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HomeMy WebLinkAboutCIVIC CENTER PARKING STRUCTURE - PDP - 35-98 - REPORTS - DRAINAGE REPORTM6, Final Drainage Report ILCivic Center Parking Struct NOLTE and ASSOCIATES, Inc. Engineers / Planners / Surveyors REFERENCES A. City of Fort Collins Storm Drainage Design Criteria and Construction Standards, City of I Fort Collins Stormwater Utility, May 1984. IB. Urban Storm Drainage Criteria Manual, Wright -McLaughlin Engineers, Inc., Denver Regional Council of Governments, Denver 1969. C. Old Town Drainage Basin Plan, City of Fort Collins, Resource Consultants and Engineers, Inc, January 1993. ID. HEC-2 Computer Program - Water Surface Profiles, U.S. Army Corp of Engineers Hydraulic Engineering Center, Davis, California, 1990. ' NOLTE and ASSOCIATES, Inc. 8 N:\FC0053Wdmin\PKGStrDrainReport.doc 1 Final Drainage Report W*4 Civic Center Parking Structure NOLTE and ASSOCIATES, Inc. Engineers / Planners / Surveyors I� erosion will be minimized. The Grading and Erosion Control Plan in the construction drawings indicates the type and timing of proposed measures. Water quality of stormwater discharge from the site in the long-term will be controlled by �! downstream regional facilities to be installed with City Master Plan improvements. 5.0 CONCLUSIONS/SUMMARY 5.1 COMPLIANCE WITH STANDARDS I' The proposed storm drainage design for the site complies with the intent of the following regulations, standards, and reports: h ♦ City of Fort Collins Storm Drainage Design and Technical Criteria ♦ Old Town Master Drainage Plan for the City of Fort Collins, by Resource Consultants and Engineers, dated January 1993. 5.2 DRAINAGE CONCEPT The drainage system is designed to control storm water discharges, protect surface water quality, and to safely convey runoff from the site in compliance with local regulations and consistent with current studies of the major drainageway basin. NOLTE and ASSOCIATES, Inc. 7 N:\FC0053Wdmin\PKGStrDrainReport.doc p'riy.g�r?. ' Final Drainage Report i Civic Center Parking Structure NOLTE and ASSOCIATES, Inc. Engineers / Planners / Surveyors HEC2 WATER SURFACE ANALYSIS •' Section # Master Plan Master Plan Existing Topo Proposed `` Original Station 100-yr. WSEL 100-yr. WSEL 100-yr. WSEL 'J 76.5 --- --- 81.30 81.30 77 3170 82.2 82.23 82.23 77.5 --- --- 82.43 82.43 78 3320 82.55 82.47 82.47 f) 78.5 --- --- 82.57 82.57 1 79 �1 3580 82.8 82.78 82.80 Note, that the HEC2 analysis includes 5-year, 10-year, 25-year, 100-year, and 500-year flows. (Reference Table 5.1, Master Plan which has been included in Appendix A.) The HEC2 model was run with the area occupied by the parking structure blocked out. This area represents are small portion of the total conveyance available in the cross-section. The result is that the parking structure will have no impact on the Old Town floodplain. Output from of the analysis is summarized in Table 1. In reviewing the HEC2 model and floodplain limits developed in the Master Plan, it is apparent that this site is included in the floodplain because of a backwater condition. No part of the site, except for approximately 20 feet adjacent to Laporte Avenue, was modeled as providing conveyance for any flood discharges from the Old Town Basin. The site is still subject to flooding, however, and first floor elevations for the commercial space and parking structure have been set 1.5 feet above the 100-year flood elevation in Mason Street, BFE for Mason (Section 78) = 4982.6, the finished floor being set equal to 4984.10. 4.4 EROSION CONTROL AND WATER QUALITY Erosion control during construction of this project will consist of silt fence installed around the perimeter of the site. Once foundations are in place and floor slabs constructed, the potential for NOLTE and ASSOCIATES, Inc. 6 N:\FC0053Wdmin\PKGStrDrainReporLdoc Fort Collins Old Town Existing River= RIVER-1 Reach = Reach-1 Section 78 RS = 3320 .013 .013- )-k .013 Legend WS 500-Year WS 100-Year WS----�-- 4987 _ WS 10-Year WS 5-Year Ground • Bank Sta 4986 4985 c 0 'w W 4984 4983 MASON STREET PROFILE 4982 0 200 400 600 800 Station (ft) • :_ u_-._ _ inn n � :_ u_i _ � n FIGURE. 