Bradley M. Romine

Historical Shoreline Change, Southeast Oahu, Hawaii; Applying Polynomial Models to Calculate Shoreline Change Rates

Abstract Here we present shoreline change rates for the beaches of southeast Oahu, Hawaii, calculated using recently developed polynomial methods to assist coastal managers in planning for erosion hazards and to provide an example for interpreting results from these new rate calculation methods. The polynomial methods use data from all transects (shoreline measurement locations) on a beach to calculate a rate at any one location along the beach. These methods utilize a polynomial to model alongshore variation in the rates. Models that are linear in time best characterize the trend of the entire time series of historical shorelines. Models that include acceleration (both increasing and decreasing) in their rates provide additional information about shoreline trends and indicate how rates vary with time. The ability to detect accelerating shoreline change is an important advance because beaches may not erode or accrete in a constant (linear) manner. Because they use all the data from a beach, polynomial models calculate rates with reduced uncertainty compared with the previously used single-transect method. An information criterion, a type of model optimization equation, identifies the best shoreline change model for a beach. Polynomial models that use eigenvectors as their basis functions are most often identified as the best shoreline change models.

A Summary of Historical Shoreline Changes on Beaches of Kauai, Oahu, and Maui, Hawaii

ABSTRACT Romine, B.M. and Fletcher, C.H., 2013. A summary of historical shoreline changes on beaches of Kauai, Oahu, and Maui, Hawaii. Shoreline change was measured along the beaches of Kauai, Oahu, and Maui (Hawaii) using historical shorelines digitized from aerial photographs and survey charts for the U.S. Geological Surveys National Assessment of Shoreline Change. To our knowledge, this is the most comprehensive report on shoreline change throughout Hawaii and supplements the limited data on beach changes in carbonate reef–dominated systems. Trends in long-term (early 1900s–present) and short-term (mid-1940s–present) shoreline change were calculated at regular intervals (20 m) along the shore using weighted linear regression. Erosion dominated the shoreline change in Hawaii, with 70% of beaches being erosional (long-term), including 9% (21 km) that was completely lost to erosion (e.g., seawalls), and an average shoreline change rate of −0.11 ± 0.01 m/y. Short-term results were somewhat less erosional (63% erosional, average change rate of −0.06 ± 0.01 m/y). Maui, Hawaii, beaches were the most erosional of the three islands with 85% of the beaches erosional, including 11% lost, and an average change rate of −0.17 ± 0.01 m/y. Seventy-one percent of Kauai, Hawaii, beaches were erosional, including 8% lost, with an average change rate of −0.11 ± 0.01 m/y. Most (60%) of the Oahu, Hawaii, beaches were erosional, including 8% lost, with an average change rate of −0.06 ± 0.01 m/y. Short-term results for Maui, Hawaii, and Oahu, Hawaii, were roughly the same as those found in the long term. Short-term analysis for Kauai, Hawaii, was less conclusive with an accretional average rate, but most of the beaches were erosional. Spatially, shoreline change is highly variable along the Hawaii beaches (length scales of hundreds of meters). Areas of chronic erosion were identified on all sides of the islands.

Vulnerability Assessment of Hawai'i's Cultural Assets Attributable to Erosion Using Shoreline Trend Analysis Techniques

Abstract KANE, H.H.; FLETCHER, C.H.; ROMINE, B.M.; ANDERSON, T.R.; FRAZER, N.L., and BARBEE, M.M., 2012. Vulnerability assessment of Hawai′i′s cultural assets attributable to erosion using shoreline trend analysis techniques. Hawai‘i’s beaches are a focal point of modern lifestyle as well as cultural tradition. Yet coastal erosion threatens areas that have served as burial grounds, home sites, and other forms of cultural significance. To improve understanding of the convergence of erosion patterns and cultural uses, we mapped shoreline changes from Kawela Bay to Kahuku Point on the capital island of O‘ahu. Shoreline change rates are calculated from historical photographs using the single-transect (ST) and eigenbeaches (EX) method to define the 50- and 100-year erosion hazard zones. To ensure that shoreline change rates reflect long-term trends, we include uncertainties attributable to natural shoreline fluctuations and mapping errors. A hazard zone overlay was compared to cultural data provided by the Hawaii State Historic Preservation Division (SHPD) and the Office of Hawaiian Affairs (OHA) to identify threats to cultural features. Cultural features identified in the study include iwi kupuna (burials), Hawaiian artifacts, and Punaulua (a freshwater spring). Our analysis indicates that, except for Punaulua, all cultural features identified are vulnerable to coastal erosion at historical rates. The data produced in this study may be used as a proactive management tool to rank the vulnerability to threatened cultural features, as well as to develop protocols to appropriately manage cultural assets.

