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Master Environmental Servicing Plan July 2013�Windfields Planning Area – West of Simcoe Street

A3.0 SUMMARY OF EXISTING CONDITIONS

The preparation of a Master Environmental Servicing Plan requires a good understanding of the existing environmental conditions within and adjacent to the study area boundaries. The majority of this information was assembled and documented during the preparation of the background report for the Part II Plan. The following sections summarize the relevant information and update based upon the current work, where appropriate.

A3.1 SURFACE WATER

As noted earlier, the Windfields Planning Area is generally located within subwatersheds WS and ES of Oshawa Creek. This MESP for lands west of Simcoe Street deals with subwatershed WS which is drained by tributaries W1 and W2 and the West Branch of Oshawa Creek. The following sections describe the existing hydrologic, hydraulic and geomorphologic conditions within subwatershed WS and the noted tributaries.

A3.1.1 Hydrology

Pre-development Model Development

A hydrologic model of pre-development conditions was prepared for the study area as part of the background report for the Windfields Planning Area Part II Plan. A previous version of OTTHYMO model (DOS based) for the Oshawa Creek Watershed Study originally developed by TSH in 1995 was obtained from the City of Oshawa and was updated by using the Visual OTTHYMO version 2.0 model (Windows based). The catchments from that original OTTHYMO model were further discretized in order to compute the existing flow rates at the flow points along the watercourses within the MESP study area. Figure A3.1.1 shows the drainage boundaries for the entire Oshawa Creek watershed from the original Oshawa Creek Watershed Study (TSH, 1995). Figure A3.1.2 shows the discretized catchment boundaries for the current MESP study. For the modelling purposes, there are a total of 16 flow points along the study watercourses. The locations of the flows points are shown in Figure A3.1.2. Table A3.1.1 shows a brief description of these flow points.

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Master Environmental Servicing Plan July 2013 Windfields Planning Area – West of Simcoe Street

TABLE A3.1.1 FLOW POINTS DESCRIPTION

Flow Points

Part II Plan Flow Points

Branch Location

1 A

West Main

Main Branch at Thornton Rd.

21 B Small Tributary to Main Branch

31 C Small Tributary to Main Branch

4 D Main Branch 500m upstream of Britannia Ave.

51 E Small Tributary to Main Branch

6 F Main Branch at Britannia Ave.

7 G Main Branch at Conlin Rd before Confluence with W2

8 X2

W2 Tributary W2 at Britannia Ave.

9 H Tributary W2 at Conlin Rd

10 I West Main

Main Branch at Conlin Rd after Confluence with W2

11 J

W1

Tributary W1 at Winchester Road

12 P2 Tributary W1 at Future Road

13 K Tributary W1 upstream of Britannia Ave.

141 L Small Tributary to Main Branch

15 M Tributary W1 downstream of Britannia Ave.

16 N Tributary W1 at Conlin Road

17 Q2,3 West Main

West Main / W1 / W2 Confluence (South of Conlin Road)

18 W3

Main

West / East Main Branch Confluence

19 Y3 Goodman Creek Confluence

20 Z3 Outlet to Lake Ontario 1 where the runoff generated from the local drainage area discharges to the receiving water courses (W1, W2

or West Main). 2 Additional FPs to Part II Plan. 3 FPs south of Windfields MESP area (South of Conlin Rd. downstream to Lake Ontario), where a comparison

of flow rates under ultimate development scenario (SC 5) and pre-development conditions are investigated.

The catchment parameters including the soil curve number (CN) and initial abstraction (Ia) were maintained from the parent sub-watershed in the original Oshawa Creek watershed model. The time to peak (Tp) was revised from the original values based on the Two-Parameter equation that was presented in the Oshawa Creek Watershed Study (TSH, 1995). Table A3.1.2 presents the Visual OTTHYMO model parameters for the study area under the pre-development conditions. Detailed catchment parameters can be found in Appendix A.3.1a. The pre-development model schematic is shown on Figure A3.1.3.

Precipitation Data Consistent with the design storms used in the previous studies (e.g., Oshawa Creek Watershed Management Plan, CLOCK, 2002 and Windfields Planning Area East MESP, MMM, 2003), design storms with a 4-hour Chicago distribution from the Oshawa Creek Watershed Study (TSH, 1995) were applied in the model. In the watershed study, an areal reduction factor of 0.85 was applied to the 1- to 100-year design storm events. For reference purposes, Appendix A3.1b includes a comparison of rainfall depths for all available design storms from various sources (e.g., Environmental Canada, C ity of Oshawa’s Municipal Design Standard IDF, etc.)

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The last 12 hours of Hurricane Hazel were simulated as the Regional Storm for the study area. The saturated antecedent moisture condition (AMC III) was used to simulate the wet soil condition at the beginning of the Regional Storm. For the Oshawa Creek watershed with a total drainage area of approximately 120 km2, the total rainfall amounts were reduced by applying an areal adjustment factor of 0.963 1. To be consistent with the MESP for the Windfields Planning Area East, the same reduction factor was used in this study. Table A3.1.3 shows the total rainfall depths for the storms to be used for the pre-development hydrology model. The hyetographs of the 100-year 4-hour Chicago storm and Hurricane Hazel Regional Storm are illustrated in Figures A3.1.4a and A3.1.4b respectively. More detailed rainfall information can be found in Appendix A.3.1b.


TABLE A3.1.2 PRE-DEVELOPMENT HYDROLOGY MODEL PARAMETERS

Existing Sub Basin

VO2 Catchment #

Area (ha) CN II CN III Ia / DPSP (mm)

Tp (hr)

Main Branch

Upstream 3040 3792.00

W30 30 120.00 80 91 1.5 0.880

W3000 3000 4.03 80 91 1.5 0.074

W3001 3001 6.98 80 91 1.5 0.107

W3002 3002 19.08 80 91 1.5 0.193

W3003 3003 12.61 80 91 1.5 0.155

W3004 3004 18.43 80 91 1.5 0.253

W3005 3005 36.03 80 91 1.5 0.364

W20 20 140.00 76 89 1.5 0.720

W2000 2000 24.74 76 89 1.5 0.267

W2002 2002 17.56 76 89 1.5 0.202

W2003 2003 10.61 76 89 1.5 0.146

W2010 * 2010 10.91 76 89 1.5 0.151

W2004 2004 19.69 76 89 1.5 0.240

W2006 2006 16.72 76 89 1.5 0.177

W2008 2008 5.60 76 89 1.5 0.112 Sub-total 4254.99

Tributary W2

W2001 2001 27.89 76 89 1.5 0.341

W2005 2005 19.86 76 89 1.5 0.212

W2007 2007 24.10 76 89 1.5 0.215 Sub-total 71.85

Tributary W1

W1100 1100 22.92 79 90 1.5 0.322

W1101 1101 20.41 79 90 1.5 0.241

W1102 1102 15.07 79 90 1.5 0.253

W1103 1103 18.61 79 90 1.5 0.214

1 According to the Ministry of Natural Resources (MNR) Technical Guide – River & Stream Systems: Flood Hazard

Limit (2002), an areal reduction factor should be applied based on the equivalent circular area for the watershed with

the drainage area larger then 25 km2.