3 Fort Collins Old Town Existing River= RIVER-1 Reach = Reach-1 Section 77 RS = 3170 013 .013 >j< .013 1 4986Legend ■ WS 500-Year x WS 100-Year WS 25-Year 4985 WS 10-Year WS 5-Year Ground Ineff • Bank Sta 4984 LAPORTE AVENUE = O 4983 PARKING STRUCTURE > a w r 4982 LOST CONVEYANCE 4981 4980 0 200 400 600 800 Station (ft) t 1 111"WIM. - I..., 1 11. V.. I. - . L FIGURE 2 ■, Final Drainage Report Civic Center Parking Structure NOLTE and ASSOCIATES, Inc. t Engineers / Planners / Surveyors " `r t minor event. Major event flows will be partially intercepted by the storm sewer, and the remainder will surcharge the inlet and discharge directly into La Porte Ave. Basin B consists of historic offsite street flows from a portion of Mason Street and La Porte Ave. Flows from this basin are intercepted by an existing 10' Type R Inlet (design pt. 2) at the southeast corner of Mason Street and La Porte Ave. This inlet will be relocated approximately 17' to the east from its original location due to the widening of Mason Street. Flows intercepted by this inlet were originally piped to the storm sewer system in Mason Street via 15" RCP to an existing inlet at the northeast corner of La Porte Ave. and Mason Street. The existing 15" RCP will be replaced by the proposed 18" RCP originating from the parking structure. The inlet will now tie into the proposed 18" RCP via manhole. 4.3 IMPACTS TO THE FLOODPLAIN The Old Town floodplain was delineated in the Master Plan using the HEC2 computer program (Reference D). Input and output files are listed in the Technical Addendum, Volume 1, to the Master Plan. The HEC2 input file for Reach 6 was copied and run as part of this analysis to duplicate the results of the Master Plan, and to determine the effect this project will have on existing flood elevations. The HEC2 input and output files from the Master Plan are included in the Appendix A to this report. The HEC2 analysis was run with the detailed existing topography (historic) and compared with the proposed structure cross -sections. In order to provide more detail, three cross -sections were added to the HEC2 analysis. The original HEC2 sections were 77, 78, and 79. Nolte has added 76.5, 77.5, and 78.5. These cross -sections are shown on the drainage exhibits (Historic and Proposed grading and erosion control), included in the back pockets. NOLTE and ASSOCIATES, Inc. 5 N:\FC0053Wdmin\PKGSErDrainReport.doc I Final Drainage Report Civic Center Parking Structure NOLTE and ASSOCIATES, Inc. Engineers / Planners / Surveyors 3.2 RUNOFF CALCULATIONS The Rational Method has been used to calculate peak runoff from the 10-year and 100-year recurrence interval storms. Storm water detention is not provided, but discharges from the site will not be increased over the "existing" conditions because the site was completely paved before this project. Peak flow calculations are listed in the Appendix and summarized on the Drainage Exhibit. 4.0 DRAINAGE FACILITY DESIGN 4.1 GENERAL CONCEPT Runoff from the site will be collected on the top level of the parking structure at inlets and floor drains which are connected with the floor drains on lower levels. The structure drains will be connected to a single outlet pipe to the northwest side of the building (Design Point 1 on the Drainage Exhibit) sized to carry the 10-year storm flows. At this location, runoff will be discharged into a proposed 18" RCP that runs west on La Porte Ave. and ties into an existing inlet at the northeast corner of La Porte Ave. and Mason Street. The 100-year flows will collect on the top level and travel down the ramps to discharge into La Porte Avenue. Historically, this flow has collected in the alley directly east of the site. The proposed 100-year discharge into La Porte helps channel flows away from the alley. The 100- year flow from the parking area is 3.1cfs, it is the same as existing conditions, and will have no significant effect on the flows in La Porte Avenue. The 100-year flow at Section 77 is 370 cfs, the garage flow is 1 % of this flow. As shown in the HEC2 analysis this increase along La Porte Avenue has no significant effect. 4.2 BASIN DESCRIPTION For the purpose of calculating runoff, the entire site is considered completely impervious. The site is comprised of a single basin, identified by the letter A, as shown on the Drainage Exhibit. Potential off -site discharges from the west will be conveyed in Laporte Avenue on the north side of the property. No off -site flows will be conveyed through the site except for flood plain flows which have done so historically. Basin A consists of the parking structure and commercial space within the boundary of the property. Discharges from the site will be conveyed by a proposed 18" RCP to an existing inlet on the northeast corner of Mason Street and La Porte Ave. This storm sewer is designed for the NOLTE and ASSOCIATES, Inc. 4 N:\FC0053W.dmin\PKGStrDrainReport.doe P%rmjw NOLTE and ASSOCIATES, Inc. Engineers / Planners / Surveyors Final Drainage Repo Civic Center Parking Stri reduce flooding problems in the vicinity of the Civic Center Parking Structure. All topographic mapping has been referenced to the City of Fort Collins "Black Bolt" benchmark system. 