Thompson Creek Flood Study Report - City of Peterborough

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Thompson Creek Detailed Flood Reduction Study City of Peterborough 4.4.1.2 Model Verification Model verification consisted of simulating the flows from a measured rainfall event from the monitoring period and comparing the results with measured flows in the creek and at the outlet from the storm sewer system at the intersection of Scollard Drive and Francis Stewart Road. The event selected occurred at about 2:00 am on June 1, 2006. According to the rainfall gauge located at City Hall, about 30 mm of rainfall fell in about 3 hours. This is approximately a 1 in 2 year rainfall for that duration. In contrast, the rainfall gauge at Trent University (about 2.5 km north of the area) recorded only about 4 mm of rain. At the outlet from the sewer system, a flow of 0.75 m 3 /s was recorded. This is theoretically at or slightly above the capacity of the outlet pipe at that location. Hence this was a significant event which produced a runoff volume of about 22 mm that is consistent with the observed rainfall at City Hall. The storm must have been a localized event which passed through downtown and the study area but did not reach as far north as the University. Hence the City Hall rainfall was used in the simulation. Interestingly, as previously discussed in Section 4.2.3, the flow recorded at the instream gauge near the corner of Ashdale Crescent West was only about 0.3 m 3 /s. Theoretically, the flow downstream should be greater than upstream given the greater drainage area. However, there appears to be significant overbank storage in the area upstream of Armour Road (see for example, the discussion on Reach No. 2 in Section 4.2.3) which reduces the downstream flows. A similar “damped” response was observed at this gauge for all other significant rainfalls during the monitoring period. Only the normal seepage of flow through the Thompson Creek Dam was measured at the upstream gauge, i.e. no flow spilled over the stop logs in the dam. The results of the simulation of the verification event are summarized in Table 4.4.3. As indicated, the outflow from subcatchment 11, is equivalent to the outflow from the Scollard/Francis Stewart outfall flow. The observed and simulated flows match very closely. The flow at the downstream gauge point also matches closely since a storage element was added to the model just upstream of Armour Road. This required about 1,900 m 3 of storage. A review of the overbank topography in the area indicated that this amount of storage is available in the riparian area and its utilization would be promoted by the presence of a number of small dam structures which obstruct the creek. The model output is included in Appendix G. Based upon this close correspondence of observed and simulated flows for this significant event, the model was judged to satisfactorily represent the hydrologic response of Thompson Creek. The model parameters adopted were those originally estimated from the physiographic data as shown in Tables 4.4.1 and 4.4.2. No other flows of significance were found in the flow record from the gauge near the outlet of 14-06605-01-W01 City of Peterborough 48
Thompson Creek Detailed Flood Reduction Study City of Peterborough Thompson Creek due to the damping effect of the storage upstream of Armour Road. Hence only the one event was used in the verification process. Table 4.4.3 Comparison of Observed and Simulated Flows for June 1, 2006 Event Location Rainfall (mm) Observed Flow (m 3 /s) Simulated Flow (m 3 /s) Scollard/Francis Stewart Sewer Outlet 29.3 0.75 0.78 Thompson Creek near Ashdale Cresc. W. 29.3 0.30 0.29 4.4.2 HEC-RAS Modelling of Thompson Creek The HEC-RAS model calculates water levels in a creek and its flood plain corresponding to a set of flows. It uses information on the shape, length and roughness of the creek and flood plain and on the size of any culverts, bridges, dams, etc. which cross the creek. The following describes the development of the Thompson Creek HEC-RAS model. 4.4.2.1 Model Development As discussed in Section 4.2.2, the Otonabee Region Conservation Authority (ORCA) developed flood plain mapping of Thompson Creek some years ago using an earlier version of HEC-RAS known as HEC-2. Since the existing model was developed prior to installation of several of the culverts at the downstream end of the creek, it required a complete review, update and conversion. The existing model was provided to MMM and was used as a general basis for a new model. In general, the locations of the crosssections used are in similar locations but all have been recoded based on current topographic information, detailed field surveys of the low flow channel and current data on the water crossings (four sets of culverts from the outlet to Armour Road). Drawing FV-1 shows the location and extent of the cross-sections used. The field survey notes and cross-section plots are included in Appendix I. The model coding is also presented in Appendix I. 4.4.2.2 Model Verification Ideally, verification of the HEC-RAS model would involve comparing computed water levels with observed levels along the creek for a significant storm event. Unfortunately, only limited water level data is available for Thompson Creek. For the event of June 1, 2006, used to verify the hydrologic models, a water level measurement was available at the downstream flow gauge near Ashdale Crescent West. This indicated 14-06605-01-W01 City of Peterborough 49
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LEGEND EXISTING CATCHBASIN LOCATION
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Thompson Creek Flood Study Report - City of Peterborough

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