While wetlands serve a number of important functions, they have been incorrectly attributed to storm surge and aquifer recharge functions.
Henry Fountain, who covers science and engineering for The New York Times, is a sharp, experienced reporter and he recently showed that with an interesting article about keeping New York City and other low-lying areas safe during future storms.
An excerpt:
“[E]ven the strongest proponents of natural defenses acknowledge that they are no match for a storm like Sandy, which produced a record storm surge of more than 13 feet in New York Harbor…
“If a marsh or reef is effective in reducing storm surge, it is because it dissipates energy as water moves over grasses, roots, oyster shells and other materials, generating friction.
“But friction has an effect only up to a point, said Joannes J. Westerink, a civil engineer at the University of Notre Dame who has developed modeling software that is used to simulate surges and other effects of storms. ‘If the storm structure is such that the water has enough time to get in, then the frictional resistance will slow it down, but it won’t stop it from getting there,’ he said.”
One of the two greatest exaggerations of wetland functions and values are storm surge and aquifer recharge. The only clear cases of wetlands protecting adjacent uplands and infrastructure from storm surge and wave damage involve broad areas of mangrove or other wetlands with large shrubs or trees. The effect is simply physical: There are trees in the way of the advancing water and, as Fountain notes, for several hundred feet out into the ocean those trees physically break down the waves.
An emergent marsh with just grass will do little more than what a shallow sandy area of similar size and depth would. The shallow water breaks down waves. All waves break at approximately a water depth twice as great as the wave, so six-foot waves start breaking in 12 feet of water.
An emergent grassy wetland would provide some frictional drag on the water as it passes over the wetland, but not much more than plain sand would accomplish. Of course any vegetation, wetland or upland, on an elevated area of ground (dune or higher uplands along the shoreline) will hold the ground together with its roots, but the main point for surge protection is higher ground.
You can clearly see that on the Florida beaches during storm events when the upland sea grape and sea oats hold the sand dune against the waves better than a similar size sand dune with no vegetation would. But it is not wetlands.
The aquifer recharge concept is limited, as many southern wetlands are simply expressions of the ground water closest to the ground surface. That means rain falling on a lake, wetland or upland above a shallow aquifer will “recharge” the aquifer that is immediately below it. Wetlands do no better than uplands or open lakes. Wetlands and uplands are clearly better than impervious areas like roofs and paved areas, but wetlands are generally no better than, and often not as good as, uplands at aquifer recharge.
Further, many wetlands are based on the rain being confined above ground by some impervious layer such as clay or permanently frozen ground under the permanent tundra. Wetlands like those do absolutely nothing for groundwater recharge. The water in the wetland just evaporates, never entering the ground or groundwater.
Wetlands are in fact very good at removing nutrients from surface waters (but so are uplands and open waters). And clearly wetlands provide important habitat for many plants and animals. People should make the most out of those real wetland functions without making functions up like aquifer recharge and storm protection.
John Studt
Senior Advisor
John spent nearly 30 years in the US Army Corps of Engineers Regulatory Program, including more than 10 years as Chief of the Regulatory Program in the Corps’ Washington, DC Headquarters.