2.0 DRAINAGE BASINS AND SUB -BASINS 2.1 MAJOR BASIN DESCRIPTION The proposed development lies within the Old Town Drainage Basin. The Basin is located in the central part of Fort Collins and is almost completely developed. There is no major drainage channel, and runoff is carried within a network of storm sewers and street sections to the Poudre River. The Basin is also subject to discharges resulting from irrigation surcharge flows occurring in the basin upstream of Shields Street. Because this project is within the Old Town Basin floodplain, the proposed structures are elevated above the 100-year flood elevations.. 2.2 SUB -BASIN DEVELOPMENT As shown on the Historic Drainage Exhibit at the back of this report, the site historically drained overland to the east to an existing storm sewer in the alley, and north to an existing storm sewer in Laporte Avenue. The site has accepted sheet flows from the offsite area to the west which may infrequently overtop Mason Street. Upon development, the site will drain to the existing storm sewer system at the southeast corner of La Porte Ave. and Mason. See section 4.2 for a specific description of the storm system improvements. This change of routing 10-year flows was worked out with the City of Fort Collins stormwater division to help relieve flooding problems and tie this drainage system to future overall downtown improvements. 3.0 DRAINAGE DESIGN CRITERIA 3.1 CRITERIA This drainage report was prepared in accordance with the City of Fort Collins' Storm Drainage Design Criteria and Construction Standards Manual (Reference A). The design rainfall data used to calculate runoff for the site was obtained from the Time -Intensity -Frequency curves contained within the City's Criteria Manual. The site drainage plan was prepared with consideration given to the Master Plan for Old Town Basin prepared by Resource Consultants, Inc. There are no specific site constraints that will adversely impact the drainage.. NOLTE and ASSOCIATES, Inc. 3 N:\FC0053Wdmin\PKGStrDrainReport.doc -.� Gravel J j-PilS '.. �N� ) ` �•III I [ 1\ ��-r�—J�� �/ \ • 8M , Gq \:\\ -- _ 0h II I . 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D Ln IIM LB R 'ST,I 11 _' J IB � � * _ _ � IMU FR STL rDu I ❑"I!Sch` �,I_ I i(I r Mu Laj_E . • 1 . 1 ( ll oN r 3 ill - p7 [l / ■ ` i y i.., n L! .:. ���,r�----1,I�Jp\�. j�/�I/I II - - — - _4 ro ii 13 d r T ORADO YIlof I • 'o UNIVE IT ` • •073 1 0T•'. �'!W •r. • II11II � I • .ul _� 1 u• --- 1 f — I as P c fi.i� nil f i S065 - - _ __ _ Lt ��-I . 1• 1 1 .. B o n •• J \ p — i T �- _ I _. "lie. F,, 4,t) 4r FIGURE 1 - VICINITY MAP ■, Final Drainage Report Civic Center Parking Structure NOLTE and ASSOCIATES, Inc. Engineers / Planners / Surveyors 1.0 GENERAL LOCATION AND DESCRIPTION 1.1 LOCATION The proposed civic center parking structure and commercial facility is located on the northwest quadrant of Block 21 in downtown Fort Collins. It is located in the northeast `/< of Section 11, Township 7 North South, Range 69 West of the Sixth Principal Meridian, see Figure 1, Vicinity Map. To the south of the site is a single story commercial building; to the north across Laporte Avenue is Washington's bar, the Old Fort Collins Freight Depot and new Temporary Parking; to the east is more commercial buildings with store fronts on College Avenue; and to the west across Mason Street is the proposed Larimer County Justice Center. Within La Porte Avenue right-of-way, at the intersection with Mason Street, southeast corner, a 10' Type R storm sewer inlet and 15" RCP line exist, which discharge through an 18" RCP to a 36" RCP north along Mason Street to the Poudre River. 1.2 DESCRIPTION OF PROPERTY The parking structure lot is approximately 1.75 acres in area and is currently paved and used for surface parking. The ground slopes downward from the west to the east at approximately 1.0 percent. The overall change in grade across the lot is just over two feet, from a maximum elevation of 4982.4 to a minimum of 4980. The proposed development will include a single three-story building for parking, with a single story building for commercial uses along Mason Street. There are no landscape areas proposed, so the site will be virtually impervious. 