I. Hazard Description:
This risk assessment will discuss tornadoes and their effects in Cleveland County, Oklahoma. According to Schwab et al., “a tornado is a violently rotating column of air extending from the base of a thunderstorm.” Although it’s widely believed that a tornado always touches the ground, this is not always true because a debris cloud can confirm the presence of a tornado without the sighting of the funnel. (74-76) Tornadoes are created by thunderstorms and formed when both cold and warm air meet, which then causes that warm air to rise rapidly. In most cases of tornadoes, heavy rainfall and sometimes hail occurs before the presence of a tornado. Hail happens to be a clear indicator of the amount of energy present and can, furthermore, indicate the potential of damaging thunderstorms and tornadoes. Tornadoes are major hazards because compared to a thunderstorm, they don’t use as much energy as thunderstorms, but it makes them even more dangerous because the energy that is used is condensed to a small area and can quickly gain potential for overwhelming and volatile damages.
Cleveland county is currently the third most populated county in Oklahoma, with a total population of 255,755 people and has an area of 558 square miles Table 1.1. It is located in central Oklahoma and is south of Oklahoma county, east of Grady county, north of McClain county, and west of Pottawatomie county. The University of Oklahoma in the city of Norman lies in the central east part of the county. As far as the geography, of the 558 square miles, 539 square miles is land and the other 19 miles is water (Lake Thunderbird & Little River/Canadian River) Figure 1.1. This entire county is at risk because it’s in the heart of Oklahoma, which also happens to be the part of Oklahoma that is a high-risk area of Tornado Alley Figure 1.2.
Tornadoes are measured by the enhanced Fujita scale. The enhanced Fujita scale classifies tornadoes on a scale from F0-F5 based on the “intensity, path length and width of tornadoes” from the original Fujita-Pearson scale and in addition also uses different damage indicators based on the level of damage to different types of infrastructure/construction Figure 1.3. Oklahoma has experienced some of the worst tornadoes in history, all of them ranging from F0-F5, and some of those have happened in Cleveland county. It’s safe to state the fact that Cleveland county had and still has the potential to experience a dangerous range of tornadoes in the area and just by looking at past events, F5 tornadoes can/have occurred. According to FEMA, the duration of a tornado typically lasts no more than 10 minutes and the speed of onset is fast so there is little to no time to prepare for this disaster (FEMA).
Most Populated Counties in Oklahoma
Rank County Population (Census, Apr 2010) Area (sq miles)
1 Oklahoma 787,958 718
2 Tulsa 603,403 587
3 Cleveland 255,755 558
4 Comanche 124,098 1,084
5 Canadian 115,541 906
Figure 1.1 Cleveland County Map
Table 1.1 Top 5 Populated Counties in OK Figure 1.3 Enhanced Fujita Scale
II. Hazard History and Frequency
According to the data provided by NOAA, there have been a total of 76 tornado events (F0/EF0 or greater) that have occurred in Cleveland county between June 1, 1950 and June 30, 2018. Tornado events in this county alone has caused a total of 42 fatalities, 649 injuries, and approximately $2.724 billion in property damages. The top five major tornado events that have had the highest number of fatalities, injuries, and property damages are outlined in Table 2.1.
Rank Date Scale Deaths Injuries Prpty Dmg Disaster Declaration
1 5/20/2013 EF5 24 207 2.00B Yes
2 5/3/1999 F5 11 293 450M Yes
3 5/8/2003 F3 0 45 210M No
4 3/25/2015 EF2 0 7 50.0M No
5 11/19/1973 F3 3 28 2.5M No
Table 2.1 Top 5 Major Events in Cleveland County since 1950
EF5 Tornado: May 20, 2013 (NOAA)
Ranking number one on the table shown above, this tornado had the highest number of fatalities, injuries, and highest amount of property damage. On May 20th of 2013, there was a tornado outbreak and the strongest tornado claimed the lives of 24 people and caused 2 billion dollars’ worth in damages. There was also large hail along with damaging winds present that contributed to an additional loss in this area. Not only did this tornado destroy houses, but it also destroyed buildings and Briarwood Elementary school located in the northeast area of Cleveland county.
F5 Tornado: May 3, 1999 (NOAA)
Number two on the list, this tremendous tornado was part of another outbreak that struck Cleveland county in the late afternoon of May 3rd in 1999. As this tornado torn through the county, it caused a massive amount of property damages specifically in a housing development, which ultimately damaged about 50 homes and tossed several vehicles through the area. The damage was so extensive that it ripped houses from their foundations.
F3 Tornado: May 8, 2003 (NOAA)
The third tornado event on the list shows 45 injuries and $210 million in property damages. The worst amount of damages took place in the city of Moore. The tornado affected “two hotels, the First Christian Church, the Young Child Development Center, a Project Headstart building, an office building, and several restaurants.” (NOAA) Numerous vehicles were also tossed in the area causing significant damage.
Recent Occurrences (NOAA)
On March 25, 2015, the city of Norman was once again devastated with the presence of an EF2 tornado. This event caused no fatalities, but it did injure 7 people and caused $50 million worth of damages. The tornado began in southwest Oklahoma and proceeded to move east through the city of Moore and then northeast through the city of Norman. It’s harmful eleven-mile path, caused extensive destruction to homes and buildings in the process.
A more recent tornado event also took place in the city of Norman on May 2, 2018. Several supercells formed in the late afternoon and moved northeast towards Norman, then dissipating near Franklin Road. This EF1 tornado only caused about $10,000 in damages and affected the built structure of one home and an outbuilding, but luckily most of the damage was to the trees. In conclusion, within the past five years, Cleveland county has continued to experience a full range of tornado events and both the past and current histories of these events go to show that this county is still a high-risk tornado area.
Disaster Probability: Tornadoes
Cleveland county has a 156% chance of a tornado in any given year. To compute this probability data, you take the number of events and divide it by the number of years and then multiply it by 100 to get the probability percentage. By using this formula, you can calculate the probability of a hazard type, in this case tornadoes, occurring in any given year. As far as the recurrence, on average a tornado impacts Cleveland county every 234 days. To calculate the recurrence interval, take the number of years and divide it by the number of events, and then choose the unit of time that can be best interpreted by the reader. Table 2.2