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.

DISCLAIMER (updated November 27, 2018): Based on the methodology of sea level rise modeling used in the Hawaiʻi Sea Level Rise Vulnerability and Adaptation Report (Report) and the Hawaiʻi Sea Level Rise Viewer (Viewer), having gone through peer review and publication in the Nature journal Scientific Reports, the results of this study are sufficiently validated to be appropriately used in land management decisions as the best available information as of the date of publication of the Report, December 2017, consistent with the intent of Act 83 SLH 2014 as amended. This Report is intended to provide a state-wide assessment of Hawaiʻi’s vulnerability to sea level rise. The location of projected impacts and economic costs from damages are estimates based on a particular sea level rise scenario. The hazard and vulnerability data and maps provided herein are based on observational data and computer-based models as described in the Report and in published research (Anderson et al., 2018). As with all models, it is important to understand the methods, assumptions, limitations, and uncertainties of the methods used. The risks associated with use or non-use of the results are assumed by the user. At its November 27th, 2018 public meeting, the Hawaiʻi Climate Change Mitigation and Adaptation Commission agreed to replace the original Report Disclaimer with an updated Disclaimer, adapted here for the Viewer. The Commission’s decision on this matter is detailed in meeting minutes from November 27, 2018.

Overview

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 Global Mean Sea Level (GMSL) rise by the year 2100 (Figure 1).

Recent observations and projections suggest that 3 feet or more of GMSL rise could occur earlier than 2100 and even as early as the 2060s under more recently published extreme scenarios (Figure 2, Sweet et al. 2017). As such, questions remain around the exact timing of that rise due largely to uncertainties around global GHG emission trajectories and the future behavior of Earth’s cryosphere. 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.

Online mapping tools depicting passive sea level rise flooding impacts beyond 3.2 feet of GMSL rise are available including at the NOAA Sea Level Rise Viewer and Climate Central Surging Seas Risk Finder.

Exposure

Sea Level Rise Exposure Area (SLR-XA) Data source: Tetra Tech, Inc. combining hazard exposure data layers from the UH SOEST Coastal Geology Group 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, Figure 3). 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 GMSL rise scenario. Assumptions and Limitations : The assumptions and limitations described below 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. In addition, mapping errors may be found in some areas due to clipping (subsetting) of the original map layers using a shoreline (Special Management Area) boundary and possible modeling errors in the Annual High Wave Flooding model at reef and harbor channels (Figure 4). See descriptions of the individual Sea Level Rise Exposure models below for more information. Data access: Shapefile, GeoJSON, WMS-C (layer: “hi_tt_all_slrxa_2030”), WMS, WFS, KML, metadata Sea level rise: 0.5 ft 1.1 ft 2.0 ft 3.2 ft Back to Top

a. Passive Flooding Data source: UH SOEST Coastal Geology Group. Adapted for display and vulnerability assessment by Tetra Tech, Inc. Passive flooding was modeled by the Coastal Geology Group at UH SOEST using a modified “bathtub” approach following methods described in Anderson et al. 2018. 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 GMSL 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 Hawaii’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. More detailed hydrologic and engineering modeling may be necessary to fully assess passive marine flooding hazards at the scale of individual properties. Mapping errors may be found in some areas due to clipping (subsetting) of the original map layers using a shoreline (Special Management Area) boundary (see Figure 4, above). Data access: Shapefile, GeoJSON, WMS-C (layer: “hi_hcgg_all_slr_flood_2030”), WMS, WFS, KML, metadata Sea level rise: 0.5 ft 1.1 ft 2.0 ft 3.2 ft Back to Top

b. Annual High Wave Flooding Data source: UH SOEST Coastal Geology Group. Adapted for display and vulnerability assessment by Tetra Tech, Inc. 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 methods described in Anderson et al. 2018. 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 some harbors or other back-bay 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 (see Figure 4, above). 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. In addition, mapping errors may be found in some areas due to clipping (subsetting) of the original map layers using a shoreline (Special Management Area) boundary (see Figure 4, above). Data access: Shapefile, GeoJSON, WMS-C (layer: “hi_hcgg_all_wave_flood_2030”), WMS, WFS, KML, metadata Sea level rise: 0.5 ft 1.1 ft 2.0 ft 3.2 ft Back to Top

Vulnerability

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. Data access: Shapefile, GeoJSON, WMS-C (layer: “hi_tt_all_slrxa_econ_2030”), WMS, WFS, KML, metadata Sea level rise: 0.5 ft 1.1 ft 2.0 ft 3.2 ft Back to Top

Other Overlays

Community Plan Areas Data source: Hawaiʻi Office of Planning Statewide GIS Data Program 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. Data access: Shapefile , ArcGIS REST (layer: “Development Plan Areas”), WMS (layer: “13”), WFS (layer: “ParcelsZoning:Development_Plan_Areas”), metadata Back to Top

Publication Date: December 22, 2017

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