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