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* W


W1104 1104 15.61 79 90 1.5 0.208

W1105 1105 13.94 79 90 1.5 0.160

W1106 1106 18.54 79 90 1.5 0.246

W1107 1107 25.99 79 90 1.5 0.223

W1108 1108 18.89 79 90 1.5 0.219

W1109 1109 122.97 79 90 1.5 0.526

W1110 1110 11.17 79 90 1.5 0.168

W1111 1111 42.24 79 90 1.5 0.388

W1112 1112 8.37 79 90 1.5 0.152

W1113 1113 33.72 79 90 1.5 0.316 Sub-total 388.45

Downstream

W1001 1001 8.64 79 90 1.5 0.169

W4001 4001 1.41 76 89 1.5 0.028 2010 is the south portion of original basin # W2003

TABLE A3.1.3 RAINFALL DEPTH

Storm Event No Reduction Rainfall Depth (mm)

Areal Reduction Factor

Reduced Rainfall Depth (mm)

25 mm 4-hr Chicago Storm 25.0 1.000 25.0

1-yr 4-hr Chicago Storm 25.8 0.850 21.9







Regional (Hazel) Storm 212 0.963 204.1

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Master Environmental Servicing Plan July 2013�Windfields Planning Area – West of Simcoe Street�

FIGURE A3.1.4a 100-YR 4-HR CHICAGO STORM HYETOGRAPH�

100-Year 4-hr Chicago Design Storm (5 min Time Step)

0.85 Areal Reduction Factor Applied

Total Rainfall Depth = 71.3 mm

0

20

40

60

80

100

120

140

160

180

200

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Time (Hr)

Inte

nsity (

mm

/hr)

FIGURE A3.1.4b HURRICANE HAZEL REGIONAL STORM HYETOGRAPH�

Hazel Regional Storm (15 min Time Step)

0.963 Areal Reduction Factor Applied

Total Rainfall Depth = 204.1 mm

0

10

20

30

40

50

60

0

0.5 1

1.5 2

2.5 3

3.5 4

4.5 5

5.5 6

6.5 7

7.5 8

8.5 9

9.5 10

10.5 11

11.5 12

Time (Hr)

Inte

nsity (

mm

/hr)

Model Results

As a result, the peak flow rates at the flow points simulated by the pre-development hydrology model are listed in Table A3.1.4. A CD in Appendix A3.1c includes the Visual OTTHYMO hydrologic model for existing conditions.

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TABLE A3.1.4 PRE-DEVELOPMENT PEAK FLOW RATE Flow Point

#

VO2 ID#

Part II Plan

Flow #

Area (ha)

Existing Peak Flow Rate (m3/s)

25mm 1-Year 2-Year 5-Year 10-Year 25-Year 50-Year 100-Year Regional

1 101 A 3912 14.6 14.4 25.1 43.6 57.5 77.4 92.7 126.7 382.5

2 102 B 31 0.5 0.4 0.8 1.3 1.8 2.4 2.9 3.7 4.3

3 106 C 55 0.7 0.6 1.1 1.9 2.5 3.3 3.9 5.2 7.3

4 107 D 4009 15.0 14.6 25.4 44.1 58.0 78.1 93.5 128.0 389.2

5 108 E 22 0.3 0.3 0.5 1.0 1.3 1.7 2.1 2.8 3.0

6 110 F 4051 15.0 14.6 25.4 43.9 57.9 78.0 93.3 128.0 390.4

7 114 G 4255 15.3 14.6 25.5 43.9 57.9 78.1 93.6 129.3 404.3

8 115 X 44 0.6 0.5 0.9 1.6 2.2 2.9 3.5 4.7 6.0

9 116 H 72 0.5 0.4 0.9 1.6 2.2 3.1 3.8 5.1 9.4

10 118 I 4328 15.6 14.7 25.7 44.2 58.3 78.6 94.2 130.2 409.1

11 122 J 218 2.0 1.8 3.1 5.4 7.2 9.6 11.6 15.1 26.7

12 123 P 244 1.9 1.7 3.0 5.4 7.2 9.8 11.8 15.6 29.4

13 125 K 277 1.9 1.6 2.9 5.2 7.0 9.7 11.7 15.8 32.5

14 127 L 53 0.8 0.7 1.2 2.2 2.9 3.8 4.6 6.1 7.3

15 126 M 330 2.1 1.7 3.2 5.9 7.9 11.0 13.3 18.1 38.6

16 131 N 388 2.2 1.8 3.2 5.8 8.0 11.2 13.9 19.3 44.9

Since the existing model was developed based on the previous version of OTTHYMO model of the Windfields Planning Area Part II Plan study, a model validation was carried out to ensure the reliability of the current existing model for the subject Windfields West MESP area by comparing the flow rates generated from both models. Table A3.1.5 summarizes the comparison results at selected flow points. Completed comparison tables are included in Appendix A3.1e.

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TABLE A3.1.5 BRIEF SUMMARY OF MODEL VALIDATION Part II Plan

Flow # Location Description 1-Year 2-Year 5-Year 10-Year 25-Year 50-Year 100-Year Regional

H W2 at

Conlin Rd

Part II Plan Flows 0.45 0.86 1.61 2.21 3.09 3.78 5.10 9.06

2011 Flows 0.40 0.90 1.60 2.20 3.10 3.80 5.10 9.40

Difference (± %) -0.22 0.00 0.06 -0.09 -0.06 0.11 0.06 3.74

I Main

Branch at Conlin Rd


2011 Flows 14.70 25.70 44.20 58.30 78.60 94.20 130.20 409.10

Difference (± %) -0.01 -0.01 -0.02 -0.31 -0.02 -0.20 -0.06 0.13

N W1 at

Conlin Rd


2011 Flows 1.80 3.20 5.80 8.00 11.20 13.90 19.30 44.90

Difference (± %) -0.17 0.09 -0.02 0.03 -0.04 0.01 0.03 2.17

As shown in Table A3.1.5, the resulting flow rates of revised existing conditions model match well with those generated from the previous model of the Windfields Planning Area Part II Plan study. Consequently, it can be concluded that current revised pre-development Visual OTTHYMO model is reliable and satisfactorily computes the existing flows for the study watersheds.

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A3.1.2 Flood Plain Limits

Existing flood plain limits for the West Branch Oshawa Creek and its Tributary W1 have been previously determined as part of the Oshawa Creek Watershed Study (Totten Sims Hubicki, 1995). Flood plain limits were later determined for Tributary W2 by the MMM Group (2001) during the preparation of the background report for the Windfields Planning Area. Regulatory Flood elevations for these watercourses were determined from the available HEC-2 models and are plotted on Figure A3.1.5. The summary of the output of HEC-2 models is included in Appendix A-3.1d.

A3.1.3 Erosion/Geomorphology

An erosion and geomorphological assessment of the existing condition of the West Branch of Oshawa Creek and its Tributaries W1 and W2 was conducted. The assessment was based upon a field inspection and analysis of the topographic mapping and available HEC-2 hydraulic models. The primary objectives of the assessment were to assess channel stability, and to make recommendations for channel remediation works, if required. The results of the assessment of each of the watercourses follow.

West Branch Oshawa Creek

This assessment is focussed on the reach of the watercourse extending from Conlin Road upstream to its crossing at Thornton Road, a reach length of approximately 2 km. The West Branch Oshawa Creek has a drainage area of 4.3 Km2 at Conlin Road. The present land use within the watershed is primarily rural/agricultural, and includes part of the community of Brooklin.

Along this study reach, the stream flows within a well defined and stable channel through a deep, well-vegetated valley. The valley is heavily wooded for much of its length, except for a relatively short section towards the downstream part of the valley which consists of a meadow. Based on a visual assessment, the stream is stable with only three observed actively eroding sites. The locations of these erosion sites are shown in Figure A3.1.6. Photographs of these erosion sites are shown in Figures A3.1.7 to A3.1.9. All of these erosion sites can be considered to be a normal consequence of the natural movement of the stream, and none of the erosion sites presently pose a risk to structures or infrastructure.

A 1500 mm diameter storm outfall is located on the west bank of the watercourse just downstream of a sharp meander bend. A photograph of the outfall is shown in Figure A3.1.10. The outfall is presently not at immediate risk of being undermined by the creek, but should be monitored to determine whether the structure will require stabilization in the future. It is noted that the watercourse flows along the north limit of Conlin Road for a short distance upstream of its crossing of the road. The south bank of the watercourse has been fortified with boulders and concrete rubble to protect the road from erosion. This erosion protection appeared to be stable at the time of the inspection, however monitoring of the site is recommended.

The reach of the West Branch Oshawa Creek has an average longitudinal valley slope of 0.6% between Conlin Road and Thornton Road. The watercourse has a meandering alignment with a sinuosity of about 1.4. The West Branch Oshawa Creek has a bankfull width varying from approximately 9 m just upstream of Conlin Road, to approximately 7 m just downstream of Thornton Road. The bankfull width of the stream was determined to be approximately 6 m through the open meadow area. The reduced channel width at this section is probably due to the stabilizing influence

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of the heavy growth of grasses and herbaceous plants in this area, which provides greater resistance to channel erosion.

The bed material is variable, but can be generally characterized as a mixture of sand, gravel and cobbles. The banks are generally well vegetated and consist of clayey sand. The watercourse has a bankfull depth varying from 1.0 to 1.2 m, and is not entrenched, and overflows onto its floodplain during flood events, which contributes to its stability.

