Climate Viewer Documentation

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Sea Level Rise Scenarios

Sea Level Rise in Hawaiʻi: Present and Long-Term Impacts

Sea level rise (SLR) is a pressing global issue, and its impacts are particularly pronounced in Hawaiʻi, where the delicate balance between land and sea is threatened by the rising waters. This phenomenon is primarily driven by climate change, specifically the melting of polar ice caps and the expansion of seawater as it warms. In the case of Hawaiʻi, a state composed of islands, the implications of sea level rise are profound, affecting not only the natural environment but also the socio-economic landscape of the region.

The National Aeronautical and Space Administration (NASA), as a prominent space agency, plays a crucial role in monitoring and studying earth’s climate, including sea level rise. Their assessments contribute valuable data to our understanding of the phenomenon, providing insights that complement global estimates. NASA provides Intermediate and Intermediate High projection scenarios that offer additional granularity to the broader picture of sea level rise.

NASA’s Intermediate Scenario:

NASA’s Intermediatescenario is based on a combination of satellite observations, climate models, and historical data. According to these projections, the global mean sea level is expected to rise within a range that aligns with the mid-point of other global estimates, like the Intergovernmental Panel on Climate Change (IPCC). The Intermediate scenario accounts for various factors contributing to sea level rise, including the melting of glaciers, ice sheets, and the expansion of seawater as it warms.

In the context of Hawaiʻi, these Intermediate estimates suggest that the islands will continue to experience a rise in sea levels 16% – 20% higher than the global average (Sweet et al., 2022). This information is valuable for regional planning and adaptation efforts, providing a more nuanced understanding of the challenges that Hawaiʻi faces in the coming decades.

NASA’s Intermediate High Scenario:

NASA’s Intermediateigh scenario projects a scenario where the rate of sea level rise is at the higher end of the spectrum. This projection takes into account potential accelerations in the melting of polar ice and increased warming of the oceans. While this scenario is considered less likely, it is crucial for planners and policymakers to incorporate such possibilities into their long-term strategies to ensure resilience of critical infrastructure in the face of uncertainty.

For Hawaiʻi, the Intermediate High scenario implies a more pronounced impact, with sea levels rising faster than the global average. This heightened rate of sea level rise would amplify the challenges faced by coastal communities, necessitating more robust and immediate adaptation measures. The vulnerability of low-lying coastal areas, critical infrastructure, and ecosystems would be exacerbated, underscoring the importance of proactive planning and mitigation strategies.

Ongoing monitoring and research efforts, such as the Gravity Recovery and Climate Experiment (GRACE) and the Ice, Cloud, and land Elevation Satellite (ICESat), provide invaluable data for refining sea level rise projections. These technologies enable scientists to track changes in ice mass and sea level with unprecedented accuracy, contributing to more reliable projections.

NASA’s Intermediate and Intermediate High scenarios offer a nuanced perspective on sea level rise, aiding both the global scientific community and regional stakeholders like Hawaiʻi in understanding the potential range of future scenarios. Incorporating these projections into local planning efforts is essential to ensure that the islands are prepared for the varying degrees of sea level rise and can implement adaptive strategies to protect their communities, ecosystems, and infrastructure.

Present Sea Level Rise in Hawaiʻi:

Hawaiʻi is already experiencing the effects of rising sea levels, with the current rate estimated to be around 3.2 millimeters per year. This may seem insignificant at first glance, but the cumulative impact is substantial, particularly in low-lying coastal areas. The primary drivers of this rise are the global increase in temperature and the associated thermal expansion of seawater. Coastal erosion, saltwater intrusion into freshwater resources, and damage to coastal infrastructure are some of the immediate consequences faced by Hawaiian communities.

