Loading...
HomeMy WebLinkAboutCSU STUDENT HOUSING UNIVERSITY VILLAGE EXPANSION ADVISORY REVIEW - 20 93 - SUBMITTAL DOCUMENTS - ROUND 2 - DRAINAGE REPORTi - 11 I 1 u . Q ELEMENTAW ❑ 6Ct40M a i i 1 In FUM 6T►EET 11 i i 1 1 1 1 1 NORTHWEST -'SITE a4cDQ= s=,V a d o/emu )ADVIEW 6TPRM / / 0 0 n V 6UBOM6iCl� ALTEPO"TE MLTI.FAmO !S VEST ELMADETH 6711MV CD pip SITE PLAN �V v . imo � Nolm ---�- - -- --- =t-M=.W / PMTAIL I n Il I I f-1 SOUTHEAST SITE cow WOO aAYSTCPE ' a cc L�cn0 W M c) Q Z o CJJ 0LA-� Qg o o Q~CC C-7 OCw° 00o O u _4- - - Q c) ,—, U "P-M PRMCT e CAR S-3.s CRO N e S P*1 APPENDIX REFERENCES 1. Storm Drainage Design Criteria and Construction Standards by the City of Fort Collins, Colorado, January 1992. 2. Erosion Control Reference Manual for Construction Sites by the City of Fort Collins, Colorado, January 1991. M I V. EROSION CONTROL ' A. General Concept ' Both sites lie within the Moderate Rainfall and Moderate Wind Erodibility Zones per the City of Fort Collins Zone maps. The potential exists for erosion problems during construction, and after construction until all disturbed areas are vegetated. It is anticipated that construction will begin ' in the summer of 1993. ' The Erosion Control criteria, as established by the City of Fort Collins Erosion Control Reference Manual for Construction Sites will be utilized during final design to determine the erosion Control Performance standard ' required for these sites. An appropriate Erosion Control Plan for these sites during and after construction will be established during final design and submitted to the City prior to the start of construction. L ' VI. CONCLUSIONS A. Compliance with Standards ' All computations within this report have been completed in compliance with the City of Fort Collins Storm Drainage Design Criteria. ' B. Drainage Concept The proposed drainage concepts presented in this report and shown on the preliminary drainage and grading plans adequately provide for the transmission of developed runoff from these projects to downstream ' improvements. ' C. Erosion Control Concept The City of Fort Collins Erosion Control Standards and Specifications will be utilized during and after construction in minimize the impacts of development of this site. 7 We analyzed the results of the addition of the proposed CSU grading to the HEC2 computer model and have summarized below the results: Section 100 yr WSEL Version 11 5030.79 Original 12 5035.40 Original 13 5038.14 Original 11 5030.79 Modified 111 5034.83 Modified 12 5035.40 Modified 13 5038.14 Modified 11 5030.79 Revised 111 5034.50 Revised 12 5034.97 Revised 13 5038.14 Revised With the proposed CSU grading plan and the Revised HEC2 computer model, we see that at cross sections 111 and 12 the 100 year water surface elevations are actually lower than the original model. This is do to the encroachment imposed upon cross sections 111 and 12 by the original model and the revised model. The net conclusion is that the proposed grading on the CSU property north of Elizabeth Street and west of Park Avenue does not impact upon the original 100 year floodplain as established by RCE in January of 1992. In addition, the original 100 year floodplain elevations in this area will not be raised as a result of the proposed grading on the CSU property. High groundwater is present at the southeast site. A proposed perforated underdrain will be proposed with this site, as shown on the preliminary grading and drainage plan. This underdrain will tie into the existing storm sewer system. C� Developed subbasin 1 coincides with historic basin 1. Developed runoff from this subbasin will also sheet flow to the northeast undetained, but at a rate less than historic. This is due to the reduce size and imperviousness of the subbasin. Developed runoff from subbasin 2 will be directed to the southeast corner of the site to a detention pond located within the parking area and the landscaping. Two proposed area drains will pipe detained flows to the existing storm sewer in West Elizabeth Street. Developed release will be limited to 2 year historic. This site lies within a flood plain as defined by a HEC2 model developed by the City of Fort Collins in conjunction with Resource Consultants. RBD, Inc. reviewed the existing HEC2 model by RCE and concluded that an additional cross section should be added between existing cross section 11 and 12. A copy of the existing input and output files are enclosed. It should be noted that cross section 12 has an encroachment imposed upon the cross section to force the model to only analyze the conveyance capacity of the property south of Elizabeth Street and not Elizabeth Street and the CSU property. The 100 year water surface elevation calculated at cross section 12 was then superimposed across the CSU property even though the CSU property was not a part of the modeling calculations. We modified the existing HEC2 model by RCE by adding a cross section between section 11 and 12 and calling it cross section 111. To be consistent with the encroachment originally imposed upon cross section 12 by RCE, we also placed an identical encroachment upon the new cross section 111. The model was run and the output compared with the original model output and the results were identical. A copy of the modified input and output files are enclosed. We revised the modified HEC2 model to include the proposed CSU grading north of Elizabeth Street on cross sections 111 and 12. Also we revised the encroachment upon cross section 111 and 12 and pulled the encroachment from the south side of Elizabeth Street to the north side of Elizabeth Street to the top of the berm separating the CSU parking lot from Elizabeth Street. If we had left the encroachment alone as originally,, modeled, then there would have been no change in the 100 year water surface elevation between the original model and the revised model because CSU was not included in the calculations of either situation. 