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When Maunaloa Erupts, Lava-sheds Can Guide Response

4:13 AM · Aug 7, 2021

What To Know Before Maunaloa Erupts, Part 3... Written by Dane DuPont and Philip Ong It goes without saying that Maunaloa is huge, taking up 51% of the island’s surface area, and it's as close to a certainty as you will find that the volcano’s rift zones will erupt again one day. When Maunaloa does erupt there will be great importance and urgency in figuring out where the lava is flowing and what communities could be impacted, particularly on the steep western slopes below the Southwest Rift Zone (SWRZ) where time is of the essence. Historically, eruptions in this area also produce twice as much lava than those from its Northeast Rift Zone (NERZ) in the same amount of time, which dictates the need for an efficient, well-planned response. The USGS Hawaiian Volcano Observatory's “Lava inundation zone maps for Mauna Loa” can be a crucial component guiding that response. Some areas may need to be evacuated rapidly as the leading edge of the lava flows can reach from the SWRZ down to the ocean in a matter of hours, as in 1868 and 1950. While steep-terrain flows can be narrow, with only localized severe impact, once lava cuts off the major roads there is no easy access from one side of the flow to the other, with the south side of the lava flows more closely accessible from Hilo and the north side more closely accessible from Kailua-Kona, posing problems for residents commuting or stuck on the “wrong” side, and requiring a coordinated effort between both sides of the island. Furthermore, multiple vents opening over the duration of the eruption is a distinct possibility on Maunaloa. While multiple flows present a challenge to emergency responders, splitting the volume of the eruption between several areas might be a good thing, as each individual flow is smaller and less likely to impact island communities (though not always — in 1950, the massive amount of lava formed three different flows which reached the ocean). All in all, given the size of the mountain, historical eruption patterns, anticipated logistical issues, and limited resources, it's important to identify specific areas during any eruption in which to focus the emergency response. The Maunaloa eruptions of community concern are those that occur on its rift zones, as its more frequent summit activity has not reached inhabited areas during historic times. Additionally, Maunaloa flank eruptions have only activated one of its two rift zones at a time, naturally narrowing the area of concern for that eruption. To compartmentalize Maunaloa further, we have the “Lava inundation zone maps” most recently produced in 2017 by Frank Trusdell and Mike Zoeller. Alternatively, the “inundation zones” have been referred to as “lava-sheds”, similar in concept to a watershed but applied to lava flows. The “Lava inundation zone maps for Mauna Loa” are not to be confused with the island-wide USGS Lava-Flow Hazard Zone Map, which rates the likelihood of lava flows entering into a particular area, and gives us the commonly used terminology “Lava Zone 1”, “Lava Zone 2”, and so on. Each map has a role to play in understanding the various dynamics of Maunaloa, and lava-sheds provide the easiest way to subdivide the world’s largest active volcano based on its natural features. “Topographic ridges primarily delineate the zone boundaries, but, in some cases, we use a combination of modeling and empirical data to designate the perimeter extent.” ~ Frank Trusdell and Michael Zoeller [1] The inundation maps divide the volcano into lava-sheds based on physical barriers for lava flows descending from the rift-zone. Within the nearly 2-mile-wide rift zone, flow directions are unpredictable, with lots of previous eruptive features such as cinder cones that create a pin-ball effect as lava deflects between and around them. Only once lava has exited the rift zone, a confident determination of flow direction can be made. The lava-sheds do NOT suggest that lava will cover everything within their boundaries during an eruption, rather they allow first responders and emergency management personnel to focus their efforts in specific areas, including which communities may need to be evacuated. For specific flow paths and potential coverage, the mapped lines of steepest descent as well as the evolution of the flow-field will guide decision-makers. Before an eruption, each resident should identify their lava-shed/inundation zone. In examining the map, the particular colors of each lava-shed do not matter; the key is to watch the boundary between the rift-zones and the lava-sheds. Once lava enters a lava-shed, all residents of that area should be on alert as authorities could go into mandatory evacuation procedures rapidly depending on factors such as