Technical Memorandum

Final Report for: LACOMBE COUNTY, PONOKA COUNTY, CITY OF LACOMBE, TOWN OF BLACKFALDS MASTER DRAINAGE PLAN For the Wolf Creek and Whelp Brook Watersheds Date: August 31, 2014 #302, 4702 – 49 Avenue Red Deer, AB T4N 6L5 Phone: 403-348-8340 Fax: 403-348-8331 Lacombe County May 31, 2014 RR 3 File: N:\42\10\014\00\R01-1.0 Lacombe, Alberta T4L 2N3 Attention: Dale Frietag Manager of Planning Services Dear Mr. Freitag, Re: Master Drainage Plan for the Wolf Creek and Whelp Brook Watersheds We are pleased to submit the Master Drainage Plan for the Wolf Creek and Whelp Brook Watersheds. This document includes a summary of the undertaken methodologies and analyses, and the resulting recommended design criteria and action plan for stormwater management in the Wolf Creek and Whelp Brook watersheds. We recommend this document be submitted to Alberta Environment and Sustainable Resource Development for approval. If you have any questions or require clarification, please call me at (403) 314-6129. We enjoyed working together on this project and look forward to working with you in the future. Yours truly, MPE ENGINEERING LTD. Peter Stevens, P.Eng., FEC, FGC (Hon.) Senior Project Manager PS/pp Lacombe City; Ponoka City; Lacombe; Blackfalds Master Drainage Plan for the Wolf Creek and Whelp Brook Watersheds EXECUTIVE SUMMARY Wolf Creek and its major tributary Whelp Brook are the natural watercourses which drain significant portions of four municipalities: Lacombe County, Ponoka County, City of Lacombe, and Town of Blackfalds. Since the area was settled, efforts by individuals and government were made to improve drainage and reduce flooding (ESRD, 2013(a)). Over the past few years, significant rainfall events and various types of development in the watershed have led to renewed concerns of local flooding and erosion expressed by the general public and the municipalities. In 2012, the four municipalities formed the “Wolf Creek Watershed Study Committee” (WCWSC) to manage a coordinated effort for watercourse improvements and guidance for future development. To that end, the WCWSC retained MPE Engineering Ltd. (MPE) to prepare this Master Drainage Plan (MDP) for the Wolf Creek and Whelp Brook watersheds. The criteria for future stormwater management systems and facilities required for development in the Wolf Creek watershed are described in this MDP. The adopted guiding principles for design of the storm water management system include: minimize flooding, erosion, and water quality impacts in the watershed; avoid wetlands where possible, otherwise minimize impacts on wetlands and mitigate where required; minimize impacts on the receiving stream, Battle River, as well as downstream development; meet or exceed regulatory requirements for stormwater management. Overall, stormwater runoff will be managed with minor systems and major systems, as required in the Alberta Environment and Sustainable Resource Development (ESRD) 2006 “Standards and Guidelines for Municipal Waterworks, Wastewater & Storm Drainage Systems”: The minor systems will be consisting of buried pipe systems with a capacity to convey the 1:5 year storm event flows. The major system will consist of curbed or ditched streets with a capacity to convey up to the 1:100 year storm event, and stormwater management facilities. ii Lacombe City; Ponoka City; Lacombe; Blackfalds Master Drainage Plan for the Wolf Creek and Whelp Brook Watersheds The discharge rates from stormwater management facilities will be limited to the approved allowable release rates based on the estimated pre-development discharge rates in the watershed. The natural hydrographs at the outlets of each of the watercourses will be maintained as closely as possible to minimize the impact on the downstream receiving watercourse, Battle River. Because insufficient streamflow records exist for the Wolf Creek watershed, their pre-development discharges were estimated using a regional stream flow analysis of hydrologically similar basins with long-term stream flow records, and modelling the Wolf Creek and Whelp Brook channels using HEC-RAS. Results suggest the estimated 1:100 year pre-development unit runoff rate for the Wolf Creek subwatersheds is 2 L/s/ha. Runoff conditions of the existing and future developed areas of the Wolf Creek watershed were compared using representative hydrographs from selected subwatersheds and routed using the HEC- RAS model. Results suggest that, under post-development conditions, an allowable unit release rate of 2 L/s/ha would not increase peak flows in the Wolf Creek and Whelp Brook, and the receiving stream, Battle River. (In comparison, an allowable unit release rate of 5 L/s/ha would increase peak flows in the Wolf Creek and Whelp Brook by about 10%, and about 1% in the receiving stream, Battle