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

City of PeterboroughLower Byersville Creek Flood-Risk Mapping UpdateSTUDY METHODrepresentation of backwater effects and of impacts of flood conditions in themain outlet channels on conditions within tributary storm sewer systems.In this approach, the EXTRAN object is providing dynamic routing using the sametechnique that had previously been applied circa 1990 when the NWS DAMBRKmodel had been used to account for floodplain storage effects on the mildly slopedlower portions of Byersville Creek (especially downstream of The Parkway). In themore recent updates to the HEC-RAS model, the newly developed “unsteady flow”capabilities of HEC-RAS had been enabled in the 2003 update (GreenlandEngineering) apparently in an effort to do away with the need for a separate dynamicrouting model.The use of PCSWMM2005 EXTRAN to provide the dynamic flow routing, and thento apply the resulting peak flows in a steady-state version of the HEC-RAS modelhas been selected as the preferred approach, based on discussions with ORCA.2.3.2 Scope of PCSWMM Model UpdateTo support an update to the flood-risk mapping for lower Byersville Creek, it wasdetermined that some modifications to the PCSWWM2005 model were needed.Details are provided in Appendix C. In summary, the modifications were as follows:Modification to Subcatchment BoundariesIt was decided that some minor adjustments to subcatchment boundaries in the lowerwatershed were warranted, to better represent inflow distribution along the mainstem of Byersville Creek, below The Parkway.Also, it was decided that an increase in surface imperviousness was needed in thesubcatchment (Subcatchment 5.1; see Appendix C, Figure C-1, for map) in which issituated the property on which a Costco commercial development has recently beencompleted (in the vicinity of The Parkway and Crawford Drive).Update to Flow Routing ElementsThe PCSWMM2005 EXTRAN model has been modified to include the following(see Appendix C for detail):• Flow routing elements to better represent the possibility of flow spilling over topthe former CNR line (recreational pathway), and resulting surface flow downCameron Street, Mountain Ash Road, Orpington Road and Erskine Avenue.• Potential for lateral split of flow from the Examiner Ditch at the 90-degree bendat the former CNR, with elements added to represent overland flowsouthwestward alongside the recreational pathway; and including the culvert pipeunder the pathway near the northwest corner of Master Brand Cabinets.• Potential for spill flow from the main stem of Byersville Creek flowing eastwardalong Crawford Drive and from Crawford Drive into to drainage swale alongrear yards of homes on Mountain Ash Road.3-2087-09-01/R_3-04108226_Byersville Creek Mapping Update 2-708/11/10