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Click on the icons below to learn more about the hazards detailed in the 2020 Maui HMP.
Flooding is a frequent, dangerous, and costly hazard. From 1980-2008, flooding accounts for 40% of all natural disasters and results in an average of over 6,500 deaths annually across the world. In the U.S. flooding results in an average of 86 deaths annually. Nearly 90% of all presidential disaster declarations result from natural events where flooding was a major component.
Flooding is the most common environmental hazard in the United States, due to the widespread geographical distribution of valleys and coastal areas, and the population density in these areas. The severity of a flooding event is typically determined by a combination of several major factors, including stream and river basin topography and physiography; precipitation and weather patterns; recent soil moisture conditions; and the degree of vegetative clearing and impervious surface. Flooding events can be brought on by severe (heavy) rain. On Maui, heavy rains typically occur from October through April. On Molokaʻi, heaviest rainfall is typically from December through March. Heavy rainfall, and therefore flooding, is generally produced by the following types of storms in Maui County:
Kona Storms: These storms occur during the wettest period of the year, from November to April. Trade winds from the northeast slack during this time, allowing storms from the south to more easily approach the islands. Kona winds are generally warmer and carry moisture that is dropped evenly as rain over the entire county. The low-elevation and southern location of these storms means that drier sides of the islands get most of their rainfall (approximately 25 to 30 inches each season) during Kona storms. Because of the potential combination of high winds and heavy rains, these events can cause coastal and inland flooding over large geographic areas. Due to special hazards associated with these storms, they are addressed further under the Hurricane (Including Tropical Storm and Kona Storm) hazard profile.
Frontal Storms: Frontal storms usually occur from December through March. They originate over the Pacific Ocean as a result of the intersection of polar and tropical air masses and move eastward over the islands. Heavy continuous rainfall over a period of several hours can create hazard conditions in high sloping areas of the islands. Low-lying areas with poor drainage are prone to landslides and flash floods during these storms.
Upper Level Lows: Upper level lows and troughs can occur any time of the year. In many instances, upper level lows have little or no effect on the lower levels of the atmosphere. However, these lows are sometimes able to tap into the marine layer and induce heavy showers that can result in flash flooding.
Tropical Cyclones: Tropical depressions, tropical storms, and hurricanes hitting or passing near the Hawaiian Islands cause heavy rains, storm surge, high winds and surf. Impacts from these events include severe coastal and inland flooding. Tropical cyclones also cause severe damage due to high surf. Due to special hazards associated with these storms, they are addressed further under the Hurricane (Including Tropical Storm and Kona Storm) hazard profile.
In coastal areas, sea level rise and high surf can result in coastal flooding events. Storm surges due to hurricanes and tsunamis can also cause coastal flooding. Due to special hazards associated with these two types of coastal flooding, they are addressed as separate hazards within the Hurricane and Tsunami profiles of this risk assessment.
Types of flooding caused by inland and coastal flooding are presented below.
Inland Flooding
Flash Flooding - Flash floods occur within a few minutes or hours of heavy amounts of rainfall and can destroy buildings, uproot trees, and scour out new drainage channels. Heavy rains that produce flash floods can also trigger mudslides and landslides. Most flash flooding is caused by slow-moving thunderstorms, cyclones, Kona storms, or repeated thunderstorms in a local area. Although flash flooding often occurs in mountainous areas, it is also common in urban centers where much of the ground is covered by impervious surfaces. In Maui County, heavy precipitation at higher elevations can quickly flow downslope and cause flooding in more densely populated valley areas.
Street Flooding - Sheet flooding is a condition where storm water runoff forms a sheet of water to a depth of six inches or more. Sheet flooding and ponding are often found in areas where there are no clearly defined channels and the path of flooding is unpredictable. It is also more common in flat areas. Most floodplains are adjacent to streams or oceans; although, almost any area can flood under the right conditions where water may accumulate.
Urban Flooding - Urban flooding is usually caused by heavy rain over a short period of time. As land is converted from fields or woodlands to roads and parking lots, it loses its ability to absorb rainfall. Since sidewalks and roads are non-absorbent, rivers of water flow down streets and into sewers. Roads and buildings generate more runoff than tropical forestland. Fixed drainage channels in urban areas may be unable to contain the runoff that is generated by relatively small but intense rainfall events. Urbanization increases runoff two to six times over what would occur on natural terrain. This high volume of water can turn parking lots into lakes, flooding basements and businesses, and cause lakes to form in roads where drainage is poor or overwhelmed.
