Lower Byersville Creek Flood-Risk Mapping Update - City of ...

City of PeterboroughLower Byersville Creek Flood-Risk Mapping UpdateRESULTS OF ANALYSISculverts and the rail embankment from the flow routing model would not have anysubstantial effect on peak flows along the main channel of Byersville Creek, sincethere is no significant flow attenuation by the channel storage created by the back-upbehind the rail embankment. This can be seen by reviewing the peak flood flowvalues above and below the CPR rail line as tabulated in Appendix D. On this basis,it was not considered warranted to model the main stem flows with the CPR culvertsremoved or washed out as happened during the July 2004 event. With respect to thecomputation of water surface elevation using HEC-RAS, the effect of the CPRembankment has been included, as required by the OMNR technical guidelines.3.2.3 Mapping of Surface Flooding Using HEC-RAS: Existing SystemFigure 3.3 presents the mapping of estimated surface flooding for the RegionalStorm ("Timmins") event, as determined from the water surface elevation computedby the HEC-RAS model using peak flows from the PCSWMM2005 model.Note that Figure 3.3 is for the existing system without any modifications to existingculverts.As was done for the July 2004 event, the mapping of the surface flooding areas hasbeen carried out using the DEM supplied by the City, with the general procedure asdescribed above.Some points to note about the mapping shown on Figure 3.3.• To map the full spatial extent of flooding, computed water surface elevations ateach modelled cross-section have been projected laterally along the section linesshown on the map figure.• These projections across the DEM did in some areas result initially in surfaceflooding or ponding within isolated patches or pockets that in all likelihoodwould not in fact experience such flooding, because there is no known orapparent hydraulic connection to the floodway. Some judgment and editing wastherefore necessary to develop the mapping as shown.• Similarly, because of the presence of multiple flow paths and the fact that themain stem and Examiner Ditch flow paths go through near 90-degree turns, therewas a requirement for some interpretation and judgment in mapping the floodsurface where cross-sections from the separate HEC-RAS models overlap or areorthogonal to one another.Along the eastern limit of the mapped area, the flood surface from the ExaminerDitch has been projected eastward to Erskine Avenue, and then further eastward ontothe Lansdowne Place property. This reflects a flood surface elevation in the rangeof 192.92 m to 193.02 m along the Examiner Ditch between The Kingsway and TheQueensway, and a low road surface elevation of 192.54 m along a sag in ErskineAvenue just south of The Kingsway. In other words, the mapping of surfaceflooding extending to the east of Erskine Avenue is based on lateral spread of flood3-2087-09-01/R_3-04108226_Byersville Creek Mapping Update 3-608/11/10