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Home My WebLink AboutRETREAT AT 1200 PLUM ST. - PDP - 34-08 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORT■ NORTHERN Preliminary Drainage Report ENGINEERING The Retreat at 1200 Plum 10 These three models will verify whether the proposed detention concept is acceptable. Partial detention for Basin 1 may be provided by means of an underground storage system. The only available outfall for the proposed underground storage system is the existing storm sewer in Plum Street, which is very shallow (less than 3 feet from invert of pipe to street grade). Because of this, the underground storage will need to be set up as a pumped system. Detention for proposed Basin 2 will be provided within the top deck of the proposed parking garage. A release structure will be designed at final which will control release to the below the historic 2-year rate of 0.2 cfs for Basin H2, as calculated in Appendix A. The structure will consist of an orifice plate placed within the downstream -most inlet box on the upper deck of the parking garage, and will control release to 0.1 cfs (please see sketch provided in Appendix B). A required100-year detention volume of approximately 0.26 acre-foot has been calculated in Appendix B. Water quality treatment will be provided for Basin1 by "Filterra" tree well filters. Runoff from the rooftops within Basin 1 will be directed into the adjacent Plum St., which will allow the utilization of these tree well filters that are proposed to be placed within the curb and gutter section of the street. A drain system will be provided which will convey storm runoff from the courtyards located within the interior of the residential complex. This drain system will convey runoff mainly from the courtyards which consist of concrete and grassed areas; therefore, a water quality treatment is not necessary for water conveyed in this system. Water quality treatment for Basin 2 will be provided by means of a Stormceptor or similar type of filtration unit. 9.0 Temporary Erosion Control A temporary erosion control plan will be provided at final design which will minimize sediment leaving the project site during construction. Silt fence will be placed along the perimeter of the site to minimize sediment entering adjacent Right of Way. Inlet filters will be placed at all inlets in order to minimize sediment entering the receiving storm sewer system. 10.0 Conclusions The proposed project will comply at final design with drainage criteria outlined in the City of Fort Collins Storm Drainage Design Criteria. -5- NORTHERN Preliminary Drainage Report WENSINEERINS The Retreat at 1200 Plum 6.0 Proposed Development The project site is proposed to be developed with a multi -story residential complex and a multi -story parking garage. The proposed residential complex is to be located along the north side of Plum St. The proposed parking garage is to be constructed within the portion of the property south of Plum St. 7.0 Proposed Drainage Plan The drainage plan for the proposed project is intended to generally match historic drainage patterns. Storm runoff from the north portion of the site, noted on the Preliminary Drainage Plan as Basin 1, will be conveyed primarily by sheet flow to the historic point of discharge, which is the adjacent Plum St. A portion of storm runoff generated by the courtyards within the residential complex in Basin 1 will be conveyed by a small diameter storm line system that will drain to the existing storm line in Plum St. Runoff from the south portion of the site, noted as Basin 2, will be detained on the top deck of the proposed parking garage as described below. Release from this detention area will be conveyed by a storm line system that will drain to the existing storm line in Plum St. 8.0 Proposed Detention and Water Quality Based on discussion in a meeting which occurred October 2, 2008 with the City of Fort Collins Stormwater utility, it may be possible to not detain runoff or partially detain from proposed Basin 1 by compensating with overdetention in proposed Basin 2. Basin 2 will overdetain storm runoff, reducing the 100-year peak rate to below the historic 2-year peak rate; thereby, reducing the quantity of storm runoff in Plum St. Basin 1 will either not detain runoff or partially detain runoff so that no greater than historic volume is created by development of the proposed project. At final design, three models