|WU, WEIMING - University Of Mississippi|
|ALTINAKER, MUSTAFA - University Of Mississippi|
Submitted to: Journal of Hydraulic Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/29/2011
Publication Date: N/A
Interpretive Summary: Through the 1944 Flood Control Act and the 1953 Watershed Protection and Flood Prevention Act of 1953 the US Department of Agriculture - Natural Resources Conservation Service has assisted in the building of over 11,000 small flood control dams. Many of these dams have become unsafe because of aging components and the filling of their reservoirs with sediment. Also, their risk factor has increased because of downstream urbanization and construction of critical infrastructure such as highways and bridges. Since these structures can sustain only limited safety levels and are subject to decay, they may fail due to various triggering mechanisms, particularly with a high probability of failure under extreme conditions. These failures pose significant flood risks to people and property in the inundation area and cause an interruption of services provided by these structures. Scientists at the U.S. Department of Agriculture-Agricultural Research Service-National Sedimentation Laboratory in collaboration with other federal agencies, academia, and industry have researched the state of the art of several important issues related to dam failure, including the earthen embankment breaching processes, data collection, breach modeling, flood routing, model limitations and uncertainties, and potential future development. Major study findings are: limited availability of adequately documented historical embankment failures; lack of large-scale laboratory experiments; and the need of improved, physically-based breach models.
Technical Abstract: A large number of embankment structures, including dams, levees, dikes, and barriers, have been built by humans or formed naturally along rivers, lakes, and coastal lines around the world. These structures play very important roles in flood defense, while many are also used for water supply, power generation, transportation, sediment retention, etc. Since these structures can sustain only limited safety levels and are subject to decay, they may fail due to various triggering mechanisms, particularly with a high probability of failure under extreme conditions. These failures pose significant flood risks to people and property in the inundation area and cause an interruption of services provided by these structures. Therefore, understanding and prediction of embankment failure processes are crucial for water infrastructure management. Embankments can be classified as non-erodible (concrete), erodible (earth/rock), or the mixed type. Manmade dams may be constructed with one of these but are mostly built as homogeneous or zoned earthfill, rockfill with clay core and/or concrete face. According to the U.S. Committee on Large Dams, almost 80% of the large dams in the U.S. were formed by embankments constructed from natural erodible materials. Landslide dams are usually comprised of erodible earth and rock materials. Most levees and dikes are constructed using clay, silt or sand with a clay core or cover, often on a foundation of erodible substrata. Natural coastal barriers often consist mostly of sands. Embankment failures are very sensitive to the structures’ materials and configurations, impacting forces, and other environmental factors. In general, a concrete embankment is prone to fail instantaneously (break) when the entire structure or only a portion loses stability under certain loading conditions; thus, the breach dimensions are often determined based on structural stability analysis and the resulting flood can then be simulated using numerical tools. An earth embankment, however, is likely to fail gradually (breaching) due to erosion of its materials by water flow or wave action involving mixed-regime flows, strong sediment transport and rapid morphological changes. Therefore, determination of the earth embankment breach characteristics (width, shape, peak outflow, failure time) is quite complex and challenging, requiring the prediction of complex interactions between soil, water, and structure. In the last few decades, a number of laboratory experiments and field investigations have been carried out and many empirical, analytical, and numerical models have been developed to first understand and then simulate the earthen embankment breaching processes. This forum article provides an overview of the state of the art of several important related issues, including the earthen embankment breaching processes, data collection, breach modeling, flood routing, model limitations and uncertainties, and potential future development.