Global Estimates and Projections:

Various global estimates contribute to our understanding of sea level rise, and organizations like the IPCC, NASA, and the National Ocean and Atmospheric Administration (NOAA) provide comprehensive assessments. According to the IPCC’s latest report in 2022, the global mean sea level is expected to rise between 0.26 to 0.77 meters by 2100, depending on various emission scenarios. These estimates are based on the best available scientific data and models that take into account factors such as glacial melt, thermal expansion, and changes in land ice.

Regional Variability in Sea Level Rise:

While global estimates offer a broad perspective, regional variability plays a crucial role in understanding the specific impacts on a local scale. In the case of Hawaiʻi, the Pacific Islands are exposed to different oceanographic and climatic conditions, leading to variations in sea level rise. Some studies suggest that the Hawaiian Islands may experience a slightly higher rate of sea level rise compared to the global average due to factors such as ocean currents and gravitational effects from melting ice.

Impacts on Hawaiian Ecosystems:

The rising sea levels pose a significant threat to Hawaiʻi’s diverse ecosystems. Coastal habitats, including coral reefs, wetlands, and mangroves are particularly vulnerable. Coral reefs, vital for the islands’ biodiversity, food security and tourism, face bleaching and degradation as they struggle to adapt to changing conditions. Beaches and mangroves, acting as natural buffers and sand storage against episodic events like storms and hurricanes, may face increased stress, impacting their ability to protect coastal areas from erosion and flooding.

Socio-Economic Consequences:

The socio-economic consequences of sea level rise in Hawaiʻi are multifaceted. Tourism, a cornerstone of the Hawaiʻi economy, could be adversely affected as iconic beachfronts disappear and recreational areas are compromised. Furthermore, the loss of arable land due to saltwater intrusion and erosion could impact agriculture, challenging food security on the islands. Additionally, the increased frequency and intensity of storms associated with sea level rise may result in more frequent damage to infrastructure, disrupting daily life and straining resources for disaster response and recovery.

Adaptation and Mitigation Efforts:

To address the challenges posed by sea level rise, Hawaiʻi is actively engaged in adaptation and mitigation efforts. These include coastal protection measures such as seawalls and beach nourishment, managed retreat, sustainable land-use planning, and community outreach programs to raise awareness and promote resilience. The state is also investing in renewable energy sources to mitigate the broader impacts of climate change and reduce its carbon footprint with the goals of net-negative carbon production and achieving 100% power generation from renewable energy by 2045.

Sea level rise in Hawaiʻi is a complex and multifaceted issue with present and long-term consequences. The global and regional estimates provide valuable insights into the scale of the challenge, emphasizing the need for immediate action to mitigate the impacts and adapt to the changing conditions. The integration of scientific research, community engagement, and policy initiatives is essential to ensuring the resilience and sustainability of Hawaiʻi in the face of rising sea levels.

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The Local Tidal Datum – MHHW

Honolulu Tide Gauge:

The Honolulu Tide Gauge serves as a critical tool in monitoring sea level changes around the Hawaiian Islands, providing localized data that complements global assessments. Located at the Honolulu Harbor, this tide gauge records tidal fluctuations and long-term sea level trends, offering valuable insights into the specific conditions faced by Oahu and the broader Hawaiian archipelago. Operated by the University of Hawaiʻi Sea Level Center in collaboration with the NOAA, the gauge provides real-time data that contributes to our understanding of sea level rise in the region.

Contributions to Understanding Sea Level Rise:

The Honolulu Tide Gauge plays a pivotal role in validating and refining sea level rise projections, offering a high-resolution dataset specific to Hawaiʻi. Researchers utilize this data to assess local factors influencing sea level, such as ocean currents, episodic water levels, tides, and geological processes. The gauge enhances the accuracy of global and regional estimates, enabling policymakers and scientists to tailor adaptation and mitigation strategies to the unique conditions of Hawaiʻi.

Future Projections:

Based on the data collected from the Honolulu Tide Gauge, future projections for sea level rise in Honolulu and the surrounding areas are derived in 1 foot increments. These projections take into account a combination of factors, including global climate models, local geological conditions, and historical sea level trends. While the global estimates provide a broad framework, the localized data from the tide gauge refines projections, offering a more precise understanding of how sea level rise will impact Hawaiʻi in the coming decades.

