Welcome Grindstone Creek Flood Hazard Mapping Study The first Public - - PowerPoint PPT Presentation

Grindstone Creek Flood Hazard Mapping - Public Information Centre #2 February 2020

Welcome

Grindstone Creek Flood Hazard Mapping Study

The first Public Information Centre (PIC) was held November 8th, 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 project mailing list, please sign in.

Additional information is available at: Conservationhalton.ca/floodplainmapping Please return all comments by March 3rd, 2020

For questions, please contact:

Amy Mayes, P.Eng. Project Manager Conservation Halton AMayes@hrca.on.ca 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

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

Flooding is a natural occurrence – resulting when flow exceeds a system’s capacity. There are many types of flooding:

Grindstone Creek Flood Hazard Mapping - Public Information Centre #2 February 2020

Why Map the Floodplain?

Floodplain mapping updates increase safety and viability in our communities, benefiting all landowners and residents. Understanding the flood hazard is important for: Flood forecasting and warning Emergency preparedness and response Prioritizing potential flood mitigation projects Informing infrastructure planning Community and land use planning

Understanding the hazard is the first step in building flood resiliency

(City of Burlington, August 2014) (City of Burlington, August 2014)

Grindstone Creek Flood Hazard Mapping - Public Information Centre #2 February 2020

Roles in Flood Prevention and Management

Municipality Landowner

• Monitor watershed conditions & forecast

flooding

• Operate & manage dams & flood control

channels

• Protect life & property through natural hazard

regulation

• Review & comment on stormwater

management strategies for subwatershed studies, re-zonings and planning applications

• Implement watershed restoration activities
• Maintain models defining flood risk across the

watershed-based jurisdiction

• Coordinate and provide emergency

response

• Complete subwatershed-based

community planning

• Review, comment and approve re-zoning

and development applications

• Own and maintain stormwater

infrastructure (ponds, roads, ditches, sewers, etc.)

• Protect water quality and public health
• Plan and manage greenspaces and

natural heritage systems

• Know the risks: Is the property flood

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?

Conservation Halton Municipality & Region Landowner

This study was completed in partnership with watershed Municipalities, who participated on a Technical Advisory Committee, as each agency has a role in flood prevention and management

Grindstone Creek Flood Hazard Mapping - Public Information Centre #2 February 2020

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

• ccurring 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

Hurricane Hazel

(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 To understand the flood hazard, we needed to analyze: what peak flow will result from specific rainfall events; what are the flow pathways, water levels, and velocities associated with regulatory flows; what is the impact of climate change on flood risk; and, what is the extent of the regulatory flood hazard over the land (mapping the floodplain) 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).

Floodplain Maps

Base Data & Guidelines Hydraulic Model Flood depth and velocity Hydrology Model Flow rate and timing Building blocks of floodplain mapping

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 km2 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

• n 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

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 During the Regional event, Bronte Creek spills into Grindstone Creek adjacent to Highway 6

Incoming Spill Flows

Falcon Creek

We built a hydraulic model to assess the potential spill between Grindstone Creek, Falcon Creek and Indian Creek Results indicate no incoming spill from Falcon Creek into Grindstone Creek

Falcon Creek Grindstone Creek watershed (white line)

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.

Spencer Creek watershed

Grindstone Creek Flood Hazard Mapping - Public Information Centre #2 February 2020

Hydrology → How were peak flows predicted?

Soil type Land use

Data Inputs Hydrology Model Modelled vs. Observed Flows

Climate

To generate flow estimates, a model that considered the impact of soils, climate, drainage patterns and land use was built and refined (calibrated) to more closely match observed flow conditions at the Water Survey Canada Aldershot Gauge.

Catchments

Flow Estimates

2 4 6 8 10 12 14 16 18 20 0 1 2 3 4 5 6 7 8 9 10 11 12 Rainfall (mm) Time (hr)

Rainfall Intensity (mm/hr)

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?

Define cross sections from topography (valley shape) Account for building obstructions Survey and represent structures

Water Levels + Flooding Extents Hydraulic Model Flow Inputs

from hydrology model

(CTV 2020)

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

Grindstone Creek Flood Hazard Mapping - Public Information Centre #2 February 2020

Spills Between Watersheds

The study assessed potential spills between watersheds Spills are an overflow from one watershed to another Flows in Grindstone Creek are not reduced downstream of spills per Provincial guidelines This figure shows where flows are spilling into Grindstone Creek watershed (red arrows) and out of Grindstone Creek watershed (green arrows) during the Regional event

Grindstone Creek Flood Hazard Mapping - Public Information Centre #2 February 2020

Draft Flood Hazard Mapping

This is the Index Map for the draft flood hazard mapping Paper copies of the flood hazard map sheets are available for viewing Digital version of the current and updated (draft) mapping is available for viewing at computer stations