Since the 1980s the frequency, duration and intensity of hurricanes in the North Atlantic have been increasing. With this and the rise of sea-levels due to a warming climate it is causing for there to be more frequent flooding and an increase in rainfall rates. As this occurs more and more barrier islands and low-lying coastal areas have seen a dramatic increase in erosion from their beaches all the way from Florida to the coast of Maine. According to the Scientific Committee on Antarctic Research (SCAR) sea levels are predicted to rise by as much as 1.4 meters by the end of the century (Hegde 2010). On most of the Atlantic and Gulf coasts sea level has been rising between 2.0 – 3.0 mm per year (Hedge 2010). Action needs to be taken now in the form of not only erosion control, but also flood mitigation.
Flood mitigation is the process of controlling and managing flood water movement as the flood is occurring. In most cases flood waters are redirected in order to alleviate areas that are highly susceptible to flooding so that structures will not be damaged as severely. It is important to note that floods can never be fully prevented only managed and controlled. Erosion control is another preventative measure that is used in order to prevent the erosion of soils and sediment due to water and wind. It is currently estimated that in United States coastal areas 0.3 m to 0.6 m of sediment/soil are eroded per year with 86% of the East Coast experiencing this loss.
In recent years it has been found that older methods of protecting the coast have not been providing the best type of protection and are actually causing beach instability. These types are considered hard structural options; some examples are: seawalls, offshore breakwaters and artificial headlands. A seawall is a structure that is built parallel to the coastline and protects a shore or cliff from wave action, can promote slope stability and can help dissipate wave action on sandy coasts (Prasetya). The problem with seawalls is that overtime due to wave reflections it can inhibit offshore sediment transport, it also does not promote healthy beach stability and needs to be constructed along the whole coastline because if it is not erosion will be begin to occur on adjacent coastlines. An offshore breakwater is built parallel to the shore and absorbs waves. As the wave energy is reduced in its lee; downstream from the wind, it creates a salient or tombolo behind the breakwater which then influences longshore transport of sediment (Prasetya). Although this helps with erosion they are difficult to construct and are particularly vulnerable to strong wave action. Lastly an artificial headland is built to help promote natural beaches although this seems like the best choice overtime it causes erosion downdrift along the protected length of the coastline and has poor stability during large storm surges.
Many coastal areas have now begun to use soft structural options which dissipate wave energy by mimicking natural forces and maintain the natural contour of the coastline. A few examples of this are: beach nourishment, dune reconstruction, and coastal revegetation. For flood mitigation the most common methods being implemented are zoning, setback limits and retreat.
South Carolina is situated between Georgia to the south and North Carolina to the north. The state has 4,628 km of coastline and tidal areas according to a NOAA report. From 1851 – 2017, 39 tropical cyclones have hit the state; 24 hurricanes, 10 tropical/subtropical storms, and 5 tropical depressions (SCHC 2017).
The most recent coastal event occurred in 2017 when Hurricane Irma traveled inland and became a tropical storm. It created the third highest recorded storm surge in Charleston of 4.15 feet due to high tide. Many beaches such as Hilton Head, Edisto, and Folly Beach islands were severely damaged by the storm surge which not only caused flooding but also major coastal erosion. In 2016 Hurricane Matthew hit as a Category 1 just south of McClellanville and serve damage occurred to Hilton Head and Edisto Islands because it hit at high tide however, in Charleston the storm surge hit 3.5 only 3.5 feet due to it being low tide. If Matthew would have hit during high tide the Charleston area could have experienced a storm tide of 9 feet.
During a period from 1977 to 1988 the South Carolina Coastal Council issued permits allowing for shorelines to be built with bulkheads, seawalls, and revetments. Property owners were issued permits and were able to build these large commercial structures landward of the sand dune line (South Carolina Hazard Mitigation Plan 2018). This was changed towards the end of 1988 because it was realized that the dunes and natural areas of the beaches were not being adequately protected and valuable natural areas were being lost.
