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HomeMy WebLinkAboutDrainage Reports - 06/19/2001■ Final A rove.—i Report PROPERTY OF MRT COLLINS 7 2uU1 FINAL DRAINAGE AND EROSION CONTROL REPORT HARRIS BILINGUAL ELEMENTARY SCHOOL EXPANSION 1 r r FINAL DRAINAGE AND EROSION CONTROL REPORT HARRIS BILINGUAL ELEMENTARY SCHOOL EXPANSION Prepared for: RB+B Architects 315 E. Mountain Ave, Suite 100 Fort Collins, CO 80524 Prepared by: North Star Design, Inc. 700 Automation Drive, Unit I Windsor, Colorado 80550 (970)686-6939 April 9, 2001 Job Number 138-03 North Star �afta,`design,. inc. ' April 9, 2001 Basil Harridan City of Fort Collins Stormwater 700 Wood Street Fort Collins, CO 80522-0580 RE: Final Drainage Report for Harris Bilingual Elementary School Expansion Dear Basil, ' I am pleased to submit for your review and approval, this Final Drainage Report for Harris Bilingual Elementary School Expansion. I certify that this report for the drainage design of ' Harris Bilingual Elementary School Expansion was prepared in accordance with the criteria in the City of Fort Collins Storm Drainage Manual. ' I appreciate your time and consideration in reviewing this submittal. Please call if you have any questions. r-- Since OPr.. RE.S 1� 1 a KO '- � �Frc 9�0 �Q,'t✓ c� Q ti. ' 31306 ' Patricia Kroetch, North Star Design, Inc. 1 �I ii ' 700 Automation Drive, Unit I Windsor, Colorado 130550 970-686-6939 Phone 9 970-686-1188 Fax I ' TABLE OF CONTENTS TABLE OF CONTENTS............................................................................................................... 1. GENERAL LOCATION AND DESCRIPTION 1.1 Location................................................................................................................... I 1.2 Description of Property ............................................................................................1 ' 2. DRAINAGE BASINS AND SUB -BASINS 2.1 Major Basin Description..........................................................................................I 2.2 Sub -Basin Description.............................................................................................1 ' 3. DRAINAGE DESIGN CRITERIA ' 3.1 Regulations..............................................................................................................2 3.2 Development Criteria Reference and Constraints...................................................2 3.3 Hydrologic Criteria..................................................................................................3 1 3.4 Hydraulic Criteria....................................................................................................3 4. DRAINAGE FACILITY DESIGN ' 4.1 General Concept......................................................................................................3 4.2 Specific Flow Routing.............................................................................................3 4.3 Drainage Summary ..................................................................................................4 ' 4.4 Variance Requests .......................... :......................................................................... 5 5. EROSION CONTROL ' 5.1 General Concept............................................................... :........................................ 5 6. CONCLUSIONS ' 6.1 Compliance with Standards....................................................................................6 6.2 Drainage Concept....................................................................................................6 6.3 Water Quality and Erosion Control Concept...........................................................6 7. REFERENCES....................................................................................................................7 ' APPENDICES A Vicinity Map ' B Hydrologic Computations & Erosion Control C Figures and Tables iii 1. GENERAL LOCATION AND DESCRIPTION 1.1 Location This site is located in the southeast