Tropical Storm Debby

Transcription

1 Tropical Storm Debby Post-Debby Beach/Dune Damage Assessment Report (Draft) Sarasota Florida By Weiqi Lin P.E., Ph.D. Coastal Resources/Community Services June 28, 2012

2 BACKGROUND AND SUMMARY Tropical storm Debby formed on June 23, With maximum sustained wind speed of 45 mph, the storm was moving northeast very slowly at just 3-4 mph. In Sarasota County, Debby has caused major coastal flooding, severe beach/dune erosion and property damages due to its slow movement, location, long duration of heavy rainfall, and steady winds. The storm surge was estimated to be about 3-5 feet. On June 25 &26, 2012, Sarasota County Coastal Resources staff conducted a beach/dune damage assessment for county s coastline. Staff found that Major coastal flooding occurred on North Siesta Key. A large portion of Beach Road was flooded during storm. Severe beach/dune erosion took place on South Siesta Key, Casey Key and Manasota Key. Up to feet of dune have been eroded away. In some areas, more than 30 feet of dune was lost. At least three residential structures are threatened, sitting at the edge of the new eroded bluff on Manasota Key. On Casey Key, at least one residential structure is threatened by coastal erosion. Structural damages to decks, dune walkover structures were documented. Some dune walkover structures were washed out. There were heavy loss of sea turtle nests and native dune vegetations. Some palm trees and Austrian pine trees fell down. A recently constructed sandtube was exposed on Casey Key. A step seawall was undermined by wave action on Manasota Key. BEACH AND DUNE CONDITIONS Siesta Key Due to its low elevation, Siesta Key is very vulnerable to coastal flooding. It has a long history of coastal flooding during storms. North Siesta Key has the lowest elevation of 4 to 5 ft. NGVD and suffered major flooding during Tropical Storm Debby. Several inches to a foot height of water have been documented on most of the Beach Road (Figs. 1-3). Most of the Siesta Key public beach was covered by waves and white-caps (Fig.4). Severe erosion took place at the most south part Siesta Key. It was estimated ft. of beach on Turtle Beach Public Park has been eroded. The boat ramp at Blind Pass Lagoon was flooded. South of Turtle Beach Public Park, from Fisherman s Cove and Heaven Condo to the Blind Pass Park, most of the restored beach and restored dune vegetation was washed away (Fig.5). Rocks were again exposed in front of Fishman s Cove and Fishman s Heaven Condo, only five years after the South Siesta Beach Nourishment 1

3 project. Several feet of sands were washed onto the most southern property at 9230 Blind Pass Road (Fig.6). Casey Key Casey Key has a higher elevation of ft. NGVD. A small portion of Casey Key Road has flooding problem due to heavy rainfall. Waves over-topped a Gulf-front portion of North Casey Road. The road was covered with 2-4 feet of sand and not passable during storm (Fig.7). Debby has caused severe dune/beach erosion. It was estimated about feet of dune was eroded throughout Casey Key due to the combined effects of high wave energy and high tide. The residential structure at 2007 Casey Key Road was threatened by the retrieve of bluff (Fig.8). A recently constructed sandtube at 2209, 2305, 2309 and 2315 Casey Key Road was exposed (Figs.9-10). More than 20 feet of dune has been eroded at that location. Dune vegetations have been washed out, and structural damages to decks and dune walkover have also been observed. Many seaturtle nests were being washed out. Manasota Key Storm surges and high waves generated by Debby have caused severe erosion on most of Manasota Key. Up to 20 to 30 feet of dune were eroded away in most part of Manasota Key and North Manasota Key. Lots of sea turtle nests and dune vegetations have been washed away. Many palm trees were lying down on the beach. Big Austrian pine trees were down. Structural damages to decks, dune walkovers have been documented. Some dune walkover structures were washed out. The worst erosion took place from the Sarasota/Charlotte county line to the Blind Pass Park (Mid-beach), and from north of Blind Pass Park to south of Pearl Motel (7990 Manasota Key Road). At least three residential structures (7160, 7110 Manasota Key Road and 6116 Gnarled Oak Drive) were threatened, sitting at the edge of the new eroded bluff line (Fig.11). A step seawall at 6110 Manasota Key Road was undermined by wave action (Fig. 12). Two decks at 6210 and 6260 Manasota Key Road. A portion of Manasota Key Road at the north end of the Blind Pass Park was threatened by dune erosion (Fig.13). The bluff has been eroded all the way to the edge of that portion of the road (about ft long) and approximate up to 40 of dune was estimated to be eroded away at this location (Fig.14). CONCLUSION REMARKS In Sarasota County, Debby has caused major coastal flooding, severe beach/dune erosion and coastal property damages. The storm has significant impacts on coastal communities, coastal environments and coastal systems. In term of flooding and beach/dune erosion, Debby is even worse than a hurricane. 2

4 ATTACHED PHOTOS Figure 1. Waves over-topped the most northern portion of Beach Road, causing major coastal flooding in this area. Figure 2. Flooding at Public Beach Access#3. More than a foot of water and two feet of sand covered this portion of Beach Road. 3

5 Figure 3. Flooded Beach Road at the exit of Siesta Key Public Beach main parking lot. Figure 4. Most of the Siesta Public Beach is covered by waves and white-caps. 4

6 Figure 5. Restored beach, dune and native dune vegetations were washed out in front of Fisherman Cove and Fishman s Heaven Condo. Figure 6. Several feet of sand was washed on shore on the property of 9230 Blind Pass Road. 5

7 Figure 7. Waves washed sand on top of a Gulf-front portion of North Casey Key Road. The road was impassable during the storm. Figure 8. The residential structure at 2007 Casey Key Road is threatened by the eroded bluff. The wooden deck suffered structural damages and collapsed. 6

8 Figure 9. This photo showed the beach/dune conditions in front of 2209, 2305 and 2309 Casey Key Road on May 1, It was taken by staff during a pre-storm beach/dune condition inspection for comparison with post-storm beach/dune inspections. Figure 10. This June 26, 2012 photo was provide by Mote Maine Lab showed more than 20 feet of the dune has been eroded away by Tropical Storm Debby. The sandtube got exposed just after two years of its construction. 7

9 Figure 11. The residential structure at 7110 Manasota Key Road was threatened by beach/dune erosion. Wooden deck collapsed and dune-walkover was washed out. The immediate neighbor at 7160 Manasota Key suffered similar damages. Figure 12. The retrieved bluff also threatened this residential structure at 6116 Gnarled Oak Drive. Wooden deck collapsed and dune-walkover was washed away. 8

10 Figure 13. A step seawall at 6210 Manasota key Road was undermined by wave action. A big hole was created landward of the seawall and the deck was structurally damaged. Figure 14. The bluff has been eroded all the way to the edge of a Gulf-front portion of Mansota Key Road (about ft ) and approximate up to 40 of dune was estimated to be eroded away at this location. (End) 9