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HomeMy WebLinkAboutOES 45-02 . 32 Ciú¡ o~ REPORT TO THE Finance & Operations Committee Report Number: OES 45-02 Date: October 28, 2002 From: Richard W. Holborn, P. Eng. Division Head, Municipal Property & Engineering Subject: Krosno Creek Floodplain Mapping Study Recommendation: 1. That Report OES 45-02 regarding Krosno Creek Floodplain Mapping Study be received; and 2. That staff from the City of Pickering continue working with TRCA to facilitate a public consultation process to advise landowners within the watershed the implications of the study and recommendations for next steps to implement solutions; and 3. That staff be authorized to initiate a Request for Proposal for consulting services to conduct a Class Environmental Assessment for Krosno Creek Floodplain Remediation Strategies, subject to approval of this item in the 2003 Current Budget. Executive Summary: N/A Financial Implications: The Krosno Creek Floodplain Mapping Study was completed by TRCA and their sub-consultants at a cost of $30,000. This was budgeted and paid for in previous years. In order to address the recommendations of the study, the City, in consultation with TRCA should issue a request for proposals to hire a consultant to conduct a Class Environmental Assessment to determine flood remediation strategies and their cost. It is anticipated that $50,000 would be required to complete the Class Environmental Assessment. Staff will identify this item in the 2003 Current Budget, Account 2290-2392 and will proceed upon budget approval. Background: The Krosno Creek Watershed drains an area of approximately 6.5 km2 and outlets to Frenchman's Bay at the bottom of Liverpool Road. The watershed experienced urban development in the early 1960's specifically the Bay Ridges community south of Bayly Street. Due to improvements to the Hwy 401 transportation corridor and the expansion of the Go-Transit facilities, further urban development such as the Town Centre, Liverpool Neighbourhood and the Brock Industrial Area occurred. Report DES 45-02 Date: October 28,2002 33 Subject: Krosno Creek Floodplain Mapping Study Page 2 Presently most of the watershed is urbanized but there remains a few parcels of land to be developed. As Pickering expanded and developed in the 1960's and 1970's, it was evident that without a comprehensive storm water management plan, uncontrolled development was leading to increased flooding potential, erosion and degradation of watercourses. A Master Drainage Study was completed for Pickering in the mid 1980's to set strategies to deal with flood control, erosion and sedimentation, but since urbanization within the Krosno Creek Watershed preceded the Study, implementation of the recommendations was limited. The City is now faced with investigating retrofit strategies for storm water quality and quantity control. After the major storm event of July 1998, the City of Pickering and Toronto and Region Conservation Authority undertook a Flood Plain Mapping Study for Krosno Creek Watershed. This watershed was impacted the most from the storm and experienced property damage due to flooded basements and watercourse erosion. The main purpose of the study was to define the limits of the floodplain associated with Krosno Creek. At the same time that this study was being finalized, the TRCA was commissioned by Ontario Power Generation to prepare the Krosno Creek Preliminary Storm water Management Strategy. The purpose of the strategy is to develop a set of erosion and water quality remediation strategies for environmental enhancements, monitoring and education. The strategy was completed in February 2002. The Krosno Creek Floodplain Mapping Study was completed in March 2002. Both the TRCA and the city of Pickering are presenting the reports to their respective Committees in October 2002. The Krosno Creek Floodplain Mapping Study consisted of creating a hydrologic model for the watershed. The model determined design storm water flows for existing and future land use and evaluated the impacts of future land use on the peak flows at critical locations. This information was used to undertake a hydraulic analysis