1. Introduction
Volcán de Fuego (14.473°N, 90.88°W) is a 3800 m open vent stratovolcano with 3000 m of local relief located in central Guatemala. Fuego actively produces pyroclastic material during small Strombolian eruptions with varying frequency up to tens of times per hour [
1] depositing material ranging from ash-sized to bombs and blocks on the upper slopes. The current style of activity, including occasional larger paroxysms producing pyroclastic currents (PCs) has been continuous since May of 1999, but Fuego has been intermittently erupting for centuries. Fuego’s usual mode of Strombolian activity is punctuated with larger paroxysmal events including 50 eruptions with a Volcanic Explosivity Index (VEI) ≥ 2 since 1524 [
2]. These larger events produce PCs [
3], which fill drainages radiating from the volcano with hot debris easily mobilized during the rainy season. Our study site, Las Lajas drainage, was filled in during a series of PCs in June of 2018 including one flow, which jumped the bank of the channel and devastated the village of San Miguel Los Lotes, killing many and burying the structures in hot rock [
3]. The estimated volume deposited in this series of PCs was 15.1 ± 4.2 × 10
6 m
3 [
4]. The majority of the material originated from the bulking and subsequent erosion of pyroclastic material at the top of Las Lajas near the vent. This led to increased mobility, allowing the PC to travel much farther than earlier events in the series, reaching 12 km from the summit [
4].
Lahars are slurries of mud and debris common at Fuego and other volcanoes. They are analogous to debris flows except that they occur on the flanks of volcanoes and transport primarily volcanic material. Lahars account for about 25% of the volcano-related deaths since 1500 AD, second only to pyroclastic currents (PCs) [
5]. Most infamously, a primary lahar, induced by an eruption in 1985 at Nevado del Ruiz, Colombia, resulted in 28,000 deaths in the City of Armero [
6]. Smaller secondary lahars, induced by rainfall rather than eruption, are a much more common and persistent hazard at many volcanoes. Lahars are particularly dangerous because they do not require an eruption trigger. Many years after eruptive activity has ended, secondary lahars still present hazards controlled by the local weather. In October of 1998, for instance, Casita volcano in Nicaragua, which has no historic record of eruptions, was hit by a hurricane that caused an avalanche and lahars which killed an estimated 1600 people [
7].
Secondary lahars are generally characterized by a bouldery snout followed by a watery tail [
5,
8]. These events may be very common due to erosional activity caused by rainfall remobilizing pyroclastic materials [
9,
10]. Lahars at Volcán de Fuego, featured in this study, commonly incise deep channels that can be over 100 m deep in places with steep and unstable walls. Lahars can quickly erode such walls and bulk up creating hazards that are extremely dangerous and destructive near to a drainage as well as in fluvial fans far from the volcano [
10]. They can be an especially significant threat to populated communities, local infrastructure, and property situated adjacent to these channels. Lahars are capable of forcing boulders and logs against bridges or dams with impact forces of 10 to 1000 tons per square meter [
7]. Channel paths are often meandering and dynamic and can erode into farmland and create steep cliffs. Flooding is another concern when lahars pass beyond their confining channels. Structures may be inundated and destroyed due to a shifting or reactivated channel. Road destruction and temporary closures are a common occurrence at Fuego when the activity is high because most vehicular transport crosses channels without bridges. Even small lahars are highly erosive and can undercut banks and bridges. Lahars can also be harmful to infrastructure through deposition. They “freeze” when the flow energy is dissipated enough that the head cannot be pushed forward anymore and the water seeps away, leaving a deposit of condensed sediment and boulders [
5]. If that deposit is on or around infrastructure or agricultural lands, removal can be costly. Lahar deposition may divert streams, widen flood plains, and shift meanders forcing land use adaptation [
3,
11].
Unoccupied aerial vehicles (UAVs) are growing in popularity for use in natural hazard disaster management due to their ever-increasing accessibility and ease of use [
12]. Although they are limited in areal coverage and surveys may be impacted by weather conditions, they are versatile, low-cost tools for collecting high-resolution data with short notice [
13]. Trends in research concerning rapid onset natural hazards utilizing UAVs under 25 kg have been on the rise since 2014, when the number of journal articles went from 9 or less per year to over 180 by 2020 [
12]. UAV use in the geosciences is expanding outside of natural disaster research as well. Many areas of research that have classically involved heavy field mapping with boots on the ground can be streamlined with these tools [
12,
13,
14,
15,
16,
17,
18,
19,
20,
21].
