Impacts of Sea Level Rise

Sea Level Rise (SLR) will have many impacts on the coastal zone (1), including:

  • Land loss due to flooding and inundation
  • Land loss due to erosion
  • Increased flood damage through extreme sea level events (extreme tides, storm surges, etc.)
  • Saltwater intrusion into surface waters and groundwater impeding drainage
  • Wetland loss and change

Coastal Flooding and Inundation

Coastal inundation and flooding are terms often used interchangeably. For the purpose of long-range planning for SLR, we will distinguish between these terms (2).  Coastal flooding is the condition where dry areas become wet temporarily—either periodically or episodically. The term coastal inundation is the process of a dry area being permanently drowned or submerged. Overtime, dry areas that are repeatedly flooded may be considered inundated as the land cannot be used.

The key factors contributing to coastal flooding and inundation in Hawaiʻi are SLR, extreme tides, waves, storm surge, groundwater inundation, and heavy rainfall. As sea level rises, the frequency and severity of coastal flooding from extreme tides, high waves and storms (as well as infrequent tsunamis) will increase negative impacts to low-lying environments, ecosystems, and developed areas including coastal roads and communities. Sea level rise will raise the groundwater table leading to increased flooding, poor drainage, and storm damage on low-lying areas behind the shoreline (3). At some point in the future, areas experiencing frequent coastal flooding will become permanently drowned or submerged creating new intertidal areas and wetlands.

Hawaiʻi is exposed to large waves near-round and tsunamis due to our location in the Central Pacific.  Unusually large wave events when combined with high tides cause damaging overwash and inundation on exposed low-lying coasts.  Waves from four dominant sources impact Hawaii’s coasts: North Pacific swell in winter months, South Pacific swell in summer months, easterly tradewind waves year round, and southerly “Kona” storm waves (including hurricanes).

 

Brief 1 Figure 9

Figure 1. Dominant swell regimes and wave-monitoring buoy locations in Hawaiʻi (4)

 

Tropical storms and hurricanes bring strong winds, high waves and heavy rains. Winds and storm surge raise water levels and drive large waves on shore, while heavy rains cause flooding from the landward side. Storm surge is an elevated water level resulting from low atmospheric pressure and strong winds within a storm.  A substantial increase in the likelihood of tropical cyclone frequency has been predicted in the Hawaiian Islands (5, 6).

Currently, the Federal Emergency Management Agency (FEMA) defines the coastal Special Flood Hazard Area (SFHA) on Flood Insurance Rate Maps (FIRM) as two primary zones: Zone VE and Zone AE (Figure 10).  Zone VE, also known as the Coastal High Hazard Area, has a 1% or greater chance of experiencing an annual flooding event and an additional hazard associated with storm waves three feet in height or greater. The coastal Zone AE has a wave component of less than 3 feet in height.  Base Flood Elevations (BFEs) will vary in each zone. Changes in flood zones and BFEs can have a significant impact on building requirements and flood insurance costs.

Sea level rise is not currently included in the coastal FIRM zones.

DLNR’s Flood Hazard Assessment Tool can be used to determine what flood hazard zone a property is in. In the future, vulnerability to sea level rise will need to be incorporated in coastal hazard zones.

 

Brief 1 Figure 10

Figure 2. Coastal hazards incorporated into coastal Special Flood Hazard Zones on FEMA’s Flood Insurance Rate Maps.

Coastal Erosion

Coastal erosion will exacerbate flooding and inundation resulting in the permanent loss of beaches and dry land which will then become submerged at increasing rates due to SLR. In their natural state, beaches, dunes and other coastal environments, such as reefs and wetlands, can provide effective protection from impacts of high waves and storms. Shoreline recession and beach loss due to coastal erosion is already a serious problem along Hawaii’s shoreline, threatening shore front development and infrastructure.  Coastal erosion is the wearing away of beach and dune sediments and land by wave action and currents.  Statewide, 70% of our beaches are undergoing chronic erosion, meaning the shoreline is retreating over years to decades (7).  On Maui, the erosion problem is particularly severe with 85% of beaches undergoing chronic erosion.  Beaches are highly variable environments.  The shape of a beach and location of the shoreline at any given moment results from a delicate balance between water level, wave energy and sand supply.  In addition to chronic erosion, Hawaiʻi’s beaches are prone to short-term erosion (hours to months) due to seasonal changes in wave direction, intermittent high wave events, and periodic or episodic high water levels.

