Thompson Creek Flood Study Report - City of Peterborough

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Thompson Creek Detailed Flood Reduction Study City of Peterborough In order to use these rainfall volumes in the hydrologic models (OTTHYMO and OTTSWMM), they had to be transformed into volume-time distributions (hyetographs) as in the case of the July 2004 storm. To do this, it was agreed that a rainfall distribution pattern developed by Canada’s Atmospheric Environment Service (AES) would be used. This distribution, known colloquially as the “AES distribution,” was prepared by that federal agency by analysing the hyetographs of historical storms across Canada. Different distributions are available for each part of the country since storm types vary by location. Different distributions are also available depending upon the duration of the storm required. It was agreed (see Appendix K) that the 6 hour distribution would be the primary choice for the detailed flood reduction studies but that the 1 hour and 12 hour distributions would also be considered to determine which was most critical. The critical duration of a design storm depends upon the size of the watershed to which it is being applied. In the case of the Thompson Creek study area, the drainage area is relatively small (approximately 200 hectares). Hence it was anticipated that either the 1 hour or 6 hour distribution would be appropriate. Figure 4.5.4 shows typical hyetographs for the 1 hour and 6 hour storms (based upon the 120 mm event). It should be noted that the 193 mm event is actually the “Regional Storm” currently in use in the Peterborough area. It is based upon a historical event which occurred in Timmins, Ontario and has its own prescribed hyetograph shape. This is also shown on Figure 4.5.4. The six design storms used for flood limit mapping were based upon these patterns with the actual amounts adjusted to reflect the required totals. The actual amounts are tabulated in Appendix K. Figure 4.5.4: Hyetographs for Selected Rainfalls 30 25 Rain (mm) 20 15 10 5 0 30 60 90 0 120 150 180 210 240 270 300 330 Time (min) 360 390 420 450 480 510 540 570 600 630 660 690 720 120 mm AES Type 2 (6 hr) 120 mm AES Type 2 (1hr) 193 mm - Timmins (12 hr) 100 Year AES Type 2 (6hr) 14-06605-01-W01 City of Peterborough 57
Thompson Creek Detailed Flood Reduction Study City of Peterborough 4.5.1.3 Return Period Events for Flood Damage Estimation For the purpose of estimating average annual flood damages, it is necessary to calculate flood damages for events with a known probability. To facilitate this, flows and water levels must be calculated for events of known probability (or equivalently return period). Hence a series of rainfalls with return periods of 1 in 2 years, 1 in 5 years, 1 in 10 years, 1 in 25 years, 1in 50 years and 1 in 100 years were required. The rainfall volumes for these events were derived from the intensity-duration-frequency (i-d-f) curves from the Peterborough Airport since it has the longest period of record in the area. Table 4.5.1 shows the total rainfall amounts (mm) for one and six hour durations for the noted return periods. In order to simulate the required hydrographs with the hydrologic models described earlier, a hyeotograph (volume-time distribution) was required for each event. As in the case of the volume events discussed above, the “AES distribution” was used for both the one and six hour distribution. The 1 in 100 year six hour storm is shown on Figure 4.5.4. The actual rainfall amounts used for each storm are tabulated in Appendix K. Table 4.5.1 Total Rainfall Volumes for 1 hour and 6 Hour Storms (Peterborough Airport) Return Period 6 Hour Volume (mm) 1 Hour Volume (mm) 2 Year 37.36 23.75 5 Year 48.65 32.26 10 Year 57.49 38.96 25 Year 65.65 45.53 50 Year 76.13 51.66 100 Year 81.73 56.25 4.5.2 Results of Thompson Creek Watercourse Simulations The results of the Visual OTTHYMO and HEC-RAS simulations for the Thompson Creek watercourse are described in the following sections. 4.5.2.1 Return Period Events for Flood Damage Estimation Table 4.5.2 shows the calculated peak flows for Thompson Creek for future land use conditions (described in Section 4.4.1.1) for the six return period events (1 in 2 year to 1 in 100 year) for the six hour AES storm distribution at the locations on the creek indicated on Figure 4.4.2. It should be noted that a constant outflow of 0.1 m 3 /s was assumed from the Thompson Creek Dam based upon the flow measurements taken during the study. The model includes the proposed Waverly Heights SWM pond which attenuates flows from that subdivision before they discharge to Thompson Creek. However, it does not include the flood plain storage upstream of Armour Road. 14-06605-01-W01 City of Peterborough 58
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1500 2.97 1510 2.43 1520 2.25 1530
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LEGEND EXISTING CATCHBASIN LOCATION
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Thompson Creek Flood Study Report - City of Peterborough

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