Given the rapid morphological changes present in active lahar channels UAV-acquired structure from motion (SfM) repeat 3D mapping of the terrain is a useful and practical tool for the study of both lahar erosion and deposition. SfM has been used to map geomorphic change over time in many studies ranging from volcanic craters to river morphology [
14,
15,
16,
19,
21]. UAV-acquired data enhanced utility over satellite data for a focused study area because the cost of operation for producing high-resolution maps (spatially and temporally) is relatively low. Such focused mapping can complement higher spatial coverage and lower-resolution satellite-derived topography. Inexpensive, consumer-grade UAVs have been proven to produce quality SfM products at sub-meter resolution [
14,
15,
16,
19,
20,
21]. This permits high accuracy DEM of Difference (DoDs) models. Our study showed the capabilities of SfM DoDs for quantifying the erosional and depositional regimes of a lahar channel over the course of a rainy season.
3. Results
The average ground sampling interval of the initial photogrammetric modeling prior to export from Agisoft was 5.22 cm/pix. The average RMSE for SGCPs in the xy direction was 3.44 cm and the average check point RMSE in the xy direction was 12.66 cm. The estimates for the z direction through Agisoft Metashape Pro were not included in the error estimation because the z was manually adjusted to a tolerance of less than 1 cm. The ground sample interval during export from Agisoft was adjusted to 10 cm/pix to reduce the computational work and make a uniform format for DEM merging and DoD calculation.
SfM reconstructions were used to generate accurate DEMs and orthomosaics for four different time steps to use in spatial and temporal analysis of landform change (
Table 1). These reconstructions span approximately 3.5 km of the Las Lajas drainage with a total surveyed area of 1.6 km
2. Volume change approximations can be made by subtracting these high-resolution DEMs at different time steps. The sample DoD in
Figure 4, for instance, shows the aggradation and erosion occurring for the entire season between June and October (
Figure 4). DoDs are also calculated for time steps spanning 36 to 48 days each, during which as much as 10 m of vertical erosion was evident in some areas. The observed erosion followed normal river meander patterns, though it was remarkably rapid given the energetics of the lahars [
24].
The DEM differences allowed high-resolution profile changes to be extracted in key areas throughout the channel reach. For example, the cross-section A–A’ in
Figure 5 shows a deep cut in the apex of a meander with 7 m of vertical erosion and 12 m of lateral erosion over the course of the entire rainy season. The steep and unstable walls were often undercut and slough off in this area. In cross-section B–B’, lateral migration and vertical erosion were evident without the extensive volume loss from collapsing channel walls compared to cross-section A--A’. Cross-section B–B’ still represented a strong erosional regime, but with less of an impact due to the relatively minor lateral erosion into the outer bank. Cross section C–C’ showed a 10 m wide constriction formed from an old, indurated deposit. Further downstream, cross-section D–D’ showed greatly reduced erosional features and some aggradation as the stream path freely meandered in the wider unconfined channel.
The volume change in each zone fluctuated throughout the season (
Figure 6) and was evident primarily as either losses or gains, whose entire volume tended to cancel out when summed across the ROI (
Figure 6a,b,d). In order to quantify the dynamism of the morphological change we computed the absolute volume transfer as the MAF (Equation (5)), which is shown in
Figure 6c.
Fuego’s common secondary lahars were triggered by precipitation remobilizing material on the flanks of the volcano. Correlation between rainfall totals from May through October and lahar events was evident, but the amount of rainfall, measured at the FGLJ rain gauge, did not necessarily correspond to the qualitative intensity of a lahar (
Figure 7). Since FGLJ was a point location, it might not represent the total rainfall distribution over an area, which fed channels in a variety of watersheds. The relative lahar intensity was a qualitative intensity of the lahar assessed by INSIVUMEH scientists based largely on seismic amplitudes. Values ranged from 1 to 3 on the graph, with 3 being the strongest lahar typically seen in this area. Every lahar event had a corresponding rainfall event, but the converse was not true. Out of the 24 precipitation events with an intensity greater than 2 mm/h, 19 resulted in lahars.
