Muskoka River Flood Plain Mapping Study

The District Municipality of Muskoka

Technical Report

For

H356689-00000-200-230-0002 Rev. 0 February 12, 2020

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. The District Municipality of Muskoka

Technical Report

For

Muskoka River Flood Plain Mapping Study

H356689-00000-200-230-0002 Rev. 0 February 12, 2020

Report

Technical Report

H356689-00000-200-230-0002

B. Heppner, G. 2020-02-12 0 Final A. Breland A. Breland Schellenberg

DATE REV. STATUS PREPARED BY CHECKED BY APPROVED BY

Manager Manager

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IMPORTANT NOTICE TO READER

This report was prepared by Hatch Ltd. (“Hatch”) for the sole and exclusive benefit of The District Municipality of Muskoka (the “Client”) for the sole purpose of updating flood line mapping for particularly vulnerable portions of Muskoka River flood plains (the “Project) and may not be used or relied upon by any other party.

Hatch makes no representation or warranty and assumes no liability in respect of the use of this report by any third party. By virtue of its review of this report, a third party shall be deemed to have waived, released and discharged Hatch from any liability in connection with this report or its contents, however arising.

The use of this report by the Client is subject to the terms of the relevant services agreement between Hatch and Client.

This report is meant to be read as a whole, and sections should not be read or relied upon out of context. The report includes information provided by the Client and by certain other parties on behalf of the Client. Unless specifically stated otherwise, Hatch has not verified such information and does not accept any responsibility or liability in connection with such information.

This report contains the expression of the opinion of Hatch using its professional judgment and reasonable care, based upon information available at the time of preparation. The quality of the information, conclusions and estimates contained in this report is consistent with the intended level of accuracy as set out in this report, as well as the circumstances and constraints under which this report was prepared.

As this report is a flood mapping study, all estimates and projections contained in this report are based on limited and incomplete data. Accordingly, while the work, results, estimates and projections in this report may be considered to be generally indicative of the nature and quality of the Project, they are not definitive. No representations or predictions are intended as to become the results of future work, and Hatch does not promise that the estimates and projections in this report will be sustained in future work.

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Table of Contents

1. Introduction ...... 1-1 2. Study Area and Objectives ...... 2-1 2.1 Previous Studies and Current Situation ...... 2-1 2.2 Study Objectives ...... 2-2 2.3 Deliverables ...... 2-2

3. Background Information Collection/Review ...... 3-1 3.1 Reports Reviewed ...... 3-1 3.2 Review of Previous Flood Estimates ...... 3-1 3.3 Review of Climate and Hydrology Data ...... 3-3 3.3.1 Normal Streamflow and Climate Conditions ...... 3-6 3.3.2 Daily Streamflow Duration Curves ...... 3-8 3.3.3 Daily Specific Runoff Duration Curves ...... 3-9

4. Hydrology Study ...... 4-1 4.1 Recent Flood Events ...... 4-1 4.1.1 2013 Flood Event ...... 4-1 4.1.2 2019 Flood Event ...... 4-4 4.1.3 Comparison of 2013 and 2019 Flood Events ...... 4-6 4.2 Single Station Frequency Analysis ...... 4-9 4.2.1 River Flow Gauges ...... 4-9 4.2.2 Lake Level Gauges ...... 4-13

5. Hydrology Conclusions ...... 5-1 5.1 Conclusions ...... 5-1

6. Climate Change Uncertainty ...... 6-1 6.1 Trend Analysis ...... 6-1 6.1.1 Annual Streamflow Maxima ...... 6-2 6.1.2 Seasonal Climate Trends ...... 6-7 6.1.3 Trend Analysis Conclusions ...... 6-7

7. Recommendations for Floodplain Mapping ...... 7-1 7.1 Regulatory Flood Flow Proration ...... 7-1 7.2 Recommendations for River Flood Mapping Flows and Water Levels ...... 7-2 7.3 Recommendations for Muskoka River Basin Lake Flood Mapping ...... 7-3 7.4 Port Severn Flood Mapping ...... 7-4

8. River Features ...... 8-1 8.1 The Muskoka River Watershed ...... 8-1 8.2 Physiography ...... 8-1

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8.3 Topography and Surficial Geography ...... 8-2 8.4 Subwatershed Characteristics ...... 8-2

9. River Hydraulics Study ...... 9-1 9.1 HEC-RAS Program ...... 9-1 9.1.1 HEC-RAS 1-D ...... 9-1 9.1.2 HEC-RAS 2-D ...... 9-1 9.2 Geometric Data ...... 9-2 9.2.1 Lidar Collection ...... 9-2 9.2.2 River Cross-sections Bathymetry Collection ...... 9-4 9.2.3 1-D River Geometry Development ...... 9-5 9.2.4 2-D River Geometry Development ...... 9-6 9.3 Model Calibration ...... 9-10 9.3.1 Big East ...... 9-10 9.3.2 Huntsville Narrows ...... 9-11 9.3.3 Baysville ...... 9-11 9.3.4 Purbrook ...... 9-11 9.3.5 Springdale ...... 9-12 9.3.6 Bracebridge ...... 9-12 9.3.7 Port Carling ...... 9-13 9.3.8 Bala Reach ...... 9-14 9.4 Hydraulic Parameters ...... 9-15 9.5 Flood Levels and Sensitivity to Climate Change ...... 9-18 9.5.1 Big East ...... 9-18 9.5.2 Huntsville Narrows ...... 9-20 9.5.3 Baysville ...... 9-21 9.5.4 Purbrook ...... 9-21 9.5.5 Springdale ...... 9-22 9.5.6 Bracebridge ...... 9-23 9.5.7 Port Carling ...... 9-24 9.5.8 Bala Reach ...... 9-25 9.6 Flood Fringe Determination ...... 9-25

10. Flood Plain Mapping ...... 10-1 10.1 Lakes ...... 10-2 10.2 Rivers ...... 10-2

11. Recommendations and Mitigation Options ...... 11-1 11.1 Wave Uprush ...... 11-1 11.2 Watershed Management and Education ...... 11-1 11.3 Flood Forecasting and Tracking ...... 11-2 11.4 Mitigation Options ...... 11-2 11.4.1 Existing Lake Water Levels ...... 11-3 11.4.2 River Flood Protection ...... 11-3

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List of Tables Table 3-1: FDR 43 Summary of Simulated 100 Year Climate Inputs ...... 3-2 Table 3-2: Review of Regulatory Streamflow and Regulatory Flood Water Elevations ...... 3-2 Table 3-3: Control Structure Drainage Areas ...... 3-4 Table 3-4: WSC Gauges Selected for Streamflow Analyses ...... 3-6 Table 4-1: Comparison of 2013 Streamflow Peaks to Average ...... 4-3 Table 4-2: Reported Areas of Flooding April 2013 ...... 4-3 Table 4-3: Comparison of Spring Flow Volumes ...... 4-8 Table 4-4: Comparison of 2019 Streamflow Peaks to Average ...... 4-9 Table 4-5: Single Station 1:100-yr Flood Frequency Analysis and Recorded Flood Events ...... 4-10 Table 4-6: Spring 2019 Peak Static Lake Levels ...... 4-14 Table 5-1: Summary of Hydrologic Analyses and the Regulatory Flood Flow ...... 5-2 Table 6-1: Water Survey of Canada Hydrometric Stations ...... 6-2 Table 6-2: Annual Maximum Daily Flow Trendline Equations ...... 6-5 Table 7-1: Summary of Regulatory Flood Peaks ...... 7-1 Table 7-2: Projection for %Increase and Recommended Flood Mapping Flow ...... 7-3 Table 7-3: Flood Mapping Static Lake Levels with Projections for Climate Change ...... 7-4 Table 9-1: Summary of Manning's n Roughness Coefficients ...... 9-10 Table 9-2: Lower Big East River Calibration Results ...... 9-10 Table 9-3: Upper Big East River Calibration Results ...... 9-11 Table 9-4: South Muskoka River at Baysville Calibration Results ...... 9-11 Table 9-5: South Muskoka River at Purbrook Calibration Results ...... 9-12 Table 9-6: North Muskoka River at Springdale Calibration Results ...... 9-12 Table 9-7: Muskoka River at Bracebridge Calibration Results ...... 9-13 Table 9-8: Indian River at Port Carling Calibration Results ...... 9-14 Table 9-9: Summary of Hydraulic Conditions for Flood Mapping Modeling ...... 9-15 Table 9-10: Lower Big East Computed Water Surface ...... 9-19 Table 9-11: Upper Big East Computed Water Surface ...... 9-20 Table 9-12: Huntsville Narrows Water Surface Profile ...... 9-20 Table 9-13: Baysville Computed Water Surface Profile ...... 9-21 Table 9-14: Upper Purbrook Computed Water Surface ...... 9-22 Table 9-15: Lower Purbrook Computed Water Surface Profile ...... 9-22 Table 9-16: Springdale Computed Water Surface Profile ...... 9-23 Table 9-17: Bracebridge Computed Water Surface Profile ...... 9-23 Table 9-18: Port Carling Computed Water Surface Profile ...... 9-24 Table 9-19: Bala Computed Water Surface Profile ...... 9-25

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List of Figures Figure 2-1: Study Areas and Mapped Flood Lines within the District of Muskoka ...... 2-4 Figure 3-1: Watershed Hydrological Features and Gauge Locations ...... 3-5 Figure 3-2: Average Annual Hydrographs and Monthly Precipitation ...... 3-7 Figure 3-3: Daily Streamflow Duration Curves ...... 3-8 Figure 3-4: Daily Specific Runoff Duration Curves ...... 3-10 Figure 4-1: April 2013 Precipitation Event ...... 4-1 Figure 4-2: 2013 Spring Flood Hydrometeorology ...... 4-2 Figure 4-3: Spring 2019 Climatic Conditions ...... 4-4 Figure 4-4: 2019 Spring Flood Hydrometeorology ...... 4-5 Figure 4-5: 2013 vs. 2019 flood comparisons ...... 4-7 Figure 4-6: Flood Frequency Analysis Results – Big East R. at Huntsville (02EB013) ...... 4-10 Figure 4-7: Flood Frequency Analysis Results – North Branch at Port Sydney (02EB004) ...... 4-11 Figure 4-8: Flood Frequency Analysis Results – South Branch at Baysville (02EB008) ...... 4-12 Figure 4-9: Flood Frequency Analysis Results – Muskoka R. below Bala (02EB006) ...... 4-13 Figure 4-10: Frequency Analysis on Fairy Lake Levels ...... 4-15 Figure 4-11: Frequency Analysis on Mary Lake Levels ...... 4-16 Figure 4-12: Frequency Analysis on Lake Rosseau Levels ...... 4-17 Figure 4-13: Frequency Analysis on Lake Muskoka Levels ...... 4-18 Figure 4-14: Frequency Analysis on Bala Bay Levels ...... 4-19 Figure 6-1: Normalized Cumulative Departure from Mean Annual Flow ...... 6-3 Figure 6-2: 02EB004: Annual Maximum Flood Trendline Analysis ...... 6-3 Figure 6-3: 02EB006: Annual Maximum Flood Trendline Analysis ...... 6-4 Figure 6-4: 02EB008: Annual Maximum Flood Trendline Analysis ...... 6-4 Figure 6-5: 02EB013: Annual Maximum Flood Trendline Analysis ...... 6-4 Figure 6-6: 02EB014: Annual Maximum Flood Trendline Analysis ...... 6-5 Figure 6-7: Regression Analysis: Estimated vs. Observed Flow Increase (%/year) ...... 6-6 Table 6-8: Trendline Equations and Expected Flow Increase ...... 6-6 Figure 6-9: Winter Snowfall Trendline Analysis ...... 6-8 Figure 6-10: Average April Temperature Trendline Analysis ...... 6-8 Figure 6-11: Maximum April Daily Rainfall Trendline Analysis ...... 6-9 Figure 9-1: Lidar Point Density in Muskoka Region ...... 9-3 Figure 9-2: SMS Mesh Interpolated from Bathymetric Point Data ...... 9-4 Figure 9-3: Cross Section Cut from Combined Terrain and Bathymetric Data ...... 9-5 Figure 9-4: Cell face from Upper Big East Model Depicting Retained Terrain Data ...... 9-6 Figure 9-5: Breaklines in Mesh of Lower Big East Model ...... 9-7 Figure 9-6: Big East River Model Arrangement ...... 9-8 Figure 9-7: South Branch of the Muskoka River at Purbrook Model Arrangement ...... 9-9 Figure 9-8: Bala Reach Muskoka River at Bala Calibration Results ...... 9-14 Figure 9-9: Upper Big East River Inflow Hydrographs used for Flood Mapping ...... 9-16 Figure 9-10: Lower Big East River Inflow Hydrographs used for Flood Mapping ...... 9-17 Figure 9-11: South Branch Muskoka River at Purbrook Inflows used for Flood Mapping ...... 9-18 Figure 11-1: Huntsville Flooding from Huntsville Narrows ...... 11-4 Figure 11-2: North Branch of Muskoka River at Springdale Shores ...... 11-5

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List of Appendices Appendix A Existing Regulatory Flood Data

Appendix B Rating Curves Appendix C Models Results from River Mapping Appendix D Glossary

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1. Introduction In January 2018, the District of Muskoka (District) initiated a flood plain mapping study for selected river reaches and lakes within the District. Hatch Ltd. (Hatch) was retained to undertake the study, which represents an update and extension of previous mapping done in the 1980’s and 1990’s under the Flood Damage Reduction Program.

The local and corporate expertise of Hatch is unparalleled in its ability to undertake this key project for the District. A few highlights of Hatch’s capabilities to undertake this work include.

• An experienced regional team. Hatch has a very strong Ontario team of engineers with the required hydrotechnical engineering capabilities and more than 100 years of combined experience to be dedicated to this project.

• Hatch completed the Water Management Plan study for the Muskoka River Watershed. As well, Hatch has conducted Dam Safety Assessment studies for most of the MNRF Dams in the Muskoka area, thus we have distinct knowledge of flood history and dam operations. All files related to the studies are in the Niagara Falls office and several of the principal contributors to the studies are still with Hatch including, Alfred Breland and Stu Bridgeman.

• Experience on similar studies. The Niagara Falls office Hydrotechnical Group has applied traditional hydrologic and hydraulic applications for more than 30 years and have completed many modeling and floodplain mapping applications in Canada.

• Technological advancement. The regional team is very experienced in applying computer models for the proposed scope of work. This study will use the latest advancements in industry software including HYFRAN, HEC-RAS and ESRI GIS products.

• Hatch’s commitment to safety. Hatch will work with the District during the project to ensure safety is a number one priority and that all safety procedures are followed.

In total, Hatch offers a regional team with very significant capability and a commitment to deliver a high-quality study and report to the District.

Flooding is a significant issue in the region and, through this study, the District wishes to update information about particularly vulnerable portions of Muskoka River flood plains, in order to develop future communication and mitigation strategies for areas most affected by lake and riverine flood events.

This study will involve the collection and extension of baseline topographic mapping, hydrologic analysis, hydraulic modelling and production of updated and new flood plain maps for selected areas within the District. The floodplain mapping will allow both area municipalities and individuals to identify hazardous lands that are at risk of adverse effects and flood damage.

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2. Study Area and Objectives The study area river and lake shore lines within the District are illustrated in Figure 2-1. The study will provide updated and new flood plain mapping for these selected areas within the Muskoka River Basin and at Port Severn. 2.1 Previous Studies and Current Situation In the 1980’s and 1990’s under the Flood Damage Reduction Program (FDR) a number of communities in the province with a known history of flooding were mapped and the 1:100- year return period flood risk zones were designated. Since their creation, these flood risk zone maps have been incorporated into a wide range of land use polices by provincial and municipal governments, including transportation, land use development, regulations, and municipal plans. Since the 1990s little has been done to update the flood mapping for the growing communities.

From 2003-2005, the Ontario Ministry of Natural Resources and Forestry (MNRF) conducted studies of water operations and management of water in the Muskoka River watershed to assess the dam operating rules of the time (Hackner-Holden Agreement) and proposed improvements to the operating rules for the environment. Those studies culminated in the Muskoka River Water Management Plan (MRWMP), which was implemented in 2006. The goal of water management planning is to contribute to the environmental, social and economic well being of the people of Ontario through the sustainable development of waterpower resources, and to manage these resources in an ecologically sustainable way for the benefit of present and future generations. The study used the historical average range of flows that occurred in the basin to study and develop a new set of water management rules for average hydrologic conditions. The management of floods and flooding is not explicitly a goal of water management planning, and Water Management Plans are not designed to manage floods.

During the spring of 2013, the District of Muskoka encountered flooding across the Muskoka River Watershed causing damage to municipal infrastructure and to personal property. In 2016, Muskoka again experienced high water levels along the major rivers and lakes.

Then in spring of 2019, the Muskoka area experienced a significant freshet where the combination of a melting snow pack and significant April rain combined to cause flood flows of record throughout most of the basin. As people across Muskoka continue to recover from significant rain events and floods, mapping key portions of Muskoka’s vulnerable flood plains is essential to ensure appropriate management of the District’s shared natural environment and water system and, in turn, the continued sustainability and prosperity of this area in the face of increasing flood events.

Both the Government of Canada and the Province of Ontario recognize that climate change is having significant impacts and cost implications for Canadians as significant weather events devastate communities; damage homes and businesses; and affect municipal infrastructure not able to manage wide variations in weather.

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2.2 Study Objectives The frequency of high flows in the Muskoka River since 2012 appear to indicate that climate conditions are changing and that an update to hydrologic and hydraulic studies presented in the Flood Damage Reduction report “FDR 43 Hydrology Study for Major Lakes in the Muskoka River Watershed” (Marshall Macklin Monaghan, 1988) and other FDR reports is warranted, especially because the additional 30 years of flow records allow a better assessment of historical flood frequencies and potential trends throughout the Muskoka basin.

The hydrology study provides a review of annual peak flows and levels recorded at Water Survey of Canada gauges, and where required updates the regulatory flow values for the Muskoka River. The hydrology study also includes an assessment of the potential trends in flood flows and projected impacts of climate change on flood flows and lake water levels used for floodplain mapping.