The average bankfull velocity along the reach was calculated to be 1.6 m/s, which is well below the erosion threshold of 2 m/s for a channel with well vegetated banks. This confirms that the channel is stable, as appears to be the case from the observed condition of the watercourse.

Tributary W1

This assessment focuses on the reach of the watercourse extending from Conlin Road to Winchester Road, a distance of approximately 2.2 km. Tributary W1 has a drainage area of 388 hectares at its crossing of Conlin Road, where it is a permanently flowing watercourse. The primary land use within its catchment area is rural/agricultural.

The stream flows along a relatively straight alignment for a distance of approximately 700 m downstream of Winchester Road, after which it exhibits a more natural meandering form. From the visual inspection, the watercourse appears to be stable and no active erosion sites were observed.

Tributary W1 has an average longitudinal valley slope of approximately 1%. The natural reach of the stream is highly sinuous with a sinuosity of 1.7. The bankfull width of the stream varies from 1 to 2 m along the natural section. The bankfull width tends to be about 1 m where it flows through open meadow and where the banks are well vegetated with grasses and herbaceous plants (see Figure A3.1.11). The bankfull width tends to enlarge where it flows through a wooded area where ground cover is lacking, and the banks are more erodible (see Figure A3.1.12). The bankfull width is widest for a distance of approximately 100 m upstream of Conlin Road where the forest cover is heaviest and the stream banks are bare.

The watercourse has an average bankfull depth of approximately 0.3 m, and is not entrenched. The bed material is variable, but can be generally characterized as sandy silt or sandy clay. The banks are generally well vegetated and consist of sandy clay.

The bankfull velocity along the vegetated reach was calculated to be 0.7 m/s which is well below the erosion threshold of 2 m/s for a channel with well vegetated banks. This confirms that the channel is stable, as appears to be the case from the observed condition of the watercourse.

Tributary W2

This assessment focuses on the reach of the watercourse extending from its confluence with the West Branch Oshawa Creek to the southern edge of the woodlot located north of the future Britannia Avenue extension, a distance of approximately 900 m. Tributary W2 has a drainage area of only 77 hectares at its confluence with the West Branch Oshawa Creek. Due in part to its limited drainage area, the stream flows intermittently. The primary land use within its catchment area is rural/agricultural.

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The stream flows along a relatively straight alignment for a distance of approximately 700 m downstream of the woodlot. This section has presumably been straightened to facilitate agricultural practices. It is in a natural condition where it flows into the valley of West Branch Oshawa Creek. This section of the stream flows through dense forest with little or no ground cover.

Tributary W2 has an average longitudinal valley slope of approximately 1% along its straight alignment upstream of the West Branch Oshawa Creek valley. The channel slope down the valley slope is 3%, after which it flattens as it flows across the West Branch Oshawa Creek flood plain to its confluence with the West Branch. The bankfull width of the stream is variable, and has a width of approximately 1 m along its straightened section. The bankfull width of the stream is relatively narrow where it flows through an open meadow and has banks that are well vegetated with grasses (see Figure A3.1.13). The bankfull width becomes wider and more irregular after it enters the West Branch Oshawa Creek valley, and flows along the flood plain of the West Branch Oshawa Creek (see Figure A3.1.14).

The watercourse has a bankfull depth of approximately 0.2 m along its straightened section, and is not entrenched. That is, the watercourse overflows onto its flood plain when flows exceed its bankfull capacity, which reduces the potential for erosion of the bankfull channel, and enhances its stability.

The bankfull velocity along the straight reach was calculated to be 0.8 m/s which is well below the erosion threshold of 2 m/s for a channel with well vegetated banks. This confirms that the channel is stable, as appears to be the case from the observed condition of the watercourse.

A3.2 GROUNDWATER RESOURCES

The purpose and objectives of the hydrogeological component of the Master Environmental Servicing Plan (MESP) for the Windfields Planning Area (West) MESP are to describe the geological and hydrogeological conditions of the lands located west of Simcoe Street and to assess the hydrological role and functions of the groundwater flow system in more detail from the earlier MESP studies. Additional information that is available for this assessment includes results from more recent geotechnical investigations carried out by others at the property in 2008, and field investigations carried out by MMM in 2009. A preliminary development concept with a land use schedule and estimates of impervious areas is presented but is expected to undergo revisions through the development process. Potential impacts to groundwater resources that may result from redevelopment of this land are discussed and include consideration of mitigation alternatives to reduce impacts. The assessment also includes a water balance estimate of pre-development conditions and post-development with mitigation alternatives considered.

Previous work at the property investigated the overall Windfields Planning Area to develop a section of the background report (MMM, 2001) addressing existing groundwater conditions. The purpose of the recent hydrogeological investigations for the MESP was to obtain additional hydrogeological information from the site focused on the Western Planning area.

The existing or pre-development land use for the plan area is predominantly agricultural (horse breeding and boarding) with natural areas and with small areas of rural residential land use (farm houses, barns). The overall land area of the plan area is approximately 258.3 ha. The proposed land use for the Windfields Planning Area (West) is principally urban (see Figure A3.2.1). The southern half of the area is to be principally institutional (University of Ontario Institute of Technology (UOIT)) with a mixture of educational facilities, student residences, park land, playing

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Master Environmental Servicing Plan� July 2013�Windfields Planning Area – West of Simcoe Street�

A3.3.3 Water Balance

Water balance analyses were performed for the entire area of the Windfields Planning Area (West), which contributes to the West Branch of Oshawa Creek and its Tributary system. In addition to the pre-development water balance, two post-development water balance scenarios were examined. The first of these scenarios examined the worst case situation with no mitigation measures applied at the site. The second scenario examined the effects of mitigation measures at the property as related to maintaining pre-development infiltration to the groundwater system. Both scenarios were compared against the pre-development case. The results for the water balance calculations are described below and summarized on Tables A3.3.2 and A3.3.3. The detailed calculations are presented on Tables WB-1 through WB-5 found in Appendix A3.3a.

The following assumptions have also been made to estimate the post-development water balance including those scenarios where recharge mitigation measures are examined:

• There is no infiltration occurring on hard surface areas and evapotranspiration is significantly lower than that under pre-development conditions (10% as evaporation only), due to rapid runoff of precipitation;

• The imperviousness of the developable lands after development is calculated at about 72% of the development area or about 133.0 ha;

• Although it is anticipated that all roof runoff can and will be directed to ground, the pervious areas available for infiltration on the front lawns is expected to be small relative to the volume of water discharged from the roofs. Infiltration will occur on these areas but the contributions from enhanced infiltration via roof runoff were not estimated in these calculations. This results in a conservative evaluation that slightly underestimates post-development infiltration. Infiltration due to natural precipitation falling on these areas is however included in each water balance scenario;

• Roof runoff directed to the rear lawns was calculated to result in an effective 1.68 times normal precipitation falling on these areas. Data was obtained from Environment Canada considering an equivalent annual precipitation of 1,367 mm/year for soils with a WHC of 75 mm/year (post-development conditions). The resulting monthly Precipitation, Actual ETR and Water Surpluses under these conditions is presented on Table WB-1 (Appendix A3.3a) and they were used for areas receiving roof runoff in the water balance;

• Tilling/scarifying the underlying soils to 300 mm depth prior to placing approximately 0.25 m of topsoil across all developed (pervious) lands. This increased thickness of organic soils with additional void space will retain a greater proportion of precipitation and/or runoff over these pervious areas and therefore promote additional infiltration. An increase in the “cover” infiltration factor by 0.05 (from 0.10 to 0.15) for the landscaped areas is considered appropriate, placing this value mid-way between the factors for cultivated lands (0.10) and forested areas (0.20);

• On pervious surfaces, the water surplus generates runoff and infiltration according to an infiltration factor of 0.53 (no mitigation scenario) to 0.58 (scenario with mitigation) compared to the pre-development infiltration factor of 0.51. This slight increase is due to the anticipated flattening of grades to about 2% across most of the developed/landscaped lots (from approximately 3% existing conditions), and from the placement of a minimum of 0.25 m of topsoil with higher moisture retention and improved infiltration potential (post-development with mitigation scenario);

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• Runoff from the road network, as well as roof area not directed to mitigation is discharged directly into the storm sewer network, with the exception of runoff from rear lots backing onto natural features that are outside of the storm system.

