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Coastal erosion is the removal or transport of sand, rocks, and/or soil along a coastline by the action of waves, currents, and wind. It is a natural process that is accelerated by human activities such as construction, dredging, and mining. In the U.S., coastal erosion causes approximately $500 million in property loss annually. In Maui County, 85% of beaches are eroding, compared to 70% of beaches statewide.
Generally, coastal erosion is measured as the rate of change in the position of a shoreline and can be chronic (gradual, i.e. sea level rise) or episodic (immediate, i.e. storm events). Coastal erosion is a naturally occurring process that is caused by storm surges, coastal storms/hurricanes, and high waves (such as king tides). A healthy shoreline will naturally recover from erosion events over time. However, it may take months or years for a shoreline to recover from a significant episodic erosion event. When shorelines do not recover quickly enough, they retreat inland, causing damage to coastal buildings, infrastructure, and natural areas. The extent and severity of naturally occurring coastal erosion can be accelerated by poor land use methods as well as global sea level rise.
Beach erosion: Beach erosion occurs when sand is carried from a beach and deposited on sandbars further from shore. On a healthy beach, sand is returned to the beach over time though tidal cycles. Sea level rise, multiple storms over a short period, and/or a shoreline disrupted by hardening can result in shoreline retreat or the narrowing of a beach.
Dune erosion: Dune erosion occurs when sand or sediment from areas not typically touched by normal tidal cycles is carried from the land and deposited offshore. Often caused by storm surge from severe coastal storms, dune erosion can result in the reshaping of the shoreline when mores and sediment is removed than can be replenished by regular tidal cycles. Dunes provide protection for the lands behind them, dune erosion can ultimately make buildings and infrastructure behind impacted dunes more vulnerable to coastal storms and flooding.
Human activity: Development activities which disrupt the natural storage or transport of sand up and down a coastline can exacerbate the impacts of erosion, despite intentions to reduce the hazard. For example, shoreline hardening — armoring sections of the coast with structures such as jetties and retaining walls — can disrupt natural patterns of sand movement, hindering replacement of sand and sediment in the long-term. Sand mining and dune grading degrade beaches by decreasing sand volumes, steepening the morphology of the shoreline, and reducing the ability of beach profiles to respond to seasonal wave stresses; both practices occurred historically in Maui County but are now prohibited. Sand often accumulates where canalized streams open along the coast and is removed to reduce upstream flood hazard. When sand is transported off-site, the impacts of this type of removal may be similar to sand mining, underscoring the importance of understanding prevailing sediment transport. In addition, coral reef degradation can contribute to coastal erosion. Coral reefs provide a natural buffer to protect coastlines from waves, storms, floods. When reefs are degraded, the absence of a natural offshore barrier exposes shorelines to wave energy and violent storms. Coral reefs can be degraded by water pollution, turbidity (sediment in water that blocks sunlight from reaching coral), and warming ocean temperatures.
All of Maui County’s shorelines are susceptible to some degree of coastal erosion. The National Assessment of Shoreline Change, conducted using aerial imagery, shows that approximately 85% of beaches on the Island of Maui are eroding. However, some shorelines on the island are more susceptible to erosion than others, such as beaches with the West Maui, Kīhei-Mākena, and Wailuku-Kahului community planning areas.
The islands of Maui and Molokaʻi largely protect the Island of Lānaʻi from erosion to the north and northeast. Therefore, the majority of erosion on Lānaʻi occurs on the southwest side of the island. On the Island of Molokaʻi, coastal erosion impacts the southern shoreline, characterized by scarped dunes, fallen trees, and outcrops of basalt rock. Coral reefs, which can provide protection against coastal erosion, can be found offshore of all the islands within Maui County. However, Molokaʻi has the most extensive coral reef within the county.
Coastal erosion is an ongoing process. Therefore, previous occurrences are typically defined by areas experiencing high rates of coastal erosion over a period of time rather than singular historic events. However, certain events, such as tsunamis, can drastically change shorelines. Below is an overview of events in county history that have contributed to noteable erosion and shoreline movement:
Historically, the West Maui shoreline has been subject to varying rates of coastal erosion. From Lahaina to Papalua, the Honoapiilani Highway (State Highway 30) runs along the shoreline. Revetments and other armoring constructed to protect the highway have interfered with natural erosion and replenishment processes. In addition, degradation of the Māʻalaea Coral Reef in the last several decades has contributed to coastal erosion. Coral cover on the Māʻalaea Coral Reef has declined from 50-75% coverage in 1993 to just 8% in 2015.
