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HomeMy WebLinkAboutTHE ORCHARD AT CLARENDON HILLS SUBDIVISION - PRELIMINARY - 47-89B - - DRAINAGE REPORTTHE ORCHARD AT CLARENDON HILLS PRELIMINARY STORM DRAINAGE STUDY I. GENERAL, INFORMATION The Orchard at Clarendon Hills is a proposed single family subdivision situated in the West `l./2 of Section 2, T6N, R69W of the Sixth P. M. , Larimer County, Colorado. The 12.5 acre site is generally located about 1,000 feet east of South Shields Street and one-half mile south of Harmony Road in the southern part of the City of Fort Collins. A vicinity map indicating the. location of the orchard Property is attached after the text of this report. This 12.5 acre parcel is in addition to the 160 acres of. land originally annexed into the City of Fort Collins as the Clarendon Hills Subdivision. The original 160 acres was Master Planned in the mid-1980's, and the Master Plan Storm Drainage Report was approved during the process of platting the first filing of 61 single family lots. Although the 12.5 acres was not included in the Master Planned drainage area, it falls within the same Fossil.Creek Storm Drainage Basin that the majority of the original 160 acre subdivision falls into. Currently, Filings 1, 2 & 3 of the Clarendon Hills Subdivision have been platted and the infrastructure has been developed to accommodate construction of about 150 homes. Individual storm drainage studies were prepared for each of the three filings completed so far,. -and the storm drainage improvements that were required for each of the first three_ filings have been completed. The proposed development plan for the Orchard at Clarendon Hills contains 27 single family lots of an average size of about 1 4, 000 to 15,000 000 square feet: The effect of development of the Orchard Property on the adjacent downstream properties should be minimal, and where necessary storm runoff detention facilities and erosion control improvements will be constructed to alleviate possible adverse impact on those properties. II. DESCRIPTION OF STORM DRAINAGE Enclosed as a part of this report is the third sheet (Sheet 3 of 3) of the Preliminary Subdivision submittal. On the drawing, the five (5) storm drainage basins that currently exist are outlined. Basin No.'s 1, 2 and 3 have a total area of seven (7) acres that slope downward to the north. Basins No.'s 4 and 5. contain a total of 5.6 acres and slope to the south. There is a ridge running east to west About midway through the site that creates the split of storm drainage runoff in the two opposite directions. Sheet 3 of the Preliminary Plat submittal shows the flow patterns and a chart of the hydrologic conditions. In the case of the Orchard Property, the drainage basins after development are essentially the same as currently exist. The street grades THE ORCHARD AT CLARENDON HILLS PRELIMINARY STORM DRAINAGE STUDY are to be designed so that a minimal. area.is affected by grading (i.e. within and immediately adjacent to the rights -of -way). The site itself, the lot layout and the anticipated street design all lend themselves to a very minimal overlot grading plan. The grading is intended to minimize the resulting loss of the fruit trees currently growing in the Orchard. A varied street section has been requested to further enhance the ability to preserve as many of the trees as possible. The improvements constructed adjacent to and downstream from the. Orchard Property on the north side (i.e. Ashford Lane and Detention Pond A in Filing One, Phase 1A) will likely be shown to adequately handle storm runoff from the north sloping portion of the Orchard Property. Because of the relatively small total 100 year storm runoff from Basins 1 through 3 of about 20 cubic feet per second, it is felt that the final drainage report will show that the direct release of storm runoff from these basins into Ashford Lane will be allowable under City of Fort Collins design requirements. The storm runoff from Basins 4 and 5 flows southerly into Applewood Estates. It is anticipated that detention will be required on the south side of the Orchard Property to detain the "after development" flows and release that storm runoff at the historic release rate. There are no off site drainage basins entering the Orchard Property boundary. A small irrigation ditch crosses from west to east along the top of the ridge and another small irrigation ditch for delivery of irrigation water to Applewood Estates crosses the southwest corner of the Orchard. The ditch along the ridge will be totally enclosed in a 15 inch culvert system and the ditch in the southwest corner will be replaced with a 15 or 18 inch culvert paralleling and 10 or 15 feet from the west boundary. A diversion structure will be designed and constructed that will allow the continued delivery of irrigation water to the property to the east of the Orchard and southerly to Applewood Estates from the southwest corner of the Orchard. The anticipated improvements for the two irrigation ditches are shown on Sheet 3. A street sag curve will be constructed in the street located on the southerly portion of the Orchard Property. There, the final storm drainage report will provide for further delineation of drainage sub -basins for hydraulic calculations for the design and sizing of inlets, or catch basins. From the low point of the sag curve, a 15 or 18 inch storm sewer will be designed and constructed to carry storm runoff from the street to the south between Lots 18 and 19. C THE ORCHARD AT CLARENDON HILLS PRELIMINARY STORM DRAINAGE STUDY From there, the runoff will be released into an area that will accommodate the required volume of detention behind an earthen berm to be constructed in the southeast corner of the Orchard Property. The release rate from the detention area will be controlled so as not to exceed the historic release rate from the storm drainage basin. The location of the storm sewer and the detention area are both shown on the enclosed Sheet 3. The street grades are to be at approximately a 4 percent. slope. As a result of the slopes and the relatively small storm runoff quantities, gutter capacity will be adequate throughout the development to carry all storm runoff within.the allowable. depth and spread, of flows. - In summary, the storm drainage system that will result from the development of the Orchard is anticipated to closely.resemble the existing drainage system. Although the addition of streets, and of roof tops and driveways of homes constructed in the Orchard, will increase the coefficient of runoff it is not anticipated that the increase will be substantial in the case of the Orchard Property. Erosion control improvements will be designed and provided for on the final construction drawings. Other than rip rap improvements at the downstream end of the storm sewer culverts and seeding of the areas affected by grading after completion of the grading operation, few erosion control improvements are expected to be required for this site. When the preliminary plat and supporting documents have been approved by the City of Fort. Collins, a comprehensive final storm drainage report will be completed and provided along with the final plat and supporting documents_ If there are questions or comments regarding this preliminary information, please contact the undersigned at (303) 225-9244. N o* 15 000000,0000 mm►m110% a om Ems OW 1 . 1 Am 0 v3 —'a z ..VICINITY,. MAP _-... ., THE ORCHARD AT C LARENDON_H. ILLS