Sea Level Rise : Hawaiʻi Sea Level Rise Viewer
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|Funding provided by the National Oceanic and Atmospheric Administration (NOAA) through their 2016 Regional Coastal Resilience Grants Program.|
The Hawaiʻi Sea Level Rise Viewer (Viewer) is intended to provide an online atlas to support the Hawaiʻi Sea Level Rise Vulnerability and Adaptation Report (Report) that was mandated by Act 83, Session Laws of Hawaiʻi (SLH) 2014 and Act 32, SLH 2017. Please visit the Hawaiʻi Climate Adaptation Portal website to view the full report and for more information on climate mitigation and adaptation.
The Viewer is intended to provide map data depicting projections for future hazard exposure and assessing economic and other vulnerabilities due to rising sea levels. Users may view and download map data for:
- Coastal hazard exposure areas with sea level rise including passive flooding (still water high tide flooding), annual high wave flooding (over wash during the largest wave events of the year), and coastal erosion.
- An aggregate of the above hazard layers into a combined Sea Level Rise Exposure Area (SLR-XA). It should be noted that for the islands of Lānaʻi, Molokaʻi, and Hawaiʻi, the SLR-XA represents only the passive flooding hazard due to the lack of historical data needed to model the other two hazards.
- Vulnerability assessment layers, including potential economic impacts to land and structures and threats to coastal highways from sea level rise and coastal hazards.
- Other base maps and overlays.
The Report was developed by Tetra Tech, Inc. and the State of Hawaiʻi Department of Land and Natural Resources (DLNR), Office of Conservation and Coastal Lands (OCCL). The Viewer was developed by the Pacific Islands Ocean Observing System (PacIOOS) at the University of Hawaiʻi School of Ocean and Earth Science and Technology (UH SOEST) through a collaborative project led by the University of Hawaiʻi Sea Grant College Program (Hawaiʻi Sea Grant) in partnership with DLNR and the State of Hawaiʻi Office of Planning. Coastal hazard exposure map data were developed for the Report and Viewer by the Coastal Geology Group at UH SOEST and adapted for display and vulnerability assessment by Tetra Tech, Inc. The SLR-XA and vulnerability assessment layers were developed for the Report and Viewer by Tetra Tech, Inc. Funding for the Viewer development was provided by the National Oceanic and Atmospheric Administration (NOAA) through their 2016 Regional Coastal Resilience Grants Program and the DLNR through Act 83, SLH (2014).
Sea Level Rise Projections For Modeling
Coastal hazards were modeled using global sea level rise projections from the 5th Assessment Report (AR5) of the Intergovernmental Panel on Climate Change (IPCC), the United Nations body of leading climate scientists and governmental representatives. The IPCC AR5 identified four sea level rise scenarios based on Representative Concentration Pathways (RCPs) for Greenhouse Gas (GHG) emissions (IPCC 2014). The IPCC AR5 “business-as-usual” GHG emissions scenario, RCP8.5, was used to model exposure to sea level rise. This scenario assumes GHG emissions continue to increase at their current rate and predicts as much as 3.2 feet of sea level rise by the year 2100.
Recent observations and projections suggest that 3 feet or more of sea level rise could occur earlier than 2100 and even as early as year 2060 under more recently published extreme scenarios (Sweet et al. 2017). As such, questions remain around the exact timing of that rise due largely to uncertainties around the future behavior of Earth’s cryosphere and global GHG emission trajectories. For this reason, the Report recommends that the magnitude and rate of sea level rise be tracked as new projections emerge and the State begin planning for 3.2 feet of sea level rise now. It is also important to recognize that global sea level rise will not stop at year 2100, but will likely continue for centuries.
Sea Level Rise Exposure Area (SLR-XA)
Modeling, using the best available data and methods, was conducted to determine the potential future exposure of each island to multiple coastal hazards as a result of sea level rise. Three chronic flooding hazards were modeled: a. passive flooding, b. annual high wave flooding, and c. coastal erosion (see descriptions of individual hazard layers, below). The footprint of these three hazards were combined to define the projected extent of chronic flooding due to sea level rise, called the sea level rise exposure area (SLR-XA). Flooding in the SLR-XA is associated with long-term, chronic hazards punctuated by annual or more frequent flooding events. Each of these hazards were modeled for four future sea level rise scenarios: 0.5 foot, 1.1 foot, 2.0 feet and 3.2 feet based on the upper end of the IPCC AR5 RCP8.5 sea level rise scenario.
Assumptions and Limitations: The assumptions and limitations described for the three chronic flooding hazards apply to the SLR-XA. Not all hazards were modeled for each island due to limited historical information and geospatial data. The SLR-XA for the islands of Hawaiʻi, Molokaʻi, and Lānaʻi is based on modeling passive flooding only. Additional studies would be needed to add the annual high wave flooding and coastal erosion to the SLR-XA for those islands.
