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II I I I I I I I I I I I I I I I I I I IV, POST-PROJECT CONDITION ANALYSES 4.1 Dynamic Flow Modeling 4,1.1 Calibration of the Pre-Proiect Condition EXTRAN Model An EXTRAN model was prepared for pre-project conditions in an effort to calibrate the dynamic flow model to the results from the effective SWMM model. Calibration of the EXTRAN model parameters was conducted for the 100-year event; however, the calibrated model was used to analyze the 10-, 50- and SOD-year events for the pre-project condition in order to verify the applicability of the model to a wide range of hydrologic conditions, The primary goal of the calibration effort was to match the SWMM-computed 100-year water surface elevation in the BNRR pond exactly, while matching the SWMM-computed 100-year release from the BNRR detention pond to within an acceptable tolerance, In order to maximize the range of hydrologic conditions over which the EXTRAN model applies, the secondary goal of the calibration was to minimize the number of parameters being calibrated, while keeping their values within a range that is physically representative, It was found during the course of the modeling efforts that a more accurate representation of the existing pond was achieved when the existing pond was separated into two ponds (EXTRAN storage junctions), The two ponds were connected in the EXTRAN model by a valley-wide cross section along the proposed Centre A venue aligrunent. This modeling configuration also allowed a more direct comparison to the post-project condition model which incorporated two ponds, one each upstream of the existing BNRR and the proposed Centre A venue embankments, Inflow hydrographs to the EXTRAN model were developed from results generated by the duplicate effective SWMM model. SWMM Conveyance Element (CE) 226 represents the inflow to the upper portion of the pond from Spring Creek, CE 221 represents the combined inflow to the BNRR detention pond from the north via a 36-inch storm sewer, a 42-inch storm sewer, and Arthur Ditch, Information given in the report, "Spring Creek Hydrologic Update West of College A venue," [Lidstone & Anderson, July 1998] indicates that approximately 80 percent of all runoff from those three inflow points is directed to the BNRR pond upstream of the proposed Centre Avenue aligrunent, representing the 42-inch storm sewer and Arthur Ditch, The remaining 20 percent of runoff, associated with the 36-inch storm sewer, is contributed downstream of the proposed road aligrunent. Consequently, 80 percent of the hydrograph from CE 221 (determined on an ordinate-by-ordinate basis) was added to the discharge from CE 226 to obtain the total inflow hydrograph to the upper portion of the BNRR pond, CE 219 represents the remainder of the runoff tributary to the BNRR pond, The hydrograph from CE 219 was added to 20 percent L OPEN"I:ocsrtDl "~ocsrtDl tema-LOMR tlnal-rpL\\pd 14 ANdERSON CONsulTiNG ENGiNEERS, INC.