Nalcor Energy - Lower Churchill Project Supplemental Dam Break Analysis Final Report H332992-0000-00-124-0001 Rev. 0 May 20, 2010 This document contains confidential information intended only for the person(s) to whom it is addressed. The information in this document may not be disclosed to, or used by, any other person without Hatch's prior written consent. Nalcor Energy - Lower Churchill Project - Supplemental Dam Break Analysis Final Report Table of Contents List of Tables List of Figures Executive Summary 1. Introduction ............................................................................................................................................ 1 2. Dam Break Analysis for Construction Phase Cofferdams ........................................................................ 2 2.1 Model Setup .................................................................................................................................. 2 2.2 Dam Breach Scenarios ................................................................................................................... 2 2.3 Breach Parameter Selection............................................................................................................ 3 2.4 Results of Dam Breach Analysis ..................................................................................................... 3 3. Economic Losses Resulting from Main Dam Failure ................................................................................ 6 4. Extension of GI1190 Inundation Mapping............................................................................................... 8 5. Conclusions........................................................................................................................................... 11 Appendices Appendix A – Gull Island Construction Design Flood Inundation Mapping Appendix B – Muskrat Falls Construction Design Flood Inundation Mapping ISO 9001 H332992-0000-00-124-0001, Rev. 0, Page i Nalcor Energy - Lower Churchill Project - Supplemental Dam Break Analysis Final Report List of Tables Number Title Table 2.1 Breach Parameter Estimates Table 3.1 Assumptions for Economic Damage Assessment Table 3.2 Summary of Total Incremental Economic Damage Estimates ISO 9001 H332992-0000-00-124-0001, Rev. 0, Page ii Nalcor Energy - Lower Churchill Project - Supplemental Dam Break Analysis Final Report List of Figures Number Title Figure 2.1 Diversion Discharge Rating Curves Figure 2.2 Construction Design Flood Hydrographs Figure 4.1 Extension of GI1190 Inundation Mapping (PMF Cascade Failure Case) ISO 9001 H332992-0000-00-124-0001, Rev. 0, Page iii Nalcor Energy - Lower Churchill Project - Supplemental Dam Break Analysis Final Report Executive Summary As part of information request (IR) # JRP.162 (related comments include CEAR #289 from the Innu Nation, (IR) # JRP.96 and (IR) # JRP.145), Hatch responded to three items related to dam break, as follows. 1. Dam break analysis for construction phase cofferdams; 2. Estimates of economic damage from main dam failure; and 3. Extension of Lower Churchill Dam Break Study (GI1190) inundation mapping to include the communities of Shetshatshiu and Northwest River. The original GI1190 model was used as the basis for Item 1 above. This model was extended downstream and the Gull Island and Muskrat Falls dam structures were replaced with the cofferdams planned during the construction of each project. Both “Fair Weather” and Construction Design Flood (CDF) scenarios were simulated. It was determined that the “Fair Weather” dam break for both cofferdams did not affect downstream water levels. Dam failure under CDF conditions led to an increase in water levels downstream, and the extent of inundation expected with and without dam failure is shown on the inundation maps provided in Appendices A and B. A brief analysis of economic damages resulting from dam failure was completed in GI1190. As part of this information request the analysis was extended to include roads, transmission lines, residential homes and Blackrock Bridge. Total incremental damages in the “Fair Weather” scenarios ranged from $100 to $200 million. Total incremental damages in the Probable Maximum Flood (PMF) scenarios ranged from $50 to $100 million. The original GI1190 inundation mapping did not extend downstream to include the communities of Northwest River and Shetshatshiu. As part of this information request, the original GI1190 model was extended to Groswater Bay on the coast of Labrador. The PMF cascade failure case (which produced the highest water levels along the modelled reach) estimated a small increase in water levels at Northwest River and Shetshatshiu, in the order of 30 cm to 40 cm. ISO 9001 H332992-0000-00-124-0001, Rev. 0, Page iv Nalcor Energy - Lower Churchill Project - Supplemental Dam Break Analysis Final Report 1. Introduction This report presents the analysis conducted in response to the Information Request (IR) # JRP.162 (related comments include CEAR #289 from the Innu Nation, (IR) # JRP.96 and (IR) # JRP.145. This IR consisted of three separate items related to dam break, as follows. 