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Home My WebLink AboutDrainage Reports - 01/01/1980ONM&i consulting engineers STORM DRAINAGE STUDY FOR GREENFIELD VILLAGE FIRST FILING FORT COLLINS, COLORADO JANUARY, 1980 R M&1 consulting engineers 4710 South College Ave. Fort Collins, Colorado 80525 Telephone (303) 2 2 6 - 2 3 2 3 January 2, 1980 Greenfield Enterprises 2101 S. College Fort Collins, CO 80525 Gentlemen: Presented herein is a storm drainage report for Greenfield Village First Filing located in the City of Fort Collins. This report conforms to both the subdivision regulations of the City of Fort Collins and the Federal Housing Authority. All calculations pertaining to this report are available upon request at our office. Please feel free to contact us with your questions or comments. Very truly yours, M S I, INC. S-Z� Stan A. Myers / SAM/JMN/ j s Enclosures s,--11.E.-L.S. ' TABLE OF CONTENTS Page No. '. I. Introduction . . . . . . . . . . . . . . . . . . . . . . 1 ' II. Study Area . . . . . . . . . . . . . . . . . . . . . . . . . 1 III. Existing Storm Drainage . . . . . . . . . . . . . . . . . . 1 IV. Design Criteria and Requirements . . . . . . . . . . . . . . 2 V. Drainage Plan . . . . . . . . . . . . . . . . . . . . . 3 ' VI. Conclusions and Recommendations . . . . . . . . . . 5 ' VII. Drainage Exhibits "Vicinity Exhibit No. 1 Map" ' Exhibit No. 2 "Sub -Basins" Exhibit No. 3 "Drainage Routing" 1 1 I. INTRODUCTION The 'purpose of this report is to present the developer with a proposed series of improvements which will provide for proper management of storm water runoff from both the Greenfield Village First Filing P.U.D. itself and adjacent contributing basins. The scope of this report involves an analysis of the storm drainage char- acteristics of this property in both its historical and improved condi- tions, consisting of the following.items:. 1) Established rainfall intensity duration curves for the City of Fort Collins. 2) Storm runoff quantities and their general direction of flow. 3) The size and slope of storm drains required. 4) Size and location of detention ponds. II. STUDY -AREA Greenfield Village First Filing. is.a proposed planned unit development located in the southeast portion of Fort Collins (See Exhibit No. 1). The development contains 23.3 acres. It is the first phase of a project which will also eventually develop the 46 acres located -immediately, to :the. west (See.Exhibit No. 1). The land in these areas slopes gently from the southwest to`the northeast at a grade of + 1.0%. III. EXISTING STORM DRAINAGE FACILITIES There are no existing storm drain systems<in:the area that have an impact ..'on the: storm water management of the development. Probable contributing runoff from the areas .to the south and west is cut off by Horsetooth Road. and Timberline: -Road.. .-The-runoff currently, generated by.the. First.Filing and the future development sheet flows toward the northeast property ' corner. A drainage Swale running along the north property line channels the runoff eastward to a ponding area located approximately a half mile east of the development (See Exhibit No. 1) .. It should also be noted ' that an irrigation ditch runs along the north side of Horsetooth Road for the entire length of the development. The widening of Horsetooth Road ' will require this ditch to be relocated 25 feet north of its. present location. It. will be piped under Queensgate Road. ' IV. DESIGN CRITERIA AND. REQUIREMENTS: ' The initial and the major drainage system make up the storm runoff -drain- age system. for an urban area. The initial drainage system. should be ' designed to accommodate the runoff from a 2-year storm by utilizing the maximum, capacity of the streets, overland swales. and, if necessary, underground conduits. The major drainage system must be designed to han- dle. a 100-year storm without extensive property damage or loss of life. Areas which may be susceptible to flooding .must be examined.