portion of Old Town Fort Collins. The school site encompasses nearly the entire block bounded by E. Elizabeth Street on the north, Smith Street on the east, Garfield Street on the south and Whedbee Street on the west. A small gas station exists on the southwest corner of this block. See the location map in Appendix A. 1.2 Description of Property The entire school site consists of approximately 3.06 acres of land. The site has buildings and pavement currently existing and the site slopes from the center (existing school) to the existing streets on all sides. The slopes on each side vary from 2.8% on the east where the playground was recently reconstructed to 20% on the north which is covered in grasses and street trees. The project will consist of a removing two existing buildings and expanding the existing building to the west. Additional walks will service the new expansion and the existing parking lot on the south will be reconstructed. 2. DRAINAGE BASINS AND SUB -BASINS 2.1 Major Basin Description The proposed development lies within the Spring Creek Drainage Basin. 2.2 Sub -basin Description ' Flow from this site will enter the streets on all four sides. Runoff in Smith Street will flow north to Elizabeth and west to existing inlets at the intersection of Elizabeth and ' Whedbee. Runoff in Garfield Street will flow west to Whedbee and north to the same inlets. This storm system will convey the runoff north. .1 1 1 3. DRAINAGE DESIGN CRITERIA 3.1 Regulations This report was prepared to meet or exceed the "City of Fort Collins Storm Drainage ' Design Criteria Manual' specifications. Where applicable, the criteria established in the "Urban Storm Drainage Criteria Manual" (UDFCD), 1984, developed by the Denver Regional Council of Governments, has been used. ' 3.2 Development Criteria Reference and Constraints ' The runoff from this site has been routed to conform to existing site constraints. Since all of the adjacent streets surrounding the site and the main building on the site are already ' constructed, there is very little room for variances from existing site conditions. The streets and associated drainage systems surrounding the site exist. The nearest storm system is at the intersection of E. Elizabeth Street and Whedbee Street at the northwest ' corner of this site. The existing school sits on top of a hill in the center of the site. No offsite flows will enter this site. The expansion proposed for this site is approximately 13,500 square feet. Some of this building will replace existing impervious area (two buildings, paved area and walks) but ' there would still be a large increase in impervious area on this site. In order to offset the impervious area created by this expansion several alternative pavements are being ' proposed. In some of the walkways, grass pavers are proposed to reduce the potential concrete area. In the parking lot, the stall areas are proposed to be gravel pavers to reduce the existing asphalt area. In the service area on the south side of the expansion, ' gravel. will be utilized rather than concrete. These techniques minimize. the potential impact of the expansion and reduce the potential runoff from the site. ' No detention is proposed for this site. The existing site (based on the rainfall criteria ' adopted by the City of Fort Collins on March 26, 1999) generates approximately 5.77 cfs runoff in the 10 year storm and 15.60 cfs in the 100 year storm event. The site as it is ' proposed will generate approximately 6.09 cfs in the 10 year event and 16.00 cfs in the 100 year event. Because of the innovative techniques utilized on this site to reduce impervious areas, the additional runoff in the 100 year event is only 0.4 cfs. This ' 2 increase, when viewed on a basin wide level, is very insignificant and will have a very minor impact on downstream conveyances. 