MEMO. Erich Knechtel (TRCA) Abe Khademi, Maggie Ding, Alyssa Mohino-Barrie (WSP)

Transcription

1 MEMO TO: FROM: CC Erich Knechtel (TRCA) Abe Khademi, Maggie Ding, Alyssa Mohino-Barrie (WSP) Steve Heuchert (TRCA), Kristy Shortall (Metropia), Jonathan Karavos (Metropia), Adolfo Emer (GHD) SUBJECT: 1856 Notion Road Riparian Storage Volume & Floodplain Context DATE: First Submission-December 11, 2017; Updated January 16, 2018 This memorandum expands upon the riparian storage discussion from the meeting at TRCA s offices on November 16, 2017, in relation to the proposed development of the property at 1856 Notion Road in Pickering. The purpose of the memo is to develop the characterization of the riparian storage assessments completed to date and facilitate ongoing dialogue to inform and advance the development approval process. The minutes of the November 16 meeting are attached for reference. EXISTING RIPARIAN STORAGE AND FLOODPLAIN CONDITIONS - SUMMARY As mentioned in the previous memo (November 13, 2017) areas within the subject site are susceptible to flooding during the Regulatory Storm Event (Hurricane Hazel) associated with Duffins Creek. Key takeaway points are: The regulatory floodplain is fairly broad compared to the channel width. The topography at the west bank of the creek is too flat to confine the floodplain within the stream corridor, and the floodplain within the site is a low terrain rather than flow conveyance path, as the existing development north of the site interferes with the flow pathway. The site is outside of the 100-year and 350-year floodplain. During the Regulatory Flood, the site s riparian storage is about 1.1% of the total storage of the localized 150 m long river section. The estimation of riparian storage of the site between and masl based on the TRCA floodplain map is summarized in Table 1. The site has a total 6,920 m 3 of riparian storage at elevation m, which should be maintained under post-development conditions to provide sufficient flood protection and maintain water surface elevations, eliminating impact within the floodplain area. wsp.com

2 Table 1 Riparian Storage Estimation Existing Conditions BOTTOM ELEVATION (m) TOP ELEVATION (m) INCREMENTAL STORAGE (m 3 ) CUMMULATIVE STORAGE (m 3 ) ,354 3, ,487 7, ,463 14,281 SITE GRADING AND PROPOSED RIPARIAN STORAGE To eliminate flood risk the proposed development area will be elevated above m, which is the regulatory flood elevation (84.44 m) plus freeboard. To compensate for the lost storage associated with the placement of fill in this area, the designated Open Space / Park Block at the eastern portion of the site will be graded to provide the necessary riparian storage. Due to the limited amount open space, the depth of the proposed Open Space blocked is lowered to a bottom elevation of 82.4 masl to accommodate the required riparian storage volume. Below the elevation of 83.0 m, a total of 1, 806 m 3 storage volume is provided. The estimation of riparian storage of the site between and m is summarized in Table 2. A total 7,438 m 3 storage is provided at elevation m. Table 2 Riparian Storage Estimation Post Development BOTTOM ELEVATION (m) TOP ELEVATION (m) INCREMENTAL STORAGE (m 3 ) CUMMULATIVE STORAGE (m 3 ) ,806 1, , ,130 3, ,196 5, ,256 6, ,319 7, ,382 9,085 Page 2

