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HomeMy WebLinkAboutPLATT FARM PUD PRELIMINARY - 3 90A - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTPRELIMINARY DRAINAGE REPORT FOR THE PLATT FARM P.U.D. JAN UARY,1990 Prepared For: The Group, Inc. 401 West Mulberry Street Fort Collins, Colorado 80521 Prepared By: Engineering Professionals, Inc. 2000 Vermont Drive Fort. Collins, Colorado 80525 (303)-226-3852 i z 1.8-7 1•t-CCf L��L t• Y�7ig�L1r it St NA11Qi Z. I R 3.zc- 6Lo ,�Z ,44 'fl�,Sg Z Fi a003 I80 ,QS 141 43 35 1.15 3.8o z •it? 9.00 3 L 1'i<t 13go j 0 ).05 Zz )3.93 Z;4z 7?20 1.168 zS z.a5 4.�o z.oz 4.4z � 6 oA4 IoSo,-48 5� ,cif, 3.10 z,Sc 1<<sA 146 50 096 3�c 5.3� Zo.9s v\ee.LL A -a. ^4. � = l lea Soo ?� dd ti \c@ Lc�o� 4-g© Zcec(i.) 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U, Cc• .,o..F GM.d� v ,,.Q -U �G.rtaj JD jfc..t.1 v.,cDe! fi'J'OuTt^�A" �UG Sz l9/v � Ctoe pcv = �od VJSSuv�•�eC9, C7teo=\.25 L=l2eo t _ J �C Leo y toe Dev 3 Q wa l�Sa. T.� 1�1e �34�raS2 LZ�' Rio ® •%�lo QkW s C RR. \ � t W ✓� (t. � 1 PRELIMINARY DRAINAGE REPORT ' FOR THE PLATT FARM P.U.D. ►11; 0 1�� This report dated January,1990 presents the preliminary drainage evaluation of the proposed Platt Farm development. A hydrologic analysis of the site was performed to predict the t magnitude and consequences of storm runoffs. A hydraulic analysis then was utilized to locate and size drainage structures to pass, gather and. transport those runoffs in accordance with the City of Fort Collins' Storm Drainage Design Criteria (SDDC). The purpose of this report is to propose drainage improvements for the developed site which 1 will protect both on -site and off -site properties. Items to be covered in this report: 1.General 2.Site Description 3.Design Criteria 4. D.etention ' 5.Proposed Storm Drainage Provisions 6.Summary 7.Design Calculations 8.Preliminary Grading Plan ' 1. GENERAL The proposed Platt Farm development is located towards the southwest part of Fort Collins near the intersection of Harmony Road with Shields Street. More precisely, it is bound on the south by Wakerobin Lane, on the southwest by Regency Drive, on the west by Seneca Street, on the east by the Pleasant Valley and Lake Canal and on the north by the proposed extension of Troutman Parkway. Specifically, it is located in the Southeast Quarter of Section '34, Township 7 North, Range 69 West of the Sixth Principal Meridian, City of Fort Collins, Larirner County, Colorado. IIn it's entirety, the site contains approximately 30.6 acres. Of this, about 0.4 acres will be used 0 for Troutman Parkway ROW directly adjacent to the parcel, about 6.0 acres for interior street ROW, and about 1.3 acres for a community recreation area, consisting of a swimming pool and bath house. Proposed residential use is single family, with lots varying in size from 7100 sf to about 11000 sf. The anticipated number of lots is 92. The proposed development also will provide about 3.9 acres of dedicated open areas which can be used for hiking and bicycling. The entire 30.6 acres will be developed in two phases, with Phase 1 being the southern portion of the site, consisting of approximately 60 residential units. Sole access to Phase 1 will be off of Wakerobin Lane. Grading and utility and street construction will also be phased to initially provide only what is required to sustain Phase 1 development. In addition to completing interior utility and street construction at the outset of Phase 2, it is foreseen that construction of Troutman Parkway will occur, joining Seneca Street with Shields Street where Troutman currently terminates. 2 SITE DESCRIPTION The site itself is currently undeveloped, with surrounding areas exhibiting various forms of development, either existing or proposed. Across Wakerobin Lane to the south is a large regional detention basin that originated out of the McClellands and Mail Creek Major Drainageway Plan, prepared by Cornell Consulting Company and dated December, 1980. Construction of this detention basin required realignment of the Pleasant Valley & Lake Canal. To the southwest, across Regency Drive, is the existing Regency Park PUD containing several ® single family homes. To the west, across Seneca Street, are two new schools, Weber Junior High and Johnson Elementary. The lands to the north and east are currently undeveloped, with another regional detention pond being considered by the City of Fort Collins Storm Water Utility to be located just to the north in the McClellands basin. 2 ® 0 ® The Platt Farm site itself slopes to the east, towards the Pleasant Valley Canal. Average slope is about 1 %. Native vegetation includes grasses, weeds, and cacti. The "Report of a Soils and Foundation and Geologic Investigation" for the same general area prepared by Empire Laboratories, Inc. for Mountain States Properties in December,1978 reveals the general soil composition. Overlaying material is a silty topsoil. Next comes a layer of silty clay, followed by sandy silty clay reaching to bedrock at a depth of 8 to 13 feet. Perched groundwater tables may develop during seasonal groundwater fluctuations. ® At approximately the northern quarter point of the site (located in Phase 2), an existing 24" RCP discharges into an existing drainageway (historical). In the southwest corner of the site, along Regency Drive, is an existing (recently constructed) channel which carries runoff from (2)-42" RCP's beneath Seneca Street and