3.3 Hydrologic Criteria Runoff computations were prepared for the 10-year minor and 100-year major storm ' frequency utilizing the rational method. All hydrologic calculations associated with the basins are included in Appendix B of this ' report. Standard Form 8 (SF-8) provides time of concentration calculations for all sub - basins. Standard Form 9 (SF-9) provides a summary of the design flows for all Sub - basins and Design Points associated with this site. 3.4 Hydraulic Criteria ' All hydraulic calculations within this report have been prepared in accordance with the ' City of Fort Collins Drainage Criteria and are also included in Appendix C of this report. 4. DRAINAGE FACILITY DESIGN 4.1 General Concept Because the site sits on top of a hill, the runoff from this site flows in all four directions. ' Flow will be directed to the existing streets on all sides of the site. 4.2 Specific Flow Routing ' A summary of the drainage patterns within each basin is provided in the following ' paragraphs. Subbasin 1 contains the north quarter of the site including a portion of the proposed ' building expansion. Runoff from a portion of the roof is collected in underground drain lines and is conveyed to a dry well at the northwest corner of the site. This flow collects ' in the drywell and, if the drywell capacity is exceed, the flow will discharge through the perforated cover and flow to the existing inlet at the corner of Elizabeth and Whedbee ' Streets. Runoff from the remainder of the roof and from the area surrounding the building flows north to Elizabeth Street. Flows then continue west, and into the existing inlet at ' 3 the intersection of Elizabeth and Whedbee. ' Subbasin 2 contains the east quarter of the site including the existing playground. This basin will remain unaltered by the expansion. Runoff from Subbasin 2 flows to Smith ' street on the east boundary of the site. The flow is combined with runoff from Subbasin 1 and flows to the existing inlet. ' Subbasin 3 contains the south quarter of the site including a small portion of the proposed building expansion, the existing parking lot and the proposed main entry walks. ' Runoff from the roof in this basin is also collected in underground drain lines and is conveyed to the proposed drywell at the south side of the building. If this drywell ' capacity was exceeded, the flow would enter the grassed area surrounding the existing tree and then flow south into the parking area. Runoff from the area surrounding the ' building flows south to Garfield Street. Flows then continue west and into Subbasin 4. ' Subbasin 4 contains the west quarter of the site including the west half of the proposed building expansion and the delivery entrance. Runoff from the roof in this basin is also collected in underground drain lines and is conveyed to the proposed drywell at the ' northwest corner of the site. Runoff from the area surrounding the building flows west to Whedbee Street. Flows then combine with runoff from Subbasin 3 and continue north to ' the existing inlet at the intersection of Elizabeth and Whedbee. ' The purpose of the drywells is to retain stormwater on site and gradually recharge the soils to reduce the irrigation needs on site and facilitate water quality if possible. Once the capacity of the dry wells is exceeded, the water will discharge from the drywell ' through the perforated lid and flow overland to the streets. The owner of this project (Poudre School District) is trying to incorporate a variety of innovative drainage facilities ' to reduce runoff from the site and improve the water quality of the stormwater that leaves the site. The owner understands that the methods proposed are not widely accepted and commonly implemented but would like to provide this project as an example in an effort to encourage implementation of similar methods in similar applications. ' 4.3 Drainage Summary The storm conveyance system is a surface system and has been designed for minimum maintenance. The City of Fort Collins will be responsible for maintenance of the existing adjacent streets utilized as drainage conveyance. The drainage facilities located on this 4 site will be maintained by the owners of the site. ' 4.4 Variance Requests ' A variance is being requested to forgo the detention requirement for this site due to the increase rainfall criteria adopted by the City of Fort Collins in 1999. The amount of increase to the runoff for this site (from the existing condition to the developed condition) ' is approximately 0.60 cfs in the 100 year storm and when viewed in the context of the overall basin, this increase is very minor. 