The hydraulic study was performed to analyze the river hydraulics to develop new and updated floodway and floodline extents on the selected river reaches for floodplain mapping.

The hydrology and hydraulic studies are provided in this Technical Report.

The new and updated floodplain mapping will allow both area municipalities and individuals to identify river floodplains and lake shoreline in selected areas that are at risk of flood damage. Given the size of the area, this project is focused on Muskoka's developed areas (i.e. Huntsville, Gravenhurst, Bracebridge, Baysville, and Port Severn) and on the largest waterbodies and rivers in Muskoka River watershed (i.e. the Mary-Fairy-Vernon-Peninsula Lake chain, Lake Muskoka and Lake Rosseau-Joseph chain, the selected high-risk areas of the Muskoka-South-North Rivers and Big East River). 2.3 Deliverables This study provides updated and new floodplain mapping for selected flood-prone, high risk areas. The floodplain mapping will allow both area municipalities and individuals to further identify hazardous lands that are at risk of adverse effects and flood damage.

The main deliverables for this project include

• LiDAR Survey Data – 1508.4 km2 of topographic mapping data including

 Processed and calibrated raw point cloud (LAS v1.2, ASPRS Classes 1, 2, 9)

 Classified point cloud in LAS v1.2, classified to ground, non-ground and water

 1m Grids of Bare Earth (DEM), Hydroflattened (DEM), and full feature (DSM)

 Hill shade Geotiffs (grayscale) of Bare earth and Full feature

 Water Breaklines and 0.5 m Contours in shape files.

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• This Technical Report on hydrology, sensitivity to climate change, and hydraulic analyses providing peak static water levels for floodplain mapping

• New and updated floodplain mapping at 1 : 2 500 scale for 65 km of river reaches providing approximately 39 km2 of floodplain mapping for rivers in a GIS database

• New floodplain mapping at 1 : 2 500 scale for 1061 km of lake shorelines providing approximately 212 km2 of floodplain mapping for lakes in a GIS database

• HEC-RAS river hydraulic models for selected floodplain mapping of river reaches.

All deliverables become the property of The District Municipality of Muskoka.

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Figure 2-1: Study Areas and Mapped Flood Lines within the District of Muskoka

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3. Background Information Collection/Review 3.1 Reports Reviewed The following documents/reports have been reviewed with an emphasis on any relevant hydrologic and hydraulic information contained therein:

• Big East River Flood line Mapping Study – Town of Huntsville, (Paragon Engineering Ltd., 1989).

• FDR 43 – Hydrology Study for Major Lakes in the Muskoka River Watershed (Marshall Macklin Monaghan, 1988).

• FDR 44 – Hydrology Study for Major Lakes in the Muskoka River Watershed; Supplementary Wave Runup/Offset Analysis (Marshall Macklin Monaghan, 1989).

• FDR 45 – Water Management Improvement Study of the Muskoka River System; Technical Report, Volume II; Phase II – Operation of the System; Phase III – Improvement Measures (MacLaren Plansearch Inc., 1985).

• FDR 128 – Hydrology Study for Major Lakes in the Muskoka River Watershed; Supplementary Wave Runup/Offset Analysis (Marshall Macklin Monaghan, 1989).

• FDR 140 – Proposal for Muskoka River Flood line Mapping, Town of Bracebridge (Paragon Engineering Ltd., 1986).

• FDR 141 – Muskoka River Flood line Mapping – Phase 1, Town of Bracebridge – Technical Report, Hydraulics (Paragon Engineering Ltd., 1988).

• FDR 142 – Muskoka River Flood line Mapping – Phase 1, Town of Bracebridge – General Report (Paragon Engineering Ltd., 1988).

• FDR 143 – Flood Risk Mapping Muskoka River – Phase II, Town of Bracebridge – Executive Summary and Technical Report (Paragon Engineering, 1991).

• Muskoka River Water Management Plan (Acres, 2006). 3.2 Review of Previous Flood Estimates The previous flood line studies were all based on the hydrology studies performed by Marshall Macklin Monaghan (MMM) in 1988 and 1989. The FDR 43 (MMM, 1988) Hydrology Study analysed historical flood events and developed a hydrologic model to simulate floods in the watershed. Due to the lack of enough historical flow data within the basin the study developed a conservative theoretical estimate of the 1:100-year Annual Exceedance Probability (AEP) spring flood event, MMM performed a bivariate frequency analysis between temperature and rainfall to develop inputs to a hydrologic model to simulate a theoretical 1 in 100-year flood event and provide flood flows and water levels at key locations within the watershed. They also simulated the Timmins Storm event, but in comparison, found this event produced lower floods on the main river and large lakes under study.

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Thus, the simulated theoretical 1:100-year AEP spring flood event was selected as the Regulatory Flood for the Muskoka River and its large lakes. A summary of the climatic inputs used to simulate the theoretical 1:100-year AEP spring flood event are provided in Table 3-1. Table 3-1: FDR 43 Summary of Simulated 100 Year Climate Inputs

Period Maximum Temperature (◦C) Maximum Rainfall (mm) Total Input (mm) 1 day 19.2 33.7 128.5 3 days 17.2 45.3 244.8 5 days 15.4 51.4 249.5 Notes: Snowpack depth (April 1) = 200 mm Snowmelt coefficient = 0.206 mm/h/◦C

The FDR 128 (MMM, 1989) Wave Runup/Offset Analysis assessed the 2-year wind and wave runup combined with the flood event peak static water levels for determination of wave runup vertical offsets for flood hazards on selected lakes in the watershed.

The river flood line hydraulic studies were performed by Paragon Engineering and were all based on the FDR 43 and FDR 128 hydrology studies as inputs for hydraulic modelling (HEC2) to determine water levels on selected reaches of the Big East, North Branch, South Branch, and Muskoka River.

Based on the review of the existing documentation, Table 3-2 summarizes previous estimates of regulatory flood stream flows and regulatory flood water elevations at various locations within the Muskoka River watershed. The source data extracted from the existing documentation is provided in Appendix A. Table 3-2: Review of Regulatory Streamflow and Regulatory Flood Water Elevations

Regulatory Flood Regulator Flood Streamflow Water Elevation (m3/s) (m) Wave River Location Inflow Outflow Static Runup Big East Williamsport 285 294.30 River Cove 291 291.49 Below Confluence with Little East 326 Highway 11 Bridge 326 290.93 Tynan’s Bridge 326 290.15 Ravenscliffe Bridge 339 287.16 Outlet into Lake Vernon 339 286.68 North Branch Vernon Lake 494 356 286.68 287.48 Peninsula Lake 285.29 286.46 Fairy Lake 366 278 285.29 286.53 Mary Lake - Port Sydney 294 291 281.74 282.95

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Regulatory Flood Regulator Flood Streamflow Water Elevation (m3/s) (m) Wave River Location Inflow Outflow Static Runup High Falls Dam 319 270.50 Wilson Falls Dam 322 256.20 Birds Mill Dam 323 244.10 Base of Bracebridge Falls 323 228.47 South Branch Lake of Bays 603 226 315.84 317.18 Wood Lake 291 Matthiasville 304 293.50 Trethewey Falls 305 280.40 Base of South Falls 344 229.33 Confluence with North Branch 344 228.20 Muskoka River Confluence of North and South 647 228.20 Outlet to Muskoka Lake 661 226.51 Lake Joseph - 450 78 226.72 227.85 Lake Rosseau 450 78 226.72 227.91 Muskoka Lake 968 514 226.51 227.84 Bala Reach 221.15

3.3 Review of Climate and Hydrology Data Streamflow/water level and climate records for the Muskoka River Watershed were collected from online Water Survey of Canada (WSC) and Environment and Climate Change Canada (ECCC) sources, respectively. Data relevant to the present study was reviewed as described in the sections below.

Gauge locations and other hydrological features of the basin are shown in Figure 3 1: while Table 3-4 summarizes the WSC streamflow gauges reviewed. While additional WSC gauges exist within the watershed, the four gauges shown in Table 3-4 were selected for initial analysis and comparison as they represent primary locations of interest and have sufficient data records for flood frequency analyses.

Table 3-3 includes approximate drainage areas for each water control structure within the Muskoka River system, as estimated using the online Ontario Flow Assessment Tool (OFAT1). OFAT was developed by the Government of Ontario as an online, map-based hydrology application. The tool uses a combination of digital elevation models, water feature maps, and streamflow data to generate watershed boundaries and flow statistics for user- specified locations anywhere within the Province of Ontario.

1 https://www.gisapplication.lrc.gov.on.ca/OFAT/Index.html?site=OFAT&viewer=OFAT&locale=en-US

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Table 3-3: Control Structure Drainage Areas

Dam Name River/System Area (km2) Camp Lake Dam Big East River/North Branch 17.4 Tasso Lake Dam Big East River/North Branch 40.6 West Harry Lake Dam Big East River/North Branch 38.5 McCraney Lake Dam Big East River/North Branch 46.5 Distress Dam Big East River/North Branch 464 Axe Lake Dam Buck River/North Branch 47.8 Buck Lake Dam Buck River/North Branch 145 Fox Lake Dam Buck River/North Branch 211 Huntsville Dam and Lock North Branch 1,231 Port Sydney Dam North Branch 1,407 Clearwater Lake Dam North Branch 3.1 Devine Lake Dam North Branch 12.7 High Falls Dam North Branch 1,555 Wilson Falls Dam North Branch 1,594 Birds Mill Dam North Branch 1,600 Bracebridge Falls Dam North Branch 1,600 Burnt Island Lake Dam Oxtongue River/South Branch 57.1 Joe Lake Dam Oxtongue River/South Branch 115 Ragged Lake Dam Oxtongue River/South Branch 70.8 Tea Lake Dam Oxtongue River/South Branch 343 Livingstone Lake Dam Hollow River/South Branch 47.3 Fletcher Lake Dam Hollow River/South Branch 23.6 Kawagama Lake Dam Hollow River/South Branch 388 Baysville Dam South Branch 1,400 Wood Lake Dam South Branch 35.0 Matthias Falls South Branch 1,649 Tretheway Falls Dam South Branch 1,657 Hanna Chute Dam South Branch 1,682 South Falls Dam South Branch 1,682 Gull Lake Dam Lake Muskoka/Main Branch 10.8 Skeleton Lake Dam Lake Muskoka/Main Branch 74.3 Port Carling Dam and Locks Lake Muskoka/Main Branch 797 Bala North Dam Lake Muskoka/Main Branch 4,681 Bala South Dam Lake Muskoka/Main Branch 4,681

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Table 3-4: WSC Gauges Selected for Streamflow Analyses

Gauge Drainage Daily Q Data Range Number Gauge Name Area (km2) (Total # Years) 02EB013 Big East River near Huntsville 610 1973 – 2019 (47)

02EB004 North Branch Muskoka River at Port Sydney 1,410 1915 – 2019 (105)

02EB008 South Branch Muskoka River at Baysville 1,400 1941 – 2019 (79)

02EB006 Muskoka River below Bala 4,770 1937 – 2019 (83)

3.3.1 Normal Streamflow and Climate Conditions Figure 3-2 illustrates the average annual streamflow hydrographs for each of the four selected streamflow gauges within the Muskoka River Watershed, as well as average monthly precipitation data. Figure 3-2 normalizes the long-term average streamflow and climate conditions within the watershed, providing a description of “typical” or “normal” conditions in the basin, but does not provide information on specific flooding events.

Average annual streamflow hydrographs, for each gauge location, depict the average of all recorded daily streamflows for each day of the calendar year over the entire period of record. These hydrographs are formed by averaging all January 1 streamflow measurements throughout the period of record, then all January 2 measurements, and so on, until a series is created for the entire calendar year. For example, the average of all streamflow records made on April 1 at the 02EB004 gauge (North Branch Muskoka River at Port Sydney), from 1915 through 2016, is approximately 50 m3/s, as shown on the orange line in the figure.

The average monthly precipitation data (ECCC Muskoka climate gauge; 1981-2010 climate normals) are also shown in Figure 3-2, which illustrates the average total (rain and snowfall) precipitation patterns within the Muskoka River Watershed.

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Rainfall Depth Snow-Water Equivalent Depth 02EB013 - Big East 02EB004 - North Branch 02EB008 - South Branch 02EB006 - Muskoka R. below Bala 200 0

180 20

160 40

140 60

120 80

100 100

Flow (cms) 80 120

60 140

40 160 (mm) Depth Precipitation

20 180

0 200

Date

Figure 3-2: Average Annual Hydrographs and Monthly Precipitation

Each flow gauge shown in Figure 3-2 displays a steady increase in streamflow beginning in early March as a result of the spring snowmelt period. The hydrographs generally peak in mid- to late-April before declining steadily into the summer months when precipitation is low. A slight rise in streamflow occurs during the fall months (September through November) as rainfall typically increases at this time of the year, as shown by the climate data.

The hydrographs in Figure 3-2 also illustrate how streamflow is compounded through the basin, from upstream to downstream, and how streamflow quantities change throughout the watershed. The Big East River is shown to be a major tributary to the North Branch of the Muskoka River, contributing approximately half of its annual average streamflow. The South Branch of the Muskoka River peaks somewhat lower than the North Branch and has a slightly slower response to the spring snowmelt. The WSC gauge below Bala represents runoff from almost the entire Muskoka River Watershed, and therefore depicts the total streamflow from the gauges located upstream and on tributaries to the Muskoka River after being routed through Lake Muskoka.

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3.3.2 Daily Streamflow Duration Curves Unlike the average annual hydrographs shown in Figure 3-2, the daily streamflow duration curves illustrated in Figure 3-3 represents frequency of flows of the entire historical record of daily streamflows for each WSC gauge.

02EB013 - Big East 02EB004 - North Branch 02EB008 - South Branch 02EB006 - Muskoka R. below Bala 1000

100

10 Flow (cms)

0.1 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Percent Exceedence

Figure 3-3: Daily Streamflow Duration Curves

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Streamflow duration curves are used by hydrologists and engineers to visualize a streamflow record and analyze how frequently streamflows of a certain magnitude are recorded. To create the daily streamflow duration curves shown in Figure 3-3, recorded daily streamflows from each gauge were first sorted and ranked from highest to lowest. Next, an exceedance percentage was determined for each sorted daily streamflow value. The exceedance percentage or Weibull plotting position of each ranked streamflow value is calculated by:

= × 100% + 1 � � where p is the exceedance probability/percentage� for the nth ranked streamflow record out of a total count of N streamflow records. For example, a daily streamflow value with an exceedance probability of 10% was equalled or exceeded 10% of the time. Plotting each daily streamflow measurements against its exceedance percentage yields the duration curves shown in Figure 3-3. For example, daily streamflow on the North Branch Muskoka River at Port Sydney (WSC gauge 02EB004) was equal to or exceeded 54.9 m3/s, 10% of the time over the entire 102 years of records.

The plots in Figure 3-3 show general consistency between streamflows recorded at the North Branch and South Branch Muskoka River gauges (i.e. orange and grey lines, respectively). However, the two duration curves diverge at the lower exceedance percentages, indicating that the less frequent - high streamflow events (i.e. floods) have higher peak flows on the North Branch than the South Branch. The green line representing streamflows recorded at the Big East River near Huntsville gauge (02EB013) has the same general shape of the North Branch gauge, suggesting hydrological similarity between the two locations. This is logical since the Big East River is a major tributary to the North Branch Muskoka River and comprises much of its annual streamflow, as discussed previously.

However, the magnitude of the high peak flow flood events recorded at the Big East River gauge converge with those of the North and South Branch gauges. This indicates that the Big East River experiences relatively high peak flood magnitudes in relation to average flows observed on the Big East River; which suggests that runoff occurs at a faster rate within the Big East River basin (i.e. Algonquin Park highlands).

The duration curve for the Muskoka River below Bala (02EB006) is somewhat “flatter”, as the rate of streamflow accumulating to this location is generally attenuated (slowed) by the various lakes and storage areas throughout the entire upstream watershed.

3.3.3 Daily Specific Runoff Duration Curves The daily specific runoff duration curves in Figure 3-4 represent the streamflow records for each gauge normalized by contributing river basin drainage area in terms of exceedance percentages. It is equivalent to the data shown in Figure 3-3, but with daily streamflows divided by the contributing river basin drainage area (listed in Table 3-3). Since runoff is typically proportional to watershed area, normalizing by drainage area helps to illustrate the runoff characteristics of each location in the watershed, and facilitates further comparison.

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02EB013 - Big East 02EB004 - North Branch 02EB008 - South Branch 02EB006 - Muskoka R. below Bala 1000.0

100.0 ) 2

10.0

1.0 Specific Runoff (L/s/km Runoff Specific 0.1

0.0 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Percent Exceedence

Figure 3-4: Daily Specific Runoff Duration Curves

Figure 3-4 illustrates hydrological consistency throughout the Muskoka River Watershed. When normalized by drainage area, the duration curves overlap across most exceedance percentages, indicating that precipitation-runoff characteristics are similar in each sub-basin. The curves separate somewhat at low exceedance percentages (i.e. flood events), with the Big East and North Muskoka rivers exhibiting a larger peak flow runoff response relative to the South Muskoka River and Muskoka River Main Branch. This suggests that peak flows in the South Muskoka River are more attenuated, relative to streamflows in the North Muskoka River, by the large lake areas in the south reach of the watershed (e.g. Lake of Bays). The Muskoka River below Bala gauge (02EB006) shows even more attenuation of peak flows due to the cumulative routing effects of flows through the flatter river reaches in the lower part of the basin and the large areas of Lake Muskoka and Lake Joseph-Rosseau.

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4. Hydrology Study 4.1 Recent Flood Events

4.1.1 2013 Flood Event The 2013 flood event was analyzed to form an understanding of the watershed’s hydrological response to significant snowmelt and/or rainfall occurrences and for comparisons to the 1:100-yr estimates derived during the statistical flood frequency analysis. Recorded climate and streamflow data from the 2013 flood event in the Muskoka River Watershed are depicted in the following figures.

Figure 4-1 illustrates the climate conditions leading up to the significant flooding that occurred in late April of 2013.

