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Lava flow hazard prediction and monitoring with UAS: a case study from the 2014–2015 Pāhoa lava flow crisis, Hawai‘i
© The Author(s). 2017
Received: 2 May 2017
Accepted: 20 October 2017
Published: 2 November 2017
Accurately predicting lava flow path behavior is critical for active crisis management operations. The advance and emplacement of pāhoehoe flows modifies and inverts pre-existing topography, prompting the need for rapid and accurate updates to the topographic models used to forecast flow paths. The evolution and velocity of pāhoehoe flows are dependent on macro and micro topography, the slope of the descent path, effusion rate, and rheology. During the 2014–2015 Pāhoa crisis on the island of Hawai‘i, we used a low-altitude unmanned aerial system (UAS) to quickly and repeatedly image the active front of a slowly advancing pāhoehoe lava flow. This imagery was used to generate a series of 1 m resolution bare-earth digital elevation models (DEMs) and associated paths of steepest descent over the study area. The spatial resolution and timeliness of these UAS-derived models are an improvement over the existing topographic data used by managers during the crisis. Results from a stepwise resampling experiment suggest that the optimum DEM resolution for generating accurate pāhoehoe flow paths through lowland tropical forest environments is between 1 and 3 m. Our updated models show that future flows in this area will likely be deflected by these newly emplaced flows, possibly threatening communities not directly impacted by the original 2014–2015 lava flow. We demonstrate the value of deploying UAS during a dynamic volcanic crisis and suggest that this technology can fill critical monitoring gaps for Kīlauea and other active volcanoes worldwide.
Pāhoehoe lava flows pose major threats to communities living near active basaltic volcanoes worldwide (Hamilton et al. 2013; Del Negro et al. 2016). Estimating lava flow hazards is important prior to eruptions for regional planning purposes, but becomes critical during times of heightened eruption activity (Gonzalez et al. 2015; Poland et al. 2016; Jenkins et al. 2017). Pāhoehoe lava flows generally move slower than ‘a‘ā flows; however, they can travel greater distances and spread out over larger areas, posing a more significant long-term threat (Self et al. 1998).
The behavior of an advancing flow is influenced by external and internal factors. External factors include slope gradient, surface roughness, and other topographic and landscape features, while internal factors include variations in lava supply, viscosity, yield strength, rates of inflation, and the geometry of the feeder channels or tube system (Mattox et al. 1993; Kauahikaua et al. 1998; Hon et al. 2003). Over days or weeks, new lobes can break out from the flow front or along margins further upslope, while older lobes can become barriers, deflecting younger flows and altering the original topography (Walker et al. 1991; Hon et al. 1994; Anderson et al. 2012).
Rapid acquisition of high quality topographic data is crucial for monitoring and forecasting lava flow behavior during effusive volcanic crises. Digital Elevation Models (DEMs) are the primary data layer used in models to estimate future lava flow paths and provide flow hazard assessments. The accuracy of the modeled results, either from the paths of steepest descent method (Kauahikaua 2007) or other physics-based lava flow models (e.g., FLOWGO, SCIARA, DOWNFLOW, MAGFLOW), depends strongly on how well the DEM represents the physical environment, which can be difficult to determine in heavily vegetated areas (Harris and Rowland 2001; Crisci et al. 2004; Favalli et al. 2005; Negro et al. 2008). As lava flows change the landscape, subsequent flows will travel along new paths of steepest descent, requiring updated DEMs to reflect the dynamic environment. (Kauahikaua 2007; Favalli et al. 2009).
Here we present work done to generate pāhoehoe lava flow paths, based on high resolution topographic models extracted from unmanned aerial system (UAS) imagery collected over the June 27th lava flow during the 2014–2015 eruption event near Pāhoa on the island of Hawai‘i. In coordination with Hawai‘i County Civil Defense (HCCD) and the U.S. Geological Survey Hawaii Volcano Observatory (HVO), we mapped the pre- and post-flow topography and developed a computational workflow to merge multiple DEMs, filter them of vegetation, and generate projected paths of steepest descent. Future flow paths were also generated to show the impact of the June 27th flow on a potential lava flow in the future that might inundate the area based on the post-flow topography. We compare our UAS-derived results to paths generated from an existing United States Geological Survey (USGS) 10 m DEM used by responding agencies during the 2014–2015 Pāhoa lava flow crisis.