A3.3.3.1 Pre-Development Water Balance

Under pre-development (existing) conditions the site is considered essentially pervious over its full 243.4 ha area (the existing farm structures and drives have been ignored). The water surplus under the pre-development conditions is calculated at about 260 mm/year over the 243.4 ha of the MESP area, or 265 mm at the future development lands (see Section 3.3.1). The infiltration factor across the MESP lands ranges between 0.51 to 0.62 (see Table A.3.3.1). Therefore, pre-development infiltration across the 243.4 area examined by this water balance and leading towards the West Branch Oshawa Creek and its Tributaries is estimated at about 137 mm/year (333,107 m3/year), which is consistent with the reported infiltration values for these types of soils. Most groundwater recharge occurs during the spring melt period when soil moisture content is high. The remaining 123 mm/year (330,369 m3/year) would be available for surface runoff, most of which occurs during the spring melt period.

The major contribution of water to the West Branch Oshawa Creek and its Tributary occurs, as expected, in the late winter and spring seasons. Water surpluses from the site during the growing season (May through September) are negligible as the ETR remains high and the soil moisture goes into a deficit. Soil moisture begins to recover in September.

The pre-development water balance for the site is summarized on Table A.3.3.2, with the detailed, monthly water balance calculations presented in Appendix A3.3a on Table WB-2.

A3.3.3.1.1 Water Budget Considerations

Regional studies of water budgets for streams draining south from the Oak Ridges Moraine have common findings. These findings include conclusions that most baseflow results from discharge to the head water reaches of the subwatershed from the Oak Ridges Aquifer Complex and that 60 % or more of base flow originates in the upper reaches above an elevation of 220 to 230 masl (e.g., Duffins Creek, Gerber and Howard, 1998). The next most significant area providing discharge and baseflow occurs below the Iroquois Shoreline where relatively thick sand deposits occur. The portion of the Oshawa Creek watershed being studied herein exhibits groundwater conditions that are less sensitive to development on a subwatershed basis than the headwater areas to the north where recharge estimates of 300 to 400 mm/year are indicated along the Moraine.

As discussed above, the annual recharge estimates for the Windfields Planning Area (West) are considered to be in the range of 137 mm/year. Recharge to deep sediments through the Newmarket Till is believed to be 30 to 40 mm/year (Gerber and Howard, 1998).

The results of hydraulic conductivity testing of monitoring wells and Hazen estimates using grain size results were shown on Tables A3.2.1 and A3.2.2. The mean hydraulic conductivity for the testing indicated that hydraulic conductivities in the order of 3x10-6

m/sec (94.6 m/year) are typical of the soils encountered at the site. This hydraulic

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Master Environmental Servicing Plan July 2013�Windfields Planning Area – West of Simcoe Street�

conductivity value may not be fully representative of the Halton Till complex but represents a range of values obtained across the study area. Localized more permeable regions may be present between borehole locations that transmit groundwater more efficiently. Groundwater flow gradients across the site vary from 0.015 to 0.045 (average 0.030) under spring conditions and are likely higher close to discharge points. The saturated thickness of the Halton Till Complex upper aquifer ranges from approximately 3 to 7 m (average 5m). Assuming a discharge length of about 10.3 km (estimated perimeter along the east side of the West Branch of Oshawa Creek and the two main tributaries W1 and W2) the flow capacity of this aquifer is of the order of 146,000 m3/year) under spring season water level conditions.

The tableland site area within the Planning Area is in the order of 184.6 ha (flow through the Halton Till Complex generally discharges along the perimeter of the valleys; hence the valley area is being ignored in this calculation). The net infiltration into the Halton Till Complex is estimated to be 97 mm/year (137 mm/year into the ground surface less 40 mm/year leakage into the deeper sediments); a recharge volume of approximately 180,000 m3/year is estimated. This volume of water is more than the Halton Till Complex appears to be able to transmit on an annual basis (by a factor of about 1.2 considering early season water levels and gradients). Although this conceptual water budget does not completely balance, the order of magnitude of the components are representative and it serves to support the seasonal function of the groundwater system in providing intermittent flow to tributaries and perennial moist ground conditions to bottom land communities. Storage of water in the groundwater system spreads out the discharge through most of the year in several important habitat areas.

A3.3.3.2 Post-Development Water Balance with No Mitigation

Water balance calculations were carried out for the scenario with no mitigation measures. The proportion of imperviousness within the areas (residential, commercial, industrial, institutional) are estimated to be 72% (ranging from 65% to 90% impervious depending on proposed land use), which includes the areas proposed for buildings, roads, parks and schools, and the stormwater management ponds. Total impervious area was calculated at about 133.0 ha, with the remaining 110.4 ha considered pervious (see Table A.3.3.1).

This first scenario, examines the worst-case condition, where there will be no mitigation measures incorporated. This assumes that all impervious area runoff (less impervious surface losses to evaporation), including roof runoff that would be expected to be discharged to pervious ground surfaces, is sent directly to the stormwater management system. This scenario considers that groundwater infiltration is supplied only by precipitation which falls upon the pervious areas. The potential maximum loss of infiltration from the proposed residential development was calculated to provide a worst-case estimate of the potential impacts on infiltration due to the introduction of hard surfaces. The detailed monthly water balance calculations are presented on Table WB-3 in Appendix A3.3a.

37


TABLE A.3.3.2: PRE- AND POST-DEVELOPMENT WATER BALANCE – NO MITIGATION

Parameters

Pre-Development

mm/year m 3/year

Post-Development

mm/year m 3/year

Change

m 3/year %

Precipitation 817.0 1,988,923 817.0 1,988,923 0 0.0%

Total AET 553.1 1,346,590 241.5 587,854 -758,736 -56.3% Evaporative Losses at 10% Precipitation 0.0 0 44.6 108,691 108,691 N/A

Infiltration 136.8 333,107 70.2 170,802 -162,305 -48.7%

Runoff 123.4 300,369 458.5 1,116,196 815,826 271.6%

Notes:

Evaporative losses are losses of precipitation though simple evaporation on impervious surfaces (such as from ponding at shallow depressions on paved areas).

As indicated in Table A.3.3.2 under this worst-case scenario, the water balance method estimates a little bit under a 50% reduction in groundwater infiltration at the site whereas runoff to the tributaries is calculated to increase in the order of 270%. The change to the local groundwater recharge function assumes that all runoff from hard surfaces is conveyed to the storm sewer network. Some additional loss of groundwater flow may occur due to foundation drains and permeable backfill surrounding services, however, most of recharge loss is anticipated to be due to rapid runoff from impervious surfaces.

A3.3.3.3 Post-Development Water Balance with Mitigation

A post-development water balance analysis was carried out with simple mitigation measures consisting of enhanced infiltration techniques, specifically located along the boundaries of the natural environment areas. The choice of mitigation measures can be constrained by site conditions (e.g., site soils, location and orientation of natural features) and design constraints (e.g., site grading, draft plan configuration).

The improvements to recharge and runoff contributions of the following mitigation measures were examined:

• Direct residential roof runoff to ground. As described earlier, enhanced infiltration from runoff to the rear of detached lots was explicitly calculated in the water balance using monthly data from a Thornthwaite-Mather analysis provided by Environment Canada with annual precipitation multiplied by a factor of 1.68;

• Placement of approximately 0.25 m of topsoil across all developed lands (potentially deeper on park-lands and school playing fields if agreeable to the City of Oshawa and the local school boards); and,

• Rear lot runoff originating at lots backing onto the natural areas was modelled as being directed into shallow infiltration structures such as infiltration swales or trenches (with perforated pipe and clear stone. Along the edges of the naturalized areas, the secondary permeability offered by the upper weathered zones of the native soils are anticipated to remain relatively intact as these areas should not be subjected to construction traffic and site grading, and for the purpose of this water balance scenario,

38


are assumed to fully infiltrate this intercepted runoff into the shallow groundwater system.

Table A.3.3.3 summarizes the results of the water balance assessment for the site including pre-development and the post-development conditions with the implementation of mitigation measures described above. The detailed calculations are found on Table WB-4 in Appendix A3.3a.