Total P - Beatrice Climate Total P - Muskoka Total P - Algonquin E. Snow on Ground - Beatrice Climate Snow on Ground - Muskoka A Snow on Ground - Algonquin E. 60 30 Temp. - Beatrice Climate

50 20 C ) °

40 10

30 0

20 -10

10 -20 Mean DailyTemperature (

Total Precip (mm) or Snow on Ground (cm) Ground on Snow or (mm) Precip Total 0 -30

01-Apr 02-Apr 03-Apr 04-Apr 05-Apr 06-Apr 07-Apr 08-Apr 09-Apr 10-Apr 11-Apr 12-Apr 13-Apr 14-Apr 15-Apr 16-Apr 17-Apr 18-Apr 19-Apr 20-Apr 21-Apr 22-Apr 23-Apr 24-Apr 25-Apr 26-Apr 27-Apr 28-Apr 29-Apr 30-Apr Figure 4-1: April 2013 Precipitation Event

Figure 4-1 shows that the Muskoka River Watershed received significant precipitation during the month of April 2013, some of it on an existing snowpack. The Beatrice Climate, Muskoka, and Algonquin Park East Gate climate gauges recorded monthly total precipitation of 175 mm, 138 mm, and 153 mm, respectively, during April of 2013. As a comparison, approximately 77 mm is shown to be a normal amount of total precipitation for the month of April in the Muskoka area (see Figure 3-2). In particular, rainfall recorded on April 18 ranged from 28 mm to 55 mm in one day across the watershed.

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Figure 4-2 illustrates the hydrologic streamflow response to the April 2013 spring precipitation (total precipitation recorded at the Beatrice Climate station is included for comparison to the streamflow responses).

Total P - Beatrice Climate 02EB013 - Big East 02EB004 - North Branch 02EB008 - South Branch 02EB006 - Muskoka R. below Bala 500 0 400 20 300 40 200 60 Flow (cms) 100 80 0 100 Total Precipitation (mm)

Date

Figure 4-2: 2013 Spring Flood Hydrometeorology

Figure 4-2 shows how the significant precipitation received over the basin in April (combined with snowmelt runoff) resulted in correspondingly significant streamflows. Streamflows measured on the Big East River increased by over 200 m3/s in a span of only five days. Streamflow responses on the North and South Branches were similar, though flow peaks were attenuated by the lakes within their respective systems. Flows from the North and South branches are compounded and routed through Lake Muskoka before being captured by the 02EB006 gauge which also includes runoff contributions from additional Lake Muskoka tributaries.

The 2013 flood flows can be compared to the average annual hydrographs shown in Figure 3-2 to further illustrate the significance of this event, relative to normal spring streamflows. The comparison is shown in Table 4-1 indicates that the peak flow on the Big East river was 5.5 times the normal spring peak; whereas the peak flow below Bala was only 2.25 times the normal spring peak; further illustrating the attenuating effects of the rivers and lakes on the peak flows as water passes downstream through the river basin.

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Table 4-1: Comparison of 2013 Streamflow Peaks to Average Avg. Annual Peak Flow 2013 Peak Flow Gauge Number Gauge Name (m3/s) (m3/s) 02EB013 Big East River near Huntsville 42.0 230 02EB004 North Branch Muskoka River at Port Sydney 79.6 201 02EB008 South Branch Muskoka River at Baysville 47.8 139 02EB006 Muskoka River below Bala 172 390

4.1.1.1 2013 Flood Observations Many areas in the watershed reported flooding in low areas due to this significant spring flood event. Table 4-2 provides a listing of some of the areas on the Muskoka Rivers that reported flooding. Table 4-2: Reported Areas of Flooding April 2013

River/Lake Area Flooded Big East River Many houses on Bridgeview Lane and Glen Acres Road River Cove Drive. Trailer Park on Silver Sands Road. North Branch Deer Lake Resort Park on Vernon Lake. Springdale Shores Road above High Falls. Wilson Falls road, River Road and Timber Mart yard in Bracebridge. Bracebridge Bay Park in Bracebridge. Lady Muskoka Docks at Riverside Inn in Bracebridge. South Branch White Eagle Trail and Island Way roads - Purbrook area.

4.1.1.2 ECCC - Canada's top ten weather stories of 2013 Web link: 8 Spring Flooding in Ontario’s Cottage Country

ECCC reported:

“A burst of spring weather in mid-April pushed temperatures into the 20s across southern and central Ontario. The unseasonably warm and unstable air triggered Canada’s first tornado of the season on April 18 around Shelburne, Ontario. More significantly, the warm, moist air led to major flooding north and east of Georgian Bay in Ontario’s cottage country. In addition, copious amounts of warm rain melted a later-than-normal snowpack in Algonquin Park and the surrounding woodland. With rain coming down in torrents − nearly 90 mm in two days − steam billowed from the ground. The ensuing melt water and rains funneled quickly into rivers, lakes and streams causing some of the highest and fastest rising water levels in recent memory …

The historic flood was due to a combination of partially frozen ground, later-than-usual snowmelt, persistent lake ice and, largely, heavy warm rains over two or more days. Before temperatures shot above 20°C, early spring temperatures were averaging as

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much as five degrees colder than normal. That left the still frozen ground unable to handle the sudden overflow of water. A protracted warm spell in the final two weeks of April saw temperatures climb two and a half degrees warmer than normal. Just north of Bracebridge, a weather station in Beatrice, Ontario with a 137-year record lost almost 48 cm of snowpack in three weeks before nearly 100 mm of rain soaked the region over three days, including 55 mm on the 18th – the wettest April day ever. The total monthly rainfall of 169 mm also set a new April record.”

4.1.2 2019 Flood Event As with the 2013 flood event, the 2019 flood was analyzed to understand the watershed’s hydrological response to the climatic conditions observed during the spring of this year, especially given the record high flows and overbank inundation sustained during this event.

Figure 4-3 illustrates the climate conditions leading up to the significant flood that occurred in April and May of 2019.

Figure 4-3: Spring 2019 Climatic Conditions

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Analysis of climatic conditions observed during the spring of 2019 show how the snowpack began to melt in mid-March as temperatures increased. The amount of snow accumulated over the winter was significant, representing a large volume of water available for runoff in the spring. Additionally, a series of precipitation events occurred during the month of April 2019, some of it falling on the existing snowpack. The Beatrice Climate, Muskoka, and Algonquin Park East Gate climate gauges recorded monthly total precipitation of 133 mm, 130 mm, and 130 mm, respectively, during April of 2019. As a comparison, approximately 77 mm is shown to be a normal amount of total precipitation for the month of April in the Muskoka area (see Figure 3-2).

Figure 4-4 illustrates the hydrologic streamflow response to the 2019 spring snowmelt and precipitation (total precipitation recorded at the Beatrice Climate station is included for comparison to the streamflow responses). Note that the plotted streamflows shown for gauges 02EB004 and 02EB008 (North Branch and South Branch Muskoka River, respectively) are provisional values from WSC’s real-time data repository. At the time of composition, this provisional data has not yet been through WSC’s complete assurance/quality control process and is subject to change once approved. Data shown for the Muskoka River, Bala Reach is provided by Ontario Power Generation and corresponds to the 02EB006 WSC gauge location (although WSC no longer publishes real-time data from this site).

Figure 4-4: 2019 Spring Flood Hydrometeorology

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Figure 4-4 shows how the snowmelt and spring precipitation combined to yield significant streamflows throughout the Muskoka River watershed. The coincident snowmelt and incoming precipitation during the week of April 15 yielded a rapid streamflow response with the Big East, North Branch Muskoka, and South Branch Muskoka Rivers all seeing significant streamflow responses around this time. As with the 2013 flood, the Big East River peaked first, followed by peaks on the North Branch and then the South Branch Muskoka Rivers, indicating similar hydrological responses in both flood events. Further comparisons between the 2013 and 2019 flood events are included in the next section.

4.1.3 Comparison of 2013 and 2019 Flood Events The flood events of 2013 and 2019 are among the largest ever recorded in the Muskoka River basin and caused significant overbank flooding within the watershed. Comparing these recent floods provides valuable information on how the watershed responds to snowmelt and spring rainfall events. Figure 4-5 provides a comparison of the 2013 and 2019 floods at each of the Big East, North Branch, and South Branch rivers WSC gauge locations. Again, note that North Branch and South Branch flow data for 2019 is provisional at the time of composition and is subject to change upon approval. Muskoka River below Bala flow data for 2019 was provided by OPG. Annual average hydrographs are included in the figure for additional context.

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Figure 4-5: 2013 vs. 2019 flood comparisons

The comparison shows the difference in flood response between the two events, which is particularly prominent on the Big East River. In 2013, streamflows on the Big East peaked very rapidly, due to the large amount of rainfall on April 18, 2013, but also receded quickly. In 2019, the peak streamflow magnitude was smaller, but the overall flood duration and volume was significantly larger. As a comparison, the 2013 flood saw higher than average flows on the Big East River for approximately 16 days in mid- to late-April. In 2019, higher than average flows persisted for approximately 55 days, continuing through April, May, and into

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June, as shown in Figure 4-5. Comparisons for the other three gauges illustrate how 2019 peak flow magnitudes were somewhat higher than in 2013 and how the duration of the high flows persisted for a longer time period.

Streamflow volumes of the two floods are compared to each other and against the annual average values in Table 4-3. The volume comparison complements the peak streamflow comparison shown below as both can be important factors on peak lake levels and impacts associated with a particular flood. Note that Table 4-3 shows total April and May streamflow volume for each of the three gauges, calculated as the sum of daily streamflows in those two months and converted from m3/s to million m3. Corresponding runoff depths are also expressed in the table, calculated as the total volume divided by the gauge’s drainage area and converted to mm. A comparison could not be made for the Muskoka River below Bala location as the flow data provided by OPG is missing the last two weeks of May 2019. Table 4-3: Comparison of Spring Flow Volumes Total April + May Flow Volume, million m3 (Runoff depth, mm in brackets) Gauge 2013 2019 Avg. Annual Number Gauge Name Flood Flood 02EB013 Big East River near Huntsville 139 176 235 (228) (289) (385) 02EB004 North Branch Muskoka River at Port Sydney 288 362 469 (204) (257) (333) 02EB008 South Branch Muskoka River at Baysville 198 329 403 (141) (235) (288)

The comparison illustrates how the 2019 flood volumes were somewhat higher (i.e. 20-33%) than during the 2013 event, largely due to the significantly larger snowpack on the ground just before the melt (50 cm on April 1, 2019 vs 20 cm on April 7, 2013) combined with the persistent precipitation that continued throughout the freshet of 2019. Correspondingly, the extended period of high-water levels resulted in record high water levels on lakes and rivers causing significant flooding of roads and houses throughout the watershed.

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Table 4-4 shows a comparison of streamflow peaks between the 2019 and 2013 floods, with average annual values shown for reference. The comparison highlights how the 2019 flood peak on the Big East River was lower than observed during the 2013 event, but the 2019 flood had a higher peak magnitude than the 2013 flood on the North Branch, South Branch, and Main Branch Muskoka Rivers. Table 4-4: Comparison of 2019 Streamflow Peaks to Average Avg. 2019 Peak Annual 2013 Peak Flow Gauge Peak Flow Flow (m3/s) Number Gauge Name (m3/s) (m3/s) 02EB013 Big East River near Huntsville 42.0 230 168 02EB004 North Branch Muskoka River at Port Sydney 79.6 201 234 02EB008 South Branch Muskoka River at Baysville 47.8 139 165 02EB006 Muskoka River below Bala 172 390 435* *Note The 2019 data for 02EB006 was provided by OPG. 4.2 Single Station Frequency Analysis

4.2.1 River Flow Gauges A single station frequency analysis of annual peak flow was performed for the selected streamflow gauges. The annual peak streamflows (maximum daily flow for each calendar year) recorded at each WSC gauge were analyzed to determine approximate streamflow values corresponding to specific “return periods” (i.e. probabilities). This process, known as a flood frequency analysis, is a standard exercise used by hydrologists to understand and quantify the Annual Exceedance Probabilities (AEP) of flood events on a given river. Familiar terms such as “the 1:100-year flood” are related to flood frequency analyses, though the meaning of these terms are often misconstrued. For example, a “1:100-year flood” (or “100- year flood”, “1 in 100-year flood”, etc.) is not guaranteed to happen only once every 100 years. The statistical flood frequency analysis process yields streamflow values that correspond to a variety of “return periods”, or probability of exceedance in a given year. These values are determined by fitting a statistical distribution to the series of recorded annual peak streamflows, and often assumes this distribution is applicable for events that have not been recorded yet. As such, the “1:100-year AEP” flood estimate actually represents the streamflow that has a 1 in 100 (1%) probability of being exceeded in any given year.

The statistical hydrology program, HYFRAN PLUS, was used to perform frequency analyses for each series of peak annual flows (i.e. the largest recorded streamflow in each year). This program allows for fitting of multiple statistical distributions to the observed records to determine which is most appropriate, in each case.

Three-Parameter Lognormal distributions were shown to fit well for all streamflow gauges analyzed; statistical fitness tests available within HYFRAN PLUS indicate that each gauge record can be characterized by a Three-Parameter Lognormal distribution, at a 5% significance level. This distribution is also recommended for application in flood frequency

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analyses in the Ontario Ministry of Natural Resources’ Technical Guide on Flooding Hazard Limits. Figure 4-6, Figure 4-7, Figure 4-8 and Figure 4-9 illustrate the statistical distributions with recorded annual peak flows for each gauge.

A summary of 1:100-yr AEP estimates from the single station frequency analysis is provided in Table 4-5, alongside recorded peak daily flows during the 2013 and 2019 floods for comparison. Results show that the recorded highest flood of record (2013 or 2019) are close to the 1:100-yr AEP flood estimate for the four-gauge locations analyzed and considered within the 95% confidence limits of the analysis. However, the highest flood of record exceeds the 1:100-yr AEP estimate at all gauge locations. Table 4-5: Single Station 1:100-yr Flood Frequency Analysis and Recorded Flood Events

Recorded Peaks (m3/s) 1:100-yr 95% AEP Confidence Gauge 2013 Flood 2019 Flood Estimate Range Number Gauge Name (m3/s) (m3/s) 02EB013 Big East River near Huntsville 230 168 220 168 – 271 02EB004 North Branch Muskoka River 201 234 214 191 – 237 at Port Sydney 02EB008 South Branch Muskoka River 139 165 147 128 – 165 at Baysville 02EB006 Muskoka River below Bala 390 435* 417 372 – 461 *The 2019 data for 02EB006 was provided by OPG.

Figure 4-6: Flood Frequency Analysis Results – Big East R. at Huntsville (02EB013)

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Figure 4-7: Flood Frequency Analysis Results – North Branch at Port Sydney (02EB004)

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Figure 4-8: Flood Frequency Analysis Results – South Branch at Baysville (02EB008)

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Figure 4-9: Flood Frequency Analysis Results – Muskoka R. below Bala (02EB006)

4.2.2 Lake Level Gauges Lake levels are a secondary element in flood routing, in which the primary elements are inflow volume and lake outflow. The outlet control of lakes may vary with the outlet geometry of the lake, human intervention or submergence due to backwater effects in the downstream channel.

In this situation the exceedance probability of high lake levels should not be based on the series of annual maximum lake levels, but on the series of floods that cause the extreme lake levels.

The Water Survey of Canada station 02EB006, the Muskoka River below Bala, was selected as a representative flood frequency model for the Muskoka Lakes, since it has flow data from 1937 to 2019. Frequency analyses were undertaken for the annual maximum daily flow series at station 02EB006. All the frequency distributions fitted were negatively skewed and were upper bound at flows only slightly higher than the flood of record in 2019. The exception was the 3-parameter Log Normal distribution, which was upper bound at a flood level two orders of magnitude higher than the 2019 flood. Thus the 3-parameter log Normal distribution was used to establish the reference frequency distribution for the Muskoka Lakes.

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The annual maximum lake level series were of different lengths, and lake level distributions may differ slightly from year to year, so the exceedance probabilities for each year from station 02EB006 could not be transferred directly to the lake level series. To overcome this the maximum lake level series for each lake and the flood series for the same period were ranked from high to low and a ranked regression analysis was undertaken to relate lake levels to equivalent flood flows.

Using the fitted regression equation, the annual maximum water level for each year was related to the equivalent flood value at station 02EB006 for each year. The resulting 02EB006 flood value was then used to extract an exceedance probability for each lake level from the 82-year flood frequency distribution.

Finally, the series of lake levels and exceedance probabilities was used to plot effective lake level frequency plots and create lake level frequency tables. It was found that the 2019 flood event was greater than the estimated 100-year maximum lake level for all lakes. Thus, the peak static lake levels for the 2019 flood, see Table 4-6 would be considered the highest flood of record for regulatory flood purposes on these lakes. The results of the frequency analysis for each of the lake gauges is available in Figure 4-10 to Figure 4-14. Table 4-6: Spring 2019 Peak Static Lake Levels 2019 Peak Static Lake Gauge Level WSC Gauge Location Reading (m) 02EB016 - Fairy Lake 7.68 284.97 02EB021 - Mary Lake - Port Sydney 9.53 281.60 02EB020 - Lake Rosseau 9.50* 226.62* 02EB018 - Muskoka Lake at Beaumaris 10.50 226.45 02EB015 - Bala Bay 226.08 226.08 * estimated level - gauge was out of commission during the flood

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Figure 4-10: Frequency Analysis on Fairy Lake Levels

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Figure 4-11: Frequency Analysis on Mary Lake Levels

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Figure 4-12: Frequency Analysis on Lake Rosseau Levels

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Figure 4-13: Frequency Analysis on Lake Muskoka Levels

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Figure 4-14: Frequency Analysis on Bala Bay Levels

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5. Hydrology Conclusions 5.1 Conclusions A comparison of the results of the previous hydrologic study FDR 43 produced by MMM in 1988 with the available historical data up to 2019 provides interesting information. Despite the fact that the 2013 and 2019 flood events were driven by hydrologic inputs with more total runoff depth (see Table 4-3) than the total input (rainfall plus snowmelt) (see Table 3-1) for the simulated 1:100-year AEP flood event used in the FDR-43 deterministic hydrologic modeling, the modeling produced higher predicted flows and lake levels throughout the watershed than the 2013 and 2019 flood events. This is due to the conservative approach used for the FDR-43 simulated 1:100 AEP-year spring flood event with an intense 200 mm of snowmelt combined with 45 and 51 mm of rainfall all within the 3-day and 5-day simulated storm event periods, respectively.

