: DESIGNATED FLOOD LINE – CARROT CREEKpbtech.org/.../6.4_Designated_Flood_Lines_-_Carrot_Creek.pdf · Flood Line along Carrot Creek and update the Land Use Bylaw accordingly. Background

City Council Agenda

June 16, 2014 / Page 1 File No.: 608-2-1

CITY COUNCIL AGENDA REPORT

Subject: DESIGNATED FLOOD LINE – CARROT CREEK Recommendation(s) That Administration bring forward a Land Use Bylaw amendment in October 2014 establishing the Designated Flood Line for Carrot Creek based on the “Factor of Safety” line as designated on Attachment 1 of the June 16, 2014 Agenda Report.

Purpose of Report Administration is requesting Council direction in order to establish the Designated Flood Line along Carrot Creek and update the Land Use Bylaw accordingly. Background and Discussion A number of reports on the flood line around Big Lake, along the Sturgeon River and Carrot Creek have been developed over the years. These formed the basis for establishing the City’s Designated Flood Line along the Sturgeon River and Big Lake, which was approved by Council in 2011. A brief summary of these reports is included in Attachment 2. With development interest growing in the City’s west end along Carrot Creek, the Designated Flood Line needs to be extended north along Carrot Creek’s west side and captured in the City’s Land Use Bylaw. Through the reports, four separate ‘flood lines’ have been determined based on different ‘elevations.’ The term “elevation” may be misleading as this Designated Flood Line does not remain at the same ‘above sea-level’ elevation throughout St. Albert but may actually vary in height at different points due to topography, fill and development, and other natural changes to the river basin. The four separate ‘flood lines’ include:

City Council Agenda

June 16, 2014 / Page 2 File No.: 608-2-1

Flood line Description ‘Original’ 1:100 flood line Prior to 2004, the City of St. Albert used the Canada Alberta Flood

Damage Reduction Agreement as the basis for flood management. This was a map developed in 1991 that indicated where the 1:100 year flood elevation was located (the “original” 1:100 flood line). Permitted development related to this elevation was done in accordance with the Canada Alberta Flood Damage Reduction Agreement.

‘Updated’ 1:100 flood line In 2004, the Big Lake Basin Task Force, comprising representatives from municipalities within the Big Lake Drainage Basin, prepared the Big Lake Stormwater Management Plan. This provided recommendations about floodplain delineation and a hydrological model of the basin to estimate current and future flow rates. A revised 1:100 year flood elevation (the “updated” 1:100 flood line) was proposed to reflect development and fill within the drainage basin and other natural changes within the basin. This revised elevation is about 0.5 m higher than the original. This line formed the basis for the current Designated Flood Line.

‘Factor of Safety’ flood line This ‘flood line’ was identified in the 2004 Study to allow for uncertainties in the analysis and to provide a zone above the design flood that would protect riparian habitat and provide a buffer to reduce flood velocities and damage from both the channel and overland stormwater draining to the channel. Estimates of increases in water levels as a result of land use changes in the overall basin and siltation were estimated at between 0.14 m and 0.3 m, and this line, approximately 0.5 m higher than the ‘updated’ 1:100 flood line, was recommended.

‘Freeboard’ line This line is not so much a ‘flood line’ as a construction level elevation. All building openings, such as basement windows would need to be higher than this elevation.

Designated Flood Line for Carrot Creek The existing Designated Flood Line is based predominantly on the ‘updated’ 1:100 flood line. This was appropriate along the Sturgeon River given the existing development and fill that had already been located along the river over time, and the freeboard elevation was sufficient to allow for any potential rise in water levels. In reviewing the potential Designated Flood Line along Carrot Creek, both the ‘updated’ 1:100 and the ‘Factor of Safety’ lines were assessed, see Attachment 1. Administration is proposing that the Factor of Safety elevation line be used as the basis for the Designated Flood Line along Carrot Creek for a number of reasons:

• Carrot Creek is a natural drainage channel for a very large man-made catchment area that extends as far north as Morinville. There are a number of upstream considerations that need to be assessed and addressed for the potential impact on Carrot Creek during a major storm event, that were not taken into account in the 2004 study when assessing impact. The City is currently collaborating on a study to assess the impact development in the City and the County will have on Carrot Creek.

