Number and Facility Type of Natech Events
Natech events identified with the NRC record totaled almost 10,000 between 2009 and 2019 giving a total of over 26,000 natech events between 1990 and 2019. Rain related events make up a quarter of events, followed in order by hurricanes, weather, wind and floods. These results differed somewhat from those by Luo et al. (2020) who identified a greater number of hurricane, storm and flood events. The discrepancy in hurricane events is in large part due to several thousand reports associated with ongoing releases from a single offshore platform destroyed during Hurricane Ivan in 2004 which were treated as duplicate reports and are not included in this assessment. Evaluation of the higher number of storm and flood related events reported by Luo et al. (2020) suggest that they are largely attributed to events incorrectly identified as natechs due to their description (e.g., a spill affecting a storm drain).
The largest number of natechs are associated with releases from fixed facilities many of which are caused by rain, followed by floods and hurricanes. Second most common are releases from transformers associated with wind, storms and lightening. Third most common are releases from storage tanks equally due to hurricanes, rain and floods. These findings are generally consistent with those of Sengul et al. 2012 with the exception that releases from platforms due to hurricanes were more prominent during the period from 1990 through 2008 (approximately 12% of the total), compared to 2009 to 2019 (only 3%). This is noteworthy given that subsequent to significant damage to offshore platforms during the 2004 and 2005 hurricane seasons the American Petroleum Institute issued updated guidance pertaining to hurricane risk to offshore platforms and significant investment in upgrades of offshore platforms has been reported (Cruz & Karausmann 2013).
Materials and Size of Releases During Natech Events
Petroleum releases are a majority (76%) of the total reports between 2009 and 2019 while chemical releases are only 11% of reports and wastewater releases were 5% of the total. This represents an increase in the fraction of petroleum events compared to Sengul et al. (2012) who reported petroleum releases as only 60% of the total. Given the wide variation in the size of releases, many of which are small, the quantity of material released due to natural hazards could give a better indication of risk than the number of releases alone. However, due to the limitations of the NRC database, quantities are not expected to be accurate, particularly for offshore events where estimation of the volume of a release is more difficult. For example, a total of about 90,000 gallons of petroleum is listed as being released from offshore platforms due to various hurricanes during 2009 to 2019. In contrast, recent estimates of crude oil released between 2009 and 2017 from a single platform destroyed during Hurricane Ivan in 2004 range from 0.7 to 3 million gallons (Skytruth 2017). When reported, spill quantities do provide a useful qualitative indicator of event severity.
Volume of petroleum released was reported in approximately two thirds of total records. Total petroleum released was approximately 3 million gallons during the period from 2009 to 2019. Gasoline, crude oil and diesel/#2 fuel oil were released in the largest volumes, about 650,000 gallons each. In contrast the record from 1990 to 2008 was dominated by several large crude oil releases primarily occurring during Hurricanes Katrina and Ike. Although most petroleum releases are reported in gallons, a single large release of asphalt totaling 2 million pounds due to flooding was also reported. In the period from 2009 through 2019 storage tank releases of petroleum were the largest in volume averaging over 4,000 gallons in size followed by pipelines (2,000 gallons) and vessels at 1,000 gallons. Hurricanes, lightning and floods accounted for most of the larger events and the majority of petroleum releases as illustrated in Fig. 1. While the large volume of petroleum released during hurricane and lightning related events is consistent with observations by Sengul et al. (2012), the large volume of petroleum released due to flooding was not observed in the earlier record and is associated with a number of large releases between 2015 and 2017.
The amount of chemical released was reported for only one third of events. Chemical quantity was most often reported by weight and totaled 1,800,000 pounds between 2009 and 2019. These releases were predominantly from flaring (sulfur dioxide, carbon monoxide) or other air releases from fixed facilities due to hurricanes, lighting and other weather. This differed from observations from Sengul et al. (2012) who noted that several large flood related releases dominated the mass of chemicals. Anhydrous ammonia stands out as the material released most commonly (over 100 releases) and in the greatest amount with a source other than flares. These releases commonly represent damage to refrigeration systems at storage tanks and other fixed facilities. Chemical releases reported by volume made up one third of events and totaled approximately 650,000 gallons. The majority of the total was from a small number of large volume releases from storage tanks during hurricanes in 2012 and 2017. Sengul et al. (2012) also observed substantial volumes of chemicals released by hurricanes but in that time period cold, flood, rain and weather also resulted in sizable releases. Although reported less frequently, the NRC database also includes reports of releases of 14,000,000 pounds of solid bulk materials such as coal ash, largely due to storms and hurricanes, 154,000,000 gallons of wastewater, attributed primarily to rain and to a lesser extent hurricanes, and 52,000 million cubic feet of natural gas primarily attributed to the impact of storms on fixed facilities. In all cases the volumes of these materials were highly dependent on a small number of very large events.
