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Home My WebLink AboutDrainage Reports - 09/01/1978OF roar OWLum U` nzruw STORM DRAINAGE STUDY FOR LARKBOROUGH SUBDIVISION FORT COLLINS, COLORADO SEPTEMBER, 1978 ov + �COLM COLOPADO .a M&I consuffing engineers 4710 South College Ave. Fort Collins, Colorado 80525 Telephone (303) 226-2323 September 22, 1§78 Art Builders, Inc. 582 Mohawk Drive Boulder, CO 80303 Attn: Mr. Art Hausman Dear Art: Presented herein is a storm drainage report for Larkborough, located in the City of Fort Collins. This report conforms to both the sub- division 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 & I, INC.X . 4z' John R. Hutchinson TABLE OF CONTENTS I. Introduction II. Study Area III. Existing Storm Drainage IV. Design.Criteria.and Requirements V. Drainage Plan VI. Conclusions and Recommendations Exhibit No. 1 "Contributing Off -Site Basins" Exhibit No. 2 'Design Flows" Exhibit No. 3 'Drainage Channel Crossection" Exhibit.No. 4 "Intensity Duration Curves" Page No. 1 1 1 2 3 5 111 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 Larkborough Subdivision itself and adjacent contributing basins.. The scope of this report involves an analysis of the storm drainage characteristics of this property in both its historical and improved conditions, 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 and culverts. II. STUDY AREA The area on which this study is focused is Larkborough Subdivision and its contributary basins. Larkborough is located in the southern portion of Fort Collins, Colorado. Exhibit Number 1 displays Larkborough Sub- division's geographic location. III. EXISTING STORM DRAINAGE Larkborough Subdivision is bounded by the Colorado and Southern Railroad on the east and Harmony Road to the south. The general topography of the area is comprised of gently rolling features, basically sloping from west to east. Approximately 80 acres drain to an existing 20" steel pipe running under the Colorado and Southern Railroad tracks. The re- mainder of the subdivision slopes to Harmony Road, where the runoff is intercepted by the barrow ditch and transported under Harmony Road .. through an existing 12" C.M.P. eventually flowing to Mail Creek. -1- V. 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 de- signed to accommodate the runoff from a 2-year storm by utilizing the maximum capacity of the streets, overland swales and, if necessary, under- ground conduits. The major drainage system must be designed to handle a 100-year"storm without extensive property damage or loss of life. Areas which may be susceptible to flooding must be examined. For purposes of the major .storm analysis, it is assumed that the storm sewer has been blocked and rendered ineffectual. Floor elevations should be .set to allow proper drainage between and around these houses without flooding.., (See Exhibit No. 2). The City of Fort Collins requires an improved site to limit its runoff to the amount resulting from a 2-year frequency storm occurring 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 maxi- mum 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 design of borrow ditches and stormsewer 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. (See Exhibit No. 4)._ The ponding elevations, capacities.of the channels and piping were deter- mined by Manning's Formula, which equates a constant, a roughness factor; 1 the area of the channel or pipe, the hydraulic radius and the slope, to the capacity of the channel or pipe. -2- V. DRAINAGE PLAN Offsite areas to the west of Larkborough Subdivision will contribute a substantial amount of runoff. (See Exhibit No. 1). The runoff from Basins A and B is presently intercepted by the Pleasant Valley and Lake Canal. However, for purposes of this analysis, it has been assumed that as Basins A and B develop, detention ponds will be constructed limiting the runoff from these areas to the 2-year historical value. The City of Fort Collins' present policy is to discourage any runoff from being deposited in irrigation ditches. This runoff is to be transported . under the canal in order to maintain the integrity and flows of the natural basin. Therefore, Basins A and C will be treated as a single basin, as will Basins B and D. The runoff contributed by Basins A and C will enter the subdivision in the northwest corner, an area that has been designated as a park site. A storm sewer has been designed to carry the 2-year historical flow through the subdivision to the main detention pond. (See Exhibit No. 2). _ In the event of flows exceeding the 2-year storm, detention will occur in the park site. Basins Band D will also contribute a considerable quantity of runoff. This water will be routed through Larkborough in a channel which will also serve as the major drainage relief for the majority of the subdivi- sion. In the future, offsite flows will be limited to 2-year historical values,' but until this occurs it was decided that in order to provide adequate protection against flooding,.the channel should be designed to at least handle flows attributable to a 100-year frequency storm.. In fact, to . provide free board as well as additional protection, the channel has the capacity for flows well in excess of this value of a 100-year storm.. (See Exhibit No. 3)• The onsite runoff flows