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