Memorandum
#1, 6325 – 12 Street SE, Calgary, AB T2H 2K1 T: 403.254.0544 F: 403.254.9186
To:
City of Grande Prairie
Attention:
Richard Sali (City) and Mike Harvard (City)
Cc:
Brian Morrison (ISL)
Reference:
Crystal Lake and Ivy Lake Bathymetry Survey – Inlet and Outlet Inspection Findings
From:
Garnet Dawes, P.Eng., DBIA, Keegan Seddon, CST
Date: Project No.:
November 7, 2017 26803
1.0 Introduction The City of Grande Prairie (the City) is in the process of determining the continued effectiveness of Ivy Lake and Crystal Lake as stormwater management facilities. Crystal Lake (semi natural) and Ivy Lake (manmade) are both located in the northeast sector of the City (respectively) and were designed to manage stormwater from their surrounding neighborhoods. As part of their function, these stormwater management facilities are intended to accumulate sediment. With this, the constructed ponds have naturalized over time. Sediment, detritus vegetation and associated materials, and other debris have accumulated, potentially impacting the stormwater management function and capacity of the ponds, thus reducing their effectiveness as stormwater systems in future. This assessment has been undertaken to document any reductions in volume due to sediment accumulation and to verify potential due diligence measures needed by the City to mitigate potential risks to private and public lands. ISL was engaged to undertake a field survey and assessment of both Crystal and Ivy Lakes. Included as part of the assessment was the analysis of the inlet and outlet structures in both lakes, and recommendations for remediating each structure to ensure the lakes are functioning at an optimal capacity.
2.0 Sediment Accumulation and Lake Function Topographic and bathymetric surveys were completed for both lakes between the dates of May 25 and May 27, 2017. The survey was completed using an unmanned remote controlled survey boat which utilizes a high precision GNSS receiver and a single beam echo sounder. A calibration equipment document is provided for reference in Appendix A following this report. Topography from water’s edge to the 1:100 year HWL was also surveyed to completely model the shape of each lake. Sediment depths were obtained through physical measurements, as well as spot checks on depth obtained from the sonar unit, to verify its accuracy. Upon review of sediment depositions within both Ivy Lake and Crystal Lake, ISL can provide the following statements pertaining to the functionality of each Stormwater Management Facility: Crystal Lake Survey Results Crystal Lake Normal Water Level: 681.75m 1:100 Year High Water Level: 682.20m Top of Sedimentation: ~680.25 (With more sediment accumulation in the west portions of the Lake) Normal Water Volume: 550,910m³ 1:100 year max storage volume: 717,589m³ (Active Storage Volume 166,679m 3)
islengineering.com ISL is proud to be Bullfrog Powered | A Green 30 Employer | One of Canada’s Best Small and Medium Employers M:\26800\26803 - Ivy and Crystal Lake Bathymetry and Assessments\03_Reports\32_Working\26803 Crystal Lake Inlet Outlet Report Draft_Version5.docx
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