Urban flooding often occurs where there has been development within or near stream floodplains. This is partly a result of the use of waterways for transportation purposes in earlier times. Sites adjacent to rivers and coastal inlets provided convenient places to ship and receive commodities. The accessibility to the water led to development near waterways which has generally increased in density overtime, leading to increased flooding in the ensuing urban areas. Urbanization intensifies the magnitude and frequency of floods by increasing impermeable surfaces, amplifying the speed of drainage collection, reducing the carrying capacity of the land and, occasionally, overwhelming sewer systems.
Riverine Flooding - Periodic flooding of lands adjacent to non-tidal rivers and streams is a natural and inevitable occurrence. When stream flow exceeds the capacity of the normal watercourse, some of the above-normal stream flows onto adjacent lands within the floodplain. Riverine flooding is a function of precipitation levels and water runoff volumes within the watershed of a stream or river. The recurrence interval of a flood is defined as the average time interval, in years, expected to take place between the occurrence of a flood of a particular magnitude and an equal or larger flood. Flood magnitude increases with increasing recurrence interval.
Coastal Flooding
High wave flooding - High wave flooding, or high surf, is a frequently occurring coastal hazard within the Hawaiian Islands. Waves reaching the shoreline are determined by the wave energy within the approaching swell, which is a function of wave height and wave length (the distance between wave crests), as well as the shoreline aspect, slop, morphology, and geology. Offshore characteristics, such as seafloor depth and barriers, such as sandbars, islands, rocks, and reefs, also impact wave energy. When deep ocean swells encounter shallow island margins as they approach the shoreline, friction caused by the shallower seafloor cause the tops of swells to stack up on their bottoms, building wave height. Because the change from deep ocean seafloor to the island margins around the Hawaiian Islands is abrupt, surface waves grow very tall, very quickly. Large waves tend to travel in sets, and after breaking they rush up onto the beach temporarily elevating the sea surface near the shoreline. Storm events that produce large swells may cause high wave coastal flooding.
King Tides - King Tides, for which the scientific term is perigean spring tides, are the highest high tides of the year. Unlike high wave flooding, which is typically a product of storm events and large swells, King Tides are a product of the moon’s orbit around Earth and the Earth’s orbit around the sun. In Hawaiʻi, King Tides tend to occur during the summer (e.g., July and August) and winter (e.g., December and January) months in conjunction with full moons and new moons. King Tides occur when the moon is at its closest point to Earth during its monthly orbit, as the gravitational pull is stronger, and when the Earth, moon, and sun are in alignment, as the sun and the moon’s individual gravitational pulls produce largest tides. King Tides can cause coastal flooding and inundation of low-lying coastal areas.
Sea Level Rise - While sea level rise is more passive than other types of flooding, over time rising sea levels will inundate buildings and infrastructure that is not currently inundated. More imminent threats occur when rising sea levels contribute to the intensity of other coastal hazards, such as high surf, high tides, coastal erosion, and storm surges. For instance, along the U.S. coastline, high tide flood is 300% to 900% more common than it was 50 years ago. Sea level rise is an effect of climate change. Rising sea levels are mostly due the combination of meltwater from glaciers and ice sheets and thermal expansion of seawater as it warms. Globally, mean sea level has risen approximately eight to nine inches since 1880. Moreover, according to NOAA the rate of global sea level rise is accelerating, doubling from 0.06 inches per year throughout most of the twentieth century to 0.14 inches per year from 2006 to 2015.
All areas of Maui County are subject to flooding impacts. Flooding in Maui County typically occurs along within low-lying coastal areas and within floodplains associated with each island’s principal flooding sources. Most flooding sources, as listed in the previous section, include streams and gulches that flow from higher elevations in mountainous areas to the sea. Urban flooding also occurs in dense, developed areas. Of the nearly 745,000 acres that comprise Maui County, approximately 24,800 acres are in the 1% annual chance floodplain (all zones). You can read more about Maui’s floodplain areas and locations of risk, including federal regulatory mapping products, in the Flood section of the HMP Risk Assessment. You can view future risk of Sea Level Rise (SLR) Using the University of Hawaii Shoreline Viewer. Finally, you can view data and photo documentation of King Tide impacts via the University of Hawaii King Tides Project map.
