17331 swm st.thomas aquinas may 18, 2017

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1109 BRITANNIA ROAD EAST, MISSISSAUGA, ON, L4W 3X1 TEL: (647) 496-8055 www.floradesigns.net Page - 1 of 10

Development Engineering Department, Town of Oakville 1225 Trafalgar Road Oakville, ON L6H 0H3. RE: Stormwater management brief for the proposed re‐zoning and lot addition proposal for

St.Thomas of Aquinas Catholic Secondary School located at 304 ‐ 332 Rebecca Street, Town of Oakville, Ont., Issued for Re‐Zoning – May 24, 2017.

Dear Sir/Madam, This development application proposes to add adjoining lands previously occupied by four residential dwellings and a portion of a former right of way fronting Rebecca Street to the existing St.Thomas of Aquinas Catholic Secondary School site located at 304 ‐ 332 Rebecca Street, Town of Oakville, Ontario. This development application proposes to maintain existing grades and overland flow paths of approximately 5,154.72 m2 lot area added to an existing 55,703.51 m2 school site. No other alteration is proposed to the existing storm sewer infrastructure serving the current establishments on private property. Pre‐development Conditions Currently the school site is well developed. A fully functional network of storm sewers, sanitary sewers and watermains are serving the existing establishment on the site. Per the original design prepared by Valdor Engineering (SP.1617.028/06, Revised dated January 2009), the portion of the rear yards of all four lots and the entire lands occupied by pedestrian right of way was accounted as contributing stormwater discharge to the private storm sewer infrastructure belongs to the school site. A drainage area plan from the original stormwater management report prepared by Valdor Engineering is included in Appendix. Furthermore, based on review of the existing topographic survey, we have concluded that, the entirety of the lands, including front yards of all four residential lots and the entire lands occupied by former pedestrian right of way slope towards the south property line. The stormwater flows from this portion of lands travel towards the south property limit through overland sheet flow, which subsequently generates uncontrolled discharge into the existing catch basins and private storm sewer infrastructure belonging to the school site. Please refer to the pre‐development drainage area plan included in Appendix. Post‐development Conditions In the post‐development condition, this development application proposes to maintain the existing grades and flow paths of the land area proposed to be added to the school site. Please refer to the post‐development drainage area plan included in Appendix. In an attempt to quantify the increase in the post development stormwater flow conveyed by the private infrastructure of storm sewers belongs to the school site, a post‐development composite runoff coefficient of 0.28 is calculated in accordance with the Development Engineering Procedures


and Guidelines Manual provided by the Town of Oakville for the entire site based on actual pervious and impervious surface treatments. Based on the pre‐development site area of 0.284 ha having a composite runoff coefficient of 0.31 per the design documents prepared by Valdor Engineering and the post‐development site area of 0.515 ha having a composite runoff coefficient of 0.28, the calculated value of storm runoff during the 5‐year and 100‐year rainfall events are summarised in the Table‐1 below and detailed post‐development peak flow calculations are included in Appendix. Table‐5 Peak Flow Summary

Pre‐development Condition Post‐development Condition Storm Event Drainage

Area (ha)

Runoff Coefficient

(C)

Peak Runoff from (m3/s)

Drainage Area (ha)

Runoff Coefficient

(C)

Peak Runoff from (m3/s)

5‐year 0.284 0.31 0.028 0.515 0.28 0.046

100‐year 0.284 0.31 0.049 0.515 0.28 0.081

The above noted calculations concludes that, the amount of increase in the post‐development stormwater runoff in comparison to the total discharge from the school site is a bare minimum. Also, the stormwater discharge from the entire school site is controlled to the allowable release rate. Based on above, we conclude that, this improvement is not anticipated to create significant negative impacts on the existing stormwater management controls, local surface ponding depths and municipal infrastructure of storm sewers. Therefore, this development application does not propose any additional on‐site storm controls and no other engineering design justification is produced to support this application. We trust that the above information is sufficient to support this re‐zoning proposal. Should you have any questions, please feel free to contact the undersigned.

Sincerely,

Chirag C. Patel, P.Eng, PMP

Project Manager

FLORA DESIGNS INC.