1 A variance is also being requested for the requirement to provide water quality in the ' form of extended detention. With the development of the expansion several measures are being proposed to minimize the impervious areas on site. Eco-stone pavers are proposed ' (instead of concrete) on each side of the walks on the south and west sides of the building. Gravel pavers are proposed in the parking bays of the reconstructed parking ' lot. On the all sides of the building, the roof drain discharge will flow across grassed areas before reaching the streets. This will allow sediment to be removed from the runoff prior to reaching the streets. ' 5. EROSION CONTROL ' 5.1 General Concept ' This site lies within the Moderate Rainfall and Wind Erodibility Zone per the City of Fort Collins Zone Maps. The potential exists for silt movement from the site and into the ' existing storm system during construction. Silt fence is being utilized to minimize this potential during construction. This project is an infill project and construction will utilize the majority of the site. Because of this, most Best Management Practices (BMP's) are not effective and therefore ' very few erosion control measures are being proposed. Most of the construction traffic (concrete trucks, etc) will utilize the existing parking lot which is proposed to remain ' until the majority of the building is constructed thus minimizing the possibility of mud tracking. Silt fence is utilized around the areas of disturbance to minimize wind and stormwater erosion. Watering of disturbed areas is being proposed to also minimize the wind erosion. The erosion control escrow amount is $2858. 1 5 J 6. CONCLUSIONS 6.1 Compliance with Standards All computations that have been completed within this report are in compliance with the City of Fort Collins Storm Drainage Design Criteria Manual. 6.2 Drainage Concept The proposed drainage concepts presented in this report and on the construction plans adequately provide for the detention and transmission of developed on -site runoff to the existing streets and into the existing storm system. The impact of the development of this site on existing downstream facilities is very minor and therefore on site detention is not being proposed. If, at the time of construction, groundwater is encountered, a Colorado Department of Health Construction Dewatering Permit would be required. 6.3 Erosion Control Concept Since the site is less than 5.0 acres, a NPDES permit is not required. During construction, erosion control shall be maintained to minimize erosion on this site and transport of silt downstream. :9 7. REFERENCES 1. City of Fort Collins, "Storm Drainage Criteria Manual", (SDCM), dated March, 1986. 2. Urban Drainage and Flood Control District, "Urban Storm Drainage Criteria Manual", Volumes 1 and 2, dated March, 1969, and Volume 3 dated September, 1992. `l I 1 1 APPENDIX A ' VICINITY MAP L 1 .1 A 1 I 1 1 1 i 1 1 1 1 1 1 1 1 1 1 I 1 m ti W 133211 S 2/3/10 S 1.iiX[ HL/M5 Sr 33 03M J Q V 3nN3A V 39377 W 1 t I I I 1 I E 1__ 1 I 1 .1 APPENDIX B HYDROLOGIC COMPUTATIONS EROSION CONTROL B k } § CL � � 3_ @ �k ® z 02 } }§ b o f �\ LL j \ E .4 _ §7 ]§�ƒ :0 = =E. \t \ %f � J@ 2 d( 22 // @ }()& 0 0< |§ a. uc ww .� k�0 )f# � : k m ; ■ E # \<k J)\cq / ! }2k ° ; 2 ; /)F G G ■ ■k2! B I oil 0 w u }§§k w \ rl N }IXk \f}\ ) \f() Si m � m m Okl�.�;�m #s 0e.�, 11 e2§6 , coco \ §a8§ (0. ESS` § O�4) a$-(D (%/6 IEO k¥§/ g§«6 SO £aE! cck/k» � §0 f 9§!— f—)}�) . o r < < -a k/ g/ B . \ m L.a a O 7. 