|Above: The Old River Control Structure on the Mississippi River as seen during a flood on May 17, 2009 (left) and at normal waters levels on December 31, 2009 (right). The May 2009 flood was the 14th highest on record, and similar in height to the great flood of 1973 that nearly destroyed the Old River Control Structure’s Low Sill Structure. Image credit: USDA Farm Service Agency, via Google Earth.|
The Old River Control Structure (ORCS), the Army Corps of Engineers’ critical defense against the risk of the Mississippi River undergoing an economically catastrophic change in its course, is under increasing threat due to flood heights that have escalated in recent decades. The higher floods are from increasing sedimentation of the river’s channel. This was due to the construction of the structure, plus other river engineering efforts.
As detailed last week in Part I of this series, America’s Achilles’ Heel: the Mississippi River’s Old River Control Structure, the ORCS was built to act as a bulwark against the Mississippi River’s natural inclination to carve out a new channel to the Gulf of Mexico. The river wants to forge a path down the Atchafalaya River that is half as long and twice as steep as the river’s current channel, which takes it past Baton Rouge and New Orleans. The ORCS consists of four major water control structures which allow no more than 30% of the Mississippi River to flow into the Atchafalaya River: the Low Sill Structure and Overbank Structure (built in 1963), the Auxiliary Structure (completed in 1987), and the Sidney A. Murray Junior Hydroelectric Plant (completed in 1990).
However, the construction of the ORCS is contributing to its potential failure: the Mississippi River is forced to slow down when it reaches the structure, and this slowdown has resulted in the deposition of a prodigious amount of sediment that has clogged up the river's channel. Additional sedimentation has been caused by the construction of smaller wing dikes that stick out from the river’s banks into the main current (to stabilize the channel and improve navigation safety during low river flows), from armoring of the levee banks with articulated concrete and steel mattress revetments (to prevent erosion of the levees), and from a series of channel modifications done in the 1930s, when necks of 16 meander bends were sliced through to create a shorter path for the river. The net result: higher flood heights due to the clogging of the river channel, which increases the chance of the structure’s failure during a major flood.
|Figure 1. Changes in discharge-stage rating curves at the Mississippi River Tarbert Landing gauging station (about 10 km or 6 mi downstream from the Old River Control Structure). For the same flow rate of about 28,000 cubic meters per second, flood heights in 2015 were 2.2 meters (7.2 feet) higher than in 1988, due to increased sedimentation from river engineering. Image credit: Wang and Xu, 2016, Long-term geomorphic response to flow regulation in a 10-km reach downstream of the Mississippi–Atchafalaya River diversion, Journal of Hydrology: Regional Studies.|
Since 1988, flood heights have increased by 7’ for the same rate of flow
A 2018 paper by Sanjeev Joshi and Y. Jun Xu of the Louisiana State University School of Renewable Resources, “Recent changes in channel morphology of a highly engineered alluvial river – the Lower Mississippi River”, and a 2016 paper by Bo Wang and Y. Jun Xu, Long-term geomorphic response to flow regulation in a 10-km reach downstream of the Mississippi–Atchafalaya River diversion, found that enough river sediment has accumulated in recent decades along the 30 miles of river channel below the ORCS to cause a significant increase in flood heights. The researchers discovered that between 1992 and 2013, the river grew shallower by about 30 - 33’ (9 - 10 meters) and its width decreased by up to one-half mile (800 meters) at transects just downstream from the ORCS. This sedimentation caused the channel’s cross-sectional area to shrink between 8 - 14%, resulting in a plugging effect that led to an 11 - 12% increase in flood heights at a gage just downstream from the ORCS. Floods were 7.2’ (2.2 m) higher in 2015 compared to 1988 for the same flow rate at that location. This is a dangerous trend that represents an increasing threat for the ORCS to fail in future floods.
The research echoes similar findings from Munoz et al., 2018, Climatic control of Mississippi River flood hazard amplified by river engineering. They constructed a 500-year history of floods on the Mississippi River by studying sediment cores, and found that the magnitude of the 100-year flood (a flood with a discharge rate having a one percent chance of being exceeded in any year) has increased by 20 percent over past five centuries, with about 75 percent of this increase attributed to human river engineering. They discussed that while the river engineering has saved billions of dollars in prevented flood costs, it has also led to higher floods, acceleration of coastal land loss due to lack of river sediment being deposited at the coast, and has helped create oxygen-depleted “dead zones” that kill marine life where the river flows into the sea.
|Figure 2. Between 1985 and 2015, 30 sandbars containing an estimated 530 million metric tons of sand formed in a 100-mile stretch of river between the Old River Control Structure (ORCS) and Vicksburg, Mississippi. The U.S. Army Corps of Engineers operates four gauging stations at Greenville (RK 855), Vicksburg (RK 701), Natchez (RK 585), and Red River Landing (RRL, RK 487). The long-term river stage records from these stations were used for determining sandbar surface area at different river water levels. A portion of the Mississippi River is diverted into the Atchafalaya River through the ORCS at RK 505. Image credit: Bo Wang and Y. Jun Xu, 2017, Dynamics of 30 large channel bars in the Lower Mississippi River in response to river engineering from 1985 to 2015, Geomorphology.|
Danger from sandbars upstream
River engineering has forced the river to dump a massive amount of sand upstream from the ORCS. A total of 30 large sandbars containing about 530 million tons of sediment have formed along a 100-mile stretch of the river upstream from the ORCS since 1985 (Figure 2). Future floods could dislodge that sand and wash it downstream, where it would raise the river bottom just south of the control structure, increasing the plugging effect and boosting flood heights to even more dangerous levels.
