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3. Katrina Case Study - Background
In their natural state, wetlands are caught in the balance between subsidence and flooding. Subsidence in the most general sense is the sinking of soil relative to sea level. Subsidence is a natural part of wetlands due to soil compaction – air pockets in the soil collapse under the weight of the soil above. In addition, some soil is eroded away by water and wind. Humans contribute to subsidence by extracting oil and natural gas (the soil above collapses when the oil below is removed). Although wetlands are sinking little by little, regular flooding brings in new sediment to rebuild the lost soil and maintain the soil levels in the wetlands. As the plant life in wetlands die, the decaying organic matter also contributes to the overall building of wetland soils. In its natural state, these two forces – soil loss through subsidence and soil building through flooding – are in balance, or in fact, wetlands grow over time as new sediment pours in from rivers and streams.
However, human developments strongly encourage the building of levees, dams, and canals to control flooding and improve water traffic. This prevents the soil building part of the equation, leaving an overall loss of soil year after year. The sediments in the canals shoot out to sea rather than rebuilding wetland soils. Wetlands shrink and eventually disappear. Land sinks. This is why much of New Orleans is below sea level. In fact, many other urbanized delta regions such as the Sacramento River Delta are experiencing the same subsidence problems.
Wetlands provide a natural sponge to soak up flood waters and act as a speed bump, slowing down hurricanes and blocking storm surges. Without much of the wetlands around Louisiana gone and with large areas of developed land below sea level due to subsidence, the effects of Hurricane Katrina were amplified upon the city of New Orleans.
So what can we do? First and foremost, we can educate ourselves and others about the science behind subsidence in order to understand the problem and not make the same mistakes in the future. Secondly, we can protect the few wetlands that remain as stewards of the environment. Finally, we can engage ourselves in rebuilding wetlands through habitat restoration efforts, many of which welcome teachers and their students to participate.