October 25, 2016 Iris Hill Town Administrator Town of Edisto Beach 2414 Murray St Edisto Beach SC 29438 RE: Hurricane Matthew Beach Damage Assessment and Recommendations [CSE 2416] Dear Iris, Hurricane Matthew impacted the shoreline of Edisto Beach on 7-8 October 2016. The storm produced high water levels (surge), increased wave energy, and hurricane force winds as it passed within miles of Edisto Beach. The storm resulted in extensive damage to the beach and beachfront structures as well as the interior and marsh side of the island. Typical damage included complete loss of dunes, overwash of sand onto properties, under houses, and into state highways, damage to structures, flooding, exposure of septic systems, damage to beach accesses, and damage to state park infrastructure. The extent of damage was severe enough to warrant the deployment of over 175 National Guardsmen to Edisto Beach to ensure public safety and restoration of safe access to the island. Figure 1 shows the path and wind radii of the hurricane and its approach near Edisto Beach. The tide station at Fort Pulaski, GA (just south of the Savannah River) showed still water levels over +8.5 ft NAVD (Figure 2). This is a surge of ~7.5 ft above the predicted water level during the early morning hours of 8 October. The surge was likely higher along the outer coast. For reference, the dry sand elevation at Edisto is typically +7 ft NAVD, and dune height prior to the storm was commonly less than 12 ft NAVD. In addition to the surge, wave energy created severe runnup and overtopping of the dune along the entire ocean-facing shoreline of the beach. Videos available on social media showed overtopping of the dune and water reaching Palmetto Blvd on 7 October, the day before the storms closest approach. Following the storm, there was significant visible damage to the beach, oceanfront structures, and public and private infrastructure.
Figure 1. Hurricane Matthew track and wind velocity radii. Hurricane force winds are shown in red and tropical storm force winds in yellow. Edisto Beach is marked with the star in the lower image. Figure 2. Water levels during Hurricane Matthew at Fort Pulaski, GA
General observations of the beach damage are as follows: Thick sand deposits on Palmetto Blvd and into the second row houses estimated to be up to 4 ft deep. Sand overwash was extensive and mostly continuous for the first 6 ½ blocks (between Atlantic and Portia Streets). In some cases, sand deposits reached the landward side of the 2 nd row houses. Overwash continued in distinct fans between the 600 and 1000 blocks. Overwash was most severe where no dunes were present (through parking areas under houses and beach accesses). The dune was mostly eliminated along the first 6 blocks. Most oceanfront homes in this area had damage to the understory and parking surfaces. Plumbing and septic systems are exposed in many of these areas. A drop in sand elevation of over 4 ft was observed based on paint marks on house pilings. The post-storm berm crest was positioned near the seaward edge of Palmetto Blvd along the first 3 blocks. High tides over the next week nearly reached the road. The dune was overtopped along the area near the Point (Neptune St) and large sand volumes were deposited under house and in yards. A seawall which has been buried over the past several years was exposed near groins 25 and 26 and reports of displaced armor stones were noted. Photos from 9 October documenting the damage are provided at the end of this letter. CSE completed a post-storm survey of the beach to document changes to the beach profile and volume losses. Profiles were obtained from the landward sand limit to ~1,500 ft offshore, well beyond the local depth of closure. Profiles were obtained near the center of each groin cell and at 600 ft spacing in the state park (Figure 3). The surveyed area encompassed the shoreline between the northern end of the state park camping area to the South Edisto River, which are the approximate limits of the 2006 Edisto Beach Nourishment Project and have previously been determined to be a qualifying engineered beach by FEMA. The post storm survey data are compared to a recent comprehensive monitoring survey CSE completed for the Town of Edisto Beach in August 2016 that included profiles at the same locations as the post-storm survey. Table 1 provides volume change values for each profile and summed for the entire beach. Overall, the only area of the island showing a net gain of sand is the southern-most monitoring station at the State Park, ~300 ft north of Groin 1, and along the point at groin cell 28. The accretion measured at these locations is likely due to a sediment transport direction from north to south, piling sand along the northern side of Groin 1, as depositing some sand lost from the Town s front beach around the point. Overall, the island lost 155,951 cy of sand between the pre and post-storm surveys, including a loss of 145,935 cy along the Town portion of the beach and 10,016 lost in the state park. Erosion was most severe along the northern 10 groin cells, averaging 11.3 cy/ft. Cells 11-24 lost an average of 6.8 cy/ft. Erosion was also severe in cells 26 and 27, losing 11.7 and 26.6 cy/ft, respectively.
