Welcome Grindstone Creek Flood Hazard Mapping Study The first Public Information Centre (PIC) was held November 8 th , 2018 and focused on obtaining public input to inform the study. The purpose of this evening’s PIC is to: Present draft study results (Flood Hazard Mapping) Answer questions on what this means for you and your property Identify what you can do to limit your flood risk Obtain public input on draft mapping Thank you for attending. If you would like to be included on the For questions, please contact: project mailing list, please sign in. Amy Mayes, P.Eng. Project Manager Additional information is available at: Conservationhalton.ca/floodplainmapping Conservation Halton AMayes@hrca.on.ca Please return all comments by March 3 rd , 2020 Phone: 905.336.1158 x 2302 DISCLAIMERS All information provided will be subjected to Freedom of Information and Protection of Privacy Act, and with the exception of personal information, it may be released upon request. Public information will be included within the final study report. The Grindstone Creek Floodplain Mapping Program received support through the National Disaster Mitigation Program. The views expressed in this material are the views of Matrix Solutions Inc. and Conservation Halton, and do not necessarily reflect the views of the Province or the Government of Canada. Grindstone Creek Flood Hazard Mapping - Public Information Centre #2 February 2020
Flooding Types Flooding is a natural occurrence – resulting when flow exceeds a system’s capacity. There are many types of flooding: Riverine Flooding – when a river overflows its banks Urban/Pluvial Flooding – when excess rainfall overloads the local drainage system Sewer Backup – when flows exceed the design capacity of the pipes and back up into homes or onto streets Coastal Flooding – when storm surge, waves, tides or other factors increase water levels along the shore Seepage – when water pressure is high enough to force groundwater through foundation cracks The Grindstone Creek Flood Hazard Study identified Riverine Flooding Grindstone Creek Flood Hazard Mapping - Public Information Centre #2 February 2020
Why Map the Floodplain? Understanding the hazard is the first step in building flood resiliency Understanding the flood hazard is important for: Flood forecasting and warning Emergency preparedness and response (City of Burlington, August 2014) Prioritizing potential flood mitigation projects Informing infrastructure planning Community and land use planning Floodplain mapping updates increase safety and viability in our communities, benefiting all landowners and residents. (City of Burlington, August 2014) Grindstone Creek Flood Hazard Mapping - Public Information Centre #2 February 2020
Roles in Flood Prevention and Management This study was completed in partnership with watershed Municipality Municipalities, who participated on a Technical Advisory Committee, • Monitor watershed conditions & forecast Landowner as each agency has a role in flood prevention and management flooding • Operate & manage dams & flood control channels • Protect life & property through natural hazard regulation • Coordinate and provide emergency • Review & comment on stormwater Conservation response management strategies for subwatershed • Complete subwatershed-based Halton studies, re-zonings and planning applications community planning • Implement watershed restoration activities • Review, comment and approve re-zoning • Maintain models defining flood risk across the and development applications watershed-based jurisdiction • Own and maintain stormwater Municipality infrastructure (ponds, roads, ditches, & Region sewers, etc.) • Protect water quality and public health • Plan and manage greenspaces and Landowner • Know the risks: Is the property flood natural heritage systems susceptible? Is flooding expected? • Make a plan: What can you do to protect your family and your property? • Get a kit: Do you have supplies for 72 hours? Grindstone Creek Flood Hazard Mapping - Public Information Centre #2 February 2020
Hurricane Hazel The Regulatory Flood Hazard is defined by the floodplain associated with the greater of: Hurricane Hazel or 1:100 year storm event (a storm with a 1% chance of occurring in any given year) Hurricane Hazel was a historic storm event that struck Toronto in 1954 81 Ontarians lost their lives and 4,000 families were left homeless (MNRF 2002) The effects of the hurricane in Toronto were significant due to: heavy rainfall in weeks preceding the storm extent of floodplain development lack of flood mitigation measures (hurricanehazel.ca) Grindstone Creek Flood Hazard Mapping - Public Information Centre #2 February 2020