Lava flow monitoring techniques
Active lava flows are monitored and mapped with a variety of techniques, including ground-based surveys and imagery collected by satellites and manned aerial platforms (Poland 2014; Orr et al. 2015; Patrick et al. 2015). Ground-based surveys employing lidar or visible-light imagery with Structure-from-Motion can collect detailed three-dimensional data, but are limited in their spatial coverage and, in the case of lidar, high cost (Cashman et al. 2013; Hamilton et al. 2013). These limitations make ground-based approaches viable for studying isolated features, as opposed to landscape-scale phenomena. Satellites have become essential platforms for remote sensing of active lava flows, providing datasets of large spatial coverage for hazard monitoring (e.g., Higgins et al. 1997; Wright et al. 2008; Harris et al. 2011; Ganci et al. 2012; Patrick et al. 2016). Intermittent cloud cover, coarse spatial image resolution, and long re-acquisition intervals all present problems for these platforms when monitoring dynamic lava flows. Manned helicopters and fixed-wing aircraft provide high-resolution aerial imagery but are expensive to operate, can have difficulties acquiring consistent data, and are normally restricted to higher flight altitudes (above 150 m) for safety reasons.
UAS technology provides several advantages over satellite, manned aircraft, and ground based surveys for data capture over active lava flows. These advantages include (1) low-cost capability for repeat aerial surveys with high temporal resolution; (2) low altitude flight operations for acquiring cm-scale spatial data; (3) automated mission planning and flight operation for consistent datasets; (4) minimal risk to human life in the event of a crash or accident; and (5) the ability to cover an area in high detail without requiring intensive field operations for personnel (Harwin and Lucieer 2012; Westoby et al. 2012; Hugenholtz et al. 2013). For these reasons, small UAS platforms allow frequent data collection over active flows and can be deployed on short notice.
The 2014–2015 Pāhoa lava flow crisis
Hawai‘i Island’s youngest volcano, Kīlauea volcano is one of the most active volcanoes on earth. In 1983, Kīlauea began an eruption focused at Pu‘u ‘Ō‘ō that continues to erupt at the time of writing (July 2017). Between 1983 and 1991, lava flows from Pu‘u ‘Ō‘ō repeatedly impacted communities in East Hawai‘i, burying 184 structures and completely destroying the town of Kalapana. Between 2000 and 2012, an additional 30 homes were destroyed by lava flows to the South-East of Pu‘u ‘Ō‘ō (Kauahikaua et al. 2003; Orr et al. 2013). On June 27th, 2014, a new vent formed on the northeast side of Pu‘u ‘Ō‘ō crater. Pāhoehoe lava from this vent, dubbed the June 27th lava flow, reached the outskirts of the town of Pāhoa in October 2014 and threatened to isolate over 10,000 people in the surrounding area, cutting their ready access to power, water, and critical infrastructure. The June 27th flow continued to threaten communities around Pāhoa until March, 2015, when breakouts ~15 km upslope diverted lava supply away from the front (Poland et al. 2016). The crisis lasted approximately seven months based on disaster declarations, but the June 27th flow was not officially declared inactive until June, 2015.
UAS flight operations
During the Pāhoa crisis, HCCD erected a temporary flight restriction (TFR) around the active flow front, restricting all manned and unmanned aircraft to those directly supporting relief operations. We were granted access to the TFR airspace after receiving permission from both the FAA and Civil Defense with our public Certificate of Authorization and secured ground access from land owners and stakeholders within the study area. During flight operations, we communicated with nearby aircraft using a ground-to-air aviation radio and kept a distance of 1000 ft. (305 m) horizontal and 500 ft. (152 m) vertical to other aircraft. These precautions were important for safely integrating UAS into congested airspace where multiple tour helicopters frequented the outskirts of the flow every hour.