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• Recommended release rates into Carrot Creek are very low; much lower than typical release rates, therefore there are capacity concerns for the creek.

• The land adjacent to Carrot Creek is very flat, elevation changes are farther apart than along the Sturgeon River.

• The loss of developable land is minimal if the ‘Factor of Safety’ line is used, 2.6 ha of land, but provides a greater reduction of risk while minimizing the impact on development potential.

• The City requires a 50 m buffer along the Creek to extend Red Willow Park. The ‘Factor of Safety’ line allows for a small portion of this buffer to be located below the proposed floodline, which therefore could be acquired as ER. This would allow for slightly more MR land to be made available throughout the adjacent neighbourhood but would have a minimal impact on the area for trails, parks or recreation sites.

Stakeholder Communications or Engagement Once amendments to the Land Use Bylaw are prepared, standard circulation to various stakeholders and affected landowners will be undertaken. Implications of Recommendation(s) a) Financial:

• None at this time

b) Legal / Risk: • None at this time

c) Program or Service: • None at this time

d) Organizational: • None at this time

Alternatives and Implications Considered If Council does not wish to support the recommendation, or to add to it, the following alternatives could be considered:

a) Alternative 1. Council could provide direction to base the Designated Flood Line on the ‘new’ 1:100 year flood line, which is 0.5 m higher than the ‘old’ 1:100 year flood line used by the Province. This would be the same approach used along the Sturgeon River currently defined in the Land Use Bylaw. This would result in more Municipal Reserve required to maintain the 50 m buffer along Carrot Creek (Red Willow Park) and increase potential developable land by 2.6 ha. The anticipated rise in water levels

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June 16, 2014 / Page 4 File No.: 608-2-1

could be accommodated by the Freeboard elevation. As Carrot Creek has more sensitive issues related to its catchment size and release rates, there may be greater risk for flooding.

b) Alternative 2. Council could provide direction to adjust the Designated Flood

Line around Big Lake to the ‘Factor of Safety’ line, similar to that proposed along Carrot Creek. It is currently based on the ‘updated’ 1:100 flood line. There isn’t a significant difference between the two lines and much of the area is or is expected to be part of the Lois Hole Provincial Park, which would limit development.

Strategic Connections a) Council’s Strategic Outcomes and Priorities (See Policy C-CG-02)

• CULTIVATE ECONOMIC PROSPERITY: A diversified, robust and resilient economic foundation to support growth and community service delivery.

• CULTIVATE A HISTORIC, CREATIVE, AND ACTIVE COMMUNITY: A vibrant and involved community with a variety of culture, recreation and heritage opportunities.

• CULTIVATE A GREEN COMMUNITY: A healthy natural environment for future generations that preserves and promotes enjoyment, conservation and responsible development.

• CULTIVATE EXCELLENCE IN GOVERNMENT: A responsive, accountable government that delivers value to the community.

• CULTIVATE SUSTAINABLE INFRASTRUCTURE AND SERVICES: A growing community that has balanced development and management of civic facilities, transportation networks and related services.

b) Long Term Plans (e.g. MDP, Social Master Plan, Cultural Master Plan, etc.)

• IDP • MDP • Utility Master Plan • Environmental Master Plan

c) Corporate Objectives (See Corporate Business Plan)

• Deliver programs and services that meet or exceed our standards d) Council Policies N/A e) Other Plans or Initiatives (Business Plans, Implementation Strategies, etc.)