The SLR-XA is an overlay of three hazards and does not account for interactive nature of these hazards as would be expected by natural processes. As with the individual exposure models, the SLR-XA maps hazard exposure on the present landscape. The modeling does not account for future (unknown) land use changes, including any adaptation measures. The SLR-XA also does not include impacts from less frequent high wave events (e.g., a 1-in-10 year event), storm surge, or tsunami.
Sea level rise:
a. Passive Flooding
Passive flooding was modeled by the Coastal Geology Group at UH SOEST using a modified “bathtub” approach following methods described in Cooper et al. 2013. The passive flooding model provides an initial assessment of low-lying areas susceptible to flooding by sea level rise. Passive flooding includes areas that are hydrologically connected to the ocean (marine flooding) and low-lying areas that are not hydrologically connected to the ocean (groundwater). Data used in modeling passive flooding include the global sea level rise projections discussed above in Sea Level Rise Projections for Modeling, digital elevation models (DEM), and the mean higher high water (MHHW) datum from local tide gauges. DEMs used in this study are freely available from NOAA and the U.S. Army Corps of Engineers (USACE). DEMs are derived from aerial light detection and ranging (LiDAR) data. The horizontal and vertical positional accuracies of the DEMs conform to flood hazard mapping standards of the Federal Emergency Management Agency (FEMA 2012). The IPCC AR5 RCP8.5 sea level rise scenario was used in modeling exposure to passive flooding from sea level rise at 0.5, 1.1, 2.0, and 3.2 feet.
Passive flooding was modeled using the DEMs in geographic information systems software to identify areas below a certain sea level height (flooded by sea level rise) when raising water levels above current Mean Higher High Water (MHHW) tidal datum. In other words, water levels are shown as they would appear during MHHW, or the average higher high water height of each tidal day. The area flooded was derived by subtracting a tidal surface model from the DEM.
Assumptions and Limitations: In many areas around the State, representing sea level rise from passive marine flooding will likely produce an underestimate of the area inundated or permanently submerged because the model does not account for waves and coastal erosion, important processes along Hawaiʻi’s highly dynamic coasts. For this reason, coastal erosion and annual high wave flooding are also modeled to provide a more comprehensive picture of the extent of hazard exposure with sea level rise.
The passive flooding model does not explicitly include flooding through storm drain systems and other underground infrastructure, which would contribute to flooding in many low-lying areas identified in the model. The DEMs used in the modeling depict a smoothed topographic surface and do not identify basements, parking garages, and other development below ground that would be affected by marine and groundwater flooding with sea level rise. As with the other models in the Report and Viewer, the passive flooding model is intended to provide an initial screening tool for sea level rise vulnerability. More detailed hydrologic and engineering modeling may be necessary to fully assess passive marine flooding hazards at the scale of individual properties.
Sea level rise:
b. Annual High Wave Flooding
Hawaiʻi is exposed to large waves annually on all open coasts due to our location in the Central North Pacific Ocean. The distance over which waves run-up and wash across the shoreline will increase with sea level rise. As water levels increase, less wave energy will be dissipated through breaking on nearshore reefs and waves will arrive at a higher elevation at the shoreline.
Computer model simulations of future annual high wave flooding were conducted by the Coastal Geology Group at UH SOEST using the XBeach (for eXtreme Beach behavior) numerical model developed by a consortium of research institutions. The model propagates the maximum annually recurring wave, calculated from offshore wave buoy data, over the reef and to the shore along one-dimensional (1D) cross-shore profiles extracted from a 1-meter DEM. Profiles are spaced 20 meters apart along the coast. This approach was used to model the transformation of the wave as it breaks across the reef and includes shallow water wave processes such as wave set-up and overtopping. The RCP8.5 scenario was used to model exposure to sea level rise of 0.5, 1.1, 2.0, and 3.2 feet.
Historical data used to model annual high wave flooding include hourly measurements of significant wave height, peak wave period, and peak wave direction, and was acquired from offshore wave buoy data from PacIOOS. Maximum surface elevation and depth of the annual high wave flooding is calculated from the mean of the five highest modeled water elevations at each model location along each profile. Output from the simulations is interpolated between transects and compiled in a 5-meter map grid. Depth grid cells with values less than 10 centimeters are not included in the impact assessment. This was done to remove very thin layers of water excursions that (1) are beyond the accuracy of the model and (2) might not constitute a significant impact to land and resources when only occurring once annually. Any low-lying flooded areas that are not connected to the ocean are also removed.