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The CRC sea level rise impacts viewer

This viewer shows results of flood modeling (ʻExposureʻ) from a variety of sources in the legend on the right side of the window. By default, the “Passive Flooding” layers “Marine Connected Flooding” and “Low-lying Areas” are toggled on. Each section can be minimized by clicking the horizontal arrow head next to the section name.

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Passive Marine and Low-lying flooding

Passive flooding was modeled by the Climate Resilience Collaborative at the University of Hawaiʻi at Mānoa School of Ocean and Earth Science and Technology. Using a modified “bathtub” approach following methods described in Anderson et al. 2018, passive flooding describes using color to indicate potential water depth, those portions of a digital elevation model (DEM) equal to or less than the sea level scenario at MHHW tidal datum. 

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 areas that are not directly hydrologically connected to the ocean (low lying)*. Data used in modeling passive flooding include the DEM at 2 meter resolution and the MHHW datum from local tide gauges. DEMs used in this study are modified from freely available sources like the NOAA, U.S. Army Corps of Engineers (USACE) and U.S. Geological Survey (USGS). DEMs are derived from aerial based light detection and ranging (LiDAR) data. Edits are made to correct streams and canals that were mischaracterized in the original data. The horizontal and vertical positional accuracies of the DEMs conform to flood hazard mapping standards of the Federal Emergency Management Agency (FEMA 2012). Passive flooding is modeled at 0 – 10 ft sea level rise scenarios. Please refer to SLR scenario documentation to apply an appropriate timeline.

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 a tidal datum surface. 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.

Note that 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 (see drainage backflow and storm drain structure failure layers and descriptions). The DEMs used in the modeling depict a smoothed topographic surface and do not identify basements, parking garages, and other development or infrastructure below ground that would be affected by marine and groundwater flooding associated with sea level rise. 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.

*Low lying areas may be indirectly connected to the ocean through soils and sediments.

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Groundwater Inundation

Groundwater Inundation (GWI) refers to flooding that occurs as groundwater is lifted above the elevation of the ground surface and/or buried infrastructure. GWI is one of the more difficult flood mechanisms to manage owing to its ability to evade coastal defenses designed to mitigate direct marine flooding (e.g., seawalls, revetments, and other methods of shoreline hardening). Simulations of groundwater levels within the Koʻolaupoko Moku makai of the 10m elevation contour were produced using a 3D numerical model (MODFLOW). The methodology for model construction was based on Habel et al. (2017) and was further expanded in accordance with Habel et al. (2020) and by incorporating aquifer drain modeling in alignment with Whittier et al. (2009). Calibration included comparing the model’s outputs with 86 discrete water-level observations from the Hawai’i Department of Health Leaky Underground Storage Tank records, five discrete water level observations from Ghazal et al. (2023), and five sets of continuous monitoring data obtained as part of ongoing monitoring efforts in the Ko‘olaupoko Moku region. Following calibration, the simulated mean residual water level and root-mean-squared error were 0.09 m and 0.30 m, respectively. Steady-state groundwater levels were simulated, considering one foot increments of sea level rise up to 10 ft. Areas vulnerable to GWI were identified through a comparison of simulated water table elevations to a DEM. Flood depths were calculated where the simulated water table exceeds ground surface, and depths to groundwater where the opposite occurred. The total vertical error in GWI simulation, considering LiDAR and calibrated MODFLOW errors, was measured at 0.42 meters.

Figure 5. Schematic diagram showing passive marine and groundwater flooding from current sea level (blue) to future sea level (red) (adapted from Rotzoll and Fletcher 2012).

Note that in certain wetland areas featuring elevated groundwater levels, an increase in sea level may counterintuitively lead to a decline in groundwater levels. This phenomenon is attributed to heightened exposure to open ocean waters and increased opportunities for drainage.

