The 2020 hurricane season is off to an early start so it is not too early to prepare for the possibility of the Tampa Bay area experiencing a direct hurricane hit. Tampa has experienced strong storm surges in the past and the question is when will it happen again?
Will History Repeat?
Since Tampa was a small town associated with a US Army post, Fort Brooke in the 1820s, there have been only two major hurricanes that directly impacted the Tampa Bay area with high winds and storm surges. The highest reported storm tide in Tampa Bay occurred in September of 1848 during the “Great Tampa Gale” when the tide was reported to have been 15 feet above low tide or about 12.75 feet above MHHW. During the October Hurricane of 1921 the tide was reported to have reached 10.5 feet above low water or about 8.25 feet above MHHW. Both of these storms were category 3 hurricanes and so far, Tampa Bay has escaped a direct hit from any category 4 or 5 storms. Hurricane Elena, in August of 1985, although not directly hitting Tampa Bay, did create a storm surge that was 4.0 feet above Mean Higher High Water (MHHW) or 6.26 feet above Mean Lower Low Water (MLLW) as Elena hovered offshore over the Gulf of Mexico to the northwest of Tampa Bay.
Although these catestrophic events are rare, they can and will happen again. I would estimate that there is a 1-2% risk for a major hurricane impacting the Tampa Bay area in any given year and the question is will it be this year? Even though there has never been a Category 4 or 5 hurricane to directly hit Tampa Bay, it is not impossible and the devastation would be extreme.
Storm Surge Simulations
In the paper: Hurricane Storm Surge Simulations for Tampa Bay (1), authors Robert H. Weisberg and Lianyuan Zheng took a close look at the flooding potential caused by wind stress and atmospheric pressure induced storm surge for a category 2 hurricane and for a catastrophic category 4 hurricane where storm surges of 4-6 meters (13-20 ft) are likely. The paper describes 11 different scenarios with 8 different tracks. Storm tracks E1, E2, E3, and E4 are for eastward moving hurricanes making landfall at Indian Rocks Beach, Sarasota, Tampa Bay mouth, and Tarpon Springs, respectively. Tracks D2, D3, D4, and D5 are for hurricanes paralleling the axis of the bay or paralleling the coastline from the northwest or the southeast, respectively.
Most people believe that a hurricane moving northeastward directly up Tampa Bay would be the “worst case scenario”, however, this is not the case. The paper, in fact, shows that a storm transiting up the bay axis from southwest to northeast actually results in the smallest surge!
While no storm scenario is a good one for the Tampa Bay area, the worst case is when the hurricane center is located north of the Tampa Bay entrance such that the maximum winds associated with the eye wall occur at the mouth of the bay with storms making landfall farther north result in lower surges. Storms that make landfall to the south of the bay tend to reduce storm surge along Pinellas County beaches, although localized surges can still occur within the bay.
Storm surge is also sensitive to the direction that storms approach the bay. Storms that approach from the south produce lower surges than those approaching from the north. The speed at which the hurricane approaches is also important. If a storm moves fast enough, the surge may not have enough time to reach the full storm surge potential. Wind speed is also key as surge height tends to increase with the square of the wind speed. A category 2 storm might produce limited flooding, while the flooding potential for a category 4 storm could be “catastrophic”.
Additional factors in surge development include the effects of tides, rivers, and waves. Tide ranges for Tampa Bay tend to be small (0.5 m to 1 m) when compared to the surge potential by winds and pressure. Flooding by heavy rains can be locally important with additional inundation and damage caused by waves.
Below is the storm surge expected by a Category 2 hurricane, approaching from the west at 5 m/s (10 knots) and making landfall at Indian Rocks Beach. The asterisk denotes the landfall location while the filled circles show the storm center. The bold lines are surge elevation contours at 1-m intervals.
Cat. 4 Hurricane Scenari0
Below is the storm surge associated with a Cat 4 hurricane making landfall at Indian Rocks Beach. The asterisk denotes the landfall location while the filled circles show the storm center. The bold lines are surge elevation contours at 1-m intervals.
How do Waves add to Storm Surge?
During hurricanes Ivan in 2004 (Alabama) and Katrina in 2005 (SE Louisiana and Mississippi) heavy waves combined with storm surge destroyed sections of bridges. The contributions of the waves to the storm surge and the increased water elevation by storm surge to the waves was described in another paper: Coupling of surge and waves for an Ivan‐like hurricane impacting the Tampa Bay, Florida region (2)
The paper describes the important interaction between waves and storm surge in shoaling water where the surge height will be elevated by the wave‐induced forces and where changes in water level by storm surge will greatly impact the wave field. It was found that storm surge is increased by wave stress by 0.3–0.5 m. In addition, the greatest surge enhancement by waves occurs in advance of the actual peak in the storm surge because the wave speed exceeds the storm speed of advance.
On the other hand, there is also an effect of storm surge on waves. The Significant Wave Height (the average of the ⅓ highest waves) can increase with surge elevation by about a 1.0–1.5 m and hence perhaps as much as a 2–3 m increase in the maximum wave height. Such enhancement of waves for a major hurricane could be sufficient to initiate large‐scale damage within the Tampa Bay region prior to the onset of hurricane strength winds. About 3 hours prior to landfall, both the inundation and the significant wave heights would be high enough to begin causing massive damage to low-lying buildings that are open water exposure.
How Vulnerable is Tampa Bay?
Particularly vulnerable would be the northeast section of St. Petersburg and the northern sections of Old Tampa Bay, including the Courtney Campbell Causeway, the Howard Frankland Bridge, and the Gandy Bridge. As the storm makes landfall, the situation worsens as the inundation continues to increase along with the significant wave heights, even until the center passes Tampa Bay.
Therefore, it is not only the wind speed (Saffir‐Simpson category) that matters, rather it is the complex mix of wind, tide, surge and waves that combine to cause major destruction. The entire coastline of Tampa Bay with low elevation and open to water exposure is susceptible to such damage. The eastern shore of Tampa Bay is at risk, even after the storm center passes beyond the Bay since the bay would already be filled with storm surge water and waves which would then shift direction to attack the eastern shore.
It is also very likely that the wave forces acting vertically beneath bridge spans would destroy spans of the Courtney Campbell Causeway, the Howard Frankland Bridge, and the Gandy Bridge that are closest to the water. McDill Air Force Base and Tampa International Airport would be damaged along with Tampa General Hospital, as well as some other critical areas of infrastructure.
- Weisberg, R.H., Zheng, L. Hurricane storm surge simulations for Tampa Bay. Estuaries and Coasts: J ERF 29, 899–913 (2006).
- Huang, Yong & Weisberg, Robert & Zheng, Lianyuan. (2010). Coupling of surge and waves for an Ivan‐like hurricane impacting the Tampa Bay, Florida region. J. Geophys. Res. 115. 0.1029/2009JC006090.
Hurricane Storm Surge Simulations for Florida’s Tampa Bay Region http://ocgweb.marine.usf.edu/Posters/TBHurricane.pdf
Mariner’s Wave Guide http://oceanweatherservices.com/featured_blog_posts/a_mariners_guide_to_wavesStorm
Surge Overview, NHC:
NOAA Blog Inside the Eye
Storm Surge–Plain and Simple (Part 1)
Ocean Weather Services