Maui County is susceptible to a variety of flooding, whether coastal, riverine, or flash. Often, events resulting in flood can result in multiple flooding types (i.e., coastal and riverine) at the same time. NOAA’s National Centers for Environmental Information’s Storm Events Database and the Spatial Hazard Events and Losses Database for the United States (SHELDUS) were used to inform historic flood events in Maui County from 1971 to March 2020 During this period, 137 flood events were reported, which resulted in two deaths and one injury. Five of these events were described as resulting in damages. However, it is likely that many damages, especially those to private businesses and residences, have gone unreported. There have been eight Presidential Disaster Declarations for flood-related events in Maui County since 1963 (not including declarations related to hurricanes, tropical storms, and tsunamis). This figure includes the 2007 Kona storm event, which caused flooding and landslides throughout eastern Maui.
In addition to the flood events described above, Maui County experiences flooding from specifically high surf/high wave and King Tide events. The NCEI Storm Events Database reported over 552 days with high surf events in Maui County between 2004 and April 2020, indicating that high surf events are a common occurrence in Maui County. No deaths, injuries, or damages were reported with these events. Not all reported high surf events result in coastal flood events. University of Hawaiʻi Sea Grant provides data on King Tide flood occurrences in Hawaiʻi from July 2015 to December 2019; 446 areas were recorded as being impacted by 16 King Tide events in this period for Maui County. Some community planning areas were documented as experiencing more King Tide flooding impacts than others, namely Kīhei-Mākena and West Maui.
Maui County participates in the National Flood Insurance Program (NFIP) and has over 12,500 policies in force providing almost $2.8 billion in flood insurance coverage. According to data from FEMA, Maui County has had 579 paid claims since January 1, 1978. This is a notable increase from the 2015 Hazard Mitigation Plan, which recorded 292 claims between January 1, 1978 and January 31, 2015. Conversely, the number of policies has stayed roughly the same, with 12,567 polices in 2015 and 12,575 in 2020. Total losses paid are approximately $6.9 million, an average of roughly $11,960 per claim.
Flood extent, or magnitude, can be defined in several ways including peak flow or discharge rate (cubic feet per second), height of flood waters, and damages. The largest peak flow recorded for a stream in Maui County was 107 cubic feet/second on Honopou Stream near Huelo. Greater floods are possible, especially with increasing precipitation and sea level rise due to climate change and development pressure along coastal areas. Increasing impervious cover results in increased runoff volumes and consequently, increased flooding. In addition, development within floodplains can, over time, increase base flood elevations as well as increasing the number of people and businesses located in flood hazard areas, resulting in more property damage, injuries, and loss of life.
Future Probability
Historic events can also be used to infer the likelihood of flood events. In the 49-year period from 1971 to March 2020, 137 flood events were reported in Maui County. This indicates three flood events occurring within Maui County per year, on average. Further, it is likely that flood events will increase in the future with climate change, as ENSO extremes become more frequent, meaning extreme rain events, including hurricanes and Kona storms, will also become more frequent. Coastal flooding due to high waves and King Tides is expected to increase overtime as sea levels continue to rise with climate change. Based on historic events and expected future conditions, the flood hazard was assigned a probability of highly likely (greater than 90% annual chance) in Maui County.
With a growing population and increasing development, Maui County is susceptible to increased flooding. Flooding can result in a variety of impacts, such as death and injury, property damage, inability to access areas and road closures. All current and future buildings, infrastructure, and populations in Maui County are considered at risk to flood hazards, including areas outside federally designated boundaries of the 1% annual chance flood hazard extents. Specific impacts are described below.
Buildings and Infrastructure: Floods can impact coastal and inland areas of Maui County. Riverine, sheet flow, urban flooding, and flash flooding may impact inland areas, while coastal flooding may be caused by high waves, King Tides, sea level rise and storm surge. Building contents can be lost, damaged, or destroyed, and structures themselves can be compromised by floodwaters. Pressure from floodwater, especially as seepage through soil, can damage building foundations. During coastal floods, waves may batter structures or erode soils, undermining building foundations. After a flood, wooden structures may rot. Mold is common in structures that have been inundated by floodwaters and can grow within 24 to 48 hours after a flood. Mold can be costly to safely and properly treat and presents serious public health risks if left untreated. Building built before the 1981 may also have increased vulnerability as they may be built in more flood-prone areas and not to current building code standards.