Much of the developed low-lying coastal plains on Kauaʻi, Oʻahu, and Maui are underlain by beach and dune sand deposits.  A “healthy” beach can be sustained along a retreating coast, if the shoreline is allowed to erode into dunes and older beach deposits, releasing sand to nourish the beach.  Chronic and short-term beach erosion becomes a problem when it threatens upland development.  Unfortunately, we are left with a legacy of dense development fronting eroding shorelines.  In many locations, the historical response to beach erosion has been to install seawalls and other coastal protection structures, which has worsened problems of beach erosion and beach loss.

Sea level has been rising around Hawaiʻi over the past century. There is evidence that sea level rise is already contributing to the overall trend of beach erosion in Hawaiʻi (8).  The extent and rates of coastal erosion are sure to increase over the coming decades with predicted increases in sea level rise rates.  A recent study by University of Hawaii researchers found that approximately 92 and 96 percent of Hawaiʻi shorelines studied are projected to retreat by 2050 and 2100, respectively (9). Due to increasing sea level rise, the average shoreline recession by 2050 is nearly twice the distance predicted by historical extrapolation alone, and by 2100 it is nearly 2.5 times the historical extrapolation.   The DLNR is using these forecasts of future erosion hazard exposure to assess vulnerabilities to coastal communities, natural resources, and economic sectors.

The cumulative effects of SLR on coastal erosion, waves and storms surge are contributing to coastal flooding and will ultimately permanently inundate many coastal areas around the State.  Vulnerability and risk assessment provide important information needed to design appropriate strategies and use the right tools to adapt to climate change


References

  1. Wong, P.P., I..J. Losada, J.-P. Gattuso, J. Hinkel, A. Khattabi, K.L. McInnes, Y. Saito, and A. Sallenger, Coastal systems and low-lying areas. In: Climate Change 2014: Impacts, Adaptation, and Vulnerability. Part A: Global and Sectoral Aspects. Contribution of Working Group II to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. . 2014: Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. p. 361-409.
  2. Flick, R.E., et al., “Flooding” versus “inundation”. Eos, Transactions American Geophysical Union, 2012. 93(38): p. 365-366.
  3. Rozoll, K. and C. Fletcher, Assessment of groundwater inundation by sea level rise. Nature Clim. Change, 2012. 3: p. 477-481
  4. Fletcher, C.H., Romine, B.M., Genz, A.S., Barbee, M.M., Dyer, Matthew, Anderson, T.R., Lim, S.C., Vitousek, Sean, and C. Bochicchio, and Richmond, B.M., National assessment of shoreline change: Historical shoreline change in the Hawaiian Islands: U.S. Geological Survey Open-File Report 2011–1051. 2012. p. 55.
  5. Murakami, H., et al., Projected increase in tropical cyclones near Hawaii. Nature Clim. Change, 2013. 3(8): p. 749-754.
  6. Kossin, J.P., K.A. Emanuel, and G.A. Vecchi, The poleward migration of the location of tropical cyclone maximum intensity. Nature, 2014. 509(7500): p. 349-352.
  7. Sea Level Trends. National Oceanic and Atmospheric Administration.  November 2015
  8. Romine, B.M.F., C.H.; Barbee, M.M.; Anderson, T.R.; and Frazer, L.N., Are beach erosion rates and sea-level rise related in Hawaiʻi? . Global and Planetary Change, 2013: p. 108.
  9. Anderson, T.R., et al., Doubling of coastal erosion under rising sea level by mid-century in Hawaii. Natural Hazards, 2015. 78(1): p. 75-103.