Kanaha Beach in northern Maui, located between Kahului Harbor and the Town of Pāʻia, has experienced severe erosion since 1950. Historic sand mining and construction of Kahului Harbor in 1910 are likely to have contributed to increased erosion rates. A revetment constructed in the 1970s to protect the wastewater facility, and another constructed at Baldwin Beach, have caused a loss of beach on the seaward side of these structures. Further, the general area is exposed to high wind and wave energy.
In the Kīhei area, varying rates of erosion have impacted shorelines. Coastal armoring has contributed to severe coastal erosion in the Kalama Park/Halama Street area. In addition, several beaches along the Kīhei Coast were subject to severe erosion during the 1980 and 1982 Kona storms.
Resources: Hawaiʻi Shoreline Study web map - Climate Resilience Collaborative (hawaii.edu)
The extent, or severity, of coastal erosion can be measured in several ways, one of which is the rates of erosion experienced in specific areas. Based on the study conducted by the Coastal Geology Group, the Spreckelsville area in Wailuku-Kahului is experiencing the most severe erosion within the county, with an average annual erosion hazard rate of -1.7 feet per year. Further, it can be expected that sea level rise associated with a changing climate will exacerbate coastal erosion rates by causing shorelines to retreat farther inland. One study found that historically, coastal erosion on the Hawaiian Islands has been two orders of magnitude higher than sea level rise.
Coastal erosion projected from sea level rise is further described in the Vulnerability Section of this hazard’s profile. Coastal erosion severity may also be measured in terms of the percentage of shoreline within the county experiencing erosion. According the 2015 Maui County Hazard Mitigation Plan Update, over the long term, the Island of Maui can expect 85% of its beaches to erode within only a 14% accretion (sediment deposit) rate.
Buildings and Infrastructure: Buildings, utility lines, roads, and bridges are vulnerable to coastal erosion. Erosion can undermine roads, scour bridges, and expose underground utilities. Severe erosion during major storm events may destabilize and/or damage infrastructure. When utility lines are damaged by erosion, functionality of water, sewer, gas, and electricity systems may be compromised. Because of their geographic extent, roads and utility lines can have a greater chance of being impacted by coastal erosion events and are difficult to move out of hazard areas. Furthermore, if any section of a road or utility line is damaged, the entire system may be impacted. For instance, coastal erosion that results in one area of a road or a bridge being impassible may block accessibility to other roads or locations.
Health and Safety: Life safety, health, warning, and evacuations affect all current and future populations with in the coastal erosion hazard areas. Public health is impacted when erosion occurs causing vulnerabilities to water and wastewater treatment facilities, which may cause sewage spills and contamination of water.
Household Composition: While time-critical evacuation for coastal erosion is unlikely, it may be required as a result of other hazards impacting areas of severe coastal erosion, such as coastal flooding, high surf or storm surge. Special accommodations should be considered for people with access/functional needs and other dependent populations, such as seniors and children, to support advance warning and evacuation efforts during an emergency.
Socioeconomic status: Low-income households living in coastal erosion zones may lack funds to support mitigation efforts as well as a financial buffer to recover from a severe erosion event.
Access to information: Populations with language and technology barriers may experience delays in receiving and acting upon urgent hazard warning information.
Housing characteristics: Homes built before 1950 and prior to the enactment of modern building codes and setback regulations are more susceptible to structural damages and even permanent losses to homes and property as a result of coastal erosion.
Access to lifelines: Populations with limited vehicle access or transportation routes, especially those living in isolated areas, are more likely to experience mobility challenges during an evacuation, and have difficulty accessing needed supplies and services before, during and after a hazard event.
Economic Impacts: Loss of sandy beaches due to coastal erosion can negatively impact tourism, as visitors tend to avoid shrinking, narrow beaches due to crowding. Further, coastal erosion may cause beachfront property values to decline as the beach is lost and the building itself becomes at risk to flooding or structural damage. Declining property values may impact the county’s tax base. Additionally, the county may need to allocate increasing amounts of funding toward beach renourishment or hardening structures to protect beaches, roads, and structures as the shoreline retreats inward.
Climate Change Impacts: Sea level rise attributed to climate change is expected to increase the frequency and intensity of coastal erosion. Studies estimate that sea level rise will increase coastal erosion by two orders of magnitude. In addition, warmer oceans resulting from climate change are contributing to increased coral bleaching episodes, and ocean acidification is expected to further reduce coral growth and health. As coral reefs provide valuable offshore protection against wave and storm action, coastal erosion is expected to increase in places where coral reefs experience further degradation.