This points to the difficulty of calibrating, estimating and combining hydrologic model inputs (climatic storm data) for deterministic modeling of design events for watershed runoff, and to the fact that flow records of sufficient length are a better source for estimating the likely frequency of flood events for a river basin.

However, this does indicate that the flood modeling for FDR 43, was done in a conservative manner and that the predictions of regulatory flows and lake water levels were, and still are, reasonable given the flooding that has occurred in the Muskoka River Watershed within the historical record. Therefore, it can be concluded that the simulated 1:100-year flood flows and water levels in the FDR series of documents still provide appropriate predictions where reliable records of flow and water level data are not otherwise available for estimates of the regulatory flood.

The current single station frequency analyses are based on more than enough data (44 to 102 years) to provide a reasonable estimate for 1:100-yr AEP flood.

However, the 2013 and 2019 recorded peak flows throughout the basin exceeded the current 1:100-year AEP flood estimates by single station frequency analysis. Thus, these recorded maximums, would by MNRF definition, become the regulatory flow throughout the basin.

Our review and analysis of the available hydrologic information indicates that predictions for the regulatory flood flows in FDR 43 are higher than what is predicted by the current single station frequency analysis for the 1:100-yr AEP flood, and higher than 2013 and 2019 peak water levels and flows throughout the basin. Therefore, Hatch concludes that the flows and water levels in the FDR series of documents can still be used for floodplain mapping purposes where current hydrologic data and analyses do not provide enough information for updating the floodplain mapping.

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Table 5-1 provides a summary of the hydrologic studies and conclusions for the regulatory flood flow2, according to the MNRF definition, at the four flow gauge locations analyzed. Table 5-1: Summary of Hydrologic Analyses and the Regulatory Flood Flow Max. FDR 43 FFA Regulatory Recorded Existing 1:100- Flood Gauge Flow (m3/s) Regulatory yr AEP Flow Number Gauge Name and year Flow (m3/s) (m3/s) (m3/s) 02EB013 Big East River near Huntsville 230 (2013) 285 220 230 02EB004 North Branch Muskoka River at Port Sydney 234 (2019) 291 214 234 02EB008 South Branch Muskoka River at Baysville 165 (2019) 226 147 165 02EB006 Muskoka River below Bala 435 (2019) 514 417 435

The results of the daily specific runoff analysis in Section 3.3.3 confirms that the regulatory flows in Table 5-1 can be used for prediction of flood flows on each river branch using a ratio of drainage areas on each branch as was done in the previous flood line mapping studies. The regulatory flood flows at the gauges represented in Table 5-1 will be used where applicable for proration of flows for the hydraulic studies, and is described in Section 7 Recommendations for Floodplain Mapping.

2 MNRF Technical Guide - River & Stream Systems: Flooding Hazard Limit (2002)

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6. Climate Change Uncertainty The study “Planning for Climate Change in Muskoka” (Peter Sale, Richard Lammers, Norman Yan, Neil Hutchinson, Kevin Trimble, Paul Dinner, Piret Hurrell, Jan McDonnell, and Scott Young, 2016.) was produced by the Muskoka Watershed Council to examine the likely impacts that mid-century climate may have on the Muskoka lakes and waterways. The main statement from that document that reflects potential changes on flooding in the Muskoka area is quoted here

“While it is not yet possible to precisely define future climates, the growing expertise in climate science makes it possible to set out plausible and likely climatic conditions for future periods, given specific assumptions about how the global economy grows and its pattern of energy use changes. We have used data generated by the Intergovernmental Panel on Climate Change (IPCC CMIP5 Project) to extract information on the mid- century values for temperature and precipitation in Muskoka that are expected if the world follows each of two plausible scenarios for economic growth and greenhouse gas emissions reductions. We have compared these data with equivalent measurements of temperature and precipitation in our current climate. While there will still be warm years and cold ones, wet ones and dry ones, the typical year at mid-century is likely to be 3-4oC warmer each month than at present, and about 10% wetter. … The mid-century projections for Muskoka reveal about a 10% increase in the total amount of precipitation in a typical year. Nearly all of this increase is likely to occur in late fall to early spring (November through April) for a 17% increase in the total for those 6 months (524 mm compared to 446 mm at present).”

Though these predictions are for average conditions it does indicate that floods will be more frequent and potentially of greater magnitude. As well, the historical flooding in the Muskoka basin over the past seven years does indicate that there has been an increase in frequency and magnitude of significant floods. In an effort to understand what the most recent historical data may reveal about potential increases in floods Hatch performed a trend analysis on climate and flow data in the Muskoka area, and is discussed in the following. 6.1 Trend Analysis Flood and water level frequency analyses have been undertaken for rivers in Muskoka to estimate critical floods within the Muskoka River floodplain. These flood estimates pertain to current climatic conditions, but there are concerns that flood flows could increase in magnitude and frequency in the future as a result of climate change.

Current climate change predictions available from Environment Canada are limited to annual and maximum day precipitation and temperature changes. These forecasts apply to the year as a whole and the summer, when highest temperatures and maximum one day rainfall occur.

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The highest floods in the Muskoka River generally occur in April as a result of snowmelt and rainfall on snowmelt. The climate parameters contributing to these flood events are not currently covered by Environment Canada’s Climate Data for a Resilient Canada ClimateData.ca website.

To gain an understanding of recent frequency of events in Muskoka and the possible future increases in flooding, a trend analyses have been undertaken for the following parameters in the Muskoka area

• annual maximum daily streamflow

• winter snowfall

• April temperature

• maximum daily rainfall in April.

The first of these parameters will identify whether there is an existing trend in annual flood maxima. Trends in the other three parameters may explain why a trend in annual flood maxima exists.

6.1.1 Annual Streamflow Maxima Table 6-1 shows the Water Survey of Canada (WSC) hydrometric stations used in the trend analyses. Table 6-1: Water Survey of Canada Hydrometric Stations

Drainage Station Year Year Station Name Latitude Longitude Area Number From To (km2) 02EB004 North Branch Muskoka River at Port Sydney 45.21286 -79.27528 1915 2019 1410 02EB006 Muskoka River below Bala 45.0225 -79.67833 1937 2019 4770 02EB008 South Branch Muskoka River at Baysville 45.14797 -79.1135 1941 2019 1400 02EB013 Big East River Near Huntsville 45.39272 -79.15994 1973 2019 610 02EB014 Oxtongue River Near Dwight 45.31197 -78.98939 1981 2019 605

Before analyzing annual streamflow maxima from the WSC hydrometric stations in Muskoka for trend it is necessary to select the period of record to analyze. Figure 6-1 shows a normalized cumulative departure from mean annual flow for the three longest records in Muskoka. Figure 6-1 shows that a major change in the hydrological regime in Muskoka occurred around 1965. Consequently, the trend analysis for annual flood maxima has been conducted for the period 1965 to 2019.

Figure 6-2 to Figure 6-6 show the annual maximum daily flow trendline plots for the five WSC stations in Table 6-1.

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Figure 6-1: Normalized Cumulative Departure from Mean Annual Flow

Figure 6-2: 02EB004: Annual Maximum Flood Trendline Analysis

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Figure 6-3: 02EB006: Annual Maximum Flood Trendline Analysis

Figure 6-4: 02EB008: Annual Maximum Flood Trendline Analysis

Figure 6-5: 02EB013: Annual Maximum Flood Trendline Analysis

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Figure 6-6: 02EB014: Annual Maximum Flood Trendline Analysis

The regression analysis using drainage area and lake area yielded the following flow increase equation:

Flood increase = 5695 A-1.693 L0.4635 %/year R2=0.983

Table 6-2 presents the data used in the regression analysis and the results obtained. Table 6-2: Annual Maximum Daily Flow Trendline Equations

WSC Drainage Area Lake Area Observed Flow Estimated Flow Station (km2) (km2) Increase (%/year) Increase (%/year) 02EB004 1410 186 0.25 0.30 02EB006 4770 902 0.09 0.08 02EB008 1400 277 0.34 0.36 02EB013 610 69.6 0.89 0.79 02EB014 605 99.4 0.97 0.94

Figure 6-7 shows a plot of estimated percent increase in flow per year from the regression analysis vs. observed flow increase.

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Figure 6-7: Regression Analysis: Estimated vs. Observed Flow Increase (%/year)

Table 6-8 indicates that the expected future increase in flood flows is inversely related to drainage area. This suggests that the events causing the floods will become more intense, but that attenuation by drainage area size and lake storage will reduce the maximum daily flow indicating that the volume increases would be fairly small.

Table 6-8 also gives the trendline equations and % increase in expected flood flows by 2050. Table 6-8: Trendline Equations and Expected Flow Increase

Flow WSC Drainage Trendline Equation Flow Increase Increase by Station Area (km2) (Median Flow vs. Year) (%/year) 2050 (%) 02EB004 1410 Flow = 0.3045Year – 476.75 0.25 8 02EB006 4770 Flow = 0.2442Year – 210.01 0.090 3 02EB008 1400 Flow = 0.2713Year – 452.23 0.34 11 02EB013 610 Flow = 0.7723Year – 1437.9 0.89 28 02EB014 605 Flow = 0.4686Year - 876.17 0.97 30

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6.1.2 Seasonal Climate Trends Figure 6-9 provides the results of the trend analysis for winter snowfall and indicates that winter snowfall will decrease over time.

Figure 6-10 provides the results of the trend analysis for average April temperature and indicates that average temperatures will increase, accelerating the rate of snowmelt

The trendline analysis in Figure 6-11 for maximum daily rainfall in April suggests that the maximum one-day rainfall in April will continue to increase significantly, resulting in increased snowmelt and higher flood peaks.

6.1.3 Trend Analysis Conclusions Based on this trend analysis, the increase in spring flood peaks observed in the Muskoka rivers is likely due to an increase in April temperatures and one day rainfalls, which increase the rate and depth of snowmelt. However, these flashier spring floods will be quickly attenuated by drainage area size and lake coverage.

Thus, as indicated in Table 6-8, the percentage increase in peak flows above current flood flows at the lower portion of the Muskoka river system, will not be as high as the increase in the upper reaches.

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Figure 6-9: Winter Snowfall Trendline Analysis

Figure 6-10: Average April Temperature Trendline Analysis

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Figure 6-11: Maximum April Daily Rainfall Trendline Analysis

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7. Recommendations for Floodplain Mapping 7.1 Regulatory Flood Flow Proration The regulatory flood flow estimates in Table 5-1 were prorated along the river branches for the reaches selected for floodplain mapping. Each of the selected river reach locations was paired with an index WSC gauge based on proximity and hydrologic similarity, and a simple proration based on drainage area ratios was performed to yield the estimates for regulatory flood flow peak at the selected river reaches.

Drainage areas for the river reaches were estimated using OFAT. Regulatory flood peak flows from the index WSC gauges were prorated to the selected locations using the equation:

= �� where: ��

Qi = regulatory flood peak flow at the river reach location

QWSC = regulatory flood peak flow at index WSC gauge

Ai = drainage area at river reach location

AWSC = drainage area at WSC gauge location

This simple proration was applied for all the selected locations except for the river reach between Lake Vernon and Fairy Lake, where the hydrologic characteristics are somewhere between those at 02EB013 and 02EB004, as indicated by the FDR-43 deterministic modeling. Thus, an average of the prorations for the two WSC gauges was applied to this location.

Table 7-1 summarizes the estimates for regulatory flood flow peak for each of the selected reaches. Table 7-1: Summary of Regulatory Flood Peaks Regulatory Flood Drainage Flow Peak River Reach Location Area (km2) Index WSC Gauge (m3/s) Bala 4,681 02EB006 435 Baysville 1,401 02EB008 167 Big East Upper - Williams Port 608 02EB013 230 Big East Upper - Little East 96.2 02EB013 36.3 Big East Lower - Highway 11 716 02EB013 270 Big East Lower - Ravenscliffe (local) 27.3 02EB013 10.3 Bracebridge – North Branch 1,602 02EB004 266 Bracebridge - South Branch 1,684 02EB008 201 Huntsville 1,106 02EB013 / 02EB004 ≈301 North Branch 1,496 02EB004 248

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Regulatory Flood Drainage Flow Peak River Reach Location Area (km2) Index WSC Gauge (m3/s) Port Carling 798 02EB006 70 Purbrook 1,562 02EB008 186

7.2 Recommendations for River Flood Mapping Flows and Water Levels The Federal Hydrologic and Hydraulic Procedures for Flood Hazard Delineation (Natural Resources Canada, 2019) suggest that considerations should be given for incorporating climate change projections into flood mapping. Integrating future climate conditions into flood mapping is an emerging area which has been applied in multiple jurisdictions across Canada using different qualitative (e.g. adding ‘freeboard’, a vertical distance applied to account for uncertainty) and quantitative (e.g. modelling) approaches.

In Ontario there has been no legislated or recommended requirements by the Ministry on incorporating climate change projections. Whereas, in British Columbia, the Professional Practice Guidelines Legislated Flood Assessments In A Changing Climate In BC (Engineers And Geoscientists British Columbia, 2018) provide the following:

• The following procedures are recommended when it is necessary to project expected flood magnitudes for design of protective works or mitigation procedures:

 By time series analysis of historical precipitation and flood records, determine whether any statistically significant trend is currently detectable in storm precipitation and in flood magnitude and/or frequency.

• If a statistically significant trend is detected, follow one of these recommended procedures:

 Adjust expected flood magnitude and frequency according to the projected change in runoff during the life of the project, or by 20% in small drainage basins for which information of future local conditions is inadequate to provide reliable guidance.”

Based on the Federal recommendations, the recommendations provided in the BC guidelines, the climate trend findings documented in this report, and in discussion with District of Muskoka on the future use of this Muskoka River Flood Plain Mapping Study, Hatch provides the projections in Table 7-2 for increases to the regulatory flood flow peaks and the recommended projected peak flows to be applied in the hydraulic modeling to account for climate change trends and uncertainty for floodplain mapping.

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Table 7-2: Projection for %Increase and Recommended Flood Mapping Flow

Projection for Projected Peak Model Inflow Location % Increase Flow (m3/s) Williams Port 20 276 Big East Upper Little East River 20 44 Big East at HWY 11 20 324 Big East Lower Local Inflow 20 12 Huntsville Narrows Lake Vernon Outlet 18 355 Baysville Baysville Dam 15 192 Purbrook Purbrook Bridge 15 214 Springdale Duck Chutes 15 286 Bracebridge Dam 15 306 Bracebridge South Falls Dam 15 231 Port Carling Port Carling Dam 10 76 Bala Bala Dam 10 479

The projected peak flow in Table 7-2 were applied in the hydraulic modeling to develop the floodline and floodway for the river floodplain mapping. Note that 2-D hydraulic models were created for two specific river sections (Big East River and South Branch Muskoka River at Purbrook). These models require full hydrograph inputs, rather than simply a peak flow as used in the steady-state 1-D models. Therefore, the peak flow in Table 7-2 were distributed into flood hydrographs as model inflows for these two specific 2-D models; the selected regulatory floods at each location (i.e., the 2013 flood for the Big East River and the 2019 flood for the Muskoka rivers) were used as index hydrographs and increased by the required percentage to provide the projected flood hydrograph flows within the HEC-RAS 2-D models. This method is used to transfer the flood hydrograph shapes to the specific hydraulic model input locations, thereby ensuring that similar flood timing and volume are represented in the 2-D hydraulic models. 7.3 Recommendations for Muskoka River Basin Lake Flood Mapping To determine the recommended static lake levels for flood mapping in the Muskoka River Basin, the results of the lake level frequency analysis in Section 4.2, and the projected peak flows in Table 7-2 were compared with the modeled lake outflows and corresponding static lake levels provided in the previous regulatory flood documents (FDR-42, FDR128, and FDR- 143). For all lakes the previous assessments provided higher outflows and static lake levels than the 2019 flood event. As well, most of the previous static lake levels in the FDR documents also accommodate the recommendations for the projected increased flows in Table 7-2 for flood mapping; this applies for all lakes except for Lake Muskoka, where it was found that the ARSP model rating curve for Lake Muskoka (see Appendix B) estimates that a 10% increase to the 2019 flood peak outflow would result in a Lake Muskoka projected peak static level of el. 226.7 m at the projected 479 m3/s peak outflow.

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Accordingly, the recommended Static Lake Water Levels for flood mapping are provided in Table 7-3 to account for climate change uncertainty for the major inland lakes in the Muskoka River Basin assessed in this study. Table 7-3: Flood Mapping Static Lake Levels with Projections for Climate Change Static Lake Level for Flood Mapping Lake (m) Vernon Lake 286.68 Peninsula Lake 285.29 Fairy Lake 285.29 Mary Lake - Port Sydney 281.74 Spence Lake 269.40(1) Lake Joseph - 226.70 Lake Rosseau 226.70 Muskoka Lake 226.70(2) Bala Bay 226.30 (1) FDR-143 calculated flood level Hanna Chute Forebay (2) ARSP model calculated Lake Muskoka flood static level with increased flows

Along shorelines subject to wave action, winds can drive water farther inland, beyond the limit of the static flood line. Land developers and Planning authorities must add the area covered by wave uprush to the area covered by the static flood line to avoid the Hazardous Lands that could be unsafe for development due to naturally occurring processes.