The sinuosity of each 500 m section was calculated separately and suggests that the upper sections (a–e) possessed much more variability in SI than the lower sections (f–i) (
Figure 8). The average sinuosity of the entire channel was between 1.4 and 1.5, which, according to Rosgen [
24], is at the boundary between sinuous and meandering. Meandering was common in sections a–f, as opposed to the strongly sinuous classification below the indurated deposit knickpoint in section f at km 2.6. The tortuosity of the channel was evident when the stream path was mapped onto the DEM (
Figure 9a). The SIs for each section and each period compiled into one figure showed significant variability over time (
Figure 9b–d).
SI variation over time is another useful metric to quantify the geomorphic dynamism of a rapidly changing fluvial system. The amount of change occurring in the sinuosity index in each section was calculated using a relative percent range (Equation (4)). The data showed a large shift in sinuosity (ΔSI) from section e to f. Although all ΔSI values were positive, the greatest relative percent range was in section d with 20% and the smallest changes in SI were in sections f and g (
Figure 10).
The slope of the channel was measured and compared for each of the 500 m channel sections using the vertical drop from DEMs and horizontal distance from the tortuous flow path. Slopes changed very little over time (less than 1 degree), and this was largely attributable to increasing flow distance due to increasing sinuosity. Slope values for the ROI ranged between −4.6 and 9.0%, as measured in the June survey (
Figure 11)
4. Discussion
Quantified erosional and depositional volume rates in the Las Lajas ROI provide insights into processes that occur over both spatial and temporal domains. During the three intra-survey measurements, we note that both the amount of rainfall and number of lahars were relatively constant (
Figure 7). Assuming geomorphic change is principally related to the lahar activity, we expected similar volumetric transport when averaged over approximately one-month intervals in the rainy season.
Notably, ΔV
t21 and ΔV
t32 showed opposite behaviors in terms of the net volume change (
Figure 6a; sections a–f). We considered the possibility that a single significant event, such as a large wall collapse, might be considerably impactful and could have dammed up the channel following the July survey (t2), resulting in sections b, c, and d showing exceptional deposition. (
Figure 6b,d). Section d showed a high MAF with a substantial gain and an even greater loss (for a slight net loss). The DoD illuminated that the loss was from a large portion of the channel wall collapsing and a gain from the subsequent damming and upstream and capture of sediments. Another plausible explanation is inconsistency in sample intervals; data below section e was collected prior to a September 1 small lahar, which preceded the surveys in Zones A and B. That small lahar might, thus, have caused the geomorphic change that was only recorded below Zone B (section e and below) (
Figure 6a). It is reasonable that a small lahar with shorter runout may have deposited more material in upper sections of the ROI (
Figure 12). The net volume changes calculated for DoD 3 showed a steady increase with increasing flow distance out through section i, suggesting that during this time interval (July through August), some lahars retained enough energy to carry and drop large quantities of material at the lowest extent of the ROI (
Figure 6a).
Although the meanders in the stream path appeared relatively dynamic throughout the 2021 rainy season, it is notable that the meandering flow was confined to the gross channel topography that existed prior to 2018. Google Earth time-lapse imagery from 2006 (prior to the construction of the La Reunión golf course), 2018 (prior to the PC), and 2021 each showed features associated with a terraced floodplain and in-filling with potential PC and lahar flow deposits. Las Lajas drainage was relatively stable within the wider channel for some time prior to 1969 according to Google Earth and these early images showed no evidence of recent migration in the form of scoured vegetation and slide scarps (
Figure 13A). The floodplain was effectively unimpacted until the catastrophic 3 June 2018 PC event, which overtopped the Las Lajas channel and flowed towards San Miguel Los Lotes [
3]. The addition of material into the system filled and dammed the channel temporarily, which diverted the flow (
Figure 13C).
Analysis of imagery since 2018 showed that loose PC material within Las Lajas was downcut quickly, resulting in the development of a meandering path over time without any influence from the surrounding or buried topography apart from the outer banks of the old channel. The sinuosity indices of sections a through e were higher and more widely varying over time compared to those from sections f to i (
Figure 8 and
Figure 10) and we suggest this was because tall unstable walls are easily undercut and collapse more frequently, which diverts the flow within the channel and adds to the material available for bulking.
Observations of how the meanders were confined to the pre-2018 terrace suggest an opportunity to evaluate risk when developing infrastructure and planning land use. The golf course was built partially within the previous channel infill and it is unsurprising that these deposits were impacted by subsequent lahars. In the case of San Miguel Los Lotes, the PC did jump the channel and devastated the community, which was located outside of the Las Lajas drainage proper (
Figure 1).
