once the largest study of fluid mechanics in the world…
“This effigy of Old Man River is expected to make him behave better.” — Popular Science, 1948
by KRISTI DYKEMA CHERAMIE
For 27 days in January 1937, rain drenched the northeastern United States. The unusually warm, wet weather thawed the frozen ground and sent torrents of water sheeting into the Ohio River. The effect was dramatic: towns throughout the region reported water levels quickly approaching, then passing, flood level. In some areas the water crested as high as 20 to 28 feet above flood stage. With national reports tallying the displaced at over one million people, the event confirmed the growing national fear that the great rivers that had contributed to the nation’s success might also threaten its future.
The country had already endured what was supposed to be the last of the “Great Floods,” only ten years earlier, when the lower Mississippi River Basin suffered the most destructive inundation in U.S. history. In the aftermath of what then Secretary of Commerce Herbert Hoover called “the greatest peace-time calamity in the history of the country,” Congress passed the Flood Control Act of 1928. This sweeping legislation called for the immediate implementation of a plan to control the waters of the mighty Mississippi. It was as if the nation had declared war against the river: In the next decade, the Army Corps of Engineers built 29 dams and locks, hundreds of runoff channels, and over a thousand miles of new, higher levees. It appeared that efforts to prevent another Great Flood would be successful.
river channel with mesh used to simulate dense foliage…
But as in so many battles, the combatants misread the enemy. The 1928 plan focused on single targets, presuming that the “menace to national welfare” was the Mississippi River itself; the Corps of Engineers failed to see the river as part of a system of interconnected, aggregating threats. When several rivers in the Northeast flooded in the winter of 1936 (in particular the Connecticut, Allegheny and Monongahela), displacing hundreds of thousands of people in Massachusetts, Pennsylvania and New York, and even reaching far enough to evacuate the National Headquarters of the American Red Cross in Washington D.C., the public felt double-crossed. A New York Times editorial called for a more comprehensive approach: “If the floods have taught us anything, it is the need for something more than a dam here and a storage reservoir there. … We need a kind of protection which considers something more than the exigencies of Johnstown, Pittsburgh and Hartford — considers the social and economic future of a nation and a continent.”
Congress obliged the new national consciousness with the Flood Control Act of 1936, which declared flood control a “legitimate federal responsibility” and provided a substantial increase in federal funding for a comprehensive network of levees, dams, reservoirs and dikes. Significantly, it handed complete responsibility for flood control to the Army Corps of Engineers, a division of the War Department (later the Department of Defense), and mandated that the economic benefits of construction outweigh the costs. In essence, the act was driven by commerce but framed as national defense.
As construction began on control structures throughout the Mississippi River Basin, and as floodwaters rushed into the Ohio River Valley in January 1937, a district engineer in Memphis, Tennessee, Major Eugene Reybold, raised concerns about this approach. Although the scope of flood control had expanded beyond the Mississippi, the work was limited by current field research methods; engineers found it difficult to track what was being done at various points along the river and thus impossible to predict how isolated “solutions” might affect one other. To understand the Mississippi River Basin as a dynamic system of interconnected waterways, the Corps needed new, more sophisticated scientific tools.
Reybold came up with a radical idea: a large-scale hydraulic model that would enable engineers to observe the interactive effects of weather and proposed control measures over time and “develop plans for the coordination of flood-control problems throughout the Mississippi River Basin.” Only a physical model of all lands affected by the Mississippi River and its tributaries could meet the three major goals of the Army Corps: “… to determine methods of coordinating the operation of reservoirs to accomplish the maximum flood protection under various combinations of flood flow; to determine undesirable conditions that might result from non-coordinated use of any part of the reservoir system, particularly the untimely release of impounded water; and to determine what general flood control works were necessary (levees, reservoirs, floodways) and what improvements might be desirable at existing flood control works.”
