Coastal Subsidence Study 2005 - Laserfiche WebLink

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This study commenced in July of 2005 to help address the many shoreline and hazard issues associated with the Kapoho area. This report helps to determine the extent of inundation and provide suggested solutions, alternatives and options. This is not an easy task, since the area is at high risk from natural hazards, which is compounded by the issue of subsidence. In addition, many homeowners and landowners have invested much time and money into their property. Many are attached to the property, both financially and emotionally. A major task of this study was to determine the magnitude of the subsidence problem so that options and alternatives could be created that are related to the dynamics of the area. Thus it was very important to determine if he subsidence at Kapoho was simply episodic such as occurred during the 1975 Kalapana event, or if there is a possibility that the subsidence is continuous and episodic. The later would be significantly more difficult to plan for. Prior to this study, there was some 2 evidence of on going subsidence in a letter from the Hawaii Volcano Observatory and in documents from the Hawaii County Planning Department. However, the information was not sufficiently specific in terms of the methodology and location to determine if on going subsidence was applicable to the study area. Thus planning decisions based on this evidence for the Kapoho Beach Lots and Vacationland area could not be made. During the spring of 2006, a six month extension to the study contract was granted so that the data collection from satellite measurements to determine the extent, level, or magnitude of subsidence could be further evaluated. This would allow the period of study to be extended from 26 months to 37 months (satellite measurements from February 2003 to March 2006). With the extended period of study, a greater level of confidence in the measurements was allowed, as well as providing insight into any subsidence trends, or temporal variability. This report is divided into three parts to meet the three main objectives of the study. Each of these objectives are covered in a specific chapter of this report. In Chapter 2 and Appendix A, the subsidence was measured by satellite and analyzed to determine if the problem is episodic only, or episodic and continuous. Specifically, 2 September 17, 2001 Letter from Don Swanson of the United States Geological Observatory to Don Swanson of the Hawaii County Planning Department. Based on leveling surveys along Highway 137 in 1976, 1986, 1987, 1989, and 1995, the Highway dropped about 0.4-0.5 inches each year relative to Hilo, totaling 8.25 inches between 1976 and 1995. When combined with relative sea-level rise at Hilo of .16 inches per year, the relative sea-level rise for the area near the highway should be .56-.66 inches per year. Thus total subsidence along the highway was about 13-16 inches in the 24 year period after 1976. In the letter, there is a question if the measurements along the highway are representative of the coastline. Although no firm answer is provided, it is stated that the measurements are probably reasonable estimates of the shoreline as well. Since a definite answer is not provided for the shoreline area, it was a major point of this study to determine if the coastal area is also subsiding on an ongoing basis. This was confirmed in this report (See report and Appendix A). 6