Assumptions and Limitations: Annual high wave flood modeling covered wave-exposed coasts with low-lying development on Maui, Oʻahu, and Kauaʻi. Annual high wave flooding was not available for the islands of Hawaiʻi, Molokaʻi, and Lānaʻi, nor for harbors or other back-reef areas throughout all the islands. Additional studies would be needed to add the annual high wave flooding for those areas. The maximum annually recurring wave parameters (significant wave height, period, direction) were statistically determined using historical wave climate records and do not include potential changes in future wave climate, the effects of storm surge, or less-frequent high wave events (e.g., a 1-in-10 year wave event). In some locations, the extent of flooding modeled was limited by the extent of the 1-meter DEM.
Changes in shoreline location due to coastal erosion are not included in this modeling. As shorelines retreat, annual high wave flooding will reach farther inland along retreating shorelines. Waves are propagated along a “bare earth” DEM which is void of shoreline structures, buildings, and vegetation, and waves are assumed to flow over an impermeable surface. The DEM represents a land surface at one particular time, and may not be representative of the beach shape during the season of most severe wave impact, particularly for highly variable north and west-exposed beaches.
Undesirable artifacts of 1D modeling include over-predicted flooding along some transects with deep, shore-perpendicular indentations in the sea bottom such as nearshore reef channels. The 1D modeling does not account for the presence of nearby shallow reef which refracts and dissipates some of the wave energy traveling through the channel toward the shore. Wave flooding modeling may be improved in future efforts by employing more complex and data-intensive 2D modeling and through local field experiments.
Sea level rise:
c. Coastal Erosion
Data source: UH SOEST Coastal Geology Group
Studies of historical shoreline change using aerial photographs and survey maps show that 70% of beaches on Kauaʻi, Oʻahu, and Maui are eroding (receding landward) (Fletcher et al. 2012). Beaches exist in a delicate balance between existing water levels, wave energy, and sand supply.
Coastal erosion modeling was conducted for sandy shorelines of Kauaʻi, Oʻahu, and Maui by the Coastal Geology Group at UH SOEST. The methods are described in Anderson et al. (2015) and combine historical shoreline change data with a model of beach profile response to sea level rise from Davidson-Arnott (2005) in order to estimate probabilities of future exposure to erosion at transects (shore-perpendicular measurement locations) spaced approximately 20 meters apart along the shoreline. The model accounts for localized alongshore variability in shoreline change by incorporating trends from the historical erosion mapping studies. The modeling is shown schematically in the following figure.
Historical data used to model coastal erosion consisted of: (1) historical shoreline positions and erosion rates measured from high-resolution (0.5 meters) ortho-rectified aerial photographs and NOAA topographic charts dating back to the early 1900s (Fletcher et al. 2012, Romine et al. 2013), and (2) beach profile field survey data (Gibbs et al. 2001, Fletcher et al. 2012). The vegetation line was identified in the most recent aerial photography dating from 2006 to 2008.
The output of the modeling is the estimated exposure zone to future erosion hazards. Based on the model and IPCC RCP8.5 sea level rise scenario, there is an 80% probability that land impacted by erosion would be confined within the exposure zone at that particular time. The exposure zones extend landward from the current-day shoreline (vegetation line) up to the 80% cumulative probability contour for each of the four sea level rise scenarios, which incorporates the uncertainty (upper and lower bounds) of the IPCC RCP8.5 sea level rise projection.
Assumptions and Limitations: Historical shoreline change data and beach profiles needed to model coastal erosion are available only for sandy shores of Kauaʻi, Oʻahu, and Maui. Exposure was not modeled for less-erodible rocky coasts and bluffs, though the latter can be prone to sudden failure in some areas. In addition, modeling did not account for:
- Existing seawalls or other coastal armoring in the backshore;
- Increasing wave energy across the fringing reef with sea level rise;
- Possible changes in reef accretion and nearshore sediment processes with sea level rise; and
- Possible changes to sediment supply from future shoreline development and engineering, such as construction or removal of coastal armoring or other coastal engineering.
Sea level rise:
Potential Economic Loss
Data source: Tetra Tech, Inc.
Vulnerability was assessed for the main Hawaiian Islands using the outputs of the coastal hazard exposure modeling. Potential economic loss was based on the value of the land and structures from the county tax parcel database permanently lost in the SLR-XA for each projected sea level rise height.
Potential economic loss was analyzed individually for each hazard (passive flooding, annual high wave flooding, or coastal erosion) at the parcel level and subsequently aggregated in 1-hectare (100 square meter or 1,076 square foot) grids. For the islands of Hawaiʻi, Lānaʻi, and Molokaʻi, the potential economic loss was based solely on passive flooding. Potential economic loss in the SLR-XA area was determined from the highest loss value of any one hazard within the 1-hectare grid, thus avoiding double counting a loss of a particular asset from multiple hazards. Those maximum values for each sector are then summed to determine the total economic loss to property in each grid.