`. Floor ' - elevations should be .setto allow proper drainage between and around . these houses'without-.flooding.. (See Exhibit No. 3.) ' The City of .Fort Collins.requires an improved site to limit its runoff to the amount resulting from a 2-year frequency storm' occuring while the site"is in its natural state. To accomplish this; it is usually_neces-,` sary to design a detention pond with a total storage capacity of the max- imum difference between, `the.100-year storm runoff from.the developed area and the 2-year historical flow. An outlet structure is designed to. limit "the flow to'the 2=year' historical runoff value. . ' _The amount of storm water runoff for the area, individual .basins and. streets was. determined by the. rational method: This _•procedure.;equates ' the design peak flow to the total area, amount of rainfall, on that area, and a multiplier that is. determined by-the._physical characteristics of ' the area under study. The rational .method is commonly employed .in the -2- 11 design of borrow ditches- and storm sewer systems. The bulk of the pro- cedures, methods and numerical constants were obtained from the Urban Storm Drainage Criteria Manual,. published by the Denver Regional Council of Governments. The rainfall intensity duration curves used in this report were developed by. the City of Fort Collins Engineering Department. The ponding elevations, capacities of the channels and piping were deter- mined by Manning's Formula, which equates a constant, a roughness factor, the area of. the channel or pipe, the hydraulic radius and the. slope, to the capacity of the channel or pipe. V. DRAINAGE PLAN . The drainage plan 'developed. for the First. Filing of Greenfield Village P.U.D..incorporated. a.preliminary analysis of the total future develop- .ment as well. This was done because .the First Filing is situated at the lower end. of the affected basin (See Exhibit No. 2). This means that drainage facilities in the FirstFiling must be .sized at this, time to handle runoff quantities generated.by the. .future development :area under fully.developed.conditions. ' Exhibit No. 2 shows the sub -basins involved in the study. ..,Sub -..basins "C", 'E", "F" and "G" will be the only -sub -basins which directly effect the drainage facilities -in the First Filing. The other sub -basins will_ have indirect -effect 'the an on sizing of. the outf all. lines .of the deten- tion ponds in the!First Filing. - This effect will be explained in detail later in this report. ' Sub -basin "G" located in the southern landscaped' portion- of the First. Filing has been treated in -a unique manner in. this 'study (See:Exhibit No. . ' 3). Since the subbasin.will-be. made - up .of landscaped. areas. and decora- 'tive-ponds, the runoff generated will remain at or below the historic rate. .The grading • incurred in developing•the 'decorative ponds .will in its elf,provide"-additional.' storage capacity. Sub -basin "E" (See Exhibit No. 3) comprises, the western portion of the First Filing. The,-sub-basin'.s corresponding pond will gather, runoff . from one principal Swale. This drainage -Swale'will.- run between lots 5 and 6.And across -Chelsea. Road in a textured concrete valley pan.. and. into - the pond. The swale--and valley -pan have been designed to channel the: majority -of the runoff -from .this sub basin into .the pond.. In .addition.. to this runoff the swale and valley' pan have been sized to accommodate . the cumulative outfall from detention areas .to.bedesigned for sub -basin 7D1" and approximately one.third of,'the sub -basin of 'the.proposed commer- cial area. The pond.itself has-been designed. to detain runoff generated by the 100 year developed storm without inundating the swimming pool area, allowing 0.5 ft of freeboard.':-. The tennis court will' be within the, limits, of the maximum ponding level; _however; if the pond is graded as proposed this . court should not be.affected by any storm whose.:'frequency is less.than 10 years. -,.As, stated:earlier, .the:.outfall "rate of this. pond will be effected by -the -'cumulative..outfall flowing' -into -.the pond •from sub-basins:"D1" and' the commercial` area. ' This .will 'increase the 'design outfall from L 18 - to,:. .3.24 cfs. The outfall pipe running t• f th pgnd has en sized �o �� Sab�arl4 Wv .nevPl Breve Accommodate -this' quantity•of flow. swale running directly over the outfall-pipe will bane flows in: the event of pond vert s. U� ota off► re�aoQse Aow �� v�/v Sub -basin "C"-Js: actually .not .within`.the subdivision boundaries of the. F irst Filing of Greenfield Village .