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HomeMy WebLinkAboutNORTHGATE COMMONS PUD, PHASE 1 - PRELIMINARY - 38-96A - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTt by 1� TERRY LAI ti� r r PROJECT SITE z OZ -o 9n � 9� �O 9Q EAST WLLO% LANE CgCy f Cq pOUO� f R/lz;y Ep0 a 0 W Y Q J I COUNTRY CLUB ROAD ol SCALE: 1'=1000' SPAULDING LANE m W UUL W J J U VICINITY MAP D. Erosion Control Concept Per the City of Fort Collins Erosion Control Reference Manual for Construction Sites, at the time of final design of the site, the erosion control performance standard will need to be calculated and appropriate measures taken to control erosion from the site. 7 I VI. A. General Concept The Northgate Commons P.U.D. lies within the Moderate Rainfall Erodibility Zone and the Moderate Wind Erodibility Zone per the City of Fort Collins zone maps. Per the City of Fort Collins Erosion Control Reference Manual for Construction Sites, at the time of final design of the site, the erosion control performance standard will need to be calculated and appropriate measures taken to control erosion from the site. -- -- �'; All construction activities must also comply with the State of Colorado permitting process for Stormwater Discharges Associated with Construction Activity. A Colorado Department of Health NPDES Permit will be required before any construction grading can begin within.this development. VIE. 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. The owners of the Northgate Commons P.U.D. will need to maintain their on -site storm drainage facilities on a regular basis. B. Drainage Concept The proposed drainage concepts adequately provide for the transmission of developed on -site runoff to the two proposed detention ponds. The sizes, location and release rates of each pond will allow the Northgate Commons P.U.D. to develop in conformance with the Greenbriar-Evergreen Basin Master Planning concepts, once approved by the City of Fort Collins. Each of the on -site ponds ' shall provide one foot of freeboard and an emergency overflow outlet in the event the outlet becomes plugged. C. Storm Water Quality I The wetlands area proposed within the on -site detention pond is an excellent way to allow storm water pollutants an opportunity to be filtered out of the storm water as the pollutants are carried by the storm water across the site. Periodic maintenance may be required to remove sediment deposits as they accumulate in the main on -site detention pond. 6 Preliminary First Filing Development The first phase of the Northgate Commons P.U.D. will include the construction of both Ponds 1S and 1N. The storm .water runoff from the site will reach these ponds by several methods, i.e. grass swales, pipe; curb cuts, cross pans etc. There will be a partial storm drainage system built to drain the developed Phase 1 into Pond 1S. A temporary swale along the north pavement limit of Phase 1 will divert I1I the upstream runoff into Pond 1S as well. — ----- --The storm drain system is-being-proposed-with-Phase-1—Inlets-will-be-located-in l the parking areas and storm drain pipes will convey runoff to the detention area. These inlets and storm drain will be sized at the time of final design. A. General Concept The State of Colorado requires Stormwater Management Plans as part of their permit process. The Northgate Commons P.U.D. site development is anticipating construction beginning in the Spring of 1998. Therefore for this study, we have sought to find various Best Management Practices for the treatment of storm water runoff at this preliminary design phase which could be implemented in the final design process. B. Specific Details It is our understanding at this preliminary phase, that the concept of storm water quality should address the treatment of the initial first flush runoff. By capturing the initial first flush runoff, the pollutants can be filtered out of the storm water runoff. The Northgate Commons P.U.D. is being proposed with a wetlands area within the on -site detention ponds. This wetlands area will serve many purposes. Because the area is held flat, the runoff will be slowed down in these areas and this will allow pollutants and other transported materials to be filtered out. In addition, the wetlands native grass species also "treat" the polluted runoff water and allow for a much cleaner discharge from the Northgate Commons P.U.D. Because water quality is of such importance, we are not proposing to construct concrete trickle pans in the bottom of any drainage areas. 5 ,I Refer to the Appendix for supporting calculations for the supporting tables for the most current SWMM model. B. Specific Details IOverall Site Development Per the Greenbriar-Evergreen Basin Master Drainage Plan, the Northgate Commons site must have on -site detention. The Master Plan showed one pond - ----- within-the-Northgate-Commons-site-which-releases-13-8-cfs.