Nalcor Energy - Lower Churchill Project Supplemental Dam

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

H332992-0000-00-124-0001 Rev. 0

May 20, 2010

Nalcor Energy - Lower Churchill Project - Supplemental Dam Break Analysis Final Report

ISO 9001 H332992-0000-00-124-0001, Rev. 0, Page i

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 ii

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 iii

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 iv

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 1

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 2

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 m3/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 m3/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 3

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

Dam

Flow Scenario

Average Breach Width (m)

Time of Formation (hrs)

Final Breach Bottom

Elevation (m) Side Slope

“Fair Weather” 60 2.4 5 1H:1V Muskrat Falls

Construction

Design Flood

125 3.6 5 1H:1V

“Fair Weather” 32 1.4 35 1H:1V Gull Island

Construction

Design Flood

121 2.8 35 1H:1V

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. This was due to the limited storage volume upstream of each dam and rapid drop in upstream water levels as a result of dam failure, leading to a reduction in the diversion flow. These conditions led to a total flow through the dam that is not significantly more than the average flow condition.

The Froelich equation to estimate the peak outflow from a dam failure (Wahl, 2004) was used as a check for this result. Based on the total inundated volume and the height of water at the dam, the resulting peak flow estimate was in accordance with the model results. This small increase in flow has a negligible effect on water levels; therefore, inundation mapping has not been produced for the “Fair Weather” scenario.

Inundation mapping has been produced to illustrate the extent of flooding that would be expected to occur as a result of cofferdam failure under CDF conditions. The maps also illustrate the amount of inundation expected under the same flood conditions, but without the failure of the cofferdam. Information boxes have been provided at selected cross sections, summarizing the following results: distance downstream of the cofferdam in question, peak flood elevation, peak breach elevation, incremental depth of flooding, breach floodwave arrival time, and time to peak water elevation. Inundation maps for Gull Island and Muskrat Falls are included in Appendix A and B, respectively.

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Figure 2.1

Diversion Discharge Rating Curves

Information Request # JRP.162 - Supplemental Dam Break Analysis


Muskrat Falls - During Construction

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Figure 2.2

Construction Design Flood Hydrographs

Information Request # JRP.162 - Supplemental Dam Break Analysis


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ISO 9001 H332992-0000-00-124-0001, Rev. 0, Page 6

3. Economic Losses Resulting from Main Dam Failure As part of GI1190, a brief analysis was completed to assess the incremental economic damages expected as a result of dam failure. This analysis included an estimate of the replacement cost of residential homes affected by the flooding. Other potential damages were identified but not quantified. As part of this IR, an attempt was made to extend the quantitative estimate of economic damages by including three more infrastructure items: roads, transmission lines, and Blackrock Bridge. Road and transmission lines (only the 138 kV transmission line was considered, not transmission within the community of Happy-Valley Goose Bay) within the incrementally inundated areas were included, and Blackrock Bridge was included in the estimate of economic damage if the water level was higher than the bridge deck.

As the estimates provided here are only based on the four infrastructure items described above, they should be considered ‘order of magnitude’ estimates. Damages to Nalcor property and energy lost as a result of dam failure have not been included.

Table 3.1 summarizes the estimated cost of replacement of roads, the 138 kV transmission line, residential homes, and Blackrock Bridge. Table 3.2 summarizes the order of magnitude estimates of total incremental economic damages for each of the scenarios considered in GI1190.

Table 3.1 – Assumptions for Economic Damage Assessment

Item Cost Unit 138 kV Transmission Line 400,000 $/km Road 500,000 $/km Residential Home 100,000 $/home Blackrock Bridge $29,300,000 $


ISO 9001 H332992-0000-00-124-0001, Rev. 0, Page 7

Table 3.2 – Summary of Total Incremental Economic Damage Estimates

Flow Scenario Dam Failure Scenario Damage Estimate Post Gull Island, Failure of Gull Island Main Dam $142,000,000

Post Gull Island and Muskrat Falls, Failure of Gull Island Main

Dam

$141,000,000

Post Gull Island and Muskrat Falls, Failure of Muskrat Falls

North RCC Dam

$124,000,000 “Fair Weather”