5 ridge, also historically drains to the New Mercer Canal. Historic Basin H-3, located in the southwest corner of the site, drains naturally to the regional detention pond. Developed basins within the site have been divided into 4 subbasins; NW1, NW2, NW3, and 26. Subbasin NW1 is a small portion of the northern edge of the site, entirely composed of landscaping, and will sheet flow northeasterly, eventually into the New Mercer Canal. Subbasin NW3 is a small portion of the site between the parking lot and the northern edge of the New Mercer. Developed flows from this basin will sheet flow to the New Mercer Canal at less than historic conditions. Subbasin NW2 is the easterly end of the project. Developed flows from this basin will be directed to a low point in the parking area, and on to a water quality pond and outlet structure that will help detain low flows and improve water quality prior to flows being released into New Mercer Canal. From previous discussions with the ditch company, there concerns deal primarily with the 2 year or minor storm events. ditch company inputs and approvals will be sought for this project and concept prior to final design. The historic release from basins NW1 and NW2 to the New Mercer Canal are approximately 1.07 cfs (Historic basin H-1). Total developed release from NW1 and NW2 will be limited to 0.28 cfs for the 2 year storm event. An emergency overflow weir will be provided for larger storms. Developed subbasin 26 represents the majority of the site (Basin 26 coincides with basin 26 of the SWMM model). Developed flows from this basin will be directed to the southwesterly corner of the site, and released into the regional detention pond located south of this project. The existing SWMM model was revised to reflect the development of this site. During the 100 year storm event, the revised model indicated that approximately 0.1 acre feet of additional detention storage was required. This additional storage requirement was found to raise the existing 100 year water surface elevation from 54.80 to 54.82. For the 2 year event, no change was found in the detention storage requirements. SOUTHEAST SITE - Historically, the majority of the site drains to the southeast. A small portion of the northern end of the site drains to the northeast to West Plum Street. A small basin (Historic basin 0-1) will also drain across this site. No detention will be provided for this upstream historic drainage basin. 4 Developed runoff from the southeast site will be released at or less than a 2 year historic rate. A small part of the northern end of the site will runoff ' undetained to the northwest at less than historic runoff. The majority of the site will be routed to the southeast corner of the site, and detained and released at historic rate into the existing storm sewer system located in Elizabeth Street. It is anticipated that there will also be a groundwater interception system tied into the existing storm sewer system. IC. Hydrological Criteria The rational method was used to determine peak runoff from these two developed sites, and surrounding off -site tributary areas. The 2 year and 100 year rainfall criteria, which was obtained from the City of Fort Collins, 1 is the criteria which was used. A copy of this criteria is included in the appendix of this report. A copy of the SWMM model for the West Plum Street basin was obtained from the City of Fort Collins, and the 100 year model was updated to reflect ' the improvements to the Northwest site to analyze detention requirements and peak runoff. During final design, the 2, 5, 10, 25, 50 and 100 year SWMM models for West Plum Basin will be updated, and a revised model provided to the City Stormwater Utility. D. Hydraulic Criteria All calculations within this study have been prepared in accordance with the City of Fort Collins Drainage Criteria. E. Variances from Criteria No variances are being sought for this project. 