Hurricane Dennis brought heavy rain to northwestern Florida, causing moderate flooding in the central panhandle. Storm surge from the hurricane caused heavy property damage along the coast from Carrabelle to St. Marks.
Dennis came ashore along the western Florida Panhandle west of Navarre Beach at 2:25 p.m. CDT on July 10, 2005, as a category 3 storm on the Saffir-Simpson Scale with winds as high as 120 mph. Flooding was highest in the Ochlockonee River basin, where stream flow (measured on the Ochlockonee River near Havana) was at levels with recurrence intervals longer than 5 years. Rainfall ranged from 1.47 to 7.08 inches at U.S. Geological Survey (USGS) stream gages in the Florida Panhandle.
The National Weather Service recorded total rainfall of 6.61 inches at Tallahassee, 3.46 inches at Panama City, and 2.07 inches at Apalachicola. Several Data Section employees at the USGS office in Tallahassee reported 6 to 8.5 inches in rain gages at their homes in different parts of Leon County. Rainfall was high in the Tallahassee area and lower in other areas because as the hurricane approached land and moved onshore, squalls moving in a southeast to northwest direction provided consistent heavy rainfall to certain areas, whereas areas between the bands of squalls received much less rain.
Hurricane Dennis caused a higher than expected storm surge in the St. Marks, Aucilla, and Wakulla River basins, and this storm surge, in combination with runoff from an average of 6 to 8 inches of rain in the vicinity of Tallahassee and the Big Bend area, created moderate flooding. Flows for most streams were in the 2- to 5-year recurrence-interval range throughout the central panhandle. At the St. Marks River near Newport, 14.4 mi upstream from the mouth near the Gulf of Mexico, flows were in the 2-year recurrence-interval range. Near Pensacola, Bayou Marcus Creek rose to levels just shy of those associated with Hurricane Ivan in September 2004 (6.82 ft and 1,500 cubic feet per second [cfs], a record high for the 7 years that gage has been in operation). However, the Ochlockonee River near Havana peaked on July 14 at about 17,000 cfs, a flow with a recurrence interval of about 8 years over the period of record, which began in 1926. On July 14, the highest flow measurement on record at Ochlockonee River near Concord was recorded at a gage height of 41.30 ft and a flow of 21,300 cfs.
At the gage at Wakulla Spring near Crawfordville (above), the associated storm surge and increase in stage is a representative example of what the Big Bend area underwent.
The storm surge at the gage near the mouth of the Aucilla River near Nutall Rise (below), 2.6 mi upstream from the Gulf of Mexico, was 7 to 8 ft. This gage is adjacent to St. Marks National Wildlife Refuge.
Surprisingly, there was no significant storm surge at some of the western panhandle gages, at least not in comparison with that produced by Hurricane Ivan. At the Escambia River near Molino, for example (below), a stage increase of about 1 to 1.5 ft was observed, with a sharp spike at landfall; in contrast, Hurricane Ivan produced a stage increase of 6 ft. For more information on the hydrologic impacts of Hurricane Ivan, see USGS Open-File Report 2005-1277, "Hydrologic Effects of the 2004 Hurricane Season in Northwest Florida," by Richard Jay Verdi.
Coastal basins in the Big Bend area showed significant surge, whereas those near the hurricane's landfall did not, probably because of (1) the angle at which Hurricane Dennis approached in comparison with that of Ivan, (2) the relative shallowness of Apalachee Bay, and (3) the occurrence of hurricane landfall near high tide in Apalachee Bay. Hurricane Ivan came straight up from the south, and so the storm had a chance to push water well ahead of it, whereas Dennis came in at an angle from the southeast and, as is often the case with the east side of the storm being more severe, affected the Big Bend area more severely than areas farther west. Hurricane landfall occurred between Santa Rosa Beach and Navarre Beach an hour and a half before high tide in Apalachee Bay.
The USGS office in Tallahassee lost data at two gages as a result of the stormat Brushy Creek near Bratt, a 25-mi2, basin site northwest of Pensacola, and at Escambia River near Gonzales, an auxiliary-stage gage for the Escambia River near Molino.
in this issue:
Hydrologic Impacts of Hurricane Dennis