DYNAMICS OF SANDY SHORELINES IN MAUI , HAWAII : CONSEQUENCES AND CAUSES

Beaches serve as important recreational, cultural, and ecological resources, and as an indispensable economic asset. The dramatic degradation of sandy beaches over the last several decades has become widely recognized as a serious problem in the Hawaiian Islands. A key component of Maui County's beach preservation strategy is the quantification of site-specific erosion hazards. The study reported here investigates shoreline change to provide erosion hazard rates for all significant sandy shoreline on the island, which will serve as the basis for improved regulations governing siting of coastal construction. Horizontal movement of the landward and seaward boundaries of the beach from orthorectified aerial photographs and topographic surveys (T-sheets) is used to develop a multidecadal database of shoreline movement every 20 m along the coast of the island of Maui. Annual erosion hazard rates (AEHRs), calculated using a reweighted least squares regression and smoothing routine, average -0.26 m y. Island wide changes in beachwidth show a 26% decrease. Erosion rates on Maui's north shore are double those on the western and southwestern sides of the island. Although experiencing erosion rates twice as large, beachwidths on the relatively undeveloped north shore have decreased half as much as those on the western side of the island. In-depth studies of two sites along Maui's coast suggest that interannual to century scale shoreline sediment dynamics are strongly influenced by Pacific Decadal Oscillation and El Niño/Southern Oscillation related storm variability. Human impacts and other factors are likely to be important as well. INTRODUCTION Sand beaches are a common feature of many coastlines in the Hawaiian Islands. They are of vital importance to the state of Hawaii, serving as a key attraction for the visitor industry, which provides more that 60% of all jobs and brings in several times more income than from all other sources combined. Their prevalence is perhaps part of the reason why they have not been widely recognized as a vital resource until recently. This lack of recognition has contributed to the use of shoreline hardening as the management alternative of choice for mitigating erosion problems. Most of the 6 km of beach that has been lost on Maui is in front of coastal armoring. Recognizing the importance of their beaches, and aware of the serious loss of this resource, Maui County has taken the lead within the state in implementing measures designed to prevent further loss and sustainably manage their remaining beach resources. Although other management options such as beach renourishment may be feasible for some areas, the high and recurring costs involved and lack of identified suitable sand sources and the uncertainty of funding, at present preclude this approach in most cases (Bodge 1998, 2000). A cost1) Department of Geology, University of Hawaii, 1680 East-West Rd., Honolulu, HI. 96822, USA. jrooney@hawaii.edu, fletcher@hawaii.edu, mbarbee@hawaii.edu, eversole@soest.hawaii.edu, siangl@hawaii.edu 2) U.S. Geological Survey, Pacific Science Center, UC Santa Cruz, 1156 High St., Santa Cruz, CA 95064, USA. brichmond@usgs.gov, agibbs@octopus.wr. usgs.gov effective way for Maui and other counties to protect many of their beaches is with a policy of adaptation and avoidance, altering development patterns to allow natural erosion/accretion cycles to continue without interference. Such a policy requires that erosion hazard zones are identified so that human activities there may be modified to avoid future damage to the beach as well as to reduce homeowner hazard expense (Bay and Bay, 1996; Fletcher, 1998). Accordingly, the Maui Planning Department (MPD) commissioned the present study to quantify historical shoreline change and identify erosion hazard areas. Physical Setting Sandy beaches on Maui are composed of variable percentages of coralline algae, foraminifera, coral, mollusc and echinoderm fragments, with volcanic grains generally contributing a minor fraction to the total volume (Rooney, 2002). The largest reservoirs of beach sand, in Maui and other Hawaiian Islands, are typically found on the coastal plains, where they were deposited during a period of (~2 m) higher than present sea level approximately 2,000 to 4,000 years ago (Calhoun and Fletcher, 1996; Grossman and Fletcher, 1998; Harney et al., 1999). Hawaiian beaches are the exposed and eroding edge of these coastal plain deposits. Beach dynamics dominated by longshore transport characterize Hawaiian littoral systems (Calhoun et al., 2002; Eversole, 2002; Norcross et al., 2002; Rooney, 2002). Although the amount of sand released by erosion from coastal plain sediments to the beaches is relatively small compared to annual longshore sediment fluxes, it is the primary source of material for maintaining long-term sediment budgets on Hawaiian beaches (Harney et al., 1999, Rooney and Fletcher, 2000). Hawaiian littoral sediment dynamics are largely driven by four types of seasonal waves (Figure 1). These include North Pacific swell, originating with storms in the North Pacific, generally