Armoring on Eroding Coasts Leads to Beach Narrowing and Loss on Oahu, Hawaii

Coastal armoring (defined as any structure designed to prevent shoreline retreat that interacts with wave run-up at some point of the year) has, historically, been a typical response to managing the problem of beach erosion on the island of Oahu, Hawaii. By limiting the ability of an eroding shoreline to migrate landward, coastal armoring on Oahu has contributed to narrowing and complete loss of many kilometers of beach. In this paper, changes in beach width are analyzed along all armored and unarmored beaches on the island using historical shoreline positions mapped from orthorectified aerial photographs from as early as the late 1920s. Over the period of study, average beach width decreased by 11%±4% and nearly all (95%) documented beach loss was fronting armored coasts. Among armored beach sections, 72% of beaches are degraded, which includes 43% narrowed (28% significantly) and 29% (8.6km) completely lost to erosion. Beaches fronting coastal armoring narrowed by −36%±5% or −0.10±0.03m/year, on average. In comparison, beach widths along unarmored coasts were relatively stable with slightly more than half (53%) of beaches experiencing any form of degradation. East and south Oahu have the highest proportion of armored coast (35% and 39%, respectively) and experienced the greatest percent of complete beach loss (14% and 12%, respectively). West and north coasts, with relatively little armoring (10% and 12% armored, respectively), experienced little complete beach loss (2% and 6%, respectively). However, beaches are still significantly narrowed compared to historical patterns on west and north coasts (61% and 70%, respectively). We find at these sites that cultivation of coastal vegetation may be a factor in beach narrowing on Oahu, along with beach erosion. Increased ‘flanking’ erosion (accelerated shoreline retreat adjacent to armored sections) is documented at several beaches, often requiring extension of armoring structures to protect abutting coastal properties, a process that leads to alongshore seawall proliferation.

Bringing Sea-Level Rise into Long Range Planning Considerations on Maui, Hawaii

Maui’s coastal lands, along with many others worldwide, are under tremendous pressure from expanding development and accelerating coastal erosion. While it may be perceived by the public that the lands most at risk from sea-level rise are those immediately bordering the coastline, the threat to low-lying areas from a rising water table inland of the coast may also be great. Maui planning officials have begun to recognize that regardless of the uncertainty over projected rates of sealevel rise, threats associated with rising sea level should be identified and mitigated through a combination of modeling, mapping, and direct observation. This paper provides a review of current sea-level rise science and describes the scientific and management approaches being undertaken by Maui County to better understand potential risks associated with rising seas and account for these projections in long-range planning. INTRODUCTION In 2003, Maui County became the first county in the state of Hawaii to adopt a science-based approach to determining construction setbacks on coastal properties (Norcross-Nu’u and Abbott 2005). High-resolution annual erosion rate data spaced at

Modeling multiple sea level rise stresses reveals up to twice the land at risk compared to strictly passive flooding methods

Planning community resilience to sea level rise (SLR) requires information about where, when, and how SLR hazards will impact the coastal zone. We augment passive flood mapping (the so-called “bathtub” approach) by simulating physical processes posing recurrent threats to coastal infrastructure, communities, and ecosystems in Hawai‘i (including tidally-forced direct marine and groundwater flooding, seasonal wave inundation, and chronic coastal erosion). We find that the “bathtub” approach, alone, ignores 35–54 percent of the total land area exposed to one or more of these hazards, depending on location and SLR scenario. We conclude that modeling dynamic processes, including waves and erosion, is essential to robust SLR vulnerability assessment. Results also indicate that as sea level rises, coastal lands are exposed to higher flood depths and water velocities. The prevalence of low-lying coastal plains leads to a rapid increase in land exposure to hazards when sea level exceeds a critical elevation of ~0.3 or 0.6 m, depending on location. At ~1 m of SLR, land that is roughly seven times the total modern beach area is exposed to one or more hazards. Projected increases in extent, magnitude, and rate of persistent SLR impacts suggest an urgency to engage in long-term planning immediately.

Measuring Historical Shoreline Change, Applying New Polynomial Change Models: Southeast Oahu, Hawaii

Digital aerial photo mosaics and NOAA topographic survey charts (t-sheets) are used to map historical shoreline positions on southeast Oahu, Hawaii. The new PX (Polynomial in alongshore X) and PXT (Polynomial in X and Time) shoreline change rate methods are applied to calculate shoreline change rates from the time series of historical shoreline positions. These new methods utilize all historical shoreline data from a beach to calculate shoreline change rates and can find acceleration in the shoreline change rate with time. The methods are shown here and in previous works to produce more parsimonious models and more statistically significant and defensible rates than the previously used ST (Single-Transect) shoreline change rate calculation method. The ability to model acceleration in shoreline change rates with time provides insight into shoreline change processes, which was previously theoretical or observed in only small-scale studies. An overview of the methods is presented along with results from shoreline change analysis of four beach study sites on the southeast Oahu, Hawaii, shoreline.