HARVIE HEIGHTS CREEK PRELIMINARY DEBRIS -FLOOD AND DEBRIS-FLOW MITIGATION DESIGN FINAL PROJECT NO.: 1286 -006 May 25, 2015 081,&,3$/',675,&72)%,*+25112 +$59,(+(,*+76&5((. 35(/,0,1$5<'(%5,6)/22'$1''(%5,6)/2: 0,7,*$7,21'(6,*1 ),1$/ 352-(&712 ',675,%87,21 '$7( 0D\ 0'RI%LJKRUQ1R FRSLHV '2&80(1712 0' %*& HFRS\ Suite 500 - 980 Howe Street Vancouver, BC Canada V6Z 0C8 Telephone (604) 684-5900 Fax (604) 684-5909 May 25, 2015 Project No.: 1286006 Dale Mather, Flood Recovery Director Municipal District of Bighorn No. 8 PO Box 310 2 Heart Mountain Drive Exshaw, AB T0L 2C0 Dear Mr. Mather, Re: Harvie Heights Creek Preliminary Debris-Flood Mitigation Design – FINAL Please find enclosed a preliminary engineering report that provides conceptual-level designs and cost estimates for measures that are intended to reduce risks associated with floods and debris floods at developed areas of the Harvie Heights Creek fan. Risk reduction is being planned for a debris flood with a magnitude similar as the 2013 event, which has been assigned a return period in the range of 300 to 1000 years. This report is intended to be supporting documentation that is required for the Alberta Community Resilience Program Grant Application. We hope this report meets your requirements. Should you have any questions or comments, please do not hesitate to contact me at the number listed above. Yours sincerely, BGC ENGINEERING INC. per: Matthias Jakob, Ph.D., P.Geo. Senior Geoscientist Municipal District of Bighorn No. 8, Harvie Heights Creek May 25, 2015 Preliminary Debris-Flood and Debris-Flow Mitigation Design – FINAL Project No.: 1286006 EXECUTIVE SUMMARY This report describes the preliminary engineering design for debris-flood mitigation on Harvie Heights Creek, located on the northeast side of the Bow River valley within the Municipal District of Bighorn No. 8 (the MD). The report is focused on mitigation measures recommended to reduce economic losses to the Hamlet of Harvie Heights. The report summarizes the Harvie Heights Creek hazard and risk assessments (BGC 2015a; 2015b) and presents the mitigation design intent and basis. Details are provided regarding the proposed preliminary debris-flood and debris-flow mitigation design elements, including the purpose of the proposed mitigation, technical details, cost estimates, and recommended further site investigation, and design work. A risk-based approach to mitigation design has been selected in contrast to a hazard-based approach (in which a return period is selected and mitigated against with no evaluation of potential consequences). Risk is estimated based on the consequences of the various hazard scenarios. Through iteration, the “design event” is then defined as the lowest return period hazard scenario that will reduce life loss risk or potential economic losses to levels deemed tolerable by the MD. The following individual geohazard risk tolerance criteria were selected in consultation with the MD: Maximum 1:10,000 (1x10 -4) risk of fatality per year for existing developments Maximum 1:100,000 (1x10 -5) risk of fatality per year for new developments. These targets are in line with standards applied in some jurisdictions in British Columbia and internationally. BGC’s risk assessment indicated that the existing loss of life risk is tolerable for all parcels of land in the Hamlet of Harvie Heights (BGC, 2015b). Long-term debris-flood mitigation is therefore proposed to reduce economic risks, to meet resident and stakeholder expectations regarding protection from the 2013 event, and to fulfill the principle of further reducing residual loss of life risk to “as low as reasonably practicable” . Additionally, risk reduction on Harvie Heights Creek will benefit transport of goods and services on Trans- Canada Highway and CP Rail, whose economic consequences of blockages were not quantified by BGC. For preliminary design, an event with an estimated return period of 300 to 1000 years, which is similar to the 2013 debris flood, has been selected as th e ‘design event’. Events with greater magnitude are conceivable. However, the extra capital expenditures needed to completely eliminate risk for the largest credible event do not appear warranted given the results of BGC’s risk assessment. Therefore, in case of an event exceeding the magnitude of that in 2013, some damage can still be expected. \\bgc-van-file\data\BGC\Projects\1286 MD Bighorn\006 DFHRA - Harvie Heights Creek\07 - Long-term Mitigation Concepts\Reporting\2_FINAL\01_Text\HarvieHeights_Mitigation_Report_Final.docx Page i BGC ENGINEERING INC. Municipal District of Bighorn No. 8, Harvie Heights Creek May 25, 2015 Preliminary Debris-Flood and Debris-Flow Mitigation Design – FINAL Project No.: 1286006 Technical parameters for the debris-flood design event, including sediment volume