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11 A spillway generally consists of various combinations of crest control structure, conveyance element (or chute), terminal structure, and downstream channel. As most small dams are embankment dams, only spillways typically associated with embankment dams are discussed in this article. Therefore, spillways typically associated with concrete dams, such as free overfall drop spillways or overtopping spillways, are not discussed. The following sections provide examples of various crest control, conveyance, and terminal structures for embankment dam spillways. Crest Control Structures A major component of a spillway is the crest control structure that regulates outflows from the dam. A crest control structure can be categorized as either a controlled (gated) or uncontrolled (ungated) crest structure. Most crest structures on small dams are uncontrolled for simplicity, economical construction cost, and ease of maintenance and operation. As the focus of the Western Dam Engineering Technical Note is predominately on small dams, uncontrolled crest structures are discussed herein. There are six common types of uncontrolled crest structures and these are described in the following subsections of this article. For a crest control structure to realize its theoretical discharge, the hydraulic conditions both upstream and downstream must be acceptable. The approach conditions upstream should not choke the flow to the crest and the downstream conveyance element should not cause excessive submergence of the crest. In some cases, sub-optimal downstream hydraulic conditions are acceptable to achieve efficiencies in design and construction cost. Ogee Crests An ogee crest has a control weir that is curved in profile and is designed to closely profile that of the lower nappe of a ventilated sheet (of water) falling from a sharp crested weir. An ogee crest can also be designed so that negative pressures on the crest are within an acceptable range and the potential for cavitation is minimized. A correctly designed ogee crest control structure has high discharge efficiency and is used on most spillway control crests for large dams and commonly on small dams. An example of an ogee crest control structure is shown on Figure 1 and Figure 2. Figure 1: Ogee Crest Figure 2: Ogee Crest Broad Crested Weir and Concrete Sill Crests The crest structures on many small dams, especially where space permits and where minimal discharge capacity is required, may be simple concrete sills or broad crested weir control structures. Occasionally, broad crested weirs will have a slightly rounded upstream edge to provide a more efficient inlet flow transition. Where concrete broad crested weirs are not located significantly above the channel bed to develop hydraulic control, these weirs are referred to as ‘sills’. Examples of a broad crested weir and a concrete crest sill are shown on Figures 3 and 4, respectively. Figure 3: Broad Crested Weir