Science & Environment

Freshwater wetlands 'vulnerable in hurricanes'

Satellite image of Hurricane Katrina (Image: NOAA)
Image caption Hurricanes in 2005, including Katrina, destroyed 527 sq km of wetlands in the US Gulf state of Louisiana

Freshwater coastal wetlands are more vulnerable to erosion during hurricanes than habitats with higher levels of salinity, a study has suggested.

US researchers say freshwater marshes have shallower root systems, leaving them at risk from wave erosion during storm surges.

They added that the results could have implications for wetland restoration projects in hurricane-prone areas.

The findings appear in the Proceedings of the National Academy of Sciences.

"During the 2005 hurricane season, the storm surges and waves associated with Hurricanes Katrina and Rita eroded 527 square kilometres of wetlands within the Louisiana coastal plain," the researchers wrote in their PNAS paper.

"Low salinity wetlands were preferentially eroded, while higher salinity wetlands remained robust and largely unchanged."

The team said that both freshwater and salt marshes within their study area were exposed to similar conditions during Hurricane Katrina, which struck the US Gulf coastline in August 2005.

"We hypothesise that wave shear stresses generated during the hurricane exceeded the shear strength of the low salinity wetland soils, resulting in failure, whereas greater soil shear strength in the saline wetlands largely precluded erosion," they suggested.

"Soil shear strength and the resistance of the soil to erosion are determined by the properties of the vegetation.

"We propose that resistance to erosion is primarily a function of rooting characteristics, which depend on the dominant species of vegetation - as controlled by salinity."

The scientists identified a "weak zone" about 30cm below the surface in freshwater wetlands, which coincided with the base of the root system of the plants growing in the habitat.

However, in the salt marshes, plants' roots were found to penetrate the soil to depths of about one metre.

As a result of the deeper root system, the vegetation was better suited to withstand the pressure exerted by storm surges and wave action during a hurricane.

The team concluded that the findings could play a part in shaping restoration programmes and land management schemes in regions that are prone to tropical storms.

They wrote: "The dramatic difference in resiliency of fresh(water) verses more saline marshes suggest that the introduction of freshwater to marshes as part of restoration efforts may therefore weaken existing wetlands rendering them vulnerable to hurricanes."

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