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WESTERN DAM ENGINEERING NEWSLETTER, VOLUME 3, ISSUE 3, JULY 2015
SPILLWAYS, DESIGN CONSIDERATIONS FOR OUTLET CONTROL, INSTRUMENTATION FOR SMALL EARTH DAMS
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<br /> <br /> <br /> <br />2 <br />Spillways: Spilling the Right Way <br />Introduction <br />Unlined earthen spillways are common features of <br />small earthen dams. They can be principle, emergency <br />or auxiliary spillways made to pass rare flood flows <br />around the embankment. While many spillways may <br />never pass a significant flow or volume of water, one <br />large event could result in significant erosional <br />consequences. <br /> <br />Historically there was little engineering design of the <br />spillways for small dams. The spillway was often <br />situated in the dam borrow area or incorporated as <br />part of an auxiliary dam. Today there has been much <br />more research conducted on spillway performance and <br />there are many tools available for spillway design and <br />evaluation. Some of the most popular tools are <br />discussed in this article. Spillways should be designed <br />to experience flow from a known recurrence interval <br />storm and earthen spillways can expect to suffer some <br />erosion damage during events that cause them to flow. <br />However, where erosion enlarges enough to <br />destabilize the structure or cause an uncontrolled <br />release, it could lead to a dam failure with downstream <br />consequences. <br /> <br />Inadequate spillway capacity is one of the most <br />common safety deficiencies in small dams and can <br />occur due to original design deficiencies or changes in <br />conditions. The necessary capacity may have changed <br />due to a change in the watershed, downstream <br />channel, design flood, hazard class, or spillway <br />condition. Earthen spillways are common at small dam <br />sites and present additional deficiencies regarding <br />erosion and stability as compared to concrete lined <br />spillways. Spillway erosion occurs when a precipitation <br />event increases the reservoir elevation above the <br />spillway crest, resulting in water flowing down the <br />spillway channel. Due to the force of the flowing <br />water, erosion of the vegetation on the surface of the <br />spillway will begin to occur. After the water has <br />removed the vegetation, erosion of the soil will enlarge <br />and deepen the eroded area. As the flow area <br />increases in size and depth, the flow becomes more <br />turbulent, increasing the rate of erosion. With <br />continued flow, headcutting begins as the eroded area <br />continues to grow and progress upstream. Depending <br />on the configuration of the dam, the headcutting could <br />proceed to the spillway crest, eventually reaching the <br />reservoir and creating an uncontrolled release as the <br />embankment erodes away or concrete structures <br />destabilize, allowing them to slide or overturn. A <br />typical event tree of a spillway erosion or headcutting <br />failure mode is described below <br /> <br />Spillway Erosion Failure Mode <br /> Reservoir level reaches spillway crest and begins to <br />flow <br /> Vegetation (if present) is removed or eroded <br /> Concentrated flow erosion begins (downcutting <br />forms headcut) and worsens <br /> Headcut advancement begins (deepens and <br />advances towards spillway crest/control <br />section) <br /> Intervention is attempted and unsuccessful <br />(more likely to be successful if attempted <br />early) <br /> Headcut advances through crest of <br />spillway or headcut undermines control <br />structure/section and flow control is lost <br /> Headcut advances into reservoir pool <br />and breaching occurs <br /> <br />The erodibility of a spillway is a function of the <br />geology, channel geometry, and the expected volume, <br />velocity and duration of the flood flow. Fine granular <br />materials such as silts and sands are more likely to <br />erode as compared to cohesive clayey materials. Soils <br />with cohesion have a plasticity or inherent ‘stickiness’ <br />that holds the particles together. The performance of <br />rock is more complex to predict due to weathering, <br />fracturing, joints, process of formation, and strength. <br />In general, vegetation improves the performance of <br />spillways to a certain point, if it is uniform grass or <br />ground cover. Discontinuities and obstacles such as <br />trees, shrubs, groins, roads, paths, ditches, and <br />changes in slope will concentrate flow and create areas <br />prone to turbulent flow, leading to erosion. <br /> <br />Areas that experience hydraulic jumps are particularly <br />susceptible to erosion due to pressures created by the <br />energy change. This occurs most commonly at the end <br />of the spillway where the flow meets the downstream <br />river channel, stilling basin or areas of change in slope <br />of the spillway. Narrow steep channels will increase <br />the depth and speed of flow, increasing the likelihood
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