Because of their geographic extent, roads, and utility lines can have a greater chance of being impacted by flood events. Further, if any section of a road or utility line is damaged or flooded during a hurricane, the entire system may be impacted. For instance, a hurricane that causes one area of a road or a bridge to be impassible may block accessibility to other roads or locations until waters recede, debris is removed, or in the case of damage (e.g., undermining, scour, or washing out), until repairs can be completed. The majority of reported flood impacts in Maui County are road closures.
Additional information about the exposure of buildings and infrastructure, including critical facilities, to various flood hazard types in Maui County can be found in the HMP Hazard Profile section for flood.
Health and Safety: The public often underestimates the dangers presented by floodwaters. Flooding is often localized to certain parts of a community (e.g., certain roads, intersections, or neighborhoods), and floodwaters can prevent normal access to buildings and facilities. In Maui County, entire communities can be inaccessible due to floodwaters. This presents a danger when motorists and pedestrians attempt to traverse floodwaters. Motor vehicles and pedestrians can get swept up in flood currents, increasing the risk for drowning. Even in shallow waters, fast-moving currents can carry individuals or vehicles into deeper waters, where pressure from flowing water can prevent drivers from escaping submerged vehicles. As little as six inches of floodwater can move a vehicle, and as little as two inches can move a person. In addition, floodwaters often conceal conditions that are a danger to those on foot, including electrical wires, debris, nails, and open manholes hidden beneath the surface. In addition, roads and bridges can be weakened by flood impacts, making them unsafe for travel.
Floodwaters often contain contaminants such as bacteria and chemical hazards. Flooding often results in combined sewer overflows, resulting in sewage in floodwaters. Individuals traversing floodwaters or children playing in floodwaters contract diseases, injuries, and infections. Floodwaters may also contaminate drinking water sources and increase likelihood of waterborne diseases. Stagnant pools of water and saturated grounds can lead to an increase in the number of mosquito-borne diseases such as malaria and dengue and West Nile fevers. Structures exposed to flooding may develop mold or wood rot. Mold growth can occur in flooded structures within 24 to 48 hours of inundation. People with asthma, allergies, or breathing conditions may be at a higher risk to mold.
Structures exposed to flooding may develop mold or wood rot. Mold growth can occur in flooded structures within 24 to 48 hours of inundation. People with asthma, allergies, or breathing conditions may be at a higher risk to mold.
Economic Impacts: Flood events can have far-reaching impacts in Maui County. Flood losses occur directly due to damaged structures and infrastructure. Flood damage to businesses is difficult to estimate. Businesses that are disrupted by floods often have to be closed. They lose their inventories, customers cannot reach them, and employees are often focused on protecting or cleaning up their flooded homes. Business can be disrupted regardless of the business being located in the floodplain when customers and clients cannot reach their location. Business interruption is also forgone sales and tourism taxes for the county. As with flooded roads, public expenditures on flood fighting, sandbags, fire department calls, clean-up and repairs to damaged public property affect all residents of the city, not just those in the floodplain.
Climate Change Impacts: Climate change is expected to exacerbate inland and coastal flooding. Climate change is associated with more frequent ENSO extremes, which will lead to more frequent and severe storms in Maui County. In coastal areas, high wave flooding will combine with sea level rise for a more severe impact that reaches further inland. King Tides will have a similar impact when combined with projected future sea level rise. Other potential impacts of sea level rise in Maui County include increased rates of coastal erosion and saltwater intrusion into surface water and groundwater systems.
Coral reefs provide substantial protection against coastal flooding and erosion by dissipating wave energy. Climate change, as well as acute human impacts, degrade coral reefs through warming ocean waters and degrading water quality, which stress corals and lead to coral loss and bleaching episodes. On Maui, a study by the Hawai’i Division of Aquatic Resources found that coral cover declined from 35% in 1999 to 27% in 2006 across Maui’s nine coral reefs.