- This -one -pond was_— '� proposed to have a storage capacity of 15.5 ac-ft. The Overall Drainage Plan for the Northgate Commons P.U.D. includes utilizing proposed permanent pool ponds within the site for retention. Additional volume above the permanent pools will be provided within these ponds which allow for the permanent detention of the 100-year runoff for the drainage area which drains to it. There will however, be an emergency overflow provided for this pond along 'I Willox Lane. Based on grading and wetland requirements, the original design proposing one pond will need to be revised to incorporate two ponds. This has been shown on the Overall Drainage Plan for the Northgate Commons P.U.D. Pond 1N will be located in the north portion of this site. This pond will outlet through weir wall structure into the lower Pond 1S. Once an approved copy of the Master Plan is available, the release rate will be adjusted based on the proportional tributary area and the original release from Regional Pond 6. This pond will has been proposed with minimum volume as indicated on the Overall Drainage Plan. Pond 1S will be located in the south portion of this site. This pond will outlet '! through an orifice controlled pipe, and discharge into an existing off -site storm drain. This pond, as well, will need to be revised once the Master.Plan is completed. The volume required for this pond has been shown on the Overall Drainage Plan for Northgate Commons P.U.D. Both ponds will contain wetlands area which will help filter out pollutants from the site. The limits of these wetlands were developed by the planner for this project, based on an analysis by a private wetlands consultants. The City of Fort Collins Stormwater Utility has requested that a SWMM Model Update be submitted once the Master Plan has been finalized for the Greenbriar- Evergreen Basin. The SWMM Model is a conservative approach to ensure all detention ponds within the City of Fort Collins have sufficient capacity. 4 I On -site runoff within Northgate Commons P.U.D. was calculated using the rational method. D. Hydraulic Criteria All calculations within this study have been prepared in accordance with the City of Fort Collins Storm Drainage Criteria and are included in the appendix. E. Variances from Criteria No variances from the City of Fort Collins Storm Drainage Criteria are being sought for this project. IV. DRAINAGE FACILITY A. General Concept As development occurs within the Northgate Commons P.U.D., the drainage concepts shown on the Overall Drainage Plan in the back pocket of this report should be followed. At final design, the Greenbriar-Evergreen Basin Master Plan will be revisited and revised to reflect the split of Basin 6, effectively creating two Cponds, with two releases, as opposed to one. There are two ponds shown within the site to handle the stormwater runoff. Both of these ponds contain wetlands, investigated by a private consultant, Theodore Boss. Also planned are several permanent wet ponds which will be fed by a source as yet to be. determined. Water in this pond will be utilized for irrigating the site and to create an aesthetic drainage feature within the development. The permanent wet ponds will need to be sized in excess of the required amount of permanent water storage to allow for the periodic inflows of stormwater and the ultimate inflow of the 100 year storm. On -site stormwater will be conveyed to the two on -site ponds through the use of storm drain pipes, inlets, curb cuts and cross pans. In the future, when the SWMM model is updated, Basin 6 will be revised to match proposed developed conditions. This will include revising Basin 6 into two areas to match proposed site layout, removing Pond 6 from the model, and replacing it with detention pond rating curves for Ponds IN and 1S within the Northgate Commons P.U.D. Pond 6 was previously master planned with a release rate of 13.8 cfs. Because of the changes in tributary area, the creation of two ponds, the peak release rates for each pond will be proportional to the respective tributary area. However, the peak combined release rate from the two ponds still must not not exceed the Maximum Release Rate from Basin 6 per the Greenbriar-Evergreen Basin Master Drainage Plan. 3 �i I B. Sub -Basin Description �j Historic drainage patterns on the subject site are in a easterly direction. Flows are ' proposed to drain into Detention Pond 6 and then be released at a .planned rate into an existing curb inlet along Willox Lane. When the Northgate Commons P.U.D. site is developed, off -site flows will be conveyed via curb & gutter and on -site ,I storm drains to the pond. There are two storm drains proposed, one with Phase 1 and one with Phase 2. With the development of Phase 1, a temporary swale will be constructed along the north pavement line of the phase to drain undeveloped —----- flows -into -the-detention-pond-area.