1.3 PREVIOUS REPORT AND STUDIES The site is in the drainageway known as Old Town Basin. The flood prone areas were delineated in a master planning study, and are documented in a report entitled, Old Town Master Drainage Basin Plan, City of Fort Collins, (Master Plan) prepared by Resource Consultants and Engineers, Inc., dated January 1993 (Reference C). This report includes a hydrologic model evaluation of the downtown area storm sewer system, delineation of the floodplain, and recommendations for future improvements to alleviate flooding problems. The Civic Center Parking Structure is adjacent to Reach 6 of the Old Town Basin drainageway. The Master Plan calls for a future diversion channel to be constructed upstream (west) of Mason Street which will significantly NOLTE and ASSOCIATES, Inc. 1 N:\FC0053\AdminIPKGSLrDrainReport.doc I I 1 M 6k NOLTE and ASSOCIATES, Inc. Engineers / Planners / Surveyors Final Drainage Report Civic Center Parking Structure CERTIFICATION I hereby affirm that this report for the final drainage design of the Civic Center Parking Structure, was prepared under my direct supervision in accordance with the provisions of the City of Fort Collins Storm Drainage Design Criteria for the owners thereof I understand that the City of Fort Collins does not and will not assume liability for drainage facilities designed by others. Patrick W. McNamara, P.E. & P.L.S. State of Colorado No. ii N:\FC0053\Admin\PKGStrDrainReport.doc r, " Final Drainage Report r**Mj , Civic Center Parking Structure NOLTE and ASSOCIATES, Inc. Engineers / Planners / Surveyors TABLE OF CONTENTS PAGE CERTIFICATION............................................................................................................ ii 1.0. GENERAL LOCATION AND DESCRIPTION.............................................................. I 1.1 Location................................................................................................................1 1.2 Description of Property.........................................................................................I 1.3 Previous Report and Studies................................................................................. I FIGURE1 - VICINITY MAP...........................................................................................2 2.0 DRAINAGE BASINS AND SUB-BASINS.....................................................................3 2.1 Major Basin Description.......................................................................................3 2.2 Sub -Basin Development.......................................................................................3 3.0 DRAINAGE DESIGN CRITERIA...................................................................................3 3.1 Criteria..................................................................................................................3 3.2 Runoff Calculations..............................................................................................3 4.0 DRAINAGE FACILITY DESIGN...................................................................................4 4.1 General Concept....................................................................................................4 4.2 Basin Descriptions................................................................................................4 4.3 Impacts to the Floodplain......................................................................................4 4.4 Erosion Control and Water Quality......................................................................8 5.0 CONCLUSIONS/SUMMARY.........................................................................................8 5.1 Compliance with Standards..................................................................................8 5.2 Drainage Concept..................................................................................................9 REFERENCES...............................................................................................................10 APPENDIX DRAINAGE EXHIBIT i N:\FC0053Wdmin\PKGStrDrainReport.doc CIVIC CENTER PARKING STRUCTURE FINAL DRAINAGE REPORT for Parsons and Associates 432 Link Lane Plaza Fort Collins, Colorado 80524 (970) 221-2400 by Nolte and Associates, Inc. 7000 S. Yosemite Street, Suite 200 Englewood, Colorado 80112 (303) 220-1001 July 1998 Revised: September 1998