A Watershed Year: Anatomy of the Iowa Floods of 2008

22. Flood Barriers

22 Flood Barriers Nathan C.Young A. Jacob Odgaard Flood barriers have been heavily featured in news stories and discussions involving the 2008 floods on the Iowa and Cedar Rivers. Permanent and temporary flood barriers were used to combat flood damage during the event. But in many cases, these barriers were overtopped or failed, causing significant damage to public and private property. Following the event, flood barriers became major components of proposed flood mitigation plans. However, while flood barriers can be effective in protecting lowland areas from flooding, the financial, ecological, and social costs associated with them can be prohibitive. As Iowans recover from the flood and seek protection from future disasters, it is important for the public and policy makers to understand the capabilities, value, and limitations of flood barriers in reducing flood risks. Flood barriers are permanent or temporary structures designed to contain floodwaters and prevent them from inundating adjacent low-lying areas.1 Several types of flood barriers are depicted in figure 22-1. The most common permanent structures are earthen levees. Levees are large soil embankments constructed along the bank of a river. Levees extend across a floodplain, connecting areas of higher elevation. Levees feature gently sloping sides to ensure soil stability and create mass sufficient to hold back floodwaters. 200 looking back,looking forward In urban environments and elsewhere where property is valuable and space is limited, construction of levees may not be feasible. A second type of permanent flood barrier, the flood wall, may be used in these areas. Flood walls, like levees, are constructed next to rivers and streams. They are vertical walls constructed from metal, concrete, or masonry, which are anchored into the ground and commonly feature structural supports to withstand the pressure of floodwaters. Where permanent structures are not feasible or justified, temporary emergency barriers may be built at the time of flooding. Sandbag levees were extensively used during the 2008 floods. These consist of sand-filled burlap or plastic bags stacked in various geometries to withstand the pressure of anticipated flood depths. They are typically covered in plastic sheeting to decrease leakage. Temporary barriers can also be constructed from soil, rock, and human-made FIGURE 22-1 Flood barriers are constructed along river channels to prevent floodwaters from affecting adjacent property. Levees and flood walls are permanent flood barriers. They are typically designed to protect against 100-year flood elevations plus an additional height, called freeboard. Freeboard, typically 1–3 feet, accounts for the possible effects of waves, settlement of the embankment, and error in flood elevation estimates. Temporary flood barriers, such as sandbag levees, are constructed when floods threaten and are used in areas where permanent barriers are not feasible. Hybrid flood barriers combine permanent and temporary components to reduce cost and maintain community connectivity with the river. Illustration by Michael Kundert. Levee Sandbag Levee Flood Wall Hybrid Flood Barrier Permanent Levee Temporary Flood Wall Freeboard flood barriers 201 materials. Temporary barriers are versatile and can be relocated or augmented when necessary to adjust to rapidly changing flood conditions. Hybrid flood barriers, which have both permanent and temporary components , can be useful in providing flood protection in urban areas without necessitating high levees or flood walls that cut off a community’s connection to the river. Hybrid barriers feature low permanent levees constructed to protect against more frequent, less severe flood events. These levees eliminate the need to regularly construct temporary flood barriers for small floods, while maintaining access to and even enhancing enjoyment of the river by facilitating construction of walkways and trails. When more severe floods threaten, protection is augmented by placement of temporary sandbag levees or removable flood walls on top of the lower permanent structures. Flood barriers can significantly decrease the risk of flood damage. However, contrary to common thought, they are not intended to prevent all flooding at all times. Foolproof protection by flood barriers is neither possible, nor is it their intent. For one thing, flood barriers must be carefully designed and maintained to ensure they do not fail or create flood problems for the areas they are intended to protect. One significant consideration is interior flooding. Interior flooding occurs when flood barriers prevent local runoff originating from the barrier’s landward side from draining to the river. To prevent ponding of water on the landward side of a levee or flood wall, land is reshaped and tiled to drain local...