TABLE A.3.3.3: PRE- AND POST-DEVELOPMENT WATER BALANCE – WITH MITIGATION�CONSISTING OF THICKER TOPSOIL PLACEMENT, RESIDENTIAL ROOF TO REAR�

LAWNS AND INTO SHALLOW INFILTRATION STRUCTURES�

Parameters

Pre-Development

mm/year m 3/year

Post-Development

mm/year m 3/year

Change

m 3/year %

Precipitation 817.0 1,988,923 817.0 1,988,923 0 0.0%

Total AET 553.1 1,346,590 246.2 599,237 -747,354 -55.5% Evaporative Losses at 10% Precipitation 0.0 0 44.6 108,691 108,691 N/A

Infiltration 136.8 333,107 94.4 229,788 -103,319 -31.0%

Runoff 123.4 300,369 429.5 1,045,596 745,227 248.1%

Notes:

Evaporative losses are losses of precipitation though simple evaporation on impervious surfaces (such as from ponding at shallow depressions on paved areas).

All water entering the infiltration devices is presumed to infiltrate (see text).

With mitigation consisting of roof runoff onto lawns, placement of thicker topsoil, and the rear lot runoff directed into infiltration devices at those properties that back onto the natural features, the water balance method estimates a 31.0% reduction in groundwater infiltration to tributaries which is a significant improvement from the 48.7% loss calculated under the worst case scenario. The increase in post-development runoff for the mitigated scenario is also reduced from about 270% to 248%. Volumetrically, infiltration is increased by approximately 59,000 m3/year while runoff is reduced by about 70,600 m3/year compared to the unmitigated scenario (the difference being made up in additional evapo-transpiration at the lawns and gardens).

A3.3.4 Discussion of Water Balance Results

The preceding tables and discussion present the potential impacts and results of the proposed mitigation measures on improving the post-development water balance for the site. From the tables it can be seen that with the currently proposed mitigation measures it can be anticipated that recharge across the properties will still be reduced from the pre-development condition.

Relatively simple measures were incorporated into the water balance to mitigate some of the losses in recharge. These simple measures included striving for relatively flat grading to reduce rapid runoff and, encouraging lot level infiltration by directing roof drainage to permeable surfaces such

39


as lawns and gardens. Two additional measures were incorporated into the water balance that consisted of placing thicker topsoil on pervious areas within the development lands (0.25 m thickness), and intercepting the rear lot runoff at properties backing onto the natural areas and directing this runoff into infiltration structures as noted in Section A3.3.3.3.

Enhanced infiltration measures that attempt to recharge water directly to the subsurface have the potential to recover a larger proportion of the lost recharge function. Enhanced recharge however, requires favourable hydrogeological conditions including a relatively permeable subsurface, simple geological conditions and a relatively deep water table. Conceptually these occur where the water table is deeper than 3 m from finished grade either naturally or due to the introduction of suitable fill, and the local soil is sufficiently permeable (>15 mm/hr infiltration rates) to allow a relatively rapid transfer of the infiltrating water.

Additional best management practices (BMPs)/Low Impact Development measures (LIDs) may therefore be feasible at specific areas within the MESP lands. As noted in Section A3.2.3.4 there are areas within the Planning Area identified where there may be the potential, for example, to incorporate additional infiltration measures. Additional investigation to delineate and determine the feasibility of additional measures would be carried out in support of later studies (e.g., Functional Servicing Study, Draft Plans of Subdivision) as the detailed development concepts and proposed site grading become available. It is noted that when the further investigation of incorporating Best Management Practices (BMPs)/Low Impact Development Measures (LIDs) within the planning area, specifically the public realm (i.e. residential streetscape design, parks and open space), are conducted that consideration be given to how these facilities, in the future, will function and be maintained and by whom.

Figure A3.2.2 identifies borehole and test pit locations where “sandy” deposits to a minimum of about 1.5 m below existing grades were identified on the soil logs (sandy deposits being those classified as Silty Sand/Sandy Silt and coarser). These are areas currently believed to have potential for additional mitigation techniques. For example, unconfirmed areas with potential to include infiltration concepts though not currently included in the water balance calculations include:

• Infiltration galleries may be feasible to convey additional volumes of roof runoff from the industrial zone lands at the northwest of the plan area (by BH-8), where sandy deposits were logged to 6 m depth and the water table was measured at greater than 3 mbgs in 1999;

• Additional rear yard infiltration swales or perforated underground drainage pipes (to remove year yard runoff) may be feasible in the central portions of the site where sandy deposits are also identified but with the water table at approximately 2 mbgs (e.g., in area of BH-4, BH-112, BH-108);

• Additional mitigation measures may be required to support groundwater discharge into the central woodlot. We understand that the City of Oshawa mandates that collection of foundation subdrain seepage is directed into a third pipe system. Directing an appropriate portion of this to outlets leading into or alongside this woodlot should be investigated in subsequent stages of the development process. Additionally, the City should be contacted to identify if they would permit collection of roof runoff into a similar system for use in mitigating infiltration reductions in appropriate locations;

• Infiltration swales or infiltration galleries that direct a portion of the roof runoff from the future UOIT institutional buildings may be feasible on the actual UOIT campus lands south of the Britannia Avenue collector and backing onto the watercourse. The effectiveness of these infiltration measures may be limited at times of seasonally high water levels or where surficial sands are too thin to support such measures. The feasibility of this option would be addressed

40

Master Environmental Servicing Plan� July 2013�Windfields Planning Area – West of Simcoe Street�

during draft plan investigations, when the proposed land use configuration of the UOIT campus lands are better defined; and

• Bio-retention areas incorporated into the residential streetscape designs and within commercial and institutional development blocks may be used to store, treat and infiltrate runoff in areas of the MESP lands where suitable (sandier) soils are present and where the water table is at least 1 m below the base of these measures.

In areas where the grading plan leads to cuts that will remove surficial sandy soils suitable for infiltration, these soils should be stock-piled separately. These sandy soils should then be re-used as surficial fill where they may be best employed to provide mitigation against infiltration reductions.

Additional LIDs not restricted by soil and groundwater conditions, and therefore not having the potential to improve infiltration characteristics, but in reducing runoff, at the Planning Area may include:

• Use of green roofs to improve evapo-transpiration and reduce runoff from the site. These measures are not practical at residential dwellings but may be considered at institutional and commercial buildings; and,

• Collection of roof-water runoff for use in grey-water systems may also be feasible at commercial and institutional buildings where large numbers of people would be expected to use the facilities, as well as at select buildings within the proposed industrial zoned lands (depending upon intended building use and projected staffing levels).

The above two measures should be encouraged, but implementation will be a decision made through the site plan development process.

A3.4 AQUATIC ENVIRONMENT

The aquatic habitat in the study area is located within the Oshawa Creek subwatershed (Figure A2.2) and includes:

• West Branch of Oshawa Creek: The West Branch of Oshawa Creek is a coldwater watercourse that flows generally southward through the western portion of the study area, and is known to provide spawning habitat for salmonids (Rainbow Trout and Chinook Salmon).

• Tributary W1: Tributary W1 is a small intermittent tributary that originates north of Winchester Road between Thornton Street and Simcoe Street within agricultural lands. This tributary flows from north to south through the eastern portion of the study area, and discharges into the West Branch of Oshawa Creek approximately 550 m downstream of Conlin Road.

• Tributary W2: Tributary W2 is a small intermittent tributary that originates within the boundaries of the study area. Tributary W2 outlets from a wetland located approximately 400 m south of Winchester Road and 800 m west of Simcoe Street. Tributary W2 discharges into the West Branch of Oshawa Creek approximately 100 m upstream of Conlin Road.

• Ephemeral Drainage Features: There are four ephemeral drainage features located in the study area that will be potentially impacted by the proposed development. Three of these features drain directly to the West Branch of Oshawa Creek, while one drains to Tributary W1. These ephemeral drainage features strictly provide an overland flow function, and offer limited indirect fish habitat.

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B1.0 STORMWATER MANAGEMENT

B1.1 DEVELOPMENT ASSUMPTIONS

For the purposes of the stormwater management analyses, the proposed development lands were

divided into four (4) separate areas, described below and shown on Figure B1.1.