directs that runoff into the regional detention pond to the south via (2)-42 RCP's beneath Wakerobin Lane. In the southeast corner, along Wakerobin, is a drainage swale constructed as part the Wakerobin street construction. This swale takes street runoff from a curb chase at a low point in Wakerobin and directs it towards the east into a 36" culvert which empties beneath Wakerobin to the regional detention pond to the south. Historically, the site proposed as Platt Farm consists of two separate basins, the northern portion of the site being tributary to the McClellands drainage basin and the southern portion belonging to the Mail Creek drainage way. Proposed grading for the development will follow closely the existing drainage patterns of the site, with no cross -over of developed runoff between historical basin boundaries. 3. DESIGN CRITERIA This drainage evaluation has been developed along the guidelines set forth in the City of Fort 1 Collins' Storm Drainage Design Criteria (SDDC), dated May,1984, The Rational Method of predicting storm runoff was utilized. Runoff coefficients, frequency adjustment factors, rainfall intensities, n-values and other variables were those recommended in the manual. t Offsite runoff which is directed onto the site from the west has been quantified in previous drainage reports. Discharge from Johnson Elementary School was addressed in the "Final Drainage Report for Poudre School District R-1 Elementary School", prepared by Engineering Professionals, Inc., and dated April 1981. Runoff from that portion of the Johnson Elementary school site historical to the Mail Creek basin runs undetained in an open channel along the west side of Seneca Street and the east side of Regency Drive to the regional detention pond south of Wakerobin Avenue. Runoff from the school site tributary to the McClellands basin is ' detained and currently released at the 2-yr. historical rate. It is possible at some future time that the release rate might be increased to the historical 100-yr. rate, as proposed in Engineering Professionals' April 1987 report. This probably would not occur until improvements in the form of construction of the regional detention facility to the north, as studied by the City Storm Water Utility, takes place. 1 3 1 ® 0 I On the Weber Junior High school site across from the southwest portion of the proposed Platt ' Farm site, an open channel carries runoff to a pair of existing 42" RCP's beneath Seneca Street at the intersection of Regency Drive. This channel was sized by Parsons & Associates for the "Final Storm Drainage Report for the Villages at Harmony West PUD" in July, 1983 for 274 cfs. ' It is assumed in this report this is the anticipated runoff for the 100-yr. storm under developed conditions. That report indicated once the pipe crossing beneath Seneca Street was reached, runoff in excess of pipe capacity was assumed to enter the street system and cross over to Regency Drive, flow southwards to Wakerobin Avenue, then to .a sump condition to the east in Wakerobin where the excess would spill into the regional detention pond to the south. Existing construction at the intersection of Seneca Street and Regency Drive allows for a t headwater to pipe diameter ratio (HW/D) of 1.72. This indicates that under inlet control 180 cfs would be accepted by the (2)-42" beneath Seneca Street. This is carried along the east side of Regency Drive, under Wakerobin Ave., into the regional detention pond. However, the crossing at Wakerobin provides a HW/D ratio of only 1.58, allowing just 170 cfs to pass. Thus 10 cfs plus any additional runoff added to the channel between Seneca Street and Wakerobin from this development will be added to Wake Robin during the 100-yr. storm event. 4. DETENTION That portion of the Platt Farm site that is tributary to the Mail Creek basin (Phase 1) historically drains to the regional detention pond adjacent to the site on the south side of Wakerobin Avenue. No detention is proposed for Phase 1 as adequate provisions have been made in existing improvements to transport the 100-yr. developed runoff from Phase 1 to the existing . detention pond. Phase 2 construction will cause developed runoff to be added to the McClellands basin. A regional detention pond is being considered by the City of Fort Collins Storm Water Utility for this basin, somewhere to the north of the site. Location, size and timing of this detention facility have yet to be determined and it is possible that Phase 2 construction may occur prior to the pond being constructed. Should in fact Phase 2 be ready to proceed prior to the pond being in place, two avenues exist for providing detention of developed runoffs from Phase 2. The first would provide detention on site for any runoff generated in excess of the 2 year historical runoff. The historical 2-yr. runoff is 3.02 cfs. With 3.02 cfs as the release rate, on -site detention volume required is 1.26 acre feet. In actuality, the allowable release rate would be increased to pass undetained the 2-yr. release rate of 3.8 cfs from Johnson Elementary School, for a total release rate of 6.82 cfs. Providing for 1.26 acre feet for runoff detention would severely restrict the flexibility of the site currently intended to house a community recreation center. Final site layout on this parcel would likely be dictated by the detention storage requirements, possibly to the point of making construction of the rec center impractical until the regional detention pond to the north is in place. 