1 ' IV. DRAINAGE FACILITY DESIGN NORTHWEST SITE - Historically, the northwest site is divided into three 1 basins; depicted on the preliminary grading and drainage plan, as historic basins H-1, H-2, and H-3. Historic basin H-1 is north of the existing ridge that bisects the site, and drainage from this basin is generally directed to the ' northeast and to the New Mercer Canal. Historic basin H-2, south of the 3 DRAINAGE BASINS A. Major Basin Description The Northwest site is located within the Canal Importation Basin as shown on the City of Fort Collins Stormwater Basin Map. It is also within the West Plum Street Drainage Basin. A SWMM model exists for the West Plum Basin, and this existing model has been updated to reflect development of this project. No major drainageways exist on -site. There is a regional detention pond located south of this project, and west of Rams Village. The southeast site is located within the Spring Creek Major Drainage Basin. Detention for the site will be determined from historical condition. No major drainageways exist on -site, however, part of this site have been identified as being a part of a floodway as determined by a HEC2 analysis performed by RCE on 1/21/92. A complete copy of the analysis of this flood information, and the impacts of the development of the southeast site have been included in the appendix. DRAINAGE DESIGN CRITERIA A. Regulations The City of Fort Collins Storm Drainage Design Criteria is being used for the subject sites. B. Development Criteria Reference and Constraints The Northwest site is naturally divided approximately in half by an existing ridge line. The majority of the site historically drains to the New Mercer canal. The intent of the drainage concepts presented within this report would have the majority of the site being directed to a low point in the southwest corner of the site and released and detained into the existing regional detention pond. A smaller portion of the site will be graded to drain to the east, and released into the New Mercer Canal at less than the historic rate. A water quality pond and outlet structure will be constructed and maintained upstream of the release point. Ditch Company input and approval is currently being pursued. 2 ',7 LL 1 I. d 1 I 1 Il I PRELIMINARY DRAINAGE AND EROSION CONTROL STUDY FOR COLORADO STATE UNIVERSITY HOUSING NORTHWEST AND SOUTHEAST SITES FORT COLLINS, COLORADO GENERAL LOCATION AND DESCRIPTION A. Location The project area is divided into two sites. The northwest site consist of a vacant tract of land bordered on the west by Skyline Drive, on the north by residential housing and surrounded on the south and east by the New Mercer Canal. The Ram Village Apartment Complex also borders this site to the south and east. The property is vegetated with dense grass and weeds. Drainage is generally from west to east. A portion of the New Mercer Canal has recently been relocated to the easterly edge of the site, and the old ditch section filled in. The northwest site contains 7.4 acres. There are 80 family units proposed for this site, with a density of 10.8 units/acre. The southeast portion of this development is located at the northwest corner of West Elizabeth Street and City Park Drive. The area is bordered on the west by Larimer No. 2 Canal, and on the north by West Plum ,Street. The Ram Village Apartment Complex is located north and west of this project site. The area is presently vegetated with grass and weeds. The southeast site contains 8.69 acres, of which 6.54 acres is proposed for 120 units of graduate/single student apartments at a density of 18.34 units/acre. Both the Northwest and the Southeast sites can be further described as being a portion of the north half of Section 15, Township 7 North, Range 69 West of the 6th P.M., City of Fort Collins, County of Larimer, State of Colorado. 1 TABLE OF CONTENTS DESCRIPTION PAGE I. GENERAL LOCATION AND DESCRIPTION 1 A. LOCATION 1 II. DRAINAGE BASINS 2 A. MAJOR BASIN DESCRIPTION 2 III. DRAINAGE DESIGN CRITERIA 2 A. REGULATIONS 2 B. DEVELOPMENT CRITERIA REFERENCE AND CONSTRAINTS 2 C. HYDROLOGICAL CRITERIA 3 D. HYDRAULIC CRITERIA 3 E. VARIANCES FROM CRITERIA 3 IV. DRAINAGE FACILITY DESIGN 3 V. EROSION CONTROL 7 A. GENERAL CONCEPT 7 VI. CONCLUSIONS 7 A. COMPLIANCE WITH STANDARDS 7 B. DRAINAGE CONCEPT 7 C. EROSION CONTROL CONCEPT 7 REFERENCES 8 APPENDIX VICINITY MAP 1 NORTHWEST SITE HYDROLOGY 3 NORTHWEST SITE DETENTION 7 NORTHWEST SITE OUTLET TO THE NEW MERCER CANAL 25 SOUTHEAST SITE HYDROLOGY 30 SOUTHEAST SITE DETENTION 33 SOUTHEAST SITE DESIGN OF INLETS, STORM SEWER AND SWALES 38 SOUTHEAST SITE HEC2 ANALYSIS 40 CHARTS, TABLES, FIGURES 75 RMINC. Engineering Consultants 209 S. Meldrum Fort Collins, Colorado 80521 303/482-5922 FAX:303/482-6368 June 1, 1993 Mr. Glen Schluetter City of Fort Collins Utility Services Stormwater 235 Mathews Fort Collins, Colorado 80522 RE: Preliminary Drainage and Erosion Control Study Colorado State University Housing Projects Northwest and Southeast Sites Dear Glen: We are pleased to submit to you, for your review and approval, this Preliminary Drainage and Erosion Control Study for the Colorado State University Housing Projects. All computations within this report have been completed in compliance with the City of Fort Collins Storm Drainage Design Criteria. We appreciate your time and consideration in reviewing this submittal. Please call if you have any questions. Respectfully, RBD Inc. Engineering Consultants Roger A. Curtiss, P.E. Project Engineer Other Offices: Denver 303/458-5526 • Vail 303/476-6340 PRELIMINARY DRAINAGE AND EROSION CONTROL STUDY FOR COLORADO STATE UNIVERSITY STUDENT HOUSING NORTHWEST AND SOUTHEAST SITES FORT COLLINS, COLORADO June 1, 1993 Prepared for: Aller Lingle Architects, P.C. 748 Whalers Way Building E, Suite 200 Fort Collins, Colorado 80525 Colorado State University Facilities Services Fort Collins, Colorado 80523 Prepared by: RBD, Inc. Engineering Consultants 209 South Meldrum Fort Collins, Colorado 80521 (303) 482-6368 RBD Job No. 274-005