Maintaining direct line of sight with the UAS at all times, as required by FAA regulations, was challenging for operational planning, as the lava flow was surrounded by thick forest with canopy heights >7 m. Flight operations were conducted from new fire-breaks surrounding the perimeter of the flow (Fig. 1) and other clearings. 5581 aerial images were collected from 23 UAS missions between December 14, 2014 and July 26, 2016.
Prior to flights, we placed a series of ground control points (GCP) across the flow field study area to provide positional control and georeference our UAS imagery and derived DEMs (Fig. 1). The three-dimensional coordinates (WGS84, UTM Zone 5 N) for GCP locations were surveyed using a Trimble Geoexplorer 6000 differential GPS with a Zephyr model external antenna mounted on a 2 m pole (Küng et al. 2011; Westoby et al. 2012). Occupation times with the Trimble GPS averaged 10 min and data were post-processed with Trimble Pathfinder Office software using nearby CORS stations. Based on the differential correction report, 71.5% of the corrected positions had an accuracy of 5–15 cm and 99.1% had an accuracy of 30–50 cm or better. Overall, 49 GCPs were deployed across the roughly 5.87 sq. km study area and at least 5 GCPs were included in each UAS flight, distributed across the flight coverage area as evenly as possible. Please refer to Additional file 1: Table S1 for corresponding flights and GCPs used.
To independently assess the accuracy of the UAS derived DEM, an additional 32 points were surveyed across the study area via GPS. These points were collected using the previously described survey technique but were not included in the UAS image processing steps. Based on the differential correction report for these data, 75.35% of their corrected positions had an accuracy <5 cm and an additional 24.65% had an accuracy between 5 and 15 cm. Absolute elevation values from the GPS survey were then compared to extracted elevation values at the same XY positions for the UAS-derived 1 m DEM and an older 10 m DEM of the Puna district, acquired from the USGS National Elevation Dataset (NED) program and used during the active disaster operations.
Data analysis and processing
Bare-earth elevation models filtered of vegetation were required for the determination of accurate flow paths. Because passive optical remote sensing techniques rely on reflected light, dense foliage and canopy shadows can conceal the complex topography lying underneath (Dandois et al. 2015; Fraser et al. 2016). In order to generate bare-earth DEMs of the study area, situated within a lowland tropical forest landscape, we needed to filter out surface features such as buildings and vegetation. We employed point cloud filtering techniques using LAStools (rapidlasso GmbH), a lidar software package consisting of a series of powerful batch script algorithms to process point cloud data. The following is a detailed description of the parameters and settings used for each script in LAStools to extract the bare-earth elevation values by filtering the point clouds of vegetation (highlighted in green on Fig. 2). Please refer to cs.unc.edu/~isenburg/lastools/ and Isenburg et al. 2006 for additional information. We employed the (1) lasThin tool to thin the point cloud, keeping only the lowest elevation points at a 1 m grid size; (2) lasGround_New tool for bare-earth extraction, classifying the points into ground points (class = 2) and non-ground points (class = 1) using a step size of 1 m (town or flats setting) and an initial search for ground points as ‘-fine’; and (3) Las2DEM tool to produce a continuous bare-earth digital elevation model (DEM) from the point cloud using the remaining elevation values at a step size of 1 m and exported as a tiff raster format. We chose a 1 m grid size for our pre-flow raster because finer grid sizes detected too much vegetation, which interfered with generating a low-noise DEM from which paths of steepest descent could be generated. Processing time for SfM models typically took several hours on an ASUS computer (CPU: Intel Core i7 3.0 Ghz; 32GB RAM; GPU: NVIDIA Geoforce GTX 970) with additional time required to filter vegetation and run flow path calculations (1–2 additional hours). Pre-lava flow paths were calculated from the DEM generated by UAS overflights in early December, 2014, after an upslope breakout formed a new lobe that ran parallel to the original flow margin and cut off lava supply to the original flow front (Poland et al. 2016; Patrick et al. 2016). Alternative point-cloud processing software or programming libraries (e.g., CloudCompare, QT Modeler, MATLAB, OpenCV) could also be used in place of LAStools.