to determine Regulatory water surface elevations, from which a Regulatory Floodplain was plotted. The Krosno Creek Floodplain Mapping Study has determined that the existing watercourse, consisting of natural open creek, an engineered concrete lined channel and storm sewers, has limited capacity. The model and analysis identifies a total of seventy-nine (79) existing structures (buildings) that will experience flooding (0.01 m to 2.13m) under the Regulatory event. The Regulatory event is described as either the Regional storm (Hurricane Hazel) or 100 year return storm, depending on location in the watershed. Provincial Policy states that the Regional Floodplain be delineated using the greater of the peak flows from the 100 year return storm event and the Regional Storm event. 34 Report OES 45-02 Date: October 28,2002 Subject: Krosno Creek Floodplain Mapping Study Page 3 The Study has also determined that the watercourse crossings (culverts) under the roads exhibit flooding under the five (5) year rainfall event, with the exception of Reytan Boulevard, which overtops at the ten (10) year rainfall event and Liverpool Road, which overtops at the Regional storm event. Future development within the Krosno Creek Watershed is expected to increase peak flows by 17%, which would result in a rise in water surface elevations by 0.5m, without storm water controls in place. Recommendations Based on the results of the Krosno Creek Floodplain Mapping Study, it is recommended that the City of Pickering initiate a Class Environmental Assessment to determine a number of alternative flood remediation strategies to mitigate the existing flooding problem in the watershed. As the cost of such a study would be approximately $50,000, staff will include the expenditure in the proposed 2003 Current Budget and only proceed upon approval of the project. Attachments: 1. 2. Letter from TRCA dated March 28, 2002 Krosno Creek Floodplain Mapping Study Executive Summary Prepared By: Ever Bunt Director, Operations & Emergency Services "'"-- RH:ds Attachments I :\CO UN CIL \0 ES -4 5 -02. docOct -02 Copy: Chief Administrative Officer Recommended for the consideration of Pickering City Council ..~ ~ 0 n s e r va tÎÖnENT # 1 TO REPORT# a;:s 4- <;-0;;" , TORONTO AND REGION 35 March 28, 2002 Richard Holborn Division Head Municipal Property and Engineering Division Operations and Emergency Services Dept. City of Pickering One The Esplanade Pickering, ON L1V 6K7 (;IT,,( , ' " ,-,'('¡"V;ç1)!~~G MUNiCIPAL pf\OPEP.1 v", dJ'" I~¡;:_f\h Dear Mr. Holborn: Re: Krosno Creek Floodplain Mapping Study We are pleased to submit four (4) copies of the fmal Krosno Creek Floodplain Mapping Study report for distribution at your discretion. In order to move forward on the recommendations of the study, we would like to meet with City staff as soon as possible to discuss the next steps for this project and to develop a work plan. It would also be useful to meet withMr. Nick Eyles, of the University of Toronto, to discuss the work they have been carrying out regarding Frenchman's Bay and to coordinate this work with any future initiatives in the watershed. The key recommendation coming out of the Floodplain Mapping Study is that a full environmental assessment (EA) be carried out to evaluate possible solutions for the current flooding problem. We would recommend that the City bring this matter to the attention of Council, identify the EA as a high priority, and provide any available funds to this project to ensure its initiation in 2002. Should you have any questions, please contact the undersigned at (416) 661-6600, extension 5212. Yours sincerely, t,l ,p,>:" ~/{< /,?,.,¡ /'}/'/ ,¿j/' ( , 'tf/ ,IF --1'1/ /C::C //7 .6-V ~-V-' ,../! ~r\..-'>--" / / / Glenn MaeMìllan, c.E.T. Supervisor, Water Management Resource Science Section Watershed Management Division Ene!. 5 Shoreham Drive, Downsview, Ontario M3~'1S4 (416) 661-6600 FAX 661-6898 www.trca.on.ca m,"" 11'(1;/ ""~' ' 36 ATTACHMENT ft ",c2 Iy-Jf DEJ (¡ ç-Q;) Toronto and Region Conservation Authority Floodplain Mapping Study City of Pickering ') .......,d , ,/ ',-,) , ,..-) (/ .. ,-.-¡ ",~j/ ' , , .......~')__I.'"" "..""" I ~ ' '-. Pickering \. \_I'~ Frene'hman's \. .