Figure 3. Location of beach profiles (dotted lines) surveyed annually and following Hurricane Matthew. Note that only the lines at the center of each groin cell were surveyed following Matthew. See the line numbers in the Table 1. Iris Hill, Town of Edisto Beach October 25, 2016
The volume loss resulted in erosion of the primary dune along most of the beachfront. Especially at the northern end of the island, the dune was completely lost and the active beach shifted over 100 ft landward in some areas. Where houses were positioned on the new active beach, many of the concrete pads used for parking areas under houses were undermined and collapsed. Table 1. Beach volume changes occurring during Hurricane Matthew. Background Edisto Beach is positioned at the southern end of a littoral cell encompassing the area between the North and South Edisto Rivers. It receives only a very limited sediment supply from the beaches to the north (Edingsville and Botany Bay) because these beaches are overwashing into the marsh-filled lagoon. As the beaches retreat due to the overwash, relict marsh muds are exposed, eroded, and the sediments are dispersed in the ocean. Edisto Beach has battled erosion since the 1940 s, when the first groins were built at the eastern end of the town beach. Additional groins were built sequentially to the west, with the last being built in 1975. The timber groins deteriorated and armor stone was added in the 1960s to maintain the seaward ends of the structures. The first nourishment project of Edisto occurred in 1954, and added ~850,000 cy of sand and mud obtained from dredging the marsh on the landward side of the upland beach, creating the Yacht Basin still existing today. Repairs were made to some of the groins in 1988, and more extensive repairs were completed in 1995 including addition of stone, restacking stone, and adding concrete grout to fill voids in the stones. The 1995 project was coupled with addition of 150,000 cy of sand dredged from the shoal on the north side of the South Edisto River delta. Following the 1995 repairs, shotcrete was added to reinforce timber sheeting and concrete caps were added to some structures at the northern end of the island.
The Town completed a large-scale nourishment project in 2006, adding ~877,000 cy of sand from the same offshore shoal used during the 1995 project. The fill extended from the northern end of the state park (~3,300 ft north of groin 1) to groin 28 at Billow St. In-place fill volumes ranged between ~20 and ~55 cy/ft along the Town portion of the beachfront. Following nourishment, the Town has continued to install sand fencing and native vegetation as opportunities arise, which has facilitated dune growth along much of the beachfront. The Town has also monitored the beach annually to document the volumes of sand remaining in each groin cell and over larger reaches. As of August 2015, approximately 51 percent of the nourishment sand remained within the fill limits. The northern end of the beach, between groins 1 and 8 has been the most erosional since 2006, but the majority still retains more sand than the pre-nourishment condition. The Town of Edisto anticipated the need for periodic nourishment on an approximately 10-year interval. With this schedule in place, the Town began planning efforts for the next project in 2012, with an assessment of the groin field seeking to determine the feasibility and impacts of lengthening certain groins. Additional analysis was completed, including determining erosion rates since the 2006 project and a design for the next project was assembled. The design calls for nourishment of up to 835,000 cy of sand and lengthening of up to 26 groins. The beach fronting the Town portion of the project area (being paid for by the Town) calls for up to 650,000 cy of nourishment. On behalf of the Town, CSE submitted a joint federal and state permit application in 2015. The permit application referenced several USACE documents prepared during a feasibility study for a federal storm damage reduction project, the design of which is very similar to the local project. Hurricane Joaquin impacted the beach in early October 2015, resulting in loss of 95,900 cy of sand from the beachfront which was determined to be eligible for FEMA Category G reimbursement. The Town has been coordinating with FEMA to include this volume in the planned nourishment project. Recommendation Actions The most immediate need for the Town of Edisto Beach is construction of an emergency berm to protect areas imminently threatened to