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Emergent and Shallow Groundwater

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Drainage Backflow

This layer shows flooding from passive marine flooding coupled with storm drain locations that are at or below the passive SLR scenario. The layer reveals the potential impact of urban coastal drainage infrastructure on the extent of passive SLR flooding.

Storm-drain backflow is similar to direct marine flooding, as both involve flood waters originating from the ocean. However, storm-drain backflow specifically occurs due to the presence of gravity-flow drainage networks, which are commonly used in coastal cities globally. These drainage systems rely on differences in elevation between the drainage and outflow areas (ocean waters). In coastal regions with lower elevations, high tides can decrease these elevation differences, leading to a potential slowdown or even reversal of drainage.

Methods:

Areas prone to flooding due to storm-drain backflow were identified using a modified version of the bathtub approach. This method was adapted to exclude flooded regions lacking direct surface connections to drainage systems. Exclusions were implemented for flood areas not interfacing with drainage infrastructure, based on the assumption that such areas wouldn’t experience drainage-facilitated flooding. To identify locations where drainage infrastructure facilitates water flow from the marine environment, we use geospatial data that characterize drainage inlet locations. These datasets, endorsed by the responsible agency, have high confidence in the spatial accuracy of the data such that it is extensively used for planning, management, and operational decision-making.

Note that groundwater contributions were not considered in the simulation of storm-drain backflow. Additionally, dynamic effects, such as variations in flow rate due to conduit radii (i.e. pipe size), were omitted from the simulation. This omission may produce a slight overestimation in simulated flood depth and area since water is assumed to be able to flow unrestricted.

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Annual High Wave-Driven 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.

We use the phase-resolving numerical model Boussinesq Ocean and Surf Zone (BOSZ) to simulate wave-driven flooding for annually-recurring wave conditions under a range of SLR levels. Modeling the annually-recurring wave conditions is relevant to this modeling effort since it represents wave conditions that, on average, could occur in any given year.     

The simulations are done over a high resolution (5 m x 5 m horizontal grid spacing) digital surface model, and the final product is projected onto a 2 m x 2 m grid. 

Histograms of wave directions generated by lengthy hindcast records (40-yr-long) are used to determine the dominant wave directions that need to be modeled. Our directional bands are 30-degrees wide (+/- 15 deg). If for a given domain we clearly identify multiple wave directions from which the wave amplitudes are sufficiently high, we will run our model simulations separately for each of these wave directions. The annually-recurring wave height is determined from a Generalized Extreme Value (GEV) analysis.  

All simulations use the MHHW tidal datum. We directly calculate the MHHW from a lengthy (18.6-yr-long) record of water level at the nearest tide gauge. Sea level rise scenarios (0-10ft) are added as a water state assuming the same tidal epoch and wave direction and amplitudes. Each SLR value requires a separate model simulation.

The flood depth at a given grid cell is the mean of the five highest water depth values out of the entire simulation.

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Compound Flooding

The compound flooding layer shows depth of floodwaters from a simulated historical event (described below) with passive sea level elevated (0 – 10 ft scenarios). 

Compound flooding refers to flooding that occurs when multiple flood drivers occur simultaneously or within close succession, resulting in more significant impacts. In Hawai’i, these events are often a result of heavy rainfall coinciding with high tide. Additionally, in the coastal zone these impacts are exacerbated by climate change due to SLR and the increasing likelihood of more frequent and intense storms reaching the islands.

Model simulations of floodwater depth and extent for a domain within the Kona Moku were produced for a historical compound flood event using the Noah-MP column land surface model (LSM) and terrain routing modules of the Weather Research and Forecasting Model Hydrological modeling system (WRF-Hydro) modeling system. Static geographical input into the Noah-MP LSM includes a 10 m topobathy bare earth composite Digital Elevation Model (DEM), Coastal Change Analysis Program (C-CAP) land cover data at 2.4 m resolution, and gridded Soil Survey Geographic Database (gSSURGO) soil texture data at 30 m resolution. Meteorological data from the Hawaii configuration of the Weather Research and Forecasting model (WRF-ARW) and Quantitative Precipitation Estimates (QPE) from the Multi-Radar Multi-Sensor System (MRMS) were provided at 1 km resolution and used as the dynamic forcing input into the model. 