(See Exhibit No.,2):. ItSLLaccompanying - , pond, ..however, will lie in the _..extreme.,northwest coiner of the First ,Filing. -This-.pond has been designed to:detain the "runoff: generated by a 100 year storm under developed' conditions allowing a -freeboard of 0.5_ft when:it'is. outfalled..at a'two year historic rate. As in the pond located in sub -basin '"E"; this,- pond's outfall has been oversized. to accommodate the cumulative'outfall .fromithe proposed,:.ponds °which. will be eventually ' installedabove it. The sub -basins contributing ;this extra outfall are "A"; "B"; .'`D2" and approximately':.two."thir'ds of the 'proposed commercial area (See Exhibit No.. - -4- n 2). The design outfall was increased from 0.31 cfs to 5.00 cfs. -As be— fore, the proposed outfall pipe and swale have been sized to channel this additional quantity as well as flows generated in the event the pond over — tops. Sub —basin "F" comprises the major drainage area in the First Filing. The total area of the sub —basin is over 11 acres. This area will surface drain to two catch basins in a low area in Queensgate: Road which will empty into the proposed pipe.system running between ponds "E" and "F". In the event of a larger storm the runoff water will flow over:the crown of the street in this- low area and through a' depressed curb section on the north. side of: the street. From here the. water will flow in a swale be— tween lots 21 and 22 and `into -Pond "F". The proposed outfall rate for Pond "F" is 9.70 cfs. The figure is the re— quired two year historic outfall rate for the entire development. Using this outfall rate the required capacity for. Pond "F".is 70875 ft3. The proposed design capacity, however is 98820 ft3, As this development. expands there is a.high probability that detention. areas will be in short supply in future phases.. It is ,essential, therefore, to make maximum use .of.the.facilities proposed in this, the First Filing. In orderto maxi— mize the additional storage available in Pond "F"' the outfall rates. from. Ponds."C" and."E" were increased. Outfall of Pond '"C" was increased from 5.00 cfs `to 6.15 cfs'while Pond`"E"'s outfall was increased from13.24 cfs -to 4.02 cfs. .-These increases will maximize the .capacity of. Pond "F" and still allow a freeboard of` 0.5 ft..: The outfall will be. piped + 400 feet, east-where, the outfall pipe will daylight.,, into ,an open channel,.which in turn empties -into -the ponding..area to'the east..(See Exhibit.No..:l): VI. CONCLUSIONS AND.RECOMMENDATIONS This drainage:'design'will.satisfy all hydraulic. considerations as - well as. governmentalregulations in providing. this future;'community with ?both 'a safe and' -equitable • solution. 'to-: the-: problem`. of handling: storm '.water runoff No Text PROSPEO�T STREET' i FORT COLLIN i —.i DRAKE ROAD 4. Z4` I , 1 ILI e W ,.0 ?8 VILLhGE P.U.D. a0jGREENFIELD� O (� FIRST FILING EXISTING SWALE ,. �W PONDING AREA IJ 0: m i HORSETOOTH ROAD °ini" HARMONY ROAD - EXHIBIT NO. 1 :] A i �I 0 ci D2 rUTURE mi DEV�LOPME r COMMERCIAL BASIN D1 - — - -- -- - ----HORSETOOTH ROAD -- EXHIBIT NO. 2 G 15'®0.85% 02: 4.67 CIA POND "C" DRAINAG SWALE PACITY: 0.32 FT POND "F" 02 :8.70 c1s CAPACITY: 2.27 AC.FT 17 18 19 16 20 14 15 55 56 21 s MFR 57 '` �` 0�10 23 �gNFFJT 100 YR. '• ONDI G LEVEL 22 24 ! A o� 25 e 58 ` 54 ' 's • 26 e 59 12 �� 27 I 53 POND "E 8 60 11 PACIT �. 22 AC. A «Cn 52` B ASI "E" - f1 28 r 10 C � i 61 y 50 F<S, 49 9 Fq 51 29 90 - 48 62 q� 8 47 0 30 -- - - - - G -- -- 63 �\ 46 31 6 100 YR.'S�{ �. 64 PO DING LEVE G% 32 02: 2.07 cfs ;1 DRAINAGE SWALE 5 65 33 66 ��GP 40 o z 4 J 41 aN 34 42 3 43 0 35 2 44 36 39 1 45. 38 37 SUB -BASIN "G" HORSETOOTH ROAD EXHIBIT NO, 3 JAN L jqw