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N co IVI , :IM } , k % E /$ \ 2` — t fk�§ / Lo -§f& • .1ec Cc .0 E2GJ \ CyL) 9 . 0 o $ \ 2 f tii § mo k North Star Design, Inc 1194 W. Ash St. Suite B Windsor, CO 80550 LOCATION: Harris Elementary School ITEM: Water Quality Pond Sizing COMPUTATIONS BY: ppk SUBMITTED BY: North Star Design, Inc From Urban Strom Drainage Criteria/ Manual, March 1969 (Referenced figures are attached at the end of this section) Use 40-hour brim -full volume drain time for extended detention basin Water Quality Capture Volume = WQCV = (required storage/12)'(tributary drainage area) MAJOR BASIN Trib. area (ac) % Imperv. Req. Storage (in. of runoff) from Fig. 5.1 WQCV (ac-ft) DWO (ft) req. area/row (In2/row) from Fig. 5-3 EX 3.06 34.0 0.15 0.038 PRO 3.06 36.0 0.16 0.041 1 /16/01 WQVOL.xIs Cost Est .1 EROSION CONTROL COST ESTIMATE Project: HARRIS ELEMENTARY Prepared by: PPK ITEM IQUANTITY UNIT COST/UNIT TOTAL COST Silt Fence 635 LF $3 $1,905 Straw Bale Barrier 0 EA $150 $0 Gravel Inlet Filter 0 EA $150 $0 Construction Entrance 0 EA $550 $0 Subtotal Contingency (50%) Total $1,905 $953 $2,858 CITY RESEEDING COST Reseed/Mulch 3.06 ACRE $615 $1,882 Subtotal Contingency (50%) Total $1,882 $941 $2,823 EROSION CONTROL ESCROW AMOUNT $2,858 Page 1 I F ' APPENDIX C ' FIGURES AND TABLES 1 C No Text 4977___\ A I , ---. v,.--------------____---- — — --- -------------- ____—_ _ 190D— �i i — \ — i ' EAST ZbIZABETH ST SF I \ yI n \ i ! \ \ � 1 tub f a M i -- -__ - - ---- - - -- --- — -- --- --_Y=EWNIsi 1E - _.985 -- -- --- __------' rur _ _ - air or u op L try CULART f 90EwA1N - - MM METAL GRATE — FWH I/ ♦ 95 .b 17 ( 3 ♦ MM SPOUT Court _ e6HIHtlHd 4rbi r t � L T E 20 10 0 20 SCALE: 1- se 20' CALL UTILITY NOTIFICATION CENTER OF COLORAD0 1-800-922.1987 m534.6700 III ceu 2—BUSINESS DAYS N ADVANCE B M RJ DIG, GRADE CN EFCAVAIT FM ME MARKING DF ADERIACWO MEMBER V11I11E5. AD Qj Li DESINGIll PONT BASN CRITERIA 0.60 O. RUNOFF COEFFICIENT AREA IN ACRES ROW DIRECTION WEEMENO BASIN BOUNDARY EXISTING PIPES --------------- EXISTING 5' CONTOUR --------- - EXISTING 1' CONTOUR PROPOSED 5' CONTOUR PROPOSED 1' CONTOUR —R © SILT FENCE O- CONSTRUCTION ENTRANCE DRAINAGE SUMMARY TABLE OE4O1 POINT ARG [EN. 1 All AGES CID 1 CIW j To (ID) WH To (IN) YIN 65 CF! 1 1 I DTI 1 1 3 I DGJ OAA I O.Y I I1; I Mil I 1.J2 3101 ! 1 J 1 1.02 bell 1 ANTI 1 5.0 1 As 1 :15 9 { 1 e 1 an I Dow I art !.a 1 a91 2J3 to 0 N Y a n \ a II n 'o a a �i the City r Collins Montreal n Itym this Cu Vd Mgeela must be MAIM at LL F , v le a Z I Nn 9 Wa e M t r e Pool Nry a \ (eturned wol ori measures All be .°e toing the pproe Nobel twoop , the constructionv r' IM I the lippowart Z !1 reportt barrel ohill construction al and Makdo cards! . Z J e} ee vpel Mr e a IM a M ow e . W disturbancea All verni 0011the arend requir yor memall p 0 m ~ Ir1 9� J 9 ( - 9 9 - n D y 9 o J J soils in areas aM Y ee trip QO 'i car more than thirty q (J0) ap `swrool (..er u / ana. p1 em ) rma as amerelee app ovM py the J a Z me prop , erM ma commonly" It a times con..uct„ a.N ,m s Q w o a eel erm rev ee Immediately a ' N Fort Cl EngMsoldier D p° I adjacent cp 1 1 M by the City of O ,. temporaryAll or (as necessary ) after continuede a, Z O w p monthly school 063 be coconuts and disposal of In 0 mal and atioonnopam .att 0 All No $011 isockpi M61 el� too (10) feet in years All sail stockpile Model be prothinted M1mm Ali monsoon by surric, rouill - i old alliclow art MCn9. Mr "I hall p e .emoe" ow DO e» Mal be ewer e old miV city aa.an.e wmolw n. starting, mwp nv As depoesting r was > v, other material r 1 sot by or town mr'M'SI. Mr transitions a of roo la Ma the Z Mowed city by the amtha.to, ICE Q City of Fort Collins, Colorado p UTILITY PLAN APPROVAL Q F- APPROVED: Day G9Feer Dole CHECKED BY: SHEET WMer A efteMater unity Data 3 CHECKED BY: llwmeoter ul ty Cple - CHECKED BY: Pvb A Aenealbn Dale PRELIMINARY CHECKED BY: 3 OF 4 AOI yc W L'LWSRX/CI)GAV Task, total Dne APRIL 9, 2001 CHECKED BY: Je! N^ 139-02 Opts