A levee (or dam) is a hydraulic structure, which is a soil embankment of a trapezoidal section for regulating water flows, sometimes for protection from avalanches, etc. The upper part of a levee can be used for roads and other communications
The height of the levee designed for railways or highways should be so large that the roadway is not flooded with high water, and the roadway rises above the high waters for at least 1 m. The width of the levee above is determined by the established size of the roadway, and for river and sea levees – stability condition. The width of the base of the levee depends on the width of its crest, the height of the levee and the permissible steepness of the slopes, depending on the nature of the material. For the construction of railway and highway dams, they prefer soil that does not retain water, sandy, gravel; clay is not suitable for this purpose.
The slopes of earthen levees give a slope single or one and a half, that is, a width of 1 or 1.5 m per meter of height. In the slopes of high levees, ledges are arranged every 2-8 m in the form of horizontal platforms, the so-called berm, with a width of 0.5 m, which increase the stability of the levees and make it difficult to wash the slope with water. Levees are poured in layers of 0.25-0.5 m in height, with each layer being rammed for greater compaction of the earth. If the soil used for filling the levee is inconvenient for sowing, then the slopes twitch or become covered with a layer of plant soil with a thickness of 15-20 cm, which is sown with grass; river levees are often planted with shrubs of willow or other rapidly growing species to fix the surface. The slopes of the levee, subject to the pressure of flowing water or the impact of waves, are stone-blasted to the horizon of high waters. Often also the slopes of the levee to the side of the water become more gentle compared to the opposite (upland) slope. To strengthen the slopes of the levees, fascines, stakes, clogged in rows, wattle from brushwood, etc. are also used. If the levee is built on swampy soil, water can seep under the base of the levee and contribute to its destruction. In this case, you must first dig a moat of sufficient depth and width in the swamp soil, ram it with clay and erect a structure on this artificial basis. During the construction of the railroads in Polesie, the artificial foundations for the levee in the swamps were made of wooden grillages. When roads cross vast hollows of the soil, where the construction of a levee of considerable length and height becomes unprofitable, they are replaced by viaducts. Recently, in order to increase the stability of river levees, they began to make the embankment core of stone or concrete.
There are the following types of levees:
By the method of construction:
By the dam material:
By the deadlines for which are being built:
Damage from levee destruction can be extremely large. This is due to the fact that the destruction of the levee structure itself is often only a small part of the total damage, which includes losses from the destruction of associated structures (since the levee is almost always only part of the hydraulic system), losses of enterprises, production of which can be paralyzed as a result of termination revenues from hydroelectric power stations, losses from destruction caused by a catastrophic spillway in the downstream of the levee.
Some major dam failures:
|March 12, 1928||St. Francis Dam||San Francisco Canyon, Coast Ranges, USA||about 600 people|
|December 2, 1959||Malpasset Dam||Cote d’Azur, France||423 people|
|October 9, 1963||Wyont Dam||Monte Tok, Belluno, Italy||2500 people|
|August 7, 1975||Banqiao Reservoir Dam||Zhumadian, China||171 thousand people|
The safety of hydraulic structures is regulated by the law. Levees should be designed in accordance with applicable regulatory documents: building codes and regulations and state standards.
Safety measures should be carried out from the design stage. During the construction of the levee, a check should be carried out on the compliance of the work, the properties of the foundations and building materials with the design data. During the operation of the facility, it is required to carry out field observations – monitoring of the levee using instrumentation. The installation of equipment in the structure should be provided even at the stage of design work and provide, depending on the class of the structure, monitoring precipitation, horizontal displacements, parameters of the filtration flow in the levee body, temperature, stress-strain state, etc.
In addition to hardware monitoring at all levees, field visual and geodetic observations should be performed. Such observations make it possible to establish the actual state of the structure and determine its compliance with design forecasts and prevent negative processes in a timely manner.
There are two cases of non-compliance of levees with design and regulatory requirements:
The potentially dangerous condition does not cause rapid destruction of the structure but requires the adoption of immediate measures to eliminate the causes of the condition. The pre-emergency state means that levee destruction can occur in a matter of hours, evacuation of the population and emergency rescue operations are required.
Taking measurements, having an emergency action plan and the preparedness of the hydraulic system personnel for emergency situations can prevent accidents and avoid tragic consequences.