- B'ay'~", ,""'" " , "-.. \ . " . .. - \ ", \\ . I --..~~ Ajq~ '~ ( ";¡ '\ ... ' --- /- ~.-, , " ':;;f,c-.::::1e= , it'""' "-""""-" LillI::! ,~ .t"~"""'¿-d,".""1'h-_, " '. Pîëi<ERtRc K:öns êrvåfío n ~* i&= If"'" iiJ 1i, """'" \""" F 1'\ ~? 7'"""- Prepared by: Toronto and Region Conservation Authority in association with The City of Pickering Date: November 2001 Revised: March 2002 () , '~ ","""() f"'" ++ oES 4-5 - 0.:( IX ~ ¡i,:.:\"' \ I tt_--~. ;t o¡: S 37 EXECUTIVE SUMMARY Background The Krosno Creek Watershed is located within the City of Pickering. The watershed drains an area of approximately 6.5 1an2, outletting to Frenchman's Bay at Liverpool Road. The watershed boundaries are generally formed by Liverpool Road in the west and Brock Road in the east. The northern limit of the watershed is Finch" A venue, and the watershed extends southerly to Frenchman's Bay. In the early 1960s, development within the watershed centred around the Bay Ridges Community south of Bayly Street. Further urbanization occurred north of Highway 401 as a result of improved transportation corridors and the expansion of the GO Transit system into the Pickering area. In the north, the Town Centre and Liverpool Communities were developed and expansion in the east occurred within the Brock Industrial Area. As a result, approximately 71 % of the Krosno Creek W.atershed has been urbanized to date. The absence of significant stormwater treatment and control within the Krosno Creek watershed, has contributed to the degraded state of water quality within the existing Krosno Creek and the downstream receiving Hydro Marsh and Frenchman's Bay. In addition, local flooding problems and increased streambank erosion continue to pose problems for both the area residents and Municipal Works staff. Upon request from the City of Pickering, the Toronto and Region Conservation Authority (TRCA) was commissioned to undertake a floodplain mapping study for the Krosno Creek watershed. The purpose of the study was to define the Regional F100dline and to identify flood hazard areas associated with Krosno Creek. Purpose of the Study Consistent with the Terms of Reference, the purpose of this study was to accurately define the existingandfuttlfe floodlêvêls andtoidetitìfyflöod hazard areas associated with Krosno Creek. The specific study objectivesind1ude t1l~fol1øwingf . to determine design flows for existing and future land use scenarios at critical locations within the study area and to evaluate the impact of projected future land use changes on peak flows; , . to develop a hydraulic model (HEC-RAS) for the open channel portion of Krosno Creek, downstream of Highway 401, in order to determine flood levels and to identify flood hazards associated with established water levels; . to delineate the floodplain limits through the preparation of floodplain mapping and supporting documentation in accordance with Provincial standards; and 3,8 . éÀ ¡CREPORT #C£S 4- 5' -0 d 30(5 . to identiîy future study requirements and to outline the next steps in developing a comprehensive remediation strategy. Scope and Approach The scope of the work completed for the study related to three components: hydrologic analysis, hydraulic analysis, and floodplain mapping. . collection and review of relevant background infonnation relating to hydrològy, hydraulics and mapping; . . field reconnaissance to collect infonnation relating to the hydrologic analysis (i.e., local topography and land use characteristics, major system flow paths and status of roof leader connections); . field reconnaissance to collect. infonnation relating to the hydraulic analysis (i.e., culvert . dimensions, channel type, channel roughness and overbank roughness); . preparation of digital base mapping (south of Highway 401) using 1 :8000 aerial photography taken in April 1993 (mapping prepared by J.D. Barnes Limited); . develop a hydrologic model for the watershed using the Visual OTTHYMO computer model to obtain peak flow estimates for the 2, 5, 10,25,50, 100 year and Regional stann events, for bOtll existing and future land use scenarios, at critical locations within the watershed; . develop a