The historical flood event selected was the 5-7 December 2021 Kona storm, which produced widespread flooding due to the compounding effects of heavy rainfall, exceptionally high tides (‘King tide’), and a storm surge of up to 8 in higher than the predicted astronomical tide. To capture the elevated water levels, time-varying tides from the Honolulu tide gauge were imposed along the coast using a coastal boundary module from an expanded version of the model code, WRF-Hydro-CUFA (Coastal Urban Flood Applications), developed in Son et al. (2023). 

Floodwater depth and extent were simulated on a 10 m x 10 m grid and validated using observations from the USGS Mānoa-Pālolo Drainage Canal streamgage and point observations collected from social media platforms. Future flood simulations were produced at 1 ft increments of SLR up to 10 ft and used to create maps at the time of maximum flooding.

Citations:

Son, Y., Di Lorenzo, E., & Luo, J. 2023. WRF-Hydro-CUFA: A Scalable and Adaptable Coastal-Urban Flood Model Based on the WRF-Hydro and SWMM Models. Environ. Model. & Software 167, 105770-. https://doi.org/10.1016/j.envsoft.2023.105770

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Future Erosion Hazard Zones

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Flooded Roads

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Stormwater Structure Failure

Stormwater inlets include structures that are intended, through gravity, to move water into the ocean from land. These are common structures in the built environment throughout Hawaiʻi as entry points for the storm drain network intended to drain upland sources of flooding in the built environment. When sea level rise exceeds the elevation of the stormwater structure, the structure will no longer be able to receive water – it fails. Coupled with drainage backflow, these locations provide an exit point for marine flooding that can be significantly more inland than direct passive marine flooding. With each increment of sea level rise, stormwater structures are highlighted that are at or below the selected sea level scenario. Infrastructure managers can use this information as a first assessment of potential drainage network failure. Please also see the drainage backflow layer and description for more information about potential flooding through existing infrastructure.

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Critical Facilities

Critical facilities include public infrastructure for emergency services (Hospitals and Clinics, Fire and Police) as well as Public Schools. The locations of these are provided as overlays to show facility locations threatened by each modeled exposure. Note that both direct (to the facilities themselves) or indirect (to the transportation, wastewater or electrical infrastructure that they may rely on) can be visualized.

Information and metadata about each dataset is available from the source department cited in the links:

Hospitals and Clinics: Source data Metadata 

Fire Stations: Source data Metadata

Police Stations: Source data   MetadataPublic Schools: Source data   Metadata (none)

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Wastewater Infrastructure

Wastewater infrastructure includes treatment plants, pump stations and known cesspool locations. The locations of these are provided as overlays to show facility and point source (cesspools) locations threatened by each modeled exposure. Note that both direct (to the facilities themselves) or indirect (to the transportation, or electrical infrastructure that they may rely on) can be visualized. Facilities impacted directly or indirectly by fooding may become pollution sources due to reduced capacity, failure or submergence. 

Information about each dataset is available from the source department cited in the links. Metadata is limited for this data:

Treatment Plants: Source data   Metadata

Pump Stations: Source data   Metadata (none)Cesspool locations: Source data Metadata

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Electrical Infrastructure

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Oahu Shoreline Setback

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Geology

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. The Geology layer identifies surficial deposits only. It should be used as an initial screening tool and may require verification at the site level.

Sherrod, David R., Sinton, John M., Watkins, Sarah E., and Brunt, Kelly M. 2007. Geologic map of the State of Hawaiʻi: U.S. Geological Survey Open-File Report 2007-1089. http://pubs.usgs.gov/of/2007/1089/. Accessed [date].