Given that the flood line on the inland lake maps represents the static flood line, a horizontal and vertical allowance for wind setup and wave runup should be added to keep developments safe from inundation and wave forces. The report FDR128 has recommendations for horizontal and vertical allowances but these are based on average fetch and shoreline characteristics. Site specific or local area studies for specific shoreline sections of the lakes, based on current practices for wind and wave analysis, would provide better estimates of required allowances for full delineation of Hazardous Lands. 7.4 Port Severn Flood Mapping Ontario MNRF has developed technical guidelines for flood mapping on the Great Lakes, Great Lakes – St. Lawrence River System and Large Inland Lakes Technical Guides (MNRF, 1997) for flooding, erosion and dynamic beaches in support of Natural Hazards Policies 3.1 of the Provincial Policy Statement (1997) of the Planning Act. In this document Section E Part 3 describes the requirements for flood hazard mapping. For the shoreline of Georgian Bay where Port Severn is located, the technical guidelines state that the flood hazard limit is an elevation of 178.00 m GSC with a 15m horizontal wave allowance. This procedure has been applied for mapping the Georgian Bay shoreline flood hazard line at Port Severn.

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8. River Features 8.1 The Muskoka River Watershed The Muskoka River watershed is located in central Ontario’s lake country, with the main population centres being Huntsville, Bracebridge and Gravenhurst. The Muskoka River watershed belongs to the southern Lake Huron/Georgian Bay drainage basin. The watershed originates on the western slopes of Algonquin Provincial Park and extends southwesterly for a distance of some 210 km to Georgian Bay and is 62 km at its widest point. The watershed encompasses an area of approximately 5100 km2 and includes about 780 km2 of lakes (15% of the watershed). The watershed, with its numerous lakes and wetland areas, all interconnected by river reaches, is typical of the many rivers of central Ontario on the Precambrian Shield. The watershed is divided into three subwatersheds: the North and South Branches, and the Lower Muskoka subwatershed.

The Muskoka River supports a range of aquatic and wildlife ecosystems and human uses, among them power generation, swimming, canoeing, boating, angling, hunting and trapping and tourism operations. There is a total of 42 dams on the river system, 29 dams are owned by the Ministry of Ontario, one is owned by Ontario Power Generation, one by the District Municipality of Muskoka, and one privately owned. There are also three navigation locks and 11 waterpower generating stations with associated dams/water control structures. None of these dams are flood control dams. 8.2 Physiography The Muskoka River watershed crosses three north-south trending physiographic units (Chapman and Putnam, 1984). The river rises on the western slopes of the Algonquin Highlands physiographic unit. This domed area is underlain by gneiss and other metaphoric rocks of the middle and late Precambrian Age. Moving westward, there is a strip of sand, silt and clay deposits that follow the alignment of Highway 11 (i.e., from Gravenhurst, through Port Sydney and north to Huntsville) and is known as the Number 11 Strip physiographic unit. This strip formed just below a shoreline of glacial Lake Algonquin and received deposits from streams entering the lake from the adjacent highlands to the east. The western half of the watershed crosses the Georgian Bay Fringe. This physiographic unit was washed by waves from glacial Lake Algonquin leaving only very shallow, coarse soils and exposing bare rock knobs and ridges of the Precambrian Age.

The Ontario Geological Terrain Mapping Study (Mollard, 1981) identifies the dominant landforms throughout the very upper and the lower parts of the watershed as bedrock knobs, plains and ridges. Along the Oxtongue River and Tasso Lake/upper East River area in Sinclair and Finlayson Townships (i.e., within the Algonquin Highlands physiographic unit), glaciofluvial features have been mapped and consist of outwash plains or valley trains. These are features consistent with streams flowing from an ice sheet in the east into the glacial lake (Algonquin) located just to the west. The central part of the Muskoka River Basin area (extending from Gravenhurst in the south through Huntsville in the north, along the North Muskoka River valley and as far east as Lake of Bays and as far west as Axe Lake) and the

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west shores of Lake Muskoka has considerably more varied landforms (i.e., the Number 11 Strip physiographic unit). The major landform feature here consists of a glaciolacustrine plain/delta. There are pockets of ground moraine, and rocky knobs and ridges are common especially moving both east and west from the actual Highway 11 corridor. A number of organic terrain landforms are mapped throughout the watershed. 8.3 Topography and Surficial Geography The watershed reaches an elevation of over 525 m at its boundary in the northeast, dropping to the lowest elevation of 177 m at the Georgian Bay shoreline. The terrain maps for the area (Mollard, 1981) classify the topography in the extreme northeast as high local relief that is ridged and hummocky. Throughout the central section, relief is defined as moderate, with a variety of local features including gulleying, knobs, plains and undulating. In the lower sections of the watershed (west of Lake Joseph/Lake Muskoka), the topography is mapped as low local relief varying from plains to undulating and hummocky conditions.

The topography is locally rough and the river channel slopes are steep within the Algonquin Highlands. Creeks can drop as much as 150 m in their first 20 km. Approximately 60% of the total drop in the river channel occurs in the upper 30% of the river length (MNR, 1997). The river channel in the lower section flattens out into a series of lakes with interconnecting channels, and then passes through two distinct channels (Moon and Musquash) to Georgian Bay. Steep walled valleys exist in upper sections of both the North (Big East River below Finlayson Pond) and South (Oxtongue River above Lake of Bays) branches.

Generally, surface drainage in the upper and central portions of the watershed is good, providing dry soil conditions with rapid runoff occurring from the exposed bedrock areas. Wet surface conditions are restricted to isolated organic terrain sites (i.e., local low-lying areas). In the lower part of the watershed (Moon and Musquash Rivers), much of the landscape is low relief, and is traversed by a series of parallel bedrock ridges and intervening bogs.

Overall, the surficial geology mirrors the physiographic units. Soils are thin in the upper and lower stretches of the watershed that contain extensive areas of exposed bedrock. Local patches of thicker sandy, gravelly deposits are found within the upper valleys. Deposits of sand, till and silt are extensive within the Number 11 Strip physiographic unit. These have weathered to give deep, often sandy loam soils that can support limited agricultural activities (Chapman and Putnam, 1984). 8.4 Subwatershed Characteristics The watershed is divided into three subwatersheds: the North and South Branches, and the Lower Muskoka subwatershed, the North and South Branches comprise approximately the eastern two-thirds of the watershed.

The two distinct headwater branches of the river, the North and the South Muskoka Rivers, both originate on the western slope of the Algonquin Park dome in Algonquin Park. These branches flow in a generally southwesterly direction until they converge in the Lower

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Muskoka subwatershed at Bracebridge which continues through Lake Muskoka which receives the inflow from Lakes Joseph and Rosseau. Lake Muskoka drains out through Bala Bay and the downstream river reaches to Georgian Bay. The river system falls approximately 345 m over the 210 km distance from its headwaters to its outlets at Georgian Bay. Nearly 60% of the drop-in river channel elevation occurs in the upper 30% of the river length.

The North Branch originates in Algonquin Park in a number of small lakes (McCraney and West Harry Lakes), whose discharges converge to form the Big East River. Tributaries of the Big East include Cripple and Tasso Creeks, which meet the river at the site of the former Finlayson Reservoir, and the Little East River. The Big East River drains into the eastern edge of Lake Vernon. The other main inlet of Lake Vernon, located at the northern end of the lake, is the Buck River, which drains Axe, Round, Buck and Fox Lakes. Outflow from Lake Vernon flows past the Town of Huntsville into Fairy Lake, which also receives inflow from Peninsula Lake. These three lakes (Vernon, Fairy and Peninsula) are collectively known as the ‘Huntsville Lakes’. Outflow from Fairy Lake, through the Huntsville Dam, forms the North Branch of the Muskoka River, which flows through Mary Lake and a cascading series of waterfalls (High, Wilson’s and Bracebridge Falls) and hydropower generating stations. The North Branch then converges with the South Branch just south of the Town of Bracebridge.

The South Branch of the Muskoka River is also divided into two distinct headwater areas. The Oxtongue River originates in Algonquin Park in a series of small lakes (with Burnt Island, Little Joe, Joe, Canoe, Ragged, Smoke and Tea Lakes being the major ones). The river flows through Oxtongue Lake and drains into the north end of Lake of Bays, near the Town of Dwight. South of the Oxtongue River subwatershed, a number of small lakes (Fletcher, Livingstone, Rockaway and Wildcat, among others) empty into Kawagama Lake (formerly known as Hollow Lake). The outflow of this lake forms the Hollow River, which flows into the southeast corner of Lake of Bays, near the Town of Dorset. The outflow of Lake of Bays, which is the fourth largest lake in the watershed, passes through the Baysville Dam, in the Town of Baysville, and is known as the South Muskoka River from this point on. The river flows generally southwest, through the area of Fraserburg Road and converges with the outflow from Wood Lake. From here the river flows west past Rocky Rapids and through the Matthias Falls generating station, then it turns slightly north to flow through the Trethewey, Hanna Chute and South Falls generating stations prior to converging with the North Branch south of Bracebridge.

Following convergence of the two branches, the Muskoka River flows a short distance before emptying into the southern edge of Lake Muskoka, the largest lake in the watershed. Lake Muskoka also receives inflow from Lakes Rosseau and Joseph, with the three collectively known as the ‘Muskoka Lakes’. Drainage from Lakes Rosseau and Joseph, respectively the second and third largest lakes in the watershed, flows into the Indian River, past the locks and dam at Port Carling and into the north end of Lake Muskoka.

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Outflow from the main part of Lake Muskoka, passes through channel constrictions at the Coulter and Jannacks Narrows and the Wallis Cut into Bala Bay, where the Bala dams are located. Bala Bay can be drawn down somewhat by the Bala dams, but the draw down of the main part of Lake Muskoka is restricted by the channel constrictions and requires a much longer time to be lowered.

Outflow from Bala Bay (Lake Muskoka) passes through the two hydropower stations and two dams at Bala into the Bala Reach area and then over Moon Chutes. After Moon chutes the Muskoka River forks into the Moon and Musquash Rivers. The Moon flows northwest and receives input from Kapikog and Healey Lakes before emptying into Woods Bay, and subsequently Georgian Bay. The Musquash flows west through Ragged Rapids GS and swings northwest before flowing past Big Eddy GS on its way to Go Home Lake. This lake has two outlets, both of which empty into Georgian Bay.

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9. River Hydraulics Study 9.1 HEC-RAS Program Models of each of the selected river reaches were created using HEC-RAS (Version 5.0.7, March 2019) developed by the US Army Corps of Engineers Hydrologic Engineering Center. HEC-RAS Version 5.0.7 is capable of modeling both one-dimensional (1-D) and two- dimensional (2-D) hydraulic conditions.

All models, with the exception of the Big East River and the South Branch at Purbrook, were modeled in 1-D. Both the Big East River and the South Branch at Purbrook have flat marshy floodplains where major flow occurs in the floodplain with major changes in direction occurring when water levels exceed the banks of the river. Hydraulic modeling of this type of flow characteristics in the floodplain is best done in 2-D.

9.1.1 HEC-RAS 1-D In 1-D modeling, the hydraulic character of the river channel is defined by a series of user input cross-sections. These cross-sections are strategically located along the river channel to represent average hydraulic conditions in an area. If required, HEC-RAS is able to create interpolated cross sections between cross-sections based on user-specified interval.

Channel roughness parameters and other minor loss coefficients are selected to account for the effects of channel boundary roughness, meanders, debris and unanticipated obstructions. Roughness values can be varied spatially across a section. This allows the model to best simulate the effect that variations in infrastructure/vegetation might have on the local hydraulic conditions. This again is a distinct advantage when simulating overland flow over floodplain areas, such as would occur during a flood event.

Results from the HEC-RAS program can be easily exported using the new RasMapper application in a format which can be imported to ESRI ArcGIS program. This allows model floodline results to be exported into a GIS database of the river valley where the flood lines can then be layered with other maps of the area denoting infrastructure and properties.

9.1.2 HEC-RAS 2-D HEC-RAS 2-D models use an implicit finite volume solution algorithm, which uses a mix of structured and unstructured mesh. This allows for larger time steps compared to an explicit solution method. Using the finite volume method, provides increased calculation stability in situations where the cell may start completely dry and then has a rush of water through it. The finite volume solution method has the additional advantage of being able to handle supercritical, subcritical and mixed flow regimes. In addition, every cell edge in the mesh retains the details of the underlying terrain data which allows for greater cell size and smaller computation times.

The model solves either the 2-D Saint Venant equations with momentum additions for turbulence and Coriolis effects, or the 2-D Diffusion Wave equations. For this project 2-D

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Saint Venant was selected as the most appropriate set of equations. As the 2-D Saint Venant equations are more applicable across a wide range of hydraulic conditions. HEC-RAS models allows for combined 1-D and 2-D modeling. 9.2 Geometric Data The HEC-RAS model geometry of both the 1-D and 2-D models was created using a combination of LiDAR data and bathymetric data from the river reaches.

9.2.1 Lidar Collection The LiDAR was collected by Airborne Imaging between the 18th-24th of May 2018. The LiDAR captured an area of more than 1500 km2 in the Muskoka region. Figure 9-1 illustrates the areas where LiDAR was acquired, and the density of points acquired. Two flight series were flown, one where the density of points acquired were a minimum of 2.5 pts/m2 in the less populated areas of the region, and the other with 10 pts/m2 in the most developed urban areas, a point density of 12.5 pts/m2 occurred where there was overlap of the of the two-flight series.

The horizontal datum used is NAD83 CSRS (2010) and the vertical datums used is CGVD28, with Geoid Model HT2.0. The project resided in Zone 17 in the UTM coordinate system. The 95%, or 2δ, accuracy of the LiDAR data cloud acquired was 35 cm horizontally and 15 / 10 cm vertically.

The LiDAR data cloud was processed by Airbourne Imaging into a 1 m ESRI grid format. The processing provided a full feature Digital Surface Model (DSM) and removal of structures for a “Bare Earth” Digital Elevation Model (DEM) product. Delineation of water breaklines was included for delineation of water bodies and Hydroflattening postprocessing of the DEM to allow straightforward integration with the bathymetric data for the hydraulic modeling.

The processing of the LiDAR provided the following products

• LiDAR Survey Data – 1508.4 km2 of topographic mapping data including

 Processed and calibrated raw point cloud (LAS v1.2, ASPRS Classes 1, 2, 9)

 Classified point cloud in LAS v1.2, classified to ground, non-ground and water

 1m Grids of Bare Earth (DEM) and full feature (DSM), and Hydroflattened (DEM)

 Hill shade Geotiffs (grayscale) of Bare earth and Full feature

 0.5 m Contours in shape files

 Water Breaklines in shape files.

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Figure 9-1: Lidar Point Density in Muskoka Region

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9.2.2 River Cross-sections Bathymetry Collection Since LiDAR does not return laser points for any ground below the water surface it is necessary to supplement these areas with surveyed bathymetry data to create accurate river geometry for the HEC-RAS model. Bathymetric survey points at selected cross-section locations were taken in-channel using sonar data. The sonar data was collected using a HydroLite-TM (Transom Mount) survey kit combined with a Trimble® R10 GNSS system to provide an integrated survey solution. Bathymetric data was collected in the last week of August and through September of 2018.

Postprocessing and filtering of erroneous data was performed on the bathymetric data points to create surface models of the river between the banks. Using SMS 12.3 created by Aquaveo (2019) a mesh was created using the processed bathymetric point data by interpolating a mesh between the bathymetric data points. The mesh was exported as contour and raster data. Figure 9-2 illustrates an intermediate step in processing the bathymetric data. This bathymetric raster data was combined with the over bank terrain data (Section 9.2.1) using HEC-RAS RasMapper tools. This process allows for cross sections to be cut from the terrain and bathymetry (Figure 9-3).

Figure 9-2: SMS Mesh Interpolated from Bathymetric Point Data

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9.2.3 1-D River Geometry Development All the 1-D models followed the same procedure to create the geometry files for the hydraulic modelling.

Using the processed terrain and bathymetric data, 1-D river cross sections were cut from the combined data of both above and below the waterline seamlessly (Figure 9-3). This allowed the cross-section to represent the entire wetted area in flood events. In 1-D hydraulic modeling calculations are only performed at the locations of the cross sections. This facilitates the modeling of complicated river reaches by simplifying the hydraulic calculations to the required river sections.

Figure 9-3: Cross Section Cut from Combined Terrain and Bathymetric Data

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9.2.4 2-D River Geometry Development The Big East and South Branch at Purbrook river models are full and partial 2-D HEC-RAS models, respectively.

Combining the bathymetric data with the terrain data was performed using similar methods as in the 1-D models with a few additional steps. Representative river bed cross sections were developed from the bathymetric data. In HEC-RAS multiple closely spaced cross-sections were interpolated between the bathymetric cross sections then extruded to a surface. This created a fully developed channel which is bounded by the banks defined by the LiDAR surface. The combination of the fully developed channel with the LiDAR 1m Bare Earth hydroflattened product results in a complete surface elevation model in HEC-RAS with a representative channel based on collected bathymetric data.

Figure 9-4: Cell face from Upper Big East Model Depicting Retained Terrain Data

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2-D modeling results are less sensitive to mesh size in HEC-RAS than other hydrodynamic 2-D software because HEC-RAS retains underlying terrain information of each cell face. Each cell has storage elevation curve and each cell face has a rating curve into the next cell (Figure 9-4). For water to pass from one cell to the next it must cross the cell face. In HEC- RAS 2-D modeler applied breaklines lines are often used to aligns cell faces along a high topographic ridge such as a road embankment or berm. The new cell face position forces the water surface to exceed the elevations of the cell faces along the breakline before water can cross into the adjacent cell. Figure 9-5 illustrates the effect of applying breaklines at the roads on the mesh in the Big East River HEC-RAS model.

Figure 9-5: Breaklines in Mesh of Lower Big East Model

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9.2.4.1 Big East The Big East River was modeled in full 2-D and was split into two reaches; Upper Reach which extends from the Williams Port Bridge to the Highway 11 Bridge. While the Lower Reach continues from the Highway 11 Bridge to Lake Vernon. Figure 9-6 illustrates the extent of both the model reaches. As HEC-RAS 2-D modeling does not currently support bridge modeling, the reaches were modeled separately with the estimated headloss through the bridge opening based on observed water levels. The headloss at the Highway 11 bridge was added to the results of the Lower Reach model to develop a rating curve (see Appendix B) for the downstream boundary condition of the Upper reach model.