Tools to calculate paths of steepest descent are common in many Geographic Information System (GIS) software packages, usually falling under hydrology toolsets. For a more detailed discussion on how these paths are calculated please refer to Tarboton et al. 1991; Kauahikaua 2007. The pre-flow DEM was brought into ArcGIS (v10.2) where paths of steepest descent were calculated based on the lavashed concept (Kauahikaua et al. 1995) with the following hydrology workflow: (1) we filled sinks throughout the DEM to ensure proper delineation of stream networks; “sinks” are often errors in the DEM due to rounding of nearest integer values and need to be filled to avoid a discontinuous drainage network; (2) flow direction was calculated using the filled DEM as input in the Spatial Analysis extension and (3) flow accumulation was generated using the flow direction DEM. The final paths of steepest descent were based on the 1 m DEM.
We also examined the impact of DEM spatial resolution on lava flow paths. This was done by resampling our UAS-derived DEM at progressively coarser resolutions, up to 10 m resolution in 1 m increments, and calculating the resulting paths of steepest descent at each step using a comparable flow accumulation threshold (Jenson and Domingue 1988).
Like hydrologic watershed boundaries for stream networks, the contributing areas of potential lava flow paths can be delineated based on topography. Lavasheds represent the area in which a given flow path can be identified and serve as a way to identify possible paths of steepest descent at varying resolutions (Kauahikaua et al. 1995). Calculating these lavasheds can provide a more synoptic view of the potential paths a flow may travel within a confined geographic area. Lavasheds were calculated using the ArcGIS watershed geoprocessing tool, and pour points were set at every fork between paths of steepest descent producing a lavashed per path. To quantify the ratio of flow length and total area of a unit watershed, the length of each flow path per lavashed unit was divided by the total area of that given lavashed unit. The mean of all lavasheds and flow lengths were taken for each DEM. The flow path length / lavashed area was then converted to a logarithmic scale with a base of 10.
Implications for lava flow monitoring
The short-term behavior of long-lived pāhoehoe flows is difficult to predict, as advancing lava is influenced by new breakouts at the front or along the margins of an active flow and requires a continuous supply of new lava through the molten core (Kauahikaua et al. 1998; Hamilton et al. 2013). During the Pāhoa lava flow crisis, early forecasts of flow behavior were thwarted by hidden ground cracks in the Puna forest reserve (Poland et al. 2016). Accurate and up-to-date measurements of underlying topography are crucial for flow behavior analysis, and are a critical gap in eruption monitoring that UAS can fill.
An example of this can be seen around the Pāhoa Marketplace, built in 2004, where the NED 10 m DEM flow paths pass directly over a 5 m wide water diversion ditch surrounding the development (Fig. 3). The primary reason this occurs is because the 10 m DEM was generated prior to the construction of the Pāhoa Marketplace and excavation of the drainage ditch. A second reason is due to the coarseness of the 10 m DEM. Based on our resampled DEM experiment (Fig. 5) we see that DEMs with a resolution >6 m do not pick up this feature, while UAS-derived DEMs with a resolution below 5 m produced flow paths that recognized the water diversion ditch. While it is unclear how or if the ditch would influence final emplacement of the flow, it is a substantial topographic feature that coarser datasets, even up-to-date ones, do not detect. Higher spatial resolution DEMs allow for a higher number of lavasheds to be detected (Fig. 5), making them valuable for short-term hazard assessments.
Additional insight on the future behavior of a flow could be gleaned by finding the critical threshold at which a pāhoehoe flow will jump between given lavasheds (Fig. 5) based on flow parameters such as size, mass flux, and surrounding topography. How important the boundaries of a given lavashed are in constraining the direction of a flow will depend greatly on the current stage of maturity in an ongoing eruption, the type of flow (e.g., sheet flow, hummocky tube-fed flow), current topography, and the context of the response efforts (short-term or long-term hazard planning). High resolution DEMs can provide important information towards observing the boundary at which flow paths diverge (Hon et al. 1994; Hamilton et al. 2013).