Conditions


Dam and Muskrat Falls North RCC Dam

$164,000,000

Post Gull Island, Failure of Gull Island Main Dam $54,000,000


Dam

$54,000,000

Post Gull Island and Muskrat Falls, Failure of Muskrat Falls

North RCC Dam

$53,000,000 Probable

Maximum Flood


Dam and Muskrat Falls North RCC Dam

$87,000,000


ISO 9001 H332992-0000-00-124-0001, Rev. 0, Page 8

4. Extension of GI1190 Inundation Mapping The original GI1190 inundation mapping did not extend downstream as far as the communities of Shetshatshiu and Northwest River; it was previously noted that little if any incremental flooding is anticipated in these areas. As part of this information request, the model was extended and inundation mapping was prepared to illustrate the extent of inundation expected at this location.

The original GI1190 model extended part way through Goose Bay (approximate chainage -9.4 km). This was assumed to be a sufficient distance to attenuate the flood wave such that the downstream boundary condition (an average high tide level of 0.4 m) would not influence the water levels in the model. As part of this IR, the model was extended downstream to Groswater Bay on the coast of Labrador. The Probable Maximum Flood (PMF) scenario with cascade failure of both Gull Island Dam and Muskrat Falls Dam (the scenario that led to the highest water levels along the modelled reach) was simulated and resulting water levels were compared to original GI1190 results. Water levels were found to be higher than original GI1190 results, suggesting that in GI1190, the downstream boundary condition was influencing the water levels in the model for some distance upstream.

Tables 4.1 and 4.2 summarize the water levels estimated in the original GI1190 model and the extended GI1190 model for the PMF No Breach and PMF Cascade Failure scenarios. Figure 4.1 illustrates the inundation expected for these two scenarios based on the extended model.

Table 4.1 - Comparison of Maximum Water Levels from Original and Extended GI1190 Model (PMF No Breach Scenario)

Maximum Water Level (m)

Location Cross

Section Original GI1190 Model

Extended GI1190 Model

Difference (m)

Blackrock Bridge 24.1 km 7.6 m 7.6 m 0.0 m

Mud Lake 2.8 km 3.8 m 4.0 m 0.2 m

*Goose Bay -9.4 km 0.4 m 0.9 m 0.5 m

* Goose Bay refers to the body of water, not the community.


ISO 9001 H332992-0000-00-124-0001, Rev. 0, Page 9

Table 4.2 - Comparison of Maximum Water Levels from Original and Extended GI1190 Model (PMF Cascade Failure Scenario)

Maximum Water Level (m)

Location Cross

Section Original GI1190 Model

Extended GI1190 Model

Difference (m)

Blackrock Bridge 24.1 km 12.8 m 12.8 m 0.0 m

Mud Lake 2.8 km 10.0 m 10.0 m 0.0 m

*Goose Bay -9.4 km 0.4 m 4.0 m 3.6 m

* Goose Bay refers to the body of water, not the community.

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DISTANCE DOWNSTREAM OF GULL ISLAND DAM: 107.1 kmPEAK FLOOD ELEVATION: 0.9 mPEAK BREACH ELEVATION: 4.0 mINCREMENTAL DEPTH OF FLOODING: 3.1 mBREACH FLOODWAVE ARRIVAL TIME: 4.4 hrTIME TO PEAK WATER ELEVATION: 14.4 hr



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PROBABLE MAXIMUM FLOODPOST GULL ISLAND AND MUSKRAT FALLSFAILURE OF GULL ISLAND MAIN DAM AND

MUSKRAT FALLS NORTH RCC DAM

FIGURE 4.1H-332992

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ALL CONTOURS SHOWN ARE IN METERS.

COORDINATES ARE BASED ON THE UNIVERSAL TRANSVERSE MERCATORPROJECTION, ZONE 20 NORTH, NORTH AMERICAN DATUM 1983.

SURFACE FEATURES WERE PRODUCED FROM 1:50,000 SCALE NATIONALTOPOGRAPHIC SYSTEM (NTS) MAPS.

TOPOGRAPHIC CONTOURS WERE PRODUCED FROM LIDAR DATA OBTAINED IN2006 AND 1:50,000 SCALE DIGITAL ELEVATION DATA.