Natech Events with Human Impacts
During the period from 2009–2019, 33 injuries were reported during 9 natech events and at least 2,600 people were evacuated in 85 events. Flooding was associated with two events with injuries with the rest due to lighting or other weather. Of evacuation events, 38% were associated with lightning, another 38% were associated with various weather, largely cold, wind and rain and 20% were associated with more severe natural hazard primarily floods (9%), tornadoes (6%) and hurricanes (4%). Releases at fixed facilities, storage tanks and pipelines accounted for the majority of evacuations.
These results are generally in accord with Sengul et al. (2012) who identified 2 deaths, 52 injuries, and the evacuation of at least 5,000 persons in 102 separate natech events. However, due to the nature of the NRC record, these human impacts are expected to be underreported. As an illustration, while the NRC database contains multiple reports of releases at an Arkema facility triggered by flooding from Hurricane Harvey in 2017, these records do not report the numbers of injured (at least 21) or evacuated persons (which included more than 200 residents).
Trends in Natech Occurrence
The frequency of natechs caused by various natural phenomena from 1990 to 2008 as reported by Sengule et al. (2012) and from 2009–2019 are illustrated in Fig. 2. During the period from 1990 to 2003 natechs averaged 2.4% of all reports; while since they have increased, averaging 4.4% of the total. The number of all hazmat releases reported to the NRC, on average 29,000 per year, remained reasonably stable during this period. Hurricane related events increased sharply beginning in 2004 with Hurricane Ivan and have remained high during years with significant hurricane activity, accounting for much of the increase in natech occurrence.
Although the largest changes in natech occurrence are in hurricanes related events, events associated with other weather have also changed over time. The full natech time series from 1990 through 2019 was evaluated for trends. As they are not clearly distinguished in the NRC record, storm, weather and unknown cause events were evaluated as a combined total for each year. Trends were evaluated for both the US as a whole and for climate regions as defined by NOAA for tabulation of CEI data. Table 1 summarizes the average number of natech events per year, slope of linear regression of annual frequency and confidence level of a Mann-Kendall trend test. Nationally, increases (> 95% confidence level) were observed in total natechs and in subsets related to flood, hurricane, storm/weather, tornado, wildfire and wind. Decreasing trends were observed for cold, tide and lightning events in the US as a whole.
Significant trends in these types of events were observed in some but not all climate regions. The number of hurricane related events increased (95% confidence level) only in the south and southeast. Storm events increased more widely, in the Ohio valley, south and northeast (where the largest numbers of such events were observed). Tornado related events increased only in the south where over half of events were reported. Wind related events increased in the Ohio valley, northeast northwest southeast and west, but not in the south where a large number of events were reported. While flood related events increased in the US as a whole, no individual region evidenced an increase.
Overall, natechs from all causes have increased (> 95% confidence level) in the northeast, due to increased events from wind, storms and hurricanes. Increases are also observed in the northwest due to rain and wind. In contrast, in the south and southeast where large number of natechs are reported, trends are not significant at a high confidence interval. This is because increases in frequency of some types of events, most notable hurricane related, are partially offset by decreases from other causes such as rain and lightning. Similarly, in the Ohio valley region increasing trends in storm and wind events are offset by decreases in in rain and lightning related natechs.
Trends largely reflect the frequency of petroleum releases as they make up the majority of the records. Total chemical natechs in the US evidenced no significant trend. Chemical natechs associated with hurricanes varied from year to year with a pattern similar to petroleum natechs but did not evidence a significant trend. Chemical releases caused by lighting and rain decreased significantly but this results from a sharp decrease starting in 2006 associated with changes in reporting requirements for flaring releases (Sengul et al. 2012). Likewise, annual number of natech releases of natural gas had no significant trends. Trends in the quantity of various materials released over time were not evaluated quantitatively due to the incomplete and episodic nature of the data. However, in general, the quantity of material released in each year for the US as a whole had no obvious temporal trends.