Figure 9-6: Big East River Model Arrangement

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9.2.4.2 Purbrook The South Branch at Purbrook is modeled in both 2-D and 1-D which are connected just upstream of Rocky Rapids. The 2-D reach upstream of Rocky Rapids is defined by a meandering channel that transforms into a large overland flowing floodplain at high flows. The lower reach of the Purbrook model is modeled in 1-D,bounded by Rocky Rapids at the upstream and Matthias dam at the downstream end.

Figure 9-7: South Branch of the Muskoka River at Purbrook Model Arrangement

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9.3 Model Calibration This section of the report describes the methodology implemented to carry out the calibration for each of the hydraulic river models. For each of the river reaches different types of information were available for calibration; therefore, the methods for calibration differ accordingly. The main method of calibration along the river reaches in the models is the adjustment of the hydraulic roughness (resistance to flow) by means of Manning’s n to match observed water levels in the reach at a given flow. A summary of the calibrated Manning’s n values used in the models is presented in Table 9-1. Table 9-1: Summary of Manning's n Roughness Coefficients

Channel Manning’s n Over Bank Manning’s n Big East Upper 0.03 0.06 Big East Lower 0.03 0.045 Huntsville Narrows 0.05 0.06 Baysville 0.035 0.15 Purbrook 0.035 0.06 Springdale 0.035 0.06 Bracebridge 0.04 0.06 Port Carling 0.035 0.06 Bala 0.035 0.06

9.3.1 Big East Manning’s n of the river channel and the floodplain were adjusted to best match the observed highwater marks from previous spring flooding events. Table 9-2 and Table 9-3 have a summary of the calibration points and hydraulic parameters used in final calibration runs for the Lower and Upper sections of the Big East River model respectively. A final calibration run was conducted at bank full flow conditions to confirm the channel roughness parameters for lower discharge conditions. This model simulation was done with the observed bank full event on August 24, 2018, with a flow of 74 m3/s. Lower Big East Manning’s n of 0.03 and 0.045 were adopted for roughness values for the channel and overbank respectively. While in the Upper Big East, Manning’s n of 0.03 and 0.06 were adopted for roughness values for the channel and overbank respectively. Table 9-2: Lower Big East River Calibration Results

UTM Coordinates (Zone 17) Total Flow WSE Estimate WSE Modeled Easting (m) Northing (m) Event (m3/s) Estimate (m) (m) 637139 5023039 2013 Peak 263 286.86 286.93 639122 5026610 2019 Peak 209 289.60 289.73

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Table 9-3: Upper Big East River Calibration Results Total Flow UTM Coordinates (Zone 17) WSE WSE Event Estimate Estimate (m) Modeled (m) Easting (m) Northing (m) (m3/s) 639709 5026470 2016 Peak 177 290.07 289.85 639709 5026470 2017 Peak 143 289.73 289.54 639162 5026633 2019 Peak 209 289.89 289.97

9.3.2 Huntsville Narrows Water levels in the Huntsville Narrows reach is controlled by the flow out of Lake Vernon and downstream levels at Fairy Lake. Flow data is not available for the channel nor are highwater marks from previous spring flooding events. Thus, for the Huntsville Narrows model, calibration involved adjusting Manning’s n of the river channel and the floodplain to best match the regulatory static water levels of Fairy Lake and Lake Vernon at the downstream and upstream ends of the model, respectively. Manning’s n of 0.05 and 0.06 were adopted for roughness values for the channel and overbank respectively.

9.3.3 Baysville Water levels in the Baysville reach on the South Muskoka river is controlled by the flow out of Baysville dam and downstream levels at Fairy Falls. There is a Water Survey of Canada gauge (02EB008) at the upstream end of the reach which was used for calibration of the model. Both the surveyed water levels in 2018 survey and the 2013 flood flows and levels were recorded at the gauge. The hydraulic river model for Baysville was calibrated to match the levels recorded by the gauge. Table 9-4 presents the calibration results as it compares to the measured levels. The calibration was achieved by adjusting the critical depth boundary condition cross section at Fairy Falls and the Manning’s n values for the reach were selected based on physical characteristics of the river. Manning’s n of 0.035 and 0.15 were adopted for roughness values for the channel and overbank respectively. Table 9-4: South Muskoka River at Baysville Calibration Results

UTM Coordinates (Zone 17) Flow WSE Estimate WSE Modeled Easting (m) Northing (m) Date (m3/s) (m) (m) 5001114 648301 9/27/2018 24.75 312.68 312.73 5000252 647916 9/27/2018 24.75 312.46 312.52 4998364 647640 9/27/2018 24.75 312.35 312.39 5001098 648315 4/26/2013 139 314.26 314.19

9.3.4 Purbrook Manning’s n of the river channel and the floodplain were adjusted to best match the observed highwater marks from 2019 spring flooding events. Table 9-5 has a summary of the calibration points and hydraulic parameters used in final calibration runs. The 2019 flooding event was used for the Purbrook model as observation of water levels were taken on

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April 23, 2019. The headwater level of Matthias Dam on the day of the observation were used as the downstream boundary condition for the calibration simulations. Both the observed water level and the observed extent of flooding were used in determining appropriate Manning’s roughness for the reach. Manning’s n of 0.035 and 0.06 were adopted for roughness values for the channel and overbank respectively. Table 9-5: South Muskoka River at Purbrook Calibration Results

UTM Coordinates WSE Flow WSE (Zone 17) Date Estimate (m3/s) Modeled (m) Easting (m) Northing (m) (m) 646014 4987140 04/23/2019 234 293.60 293.58

9.3.5 Springdale The 2019 flooding event was used for the Springdale reach of the North Branch model as observation of water levels were taken on April 23, 2019. The headwater level of High Falls Dam on the day of the observation were used as the downstream boundary condition for the calibration simulations. Both the observed water level and the observed extent of flooding were used in determining appropriate Manning’s roughness for the reach. Manning’s n of 0.035 and 0.06 were adopted for roughness values for the channel and overbank respectively. Table 9-6: North Muskoka River at Springdale Calibration Results

UTM Coordinates (Zone 17) Flow WSE Estimate WSE Modeled Easting (m) Northing (m) Date (m3/s) (m) (m) 4996267 634357 04/23/2019 234 270.84 270.98 4994511 633784 04/23/2019 234 270.36 270.43

9.3.6 Bracebridge Observed water levels in Bracebridge were observed during the 2019 flood event and water surface elevation during low flows were used to calibrate the Bracebridge reach of the Muskoka River. Inflows locations for the North Branch and South Branch of the Muskoka River meet in Bracebridge and discharge into Lake Muskoka. The downstream boundary condition for the calibration was the Muskoka Lake level at WSC Beaumaris gauge (02EB018). Manning’s n in the reach was adjusted until the modeled water surface matched the water surface measured, presented in Table 9-7. Manning’s n of 0.04 and 0.06 were adopted for roughness values for the channel and overbank respectively.

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Table 9-7: Muskoka River at Bracebridge Calibration Results

UTM Coordinates Elevation of Flow (m3/s) WSE (m) (Zone 17) Beaumaris Easting Gauge 02EB018 North South (m) Northing (m) Date (m) Branch Branch Estimate Modeled 633148 4988593 4/28/2019 226.33 268.3 216.1 227.85 228.83 633122 4988330 4/24/2019 226.00 240.7 145.8 227.20 227.21 632844 4987823 225.61 225.53 633172 4987922 225.67 225.55 633541 4987460 225.70 225.57 633552 4986970 225.80 225.60 633477 4986353 225.76 225.63 632996 4985804 225.88 225.67 633163 4985269 225.95 225.77 633411 4984878 226.07 225.85 627569 4987538 225.54 225.44 627764 4987223 8/30/2018 225.43 57.8 39.7 225.51 225.44 628642 4986911 225.55 225.45 629127 4986913 225.60 225.46 629634 4986677 225.55 225.46 630232 4986486 225.63 225.47 631086 4986709 225.66 225.48 631689 4987337 225.67 225.50 632567 4987464 225.69 225.52 632847 4987954 225.68 225.53 632878 4988291 225.73 225.55

Flood flows from 2019 matched the observed water levels from the field visit. At the low summer flows the model tends to underestimate the water surface elevation at the upstream end of the model. This is to be expected as Manning’s n tends to decrease as depth of flow increases, and the channel in the 2019 spring calibration period did not have the fully-grown weeds that had accumulated in the channel during the summer 2018 calibration period. The selected Manning’s n are appropriate for high spring flow events.

9.3.7 Port Carling There is limited data available to calibrate the model of the Port Carling reach of the Indian River. In lieu of calibration a sensitivity analysis was performed on the model. A range of Manning’s n values for the reach were used to determine the effect channel roughness has on the flood levels in the reach. Table 9-8 represents the water surface elevation of each of the sensitivity runs. The differences in values represent the max change in water surface due to a change in Manning’s n. The difference is not significant for the modeling of this reach and

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the Mid Manning’s n was selected to represent the river. Manning’s n of 0.035 and 0.06 were adopted for roughness values for the channel and overbank respectively. Table 9-8: Indian River at Port Carling Calibration Results

Upstream Deviation Sensitivity Channel Over Bank Flow WSE Modeled from Mid Analysis Manning’s n Manning’s n (m3/s) (m) (m) Low Manning’s n 0.030 0.055 76 226.71 -0.01 Mid Manning’s n 0.035 0.060 76 226.72 0.00 High Manning’s n 0.045 0.065 76 226.72 0.00

9.3.8 Bala Reach Water levels in the Muskoka River for Bala Reach is controlled by the total flows out of Bala Bay and the downstream water level at Moon Chutes. There is an established tailwater rating curve for the area directly downstream of the Bala Dams available from the Water Management Plan background data that is based on recorded flows and periodic measurement of water levels over the history of the downstream Ragged Rapids GS operations on the river. The curve establishes the backwater effect in the Bala Reach from the Moon Chutes up to the Bala Dams. The hydraulic river model for Bala Reach was calibrated to match this tailwater curve. Figure 9-8 illustrates the calibration results. The calibration was mainly achieved by adjusting the critical depth boundary condition cross section that represents Moon Chutes. Manning’s n values for the modeled reach were tested for sensitivity and found to not have any significant impact on water levels and selected based on physical characteristics of the river. Manning’s n of 0.035 and 0.06 were adopted for roughness values for the channel and overbank respectively.

Bala Reach Discharge Curve Calibration 221.75 221.5 221.25 221 220.75 220.5 220.25 220 Established Tailwater… 219.75 219.5 Bala Dam (m) 219.25 219 218.75 218.5 Water Water Surface of Downstream Elevation 0 100 200 300 400 500 Flow in Reach (m³/s) Figure 9-8: Bala Reach Muskoka River at Bala Calibration Results

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9.4 Hydraulic Parameters The hydraulic parameters selected as the boundary conditions for floodplain mapping of each model are summarized below in Table 9-9. Table 9-9: Summary of Hydraulic Conditions for Flood Mapping Modeling Peak Type of Type of Inflow Downstream Water Surface Model Modeling Inflow Location (m3/s) Boundary Elevation (m) Williams Port 276 Rating Curve Big East River based on Big East 2-D Lower Model performance of Upper Little East River 44 Rating Curve Lower Big East Model ** Big East Big East at HWY 11 324 Lake Vernon 2-D 286.68 Lower Local Inflow 12 Elevation Huntsville Fairy Lake 1-D Lake Vernon Outlet 355 285.3 Narrows Elevation Baysville 1-D Baysville Dam 192 Fairy Falls Critical Depth Matthias Dam Purbrook 2-D Purbrook Bridge 214 Head Pond (Max 293.5 Allowed) High Falls Dam Springdale 1-D Duck Chutes 286 Head Pond (Max 270.8 Allowed) Bracebridge Dam 306 Bracebridge 1-D Lake Muskoka 226.7 South Falls Dam 231 Port Carling 1-D Port Carling Dam 76 Lake Muskoka 226.7 Bala 1-D Bala Dam 479 Moon Chutes Critical Depth * Rating curve available in Appendix B

Unsteady flow modeling is required for 2-D modeling in HEC-RAS. The inflow hydrographs for the Upper and Lower Big East models are presented in Figure 9-9 and Figure 9-10, respectively. The hydrograph used for the Purbrook model is illustrated in Figure 9-11.

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Figure 9-9: Upper Big East River Inflow Hydrographs used for Flood Mapping

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Figure 9-10: Lower Big East River Inflow Hydrographs used for Flood Mapping

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Figure 9-11: South Branch Muskoka River at Purbrook Inflows used for Flood Mapping 9.5 Flood Levels and Sensitivity to Climate Change Due to the increase in flows during recent flooding events it is prudent to evaluate how future increases in flows will affect the water levels in each of the study reaches. Each of the hydraulic models were run under two conditions, the first representing a flood equivalent to the flood of record and the second representing a potential climate change scenario in the basin. The climate change scenario mapped includes the increase in flow based on the selected change described in Table 7-21.

The discussion below details the effects of an increase in flow as a result of climate change on each of the reaches assuming all other hydraulic parameters remain constant.

9.5.1 Big East The Big East river has the largest potential increase in flow due to climate changes due to its basin flow being largely uncontrolled by large lakes. The flood of record for the basin occurred in 2013 and there is a potential for a 20% increase to that flow based on the current trends. Table 9-10 and Table 9-11 present the results of each hydraulic simulation.

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The Big East hydraulic model is bounded at the downstream end by Lake Vernon. If that boundary remains unchanged the level at the outlet of the Big East does not change with an increase to peak flood flows. Moving upstream an increase of flow due to climate change would increase flood levels at Ravenscliffe Road Bridge by 11 cm. The increase to water levels would reach 26 cm where the floodplain extends to Highway 11, overtopping both lanes of the highway and creating a floodplain on the East side of the highway. Although this area is not inundated in the flood of record event model, it may have been flooded by water arriving by culverts under roadways. The difference between the flood of record and the expected flood due to climate change is 34 cm at the downstream Highway 11 crossing, translating to an increase of 33 cm upstream of the highway.

Upstream of the highway the largest increase to water levels is at the Highways 11 bridge to about Roberts road due to the constriction at the bridge. The increase in water level decreases, but not significantly, towards the upstream end of the model.

In summary the expected water level increases due to an increases in runoff caused by climate change for the Big East river is between 11 cm and 34 cm, with the largest increase occurring at the highway 11 bridge. Table 9-10: Lower Big East Computed Water Surface

UTM Coordinates Water Surface Elevation (Zone 17, m) (CGVD 28, m) Estimate Flood of Climate Record Change Change Equivalent Event (+20% in Water Location Easting Northing Event Flow) Level (m) Lake Vernon/ Big East Delta 636742 5022557 286.77 286.79 0.02 Ravencliffe Bridge 636985 5022921 287.141 287.25 0.11 End of Ravencliffe Road 637735 5023385 287.762 287.96 0.20 Highway 11 South Bound Lanes 639448 5024528 288.8 289.06 0.26 Floodplain East of HWY 11 640060 5025656 Not Flooded 289.06 NA US of Railway 638923 5026819 289.84 290.11 0.27 Western Floodplain 638240 5025254 289.07 289.30 0.23 DS of Railway 638844 5026781 289.66 289.88 0.22 Old North Road 638230 5026106 289.16 289.37 0.21 DS HWY 11 Bridge 639118 5026609 290.14 290.48 0.34

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Table 9-11: Upper Big East Computed Water Surface

UTM Coordinates (Zone 17, m) Water Surface Elevation (CGVD 28, m) Estimate Climate Change Flood of Record Change Event in Water Location Easting Northing Equivalent Event (+20% Flow) Level (m) US HWY11 639216 5026624 290.37 290.70 0.33 Robert Rd West 640514 5026340 290.63 290.96 0.33 River Cove Drive 640514 5026095 290.6 290.94 0.34 Big Bend 641581 5027647 291.34 291.61 0.27 Upstream End 643543 5027724 292.68 292.92 0.24 US HWY11 641565 5026580 291.23 291.52 0.29

9.5.2 Huntsville Narrows The Huntsville Narrows between Lake Vernon and Fairy Lake will be affected by the increase in outflow of Lake Vernon. There is a potential for a 18% increase to flow from the flood of record, based on current trends in the basin.

Since Fairy Lake has a controlled outlet, both simulations summarized in Table 9-12 have a constant boundary condition of 285.30 m water surface elevation for Fairy Lake. The increase of flow in the reach caused by climate change will have the largest effect at the upstream end of the reach at Lake Vernon, where the increased flow will increase the energy loss in the channel between the lakes. The expected increase in water level caused by climate change is 20 cm at the upper end near Lake Vernon and 0 cm at Fairy Lake. Table 9-12: Huntsville Narrows Water Surface Profile

Water Surface Elevation (CGVD 28, m) Estimate Climate Change Flood of Record Cross Section Change Event (+18% in Water Equivalent Event Location Chainage (m) Flow) Level (m) Lake Vernon 16695 286.13 286.33 0.20 DS of Hwy 11 12819 286.08 286.28 0.20 Rail Bridge 7953 286.01 286.19 0.18 DS of Center St North 6491 285.87 286.03 0.16 US of Main Street 4559 285.67 285.78 0.11 DS of Main Street 4488 285.58 285.67 0.09 Near Park Drive 3145 285.43 285.47 0.04 Fairy Lake 442 285.30 285.3 0.00

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9.5.3 Baysville The flow in the Baysville reach of the South Muskoka River is controlled by the Baysville dam at the outlet of Lake of Bays. There is a potential for a 15% increase to flow from the flood of record, based on current trends in the basin.

The Baysville reach is controlled at the downstream boundary by Fairy Falls as a result the increase in flow caused by climate change will cause an increase in water level at that boundary. Table 9-13 summarizes the increase in water level expected due to climate change in the Baysville reach. The increase of flow will cause a 27 cm increase in level at Fairy Falls. Consequently, this increase of water level at Fairy Falls translates upstream to an increase of 31 cm at Baysville Dam. In short, the increase to the Baysville reach due to climate change can be expected to be between 27 cm and 31 cm. Table 9-13: Baysville Computed Water Surface Profile

Water Surface Elevation (CGVD 28 m) Estimate Climate Change Cross Section Flood of Record Change Event (+15% in Water Location Chainage (m) Equivalent Event Flow) Level (m) DS of Baysville Dam 10294 314.40 314.71 0.31 Dickie Street 8846 314.38 314.68 0.30 Yolanda Bay 7982 314.39 314.69 0.30 US of Fairy Falls 230 314.03 314.30 0.27

9.5.4 Purbrook The flow in the Purbrook reach of the South Muskoka River has several dams and reservoirs upstream of it that reduce the peak of the flood and spread the bulk of the flood volume over several days. There is a potential for a 15% increase to flow from the flood of record, based on current trends in the basin.

