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HomeMy WebLinkAboutDrainage Reports - 03/08/2016City of Fo Collins Approved Plans ENGINORTHERN NEERING Approved by: Date: 3 s 20 l v ' Drainage Letter Report Date: March 2, 2016 Project: College Eight Thirty Project No. 574-006 Fort Collins, Colorado ' Attn: Stormwater Staff City of Fort Collins Stormwater Utility ' 700 Wood Street Fort Collins, Colorado 80521 ' Dear Staff: This letter serves to address the stormwater impacts of the College Eight Thirty project. The project is a ' redevelopment of the Schrader's Country Store located on the northeast corner of College and Locust Street. The site currently consists of rooftop, concrete walks, and asphalt parking areas. On -site drainage currently flows out from the building to the adjacent streets. Once it reaches the streets, water flows south ' on College, then is directed east along Locust. The total impervious area of the existing site is approximately 13,997 square -feet. The existing percent impervious for the entire site is 90.0%. ' The proposed redevelopment will remove the existing structures from the site and replace them with a mixed -use building. In addition to the new building, the parkways will be modified to meet current Land Use Code requirements, including sidewalks and landscaping that is not currently present or poorly ' defined. An area of pavers is also proposed at the northeast corner of the site, which is the only area of exposed pavement within the property line. Existing drainage patterns will generally be followed. The total impervious area of the proposed redevelopment is roughly 13,340 square -feet. When the impervious value ' of 40% for pavers is considered, the effective impervious area decreases to 12,094 square feet and the proposed percent impervious for the entire site decreases to 75.7%. An exhibit depicting existing and proposed impervious areas has been provided. ' The site is located in the Old Town Drainage Basin. Historically, on -site detention has not been required so long as there is no increase in impervious area. Since the redevelopment does not propose an increase ' in runoff, detention would not have been required. However, changes to City policy now require that a project that is being completely redeveloped must provide onsite detention with a 2-yr historic release rate. However, detention is not required if the site has historically been paying fees for a higher imperviousness. ' Our project has documented the higher existing imperviousness and City Staff has looked at this property and confirmed that the historic stormwater fees were based on the higher historic imperviousness and have been paid. Therefore, the current project is exempt from the 2-year release and onsite detention _ requirements. There are no regulatory floodplains associated with the project. 301 N. Howes Street, Suite 100, Fort Collins, CO 80521 1 970.221.4158 1 www.northernengineering.com. NORTHERN ENGINEERING ' Although stormwater quantity detention is not required, stormwater quality mitigation will be addressed by both temporary and permanent Best Management Practices (BMPs). During construction, the Contractor ' will follow the appropriate and applicable City of Fort Collins standards for erosion and sediment control. Since more than 10,000 sf of the project site will be disturbed, a comprehensive Stormwater Management Plan will be prepared for this project at Final Design. Post construction water quality and erosion control ' will be achieved by a fully established and stabilized site. All areas disturbed during construction will receive permanent hardscape, landscape, or building structure. ' There are no existing stormwater BMP's currently on -site. In order to bring the site into compliance with the Low Impact Development