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Public Safety Power Shutoff

This PSPS Estimated Outage Areas feature layer displays the generalized areas where power may be shut off due to potential wildfire conditions. Using advanced technology and weather monitoring systems, we have identified and marked those regions most at risk. Due to the dynamic nature of our electrical grid and environmental conditions, areas affected by power shutoffs may vary.

Please note that the borders of the displayed areas are approximate and may change based on the grid configuration, electric system status, and changing environmental and weather conditions. The highlighted communities are at higher risk, and therefore, more likely to experience a power shutoff due to potential wildfire conditions. 

For detailed information on our PSPS program and the latest updates, please refer to the Company’s official PSPS website.

Citations:

Anderson, T., Fletcher, C., Barbee, M., Romine, B., & Lemmo, J. 2018. Modeling multiple sea level rise stresses reveals up to twice the land at risk compared to strictly passive flooding methods. Nature Scientific Reports 8: 14484 DOI:10.1038/s41598-018-32658-x

Habel, S., Fletcher, C., Anderson, T., & Thompson, P. 2020. Sea-Level Rise Induced Multi-Mechanism Flooding and Contribution to Urban Infrastructure Failure. Nature Scientific Reports, 10: 3796 DOI:10.1038/s41598-020-60762-4

Habel, S., Fletcher, C., Rotzoll, K., El-Kadi, A., & Oki, D. 2019. Comparison of a simple hydrostatic and a data-intensive 3D numerical modeling method of simulating sea-level rise induced groundwater inundation for Honolulu, Hawai’i, USA. Environmental Research Communications, 1(4), 041005. DOI:10.1088/2515-7620/ab21fe

Habel, S., Fletcher, C.H., Rotzoll, K. and El-Kadi, A. 2017. Development of a model to simulate groundwater inundation induced by sea-level rise and high tides in Honolulu, Hawaii. Water Research. ISSN 0043-135.http://dx.doi.org/10.1016/j.watres.2017.02.035

Hawaiʻi Climate Change Mitigation and Adaptation Commission. 2021. State of Hawaiʻi Sea Level Rise Viewer. Version 1.12. Prepared by the Pacific Islands Ocean Observing System (PacIOOS) for the University of Hawaiʻi Sea Grant College Program and the State of Hawaiʻi Department of Land and Natural Resources, Office of Conservation and Coastal Lands, with funding from National Oceanic and Atmospheric Administration Office for Coastal Management Award No. NA16NOS4730016 and under the State of Hawaiʻi Department of Land and Natural Resources Contract No. 64064. http://hawaiisealevelriseviewer.org. Accessed [date].

Sweet, W.V., B.D. Hamlington, R.E. Kopp, C.P. Weaver, P.L. Barnard, D. Bekaert, W. Brooks, M. Craghan, G. Dusek, T. Frederikse, G. Garner, A.S. Genz, J.P. Krasting, E. Larour, D. Marcy, J.J. Marra, J. Obeysekera, M. Osler, M. Pendleton, D. Roman, L. Schmied, W. Veatch, K.D. White, and C. Zuzak, 2022: Global and Regional Sea Level Rise Scenarios for the United States: Updated Mean Projections and Extreme Water Level Probabilities Along U.S. Coastlines. NOAA Technical Report NOS 01. National Oceanic and Atmospheric Administration, National Ocean Service, Silver Spring, MD, 111 pp. https://oceanservice.noaa.gov/hazards/sealevelrise/noaa-nostechrpt01-global-regional-SLR-scenarios-US.pdf

State of Hawaii Office of Planning. Hawaii Statewide GIS Program: Download GIS Data. Available at http://planning.hawaii.gov/gis/download-gis-data/

Tetra Tech, Inc. and University of Hawaiʻi Coastal Geology Group. 2017. Sea Level Rise – Exposure Area. https://planning.hawaii.gov/gis/download-gis-data-expanded/. Accessed [date].