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Western Dam Engineering Technical Note August 2016 14 another alternative used to provide capacity to safely discharge an increased IDF. By raising the dam crest elevation, the benefits to outflow capacity are twofold: • Additional flood storage surcharge volume is provided, which can be particularly effective in attenuating flood inflows and reducing the spillway capacity requirements associated with the short duration, high intensity storm events commonly observed across the western states. • Raising the dam increases the spillway head, resulting in increased spillway capacity. Spillway capacity is a function of spillway head (H), a crest efficiency coefficient (C), and crest length (L) [8]. The spillway head is raised to the three halves power, whereas, the spillway crest coefficient and length are raised to the power of unity (Q=CLH3/2), making the dam raise exponentially effective. A dam crest raise can be a very efficient, simple, and economical solution to providing additional outflow capacity. Dam crest raise alternatives commonly include: • Conventional upstream, downstream, or centerline embankment raises • Parapet or other conventional concrete walls • A variety of reinforced/retaining earth walls Figure 6. Dam Crest Raise with a Concrete Parapet Wall [9] Although the normal reservoir water surface elevation will not change by raising the dam crest elevation, the flood pool will change, particularly for infrequent food events. The potential impacts of these flood pool increases should be evaluated to confirm that they do not pose unacceptable consequences. Furthermore, the impact of increased spillway outflows on existing features should also be evaluated to confirm satisfactory performance. Consideration should also be given to specific site conditions including, but not limited to: • Practical dam raise heights due to topographic limits • Abutment tie-in locations and the associated dam crest length • The potential need for saddle dams • Foundation conditions • Compliance with local water laws regarding the maximum detention time of flood storage References [1] Connecticut Department of Energy and Environmental Protection (DEEP) website: HTTP://WWW.CT.GOV/DEEP/LIB/DEEP/WATER_INLAND/DAMS/CHANGES_IN_CT_ DAM_SAFETY_PROGRAM.PDF [2] FEMA, 2013. Selecting and Accommodating Inflow Design Floods for Dams. Federal Emergency Management Agency (FEMA), FEMA P-94, August, 2013. HTTPS://WWW.FEMA.GOV/MEDIA- LIBRARY/ASSETS/DOCUMENTS/3898 [3] FERC, 2015. Engineering Guidelines for the Evaluation of Hydropower Project - Selecting and Accommodating Inflow Design Floods for Dams. Federal Energy Regulatory Commission (FERC), Chapter II, August, 2015. HTTPS://WWW.FERC.GOV/INDUSTRIES/HYDROPOWER/SAFETY/GUIDELINES/ENG- GUIDE.ASP [4] Hydrometeorological Reports from the National Oceanic and Atmospheric Administration (NOAA). HTTP://WWW.NWS.NOAA.GOV/OH/HDSC/STUDIES/PMP.HTML [5] ASDSO, 2013. “Hydraulic Design of Labyrinth Weirs.” Association of State Dam Safety Officials (ASDSO) webinar, August, 2013. [6] U.S. Army website: HTTPS://WWW.ARMY.MIL/ARTICLE/53577/HEAVY_LIFTING_KEEPS_CORPS_DAM _IN_WORKING_ORDER [7] FEMA, 2014. Technical Manual: Overtopping Protection for Dams. Federal Emergency Management Agency (FEMA), FEMA P-1015, May, 2014. HTTPS://WWW.FEMA.GOV/MEDIA-LIBRARY/ASSETS/DOCUMENTS/97888 [8] Chow, V.T. 1959. Open Channel Hydraulics. [9] National Resources Conservation Service (NRCS) website: HTTP://WWW.NRCS.USDA.GOV/WPS/PORTAL/NRCS/DETAIL/AL/ABOUT/?CID=NRC S141P2_022973.