and peak discharge, were established using a hazard assessment frequency-magnitude analysis (BGC, 2015a), and are summarized in Table E-1. Table E-1. Design event parameters used for conceptual design, with a return period of 300 to 1000 years, which is similar to the 2013 event. Item Harvie Heights Creek Debris -flood peak discharge at upper fan apex (m 3/s) 470 Sediment volume mobilized at lower eastern fan area (m 3) 5,000 Mitigation options were evaluated considering stakeholder interests, cost, environmental impacts and land ownership. The proposed long-term mitigation design concept follows the same logic and objectives of the short-term design, and in general seeks to upgrade the short- term design elements. The proposed mitigation design has two principal objectives, including: To protect residences within the Hamlet of Harvie Heights from events originating from the upper portions of Tower Creek fan and Harvie Heights creek fan. To prevent flooding of businesses and a water treatment plant located in the low lying area along the Trans-Canada Highway between Harvie Heights Creek fan and Tower Creek fan. The proposed long-term debris-flood and debris-flow mitigation design consists of four elements, which ought to be designed and constructed in unison and should thus not be viewed as independent mitigation strategies: 1. North Harvie Heights Diversion Berms - A network of berms along the northern extent of the Hamlet of Harvie Heights is proposed to protect residents from Tower Creek and Harvie Heights Creek flows that avulse from their existing channels. The proposed berms follow the general alignment of the short-term mitigation ditch and berm network, and direct surface water into Quarry Creek. 2. Harvie Heights Saddle Berm – A natural ridge at the upper extent (northeast) extent of the kame terrace protects a portion of the Hamlet of Harvie Heights from direct impact by Harvie Heights Creek flows. However, a low point in this ridge is susceptible to being overtopped by creek flows. The proposed saddle berm increases the height and extent of the short-term berm, and protects against surface water flow through this topographic low point. 3. Harvie Heights Culvert Protection at Trans-Canada Highway – Flooding of the low-lying area along the highway will occur if the Harvie Heights Creek culvert beneath the highway is blocked by sediment. Two measures are proposed to protect the culvert from sediment, including increasing the height of the short-term sediment barrier at the culvert inlet, and re-grading the transition point from the natural channel to the short- term sediment basin to reduce sediment mobilization caused by knick-point erosion. \\bgc-van-file\data\BGC\Projects\1286 MD Bighorn\006 DFHRA - Harvie Heights Creek\07 - Long-term Mitigation Concepts\Reporting\2_FINAL\01_Text\HarvieHeights_Mitigation_Report_Final.docx Page ii BGC ENGINEERING INC. Municipal District of Bighorn No. 8, Harvie Heights Creek May 25, 2015 Preliminary Debris-Flood and Debris-Flow Mitigation Design – FINAL Project No.: 1286006 4. Water Treatment Plant Berm and New Culvert – To further protect the water treatment plant from flooding, a new culvert beneath the Trans-Canada Highway, and berm oriented perpendicular to the highway are proposed. These are located on the southeast side of the water treatment plant. If the existing Harvie Heights Creek culvert beneath the highway becomes blocked or if flow exceeds the existing culvert capacity, the proposed new culvert provides an outlet to the Bow River, without flooding the highway or railway, and the berm protects the culvert from sedimentation. Items 1 and 2 protect residences within the Hamlet of Harvie Heights from flows originating from the upper portions of Tower Creek fan and Harvie Heights Creek fan. Items 3 and 4 protect against flooding of low-lying areas along the Trans-Canada Highway, including the water treatment plant. The proposed design is illustrated in Drawings 01 through 03. The total conceptual-level cost estimate for all components of this mitigation strategy on Harvie Heights Creek is $1.5 million, and itemized cost tables are provided in Table 5-1 and Appendix A. \\bgc-van-file\data\BGC\Projects\1286 MD Bighorn\006 DFHRA - Harvie Heights Creek\07 - Long-term Mitigation Concepts\Reporting\2_FINAL\01_Text\HarvieHeights_Mitigation_Report_Final.docx Page iii BGC ENGINEERING INC. Municipal District of Bighorn No. 8, Harvie Heights Creek May 25, 2015 Preliminary Debris-Flood and Debris-Flow Mitigation Design – FINAL Project No.: 1286006 TABLE OF CONTENTS EXECUTIVE SUMMARY ...................................................................................................... i TABLE OF CONTENTS ....................................................................................................