NOTES:

INDICATES AREA COVERED BY THIS SHEET.

GooseBay

Lake Melville

MudLake

Grand Lake

Happy Valley - Goose Bay

Sheshatshiu

North WestRiver


ISO 9001 H332992-0000-00-124-0001, Rev. 0, Page 11

5. Conclusions The following are the conclusions related to each of the three items addressed in this information request.

1. Inundation mapping was completed for Gull Island and Muskrat Falls cofferdam failures under the CDF scenario. Inundation mapping was not completed for the “Fair Weather” scenario since the increase in water level is expected to be negligible.

2. Order of magnitude estimates of the incremental economic damage resulting from failure of the main dams under “Fair Weather” conditions are estimated to be in the range of $100 to $200 million. Incremental economic damages resulting from failure of the main dams under PMF conditions are estimated to be in the order of $50 to $100 million. These estimates are not comprehensive and only include residential homes, roads, the 138 kV transmission line, and Blackrock Bridge. They do not include losses to Nalcor property or the energy loss that would be associated with dam failure.

3. A cascade failure of both Gull Island and Muskrat Falls main dams under the PMF is expected to lead to little inundation at Shetshatshiu and Northwest River, in the order of 30 cm to 40 cm.


ISO 9001 H332992-0000-00-124-0001, Rev. 0

Appendix A Gull Island Construction Design Flood

Inundation Mapping

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200DISTANCE DOWNSTREAM OF GULL ISLAND DAM: 11.3 kmPEAK FLOOD ELEVATION: 30.0 mPEAK BREACH ELEVATION: 32.0 mINCREMENTAL DEPTH OF FLOODING: 2.0 mBREACH FLOODWAVE ARRIVAL TIME: 0.4 hrTIME TO PEAK WATER ELEVATION: 3.7 hr


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CONSTRUCTION DESIGN FLOODFAILURE OF GULL ISLAND COFFERDAM

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200DISTANCE DOWNSTREAM OF GULL ISLAND DAM: 11.3 kmPEAK FLOOD ELEVATION: 30.0 mPEAK BREACH ELEVATION: 32.0 mINCREMENTAL DEPTH OF FLOODING: 2.0 mBREACH FLOODWAVE ARRIVAL TIME: 0.4 hrTIME TO PEAK WATER ELEVATION: 3.7 hr


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DISTANCE DOWNSTREAM OF GULL ISLAND DAM: 134.1 kmPEAK FLOOD ELEVATION: 0.4 mPEAK BREACH ELEVATION: 0.4 mINCREMENTAL DEPTH OF FLOODING: 0.0 mBREACH FLOODWAVE ARRIVAL TIME: N/ATIME TO PEAK WATER ELEVATION: N/A

Lake Melville

North WestRiver

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ISO 9001 H332992-0000-00-124-0001, Rev. 0

Appendix B Muskrat Falls Construction Design Flood

Inundation Mapping

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BREACH INUNDATION

CONSTRUCTION DESIGN FLOODFAILURE OF MUSKRAT FALLS COFFERDAM

FIGURE B - H-332992

NALCOR ENERGY

1:50000

HATCH PROJECT No

DRAWING NO.SCALE


1

KEY PLAN



1.

2.

3.

4.





NOTES:

23 4

6

1

7

8 9

5

100

100200

100

100

200 200

200

200

200

200

200

200

200

200

100



qLEGEND

" BUILDING

9 TANK

W TOWER


ROAD

RIVER

CROSS SECTIONS

CONTOURS

CONTOUR (20 m)


WETLAND

WATER BODY

FLOOD INUNDATION

BREACH INUNDATION


FIGURE B - H-332992

NALCOR ENERGY

1:50000

HATCH PROJECT No

DRAWING NO.SCALE


2

KEY PLAN



1.

2.

3.

4.