Major floods can also wash a large amount of sediment into the control structures, threatening to completely block them. A 2011 study, Mississippi River and Old River Control Complex Sedimentation Investigation and HSR Model Report, warned that “there is a real possibility and threat that both Low Sill and Auxiliary could become totally buried with sediment during an extreme event,” leaving only the hydropower structure to manage the flow between the Mississippi River and the Atchafalaya River.
The great flood of 2011, the highest flood ever recorded on the Lower Mississippi River, carried a large amount of sediment into the Low Sill and Auxiliary structures, but did not bury them. Still, the amount of sediment transported was so large that the Corps was forced to perform a major dredging effort between July 2012 and March 2014 along the two structures’ inflow and outflow channels. This was the first such dredging operation since the two structures were built.
|Figure 3. Landsat image of the Mississippi River, showing the location of the Old River Control Structure (ORCS) and Widow Graham Bend, located 13 miles to the north-northwest of the ORCS. If the Mississippi broke through its west bank levee at Widow Graham Bend, it could cut a new path to the Gulf of Mexico via the Red River and Atchafalaya River. Image credit: Google Earth|
Widow Graham Bend
In Beyond Control: The Mississippi River's New Channel to the Gulf of Mexico, historian James Barnett Jr.’s fantastically detailed 2017 book, he brings up a possibility that other experts have also warned about: while the Old River Control Structure is the most likely place for the river to break through and carve a new path to the Gulf, the river could also breach its levees elsewhere and accomplish the same feat. One possible location for such a breakthrough: Widow Graham Bend, the location of a sharp meander bend in the river about thirteen miles north-northwest of the ORCS. This location was identified as early as 1882 as a possible location where the Mississippi could breach its west bank levee and send its waters southward to join with the Atchafalaya River (though it would also have to break through a second levee that lines the Red River).
The river did breach the levee at Widow Graham Bend during the great flood of 1927, but the Mississippi was not ready to jump to a new channel then. Since the flood of 1927, the critical west bank levees near the ORCS have been built to a height of 71 – 74 feet--at least seven feet higher than the all-time record flood from 2011.
A possible solution: regulate the river to flush sediment downstream
In an interview, Dr. Sanjeev Joshi, who wrote his 2017 Ph.D. thesis on the issue, said that his research demonstrated that at certain flow rates of the river, the problematic accumulated sediment could be flushed downstream. He advocated preforming controlled experiments to regulate the flow through the ORCS to see how much sediment could be removed in this way. He cautioned against dredging or dynamiting the bottom to remove the sediment, since this might destructively alter in the river’s channel, destabilizing the banks and putting increased pressure on the levees. Assuming that a solution to the excessive sediment problem could be found, Dr. Joshi stated that the ORCS and the levees surrounding it were so strong that there was perhaps just a 1% chance that the river would jump to a new channel in the next 100 years.
Flow rates of the Mississippi River predicted to increase by 11 – 60% by 2100
Dr. Joshi’s research found that climate change does not appear to have significantly changed flood volumes in the Lower Mississippi River in recent decades--the biggest floods, with flow rates between 24,000 and 26,000 cubic meters per second, accounted for about 14 to 10% of all discharge events during 1986 - 2015, and 15 - 11% of all events during 1973 - 2015. However, there is the danger that climate change could increase flow rates on the river by up to 60% by the end of the century.
A warming climate has already caused an acceleration of the hydrological cycle—increased precipitation, evapotranspiration, runoff, and river flow. These changes are predicted to intensify further as the climate continues to warm. A 2014 study by Tao et al., Increasing Mississippi river discharge throughout the 21st century influenced by changes in climate, land use, and atmospheric CO2, used a high-resolution Dynamic Land Ecosystem Model (DLEM) to examine future changes in discharge from the Mississippi River and to evaluate the relative roles of future climate, atmospheric CO2, and land use during 2011 - 2099. The model predicted that the flow rate of the Mississippi River--in both drought and flood conditions--would increase by 11 - 60% by 2100, depending upon the amount of human-caused climate change and land use change assumed.
In the higher emissions scenario, the predicted increase in precipitation and temperature in the model accounted for 49% of the increased Mississippi River flow, while changes in land use (due to an increase in urban areas and farmland) contributed 15% of the increased flow; paved surfaces deprive the land of its ability to store and slowly release water, allowing more runoff into the river. The other 36% of the increase was attributed to increased atmospheric CO2, which affects plant growth and evapotranspiration and soil processes in a manner that increases river runoff.
Part I of this series was, America’s Achilles’ Heel: the Mississippi River’s Old River Control Structure. The final part of this 3-part series on the Old River Control Structure is, If the Old River Control Structure Fails: A Catastrophe WIth Global Impact.