Standards adopted by the city, water quality mitigation and permeable pavers will be provided on -site. Water quality treatment will occur through a variety of methods. The first ' method is the use of a raised bio-retention area (Stormwater Planter Boxes) along the west side of the building that intercepts runoff from rooftop downspouts on the west side. These planter beds will have an engineered section designed to remove particulates and contaminants from the runoff. While most minor ' events are expected to infiltrate into the ground, a subdrain is provided to drain the Planter Boxes should it become overburdened. The underdrain will outfall directly into the existing inlet located at Locust Street and College Avenue. ' There will also be an area of permeable pavers installed on the northeast corner of the project site. This area of pavers will treat not only the exposed parking areas, but also the rooftop drainage by routing runoff ' into the paver subbase. A subdrain and monitoring basins will be provided for the paver subbase that ties into the same system as the Planter Boxes. The subdrain will have a water quality control structure that will slow the release of stormwater from the paver subbase and promote infiltration as well as to increase contact time across the surface area of the subbase. ' The City LID requirements specify that no less than 25% of all new paving shall be treated using permeable paving technique. In the case of this project, 2,076 sf of permeable pavers will be provided, which is 100% of the total 2,076 sf of new paving in parking areas, which far exceeds the requirement. The City LID requirements also specify that no less than 50% of any new impervious areas must be treated using LID techniques. The combination of treatment provided by the Permeable Pavers and Stormwater ' Planter will treat 11,068 sf of impervious area on the site, which is 82.67% of the new impervious areas proposed with the project, which again far exceeds the amount required by city code. A separate LID Conformance Report and Exhibit have been provided to document LID compliance. To summarize, the proposed grading concept closely matches existing elevations and drainage patterns. Existing site imperviousness has decreased from the existing condition, and a variance to the requirement ' to provide onsite detention using a 2-year historic release rate will be requested. Stormwater quality has been provided, and exceeds the city requirements for Low Impact Development paving and treatment. Therefore, it is my professional opinion that College Eight Thirty satisfies all applicable stormwater criteria. ' Please do not hesitate to contact me if you have questions or require additional information. Sincerely, O LICF O,. ti3iit •. ti ca ' Andrew Reese Nichola LEED Project Engineer Project Manager AP ' enc. EXISTING PROPOSED 0 ° a • ° n 4' a•. W 0 O Q J. O ILl r^ a • IJJI J" J 0 ,o , .J ° • J LOCUST STREET NORTHERN ENGINEERING 301 N. Hawes Street, Suite 100 PHONE 90.2214159 Fart Collins, Colorado 80521 www.norNernen8ineeri�.can V SURFACE CE AREA (SF) %IMPERV. IMPERV. AREA (SF) ROOFTOP 3,438 1D0% 3,438 CONCRETE 10,659 100% 10,559 ASPHALT 0 100% 0 LANDSCAPE 1 0 0% 0 TOTALS 13,997 TOTAL= 13,997 COLLEGE EIGHT THIRTY FORT COLLINS COLORADO IN LOCUST STREET SURFACE AREA(SF) %IMPERV. IMPERV. AREA (SF) ROOFTOP 9,879 10D% 9.879 CONCRETE 1,454 100% 1,454 ASPHALT 0 100% 0 PAVERS 1,978 40% 791 LANDSCAPE 686 0% 0 TOTALS 13,997 TOTAL= 12,124 CRIPL KISTING VS. PROPOSED 9PERVIOUS AREA 'IMIW 9GlE DRAWING REESE 1'=30' e SK-Cl IRCH 2, 2016 574-006 00 ErL Rk 0 \ \E) )§ 6 -a a y tD kkfq/ c $§■�o ) a% \}mj) u a ... ;a [ c /.2 CD 0) LO 2 3E2200 2 = a u $60 2}6■£_oo /8;"G//2 m � )m 00 i . 