Lava flow monitoring with UAS can provide high spatial resolution DEMs with the capability for frequent and cost-effective overflights to reflect new changes caused by flow activity. The datasets provide a detailed record of morphological changes across the flow field, serving as a time-series digital repository of observations on pāhoehoe behavior. This can be of high importance during a prolonged effusive crisis because eruptions can span weeks to years, and budget limitations control the frequency of helicopter overflights to monitor lava flows (Patrick et al. 2016; Orr et al. 2015b). A key observation from a HVO study on the 2014–2015 flow showed a large percentage of surface activity occurring away from the immediate flow front in the form of lateral breakouts well behind the active front (Patrick et al. 2016), underscoring the importance of extending the operational range of UAS to capture areas beyond the active front of an advancing pāhoehoe flow. Doing so would extend the benefits of higher spatial and temporal resolution across the entire flow and throughout the flow’s evolution.
Applications for other lava flow models
This work has focused on the lines of steepest descent technique, one of the simplest methods for predicting lava flow paths (Kauahikaua 2007). However, more sophisticated lava flow modelling efforts, including stochastic slope-controlled models (Harris and Rowland 2001; Favalli et al. 2005), cellular automata models (Crisci et al. 2004; Del Negro et al. 2005; Vicari et al. 2007), and other numerical simulations (Dietterich et al. 2015), also rely on high quality DEMs input layers to produce successful results. UAS provide a means of effectively generating these needed DEMs, regardless of the modeling method. For models that are computationally complex, simulation run time is an important factor to consider for the results to be of value, especially during an active crisis that demands quick and frequent updates to changing conditions. The ability of UAS to rapidly and continuously update changes in topography can reduce or even negate the requirement for running multiple flow paths in a stochastic simulation. Beyond collecting the UAS data, an important aspect of effectively incorporating this technology into hazard operations will come from automating the DEM post-processing steps (e.g., vegetation filtering, combining multiple flights) as much as possible to make new DEMs more readily accessible for given scenarios.
Successful volcano monitoring with UAS
The successful integration of UAS into active volcano monitoring programs still has challenges, including evolving aviation regulations. In the United States, the Federal Aviation Administration has released its small UAS regulation Part 107, which permits UAS flights by pilots that pass a remote pilot exam. Waivers can be acquired to do special types of flying such as night operations, flying beyond line-of-sight (BLOS), and having a single UAS pilot control multiple aircraft simultaneously commonly referred to as SWARM flights (Diaz et al. 2015). For most eruption settings, large-scale monitoring over the length of the entire flow field will require BLOS flying and/or the use of SWARM technology with multiple UAS mapping sections of the lava flow simultaneously. In the future, these types of operations can significantly enhance the monitoring capability of volcano observatories worldwide within existing monitoring programs. In addition, UAS-derived topographic datasets can be easily combined with other monitoring tools such as thermal infrared mapping and field-based measurements of pāhoehoe flow inflation and advancement (Patrick et al. 2016). The increased mapping frequency UAS flights afford, due to their low operational costs, can be a substantial boost to effective monitoring and tracking of both pāhoehoe and a‘ā lava flows at effusive volcanoes worldwide.