River, assuming coincident peaks.) To meet ESRD guidelines, stormwater management facilities will be required to meet the approved allowable release rate as well as the water quality requirements. As well, each facility will also be sized to remove 85% of the sediment loading (greater than 75 μm) from the development area prior to being released. Where feasible, each facility will have a permanent pool with a volume of at least the runoff generated from a 25 mm rainfall event. Determining whether a forebay would be functional in each storm facility will be done during the design phase. The adopted guiding principles for design are sensitive to the fact that a greater volume of runoff can be expected under post development conditions compared to pre-development conditions. That is, the duration of elevated discharge rates will be increased under post development conditions, and may iii Lacombe City; Ponoka City; Lacombe; Blackfalds Master Drainage Plan for the Wolf Creek and Whelp Brook Watersheds increase the risk of erosion in the receiving watercourses. To remedy this, Low Impact Development (LID) will be encouraged and source control practices (SCP) will be implemented where practical, to reduce the total volume of stormwater runoff. Future stormwater management facilities will be located to avoid wetlands disturbance. In circumstances where impacts to wetlands cannot be averted, the stormwater facilities will be constructed and operated to mimic the wetlands function or compensation approved by the provincial government will be provided. By adopting the above-described best management practices, the impacts of the proposed development in the Wolf Creek watershed will have negligible impact on Battle River, both quantitatively and qualitatively. The runoff characteristics of the existing watercourses through the Wolf Creek watershed under post-development conditions will be similar to those under pre-development conditions, as demonstrated by the estimated hydrographs presented herein. The design, construction, and implementation of operation plans of future stormwater management systems will need to be appropriately planned by addressing the relevant issues and design criteria presented herein. Integral to the successful operation of the stormwater management system is the conveyance capacity of Wolf Creek and Whelp Brook channels. A maintenance program to remove flow impediments (such as deadfall, debris, beaverdams) is therefore an essential component of this plan. It is recommended that this Master Drainage Plan be submitted to Alberta Environment and Sustainable Resource Development for approval. iv Lacombe City; Ponoka City; Lacombe; Blackfalds Master Drainage Plan for the Wolf Creek and Whelp Brook Watersheds TABLE OF CONTENTS LETTER OF TRANSMITTAL CORPORATE AUTHORIZATION EXECUTIVE SUMMARY 1.0 INTRODUCTION .......................................................................................................................................... 1 1.1 SCOPE OF ANALYSIS............................................................................................................................................. 1 2.0 WATERSHED DESCRIPTION ......................................................................................................................... 3 2.1 PHYSIOGRAPHY .................................................................................................................................................. 3 2.2 CLIMATE ........................................................................................................................................................... 4 2.3 WETLANDS ........................................................................................................................................................ 4 2.4 LAND USE ......................................................................................................................................................... 5 2.5 EXISTING STORMWATER MANAGEMENT SYSTEMS ..................................................................................................... 5 2.6 PROPOSED DEVELOPMENTS .................................................................................................................................. 5 3.0 STORMWATER MANAGEMENT CONSIDERATIONS ..................................................................................... 6 3.1 EFFECTS OF URBANIZATION .................................................................................................................................. 6 3.2 STORMWATER MANAGEMENT CONCEPTS ................................................................................................................ 6 3.3 STORMWATER QUALITY REQUIREMENTS

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