APPENDIX


KEY MAP


RUNOFF COEFFICIENT CALCULATIONS

In accordance with Development Engineering Procedures & Guidelines Manual, Town of Oakville

Landscape Areas 0.25

Asphalt, Concrete, Roof Areas 0.90

Pre‐Development Drainage Area

Area (m2) Area (ha) Catchment #

Imp. Total Imp. Total % Imp.

Runoff Coeff.

CA‐1‐Pre 0.00 980.00 0.000 0.098 0.00 0.25

CA‐2‐Pre 250.00 790.00 0.025 0.079 31.65 0.46

CA‐3‐Pre 0.00 1070.00 0.000 0.107 0.00 0.25

250.00 2840.00 0.025 0.284 8.80 0.31

Pre‐development Runoff Coefficient ( C )

The runoff coefficients are based on the values allocated in the school site design

Post‐Development Drainage Area

Area (m2) Area (ha) Catchment #

Imp. Total Imp. Total % Imp.

Runoff Coeff.

CA‐1‐Post 0.00 2183.42 0.000 0.218 0.00 0.25

CA‐2‐Post 250.00 790.00 0.025 0.079 31.65 0.46

CA‐3‐Post 0.00 2181.30 0.000 0.218 0.00 0.25

TOTAL 250.00 5154.72 0.025 0.515 4.85 0.28

Post‐development Runoff Coefficient ( C )

C = 0.9 i + 0.25 (1 ‐ i), Where i=Imperviousness Ratio


PRE‐DEVELOPMENT DRAINAGE AREA PLAN FROM ORIGINAL SWM DESIGN (N.T.S.) Ref: SWM Report prepared by Valdor Engineering (SP.1617.028/06, Revised dated January 2009)


PRE‐DEVELOPMENT DRAINAGE AREA PLAN (N.T.S.)


POST‐DEVELOPMENT DRAINAGE AREA PLAN (N.T.S.)


PRE‐DEVELOPMENT PEAK FLOW CALCULATIONS

Drainage Area

Runoff Coefficient

Time of Concentrati

on

Catchment #

"A" in Hector "C" "Tc" in Minute

CA-Pre 0.284 0.31 10 Rational Method Calculation:

Q = 2.78CIA Where: Q = Runoff Quantity (Flow) in litre/sec A = Drainage Area in Hectors C = Runoff Coefficient I = Average Rainfall Intensity - mm/h Event 5 yr IDF Data Set, Town of Oakville Coefficient, a = 1170.00 Coefficient, b = 5.80 ( I ) = a * (Tc + b)^c Exponent, c = -0.84

Catchment # A

(ha) I

(mm/h) C

Q (L/s)

Q (m3/s)

CA-Pre 0.284 114.21 0.31 27.70 0.028

Event 100 yr IDF Data Set, Town of Oakville Coefficient, a = 2150.00 Coefficient, b = 5.70 ( I ) = a * (Tc + b)^c Exponent, c = -0.86

Catchment # A

(ha) I

(mm/h) C

Q (L/s)

Q (m3/s)

CA-Pre 0.284 200.80 0.31 48.71 0.049


POST‐DEVELOPMENT PEAK FLOW CALCULATIONS

Drainage Area

Runoff Coefficient

Time of Concentrati

on

Catchment #

"A" in Hector "C" "Tc" in Minute

CA-1-Post 0.515 0.28 10 Rational Method Calculation:

Q = 2.78CIA Where: Q = Runoff Quantity (Flow) in litre/sec A = Drainage Area in Hectors C = Runoff Coefficient I = Average Rainfall Intensity - mm/h Event 5 yr IDF Data Set, Town of Oakville Coefficient, a = 1170.00 Coefficient, b = 5.80 ( I ) = a * (Tc + b)^c Exponent, c = -0.84

Catchment # A

(ha) I

(mm/h) C

Q (L/s)

Q (m3/s)

CA-1-Post 0.515 114.21 0.28 46.03 0.046 TOTAL 0.046

Event 100 yr IDF Data Set, Town of Oakville Coefficient, a = 2150.00 Coefficient, b = 5.70 ( I ) = a * (Tc + b)^c Exponent, c = -0.86

Catchment # A

(ha) I

(mm/h) C

Q (L/s)

Q (m3/s)

CA-1-Post 0.515 200.80 0.28 80.94 0.081 TOTAL 0.081

17331 swm st.thomas aquinas may 18, 2017

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