In early November 2012 a shoreline survey was initiated to investigate changes in beaches along the Mana Coastal Plain of west Kauai. It is the intention of the core group of this study to continue surveys through 2013, at a minimum, and possibly over the next two to three years in order to gain a more thorough understanding of the dynamics of the beaches present along the coastal plain, especially the segment including the beaches present along the Pacific Missile Range Facility (PMRF) and the community of Kekaha. This report is provided as a summary of the project to date.
The dynamic beaches of Mana have long been the subject of great interest in terms of their character, origin and considerable variability; however, the present investigation was set in motion as the result of two things – 1) severe erosion of the beach fronting the community of Kekaha, beginning especially in 2011, and 2) considerable widening of beaches riming the PMRF portion of the coastal plain.
Primary Hypothesis and Approach
The beaches along the PMRF portion of the Mana Coastal Plain are “fat.” Kekaha Beach has essentially disappeared, with minimal prospect for its return in the near future. We would like to better understand the situation in order hopefully provide a favorable solution. The residents of Kekaha would love to have their beach back!
The hypothesis I have put forth for testing is that the carbonate sand that has comprised Kekaha Beach in the past has been derived from the north shore-wave-generated longshore movement of sand along Na Pali and Mana to Kekaha. The disappearance of Kekaha’s beach and buildup of PMRF’s beaches most likely is due to a couple winter seasons of weak North Pacific swells. Mana’s longshore currents have not been strong enough to drive the sand all the way to Kekaha. Additional factors include possible stronger than “normal” summer South Pacific swells, along with trade wind-generated currents, that may have generated south shore-wave-generated longshore currents that would have transported Kekaha beach sediment westward toward Mana. A final significant factor contributing to the disappearance of Kekaha Beach, and making the return of its sand less than probable, is the substantial lava rock revetment present along State Hwy 50 fronting the community of Kekaha.
The test we have designed consists of two main forms of observation 1) periodic, systematic beach profiling and 2) wave height monitoring. Six localities have been selected at which beach profiles are being constructed on a monthly basis. Along the PMRF shoreline, profiles are being made at Kinikini Ditch, Major’s Bay and Kokole Point. Along the Kekaha shoreline, profiles are being made at the beach near the Mana Drag Strip, at MacArthur Park and the small beach in from of St Thereas’ Church. Each day the predicted heights of waves approaching the island from the four main sides of the island (N, S, E, W) are being downloaded from the Kauai Ocean Report. That data is obtained from the National Weather Service and provided by kauaiexplorer.com on a daily basis.
Winter Beach Surveys (Nov 2012-Apr 2013)
The extensive arid low-relief Mana Coastal Plain of West Kauai exhibits the State of Hawaii’s longest sand beach, extending nearly continuously for over 16 miles from the mouth of the Waimea River to the dramatic sea cliffs of the Na Pali Coast. Most of the strandline, including Polihale Beach, the beaches that rim the Pacific Missile Range Facility (PMRF) and Kekaha Beach, is dominated by whitish to yellowish-orange carbonate sand. Only the southeastern portion, occupied by Waimea Beach, is composed of greenish-gray volcanic sand derived from the river that drains Waimea Canyon. Impacted by strong, but extremely variable wave-generated longshore currents, these beaches fluctuate dramatically in width and overall sand volume throughout the year. Our present investigation, initiated in early November 2012, intends primarily to document the relatively short-term, seasonal changes observable in the carbonate sand beaches of PMRF and Kekaha. Preliminary results of surveys through the winter of 2012-13 are presented below.
Background – The Geologic/Oceanographic Setting
Our investigation is anchored by the geologic/oceanographic model that has been developed, from previous extensive scientific investigations, for the sandy beaches of the Island of Kauai. The somewhat complex model, displayed below, encompasses Kauai’s geology and geologic history, nearshore topography and bathymetry, oceanographic processes (especially the island’s wave systems), and beach sediment composition. [A more comprehensive discussion of the model is provided in the most recent edition of Kauai’s Geologic History: A Simplified Overview recently released and now available on line.] The beaches of Kauai fall into three main categories: 1) North, east and south side pocket and back reef carbonate sand beaches, 2) west side carbonate sand beaches derived from the longshore transport of reef-derived sand, and 3) the volcanic sand beach of Waimea Beach, a product of longshore transport of sand derived from the river that drains Waimea Canyon.
