Levees, those wonderful walls that protect us and our land from destructive floods, come with a dark little secret – they protect us today but doom us tomorrow.
Levees are walls that run along the shoreline of a body of water and can be either natural or man-made. Levees can naturally form by sediment settling out of the water during floods. When water is confined to the channel, it moves very fast and can carry a lot of sediment with it. This is like holding your thumb over the end of a hose – the water comes out very quickly. But during floods, the water surges over the banks of the channel and slows down dramatically. This is like taking your thumb off the end of the hose – the water slows to a measly pour. When this water slows down quickly, it can no longer carry all of the sediment. The largest and heaviest pieces of sediment, mainly sand, settle out as soon as the water leaves the channel and pile up at the channel edges. And “Whala!”, a natural levee has been made. These natural walls keep the water within the channel, not letting it escape over the banks and onto the floodplains even when the water level rises during floods or high tides.
We took this trick from nature and have been building levees (also known as dikes) out of concrete, wood, metal, dirt, or just whatever else we can get our hands on along river and delta channels for thousands of years to protect the land we want to cultivate. Great idea, right? The answer is a bit more complex than just yes or no, because along with the great immediate benefits, problems often arise months, years, or even decades after the levees have been built.
By not letting water flow onto the floodplain and create shallow water habitats, levees eliminate nurseries that are important for the growth and survival of juvenile fish, like trout and salmon. They also restrict the nutrients and mud that are naturally brought to floodplains by flowing water. Without natural nutrients, we have to turn to expensive fertilizers. If fertilizer is not managed properly, high concentrations of it can be washed into the water, causing harmful algal blooms which reduce the amount of oxygen in the water. The algal blooms thrive on the nutrients in the fertilizers and use up all the oxygen themselves. Just like us land-dwellers, marine animals need oxygen to breathe, and without enough oxygen, marine organisms suffer even further.
Perhaps the most damning, and least intuitive, consequence of levees is the cutoff of mud supply to the land. When mud is initially deposited on land, it’s very soft and has lots of water in it. Over time, this water is squeezed out by gravity and overlying infrastructure or extracted by man-made wells. Without the water, the mud becomes more compact, loses volume, and the land surface sinks. This process is sustainable if more mud is deposited on the land to make up for the lost volume. But, when levees cut off the mud supply, the land continues to sink – a process called land subsidence.
Land subsidence caused by levees and groundwater extraction is extremely common in major cities built on delta plains. Deltas are naturally built by the deposition of fertile river sediments, making them ideal for agricultural development and the eventual rise of cities. Take New Orleans for example, a city built on the Mississippi River Delta. Levees were built along the Mississippi River way back during colonial times. Now, almost half of the city has sunk below sea level and the land is continuing to sink at an astonishing rate of almost 5 centimeters per year (2 inches/year). This rate may seem slow and insignificant, but can have huge effects on land that is already so close to sea level – just watch this time-lapse video. And New Orleans isn’t alone… Vietnam’s Mekong Delta (3 cm/year), Indonesia’s Jakarta (5 cm/year), and Bangladesh’s Ganges Delta (0.5 cm/year) are just a few in a long and growing list. Now that many of the deltas we live on have sunk below sea level, our dikes are not just protecting us from floods only during high tides and large storm surges, they are protecting us from floods all the time. And breaks in these dikes can be catastrophic. Just ask people who were living in New Orleans when their levees failed during Hurricane Katrina.
The increasing rate of sea-level rise is another factor exacerbating this subsidence problem – click here to read my post about that.