NOTES:

23 4

6

1

7

8 9

5




Happy Valley -Goose Bay

qLEGEND

" BUILDING

9 TANK

W TOWER


ROAD

RIVER

CROSS SECTIONS

CONTOURS

CONTOUR (20 m)


WETLAND

WATER BODY

FLOOD INUNDATION

BREACH INUNDATION


FIGURE B - H-332992

NALCOR ENERGY

1:50000

HATCH PROJECT No

DRAWING NO.SCALE


3

KEY PLAN



1.

2.

3.

4.





NOTES:

23 4

6

1

7

8 9

5


Community ofMud Lake

MudLake

qLEGEND

" BUILDING

9 TANK

W TOWER


ROAD

RIVER

CROSS SECTIONS

CONTOURS

CONTOUR (20 m)


WETLAND

WATER BODY

FLOOD INUNDATION

BREACH INUNDATION


FIGURE B - H-332992

NALCOR ENERGY

1:50000

HATCH PROJECT No

DRAWING NO.SCALE


4

KEY PLAN



1.

2.

3.

4.





NOTES:

23 4

6

1

7

8 9

5

100

200

200

200

200

200

200

100

GoslingLake

GooseBay

qLEGEND

" BUILDING

9 TANK

W TOWER


ROAD

RIVER

CROSS SECTIONS

CONTOURS

CONTOUR (20 m)


WETLAND

WATER BODY

FLOOD INUNDATION

BREACH INUNDATION


FIGURE B - H-332992

NALCOR ENERGY

1:50000

HATCH PROJECT No

DRAWING NO.SCALE


5

KEY PLAN



1.

2.

3.

4.





NOTES:

23 4

6

1

7

8 9

5


GooseBay

qLEGEND

" BUILDING

9 TANK

W TOWER


ROAD

RIVER

CROSS SECTIONS

CONTOURS

CONTOUR (20 m)


WETLAND

WATER BODY

FLOOD INUNDATION

BREACH INUNDATION


FIGURE B - H-332992

NALCOR ENERGY

1:50000

HATCH PROJECT No

DRAWING NO.SCALE


6

KEY PLAN



1.

2.

3.

4.





NOTES:

23 4

6

1

7

8 9

5

qLEGEND

" BUILDING

9 TANK

W TOWER


ROAD

RIVER

CROSS SECTIONS

CONTOURS

CONTOUR (20 m)


WETLAND

WATER BODY

FLOOD INUNDATION

BREACH INUNDATION


FIGURE B - H-332992

NALCOR ENERGY

1:50000

HATCH PROJECT No

DRAWING NO.SCALE


7

KEY PLAN



1.

2.

3.

4.





NOTES:

23 4

6

1

7

8 9

5

DISTANCE DOWNSTREAM OF GULL ISLAND DAM: 74.6 kmPEAK FLOOD ELEVATION: 0.4 mPEAK BREACH ELEVATION: 0.4 mINCREMENTAL DEPTH OF FLOODING: 0.0 mBREACH FLOODWAVE ARRIVAL TIME: N/ATIME TO PEAK WATER ELEVATION: N/A

Lake Melville

North WestRiver

Sheshatshiu

qLEGEND

" BUILDING

9 TANK

W TOWER


ROAD

RIVER

CROSS SECTIONS

CONTOURS

CONTOUR (20 m)


WETLAND

WATER BODY

FLOOD INUNDATION

BREACH INUNDATION


FIGURE B - H-332992

NALCOR ENERGY

1:50000

HATCH PROJECT No

DRAWING NO.SCALE


8

KEY PLAN



1.

2.

3.

4.





NOTES:

23 4

6

1

7

8 9

5

Lake Melville

qLEGEND

" BUILDING

9 TANK

W TOWER


ROAD

RIVER

CROSS SECTIONS

CONTOURS

CONTOUR (20 m)


WETLAND

WATER BODY

FLOOD INUNDATION

BREACH INUNDATION


FIGURE B - H-332992

NALCOR ENERGY

1:50000

HATCH PROJECT No

DRAWING NO.SCALE


9

KEY PLAN



1.

2.

3.

4.





NOTES:

Suite E200, Bally Rou Place, 280 Torbay Rd. St. John's, Newfoundland, Canada A1A 3W8

Tel (709) 754 6933 Fax (709) 754 2717

Documents

Nalcor Energy - Lower Churchill Project Supplemental Dam