A Watershed Year: Anatomy of the Iowa Floods of 2008

10. The Coralville Dam and Reservoir: Design and Operation

10 The Coralville Dam and Reservoir Design and Operation John Castle The Coralville Dam was constructed in the 1950s by the U.S. Army Corps of Engineers (Corps) and put into operation in September 1958. The earthfilled dam is 1,400 feet long, 100 feet tall, and 650 feet wide at the base.1 The dam created the Coralville Reservoir, a large body of water that under normal conditions—when the water surface is at 683 feet elevation above mean sea level (msl)2 —reaches 21.7 miles upstream, has a surface area of about 5,400 acres, and holds 28,400 acre-feet of water.3 Inflowing sediments, primarily from agriculturalland,aregraduallyreducingthereservoir’sdepthandwater-holding capacity: by 2008 the reservoir’s total storage capacity was 14 percent less than when newly constructed (Martin 2008). At the dam’s east end stands the water control structure: three vertical slide gates, each 8.33 feet wide by 20 feet tall, that control the amount of water released from the reservoir back into the Iowa River. A 500-foot-wide emergency spillway lies at the dam’s west end. The emergency spillway crest or top is 712 feet, 31 feet lower than the top of the dam. The dam is designed to safely handle flows over the emergency spillway as large as 244,000 cubic feet per second (cfs). Downstream flooding can result either from large but regulated water release 96 why here, why now? through the dam’s gates or from uncontrolled spillway overflows. Upstream lies the Hawkeye Wildlife Area, land owned by the Corps and leased to the Iowa Department of Natural Resources that is part of the flood storage area. Lake Macbride, an impounded lake within Lake Macbride State Park on the east side of the reservoir, is also considered part of the reservoir storage area when water in the lake is above the 712-foot level, the height of the spillway separating Lake Macbride and the reservoir. The primary purpose of the dam and reservoir is reducing flooding on the Iowa River and, by storing Iowa River floodwaters, ultimately reducing flooding on the Mississippi River (which is fed by the Iowa River 83.3 miles downstream from the dam). As was demonstrated in 1993 and again in 2008, the dam can reduce flooding downstream but cannot eliminate all flooding, nor was it ever intended to do so. The dam’s secondary purposes are low flow augmentation (water storage and release during dry conditions), water quality improvement (by allowing sediment to settle out), provision of fish and wildlife habitat, and recreation. The normal reservoir level varies with the time of the year. In summer the reservoir is maintained at elevation 683 feet. The reservoir is raised to elevation 686 feet in October to provide additional water surface area for migrating waterfowl. In December, the level is returned to elevation 683 feet. In February , the reservoir level is lowered to elevation 679 feet in anticipation of spring rainfall. This adds a small measure (3 percent) of additional flood storage capacity. The reservoir is returned to elevation 683 feet by Memorial Day. Once the desired level for any given period is reached, water’s inflow (from upstream tributaries) and outflow (through dam gates) is balanced to keep the reservoir level constant. The dam is managed by the Water Control Section of the Rock Island District office of the Corps, with input from personnel at the Coralville Reservoir office (who carry out the desired operations). Management decisions are guided by a detailed regulation schedule (Corps 2001). The schedule specifies limits to release rates from the Coralville Dam according to the time of year, elevation of the reservoir, downstream level of the Iowa River, and even the level of the Mississippi River at Burlington. For example, from May 1 to mid-December, the reservoir is to be held at elevation 683–686 feet by using release rates between 1,000 and 6,000 cfs, unless well-defined flood conditions exist downstream. (In that case, a variety of other specific discharge rates are mandated.) Alternatives are also given for major flood and major drought conditions. When flood conditions exist, the dam’s release rates are lowered to reduce the coralville dam and reservoir 97 Iowa River flood crest levels expected at downstream streamgages near Lone Tree and Wapello. Flows on the Cedar River, which joins the Iowa River at Columbus Junction, also help...