4 ® 0 A drainage swale, though possibly only temporary in nature and alignment; would be required north of Troutman. Parkway to day-lite discharge from the detention pond. The alternative to on -site detention would be to initiate construction of the regional detention facility in coordination with the City of Fort Collins Storm Water Utility, It is possible this new pond then could be utilized for the required detention of Phase 2 runoffs, with no detention being provided on Phase 2 itself.This approach would allow for maximum utilization of land located in Phase 2. There is the possibility that even with construction of the regional facility, the City's Storm Water Utility may require that Phase 2 still detain runoff in excess of the 100-yr. historical runoff. In this case, needed detention volume would be reduced to .66 acre feet using a 100-yr. historic release rate of 13.02 cfs (or a total of 26.72 cfs when the 100-yr. release from Johnson Elementary is included). This would still restrict the flexibility of the proposed community rec site; though to a lesser degree than the first option, and could make this alternative somewhat less appealing. Detailed discussions with the City Storm Water Utility as to detention requirements with the regional pond in place and financing arrangements will be necessary prior to final submittal. The preliminary grading plan included with this report assumes the first option is chosen. The shape, size and location of the detention pond are preliminary. 1 5. PROPOSED STORM DRAINAGE PROVISIONS Proposed storm drainage improvements for the site can be classified into two categories; streets and storm sewers. Site grading will be such that full advantage of the 100-yr. street carrying capacities can be taken. In no instance will anticipated runoff for any of the analyzed ' sub -basins be greater than the allowable street capacities calculated in conformance with the SDDC. ' The storm sewer system has been designed to collect accumulated runoffs from sump areas within the interior street system or to allow for release from the proposed detention pond in the northwest corner of the site. The pipes placed to remove storm water from the streets have been designed to accommodate the 100-yr. developed flows. This precludes the necessity of providing drainage swales down lot lines from one cul-de-sac to another which would require either concrete pans or substantially more fill due to minimum slope requirements for open channels. As it is, the layout of the site requires substantial fill in areas to provide enough fall for drainage given the controlling existing pipe inverts. 11 5 11 The existing 36" RCP in Wakerobin has sufficient capacity under gravity flow to pass the 100- yr. runoff deposited to it by this development, but it is likely that some of the collected stormwater delivered to it will find it's way to the sump location in Wake Robin and pass into the detention pond at that point, given the relative vertical location of the street crown at this location and the invert of the 3T pipe. It is likely that Wakerobin Avenue from this sump location westwards to Regency Drive will be completely inundated during the 100 year event given the duty of carrying flows in excess of existing crossings upstream as proposed in the aforementioned Parsons & Associates report. Before the allowable street capacity can actually be reached, however, accumulated flows would overtop the back of sidewalk on the south side of Wakerobin and empty into the regional detention pond. The discharge pipe for an on -site detention pond, if required, has been sized to accommodate the 100-yr. release rate from both the Platt Farm site and Johnson Elementary -This will provide for some site reclamation of areas initially required for runoff detention storage should the regional detention pond to the north not be constructed prior to, or in conjunction with, Phase 2. Preliminary calculations require (2)-30" RCP's to be placed beneath the Troutman Parkway extension during Phase 2 construction. This is to allow.for drainage for the 35.2 acres located south of the Troutman extension, north of Wakerobin Lane, west of Shields Street and east of the Pleasant Valley Canal. Location is as yet undetermined, pending final design of the Parkway and a closer analysis of the site and probable future uses. For this preliminary analysis, it was assumed that one half of the site could eventually be directed to the north, towards Troutman Parkway. There are existing provisions to drain portions of the site beneath Wakerobin and also to the east beneath Shields Street. - N. 1 6. SUMMARY ' Proposed street improvements and accompanying drainage appurtenances will convey developed runoffs within the street. until being removed and placed into proposed or existing storm sewers leading to detention facilities, either on -site or off. Historical runoff patterns will not be adversely affected. The proposed drainage improvements will have a positive effect on the site in that concentrations of high flows are avoided or managed. Overall, the proposed drainage plan is consistent with neighboring developments, the McClellands and Mail Creek Major Drainageway Plan, and the City of Fort Collins' Storm Drainage Design Criteria. ENGINEERING PROFESSIONALS, INC., Joseph P. Marcisofsky Robert W. Sch.ildgen, P.E. 7 PRELIMINARY DESIGN CALCULATIONS