Stormwater ponds are a used to control flooding in residential and commercial areas, but research over the years has shown that there is commonly an accumulation of contaminants such as high nutrient, chemical and bacterial levels which can lead to water quality problems.
Currently the focus has changed to using more restorative measures such as dune and coastal reconstruction. Some areas such as Kiawah Island have taken it a step further and designed an inter-lagoon system. This system allows for the island to prepare for large flooding events such as storm surges, topical storms and hurricanes. When one of these events is forecast to hit the island, the lagoons are slowly drained with the lowering of tides. When the system hits these lagoons have an ability to capture rainwater to prevent mass flooding on the island although it only has the capacity to prevent a 100-year flood event.
North Carolina is located between South Carolina to the south and Virginia to the North. The state has 5,432 km of coastline and tidal areas according to a NOAA report. Since 1851, 47 hurricanes have hit the state and it is estimated the of all hurricanes in the Atlantic it has been affected by 17.5 percent. The frequency for hurricanes to pass within 80.5 km of the coast is every five to seven years, which is the highest frequency of all coastal areas along the Atlantic and gulf coasts (Verdi 2017).
The most recent hurricane to hit North Carolina was Hurricane Florence in 2018. This was one of the widest hurricanes on record spanning 350 miles or 536.3 km. The storm hit on September 13th and dropped over 34 inches or 86.5 cm in a three-day period due to how slow the storm was moving; storm surges were recorded at 3.05 m with gusts of wind hitting 100 mph. According to the NOAA Hurricane Florence caused $24 billion in damages.
Before 1970 North Carolina used seawalls in an effort to reduce flooding in coastal areas however, it was determined that this increased the amount of erosion occurring and now seawalls are banned on coastal areas. They are now being used further inland to protect canals, waterways and other structures. The focus now is to restore dunes and the natural coastline by introducing natural vegetation that was once commonly found along the coast.
As is common in most other coastal states North Carolina has a large amount of stormwater ponds to prevent flooding. In 2007 a study was performed that investigated 15% (524) of low and high density projects. Of these only 30.7% were found to be in full compliance meaning that almost 70% were reporting violations mostly due to the amount of nutrients, bacteria, and chemicals present. This number is very troublesome since along these coastal areas the water table is high and the soils and sediments are sandy allowing for these different contaminants to enter groundwater very easily.
New studies have been released showing that many areas have begun to bring stormwater ponds up to code and reducing the amount of chemicals used within the vicinity. Ponds are now beginning to be regularly checked to ensure that there is a lower risk of contamination. Currently the state is in the process of acquisitioning over 5,000 homes in high hazard areas and plans to increase the elevation of over 800 homes in flood prone areas. It has also started to implement set-back rules and better zoning laws on new construction.
New types of proposed flood mitigation and erosion control in these two states has been increasing due to the rise of sea levels and intensity of storms. In South Carolina the town of Kiawah Island has been working with the Army Corps of Engineers to perform excavations in order to create a new flushing channel and to move this material to create a closure berm on the eastern shoreline near the Stono Inlet. This would involve moving 7645.5 cubic meters of beach sand to the new flushing channel to create the berm closure (Joint Public Notice 2014). This would help to mitigate erosion of the beach and dunes, reshape the shoreline causing it to become more stable, and allow natural restoration and rebuilding of the northern part of the island. In North Carolina they have been dredging established channels and using the sand to reestablish failing coastlines. A new project that has begun since Hurricane Florence is to restore more of the coastal salt marches. It was found that these areas were the least effected and experienced far less erosion and helped to protected areas that were surrounded by them due to their ability to absorb wave action.
As coastal waters continue to rise, and storms continue to intensify it is pertinent that changes be made now to existing flood mitigation and erosion control methods. It has taken too many disasters to realize that trying to change the natural landscape of these areas has made the land even more unstable and it is through new innovative ideas in preservation and control that we may begin to see a change.
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