Assumptions and Limitations: The vulnerability assessment addressed exposure to chronic flooding with sea level rise. Key assumptions of the economic analysis for the SLR-XA included: (a) loss is permanent; (b) economic loss is based on the value in U.S. dollars in 2016 as property values in the future are unknown; (c) economic loss is based on the value of the land and structures exposed to flooding in the SLR-XA excluding the contents of the property and does not include the economic loss or cost to replace roads, water conveyance systems and other critical infrastructure; and (d) no adaptation measures are put in place that could reduce impacts in the SLR-XA.
Economic value data were not available for length of roads, water and wastewater lines, and other public infrastructure due to the variable cost of such infrastructure depending on location, and the complexity and uncertainty involved in design, siting, and construction. Additionally, environmental assets such as beaches and wetlands were not assessed economically due to the complexity in valuing ecosystem services. The loss of both public infrastructure and environmental assets from flooding would result in significant economic loss. Therefore, the total potential economic loss figures estimated in the Report and Viewer are likely an underestimate.
Sea level rise:
Data source: Tetra Tech, Inc.
Potential impacts to roads were assessed in terms of exposure to chronic flooding in the SLR-XA, but were not monetized. The SLR-XA is overlaid with these assets to determine locations impacted by chronic flooding with sea level rise.
Sea level rise:
Community Plan Areas
Community planning district boundaries for the islands of Kauaʻi, Oʻahu, Maui, Molokaʻi, Lānaʻi, Kahoʻolawe, and Hawaiʻi Island are provided by the State of Hawaiʻi Office of Planning. Community plans are required by the county charters and adopted by ordinance through the county councils and must be updated at regular intervals. The plans, developed through the planning departments of each county, are intended to provide vision, guidelines, and implementing policies for each area. Community plans can provide an important existing framework for addressing sea level rise vulnerability because the plans provide recommendations concerning land use, density and design, transportation, community facilities, infrastructure, visitor accommodations, commercial and residential areas and other matters related to development that are specific to the region of the plan.
Flood Hazard Zones
Flood hazard zones for the State of Hawaiʻi are established by the U.S. Federal Emergency Management Agency (FEMA) through the Flood Insurance Rate Maps (FIRM). Areas that fall within the 100-year flood boundary (a.k.a. base flood or floodplain) are called Special Flood Hazard Areas (SFHA) and are divided into insurance risk zones A, AE, AH, AO, or VE. The term 100-year flood indicates that the area has a 1% chance of flooding in any given year. Zones X500 and X Levee are Non-Special Flood Hazard Areas (NSFHA) and have moderate-to-low flood risk. The FIRMS are used by the National Flood Insurance Program (NFIP) for floodplain management, mitigation, and insurance purposes. The FIRMs are based on hydraulic modeling of present day flood risk and do not include future increases in flood hazards with sea level rise. See also: State of Hawaiʻi, Department of Land and Natural Resources (DLNR), Flood Hazard Assessment Tool (FHAT).
Beaches and Sand (USDA)
The beaches and sand map layer serves as a useful guide to understanding the physical setting of coastal areas around the State and how these areas may be affected by increased flooding and erosion with sea level rise. This layer is adapted from the Gridded USDA-NRCS Soil Survey Geographic (gSSURGO) database. For the purposes of the Viewer, we have highlighted beaches and sand, including dunes and other areas with sandy substrates. These areas may be more susceptible to coastal erosion. However, it should be noted that beach environments may be sustained if they are allowed to migrate landward and erode into upland deposits of beach and dune sand, releasing this sediment into the littoral system.
The geology map layer serves as a useful guide to understanding the physical setting of coastal areas around the State and how these areas may be affected by increased flooding and erosion with sea level rise. For the purposes of the Viewer, we have categorized the geology into beach and dune deposits, marine and lagoon deposits, alluvium deposits, and volcanic deposits. Volcanic and marine limestone deposits may be more resistant to coastal erosion. In contrast, deposits of sand and alluvium may be more susceptible to coastal erosion. However, it should be noted that beach environments may be sustained if they are allowed to migrate landward and erode into upland deposits of beach and dune sand, releasing this sediment into the littoral system.
State Land Use Districts
Pursuant to Hawaiʻi Revised Statutes §205, all lands in the State of Hawaiʻi are classified and placed into four land use districts: Urban, Rural, Agricultural, and Conservation. This land use district map layer serves as a useful guide in determining potential impacts of sea level rise to the various land use classifications within a community. The boundaries depicted here are not official and for presentation purposes only. A determination of the official State Land Use District Boundaries should be obtained directly through the State Land Use Commission (LUC).
Publication Date: December 22, 2017