As flood waters recede, Michigan residents are assessing and repairing damages, Midland Daily News reported Friday. Midland County faced massive floods last week when two of its dams failed, sending waters into inhabited areas. Dams are more complex than most people imagine.
According to the Midland Daily News, flooding damage in residential areas of Michigan has been devastating. “In the days and weeks ahead, the people of Edenville, Sanford, and Midland will be navigating through situations they have never experienced,” the article said. “The damage is unimaginable: Downtown Sanford businesses destroyed. Residents on Wixom Lake are turning to each other as they begin recovery efforts.”
The article said flooding began after rainfall pummeled the Edenville Dam, which forms Wixom Lake. When the dam broke, it created a domino effect. First it spilled massive amounts of water into Sanford Lake, which is formed by Sanford Dam. The Sanford Dam couldn’t hold that much water either, and it overflowed into Tittabawassee River, a nearby river that flows near the affected towns.
There are two main kinds of dams used to hold water, and they depend on gravity, the natural shape of land, and natural water flow to do their jobs.
“Dams are used for water supply, irrigation, flood control, and for the generation of hydroelectric power,” said Dr. Stephen Ressler, Professor Emeritus from the United States Military Academy at West Point. “A dam is generally classified according to two characteristics: its principal construction material—which might be concrete, masonry, soil, or rock—and its structural configuration—either an arch dam or a gravity dam.”
Dr. Ressler said that arches used in most construction are oriented vertically and meant to bear their loads vertically as well. However, with arch dams, the arch and its supports face horizontally, which is how they take their loads.
“The load is hydrostatic pressure—that’s the same pressure that’s applied to your basement walls by groundwater,” he said. “Hydrostatic pressure always acts perpendicular to a submerged surface. Here, the submerged surface of the dam is vertical, so the pressure is applied horizontally. The lateral support to this arch is provided by the river valley, and that’s why dams like the Glen Canyon Dam in Arizona are always built in steep valleys with solid rock walls.”
The other main category of dams is the gravity dam, which Dr. Ressler said gets its strength and load-bearing capacity mostly from its mass.
“There are two principal types of gravity dams based on whether the construction material is concrete or masonry, on one hand; soil or rock, on the other,” he said. He turned to the Dworshak Dam in Idaho as an example of a concrete gravity dam.
“Because the dam’s upstream face is essentially vertical, the hydrostatic pressure acts horizontally upon it. Assuming that the concrete itself is sufficiently strong, there are really only two ways that this structure can fail: It can either slide horizontally, or it can tip about its toe. As long as the base of the dam is keyed into solid bedrock, it can’t slide.”
Dr. Ressler said that on the opposite end of the spectrum, technologically speaking, are dams built from soil or rock. These are called earth dams and rock-fill dams, respectively. He said that the base of the dam is made of a clay core, which is then topped by gently-sloped layers of compacted “fill.” The composition of the fill is either primarily soil or rock, which determines which of those types of dam it is. Next, a layer of heavy stone lays on the upstream face of the base of the dam to protect from things like water erosion and ice.
“Whether we’re talking about soil or rock fill, all of these layers add both strength and mass to the structure, and therefore improve its overall stability,” Dr. Ressler said. “In addition to the armoring on the upstream face, it’s also common practice to plant vegetation on the downstream face in order to further stabilize the soil and prevent erosion. [An] elaborate concrete chute along the side of the dam [is] called a spillway, and it’s used for the controlled release of surplus water to prevent the dam from being overtopped.”
Whether it’s an arch dam or a gravity dam, and regardless of its compositional make up, dams are truly feats of modern engineering, fulfilling multiple roles to keep humanity in good supply of clean water, which is perhaps the most vital substance on Earth.
Dr. Stephen Ressler contributed to this article. Dr. Ressler is Professor Emeritus from the United States Military Academy at West Point. He earned a B.S. from West Point and an M.S. and a Ph.D. in Civil Engineering from Lehigh University, as well as a Master of Strategic Studies from the U.S. Army War College.