hydraulic model for the watershed using the HEC-RAS computer model to calculate flood levels for the 2,5, 10,25,50, 100 year and Regional stonn events; and . delineate the Regional Floodplain and prepare digital floodplainmapsheets, for the open channel portion of the watershed, in accordance with the Ministry of Natural Resources' Technical Guidelines for Floodplain Management (2001). Flood Vulnerable Sites (FVSs) I . ¡ Flood vulnerable sites were determined by comparing the actual ground elevation at the structure and the computed water surface elevations. It'W~~,ê$tÌ1Œ).atêdJl1at~appròxirnately'V9bui1dings (sites) are susceptible to flooding 'under the Regulatory event (combination of 100 yr and Regional depending on location in the watershed). The minimum level of flooding observed was 0.01 m and the maximum level of flooding observed was 2.13 m. Flood Vulnerable Roads (FVRs) The Krosno Creek channel and associated structures were originally designed and constructed to convey a 5-year return period flow. However, based on the findings ofthe hydraulic analysis, some of the crossings within the study area will overtop. under the 5-year event. It is important to note that, during the site visits, significant sediment and debris build up were observed at a number of "'" ", ' ;¿ .. t ¡i; óES /¡~ -ô¡;t 39 lj--oF 5 crossings within the watershed. Remo:)lal of thisdeþris,.and regular inspection to prevent further build up, would improve hydraulic conveyance. Floodline Delineation The Regulatory water surface elevations were derived from the hydraulic computations (Le.,HEC- RAS analysis). The Regulatory Floodplain was plotted on 1:2000 scale, digital maps in standard, fonnat according to the Provincial Flood Damage and Reduction Program specifications. Conclusions 1. The hydrologic model can be considered representative of the study area given the results of the model calibration/verification exercise. 2. The 100 year, Chicago type design storm was selected as the Regulatory event for those portions of the watershed downstream of Highway 401 and upstream of the Hydro Marsh. The Regional .Storm (i.e., Hurricane Hazel)was selected as the Regulatory event for the Hydro Marsh area to the outlet at Frenchman's Bay. The selection of the Regulatory event is based on the MNR policy which states that the Regulatory storm is the greater of the 100 year or the Regional event peak flows. 3. Given the limited capacity of the existing watercourse, .~ total of 79 structures will experience flood conditions (0.01 m to 2.13 m) under the Regulatory event. 4. All of the water course crossings exhibit some flooding under the five year rainfall event with the exception of Reytan Boulevard which overtops at the ten year and Liverpool road which overtops under the Regional event. 5. Peak flows are expected to increased by 17% under future development conditions and water surface elevations will also rise by 0.5 m without stormwater controls in place. Recommendations It is recommended that: 1. A full Environmental Assessment be carried out to assess a number of alternative flood remediation strategies (Le. diversions, stormwater controls, channel improvements) to alleviate or eliminate the existing flooding problem in the watershed. 2. Additional monitoring data be collected and used to further validate and refine the hydrologic model. ¡ ¡ t 3. Çity staff carry out an irnmediatËjJ.1§PË~ti9n.P~~:x;iêtiJ.1g..9!!gs.singsandremove any debris build up. In addition, aregûîârìnspð6ffõh'âhãfff¡!Wi~îJ.á1iêêprogram should be developed to prevent further build up of sediment anddebrisuþstrea1nofthe structures. 40 ATTACHMENT if _<?S- 5'oF5 , ;'O~Lf-5-0;) 4. The TRCA and the City ofPiç¡kering adopt a water management policy which states that future development (i.e., Greenfie1dandlr1fí11) within the watershed must provide 2-100 year, post to pre-development water quantity control. 5. The Krosno Creek Floodline mé;lps :be updated to include a draft Regulated Fill Line pursuant to Sêêtìön28ôft1îêtonservlitìÒh-..4u.tb,orities Act. Finalization and registration oftms fill line will proceed in accordance with the Conservation Authorities Act and the approved Generic Regulation.