The Purbrook reach is controlled at the downstream end by the Matthias Dam and in the middle by Rocky Rapids. Table 9-14 and Table 9-15 summarize the increase in water level expected due to climate change in the Purbrook reach. Between the Matthias Dam and Rocky Rapids there is an increase of water level caused by climate change of 0 cm to 7 cm. Upstream of Rocky Rapids the increase in flow causes an increase to water levels of 15 cm to 16 cm. The spillway facilities at the Matthias Dam can manage the selected increase in flow within the current water management plan, therefore increases in mapped water levels at the lower end of the reach near the dam are not expected due to the increase in flow modeled for climate change.

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Table 9-14: Upper Purbrook Computed Water Surface

UTM Coordinates Water Surface Elevation (CGVD 28, m) (Zone 17, m) Change Estimate Climate in Water Flood of Record Change Event Level Location Easting Northing Equivalent Event (+15% Flow) (m) Upstream of Rapids 645089 4986143 294.07 294.23 0.16 White Eagle Trail 645994 4987215 294.12 294.28 0.16 Purbook Road 646289 4988235 294.30 294.45 0.15 Upstream End of 647357 4987944 294.59 294.75 0.16 Model Southern Floodplain 646111 4985080 294.09 294.25 0.16 Near 118 Moes Road 646391 4986996 294.14 294.30 0.16 Island Way 646386 4986649 294.13 294.29 0.16 Purbrook Road 646694 4988247 294.39 294.55 0.16

Table 9-15: Lower Purbrook Computed Water Surface Profile

Cross Water Surface Elevation (CGVD 28 m) Section Estimate Climate Change Chainage Flood of Record Change Event (+15% in Water Location (m) Equivalent Event Flow) Level (m) US Rocky Rapids 13368 294.05 294.2 0.15 DS Rocky Rapids 12521 293.74 293.81 0.07 Carlson Ct 3109 293.51 293.51 0.00 Matthias Dam Headpond 275 293.50 293.5 0.00

9.5.5 Springdale The flow in the Springdale reach of the North Muskoka River has several dams and reservoirs upstream of it that reduce the peak of the flood. There is a potential for a 15% increase to flow from the flood of record, based on current trends in the basin.

The Springdale reach is controlled at the downstream end by High Falls dam. Table 9-16 summarizes the increase in water level expected due to climate change in the Springdale reach. There are no hydraulic controls within the reach and the increase in water level due to the increase in flow is cumulative moving upstream caused the natural headloss in the river. At the most upstream end of the model downstream of Duck Chutes the increase to water level is 27 cm. The spillway facilities at High Falls can manage the selected increase in flow within the current water management plan, therefore increases in mapped water levels at the lower end of the reach near the dam are not expected due to the increase in flow modeled for climate change.

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Table 9-16: Springdale Computed Water Surface Profile

Change in Water Cross Water Surface Elevation (CGVD 28 m) Level (m) Section Estimate Climate Chainage Flood of Record Change Event Location (m) Equivalent Event (+15% Flow) DS of Duck Chutes 17487 272.11 272.38 0.27 North End of Springdale Shores 10984 271.46 271.62 0.16 US Spring Dale Park Rd 8348 271.24 271.35 0.11 South end of Springdale Shores 5538 271.10 271.18 0.08 Harmony Ln 2804 270.92 270.96 0.04 Hwy 11 Bridge 768 270.78 270.78 0.00 High Falls Headpond 181 270.80 270.80 0.00

9.5.6 Bracebridge The Bracebridge reach of the Muskoka River has several dams and reservoirs upstream of it that reduce the peak of the flood. There is a potential for a 15% increase to flow from the flood of record, based on current trends in the basin.

The Bracebridge reach is controlled at the downstream end by Lake Muskoka water level. Table 9-17 summarizes the increase in water level expected due to climate change in the Bracebridge reach. There are no hydraulic controls within the reach and the increase in water level due to the increase in flow is cumulative moving upstream caused the natural headloss in the river. The increase in water level due to climate change increases from 0 cm at Lake Muskoka to 23 cm at the junction of the North Branch and South Branch of the Muskoka River in Bracebridge. In the North Branch the water level increases from 24 cm at the junction of the two rivers to 26 cm at Bracebridge Bay park. The South Branch covers a longer distance upstream and a section of rapids resulting in a larger increase due to climate change up to 33 cm directly downstream of South Falls. Table 9-17: Bracebridge Computed Water Surface Profile

Water Surface Elevation (CGVD 28 m) Cross Flood of Change Section Record Estimate Climate in Water Chainage Equivalent Change Event Level Location River Reach (m) Event (+15% Flow) (m) Bracebridge Bay Park North 27420 227.84 228.1 0.26 Shoreline Dr North 26190 227.75 228 0.25 North Branch US Junction North 24617 227.68 227.92 0.24 DS of South Falls South 16849 228.61 228.94 0.33 Near Muskoka Pine Road South 11099 228.29 228.61 0.32

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Water Surface Elevation (CGVD 28 m) Cross Flood of Change Section Record Estimate Climate in Water Chainage Equivalent Change Event Level Location River Reach (m) Event (+15% Flow) (m) Near Summer Ln South 9492 228.14 228.43 0.29 DS of Rail Bridge South 4152 227.94 228.19 0.25 Ecclestone Dr Bridge South 218 227.60 227.83 0.23 US of Junction South 116 227.60 227.93 0.33 Junction of North and South Downstream 24465 227.65 227.88 0.23 118 West Bridge Downstream 23914 227.58 227.8 0.22 US Kerr Park Downstream 22514 227.52 227.74 0.22 DS Kerr Park Downstream 20438 227.40 227.58 0.18 Nichols Country Ln Downstream 15729 227.16 227.29 0.13 DS Leslie Dr Downstream 10836 226.89 226.95 0.06 Lake Muskoka Delta Downstream 5153 226.7 226.7 0

9.5.7 Port Carling The flow in the Port Carling reach of the Indian River has several large lakes upstream of it that reduce the peak of the flood and spread the bulk of the flood volume over several days. There is a potential for a10% increase to flow above the flood of record, based on current trends in the basin.

The Port Carling reach is controlled at the downstream end by Lake Muskoka. Table 9-18 summarizes the increase in water level expected due to a flow increase in the Port Carling reach of the Indian River. There is no significant increase in water levels due to projected flow increases in this reach. This is because the flow in this reach is relatively small compared to the channel size. There may be an increase in water level in this area due to climate change, but it is not due to an increase in flow in the Indian River. The high-water levels during flood events are caused by the downstream Lake Muskoka water levels. Table 9-18: Port Carling Computed Water Surface Profile

Cross Water Surface Elevation (CGVD 28 m) Section Estimate Climate Change Chainage Flood of Record Change Event in Water Location (m) Equivalent Event (+10% Flow) Level (m) Port Carling Dam Tailwater 12985 226.7 226.72 0.02 West St 11842 226.7 226.72 0.02 US Mirror Lake 8372 226.7 226.71 0.01 DS Mirror Lake 6224 226.7 226.71 0.01 Lake Muskoka 3153 226.7 226.70 0.00

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9.5.8 Bala Reach The flow in the Bala reach of the Muskoka River is controlled by the Bala dam at the outlet of Bala Bay off Lake Muskoka. There is a potential for a 10% increase to flow from the flood of record, based on current trends in the basin.

The water levels in Bala reach are mainly controlled at the downstream boundary by Moon Chutes. As a result, the increase in flow caused by climate change will cause an increase in water level at that boundary. Table 9-19 summarizes the increase in water level expected due to climate change in the Bala reach. The increase of flow will cause a 19 cm increase in level at Moon Chutes. Consequently, this increase of water level at Moon Chutes translates upstream to an increase of 19 cm at Bala Dam. In short, the increase to the Bala reach due to climate change can be expected to be between 19 cm and 20 cm. Table 9-19: Bala Computed Water Surface Profile

Water Surface Elevation (CGVD 28 m) Cross Estimate Climate Change Section Flood of Record Change Event (+10% in Water Location Chainage (m) Equivalent Event Flow) Level (m) Bala Dam Tailwater 19142 221.53 221.72 0.19 US of White Birch Island 15899 221.51 221.71 0.20 US of Juniper Island 12891 221.49 221.68 0.19 DS of Kimberley Island 5409 221.48 221.67 0.19 US of Moon Chutes 3386 221.48 221.67 0.19

9.6 Flood Fringe Determination The only currently accepted method by MNRF for assessment of life safety hazard is evaluated according to the “2x2 rule” as defined in the “Technical Guide – River and Streams Systems: Flooding Hazard Limits” (MNRF, 2002). Permanent residents are assumed to be at risk if the product of the flood flow velocity and the flood water depth at that location exceeds 0.37 m2/s (or 4 ft2/sec, i.e. nominally a velocity of 2 ft/sec times a depth of 2 ft, from which the name of the rule is derived.) Life safety risks are also assumed to occur if the water depth exceeds 0.8 m, or if its velocity exceeds 1.7 m/s, even if the product of the two would not exceed the 0.37 m2/s limit. This accounts for the fact that people can drown once the water becomes deep enough, even at low velocities, and that they can be swept off their feet if the velocity is too high, even at relatively small depths.

Therefore, the floodway as it appears on the flood plain maps is the maximum extent of three lines produced by each of the following criteria; depth greater than 0.8 m, velocity greater than 1.7 m/s and depth x velocity greater than 0.37 m2/s. Nevertheless, some adjustments were made to the extent of the floodway along the outer edges of the floodplain areas that were deemed to be outside of the main flow path.

The flood fringe is the area between the floodway and the floodline boundary. Although the flood fringe may contain small area with depths greater than 0.8 m, they are not considered to

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be part of the floodway because there is limited flow and the area could be filled with engineered soils to reduce the flood depth and provide safe access, provided these activities are approved within municipal planning provisional rules.

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10. Flood Plain Mapping Flood plain maps can have a variety of uses including Emergency Preparedness and Response Plans (EPRPs), mitigation planning, emergency response, consequence assessment and zoning of land development. The mapping of potentially inundated areas during a flood should help authorities in identifying critical infrastructure and population-at-risk sites that might require protective measures, warnings and evacuation planning.

Flood plain maps can be used to facilitate communication during a flood event but can also convey relevant information regarding at-risk areas useful for effective long-term mitigation planning. Base map data provide the background from which flood inundation hazard information is overlain and interpreted. Clear, easy-to-interpret base maps are critical for the effective use of a flood plain map. Additional content is added to annotate key road names and key landmarks to help orient users and aid interpretation of the maps. Planimetric base maps use vector features, such as annotated road lines, political boundaries, streamlines, landmarks, etc., against a typically white background that allow users to orient themselves and interpret the maps.

The key information on a flood plain map is provided by floodline polygons that show the intersection of the peak water surface elevations from the river hydraulic models and static floodline water levels on the lakes with the ground elevations from the terrain source.

Typically, because of the coarseness of the terrain source, the resulting floodline boundaries can be very approximate. However, the floodline mapping provided in this study for urban areas is for the most part quite accurate, due to the use of Bare Earth 1m grid topographic data derived from 10 pts/m2 LiDAR survey data. The non-urban areas use a Bare Earth 1 m grid topographic data derived from 2.5 pts/m2 LiDAR survey data, and therefore less accurate, but still more accurate than topographic data derived from areal photography and very suitable for the purposes of this study. However, the accuracy of the mapping is diminished in areas where there is no clear line of site to the earth surface such as in areas with thick vegetation cover, buildings, dams and bridge infrastructure; in these areas the Bare Earth DEM topographic data only provides approximate elevations due to the interpolation methods used to estimate the elevation of the bare ground surface.

The flood plain maps prepared by this study are based on professional judgment and reasonable care, and upon information available in the Bare Earth 1 m grid topographic data. These maps were reduced to an equivalent scale or 1:2 500 for inclusion in the map sets. This scale is considered to be adequate for presenting the flood line mapping results.

Professional judgement is important for interpretation of the flood lines illustrated on the flood plain maps and will require geomatic ground survey to verify actual flood limits from the water surface elevations provided in this study and the flood plain maps.

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10.1 Lakes The lake flood plain maps were developed in ArcGIS by first identifying, through GIS processing, the contours on the Bare Earth1m grid topographic data associated with the static lake level in Table 7-3 and then the floodlines were interpreted from the contours through professional judgement. The static floodlines were combined with the additional GIS map content provided by District of Muskoka to produce map sets as follow:

• Map Set A: Lake Vernon

• Map Set B: Peninsula Lake

• Map Set C: Fairy Lake

• Map Set D: Mary Lake

• Map Set E: Lake Joseph

• Map Set F: Lake Roseau

• Map Set G: Lake Muskoka

• Map Set H: Bala Bay

• Map Set I: Port Severn

• Map Set J: Spence Lake 10.2 Rivers Flood plain mapping was prepared using combination of HEC-RAS RasMapper and ArcGIS software. Polygons were produced by HEC-RAS RasMapper to represent the floodline and floodway extents. These polygons were imported into ArcGIS and then the floodline and floodway were interpreted from the polygons through professional judgement. The floodline and floodway were combined with the additional GIS map content provided by District of Muskoka to produce the map sets as follow:

• Map Set K: Big East River

• Map Set L: Huntsville Narrows

• Map Set M: South Branch of the Muskoka River at Baysville

• Map Set N: South Branch of the Muskoka River at Purbrook

• Map Set O: North Branch of the Muskoka River at Springdale

• Map Set P: Muskoka River at Bracebridge

• Map Set Q: Indian River at Port Carling

• Map Set R: Bala Reach of the Muskoka River.

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The river flood plain maps in addition to the floodline also include

• Floodway Boundary

• Flood Fringe Extent

• Location of Cross Sections

• Water Levels.

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11. Recommendations and Mitigation Options Due to the many variables that can influence flood modeling and mapping, areas inundated during an actual flood event may differ from the areas shown on the maps. As a precaution, it is to be noted that the floodline locations and limits shown on the map sets must be considered approximate only and are to be used only as a guideline to illustrate potentially impacted areas.

Professional judgement is important for interpretation of the flood lines provided by the flood plain maps. Geomatic ground surveys will be required to verify actual flood limits from the water surface elevations provided in this study and the floodplain maps. 11.1 Wave Uprush Development should be restricted in Hazardous Lands as defined in the MNRF technical guides for flooding hazards. Along shorelines subject to wave action, winds can drive water farther inland, beyond the limit of the static flood line. Horizontal and vertical allowances must be added to the area covered by the static floodline to include the area covered by wind setup and wave uprush to delineate the Hazardous Lands that could be unsafe for development due to naturally occurring processes.

The Technical Guide for Large Inland Lakes (MNR 1996) provides the methods and requirements for determining the required allowances and delineating the Flooding Hazard Limit for inland lakes.

The report FDR128 has recommendations for horizontal and vertical allowances but these are based on average fetch length and shoreline characteristics.

Hatch recommends that site specific or local area studies for sections of lake shoreline should be incorporated into planning studies to provide suitable estimates of the required allowances for delineation of Hazardous Lands. It is also recommended that the new allowances on lakes shores be calculated based on the actual fetch length and shoreline characteristics at each site based on current practices for wind and wave analysis. 11.2 Watershed Management and Education Based on review of the water management studies and flood studies on the Muskoka River Watershed, it is very evident that the dams in the basin do not provide any measure of flood control; the available water storage that can be controlled by the dams is too small compared to the volume of water that runs off the watershed during a significant flood event. Flood events such as the 2013 and 2019 events, or larger, will happen again sometime in the future. There is nothing that can be done within the MRWMP that would reduce the water levels and flooding that occurs during these significant flood events.

It is recommended that more education and understanding of the nature of floods within the basin would help all levels of government and the public gain further knowledge on why flooding occurs and inform everyone of what areas are at risk during flood events.

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Hatch also recommends that due to the increasing frequency of wide spread flooding in the District of Muskoka there is a need for more active flood monitoring. A larger network of climate and flow gauges would provide better information preceding and during a flood event, and the data collected would provide better information for future flood studies and mitigation measure studies. More gauges and area wide geomatic surveys of flood water levels and measurement of flows would provide excellent information for future studies of flooding in the Muskoka River Basin. 11.3 Flood Forecasting and Tracking Climate monitoring is the first and most important step in flood risk management. The addition of a more extensive network of climate stations and flow gauges combined with data available from weather forecasts and radar sensing is required to keep track of snow depths, rainfall, and water flows across the basin. These would then be used for setup and calibration of a flow prediction model and monitoring of water flow and levels in the watershed. Foreknowledge of the magnitude of flooding and tracking of a flood event can help communities take the appropriate action to reduce risks and damages caused by flooding.

A forecasting and real time flood routing model of the watershed may help determine the timing and magnitude of future flood events when coupled with accurate climate forecasts. Many conservation authorities in Ontario depend on flood forecasting to warn communities and manage risks. A similar system for flood risk management in Muskoka is advised to be considered by existing and/or new watershed governance bodies, with particular consideration to be given for resources and expertise required to manage such a system. 11.4 Mitigation Options There are several mitigation options for consideration by all levels of government and the public to reduce future damages due to flooding. All levels of government may use this data to consider their approaches to Official Plans and Zoning By-laws, building regulations, emergency planning, and sizing of new or renewed infrastructure impacted by floods, such roads and bridges.

The main essential mitigation option is the restriction of future development of any vulnerable residential buildings, commercial buildings, or infrastructure on hazardous lands. Vulnerable developments within the hazardous lands are at risk of damage and the lives of the people on those lands are also at-risk during flooding events. Development plans of properties within flood plains need to be restricted and need to include floodproofing measures using established standards and procedures. The Technical Guide, River and Stream Systems; Flooding Hazard Limit - Appendix 6 (MNRF, 2002) provide recommendations for floodproofing.