Area 1.� Land West of West Branch of Oshawa Creek within MESP Study Area, and Lands

South of Britannia Avenue, East of West Branch of Oshawa Creek: This land

encompasses a total area of approximately 112 ha and is mostly owned by the University

of Ontario Institute of Technology (UOIT). As a part of the UOIT north campus, the land

east of Tributary W1 has been developed as a combination of indoor and outdoor sports

facilities and parking lots. Currently, an existing quality/erosion pond F (Pond W-1-A in

Part II Plan) located north of Conlin Road receives and treats the post-development flows

from this area and discharges to the Tributary W1. The area west of Tributary W1 and

south of Britannia Avenue will ultimately be developed in a manner similar to the existing

UOIT campus. In addition to the existing pond F, four wet ponds (i.e., Ponds G, H, J and I)

will be proposed for Stormwater Management purposes.

Area 2.� Land North of Britannia Avenue, East of West Branch of Oshawa Creek and South

of Proposed Future Road within MESP Study Area: This area (approx. 100 ha) is to be

developed as a residential area with schools and parks. As indicated in Part II Plan, five

(5) wet ponds (i.e., Ponds A, B, C, D and E) will be proposed to treat the post-

development flows from this area. In order to minimize the number of ponds along

Britannia Avenue, additional simulations will be investigated to calculate the flows based

on the scenario to combine Pond B with Pond C (namely, Pond BC) and combine Pond E

with Pond D (namely Pond DE). Section B1.6.1a discusses the scenarios with reduced

ponds along Britannia Avenue in details.

Area 3.� Land North of Proposed Future Road and South of Ontario Hydro Corridor within

MESP Study Area: The development of these lands (approx. 50 ha) is designated for

commercial land uses with very high imperviousness. It is recommend to implement

treatment train approach for such land use. As a treatment train, two or more treatment

devices/facilities could be combined, which include OGS, wet pond, wetland, bio-swale,

vegetated strip, etc.

Area 4. Land within the Tributary Watersheds Upstream of MESP Study Area: For the purposes of the stormwater management analysis, the area within the tributary watersheds upstream of the proposed MESP study area (approx. 330 ha) is considered as potential future development and included in the analysis. This area includes Highway 407 extension and existing Regional Road 3 (Winchester Road). The land uses in these lands are taken from the current Regional Municipality of Durham Official Plan (OP).

B1.2 STORMWATER MANAGEMENT CRITERIA

The stormwater management criteria applicable to the proposed Windfields Planning Area west of Simcoe Street were developed during the preparation of the Part II Plan Final Report (MMM, August 2001).

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Water Quality Control

It is recognized that Stormwater Management water quality control with an Enhanced Level of Protection (Level 1) is required for the proposed development area, as defined in the Stormwater Management Practices Planning and Design Manual (MOE, 2003).

Erosion Control

In order to mitigate existing erosion problems and prevent additional erosion in the main branch of West Oshawa Creek, it is required to capture the runoff from a 25 mm rainfall event and discharge it to the receiving water bodies over a period of 24 to 48 hours.

Water Quantity Control

An extensive analysis of flood control alternatives was undertaken during the preparation of the Part II Plan Final Report (MMM, August 2001). It was determined that quantity controls are not required for the proposed study area. The provision of storage for peak flow attenuation has the effect of reducing peak flows in the smaller tributaries through the study area, but results in an overall increase in peak flow rates in the main branch of West Oshawa Creek. The Part II Plan Final Report indicates quantity control facilities would delay the peak flows from the study area to correspond more closely with the timing of the peak flow in the main branch of West Oshawa Creek, thereby causing the increase in peak flows in the receiving watercourse. For Tributary W1, the uncontrolled post-development peak flows are less than those under existing conditions. This is due to a change in drainage boundaries resulting from the proposed development. However, if the morphology of the watercourses is affected by the proposed development, some modifications, e.g., natural channel design, may be required to the tributaries through the development area. The hydrologic models from the Part II Plan Final Report did not consider the effect of future upstream development on its tributaries (Area 4 as indicated in Section B1.1). Consequently, in Section B1.4, more detailed hydrologic modelling will be discussed which confirms the Stormwater Management water quantity control strategy for the proposed developed area.

Water Balance

Best efforts should be made to utilize a variety of suitable BMPs/LIDs to maintain local groundwater recharge and discharge for the development area. It is required to maximize infiltration over the study area to the extent possible. The potential infiltration measures are described in Section A3.3 and B4.0.

B1.3 STORMWATER MANAGEMENT STRATEGY

As previously recommended in Part II Plan, a total of ten (10) wet detention ponds are proposed to

provide the required stormwater management controls for the MESP lands west of Simcoe Street.

As previously indicated in Section B1.1, these ten (10) proposed ponds include four (4) wet

detention ponds (Ponds F, G, H and J) for the area south of Britannia Avenue and six (6) wet

detention ponds (Ponds A, B, C, D, E and I) along north side of Britannia Avenue. The number and

location of ponds was determined through an attempt to balance the following objectives:

• Provide the required stormwater management controls

• Respect biological constraints

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• Locate ponds in areas of lower topography, to facilitate drainage to the ponds

• Accommodate the expected phasing of development

• Accommodate future ownership of ponds

• Minimize the number of ponds

• Minimize storm sewer sizes

Since the water quantity control, which requires detention storage up to 100-year, is not required for

the study area, instead of a total of 6 wet ponds proposed along north side of Britannia Avenue, it is

possible to combine Ponds B, C and Ponds D, E and reduce the number of the ponds to 4. Detailed

discussion, including additional hydrologic models (Scenarios 3a and 7a) on this optimized SWM

plan, is included in Section B1.6.1a.

For Area 3, “treatment train approach” (as discussed in the following paragraph) is recommended to

be implemented in order to confidently provide 80% of the potential total suspended solids (TSS)

removal rate for the proposed development. The available treatment devices/facilities include OGS,

wet pond, wetland, bio-swale, vegetated strip, etc. Two or more of these devices/facilities could be

combined as a complete “treatment train” to meet the required quality control requirement as

defined in MOE’s Stormwater Management Planning and Design Manual.

The term treatment train is defined as a series of separate treatment devices/facilities or measures,

typically a set of source or convey control best management practices (e.g., swale infiltration, filter,

etc.) followed by an end-of-pipe BMPs (e.g., Wet Pond, OGS, etc.) for stormwater management

purposes, including water quality, quantity, erosion control, water balance, etc. Generally, such

approaches will be investigated and implemented during detail design stage. For the proposed

MESP study area, possible treatment train approaches include “swale + wet pond”, “swale + OGS”,

“infiltration + wet pond”, “OGS + wet pond”, etc. Low Impact Developments (LIDs) designs should

also be incorporated with the treatment train approach. If site permits, such designs provide

maximize infiltrations over the study area to maintain local groundwater recharge/discharge and

water balance. Section A3.3 and B4.0 discuss the local soil and groundwater conditions and water

balance in details.

As described in Section B2.3.2, the development area will be serviced with a 1 year storm sewer system, supplemented with foundation drain collectors (FDCs). All the ponds will be constructed as off-line facilities. The storm sewer system will discharge to the off-line stormwater management ponds, while the FDCs and major system flows will be directed to by-pass the ponds. For the modelling purposes, a DuHYD command was used to divert minor flow (peak runoff rate generated from 1-year storm event with no areal reduction factor applied) from the development catchment, which would be captured by the proposed storm sewer system and discharge to the receiving SWM pond for the quality and erosion controls. At each pond, where possible, the controlled discharge, pond emergency overflow, FDC discharge and major system flows will be directed down the valley slope to the receiving watercourses in a single conveyance. The storm sewer and FDC systems are described in more detail in Section B2.3.2.

Please note, in all cases, it has been assumed that upstream developments including the future Highway 407 extension and expansion of existing Regional roads (Area 4 as defined in Section B1.1) will provide their own stormwater management controls. The proposed ponds in this MESP study, including the proposed ponds for both MESP area (Areas #1, 2 and 3) and upstream future development area (Area #4), do not include provision for any upstream development.

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B3.0 STREAM CROSSINGS AND STREAM CORRIDOR PROTECTION LIMITS

The proposed development will require a number of crossings of the West Branch Oshawa Creek and its Tributaries W1 and W2 which could potentially impact upon upstream water levels and associated flood plain limits. This issue was investigated during the preparation of this MESP using the hydraulic models (HEC-RAS) developed for the relevant water courses. The following sections discuss the results of the hydraulic analysis and present recommendations for sizing of the crossings and stream corridor protection limits.