Study Objectives The focus of this study was to: assess how Grindstone Creek and its tributaries would respond to storms like Hurricane Hazel and the 1:100 year storm, as the larger of these two storms defines the mapped floodplain; and develop flood hazard maps Floodplain Maps To understand the flood hazard, we needed to analyze: what peak flow will result from specific rainfall events; Hydraulic what are the flow pathways, water levels, and velocities Model associated with regulatory flows; Flood what is the impact of climate change on flood risk; and, Hydrology depth and Base Data velocity what is the extent of the regulatory flood hazard over the land Model & Flow rate Guidelines (mapping the floodplain) and timing Building blocks of floodplain mapping The study complies with Provincial Guidelines. At the end of the study, the report and flood hazard maps will be available for public review (spring 2020). Grindstone Creek Flood Hazard Mapping - Public Information Centre #2 February 2020
Study Area Description The watershed covers a diverse landscape encompassing large wetland areas, the Niagara Escarpment, rural lands, and settlement areas watershed area - 90 km 2 settlement areas - Waterdown, Aldershot, Millgrove, Flamboro Centre, Clappison’s Corners large wetlands in the watershed headwaters the Niagara Escarpment transects the southern portion of the watershed 2020 ) Grindstone Creek Flood Hazard Mapping - Public Information Centre #2 February 2020
Previous Flood Hazard Study The previous flood risk study was completed in 1983 Modelling has since been updated on a piecemeal basis to support local floodplain alterations and major land use changes Significant technical advances since 1983 allow greater analytical complexity, giving a better understanding of flood risk Comprehensive modelling and mapping is necessary to support identification of the flood hazard Grindstone Creek Flood Hazard Mapping - Public Information Centre #2 February 2020
Incoming Spill Flows Spills occur when floods exceed the capacity of a valley system, and overflow. Excess flood flows from one watershed travel overland, and can contribute flow to another system. Spills from adjacent watersheds may impact peak flows within Grindstone Creek. Potential spills into Grindstone Creek were assessed at Bronte Creek at Highway 6 and Falcon Creek at CN Rail Bronte Creek We built hydraulic models to assess and quantify this spill Grindstone Creek watershed (white line) During the Regional event, Bronte Creek spills into Grindstone Creek adjacent to Highway 6 Spencer Creek watershed This figure shows how the flows from Bronte Creek move through the Grindstone Creek watershed to the adjacent Spencer Creek watershed (Hamilton Conservation Authority). The colours represent the water depth. Falcon Creek We built a hydraulic model to Falcon Creek assess the potential spill between Grindstone Creek, Falcon Creek and Indian Creek Results indicate no incoming spill from Falcon Creek into Grindstone Creek Grindstone Creek Flood Hazard Mapping - Public Information Centre #2 February 2020
Hydrology → How were peak flows predicted? To generate flow estimates, a model that considered the Data Inputs impact of soils, climate, drainage patterns and land use was Climate Soil type built and refined (calibrated) to more closely match observed 20 flow conditions at the Water Survey Canada Aldershot Gauge. 18 16 14 Rainfall (mm) Hydrology Model 12 10 8 6 Rainfall Intensity (mm/hr) 4 2 0 0 1 2 3 4 5 6 7 8 9 10 11 12 Time (hr) Modelled vs. Observed Flows Land use Catchments Flow Estimates Topographic information (which defines the ground surface elevation) was collected using LiDAR technology in the spring of 2018. This information has a vertical accuracy (95%) of ±6.6 cm for smooth hard surfaces. Grindstone Creek Flood Hazard Mapping - Public Information Centre #2 February 2020
Hydraulics → How Were Flood Levels Predicted? A hydraulic model was built to predict water surface elevations and flooding extents The hydraulic model evaluates the impact of bridges and culverts, valley shape, obstructions (such as buildings), and vegetation conditions to predict the water surface elevation and flow velocity associated with a given flow. Hydraulic Inputs Hydraulic Model Survey and represent structures Flow Inputs from hydrology model Account for building obstructions (CTV 2020) Water Levels + Flooding Extents Define cross sections from topography (valley shape) Grindstone Creek Flood Hazard Mapping - Public Information Centre #2 February 2020
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