and directions for Larkborough are shown on Ex- 1 hibit No. 2. The catch basins at the low points in Warbler, Hummingbird and Troutman Parkway were sized to handle 2-year improved storm flows.. -3- 'I V The culverts under Warbler and.Hummingbird were designed to carry 100-year improved storm flows. Basins 2, 3 and 4 are tributary to the detention pond. The runoff from Basin 1 will be carried in the gutter of Harmony Road to a pipe under the pavement which eventually leads to Mail Creek. The detention pond, as shown on Exhibit No..2, has been designed to meet the basic criteria as previously stated in this report. The re- quired capacity of the pond was determined employing three assumptions. They are as follows: 1) The detention area in the park site will.contribute a constant flow of.15.20 C.F.S. 2) Basins B and D will be considered to be in their historical con- ditions. 3) Larkborough will be in an improved state. Ordinarily, provisions for passing offsite runoff directly through the site with no detention are made. However, the outlet size for the Tract "B" detention pond was fixed at 16 C.F.S. by a previous agreement with the New Mercer Ditch Company. The ditch company will not allow the introduction of any more flow than the agreed upon 16 C.F.S. into their ditch. Therefore, it was necessary to provide adequate storage capacity to handle the additional offsite as well as onsite flows. The total capacity of the detention pond in Tract "B" is 15.0 ac.ft. The City of Fort Collins wishes to maintain a permanent lake on the detention pond site. The capacity of the pond was determined as the total volume between the maximum level of this lake, and the maximum high water mark of the pond. The highest level of the lake will be con- trolled by an area drain on the eastern side of the detention pond. The area drain not only regulates the maximum level of the lake, but also acts as the regulating outlet for the detention pond. -4- 11 During a major (100-year frequency) storm, there are only three areas susceptible to flooding. Exhibit No. 2 illustrates these, three points, the areas covered, and the maximum depth, as measured at the flowline of the gutter. In all three cases no damage would occur to homes in the immediate area. VI. CONCLUSIONS AND RECOMMENDATIONS This drainage design will satisfy all hydraulic considerations as well as govern mental regulations in providing this future community with both a safe and equitable solution to the problem of handling storm water runoff. The following is recommended: 1) Storm water runoff should be routed as shown on Exhibit No. 2. 2) All drainage facilities and detention ponds should be constructed as shown on Exhibit No. 2. 3) The Federal Housing Administration minimum requirements be maintained on overlot grading and street design. -5- Drakes =,• • -', .� i ' SP90 � i PRAK E- ROAD +}�yt � S J S 50<] I BAI K N� -----�` �� ,PON_TRIBIJTINi 0 -ST�E t rt. :299ti _ _ { 6 r r EXHIBIT N'O I ir'�GrnveC q it - . �p .. ': � ♦ \ - �...�_ 50591 • � ��—s - .! •yl I �� _ :Q BM 5032 'So �•, I o x I a I '� •` ` .! � ! •� � � I, W '� �, to %- - Qe 1520 C.FS. R o I7 I ¢. �® 0100` 5Q80 C.F-S I, j yS FN: �r ��- ! D . ... _ Aso el- i,33 0 .5 AC.IN ��----a �3 Q a r �. �'-m-F"' - - 5 SAC. - - 50« 136 SINi D ,t¢ soap, L RKBOROUGH BAAMN - � � o I' +.2;At~,„ $DIVISION -- - 4�G:� AC. ® - � BOAS_ - I I 0`67,,34C.FS sc ! i�•. I I 5050 N I' < q<. 1 5078 LARKBOROUGH SUBDIVISION EXHIBIT NO. 2 DESIGN FLOWS FOR 2 YR. AND IOOYR. FREQUENCY STORMS too "O"ro OFFSITE 0 . ........ 5046 ............ ............. ......... ------ 0 .......... R . .... w........ ................ . ..... SlONG STREET 5060 Z. w -TRACT A RE 4a 6.0 AC. - — — — — — — PkA�R. K ....... z BASfU 3 A Rs. "= ... .. ........... ......... ......... Z z LU ZA _j ........... ... > ----------- 5046 w ....... ... TROUTMAN TROUTMA IPARKW4Y .......... LU 70 )YR. P DING < z 5060 VL E HT FL= U) /> F GUI -EMONT STREET cr w w w , >. LU z 5058 ;r, ..ta 0 w CD 0 cm r ,w 5044 cr TRACT,."B'._... 5042 5 DETENTION 040 aB POND BASIN 2 w E R CAP = 15.0 AC. -FT. 5056' 52 1 . lmrc' I I I HIGH WATER OUTLET E .ELEV= 41.5 OFFSITE 16 C.F.S. 1 0 R. ONDI G LE% X DE. TH TO 100 R.. DING LEVEL 004 CEPTHTOFL=.90' Qp 0 _j move Qe4Z, 6 QIUCf= 97.81 Q =I Rik __5040 CIA -iij z 'V ...... 4,? ...... CAP CITY 11. C. 50! 0_5042 VE.... ...... ..... ... Cc*. 0 4-5044 JBASIN.• 4 5056 __AKlEA=7!,,AC.''. 0 h. ... ....... _j 0 .......... >ei'..... . ...... . ..... ........ .... .. . .......... 5046 .... 5048 ...... --- ........ T,R A Cr E 0 5048 FUTURE AULTI-FAMILY"AREA) LEGEND FINCH owj STREET' \ VRCID ,r 73y DESIGNATES DRAINAGE BASIN 5056 INDICATES DIRECTION 80 UANTITY OF FLOW 5058 j A$tN I IN CUBIC FEET PER 50601— A A= 14.2 C-.-- SECOND - "C.FS." ... ... .......... ........ . ... .... .. ....... CITY OF FORT COLLINS OAA STANDARD CATCH BASIN 5058 .... 5056 .. ...... .. ... . TQ.WHEE STR E Ef CITY OF FORT COLLINS •• STANDARD S' VALLEY - 5054 .............. PAN 5052— z . ........ -5046 STORM SEWER PIPE 5050—.agGo 5 MANHOLE .60 ......... ........ . 9 !046. ..................... ..... WEST HARMONY ROAD 5044 EXISTING 12 41 C.M.P.0% mu .. ....... ........ ,tHRASHER,.. STREET I SLOP MAX. S. 9n' noniuwr_e r e erur.�r HIGH WATER DEPTH 1.3' ' 2.33' - OPE 3:1 3 T' 6' MAX. SL PE MAX CAPACITY = 212. C.F.S. No Text