B3.1 DESIGN CRITERIA FOR PROPOSED CROSSINGS

There are a total of ten proposed road crossings within the study area. Three of these (OC-1, W1-2, and W2-1) are for the proposed Britannia Avenue, which crosses the West Branch of Oshawa Creek and Tributaries W1 and W2. Two crossings (W1-1 and W1-6) are at Tributary W1 in the northern part of the study area. The remaining crossings (W1-3, W1-4, W1-5, W2-2, and W2-3) are located where proposed roads will cross Tributaries W1 and W2. The locations of the road crossings are shown on Figure B3.1.

In the absence of municipal design criteria for the proposed watercourse crossings, the sizing of the crossings was guided by design criteria from the MTO Highway Drainage Design Standard (2008). Table B3.1 presents the return periods of the design flow events for the proposed crossings based on the functional road classification and total span of the proposed structures, based on MTO Drainage Design Standards.

Table B3.1�Design Flow Criteria for Proposed Watercourse Crossings�

(Based on MTO Drainage Design Standards)�

Road Name

Functional Road

Classification

Water Crossings

Return Period of Design Flow

(Years)

Total Span less than or equal to 6.0

m

Total Span Greater

than 6.0 m

Britannia Avenue

Urban Arterial OC-1, W1-2,

W2-1 50 100

East-West Road

Collector Road

W1-1 25 50

Others Local Road W1-3,W1-4, W1-5, W2-2, W2-3

10 25

In addition to giving due regard to the MTO Drainage Design Standards, the crossings were sized so that the soffit elevation of the structures were set at or above the Regulatory flood levels in order to minimize impacts on upstream flood levels.

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B3.2 DESIGN FLOWS

Hydrologic analysis has been carried out for the West Branch of Oshawa Creek and the Tributary W1 and W2, and the results have been presented in Section B1. As stipulated by the City of Oshawa, the design flows used to size the crossings at W1 and W2 are based on uncontrolled flows where the effect of stormwater management facilities is not accounted for. Scenario #5a represents the condition where there are no any stormwater management controls in the upstream areas and the MESP study area. The flows for Scenario #5a in the subject watercourses are summarized in Table B3.2 and these flows were used in the hydraulic analysis the road crossings and floodplain mapping.

Table B3.2 Design Flows for Scenario #5a (Uncontrolled)

Storm Events

West Branch of Oshawa Creek

(FP #7)

Tributary W1 (FP #16)

Tributary W2 (FP #8, #9)

Existing (m3/s)

Propose d

(m3/s)

Existing (m3/s)

Propose d

(m3/s)

Existing (m3/s)

Propose d

(m3/s)

2-Year 25.5 26.6 3.2 12.8 0.9 3.5

5-Year 43.9 45.2 5.8 20.6 1.6 5.3

10-Year 57.9 59.4 8.0 26.3 2.2 6.5

25-Year 78.1 80.0 11.2 34.6 3.1 8.3

50-Year 93.6 95.6 13.9 41.4 3.8 9.9

100-Year 129.3 132.1 19.3 53.1 5.1 12.9

Regional 404.3 404.5 44.9 46.6 9.4 10.2

FP# refers to flow points in the hydrologic modelling of the site.

B3.3 HYDRAULIC EVALUATION OF CROSSINGS

HEC-RAS models for relevant reaches of West Branch Oshawa Creek and its Tributaries W1 and W2 were developed and used to delineate the Regulatory Flood plain for these watercourses within the study area, and to size the proposed structures at the stream crossings.

B3.3.1 Crossing OC-1: West Branch Oshawa Creek at Britannia Avenue

A HEC-RAS model was developed for the reach of the West Branch of Oshawa Creek extending from Conlin Road to Winchester Road by importing the geometric data from the existing HEC-2 model for the West Branch of Oshawa Creek. Cross-sections were added to the model to represent the proposed crossing of the West Branch of Oshawa Creek at Britannia Avenue.

An 80 m long bridge is proposed to span the West Branch of Oshawa Creek in order to minimize its hydraulic impact. HEC-RAS simulations were conducted for the 2-year through 100-year events and the Regional Flood. Detailed HEC-RAS output is included in Appendix B3a. Table B3.3 presents the flood elevations for the two cross-sections upstream of the proposed structure. The minimum soffit elevation of the bridge is based on the larger of the Regional Flood level or one

123


metre above the 100year design flood level. The 100 year flood level at the upstream face of the bridge is 149.73 m and the corresponding Regional Flood level is 151.27m. Therefore, the recommended minimum soffit elevation for the proposed is 151.27m.

Table B3.3�HEC-RAS Results – West Branch Oshawa Creek�

Event Flood Flow (m3/s)

Section 2.354 Section 2.36

Without Bridge

(m)

With Bridge

(m)

Without Bridge

(m)

With Bridge

(m)

2-Year 26.6 148.40 148.45 148.65 148.65

5-Year 45.2 148.78 148.85 148.86 148.89

10-Year 59.4 148.97 149.04 149.07 149.13

25-Year 80.0 149.19 149.26 149.35 149.40

50-Year 95.6 149.33 149.42 149.53 149.58

100-Year 132.1 149.60 149.73 149.93 149.96

Regional 404.5 151.05 151.27 151.81 151.86

As can be seen from Table B3.3, the HEC-RAS model predicts that with the crossing in place, there is a small and acceptable increase in the 100-year and Regional Flood elevations upstream of the bridge.

Due to the length of the span, intermediate pier supports may be required. A detailed analysis of the bridge hydraulics and update to the HEC-RAS model will be completed during the detailed design of the structure to determine the impact of the final configuration of the bridge. The proposed Regulatory Floodline of the West Branch of Oshawa Creek is shown in Figure B.3.1

B3.3.2 Crossing W2-1: Tributary W2 at Britannia Avenue

The geometric data used in the HEC-RAS model for Tributary W2 were generated from the topographic map of the site. Two cross-sections were incorporated into the model to represent the proposed crossing of Tributary W2 at Britannia Avenue.

A structure with a total span of 11m is proposed at Tributary W2 at Britannia Avenue to minimize hydraulic impacts. HEC-RAS simulations were conducted for 2-year through 100-year events and the Regional Flood. Detailed HEC-RAS output is included in Appendix B3a. Table B3.4 presents the flood elevations for the two cross-sections just upstream of the proposed structure. The soffit elevation of the proposed structure is set at the Regulatory Flood level (100 year) of 158.37m.

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Table B3.4�HEC-RAS Results – Tributary W2 at Crossing W2-1�

Event Flood Flow (m3/s)

Section 16.068 Section 16.07

Without Bridge

(m)

With Bridge

(m)

Without Bridge

(m)

With Bridge

(m)

2-Year 3.5 157.86 157.89 159.06 159.07

5-Year 5.3 157.91 158.00 159.11 159.13

10-Year 6.5 157.94 158.07 159.15 159.16

25-Year 8.3 157.99 158.16 159.19 159.21

50-Year 9.9 158.02 158.24 159.22 159.25

100-Year 12.9 158.08 158.37 159.26 159.32

Regional 10.2 158.02 158.25 159.22 159.25

As can be seen from Table B3.4, the HEC-RAS model predicts that with the proposed crossing in place, there will be a slight increase in the 100-year and Regional Flood elevations immediately upstream of the bridge. However, the valley walls are steep enough that the increase in flood elevations does not cause a significant increase in the extent of the floodplain. The post-development Regulatory Floodlines are shown in Figure B3.1.

A detailed analysis of the crossing hydraulics and update to the HEC-RAS model will be required during the detailed design of the structure to confirm the hydraulic analysis.

B3.3.3 Crossing W1-2: Tributary W1 at Britannia Avenue

A HEC-RAS model was developed for Tributary W1 upstream of Conlin Road by importing the geometric data from the existing HEC-2 model. The four existing structures spanning Tributary W1 at local roads will be removed in the post-development condition; therefore, these structures were deleted from the existing hydraulic model. Cross-sections were added to the model at appropriate locations to represent the proposed crossings of Tributary W1 at Britannia Avenue, the East-West Collector Road and local roads in the study area.

A structure with a total span of 18m is proposed to span Tributary W1 at Britannia Avenue to minimize hydraulic impacts. HEC-RAS simulations were conducted for 2-year through 100-year events and the Regional Flood. Detailed HEC-RAS output is included in Appendix B3a. The flood elevations for the two cross-sections just upstream of the proposed structure are presented in Table B3.5. The soffit elevation of the proposed structure is set at the Regulatory Flood level (100 year) of 159.84m.

