibbs. 2003. "Mapping shoreline change using digital orthophotogrammetry on Maui, Hawaii." Journal of Coastal Research (Special Issue No. 38): 106-124.
Fletcher, C.H., R.A. Mullane, and B.M. Richmond. 1997. "Beach Loss Along Armored Shorelines on Oahu, Hawaiian Islands." Journal of Coastal Research 13(1): 209-215.
Gibbs, A.E., B.M. Richmond, and C.H. Fletcher. 2002. "Beach Profile Variation on Hawaiian Carbonate Beaches." Proceedings of the First International Symposium on Carbonate Sand Beaches, December 5-8, 2000. American Society of Civil Engineers. p. 99-110.
Gibbs, A.E., B.M. Richmond, C.H. Fletcher, and K.P. Hillman. 2001. Hawaii Beach Monitoring Program: Beach Profile Data. Open-File Report 01-308. Version 1.0. U. S. Geological Survey.
Hwang, D.J. 1981. Beach Changes on Oahu as Revealed by Aerial Photographs. Hawaii Office of State Planning, Coastal Zone Management Program. 146 pp.
Makai Ocean Engineering Inc. and Sea Engineering Inc. 1991. Aerial Photograph Analysis of Coastal Erosion on the Islands of Kauai, Molokai, Lanai, Maui and Hawaii. Prepared for Office of State Planning: Honolulu HI. Coastal Zone Management Program. 199 pp.
Neumann, C.J. 1993. Global Overview - Chapter 1. In Global Guide to Tropical Cyclone Forecasting. Geneva, Switzerland: World Meteorological Organization. http://www.bom.gov.au/bmrc/pubs/tcguide/globa_guide_intro.htm
Sea Engineering Inc. 1988. Oahu Shoreline Study. Prepared for: City and County of Honolulu, Department of Land Utilization. 61 pp.
Related References
Coyne, M.A., C.H. Fletcher, and B.M. Richmond. 1999. "Mapping Coastal Erosion Hazard Areas in Hawaii: Observations and Errors." Journal of Coastal Research 28(Special Edition): 171-184.
Moberly, R., Jr., and T. Chamberlain. 1964. Hawaiian Beach Systems. Hawaii Institute of Geophysics Report 64-2. University of Hawaii, Honolulu. 95 pp.