0CL 7 E &; _ §000m �) ooj g! )§ «� �IE \ %k =coeo �z\ 0000 A$\$0 2%/ IJk 02 2J7 20, `m Gd / J,m °a ƒm ad @ ° ca /46 ]: a- $ : $ . § k ; 3= \2 ƒ f¥:'- m x 2_ 0 :,��:; r � LU im)!a ) ��\k G 2 <k § �f<acwcs ,(®E {4E0 f { ( § `3®kG �)~ 9L ® E } � •/�k) I-@S c -11 ®ƒ & � ' ���0 § | ! 7 In | " § � | # 2§§§) f � a o LL. ƒ-a, J !� °JƒmQ _ \ / \ 0 0RAI ) } ) 0 � / 4 ®�� - _ k Z � y If LZ w \ . 10 ! )/})) 0 0 ` £af% ƒa�qdl f � )\ Icv o /aa £ k�2 ZI LO Ix d £ co is \ \ � � ! | k ° �| ■ 2 ]§\§ � e , a J 46 2| 2 JLO | | =222k ; | �)®a; } £ o . \�R2/ / ƒi®2/ 12 k ) i % 0 ci ) , \ \ ° j \ } ` ) . Q ; \ \ m G &t ) 2 k/ �2 < EXISTING RUNOFF SUMMARY TABLE DESIGN BASIN TOTAL 2-yr T� 10-yr ICT-yr QZ Q+o Q+oo AREA Cs C+o C+oo T` ` POINT ID (min) (cfs) ((,Ioefs (cfs) (acres) min min A A 0.32 0.95 0.95 1 1.00 1 5.0 5.0 5.0 0.87 1.49 1 3.20 D.- Projectsl574-0061DrainageWydrologyl574-006 Rational-Calcs Existing.xlsxlSummary Table a� ErL § ui � u@ LY 0k%}oo t aE s�- - a u k 2 } O 2 0 a — LU � \E \}\ Q } u u U tn $� §I$77 2 CL 2 _ 2E0%od U Jtko ! cm Lai. Ly\LDc�m§§ U La. IV | 00 2 ; ]«a 0CL / �`k & _ �- 0 °° �)k o0 2�. 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In particular, the project is meeting Section 3.1 of the Fort Collins Stormwater Amendments to the Urban Drainage and Flood Control District Criteria Manual, including Section3.1 (a)(1) requiring that no less than 50% of any newly added impervious area be treated ' using one or more LID techniques and Section 3.1(a)(2) requiring that no less than 25% of all new paving use a permeable pavement technology that is considered an LID technique. ' Section 3.1(a)(1) The project as a whole is proposing 0.307 acres of new impervious area. This results in a minimum required LID treatment area of 0.154 acres. The project is providing 0.233 acres of impervious area ' treated by pavers (directly or indirectly), and is treating 0.021 acres of impervious areas using rain gardens. The total project area treated by LID techniques is 0.254 acres, which is 82.67% of the total new impervious area. There is also a small area of roof (0.018 acres) that is being released into a ' landscaped area within the public ROW that is not being counted towards the requirement but that does treat runoff, making the effective treatment percentage even higher. Section 3.1(a)(2) ' The project as a whole is proposing 2,076 sf of exposed impervious paving for parking. This results in a minimum required permeable paving area of 519 sf. The project has evaluated various areas for pavers, and has determined that the only appropriate location is the exposed area of paving. The total area of ' permeable pavers provided by the project is 2,076 sf, which is 100.0% of the total new paving. This greatly exceeds the requirement of 25%. ' An exhibit has been provided to illustrate the topics outlined. Please feel free to contact me if you have any questions. ' Sincerely, tAndrew Reese Nicholas W. Haws, PE FAB Project Engineer Project Manager. 301 N. Howes Street, Suite 100, Fort Collins, CO 80521 1 970.221.4158 1 www.northernengineering.com I 1 1 PROVIDED VOLUME: 16 CU. FT SOUTH RAIN GARDEN REQUIRED VOLUME: 14 CU. FT. PROVIDED VOLUME: 1 CU. FT. Q 30 0 30 Feet IN FEET 1 inch -30 n. LEGEND RAIN GARDEN TREATMENT AREA RAIN GARDEN 7A PAVER AREA PAVER RUN-0N AREA ELII LL.G4 NORTHERN ENGINEERING 301 N. Howes Sbee[, Sidle 100 PHONE: 970.221,4158 Fort Collins Colorado 80521 wvw.nutlemnglneidnt.can SO% On -Site Treatment by LID Requirement New Impervious Area 0.307 Ac. Paver Area 0,048 Ac. Traditional Pavement Area 0.000 Ac, Other Impervious Surfaces (Roofs, concrete Walks, etc.) 0.260 Ac Required Min/mum Impervious Area to 0.154 Ac. be Treated Paver LID Treatment Paver area 0.048 Ac. Run-on area for Pavers (parking areal 0.000 Ac. Run-on area for Pavers (other -concrete 0.185 Ac. walks, etc) Impervious Area Treated by LID Pavers 0.233 Ac. Rain Garden Treatment