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Existing areas with buildings, infrastructure and properties within the floodline on the floodplain maps are possibly at risk. There are very few options available for these existing areas other than flood protection and floodproofing using established standards and procedures, or removal of the vulnerable buildings and infrastructure.

The report “An Independent Review of the 2019 Flood Events in Ontario” (Douglas McNeil, 2019) provides a very good review of the 2019 flood event within Ontario and the flooding that occurred in the Muskoka River Watershed. The report provides a number of recommendations to improve technical knowledge of flooding hazards that apply directly to the Muskoka River Basin. The report also provides recommendations for improvement of provincial and municipal policies to restrict development in hazardous lands. As well, the report implies that direction is necessary for incorporating floodproofing measures into the construction plans for structures built in the floodplain such as boathouses. The recommendations provided in the report should be implemented and would provide beneficial measures to mitigate future damages, specifically Recommendation #25 provided by McNeil

“Recommendation #25 That the MNRF review and update the appropriate technical guides, with consideration of a new category permitting development in hazardous lands along large inland lakes, rivers and streams, and along the Great Lakes/St. Lawrence River, utilizing flood protection land forms and/or other forms of flood protection and floodproofing methods with very strict requirements and conditions. Further, consideration should be given to enshrining this concept in legislation or in a regulation along with other structural methods that are now permitted in non-hazard lands or Special Policy Areas.”

11.4.1 Existing Lake Water Levels As previously stated in reports by others, and by dam owners, there are no improvements in dam operation measures beyond what is already being done that would reduce flood water levels.

The only options for reducing flood water levels at the dams is physically increasing the spillway hydraulic capacity at the dam and associated natural controls. These would be very difficult to construct and most likely not financially or environmentally feasible endeavors. A full study of proposed mitigation efforts and potential environmental impacts would be required before implementation.

11.4.2 River Flood Protection Mitigation options for flood protection of buildings along the rivers would require building of berms and improvements to the area drainage system. A full study of each of these proposed mitigation efforts and potential environmental impacts upstream and downstream is required before implementation. Examples for river flood protection are provided below.

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Figure 11-1 illustrates an off channel urban area in Huntsville that could potentially be protected from the impact of high-water levels in Huntsville Narrows. High water could flood back into this area through the urban drainage system. This type of flooding is a consequence of developing low lying land even at a significant distance from the river. A change to the drainage system could prevent this type of backwater flooding of residential properties. A portion of this land could potentially be protected from flood waters by adjusting the path of the drainage ditch between Aspdin Rd and Bickley Country Dr, behind Bickley Ford.

Figure 11-1: Huntsville Flooding from Huntsville Narrows

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Along the North Branch and South Branch of the Muskoka river and the Big East River there are several locations where individual properties, or a group of properties can be protected with a berm or by increasing the elevation of the properties themselves. Figure 11-2 is an example of one of the areas, along the North Branch, where a berm could potentially reduce localized flooding.

Figure 11-2: North Branch of Muskoka River at Springdale Shores

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Appendix A Existing Regulatory Flood Data

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FDR 43 Hydrology Study MMM 1988 Hydrologic model inputs for 100 Year Event:

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FDR 43 Comparison of Historical - Timmins – 100 Year Events:

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FDR 43 Hydrologic Modeling Results:

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FDR 43 Hydrologic Modeling Results:

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FDR 45 Water Management Improvement Study:

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FDR 128 Wave Runup Analysis Results:

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FDR 128 Wave Runup Analysis Results:

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FDR 141 and 142 Muskoka River Flood line Mapping Phase I

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FDR 143 Muskoka River Flood line Mapping Phase II:

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FDR Big East River Flood line Mapping Study:

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Muskoka River Water Management Plan - Background Data:

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Huntsville Area Data Extracted from Public Documents:

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Appendix B Rating Curves

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Rating Curve Boundary Condition at HWY 11 for Upper Big East Model

Elevation Upstream of Highway 11 Bridge (m) Flow (m3/s) 285.3 0 287.15 15.54 287.25 19.98 287.49 29.78 287.69 37.49 287.9 44.39 288.1 53.39 288.33 63.83 288.54 74.28 288.71 83.23 288.83 90.33 289 101.53 289.21 122.41 289.59 160.24 289.65 166.54 289.7 172.85 289.75 179.15 289.8 185.45 289.85 191.76 289.9 198.06 289.95 204.37 289.99 210.67 290.03 216.98 290.07 223.28 290.11 229.58 290.15 235.89 290.2 242.19 290.29 254.8 290.39 269.15 290.5 286.38 290.61 306.48 290.66 317.96

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Rating Curve for ARSP Model Muskoka Lake Water Level

Muskoka Lake WCS Gauge Water Level 02EB006 Elevation (m) Flow (m3/s) 224.40 0 224.48 20 224.56 40 224.64 60 224.72 80 224.81 100 224.89 120 224.98 140 225.07 160 225.16 180 225.25 200 225.34 220 225.44 240 225.54 260 225.63 280 225.74 300 225.84 320 225.94 340 226.05 360 226.15 380 226.26 400 226.37 420 226.48 440 226.59 460 226.71 480 226.83 500 226.94 520 227.06 540 227.18 560

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Appendix C Models Results from River Mapping

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Big East River Model Results - Lower Section

UTM Coordinates (Zone 17, m) Water Surface Elevation Easting Northing (CGVD 28 m) 639119 5026609 290.44 638924 5026820 290.10 639209 5026112 290.36 638847 5026780 289.88 638929 5026432 289.61 638644 5026539 289.59 638989 5025962 289.44 638472 5026387 289.49 638234 5026105 289.38 638574 5026039 289.41 638171 5025797 289.34 638521 5025725 289.34 639158 5025634 289.25 638009 5025480 289.31 638637 5025338 289.29 639124 5025300 289.20 639503 5025322 289.17 640063 5025655 289.06 639888 5025325 289.06 638241 5025253 289.30 637717 5025237 289.31 637882 5025045 289.31 638027 5024813 289.31 637386 5024957 289.31 637386 5024598 289.31 638529 5024987 289.30 639312 5025212 289.17 639612 5024834 289.06 639425 5024799 289.10 639454 5024529 289.06 639213 5024377 289.01 638954 5024097 288.98 639225 5023928 289.00 638904 5023862 288.91 638744 5023735 288.77 638369 5023704 288.59 638381 5023407 288.43 638054 5023490 288.17

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UTM Coordinates (Zone 17, m) Water Surface Elevation Easting Northing (CGVD 28 m) 637733 5023377 287.87 637433 5023624 287.78 637334 5023263 287.62 637400 5022928 287.56 637097 5023062 287.44 636986 5022918 287.27 636970 5022852 287.18 636848 5022743 287.00 636743 5022560 286.80

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Big East River Model Results - Upper Section

UTM Coordinates (Zone 17, m) Water Surface Elevation Easting Northing (CGVD 28 m) 639223 5026619 290.71 639412 5026339 290.82 639585 5026022 290.82 639429 5026574 290.83 639750 5025976 290.82 639610 5026533 290.85 639721 5026166 290.87 639771 5026557 290.87 639977 5026870 290.91 639977 5026545 290.89 640022 5026174 290.89 640179 5026809 290.90 640212 5026265 290.91 640319 5026545 290.94 640311 5025985 290.30 640533 5026829 290.96 640513 5026331 290.97 640508 5026100 290.94 640624 5025865 290.94 640780 5026677 290.96 640706 5026001 290.98 640871 5025997 290.98 640970 5026718 291.41 641172 5026347 291.12 641126 5026512 291.51 641180 5026623 291.51 641234 5026458 291.51 641085 5027056 291.54 641122 5026936 291.52 641287 5026809 291.53 641419 5026463 291.52 641271 5027217 291.56 641464 5026973 291.54 641576 5026578 291.52 641398 5027530 291.57 641740 5026796 291.54 641588 5027649 291.62 641662 5027270 291.60

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UTM Coordinates (Zone 17, m) Water Surface Elevation Easting Northing (CGVD 28 m) 641798 5027138 291.59 641926 5026990 291.62 641856 5027740 291.63 641868 5027406 291.65 642161 5027814 291.69 642128 5027563 291.71 642107 5027353 291.69 642210 5027151 291.67 642354 5027204 291.76 642486 5027583 291.81 642573 5027394 291.82 642647 5027777 291.82 642816 5027604 292.06 642898 5027736 292.09 643080 5027526 292.28 643178 5027406 292.39 643327 5027534 292.65 643549 5027723 292.92

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Huntsville Narrows Model Results

Water Surface Elevation Cross Section Chainage (m) (CGVD 28 m) 16695 286.33 15982 286.31 15360 286.30 14624 286.29 13798 286.29 12819 286.28 8680 286.21 7953 286.19 7586 286.16 7383 286.15 6969 286.03 6491 286.03 6239 286.02 5776 285.95 4713 285.88 4662 285.83 4559 285.78 4488 285.67 4399 285.57 4307 285.47 4139 285.56 3710 285.43 3573 285.49 3145 285.47 2866 285.43 2367 285.39 1498 285.33 1067 285.28 442 285.30

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South Branch of the Muskoka River at Baysville Model Results

Water Surface Elevation Cross Section Chainage (m) (CGVD 28 m) 10294 314.71 10001 314.72 9524 314.69 9261 314.71 8846 314.68 8628 314.69 8329 314.69 7982 314.69 7559 314.68 7396 314.68 6850 314.65 6705 314.60 6564 314.47 6443 314.42 6209 314.51 6008 314.51 4484 314.47 4403 314.44 4268 314.33 4089 314.38 3421 314.39 2891 314.40 2550 314.40 2308 314.38 1870 314.35 1037 314.35 455 314.32 230 314.30

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South Branch of the Muskoka River at Purbrook Model Results – Lower Section

Water Surface Elevation Cross Section Chainage (m) (CGVD 28 m) 13719 294.22 13368 294.20 12860 293.67 12521 293.81 10977 293.70 10777 293.67 10582 293.67 7153 293.57 6695 293.55 6347 293.56 4727 293.52 3823 293.51 3109 293.51 2413 293.51 2197 293.50 1760 293.50 1546 293.50 1380 293.50 275 293.50

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South Branch of the Muskoka River at Purbrook Model Results – Upper Section

UTM Coordinates (Zone 17, m) Water Surface Elevation Easting Northing (CGVD 28 m) 647355 4987946 294.75 647092 4987852 294.69 646957 4988167 294.58 646698 4988246 294.53 646289 4988239 294.45 646428 4987976 294.45 646646 4988010 294.47 646739 4987860 294.47 646781 4987559 294.46 646552 4987634 294.44 646236 4987762 294.42 646184 4987465 294.38 646417 4987402 294.35 646661 4987297 294.33 645993 4987214 294.28 646390 4987000 294.30 646931 4987000 294.30 646953 4986685 294.30 645114 4987188 294.26 645456 4986955 294.27 645899 4986824 294.27 646390 4986644 294.29 647835 4986865 294.30 647351 4986857 294.30 646327 4986362 294.75 645989 4986347 294.27 645820 4986580 294.27 645309 4986583 294.50 644803 4986681 294.24 645565 4985964 294.25 646105 4985908 294.26 645099 4986144 294.23 645445 4985634 294.25 645895 4985510 294.25 646120 4985079 294.25

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North Branch of the Muskoka River at Springdale Model Results

Water Surface Elevation Cross Section Chainage (m) (CGVD 28 m) 17487 272.38 17401 272.36 17246 272.30 16800 272.29 16584 272.27 16105 272.18 15888 272.19 15450 272.17 15008 272.08 14483 272.01 14254 271.99 13556 271.92 13156 271.95 12996 271.83 12839 271.83 11544 271.64 10984 271.62 10814 271.60 10665 271.62 9988 271.59 9400 271.53 8905 271.45 8348 271.35 7804 271.33 7043 271.28 6503 271.23 5969 271.20 5538 271.18 2804 270.96 768 270.78 181 270.80

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Muskoka River at Bracebridge Model Results

Water Surface Elevation River Reach Cross Section Chainage (m) (CGVD 28 m) North 27904 228.11 North 27420 228.10 North 26711 228.03 North 26190 228.00 North 24886 227.93 North 24617 227.92 Downstream 24465 227.88 Downstream 23914 227.80 Downstream 23570 227.79 Downstream 22514 227.74 Downstream 20438 227.58 Downstream 18344 227.43 Downstream 16919 227.30 Downstream 15729 227.29 Downstream 14867 227.19 Downstream 12702 227.03 Downstream 10836 226.95 Downstream 8231 226.94 Downstream 7801 226.91 Downstream 6610 226.83 Downstream 5153 226.70 South 16849 228.94 South 16553 228.93 South 15809 228.88 South 14526 228.79 South 14163 228.79 South 14061 228.77 South 13903 228.76 South 13740 228.70 South 12980 228.63 South 12527 228.60 South 11993 228.59 South 11099 228.61 South 10937 228.60 South 10733 228.55 South 10148 228.53 South 9492 228.43 South 8290 228.39

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Water Surface Elevation River Reach Cross Section Chainage (m) (CGVD 28 m) South 7363 228.37 South 6196 228.29 South 5657 228.24 South 4152 228.19 South 3770 228.16 South 3372 228.15 South 2677 228.11 South 2169 228.11 South 1903 228.09 South 1651 228.03 South 1183 228.08 South 830 228.01 South 615 228.00 South 348 228.03 South 218 227.83 South 116 227.93

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Indian River at Port Carling Model Results

Water Surface Elevation Cross Section Chainage (m) (CGVD 28 m) 12985 226.72 12146 226.71 11842 226.72 11254 226.71 9107 226.71 8903 226.71 8674 226.71 8372 226.71 7254 226.71 6546 226.71 6224 226.71 5839 226.71 5332 226.71 4995 226.71 4659 226.70 4353 226.70 4025 226.70 3614 226.70 3153 226.70

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Bala Reach Muskoka River Model Results

Water Surface Elevation Cross Section Chainage (m) (CGVD 28 m)

19142 221.72 18015 221.72 17408 221.70 16698 221.70 16107 221.70 15899 221.71 13555 221.69 13380 221.65 12891 221.68 12168 221.68 8421 221.68 5409 221.67 3386 221.67

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Appendix D Glossary

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Adverse Effects

As defined in the Environmental Protection Act, means one or more of:

• impairment of the quality of the natural environment for any use that can be made of it;

• injury or damage to property or plant and animal life;

• harm or material discomfort to any person;

• an adverse effect on the health of any person;

• impairment of the safety of any person;

• rendering any property or plant or animal life unfit for use by humans;

• loss of enjoyment of normal use of property; and

• interference with normal conduct of business.

Design Flood

The flood which controls the design of a specific flood related project, i.e. dam, diversion, bank protection, etc.

Development

Means the creation of a new lot, a change in land use, or the construction of buildings and structures, requiring approval under the Planning Act; but does not include activities that create or maintain infrastructure authorized under an environmental assessment process; or works subject to the Drainage Act.

Established Standards and Procedures

Means the following: Floodproofing standard, which means the combination of measures incorporated into the basic design and/or construction of buildings, structures, or properties to reduce or eliminate flooding, wave uprush and other water related hazards along the shorelines of the Great Lakes - St. Lawrence River System and large inland lakes, and flooding along river and stream systems. Recommendations for Floodproofing are provided in The Technical Guide, River and Stream Systems Flooding Hazard Limit - Appendix 6 (MNRF, 2002).

Fetch

Length of water surface exposed to wind.

Flood Fringe

Means the outer portion of the flood plain between the floodway and the floodline limit. Depth and velocities of flooding are generally less severe in the flood fringe than those experienced in the floodway. The flood fringe is the area where development and site alteration may be

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permitted, subject to appropriate floodproofing to the flooding hazard elevation or another flooding hazard standard.

Flood Plain

Flood plain of a river system means the area, usually low lands adjoining a watercourse, which has been or may be subject to flooding hazards.

Great Lakes - St. Lawrence River System:

Means the major water system consisting of Lakes Superior, Huron, St. Clair, Erie and Ontario and their connecting channels, and the St. Lawrence River within the boundaries of the Province of Ontario.

Hazardous Lands

Means property or lands that could be unsafe for development due to naturally occurring processes. Along the shorelines of the Great Lakes - St. Lawrence River System, this means the land, including that covered by water, between the international boundary, where applicable, and the furthest landward limit of the flooding, erosion or dynamic beach hazard limits. Along the shorelines of large inland lakes, this means the land, including that covered by water, between a defined offshore distance or depth and the furthest landward limit of the flooding, erosion or dynamic beach hazard limits. Along river and stream systems, this means the land, including that covered by water, to the furthest landward limit of the flooding or erosion hazard limits.

Large Inland Lakes

Means those waterbodies having a surface area of equal to or greater than 100 square kilometres where there is not a measurable or predictable response to a single runoff event.

One Hundred Year Flood (for river and stream systems)

Means that flood, based on an analysis of precipitation, snow melt, or a combination thereof, having a return period of 100 years on average, or having a 1% chance of occurring or being exceeded in any given year.

One Hundred Year Flood Level

Means:

• For the shorelines of the Great Lakes, the peak instantaneous Stillwater level, resulting from combinations of mean monthly lake levels and wind setups, which has a 1% chance of being equalled or exceeded in any given year.

• In the connecting channels (St. Mary’s, St. Clair, Detroit, Niagara and St. Lawrence Rivers), the peak instantaneous stillwater level which has a 1%chance of being equalled or exceeded in any given year.

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• For large inland lakes, lake levels and wind setups that have a 1% chance of being equalled or exceeded in any given year, except that, where sufficient water level records do not exist, the one-hundred-year flood level is based on the highest known water level and wind setups.

• For rivers and streams, water levels caused by the 100-year flood, resulting in a 1% chance of occurring or being exceeded in any year.

River and Stream Systems

Means all watercourses, rivers, streams, and small inland lakes or waterbodies that have a measurable or predictable response to a single runoff event.

Thalweg

Line connecting the deepest points along a stream channel.

Valleylands

Means a natural area that occurs in a valley or other landform depression that has water flowing through or standing for some period of the year.

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