will be presented which will show the results of analysis of: • Overall hyrdrologic basins and the effect of overdetaining within proposed Basin 2 and partial detention within proposed Basin 1. • Local hyrdrologic basins and the effect of overdetaining within proposed Basin 2 and partial detention within proposed Basin 1. • Plum St. and its ability to convey 2-year, 10-year, and 100-year flows. -4- a NORTHERN Preliminary Drainage Report �_ ENGINEERING The Retreat at 1200 Plum 4.0 Historic Drainage and Maior Drainage Basin Historic drainage patterns are shown on the Historic Drainage Exhibit provided in Appendix A. The site has been broken into 2 historic drainage basins in this exhibit noted as Basins H1, H2, and H3. Basin H1 is composed of the single family residential area within the north portion of the project site. Basins H2 and H3 are composed of the multi-plex residential area within the south portion of the project site. Existing imperviousness and runoff rates for Basins H1, H2, and H3 have been calculated in Appendix A. Historic runoff from both Basins H1 and H2 is conveyed generally by sheet flow into Plum St. Runoff from Basin H3 is conveyed by sheet flow to a parking lot located southeast of the site. The project site is located within the Old Town major drainage basin. 5.0 FloodDlain Encroachment The project site is not encroached by any FEMA or City of Fort Collins designated floodplain. -3- NORTHERN ENGINEERING Preliminary Drainage Report The Retreat at 1200 Plum 3.0 Site Location and Description The project site is located on the north and south sides of Plum St. between Daisy St. and Aster St., in Fort Collins, Colorado. The portion of the site north of Plum St. consists of residential single family homes, the portion of the site south of Plum St. consists of multi-plex housing. NORTHERN Mllift ENGINEERING 1.0 Scope Preliminary Drainage Report The Retreat at 1200 Plum This preliminary drainage report has been prepared for the proposed The Retreat at 1200 Plum, and is intended to provide information regarding existing and proposed storm drainage including: • Historic drainage patterns and quantities of flow • Proposed drainage patterns and quantities of flow • Proposed detention and water quality concepts 2.0 Design Criteria and References Drainage criteria outlined in the City of Fort Collins Storm Drainage Design Criteria (SDDC) has been referenced in this study, and will be complied with at final design The Rational Method has been used to estimate peak discharge rates in the initial and major storm events for both historic and proposed conditions in this study, and will also be used at final design to estimate peak discharge rates. The initial and major storm events are considered to be the 2-year and 100-year storm events, respectively. Runoff coefficients and rainfall values utilized in the Rational Method are consistent with the City of Fort Collins SDDC and will also be utilized at final design. - 1 - TABLE OF CONTENTS 1.0 Scope 1 2.0 Design Criteria and References 1 3.0 Site Location and Description 2 4.0 Historic Drainage and Major Drainage Basin 3 5.0 Floodplain Encroachment 3 6.0 Proposed Development 4 7.0 Proposed Drainage Plan 4 8.0 Proposed Detention and Water Quality 4 9.0 Temporary Erosion Control 5 10.0 Conclusions 5 APPENDIX APPENDIX A - HISTORIC AND PROPOSED DRAINAGE CALCULATIONS, EXHIBITS APPENDIX B - DETENTION CALCULATIONS NORTHERN mum nME9 O-M-4'2 Mom - ENGINEERING RratfdlrMtC e� +wn�aUe�ioe�mgomi is,C0811SM fAL 97flg1A1M y y City of Fort Collins Stormwater Utility 700 Wood St. Fort Collins, CO 80521 October 21, 2008 RE: Preliminary Drainage Report The Retreat at 1200 Plum Dear Staff: Northern Engineering Services, Inc. is pleased to submit this Preliminary Drainage Report for your review. We understand that review by the City is to assure general compliance with standardized criteria contained in the City of Fort Collins Storm Drainage Design Criteria. If you should have any questions as you review this report, please feel free to contact me at your convenience. Sincerely, NORTHERN ENGINEERING SERVICES, INC. 044..' L, Aaron Cvar, PE Senior Engineer Preliminary Drainage Report for The Retreat at 1200 Plum Fort Collins. CO October 21, 2008 Prepared for: Glenwood Intermountain Properties Prepared By: N NORTHERN ENGINEERING 200 South College Ave., Suite 100 Fort Collins, Colorado 80524 Phone: (970) 221-4158 Fax:(970) 221-4159 Project Number: 410-002