On 20 May 2019, 14 tornadoes swept through Oklahoma and Western Texas overnight, resulting in severe damage to businesses, properties and vehicles. As a result, around 50 million people were considered to be at risk from severe weather, including strong winds, hail and flash floods (Lavandera et al., 2019).
Heavy rainfall associated with the tornadoes has contributed to many river gauges across the states affected (Kansas, Oklahoma, Arkansas and Missouri) exceeding the flood stage, the river height at which flooding may occur, with some recording new maximum river heights (Figure 1) (National Weather Service, 2019).
Kansas and Oklahoma were inundated due to prolonged periods of heavy rain (Lavandera, Grinberg and Yan, 2019). Initial reports estimate 15–20 inches (381–508mm) of rain has fallen across Oklahoma during May 2019, almost three times the average rainfall for the month, with five-day rainfall totals of 11 inches (279mm) recorded (McKay, 2019; Mitchell and Navarro, 2019) (Figure 2).
Flooding along the Arkansas River in Oklahoma
In Tulsa, water levels along the Arkansas River started to rise rapidly on 21 May, when the rainfall intensity peaked and flooding occurred two days later (Figure 3). The water level continued rising and reached a crest of 22.8ft, slightly less than the historical maximum of 25.2ft recorded at Tulsa in May 1986. At Van Buren station (Fort Smith), 40.5 miles (65km) downstream from Tulsa gauge station, the water level in the Arkansas River was already close to the flood stage before the onset of heavy rain. The water level at Van Buren began to rise on 22 May, exceeding the historical maximum of 38.1ft just six days later. It is expected to reach its peak height (42.5ft) between 30 May and 1 June 2019 (National Weather Service, 2019).
JBA has conducted return period analyses using Arkansas River peak height data from 1904 to 2019. Results from the analyses suggest the return period for the flood event at Tulsa is 50 years (Figure 4).
In the Arkansas-Red Basin, a total of 60 gauge locations are still experiencing flooding as of 30 May 2019, with 22 considered to be experiencing major flooding (National Weather Service, 2019).
More than 1,000 houses were damaged by the floods due to the week-long prolonged rainfall in Oklahoma (Bosman and Williams, 2019). In addition, water levels at the Keystone Dam, which is located close to the Sand Springs suburban area in Tulsa, reached a critical stage as it received excessive rainfall (Bosman and Williams, 2019). As a result, water was released from the dam at a rate of 275,000 cubic feet per second into the Arkansas River (Reuters, 2019). The risk of flood was exacerbated by the release of dam water onto already-saturated ground, posing the greatest threat to Sand Springs, which is home to 20,000 residents. A total of 152 houses were reportedly inundated as a result of opening the floodgates at the Keystone Dam. To date, the severe weather and floods have resulted in six fatalities and the injury of 107 people more widely in Oklahoma (Reuters, 2019).
The crude oil industry has been affected by the bad weather in the US Midwest as pipelines and refineries have been forced to close. One of the pipelines forced to shut its operation was the Diamond pipeline which is responsible for transporting 200,000 barrels of oil per day from Arkansas to the refinery in Tennessee (Kumar and Kelly, 2019).
Floods in the Missouri River Basin
In the Missouri River Basin, 108 gauges were flooded as of 30 May 2019, affecting locations such as Kansas City and Jefferson City. On 22 May, a tornado hit Jefferson City and triggered the water levels in the Missouri River to rise to 32ft, 11ft higher than the flood stage, but not exceeding the historical maximum (Bosman and Williams, 2019; National Weather Service, 2019) (Figure 5).
Based on JBA’s analyses, the return period for the river height at Jefferson City is estimated to be less than 20 years (Figure 6). At Kansas City, the maximum crest is expected to reach 35ft, which is far less than its maximum recorded level of 48.9ft during the 1993 flood event. The present river height at Kansas City is estimated to have a return period of less than 10 years.
Future flood risk in the US
Based on data available for 2013, a total of 5.6 million properties were insured under the NFIP (National Flood Insurance Program). It was reported that premiums paid to the NFIP totalled USD $3.6 billion in 2017 (The Washington Post, 2019). However, that figure is likely to rise to USD $5.4 billion by 2040 and increase by three-and-a-half times to USD $11.2 billion by the end of this century. Around 30% of the increase in flood risk is likely to result from population growth, with the remaining 70% attributed to climate change (Sheppard and West, 2013). The Federal Emergency Management Agency is currently considering using industry practices to upgrade the NFIP’s risk pricing methods to better reflect the risk at different properties (The Washington Post, 2019) (FEMA, 2019).
JBA Risk Management has nationwide return period flood maps for the US at 30m resolution, with 5m maps in development. Please get in touch for more information.