The Beaches: Many of the Category I beaches occupy the heads of drowned river valleys that were cut during the several relatively recent, Pleistocene era, sea level lowstands associated with cycles of global glaciation. When sea levels were hundreds of feet lower, sometimes as much as 400 feet lower, during glaciation events every 100,000 to 125,000 years, windward (eastern side) rivers cut valleys as much as 2 to 3 miles farther seaward of the present shorelines. Subsequent sea level rise during interglacial periods, such as the present, flooded the river valleys to produce small indented bays, the heads of which are now occupied by carbonate sand beaches. In addition, fringing platform reefs, composed mainly of coralline algae and corals, developed along these shorelines, providing the skeletal material of which the beaches are composed as well as providing for the development and protection of beaches behind the reef platforms. As noted in the composite beach sediment composition histogram for Eastside Beaches, illustrated by the following diagram, Category I beaches commonly are composed of over 90% carbonate sand grains, most derived from the fragmentation, transport and deposition of the calcareous skeletons of the coralline red algae and coral that makes up Kauai’s eastside reefs. The relatively low concentration of volcanic rock fragment sand grains in Category I beaches, especially grains of the greenish mineral olivine, is a reflection of the mineraological character of the late stage Koloa Volcanics lava rocks that cover most of the eastern half of the island.
Category II beaches also are composed predominately of carbonate sand; however, they contain greater amounts of volcanic sand owing to the contribution of the Waimea Canyon Basalt shield lavas exposed all along the Na Pali coast. On the average the beaches contain nearly 12% olivine, a common constituent of the island’s main shield forming lavas. Beaches present discontinuously along the base of the sea cliffs of Na Pali and at a few major valley mouths, such as Hanakapiai and Kalalau, are relatively limited; however, those all along the Mana Coastal Plain are extensive. Perhaps the most significant characteristic of these Category II beaches is that they are a product mainly of the longshore transport of carbonate sand derived from the abrasion of the fringing reefs of the Haena area. Such sediment is transported all along Na Pali and the Mana Coastal Plain as far as the eastern extent of the beach fronting the community of Kekaha, a distance of as much as 25 miles. The Mana beaches extending for nearly 6 miles along a portion of the Pacific Missile Range Facility to the eastward extent of Kekaha Beach have been the targets of the investigation presented herein.
Category III of our classification of Kauai’s beaches is reserved mainly for one beach, Waimea Beach, which extends for nearly three miles from the mouth of Waimea River westward to Oomano Point at the eastern extend of Kekaha Beach. Waimea Beach is composed of 90% or more of greenish-gray sand derived from the erosion of lava rocks in Waimea Canyon and transported to the shore by Waimea River. From there the sand is moved principally westward along the shore to Oomano Point by longshore currents generated by ocean swells that approach the island from the south and the northeast trade winds that can move coastal waters toward the southwest. Significantly very little volcanic sand makes it around Oomano Point to mix with the carbonate sand of Kekaha Beach. Most of the volcanic sand being transported westward along Waimea Beach eventually moves offshore at Oomano Point and is lost to the island, in part moving down an old submerged river valley. The transition from the dominantly volcanic sand of Waimea Beach to the dominantly carbonate sand of Kekaha Beach is less than a quarter of a mile in length along the shoreline.
The Waves: A principal element of our model dealing with the character and origin of Kauai’s sandy beaches encompasses the ocean waves that impact the island’s shore year round. As illustrated in the following diagram, such waves are generated both by distant ocean weather as well as local winds. The largest waves that reach the Hawaiian Islands are those that are produced during the winter, principally the seven months of October through April, by North Pacific storms. Such storms generate large swells that move southward to impact mainly the northern and western shores of Kauai; although, they also have a minor influence along eastern shores. The winter swells generate huge waves and strong longshore currents that move large volumes of sand from the northwestern portion of the island southward and eastward along the Na Pali coast and around the Mana Coastal Plain, the location of the shoreline investigation being conducted at present.
Along Kauai’s south coast waves are generated principally in two ways. During summer months, the five months of May though September, ocean swells produced by South Pacific winter storms move northward to impact the island’s south shore. Such waves never reach the magnitude of the winter north shore waves; however, they do tend to generate longshore currents that move beach sediment westward and northward back along the south coast and the Mana Coastal Plain. A second south shore wave component is generated periodically during winter months by relatively local storms in the vicinity of the Hawaiian Archipelago – the Kona Storms. Such storms are relatively short lived, a couple of days or so, but can generate strong longshore currents that move a lot of sediment over a short period of time. During our upcoming summer beach surveys along the Mana Coastal Plain, we expect to see mainly the impact of the South Pacific winter storm swell waves and the longshore currents they generate.
A final principal wind and wave component of the Kauai beach model involves the waves and current generated by the northeast trade winds. Such winds are strongest and most persistent throughout the summer months; however, they do blow throughout most, as much as 75-90%, of the year. Such winds and the waves they generate impact mainly the east side beaches; however, they do generate minor to moderate longshore currents that wrap around the western parts of the island from the north and south.
