Coastal Wetlands

Hurricanes and other storms generally produce damaging winds, storm tides, and rain that flood inland coastal areas as well as erode beaches and barrier islands. Coastal wetlands help to dissipate the force of storm surges and can therefore lessen the impact of these storms on areas farther inland. Damage to these valuable coastal wetlands themselves however, can be quite severe. The effects of high winds and storm surges are most apparent as continuous marsh is broken up into pieces, channels are filled with debris, and areas of marsh are converted into open water. (Figure 8a,b)

Other types of physical damage to coastal wetlands were evident following the passage of Hurricane Andrew. Most striking was the widespread lateral compression of marsh, resulting in a series of accordion-like folds with ridges rising 2 m (7 ft) above the normal surface level.

In other areas, the marsh was scoured as portions were washed away, leaving open water. At some sites, large pieces of soil and vegetation were torn from the marsh and thrown to the tops of levees or deposited into oil and pipeline canals, effectively blocking them. Some of these pieces were as large as a small car. Other marsh sites were covered with 1.5-2 m (5-7 ft) of wrack (plant debris), which completely buried the existing vegetation. Areas that were not physically disrupted or covered with sediment or wrack also appeared to lose plant cover because the salty gulf waters driven onshore by the hurricane "burned" the tops of the plants, killing the aboveground parts.

Hurricane Andrew also introduced large amounts of sediment into these coastal marsh systems. In some cases, vegetation was completely buried while, in others, the sediment was deposited as a thin layer on the marsh surface but did not smother plants. Sediments were deposited over large expanses of the coast, and even sites as far as 130 km (81 mi) from the path of the hurricane received significant amounts.

Where did these sediments originate? Careful measurements of sediment characteristics indicated that some were introduced from outside the coastal marsh system, while others were redistributed from the bottoms of shallow basins where the marsh substrate had eroded.

Coastal marshes closest to the path of the storm east of Atchafalaya Bay had the thickest deposit of storm-generated sediment, which was 10-16 cm (4-6 inches) deep and most probably came from the bottom of Atchafalaya Bay. As the storm approached, water was pulled from the bay by the force of the storm, exposing the sediments of the shallow bay bottom. As the storm passed the bay, this water rushed back in and the resulting storm surge of 1.8 m (6 ft) mixed these sediments into the water column. As the storm surge moved over the marsh, it deposited the sediments onto the surface.

Subsidence and lack of sediment are critical factors affecting wetland loss in coastal Louisiana. To remain stable, coastal marshes must grow in height as rapidly as they sink and sea level rises, but many portions of coastal Louisiana are isolated from renourishment by sediment. Events such as winter storms and hurricanes, which suspend sediments in the water column, may partially off-set the effects of subsidence and subsequent wetland loss by supplying needed sediment.

Not all hurricanes appear to contribute as much sediment to the coastal environment as Hurricane Andrew did. Whether they do depends on their idiosyncratic nature: wind velocity, storm tide height, angle of approach to the shore, and the availability of a source of sediments. And even if hurricanes do contribute much sediment, it may not ultimately help certain coastal marshes maintain their elevation relative to increasing sea level.

In studying the effects of Hurricane Andrew, scientists have found that, in certain areas of coastal Louisiana, even significantly increased contributions of sediment cannot completely counteract subsidence patterns, and marshes will continue to be drowned and lost. (Figure 9) In other areas, additional sediment resulted only in a temporary increase in elevation that was slowly lost in following years. The increases in elevation were short-lived phenomena and these sites are returning to prestorm conditions.

Proximity to the coast was not necessarily the primary factor that determined damage to wetland vegetation. In many cases, salt marshes closest to the path of the hurricane showed the least signs of damage. On the other hand, freshwater plants in interior marshes suffered most from exposure to the moderately saline water that accompanied the storm surge as it moved inland. At research sites 20-40 km (12-25 mi) from the coast, the hurricane-induced storm surge was still 1.7 m (6 ft) high with a salinity of 10-15 parts per thousand-about half the salt in seawater. This saltwater "burned," and in many cases killed, the aboveground portions of fresh marsh plants. This phenomenon was seen as far away as the Pearl River, located on the border of Louisiana and Mississippi, over 180 km (112 mi) from the storm's eye. Most of the plants affected, however, began to resprout within six weeks of the storm.

Five days after the hurricane, the salinity of some interior marsh sites was still eight times higher than it was prior to the storm and, at one site in western Terrebonne Parish, a wedge of saltwater was still evident below the marsh surface 55 days after the storm. Results from earlier studies of Louisiana wetlands indicate that impounded marshes suffer more extensive and long-term effects from saltwater than do marshes that drain freely. In one instance, the vegetation of an impounded marsh on the chenier plain in southwestern Louisiana required four years to recover from the entrapment of saltwater driven ashore by a hurricane storm surge.

At sites where researches had prestorm data, they were readily able to see how Hurricane Andrew caused changes in plant composition. Areas with different types of storm damage, including sediment addition, wrack, lateral compression, and scoured marsh, were affected quite differently. (Figure 10)

Surfaces at the compressed-marsh sites were elevated, creating drier habitat. The most dramatic changes in species composition were in the compressed areas. Plants not typically found in coastal marshes increased significantly because of the drier conditions. (Figure 11)

Areas where sediment was deposited did not change substantially in species composition. In these sites, it is likely that the sediment will act as a fertilizer, increasing the growth of the existing vegetation. In areas with wrack deposition, plants have recolonized very slowly because the wrack must first decay enough to allow plants to grow through the thick debris.

Virtually all of the study areas in the coastal marsh had some degree of disturbance after Hurricane Andrew. Although the kinds of species present in many sites changed, this change is likely to persist only in areas of compressed marsh where increased surface elevations will last longer.

Ultimately, scientists have found that vegetation of these coastal marshes recovers fairly quickly from the impacts of hurricanes and other tropical storms. Even areas where the vegetation dies back because of salt burn generally recover over time. Only in areas with dramatic changes, such as lateral compression, erosion, and wrack deposition, will long-term adverse effects be seen. (Figure 12a,b)

Areas where the marsh was totally lost are unlikely to recover, worsening the existing problems of coastal marsh erosion and degradation. In other cases, sediment added to the marsh surface contributes to the health of these systems.

The overall firmness and "health" of the marsh strongly influence the degree to which it is affected by storms. Hurricane Andrew had a major impact on floating and weakly rooted wetlands, but a much smaller effect on firmly rooted marshes. In the future, areas of degraded marsh will be more susceptible to increased erosion from hurricane winds and associated storm surges.

Levees surrounding impoundments can also isolate marshes from available sources of sediment, which can promote subsidence and result in marsh loss. The sediment that enters an impounded marsh as a hurricane storm surge washes over the levee may be the only significant addition of sediment received by the marsh since its enclosure.

Hurricanes are valuable sources of sediment for coastal wetlands and may in the short-term be able to counteract subsidence and slow the process of the marsh's interior fragmentation and degradation. Although these sediments may not completely counteract the subsidence associated with Louisiana's coastal wetlands, they are an important addition in areas cut off from normal sediment supplies. Marsh managers should consider these often opposing effects when considering restoration and mitigation projects. The best way to protect wetlands from future hurricane destruction is to promote the inorganic and organic accretionary processes that encourage a "healthy" marsh.

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