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Due to the changes to land use and the increase in impervious surfaces there is a potential for impacts to water quality including water temperature that may adversely affect aquatic communities and habitat. The proposed stormwater management plan includes the following measures to address potential changes to water quality:

• SWM ponds will be designed to manage water quality and erosion control with an Enhanced Level of Protection (Level 1) according to the Stormwater Management Practices Planning and Design Manual (MOE, 2003), which meets the recommendations of the Oshawa Creek Watershed Aquatic Resource Management Plan (CLOCA, 2002).

• Potential impacts to the thermal regime associated with SWMP will be minimized through the construction of bottom draw outlets.

• Surface water associated with the future industrial area in the northeast corner of the study area will not be directed to SWM ponds, instead the surface water can be directed to Oil-grit separators to remove potential total suspended solids (TSS) from the post-development flows to protect the receiving aquatic habitat.

B5.1.3 Road and Utility Crossings and Fish Habitat Protection

Road Crossings

It is our understanding that the road network and proposed crossing locations within the southern portion of the planning area (south of proposed Britannia Avenue) are based on a preliminary community concept plan prepared by UOIT without input or endorsement from MMM, or review by the relevant planning agencies. The proposed crossings in the southern portion of the study area were given the same field assessment consideration as proposed crossings in the northern portion of the study area. For these proposed crossings, additional comments are provided where existing natural environment conditions warrant consideration of alternative crossing locations. As previously noted, proposed crossing locations in the north portion of the study area (including proposed Britannia Avenue and areas north) were identified in Schedule A of the Part II Plan for the Windfields Planning Area and have been reviewed with, and supported by, the agencies.

The construction of watercourse crossings associated with the proposed road network has the potential to affect fish habitat through their design and construction. The impacts of the road crossing structures to the aquatic habitat and biota are dependent on the type of crossing constructed with the use of an open footed or spanning structure potentially resulting in fewer impacts and impacts of lesser severity when compared to box culverts or corrugated steel pipes (c.s.p.). The proposed road network consists of the Britannia Avenue crossing of West Oshawa Creek (OC-1) as well as five crossings of Tributary W1 (W1-1 to W1-5) and four crossings of Tributary W2 (W2-0 to W2-3).

In order to minimize impacts to the aquatic habitat it is necessary to develop an effective mitigation plan including but not limited to type of construction to isolate the work area during construction, de-watering procedures, permitted temporary watercourse crossings, sediment and erosion control measures, equipment and fuelling plan, the clear delineation of work areas to minimize effects associated with soil disturbance, encroachment into the watercourse and/or undisturbed areas and site restoration (i.e. planting, seeding etc.).

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Potential Impacts – Britannia Avenue Crossing (OC-1)

The Windfields Planning Area Part II Plan Background Report (MMM, 2001) identified Rainbow Trout resting pools and redds during the spring spawning run located upstream and downstream of the proposed road crossing. While spring 2009 field investigations confirmed this activity as well as the migration and spawning of White Sucker upstream of the crossing. The construction of a road crossing has the potential to result in a loss of fish habitat at the crossing location as well as upstream and downstream during construction. Furthermore the design of the crossing structure can also result in adverse effects to the aquatic communities and habitat through improperly sized crossings, poor fish passage opportunities, etc. Due to the function of the West Branch of Oshawa Creek as coldwater habitat supporting a diverse fish community including the migratory Rainbow Trout population it is important to maintain the natural channel habitat and migration (meander) corridor at this location. This would be best accomplished by a crossing structure with an open bottom that is of sufficient size to fully span the watercourse and if feasible permit the migration of the channel (meandering). The use of a structure of this nature would is unlikely to result in a loss of fish habitat.

Potential impacts to the aquatic habitat associated with this type of crossing include the potential transport of deleterious substances (i.e. sediment, fuel, etc) associated with construction activities that could physically injure fish or destroy/alter their habitat, elimination of floodplain vegetation that could result in reduced productivity and shading to the watercourse at the crossing location.

Potential Mitigation – Britannia Avenue Crossing (OC-1)

The use of a full span crossing structure that spans the watercourse through the valley will allow the channel to continue to flow and meander naturally under the crossing, therefore maintaining natural fish passage at the crossing location. This type of crossing method may require the construction of footings and/or piers on the floodplain, which has the potential to result in the encroachment of equipment/personnel and the transport of deleterious substances from the construction area. In the absence of proper mitigation measures, impacts associated with the construction of a full span crossing of the West Branch of Oshawa Creek have the potential to contravene the Fisheries Act through the introduction of a deleterious substance and/or result in a HADD. However, the potential for these impacts is reduced through the preparation and implementation of a mitigation plan and construction that will likely occur at a reasonable distance from the channel.

Potential Impacts – Tributary W1 and W2 Crossings

The impacts of the road crossing structures to the aquatic habitat and biota of Tributary W1 and Tributary W2 will be dependent on the type of crossing that will be constructed. It is anticipated that a box culvert or an open bottom precast culvert will be selected as the preferred crossing structures at these locations. Both types of crossings share similar impacts, however, the extent and severity of the impacts differs between the crossing structures due to the different construction methods and their design. Both share potential impacts including the transport of deleterious substances (i.e. sediment, fuel, etc) associated with construction activity that could physically injure fish or destroy/alter habitat, elimination of floodplain vegetation that could result in reduced productivity and shading of the watercourse at the crossing location and loss/alteration of habitat during construction due to in-water work or de-watering. Typically there are more impacts to the aquatic habitat associated with a box culvert versus an open bottom precast culvert structure, as the natural channel bed is covered with a box culvert.

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The use of an open bottom precast culvert crossing will allow the channel to continue to flow in its natural channel and to a certain extent meander naturally under the crossing, provided realignment of the channel is not required to direct water into/through the crossing structure and it has been sized to account for channel meander. Therefore, fish passage is not expected to be impeded as a result of this type of crossing structure. This type of crossing will require the construction of footings on the floodplain, which has the potential to result in the encroachment of equipment/personnel and the transport of deleterious substances from the construction area, however the potential for these impacts is reduced as construction will occur at some distance from the channel. In the absence of proper mitigation measures impacts have the potential to contravene the Fisheries Act and result in the Harmful Alteration, Disruption or Destruction (HADD) of fish habitat.

The use of a box culvert will likely result in more extensive disturbances to the aquatic environment associated with the need to realign the watercourse into, and through the culvert. This will result in the hardening of the channel (i.e. riverstone) to prevent meandering in the culvert. The provision of fish passage (where necessary) through a box culvert can be achieved by countersinking and lining the culvert with properly sized substrate constructed to create a low flow channel. These impacts have the potential to result in the Harmful Alteration, Disruption or Destruction (HADD) of fish habitat and may require fish habitat compensation measures.

This assessment is dependent upon site specific impacts; the type of habitat affected and will be determined through consultation with CLOCA/DFO.

Potential Mitigation – Tributary W1 and W2 Crossings

An open bottom precast culvert for the tributary crossings would likely be considered the more effective at minimizing impacts to aquatic habitat and biota due to the limited in-water work required when compared with the use of a box culvert. It is anticipated that with the construction of an open bottom precast structure that avoids in-water work (i.e. footing excavation, realignment, etc.) and the development and implementation of suitable mitigation measures (i.e. construction methods), a HADD will not occur. In the event that a box culvert is selected as the preferred crossing structure it is anticipated that impacts to fish habitat cannot be entirely mitigated and as a result the construction of this type of crossing may result in a HADD. If a HADD does occur, fish habitat compensation measures will be required to address these outstanding impacts and achieve a net gain to the productive capacity of fish habitat.

Further mitigation measures include:

• The potential to consolidate some of the proposed UOIT campus road network crossings of Tributary W1 south of Britannia Avenue should be examined to minimize the number of crossings in such close proximity thereby minimizing impacts to the aquatic habitat of this tributary.

• The potential to shift the proposed Tributary W2 road crossing location W2-0. Although this is a headwater wetland feature it functions as indirect fish habitat. Attempts to shift the road to the north during detail design should be examined to minimize impacts to this habitat and maintain the existing hydraulic connection with Tributary W2.

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es britannia appendix h - oshawa · master environmental servicing plan july 2013 windfields...

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