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I I I I I I I I I I I I I I I I I I I the top of the box is 20 square feet; however, as indicated in documentation associated with the 1993 revisions to Spring Creek, the entrance is further limited by an inlet grate which has an open area of 14 square feet. 2.2 Post-Project Conditions In response to on-going development south and southwest of the BNRR detention pond. and at the request of the City of Fort Collins, Centre A venue was extended through the pond, The roadway crosses Spring Creek approximately 1,500 feet west of the BNRR and 1,100 feet east of Hill Pond, The roadway embankment is elevated above the IOO-year water surface elevation in order to meet the City requirement of no overtopping during the 100-year event, Two large span culverts have been installed at the Spring Creek crossing. The northern three-sided concrete box culvert has an average height of 11.6 feet and is 20,2 feet wide, which accommodates the low flow channel. The bottom of the culvert is a trapezoidal riprap-lined channel with approximately a 13-foot wide bottom and side slopes of2H: I V, The southern three- sided concrete box culvert has an average height of9 feet and is 18.4 feet wide. which provides access under Centre Avenue for the City's bicycle trail along Spring Creek, The bottom of the southern culvert is elevated an average of 2,6 feet above the Spring Creek channel invert. and is concrete-lined to accommodate the trail. A cross-sectional view of the Centre A venue culverts is provided in Figure 2,1, The roadway and main channel at the culverts is constructed with the requisite channel and embankment stabilization measures as described in Section 4,5, In addition to the culverts, approximately 16 acre-feet of compensatory storage volume has been excavated below elevation 4994 ft, NGVD from the lower (southeast) portion of the pond, directly west of the BNRR, Selected portions of the as-built drawings for the roadway and culvert improvements. prepared by the Sear-Brown Group, Inc" are provided in Appendix A of this report, As-built topography in the compensatory storage area and along Centre A venue is provided on Sheets 1 and 2, It is noted that four insurable structures were located west of Centre A venue prior to this project. These structures were removed as part of the proposed roadway extension, The effect of the project was the division of the original BNRR detention pond into two hydraulically connected detention ponds, The Extended Transport Block of the U ,S. EPA's Storm Water Management Model (EXTRAN) was utilized to evaluate the operation of the ponds and the connecting culverts, EXTRAN is a dynamic flow routing model based on the Saint-Venant r:\OPF.N\cocsrtnl cOl:srtUl km;1-LO\tIR tlnal-rpt,\~rJ 6 ANdERSON CONSulTiNG ENGiNEERS_ INC.