3 Comparing the cumulative storage volume at elevation m of the pre-development and postdevelopment condition, there is excess storage volume provided under post-development conditions (see Table 3). Although the incremental storage does not match, the riparian storage provided will continue to provide the required level of flood flow attenuation. Table 3 Comparison of Riparian Storage CUMMULATIVE STORAGE (m 3 ) Feature Elevation (m) Pre-development Post-development Regulatory Flood Elevation (RFE) ,677 7, m Freeboard Above RFE ,899 8,808 It is therefore anticipated that the overall effects of the development will have no profound effect on the watershed s water surface elevations provided the quantity control of the site is maintained, and will have no significant upstream or downstream impacts to the adjacent properties or Highway 401. HYDRAULIC ANALYSIS A new cross section (XS26.195) was added to the TRCA hydraulic model (HEC-RAS) in order to reflect the alternation of site topography. The location of cross section was revised to accommodate the new cross section and perpendicular to the contours. The location of these two cross sections are shown in Figure 1. The geometry of the new cross section in the scenario of existing condition reflects the pre-development grades shown on the TRCA floodplain map; it was revised in the scenario of post-development to represent the proposed site grading plan. The plots of XS under existing and proposed conditions are included as Figure 2 and Figure 3. The geometry of XS26.19 was obtained from the TRCA floodplain map and used in both scenarios. The comparison of hydraulic results of these two model scenarios demonstrates that the proposed grading plan will not have any adverse hydraulic impacts on upstream or downstream areas. The model output is included in the attachment. CONCLUSION The proposed development requires fill to safeguard the future community from the regulatory floodplain. The introduction of fill requires compensation for lost storage to avoid hydraulic impacts to upstream or downstream areas. The introduction of a riparian storage zone will provide the necessary compensation for the lost storage. Introduction of the proposed grading into the hydraulic model has confirmed that there will be no upstream or downstream changes in flood elevation resulting from the development. Page 3

4 The material provided in this document is intended to present a riparian control concept with an understanding that this strategy is to be examined further by senior TRCA staff, to continue the dialogue process and advance the development opportunities of the site. Respectively submitted, Maggie Ding, P.Eng. Senior Project Engineer, Water Resources Infrastructure - WSP Page 4

5 Floodplain Limit Duffins Creek Floodplain Limit Revised xs26.19 Subject Site Original xs26.19 (removed from model) New xs (NOT FOR SCALE) Figure 1 - Location of New XS and Revised XS26.19

6 88 86 Duffine Creek Catchment 26gt Plan: Existing-New XS /4/2017 RS = Legend WS Regional Storm Ground Bank Sta 84 Elevation (m) Station (m) Figure 2 Cross Section _Existing Condition

7 88 86 Duffine Creek Catchment 26gt Plan: Proposed-New XS /7/2017 RS = Legend WS Regional Storm Ground Bank Sta 84 Elevation (m) Station (m) Figure 3 Cross Section _Proposed Condition

8 ATTACHMENT - HEC-RAS Model Output_Existing and Proposed Condition HEC-RAS River: DUFFIN CREEK Reach: Reach-2 Reach River Sta Profile Plan Q Total Min Ch El W.S. Elev Crit W.S. E.G. Elev E.G. Slope Vel Chnl Flow Area Top Width Froude # Chl (m3/s) (m) (m) (m) (m) (m/m) (m/s) (m2) (m) Reach year Exisiting Reach year Proposed Reach year Exisiting Reach year Proposed Reach year Exisiting Reach year Proposed Reach year Exisiting Reach year Proposed Reach year Exisiting Reach year Proposed Reach year Exisiting Reach year Proposed Reach Regional Storm Exisiting Reach Regional Storm Proposed Reach year Exisiting Reach year Proposed Reach year Exisiting Reach year Proposed Reach year Exisiting Reach year Proposed Reach year Exisiting Reach year Proposed Reach year Exisiting Reach year Proposed Reach year Exisiting Reach year Proposed Reach year Exisiting Reach year Proposed Reach Regional Storm Exisiting Reach Regional Storm Proposed Reach year Exisiting Reach year Proposed Reach year Exisiting Reach year Proposed Reach year Exisiting Reach year Proposed Reach year Exisiting Reach year Proposed Reach year Exisiting Reach year Proposed Reach year Exisiting Reach year Proposed Reach year Exisiting Reach year Proposed Reach Regional Storm Exisiting Reach Regional Storm Proposed Reach year Exisiting Reach year Proposed Reach year Exisiting Reach year Proposed Reach year Exisiting Reach year Proposed Reach year Exisiting Reach year Proposed Reach year Exisiting Reach year Proposed Reach year Exisiting Reach year Proposed Reach year Exisiting Reach year Proposed Reach Regional Storm Exisiting Reach Regional Storm Proposed Reach year Exisiting Reach year Proposed Reach year Exisiting Reach year Proposed Reach year Exisiting Reach year Proposed Reach year Exisiting Reach year Proposed Reach year Exisiting Reach year Proposed Reach year Exisiting Reach year Proposed Reach year Exisiting Reach year Proposed Reach Regional Storm Exisiting Reach Regional Storm Proposed Reach year Exisiting