Reybold understood that such a project would require a paradigm shift in the Army Corps of Engineers. His colleague John Freeman ran a small hydraulics laboratory, the Waterways Experiment Station, in Vicksburg, Mississippi, but had been denied funding for more comprehensive research. “Field experience,” said Secretary of War Dwight Davis, “is undoubtedly of much greater value than laboratory experiments could possibly be.” Nevertheless, Freeman’s laboratory drew the attention of young, ambitious engineers who could see the benefit of fluid mechanics modeling. Reybold worked with the Experiment Station to construct a small section of the exceptionally steep Kanawha River as a pilot model. He knew that if he could simulate historic flood events and produce accurate flood hydrographs of the Kanawha, he could build support for a model of the entire Mississippi River Basin. Reybold’s plan worked; in 1943 the Corps of Engineers approved his proposal to build a comprehensive model.
What Reybold needed next was a site and a workforce. World War II had commandeered the Army’s stateside labor force and depleted its funding for civilian hiring. So as Reybold surveyed the area near Vicksburg for suitable topography on which to build the basin model, he also negotiated for the transfer of prisoners of war to a new internment camp. He settled on a large area of undeveloped land in Clinton, Mississippi, and under his supervision 3,000 German and Italian POWs began construction on a 200-acre working hydraulic model. The ambitious model would replicate the Mississippi River and its major tributaries — the Tennessee, Arkansas and Missouri Rivers — encompassing 41 percent of the land area of the United States and 15,000 miles of river. It would reflect existing topography and river courses throughout the Mississippi Basin, using the best data drawn from hydrographic and topographic maps, aerial photographs and valley cross-sections.
POWs at work August 1943…
The prisoners cleared the site of a million cubic yards of dirt and rough-graded the land to match the contours of the Mississippi River Basin. To ensure that topographic shifts would be apparent, the model was built using an exaggerated vertical scale of 1:100 and a much larger horizontal scale of 1:2000. While the existing topography offered a close approximation of the actual Mississippi Basin, some areas required significant earthmoving; the Appalachian Mountains were raised 20 feet above the Gulf of Mexico, the Rockies 50 feet. An existing stream running east-to-west provided the model’s water supply. The streambed was molded to take on the shape and form of the upper reaches of the Mississippi, and a complex system of pipes and pumps distributed water throughout the model; it was regulated by a large sump and control house sited near what would become Chicago, Illinois. To simulate flood events, Reybold needed to introduce large volumes of water over short periods of time, so he designed a collection basin and 500,000-gallon storage tower system at the model’s edge. Small outflow pipes at anticipated data collection points channeled excess water to 16 miles of storm drains.
A 20-acre section in the center of the 200-acre site would be subject to high-intensity tests. Here the engineers installed a “fixed-bed model” that enabled greater precision and control, modeling the river channels and overbank flood areas in concrete. This section represented the areas of the central and lower basin perceived to be most vulnerable to catastrophic floods: the Mississippi River from Hannibal, Missouri, to Baton Rouge, Louisiana; the Atchafalaya River from its confluence with the Mississippi to the Gulf of Mexico; and the lower reaches of key tributaries, the Missouri, Ohio, Cumberland, Tennessee, Arkansas and Ouachita Rivers.  Large concrete panels, flat on the underside and uniquely molded on top to reflect particular topographic shifts, were installed over the pipes and held in place with a secondary structural system. Although the fixed-bed model accounted for only 10 percent of the site, it represented a large enough area that the curvature of the earth played a significant role in the design and construction of the concrete panels. Engineers overlaid the traditional grid system with the conical Bonne Projection, skewing the surface of each panel to respond to the topographies of both the model site and the basin itself.
The panel surfaces were enhanced with concrete riverbeds, sheer cliffs, flat plains, tributaries and oxbow lakes, as well as railroads, bridges, levees and highways. The engineers faced the significant challenge of achieving an accurate degree of “roughness,” the measure of frictional resistance experienced by water as it passes over a particular surface. Because the concrete created an impermeable (fixed) ground, they installed 3/8″ metal plugs of varying length, called “parallelepieds,” to create drag in the water flow and simulate scouring. These brass plugs were used in conjunction with brushed and scored concrete and periodic concrete ridges to model channel roughness. To add further surface detail to “overbank phenomena” such as the vegetation observed in aerial photographs, an accordion-folded metal screen was cut to scale and placed (unfixed) at appropriate locations.
the article continues…
(DESIGN OBSERVER 3.21.11)