Identified trends are potentially affected by sharp increases in oil and gas development in the US during recent decades. To evaluate this factor, state level petroleum production data from 1990 through 2019 (USEIA 2020) was compared to natech occurrence in each state both for the total record and record excluding hurricane related events (which were highly variable from year to year). Of 30 states with oil production data, no significant positive correlation was observed between annual natech occurrence and annual production volume, with two exceptions. For Oklahoma moderate and significant correlation (R = 0.52 p < 0.05) was observed between production and natechs. Both production and natechs had similar trends of a gradual decrease through 2010 followed by increases with a sharp peak in 2015. This results in an overall trend of approximately 0.5 additional natechs in Oklahoma per year. The same pattern was observed in total natechs and the subset of the record excluding hurricanes. For Louisiana a strong and significant correlation (r = 0.7 p < 0.05), was observed for the record excluding hurricanes with both production and natechs having a declining trend amounting to roughly a halving over the record (a decreasing trend of around three natech events per year). In both states, rain related natechs accounted for these temporal patterns suggesting that, in these two areas, changes in production affected the incidence of only smaller spills. A recent review of all petroleum spills in the US (CRS 2017), also suggests that increased production has not resulted in a greater frequency of releases nationally over the last two decades. Overall, this assessment indicates that changes in petroleum production have not been a major driver of the natech occurrence summarized in Table 1.
Correlation with Occurrence of Causative Hazards and Climate Index Values
To evaluate correlation between natech occurrence and that of the underlying natural hazards, the natech record was first compared with annual reports of damaging weather events from the NOAA Storm Database from period from 1996 through 2019. Observed Pearson correlation and significance level is provided in Table 2 nationally and for US climate regions. As expected, positive correlation was generally observed between frequency of weather events and associated natechs. Exceptions were lightning related events in the upper Midwest and wind related events outside the continental climate regions (Other) where a significant inverse correlation was observed. This reflected an increase in natechs, while reports of damaging lightning and wind decreased over the same period. The increased lightening related natechs were largely strikes on residential gas meters in Minnesota while wind related events were largely vessels outside the continental US. The specific nature of these releases suggests this inverse correlation is an artifact of changes in reporting practices. On a national level, annual natech occurrence for flood, hurricane, lighting, tornado, wildfire and wind was weak to moderately correlated (p < 0.05) with the reported occurrence of these events in the Storm Database. Similar or higher levels of correlation were observed in many US regions. Notably flood, hurricane, tornado and wind related natechs, showed moderate to strong correlation in a number of regions. Overall, the level of correlation in Table 2 suggests that the interannual variability in many natechs is dependent, at least in large part, on the frequency of damaging weather. Although correlation is in most cases moderate, this is unsurprising given the variability in the population of facilities which may be exposed to any given weather event. Additionally, in the case of hurricanes, Storm Database events associated with named hurricanes are often recorded under proximal causes of damage such as wind reducing the calculated level of correlation. Where no correlation is observed across all regions (i.e., rain) this may reflect the fact that a majority of rain related events are minor in nature and may not be triggered by a damaging event.
To place the natech record in the context of a longer climate history, reported releases were also compared to individual CEI component values from 1990 through 2019 as summarized in Table 3. In addition to comparison of natechs to directly related weather indices, landslide, lightning, wind and tornado natechs were compared to extreme precipitation CEI as a general indicator of convective weather. On a national level, weak to moderate correlation (p < 0.05) are observed between flood, hurricane, tornado, wildfire and wind related natech occurrence and related CEI values. Despite discrete causative events with high interannual variability, hurricane natechs only weakly correlated with the tropical storm CEI. A similar weak correlation (r = 0.38, p < 0.05) is observed between annual hurricane natech frequency and annual Accumulated Cyclone Energy (ACE) commonly used as a measure of hurricane energy. This low correlation appears to reflect variation in the industry present in areas where hurricanes make landfall (e.g. low natech occurrence associated with Hurricane Andrew in 1992) as well as the fact that tropical storm CEI and ACE are based on winds speed while hurricane natech occurrence is heavily driven by storm surge (Santella et al 2010), the size of which are also dependent on other factors. These findings are consistent with those of Luo (2021) who observed low levels of correlation between hurricane natech occurrence and climate indices affecting tropical storm intensity and frequency. On a regional level it is notable that moderate and significant correlation is observed between flood natechs and extreme precipitation CEI in the northeast, northwest and southern regions but not in other regions. These three regions make up more than half of total flood natechs. In the northeast and south both CEI and natechs have similar increasing trends, about a 2% increase annually. In the northwest they are correlated with no overall trend. Correlation with rain related natechs in Table 3 is higher than in Table 2, similar or higher levels of correlation are observed with the CEI value measuring wet years compared to the extreme precipitation CEI. This may reflect the fact that a majority of rain related natech events are minor and more dependent on frequency of rain rather than severity. Correlation is also observed between CEI values for heat and dry years and the relatively small number of wildfire related natechs.