Civilizations have endeavored to improve their condition through modifications of their environment for millennia. Often, these advancements have resulted in unforeseen consequences due to a lack of foresight. As a southerner and lover of Mark Twain, when I thought of these concepts my mind wandered to the Mississippi River and all that has been done to tame its mighty waters. Included below is a brief history of river engineering projects that have been undertaken in the Mississippi River Basin (MRB) and some of their unintended consequences. The reader is directed to the USGS report linked at the conclusion of this entry which provides greater detail on these issues. Mississippi River Basin The Mississippi River Basin (MRB) covers portions of 31 states and two provinces of Canada. Comprising 41% of the contiguous United States, it is bound by the Rocky Mountains in the west and the Appalachian Mountains in the east. The Mississippi River itself runs 2,340 miles from its headwaters in Minnesota to its outlet in the Gulf of Mexico near New Orleans. It is both one of the longest and most engineered waterways in the world. Left solely to the physics of nature, the Mississippi River represented a wild and unpredictable force affecting great portions of the country through course migration and regular flooding. A complex system of manmade dams, levees, floodways, and dikes have tamed the river significantly by providing protections from flooding and fixing its channel more or less on the course it had around 1900 when river engineering projects reached a colossal scale. While the August 2005 flooding of New Orleans in the aftermath of Hurricane Katrina brought the city’s geographical challenges to the attention of much of the country, the devastating effects of the hurricane were exacerbated by many of these civil engineering decisions tracing back to the 18th century. Fluvial Geomorphology Basics As rivers flow downstream they tend to meander in a circuitous path, depositing sediments on the lower velocity regions inside of a bend, and scouring out the bank on the higher velocity regions towards the outside of the curve radius. This process results in a phenomenon in which the river tends to further migrate across its valley in the direction of the river’s outer edge. As the channel slope and velocity decreases along with the widening of this arc, the river will ultimately cut off the loop, shortening the length of its channel again, leaving behind an oxbow lake in its former path as the process starts all over again. In this manner the course of the Mississippi has exhibited great variability even in anthropologic times. Energy decreases as the river widens along its path from higher to lower elevations. Much of the Mississippi River delta region is a result of sediment deposits ultimately turning into solid land. High discharge meteorological events periodically raise a river’s surface level above the banks of its channel, resulting in flooding of adjacent lands. Left untouched by man, thus, a river such as the Mississippi is a dynamic and unpredictable entity of great power. River Engineering of the MRB Following a series of devastating floods, The Flood Control Act of 1928 galvanized existing localized engineering efforts within the MRB into a more comprehensive basin-wide project to be implemented by the Army Corps of Engineers. The results of this ongoing initiative are obvious. Most of the river miles within the MRB have been improved for transportation purposes. These alterations include removal of obstructions, deepening of the channel through dredging, and channel straightening for more efficient travel. Articulated concrete block mattresses or rip rap revetments cover and stabilize existing banks throughout the MRB. Dikes have been constructed extending perpendicular to the river’s path from the banks, directing flow towards the center of the channel where it creates a high velocity and deep scour zone. Levees and floodways act as measures against flooding in high discharge events throughout the basin. Levees made of compacted sediment, impermeable clays, or concrete function essentially as high water dams running parallel to the river, while floodways are simply engineered channels that may be opened to redirect water threatening densely populated areas to locations where a flood would be less devastating. All of the Mississippi’s tributaries are impounded by dams and reservoirs which are used to control discharge for navigation, flood control, irrigation, hydropower, and water supply purposes. Effects of River Engineering in the MRB These river engineering projects have resulted in significant socioeconomic benefits for the United States, including boons to transportation, agriculture, hydroelectric power, and flood control which has made the existence of cities along the rivers practical. However, these advancements come at a cost as these projects have resulted in many unforeseen negative consequences. The overall sediment load of the MRB has been reduced by more than 60% due to the combined effects of reservoirs built on the Arkansas and Missouri Rivers, dike field sedimentation, a reduction in upriver flooding, and stabilization of channel banks with revetments. Coastal wetlands and bogs which rely on this influx of silt are sinking into the Gulf at an alarming rate, currently equivalent to the area of a football field with each passing hour, or 1.2 million acres since the 1930’s. The importance of these wetlands cannot be overstated. In addition to providing habitat for an incredibly diverse array of plant and animal species, these wetlands naturally act as an overflow mechanism, providing a flood buffer for coastal communities by absorbing great volumes of water during storm surge events. In the case of the Mississippi, not only is historic New Orleans at stake, but also the entire Port of South Louisiana. These facilities serve as the point of entry to the nation’s internal waterway system and provide vital infrastructure to the country’s oil and gas industry. Looking offshore, the coastal wetlands of the MRD also support the largest commercial fishery in the contiguous states. Taken as a whole, preservation of the coastal wetlands and communities of the MRB is truly an issue of national security. Conclusion Civil engineers have played a significant role in both the advancements and ecological degradation associated with engineering of the MRB. Since it is both impractical and undesirable to return the MRB to its natural state, solutions will need to be developed which counteract the negative effects of these alterations. Civil engineers will remain at the forefront as remediation, stabilization, and water resources projects are undertaken. With population growth and climate change magnifying existing stresses on our environment, the need to build holistically with a broader understanding of how the built environment interacts with the natural one is more important than ever. Article: http://pubs.usgs.gov/circ/1375/C1375.pdf -Inge Hill
- Ingle Hill