1. Dam break analysis for construction phase cofferdams; 2. Estimates of economic losses from main dam failure; and 3. Extension of the Lower Churchill Dam Break Study (GI1190) inundation mapping to include the communities of Shetshatshiu and Northwest River. The analysis and results pertaining to each of these three items are discussed in the following sections. ISO 9001 H332992-0000-00-124-0001, Rev. 0, Page 1 Nalcor Energy - Lower Churchill Project - Supplemental Dam Break Analysis Final Report 2. Dam Break Analysis for Construction Phase Cofferdams A breach of the cofferdams during the construction phase is highly unlikely, considering the level of quality control for the design and construction that will be carried out. The cofferdam design crest elevations will be confirmed by flood and ice studies to ensure that overtopping during construction phase remains improbable. 2.1 Model Setup The HEC-RAS dam break model developed for GI1190 (April 2008) was modified to assess the consequences of dam failure during the construction phases of the project. Two separate models were developed: one including the upstream storage caused by the Gull Island cofferdam and extending to Groswater Bay on the coast of Labrador, and the second model including the upstream storage caused by the Muskrat Falls cofferdam and extending to Groswater Bay. Construction layouts for both cofferdams were obtained from Nalcor and the structure geometries updated in the model. Diversion discharge rating curves were also defined in the model and are provided in Figure 2.1. 2.2 Dam Breach Scenarios Dam breach scenarios were simulated in each model for both “Fair Weather” and during the Construction Design Flood (CDF). The “Fair Weather” dam breach scenario assumes that the dam fails during a normal operating condition. Inflows from the Upper Churchill project and local tributaries were held constant in this case. Construction design flood estimates and hydrographs were developed previously (as described in MF1130, GI1130, and GI1140). Nalcor previously selected the 1/20 Annual Exceedance Probability (AEP) flood (5,930 m 3/s) for construction of the Muskrat Falls dam, and the 1/40 AEP flood (6,070 m3/s) for construction of the Gull Island dam. The 1/40 AEP estimate for Gull Island was reduced to 4,800 m 3/s based on a reduction in the output of the Upper Churchill (as described in GI1061). The historic flood hydrograph for 1998 at the hydrometric station upstream of Muskrat Falls (peak occurred May 16, 1998) was used to develop the shape and timing of the CDF hydrographs for both structures, since this flood had the highest 31-day volume on record and would therefore provide the most conservative peak elevations during routing. The CDF hydrographs for both Gull Island and Muskrat Falls are provided in Figure 2.2. Lateral inflows along the reach were derived by prorating the CDF hydrographs based on drainage area. The cofferdams for each structure have been designed to pass the CDFs and as such, they would not be expected to overtop under these flood conditions. For this reason, failure was assumed to be a result of piping, starting at an elevation halfway between the peak water level and the bottom of the dam. The breach was conservatively assumed to be initiated at the time of the peak water level. A piping failure was also assumed for the “Fair Weather” case. ISO 9001 H332992-0000-00-124-0001, Rev. 0, Page 2 Nalcor Energy - Lower Churchill Project - Supplemental Dam Break Analysis Final Report 2.3 Breach Parameter Selection Consistent with GI1190, dam breach parameters were estimated using a revised version of the empirically based equations developed by Froelich. A thorough explanation of these revised equations can be found in the GI1190 report. Table 2.1 below summarizes the breach parameters estimated for this analysis. Table 2.1 – Breach Parameter Estimates Breach Parameters Final Breach Dam Flow Scenario Average Breach Time of Bottom Side Slope Width (m) Formation (hrs) Elevation (m) Muskrat “Fair Weather” 60 2.4 5 1H:1V Falls Construction 125 3.6 5 1H:1V Design Flood Gull “Fair Weather” 32 1.4 35 1H:1V Island Construction 121 2.8 35 1H:1V Design Flood 2.4 Results of Dam Breach Analysis Results of dam breach analysis for both Muskrat Falls and Gull Island “Fair Weather” cases indicated that the increase in water level as a result of dam failure would be negligible.