Raln Garden Area 0.001 Ac. Run-on area for Rain Garden 0.020 Ac. Impervious Area Treated by Rain 0.021 Ac. Gardens Total Impervious Area Treated 0.254 Ac. Percent of Impervious Area Treated 82.67 % 25% Permeable Pavement Requirement New Pavement Area 2,076 Sq. Ft. Required Minimum Area of Permeable 519 Sq. Ft. Pavement Total Permeable Pavement Area 2076 Sq. Ft. Actual %of Permeable Pavement 100.0 % Provided D TREATMENT EXHIBIT COLLEGE EIGHT THIRTY FORT COLLINS aMWN By COLORADO A. REESE MARCH 2, 2016 LID-1 Stormwater ' Alternative ComplianceNariance Application City of Fort Collins Water Utilities Engineering ' Engineer Name Nicholas W. Haws Phone 970.568.5414 Street Address 301 N. Howes St., Suite 100 ' City Fort Collins State CO Zip 80521 Owner Name Schrader Development Co. Phone 970.484.1225 ' Street Address 320 N. College Ave. City Fort Collins State CO Zip 80524 Section B: Proposed Project Information ' Project Name College Eight Thirty Project/Application Number from Development Review (i.e. FDP123456) FDP160002 tLegal description and/or address of property 830 S. College Ave. ' Description of Project Redevelop Schrader fueling station / convenience store into a modern mixed use building. Improve curb cut conditions, on -street parking, and pedestrian experience. ' Existing Use (check one): (- residential (*-" non-residential C mixed -use C vacant ground Proposed Use (check one): r" residential r- non-residential r mixed -use C other If non-residential or mixed use, describe in detail 31,007 sq. ft., 4-story, mixed use project with 34 residential units above main level commercial space. ' State the requirement from which alternative compliance/variance is sought. (Please include applicable Drainage Criteria Manual volume, chapter and section.) ' Section 4.5.5 - Water Quality Treatment (Fort Collins Amendments) What hardship prevents this site from meeting the requirement? Relatively small site historically utilized as fueling station and convenience store is fully developed on ' all sides with zero lotline to the north and minimal setbacks on other sides. Architecture has to be compatible with historic district to the east, which drives roof forms. The only "untreated" area is a portion of said roofs which drains through landscape area in the public right-of-way prior to entering ' the City storm drain. Attach separate sheet if necessary What alternative is proposed for the site? Rather than 100% water quality treatment and 50% LID treatment, the project proses 83% water ' quality treatment, ALL of which will be achieved via an approved LID technique. Additionally, parkway areas in the public right-of-way which were historically paved and subject to vehicular traffic will be converted to permeable landscaping. If such areas outside of the property boundary are ' considered, then the proposed alternate not only meets, but exceeds the intent of the code. Attach separate sheet if necessary page 2 The owner agrees to comply with the provisions of the zoning ordinance, building code and all other applicable sections of the City Code, Land Use Code, City Plan and all other laws and ordinances affecting the construction and occupancy of the proposed building that are not directly approved by this variance. The owner understands that if this variance is approved, the structure and its occupants may be more susceptible to flood or runoff damage as well as other adverse drainage issues. ' Signature of owner: 1 — Date:_2 -24 ' The engineer hereby certifies that the above information, along with the reference plans and project descriptions is correct. W Signature of engineer: "- Date:_ 02.26.26 PE STAMP Date complete application submitted: 1(t Date of approval/denial: Variance: 19 approved © denied Approved by:_AfJi,4 Entered in UtilityFile Data6&?#yes Qno