—Per-the-Master_Drainage_P-lan,_Northgate �i Commons P.U.D will receive off -site flows from runoff from properties north and east of the site, on the opposite side of the Larimer and Weld Canal. A. Regulations The City of Fort Collins Storm Drainage Design Criteria is being used for the subject site. B. Development Criteria Reference and Constraints The most current report on the Greenbriar-Evergreen Basin Master Plan is being utilized in this Overall Drainage Study. As can be seen from the Drainage Master Plan report (See Appendix), 1 detention pond was planned for Basin 6, within which the entire Northgate Commons P.U.D. resides. This pond, labeled as Pond 6 is to be located entirely within the Northgate Commons P.U.D. Due to topographical relief, existing wetlands and to the desires of the developer, it will be necessary to split Pond 6 so that it can effectively detain the entire developed basin runoff. These ponds will be sized to accept runoff from the entire 27.33 acres of Northgate Commons P.U.D., as well as all planned off -site flows. C. Hydrologic Criteria The SWMM hydrologic model was previously used in the Greenbriar-Evergreen Basin Master Plan. The Overall Drainage Design and Phase 1 Preliminary Drainage Design for the Northgate Commons P.U.D. are based on the May 1996 version of the Master Drainage Plan. This SWMM model will need to be updated based on further design of Northgate Commons P.U.D. and also based on the finalization of the Master Plan itself. No copy of the SWMM model was available at the time of the submittal. OVERALL DRAINAGE STUDY AND PHASE I PRELIMINARY DRAINAGE STUDY FOR THE NORTHGATE COMMONS P.U.D. FORT COLLINS, COLORADO I. GENERAL LOCATION AND DESCRIPTION The Northgate Commons P.U.D. site is located north of Willox Lane and east of North College Avenue in northern Fort Collins, Colorado. The site is shown on the Vicinity Map in the Appendix. More particularly, the site is situated in the southwest quarter of Section 36, Township 8 North, Range 69 West of the Sixth P.M., City of Fort Collins, Larimer County, Colorado. B. Description of Property The entire Northgate Commons site resides on 27.33 acres of land, more or less, most of which all of the area is currently undeveloped and being proposed for a commercial and retail plaza. The majority of the property currently consists of ' native grassland, with some evidence of prior farming. Topography at the site is generally sloping from west to east at approximately 0.45 %. ' The Larimer and Weld Canal borders the property along the north and east. This canal will not be disturbed during Phase 1 design or construction. ' II. DRAINAGE BASINS ' A. MaJor Basin Description The Northgate Commons P.U.D. lies entirely within the Greenbriar-Evergreen Basin Master Plan (Basin 6) as shown in the Master Plan Report by Lidstone & Anderson, Inc., dated May, 1996. Applicable portions of this study are included in the Appendix. At the time of this project's submittal, this Master Drainage Plan rwas in a draft stage, and had not been approved by the City of Fort Collins Stormwater Utility. In effect, this report simply incorporated the recommendations that the Master Drainage Plan had called for at the time of submittal. At the final design stage it will be necessary to review the Greenbriar-Evergreen Basin Master Plan and update the drainage facility design to reflect the most current version of 1 the master drainage plan. 11 PAGE VICINITY MAP 2 OVERALL DRAINAGE PLAN HYDROLOGY 4 PRELIMINARY DRAINAGE PLAN HYDROLOGY 10 DETENTION POND/SWALE CALCULATIONS 16 TABLES AND FIGURES 20 EXCERPTS FROM THE GREENBRIAR-EVERGREEN BASIN 23 MASTER DRAINAGE PLAN, DRAFT, MAY 1996 TABLE OF CONTENTS PAGE I. GENERAL LOCATION AND DESCRIPTION A. LOCATION 1 B. DESCRIPTION OF PROPERTY 1 II. DRAINAGE BASINS A. MAJOR BASIN DESCRIPTION 1 B. SUB -BASIN DESCRIPTION 2 III. DRAINAGE DESIGN CRITERIA A. REGULATIONS 2 B. DEVELOPMENT CRITERIA REFERENCE AND CONSTRAINTS 2 C. HYDROLOGICAL CRITERIA 2 D. HYDRAULIC CRITERIA 3 E. VARIANCES FROM CRITERIA 3 IV. DRAINAGE FACILITY DESIGN A. GENERAL CONCEPT 3 B. SPECIFIC DETAILS 4 OVERALL SITE DEVELOPMENT 4 PRELIMINARY FIRST FILING DEVELOPMENT 5 V. STORM WATER QUALITY A. GENERAL CONCEPT 5 B. SPECIFIC DETAILS 5 VI. EROSION CONTROL A. GENERAL CONCEPT 6 VII. CONCLUSIONS A. COMPLIANCE WITH STANDARDS 6 B. DRAINAGE CONCEPT 6 C. STORM WATER QUALITY 6 D. EROSION CONTROL CONCEPT 7 REFERENCES 8 INC. Engineering Consultants A division of The Sear -Brown Group 209 S. Meldrum Fort Collins, Colorado 80521 970/482-5922 December 23, 1996 Mr. Basil Hamdan City of Fort Collins Utility Services Stormwater 235 Mathews Fort Collins, Colorado 80522 RE: Overall Drainage Study and Preliminary Phase I Drainage Study for the Northgate Commons P.U.D. site Dear Basil: We are pleased to submit to you, for your review and approval, this Overall Drainage Study and Phase I Preliminary Drainage Study for the Northgate Commons P.U.D. site. 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 Prepared By: Reviewed By: (NOT AVAILABLE) Kevin W. Gingery, P.E. Water Resources Project Manager Denver303/458-5526 OVERALL DRAINAGE STUDY AND PHASE I PRELIMINARY DRAINAGE STUDY FOR THE NORTHGATE COMMONS P.U.D. FORT COLLINS, COLORADO December 23, 1996 Prepared for: Client: Wheeler Realty 1125 West Drake Road Suite 472 Fort Collins, Colorado 80526 Prepared by: RBD, Inc. Engineering Consultants 209 S. Meldrum Fort Collins, Colorado 80521 (970) 482-5922 RBD Job No. 154-012 OVERALL DRAINAGE STUDY PHASE I PRELIMINARY DRAINAGE STUDY THE NORTHGATE COMMONS P.U.D. FORT COLLINS, COLORADO A division of The Sear -Brawn Group