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12 Figure 4: Concrete Control Sill Sharp Crested Weirs A sharp crested weir is typically formed through the construction of a thin wall and placed normal to the flow in a spillway chute. The flow depth is generally two or more times greater than the wall thickness. Sharp crested weirs often have air vents on the underside of the flow so that atmospheric pressure exists on the underside of the nappe. Sharp crested weirs are seldom used on spillways for dams and are more common in canals and flood control channels. An example of a sharp crested weir is shown on Figure 5. Figure 5: Sharp Crested Weir Labyrinth Weirs Labyrinth weirs provide additional crest length across the width of a spillway to increase discharge flows, particularly at low heads. The additional crest length is obtained by utilizing a series of trapezoidal walls that provide ’cycles’ on the crest. The walls are typically thin walls that are supported on a concrete slab or other acceptable foundation. Flow patterns on a labyrinth spillway are complex. When the maximum design head for a given labyrinth weir configuration is exceeded, the discharge coefficient can decrease due to interference from adjacent labyrinth cycle(s) which can partially, or fully, submerge the weir crest. At high design heads the labyrinth discharge can begin to approach that of a broad crested weir. As with all crest structures, a labyrinth weir is sensitive to tailwater levels and when the crest becomes submerged the discharge coefficient can decrease significantly. A piano key weir offers similar discharge efficiency to a labyrinth weir, and in some cases greater discharge efficiency than a labyrinth weir. Piano key weirs are rarely constructed on small embankment dams and, therefore, are not discussed herein. An example of a labyrinth weir control structure is shown on Figure 6. Fusegates® are most often designed with a labyrinth crest shape for more efficient discharge and to minimize the number of Fusegates® required to pass the IDF. Fusegates® are a more common installation on large dams and, therefore, are discussed no further in this article. Figure 6: Labyrinth Weir Drop Inlet/Morning Glory Crests A drop inlet crest is one in which the flow enters the spillway over a horizontal lip, drops through a vertical or sloping shaft, and then flows through a horizontal or near horizontal conduit or tunnel to the downstream river channel. Drop inlets are commonly used for principal spillways and less frequently for auxiliary spillways. The drop inlet is normally located within the