9 HEC-RAS River: DUFFIN CREEK Reach: Reach-2 (Continued) Reach River Sta Profile Plan Q Total Min Ch El W.S. Elev Crit W.S. E.G. Elev E.G. Slope Vel Chnl Flow Area Top Width Froude # Chl (m3/s) (m) (m) (m) (m) (m/m) (m/s) (m2) (m) Reach year Proposed Reach year Exisiting Reach year Proposed Reach year Exisiting Reach year Proposed Reach year Exisiting Reach year Proposed Reach year Exisiting Reach year Proposed Reach year Exisiting Reach year Proposed Reach year Exisiting Reach year Proposed Reach Regional Storm Exisiting Reach Regional Storm Proposed Reach year Exisiting Reach year Proposed

10 Minutes Prepared by: Jonathan Karavos Meeting Date: November 16, 2017 Meeting Purpose: Meeting Location: TRCA Meeting, Notion Road re: Wetland, Riparian Storage, Stormwater Management TRCA Offices, Vaughan ON Participants: Kristy Shortall Metropia (KS) Jonathan Karavos Metropia (JK) Ryan Guetter Weston Consulting (RG) Abe Khademi WSP (AK) Alyssa Mohino-Barrie WSP (AMB) Julianna MacDonald Beacon Environmental (JM) Gretel Green TRCA (GG) Steve Heuchert TRCA (SH) Erich Knechtel TRCA (EK) Copy to all attendees and the following: Steven Warsh Chestnut Hill Developments Michael Bennett Weston Consulting Adolfo Emer GHD John Lopez GHD Francois Chartier EXP Chana Steinberg Beacon Environmental 1.) Wetland discussion - JM provided brief summary of the proposed Wetland replacement strategy. Have not settled on size of replacement. The previously existing wetlands were dominated by nonnative Phragmites. The proposed wetland replacement will be of better quality than the previously existing wetland. - GG advised that she is satisfied with the concept and location of the proposed wetland location. Further details such as proposed plant material, swamp thicket screening at top-of-bank and a sketch to be provided in the EIS through Draft Plan/Zoning submission. - KS asked whether the TRCA would be willing to accept an easement in favour of the TRCA over the wetland replacement lands instead of having ownership of the land. SH

11 2 advised that he will inquire with the TRCA s property group, however the TRCA typically prefers to have ownership of the property. - RG indicated that this could be maintained by the Condo Corporation. 2.) Riparian Storage - EK noted that the proposed riparian storage solution in the memo met the volume requirements, however it is TRCA criteria that the riparian storage be accommodated within the elevations shown in the TRCA flood mapping. The riparian storage proposed in the memo is below the depth of the TRCA flood mapping and would not be acceptable to the TRCA. - SH advised that the site requires a secondary access point outside of the floodplain. KS explained that the site can be accessed through the proposed entrance at the south property line, through the adjacent commercial development. - EK advised that once there is a riparian storage solution for the site, the development group will be responsible for updating the TRCA flood mapping. - EK expressed concern about some of the existing grades along the North property line near Block 6 and Block 7 that are below the flood protection elevation of 84.4masl. KS and RG advised that City staff will not allow retaining walls to address the issue so the development team will explore solutions such as transitional grading onto municipal ROW and terracing. 3.) Stormwater Management - GG asked that WSP explore additional LID measures such as rain gardens, bio-swales and measures to address evapotranspiration on the site. - GG asked that details of the watercourse at the Southeast corner of the site be included in submission materials. - EK asked whether the proposed non-permeable pavement would drain towards the proposed permeable pavement. - EK raised the topic of Water Balance and Erosion Control. It was clarified that 5mm from all site surfaces must be retained above initial extractions. Next Steps: - SH to speak to ask Property Group whether an easement in favour of the TRCA over the proposed wetland is a possibility. - WSP to investigate the incremental riparian storage volumes to determine the deficit and report to EK with Hydraulic Analysis. - JM to finalize details of proposed wetland replacement for EIS submission.