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Western Dam Engineering Technical Note August 2016 12 Dam Modification Alternatives Dam modification alternatives developed to safely accommodate an increased IDF could include one or more of the following: • Existing spillway modifications • New spillway construction • Dam crest modifications Existing Spillway Modifications Increasing the capacity of an existing spillway is one method that can be used to accommodate an increased IDF. Modifications to increase existing spillway capacity could include: • More efficient approach conditions o Sloped approach to weir crest o Rounded or curved abutments to reduce flow separation and loss of effective spillway crest length • A more efficient spillway crest o Sharp crest weir o Ogee crest weir o Rounded or curved crest shape • A longer spillway crest o Linear weir o Labyrinth weir o Curved or arced weir • Lowering the spillway crest elevation to increase available spillway head o Loss of normal storage capacity would occur, but could be offset by use of fusing elements like fusegates or fuseplugs Combinations of these potential modification alternatives should also be considered when optimizing a balance between project objectives and construction cost. Figure 2. Arced Weir Spillway [5] Figure 3. Labyrinth Weir Fusegate Spillway [6] Regardless of the adopted modification arrangement, it is critical to evaluate the performance of existing structures and features relative to the increased outflows. These features and evaluations could include: • Chute erosion potential, particularly for chutes without concrete lining • Chute slab uplift potential • Chute wall overtopping potential • Cavitation potential • Adequacy of energy dissipation Additionally, increasing spillway capacity to accommodate the IDF can increase spillway outflows associated with more frequent events, which can have unacceptable downstream consequences. This potential condition should be evaluated and considered as part of modification designs to ensure