• N/A Attachment(s) 1. Carrot Creek Proposed Designated Flood Line – ‘Updated’ 1:100 and ‘Factor of

Safety’ 2. Summary of flood line reports

City Council Agenda

June 16, 2014 / Page 5 File No.: 608-2-1

Originating Department(s): Planning & Development, Sustainability & Environment,

Engineering Author(s): General Manager Approval:

Carol Bergum, Director P&D David Hales, GM P&E

City Manager Signature:

Date:

I

STURGEON COUNTY CITY OFST. ALBERT

0 375 750 1,125 1,500187.5Meters Big Lake

G:\MAPS\FloodMapping\2014 Flood Risk Mapping - Carrot Creek\Carrot Creek DFL Delineation\Factor of Safety Line Delineation - Lidar Data\Carrot Creek Designated Flood Line - Updated_1_100andFactorOfSafety.mxd

Carrot Creek Designated Flood Line -'Updated' 1:100 and 'Factor of Safety'

City BoundaryUpdated Flood Line (lidar generated)Factor of Safety (Design) Flood Line (lidar generated)Schedule G Designated Flood (LUB 9/2005 - Bylaw 25/2011)50M Carrot Creek Buffer (MDP)Lost Developable Area with Factor of Safety2.61ha

ATTACHMENT 1

ATTACHMENT 2

Summary of Flood line reports Date Report Description Prior to 2004

Canada Alberta Flood Damage Reduction Agreement

This was a map developed in 1991 that indicated where the 1:100 year flood elevation was located (the “original” 1:100 flood line). Permitted development related to this elevation was done in accordance with the Canada Alberta Flood Damage Reduction Agreement.

2004 Big Lake Basin Stormwater Management Plan

The Big Lake Basin Task Force, comprising representatives from seven municipalities within the Big Lake Drainage Basin, prepared the Big Lake Basin Stormwater Management Plan. This provided recommendations about floodplain delineation and a hydrological model of the basin to estimate current and future flow rates. A revised 1:100 year flood elevation (the “updated” 1:100 flood line) was identified to reflect development and fill within the drainage basin and other natural changes within the basin. This revised elevation is about 0.5 m higher than the original. In addition, a “design” 1:100 flood line was proposed with an elevation 1.0 m higher than the original flood line, providing a greater factor of safety (and is often referred to as the “factor of safety” floodline).

2007 Big Lake Basin Task Force: Floodplain Delineation for the City of St. Albert – Sturgeon River

The City of St. Albert commissioned the Big Lake Basin Task Force: Floodplain Delineation for the City of St. Albert – Sturgeon River Final Report by SamEng Inc. This study delineated the Sturgeon River floodplain within the City using a series of measurement points, or cross-sections for both the original 1:100 year flood elevation, the “updated” 1:100 flood line and the “design” (factor of safety) flood level from 2004.

2010 Sturgeon River Floodplain Management Policy Review

The City of St. Albert commissioned the Sturgeon River Floodplain Management Policy Review by ISL Engineering. This study provided a critical review of current floodplain management practices and provided the rationale towards the support of policies and principles that would provide a reasonable level of flood protection in the long term. The report focused on built areas of St. Albert particularly the downtown area where the design flood line would impact existing buildings. The report recommended using the “updated” 1:100 flood line for developed areas.

2011 Land Use Bylaw Update – Designated Flood Line Map

Mapping of the Designated Flood Line was completed for the developed areas of St. Albert from Big Lake to the City’s eastern boundary. This line was based on the developed areas flood line and incorporated previously approved fill within the floodplain in the Oakmont and Riverside neighbourhoods. The mapping detail was also updated using more recent LIDAR data to refine the estimated flood lines. This mapping did not include the undeveloped areas around Carrot Creek.

2014 Flood Review Report (in progress)

The Engineering Department has commissioned a study to assess risk from flooding from the river and within older neighbourhoods. Initial work is suggesting that risk of flooding from the river is minimal, but risk of flooding in established neighbourhoods from storm events is greater due to low levels of service.