During response efforts for the Pāhoa crisis, all imagery and related data products from this project (DEM, orthomosaics) were shared with both emergency managers at Civil Defense and scientists at HVO. Although UAS data were shared as quickly as possible, final delivery of data products often took 1–2 days due to field logistics, travel time, computational processing, and coordinating with Civil Defense analysts. If UAS are to be used effectively in a future crisis, increased data workflow automation can prevent staleness of information. Other advancements in UAS technology, including connecting field operations to the internet via cloud related hardware, can allow field teams to share imagery in near-real time. Structure-from-Motion processing could be done in the cloud or imagery downloaded locally to be processed before teams even leave the field. Other approaches include computer vision based machine learning models to extract only the most relevant information from UAS imagery (e.g., classifying and extracting fresh lava), reducing bandwidth requirements and transferring only the most critical data during flight. If placement of GCPs are too dangerous for UAS crews, other technologies such as high precision real time kinematic (RTK) GPS or omitting GCPs entirely and sacrificing DEM accuracy are possible alternative approaches. The most successful future integration of UAS for volcano monitoring programs will likely include all of the above as observatories supplement existing monitoring networks (e.g., seismic networks, GPS stations, gas monitoring, weather stations) with UAS technology.
Additional applications for UAS in lava flow monitoring
Beyond continuous updates to changes in topography and generating high resolution DEMs, products derived from this work can be used to gain additional insights into the emplacement processes of active pāhoehoe flows, including detailed measurements of lava flow inflation rates, flow thickness, volumetric flow rates, and characterization of features (e.g., tumulus formation, surface vs lateral breakouts, sinuous inflation ridges, hummocky vs sheet morphology, lava partitioning) over spatially and temporally useful scales to improve emergency response efforts (Perroy et al., 2015). Consecutive Structure-from-Motion derived three-dimensional surface models of the flow, collected over hours to weeks, can effectively track the advancement of a flow, formation of a master tube system, and related breakouts. Merging these datasets with aerial thermal infrared imagery captured contemporaneously may yield new insights into flow emplacement behavior (Patrick et al. 2016).
UAS technology can provide a detailed synoptic view of a flow’s evolution and emplacement behavior, possibly spurring the development of new predictive models designed specifically for pāhoehoe lava flows. These models will need to incorporate the complexity of an actively evolving flow field including the development of a master tube system prone to leakage, identify the morphology of breakouts (e.g., sheet and hummocky lava), reflect changes in lava supply/storage through inflation rates, and rapidly update the modifications in topography. UAS provide the capability to collect this type of data both spatially and temporally.
Generate high spatial resolution DEMs from low-altitude UAS flights using low-cost optical sensors.
Better estimate future flow path behavior and reduce uncertainty in hazard forecasts.
Improve the frequency and quality of data collected by volcano monitoring programs, due to the low operational costs of UAS and their ability to fly on short notice.
Capture new topographic changes caused by pāhoehoe flow inflation, providing detailed updates to obsolete topographic datasets that were not possible before.
Supplement existing remote sensing (e.g., satellite and manned aircraft) and field based monitoring of active flows.
As UAS technology progresses and matures, we believe future monitoring efforts that utilize UAS will fill a critical gap in volcano monitoring. UAS hold great potential for improving response efforts during a volcanic crisis and are an effective tool for mapping changes for both short-term and long-term lava flow hazard assessments.
We would like to acknowledge the hard work of Nathan Stephenson and Arthur Cunningham for their help in the field with UAS operations; Victor Rasgado for his time and equipment as well as help from his students at Hawaii Community College; Donald Straney for his support with equipment and personnel funding; Daryl Oliveria and the dedicated staff at Hawaii County Civil Defense for their support, access to field sites, and close coordination during the crisis; and Tim Orr, Matthew Patrick, and the staff of Hawaii Volcano Observatory for their professionalism and field support. Parrot senseFly provided a second SwingletCAM aircraft during the crisis; their support was greatly appreciated. Suggestions from Jan Lindsay and two anonymous reviewers greatly improved the final manuscript and NRT would also like to thank Bruce Houghton and Sebastien Biass for their helpful comments.
NRT is grateful for support from the University of Hawaii at Hilo Office of the Chancellor during the crisis and from the National Disaster Preparedness Training center during the writing of this manuscript. This material is based upon work supported by the National Science Foundation Graduate Research Fellowship under Grant No. (1329626). This work was partially funded by NASA EPSCoR Grant NNX15ANZZA.
All authors have reviewed this manuscript and contributed to its content. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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