Obviously, all of these wind and wave systems must be considered when attempting to understand the character and origin of sandy beaches surrounding the Island of Kauai. Different wave systems impact the various sides of the island throughout various times of the year. The following diagram displays plots of the average predicted wave heights for the four main sides of the island throughout the 2006, a typical year. The two curves plotted across each segment represent running averages of the daily height predictions. Note in particular how the north and south shores receive their largest waves in an alternating pattern. During winter months when the north and west shores are being pounded by huge waves, periodically as much as 25-30 feet or more in height, the south short is receiving waves typically of only a couple of feet in height. However, during summer months, when the south shore is being impacted periodically by 5 to 10 ft high waves, the north shore is essentially flat, commonly with waves of only 2 feet or less. Throughout the year the east shores receive highly variable trade wind-driven waves as much as 5 to 10 feet in height.
These wave systems are surprisingly regular over a yearly cycle. As can be seen in the following diagram showing plots of north shore waves over four years, 2004-2008, the winter waves are consistently high through the months of October to April and low through the summer. These of course are the waves that drive the longshore currents that move so much sediment along the Na Pali coast to the area of the extensive beaches along the Mana Coastal Plain.
West Kauai Shoreline Study Area
The following two Google Earth acquired images illustrate the shoreline area of interest in our current study – the beaches along the central portion of the Mana Coastal Plain. In the first image note the white arrows along the Na Pali and Mana coastlines. They represent the longshore movement of sand, especially during the winter months when the north shore swells generate the southerly and easterly moving coastal currents that move carbonate sand from Haena as far as 25 miles to Kekaha. The second image illustrates the locations (red dots) of our beach surveys where at least once each month we measure the width and topographic character of the sandy beach.
West Kauai Beach Profiles
The following six beach profile plots illustrate data collected at each of our study localities over the six months, November 2012 through April 2013. Profiles, with all measurements in meters of beach width and elevation, extend generally from the landward extent of each beach, across the backshore to the berm and down the foreshore of the beach to an estimate, in the field, of the intersection of ocean level with the sandy beach. Generally, location of the berm, the break in the topography of the beach between the backshore and foreshore, is relatively easy to delineate. The surf line, however, is almost always difficult to locate with any consistency owing mainly to variations in wave and surf height. We therefore prefer to use the berm location when comparing beach width from month to month in order to document seaward growth or landward retreat of the front of the beach. Each profile below displays (bold black arrow) the overall retreat or growth of the beach berm through the six months of our study to date.
Since it is obvious that the ocean waves that impact Kauai’s shoreline throughout the year are mainly responsible for the movement of sediment along the island’s beaches, we have also been monitoring them throughout out the time of our beach survey. For the west side beaches, the north and south shore waves have the greatest impact. As seen in the diagram below, through the winter months of November 2012 to April 2013 north shore waves have been dominant whereas those along the south have been minimal. Such information plays nicely into an overall interpretation of the beach survey data shown above.
Results to Date – The Winter Months
The following three diagrams summarize the results of the first six months of our investigation, through the 2012-13 winter months. The table of beach width changes, in meters, lists the maximum changes for both the position of the berm and surf (average estimated ocean level). The berm position is the indicator we prefer for monitoring the changes in beach width.
The following close-up Google Earth image of the study area displays maximum beach width changes, listed both in meters and feet. The data illustrates the obvious movement of sediment from the PMRF beaches southeastward toward Kekaha. Of greatest significance are the dramatic decrease in beach width of almost 300 feet at Majors Bay and the simultaneous increase in beach width of over 200 feet at MacArthur Park in Kekaha. The relatively minor changes in the beaches at Kokole Point and along the Mana Drag Strip suggest that this somewhat seaward-protruding portion of the coast is a bypass zone along which a lot of sediment moves but relatively little is removed or deposited. The little beach in from of St. Thresea’s Church/School receives almost no sand from either the Kekaha carbonate beach or Waimea volcanic sand beach, representing a sort of “null point” between the two beaches.
The final illustration provided below in this summary progress report incorporates both the north shore wave heights throughout the time of our beach surveys along with the changes in beach width from month to month. Again emphasized are the significant changes at Majors Bay and MacArthur Park. Obviously, during winter months when north shore waves are dominant in terms of the generation of sediment transporting longshore currents, most coastal sediment moving along the Mana Coastal Plain is transported southeastward from the area of PMRF to Kekaha. Sediment bypasses seaward protruding coastal zones and is removed from and/or deposited in the more landward indented portions. The magnitude of the changes in beach width observed at Majors Bay and MacArthur Park testify to the dynamics of the beaches along Kauai’s west coast.
Mana Coastal Plain Shoreline Survey, Kauai
The Future – The Summer Months
As our investigation moves into the summer months of May through September, we expect sand to move back to the west and north as the summer south shore swells kick in. Surveys over the coming months will continue to test the model.