While the upper sections of our study area appeared particularly dynamic, the lower sections were more static. We observed a transition from the upper sections to the lower in section f at a natural constriction, which then became wider before going over a waterfall in section g. This waterfall, just below the bridge, was at an old, indurated deposit with resistant rock serving as a knickpoint. The local base level at the knickpoint accounted for some of the stability of the channel from the constriction above it. The slope of the channel between sections e and f decreased with distances downstream from −7.6% to −5.9%. The channel width, on the other hand, increased from ~10 m at the constriction to between 40 and 60 m before the waterfall. We assumed that much of the energy carrying the type 2 lahar material was lost here. The stability of the geomorphology was reflected in low sinuosity and the cross section (
Figure 5D) indicated an evenly mixed regime of deposition and erosion. This was also evident when comparing the high MAF to the low net volume difference. The findings in this study may inform identification of higher risk zones at a course scale in areas with high erosion and lateral channel movement. The placement of the bridge, for example, appeared to be in a low-risk zone for typical type 2 lahars. The location was advantageous, given the combination of the constriction upstream and the indurated layer as a local base level. This was the most stable section of the channel to have major infrastructure.
There were several limitations to this work, motivating future study and improvements. UAV surveys such as the featured examples were limited by weather, battery life, and altitude restrictions. Only a relatively small areal area can be effectively surveyed for analysis and it could be desirable to increase the ROI area for broader volume calculations. The lack of ground control points makes accurate geo-registration impossible. The temporal resolution, at approximate monthly intervals, allows for the generalized impact of many lahar events, but does not allow for study of individual lahar depositional and erosional statistics.
Overall, the Las Lajas channel quickly eroded the PC material at the top of the study area and deposited much of the material at the bottom or beyond. The MAF indicates where major erosion and deposition were simultaneously happening with a net volume change relatively close to zero. The relative percent range of the sinuosity index decreased dramatically where the channel was restricted and changes from erosion-dominated to deposition-dominated behavior. Future work could focus on collecting DoDs with higher temporal resolution in a channel that does not have a constriction. This could be used to investigate if the MAF and sinuosity index can robustly indicate where the change from erosion to deposition occurs.
5. Conclusions
Secondary lahars at Volcán de Fuego are frequent and capable of moving large volumes of material over the course of a rainy season. The constant addition of new material near the vent through volcanic activity, including PCs every few years, combined with heavy precipitation makes Volcán de Fuego a natural laboratory to study the impacts of lahars on the geomorphology of the channels they occupy. Analyses of DoDs in 500 m channel flow sections elucidate the movement of material in both erosional and deposition sense. Using UAV SfM, we quantified volume changes of 490 m3/day loss in the upper sections (~6 km from summit) and a 440 m3/day gain in the lower sections (~10 km from summit).
Erosion and deposition along a lahar path are not solely dependent on the slope of the channel. This was evident using time-lapse volume estimates from four distinct aerial surveys. ΔVt21, corresponding to volume changes between June and July, showed deposition in several sections that only had erosion in the other periods. The slope in these sections changed negligibly during that span and the number of lahars and amount of rainfall differed very little from the other two periods.
We propose that the MAF is a useful measure of the dynamism of a channel as a function of erosion and aggradation. The net volume change, gross erosion, and gross deposition do not tell a clear enough story of the total activity occurring in the channel. The MAF allows for a quantitative measure of the total activity occurring within the channel. The area where the regime switches from erosional to depositional is the area where the MAF has the highest value due to the shifting channel eroding and depositing at near-equal rates.
The shape of the channel and some underlying indurated deposits significantly influenced the behavior of the channel in sections e through i. In section e, there was a constriction 12 m wide made of an indurated deposit, directly after a bend in the channel, which slowed the flow and presumably shed some of the energy. The constriction opened into a 60 m wide channel that flowed over a knickpoint formed from another indurated deposit, creating a local base level and stabilizing the stream behavior concerning sinuosity index and downcutting. Below the knickpoint, lahars ran out into a wide channel and gradually deposited material as they lost momentum.
The sinuosity index showed a distinct change in character from the upper sections, which were tortuous up high and less so further down Las Lajas, starting at section f. An increase in sinuosity over time was observed in the upper sections and can be a useful geomorphological statistic because it shows how lahars are actively changing flow paths.