additional damage to property and infrastructure. Presently, the beachfront between the 100 block and the 1000 block lacks sufficient sand in the profile to protect from even a modest storm event. The Town should attempt to recover as much sand as possible from the road and yards in overwashed areas to build a berm a minimum of +11 ft NAVD (~4 ft above the normal dry sand level) and a minimum of 20 ft wide. If additional sand is available, the berm should be built to +13 ft NAVD. If insufficient sand is available, sand may be brought in from an upland source containing beach compatible material or scraped from the low-tide beach as a last resort. This effort should be considered an emergency measure to prevent further damage to property and infrastructure under FEMA Category B Emergency Protective Measures. As the damage to the engineered beach along Edisto due to Hurricane Matthew qualifies for disaster recovery funds under FEMA Category G, CSE recommends the Town requests FEMA reimbursement for lost sand as part of the upcoming nourishment project, similar to the process completed following
Hurricane Joaquin. This will provide the Town with an efficient and logical means for restoring the damaged portions of the beach, without the negative impacts of placing sand from upland borrow areas. Losses during the storm totaled 145,935 cy of sand along the Town s portion of the beach, and when combined with losses from Hurricane Joaquin, represent 27.1% of the total volume required for the previously planned nourishment of 650,000 cy plus losses during the two storms. The Town should request FEMA funds to cover dredging and placement of a total of 241,835 cy (95,900 cy for Hurricane Joaquin and 145,935 cy for Hurricane Matthew), and a pro rata share of mobilization, permitting, and environmental monitoring. The Town should also seek reimbursement for engineering services and contractor services related to removing overwashed sand from Palmetto Blvd and restoring beach access points. Local Project Scope and Opinion of Probable Cost Table 2 outlines the scope of work for the project being pursued by the Town and provides an opinion of probable cost, including permitting, construction, and environmental monitoring. The anticipated costs are based on recent projects and experience at Edisto and similar settings. The pro rata share is based on the total project volume (original Town plan plus the storm losses) and is applied to each task that is required to implement a nourishment project. Groin lengthening is not included in any FEMA related work. Table 2. Updated opinion of probable construction costs including the pro rata share of FEMA eligible sand for both Hurricane Joaquin and Hurricane Matthew. Edisto Beach 2016-2017 Nourishment Project Town Of Edisto Portion Quantity Unit Unit Cost ($) Cost ($) FEMA Pro Rata Share (%) FEMA Cost ($) Permitting Construction Monitoring State and Federal Permitting 271,850 27.1 73,671.35 Final Design/Procurement 121,033 27.1 32,799.94 Mobilization 2,500,000 27.1 677,500.00 Dredging - Town 650,000 cubic yard 10 6,500,000 Dredging - FEMA Joaquin 95,900 cubic yard 10 959,000 100 959,000.00 Dredging - FEMA Matthew 145,935 cubic yard 10 1,459,350 100 1,459,350.00 Groin Lengthening 1,675 linear foot 1,700 2,847,500 Construction Administration 367,635 27.1 99,629.09 Surveys 5 40,000 200,000 27.1 25,800.00 Total 15,226,368 3,327,750
The above assessment and recommendations are provided to document the impacts of Hurricane Matthew to the shoreline at Edisto Beach and provide an opinion on the most prudent and logical method for restoration. To facilitate the FEMA review and expedite the reimbursement process in anticipation of the planned renourishment project beginning in the next two months, the impacts of Hurricane Matthew were combined with those from Hurricane Joaquin in the opinion of probable construction costs. Please let me know if you have any questions or need additional information. Sincerely, Coastal Science & Engineering (CSE) Steven B Traynum Coastal Scientist / Project Manager
Upper: Overwash on Palmetto Blvd near Finns Restaurant. Lower: View of a collapsed house in the 100 block.
Upper: Overwash on Palmetto Blvd near the 800 block. Lower: View of a damaged dune and overwash reaching the road.
Upper: Damage to the lower level of houses was common along the first 6 blocks. Lower: Aerial view of an exposed seawall that was previously buried and no dry beach.
Upper and Lower: Typical beach condition along the first 10 blocks following Hurricane Matthew.