slope, eruptive volume, distance, and advancement rates. Due to Maunaloa’s vast size, many lava flows that descend from the rift will ultimately stop before entering areas inhabited today, but given the time needed, evacuations could be necessary before it is determined that a flow will stop short. While large eruptions may spill lava flows into several lava-sheds at the same time, an awareness of these natural divisions would allow for an appropriate, proportional response within each area. For example, if lava exits the SWRZ to the west between the 11,350 to 9,000 ft elevations, that places it into the Hoʻokena lava-shed, and excludes the Hōnaunau and Kealakekua lava-sheds to the north that are more susceptible to lava flows from the summit or radial vents. To the south, the Kaʻohe lava-shed similarly captures westward SWRZ flows issuing from 9,000 to 8,200 ft, the Kaʻapuna lava-shed from 8,200 to 7,450 ft, the Miloliʻi lava-shed from 7,450 to 6,250 ft, the Kapuʻa lava-shed from 6,250 to 5,800 ft, the Hawaiian Ocean View Estates lava-shed from 5,800 to 2,000 ft, and finally the Kalae lava-shed from 2,000 ft elevation down to sea level. At the present time of writing, no comprehensive eruption response plan has been published for Maunaloa volcano, though the State of Hawaiʻi Senate has tasked the Hawaiʻi Emergency Management Agency with producing a master plan for review by the January 2022 legislative session. Further concern arises when considering the mixed response to Kīlauea's 2018 Lower East Rift Zone eruption, with officials claiming success for no loss of human life, but residents suffering unnecessarily due to the loss of pets and movable belongings amidst evacuation procedures that changed daily, even with the benefit of several days of warning before the eruption, as well as the typically slow onset and expansion of lava flows (only 5% of the 716 recognized homes were lost in the first 2 weeks of the 3-month eruption). Incorporating the “Lava inundation zones for Mauna Loa” as a fundamental component of the master plan is a first step towards improving eruption response especially for its riskiest southwest slopes, where even small errors in the official response could lead to disastrous consequences. In the meantime, our communities can already benefit from the lava-shed maps for our preparedness, and to guide any grass-roots efforts filling gaps in the official response, should it be necessary as it was in 2018. Additional Lava-Shed Notes: One thing to note is that Kailua-Kona is outside of all inundation zones of Maunaloa, as the slopes of Hualālai shelter the town. The Kealakekua lava-shed starts just south of Kailua-Kona near the Keauhou Transfer Station and continues south. North of town, less common radial vent eruptions like the 1859 Anaehoʻomalu Bay lava flow fall into the Puako lava-shed, the volcano's northernmost reach, which also captures the western part of Saddle Road. On the Hilo side, the eastern part of Saddle Road drains into the Kaumana lava-shed, with the Waiakea lava-shed to the south capturing the rest of Hilo town. During Maunaloa's most recent 1984 eruption within the now-defined Kaumana lava-shed, knowledge of this division would have allowed responders to shift resources from Waiakea to Kaumana, had the flow continued. Further south, Keaʻau falls into the Volcano-Mountain View lava-shed that stretches east all the way to Hawaiian Paradise Park. The final set of inundation zones comprise the southeast flank of the mountain, with the Kapāpala lava-shed draining Maunaloa's NERZ to the south, skirting Kīlauea caldera and overlapping Kīlauea's SWRZ all the way to the coast. To the west, the Wood Valley lava-shed drains southward flows from Maunaloa's uppermost SWRZ from the summit to 11,200 ft, the Pāhala lava-shed from 11,200 to 10,000 ft, the Punaluʻu lava-shed from 10,000 to 8,650 ft, and once again the Kalae lava-shed from 8,650 ft elevation down to sea-level, finishing the circuit. While lava flows have spilled into the upper elevations of these southeastern lava-sheds in historic times, none have reached populated areas. Images: 1) Lava inundation zone maps for Mauna Loa, Island of Hawaiʻi, USGS 2) Lava-flow hazard zones map, Island of Hawai‘i, USGS 3) Mauna Loa eruption response times over the past 200 years, USGS 4) Shaded-relief map showing two inundation zones in eastern Island of Hawai‘i, USGS 5) Map showing how lava flows that originate from the rift segment upslope of the Hawaiian Ocean View Estates, Kula Kai, and Hawaiian Ranchos subdivisions group together to form the first approximation of a potential lava inundation zone, USGS References [1] Frank Trusdell, Michael Zoeller (2017). Lava inundation zone maps for Mauna Loa, Island of Hawaiʻi, Hawaii. Scientific Investigations Map 3387. Pamphlet.

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