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HomeMy WebLinkAboutSANCTUARY WEST - FDP - 30-07A - REPORTS - DRAINAGE REPORT�U (,i � t Vl 2�31 l� ��1� ��e. C. Erosion and Sediment Control Plan The following summarizes the overall erosion control plan for this project. The details will be shown on the construction drawings. I . Inlet protection will be used on the detention pond outlet box and any existing and proposed inlets. 2. Other erosion control measures will be used as necessary to help minimize erosion for this project such as vehicle tracking control and silt fence. 3. Straw bails will be used in the swales to keep sediments from running down into the propose detention ponds. IX. References West Vine Master Plan -Selected Plan by UPS Corporation, dated November 2002. 6 criteria establishes the methods and guidelines used to perform the erosion analysis and develop the erosion control plan. A. Discussion The clearing and stripping of land for site grading, overlot grading or for the construction of drainage structures and swales may cause high -localized erosion rates with subsequent deposition and damage to offsite properties. Uncontrolled, such erosion could destroy the aesthetic and practical values of individual sites and cause damage to downstream property. In general, erosion and sediment control measures may consist of minimizing soil exposure, controlling runoff across exposed areas and controlling sediment at drainage structures. Each of these measures is described below and shall be utilized by the developer and/or the contractor during any construction activity occurring at this site. B. General Erosion and Sediment Control Measures Minimizing Soil Exposure: Where practical, the construction area and duration of soil exposure should be kept to a minimum. All other areas should have a good cover of vegetation or mulch. Grading should be completed as soon as possible after it is begun. A temporary cover crop, a permanent vegetative cover crop or other landscaping should be established in disturbed areas. Re -vegetation may consist of native grasses, lawn grasses or various winter wheat. Decorative rock, flower gardens or shrubs may also be utilized in the final landscaping to cover the soil. Re -seeded areas should be mulched with straw or hay to protect exposed soil until vegetation is established. Controlled Runoff Across Exposed Areas: Where practical, construction may include constructing temporary swales to intercept and direct storm water around exposed areas. Swales can be constructed to control surface water, which collects on exposed areas and prevents gross erosion in the form of gullies. Riprap, straw bale dikes or other temporary erosion control devices may be installed to control storm water velocities across exposed areas. Sediment Control: Temporary and/or permanent sediment control devices may be installed at the major drainage structures and flow concentration points shown on the Drainage Exhibit. Such structures shall be used to intercept and trap sediment once it is produced and prevent it from being conveyed offsite onto downstream property. The FEMA FIRM panel # is 0869C0978F (see Appendix) and the City Is, Master plan that includes this site is the West Vine Major Drainage Basin that was completed in 2002. A floodplain report was prepared be URS and has been submitted to the City of Fort Collins. A paved roadway (Briarwood Road) is being proposed that is located within the Southern Branch's floodplain and floodway as well as modifications to the Hollywood -Irish Pond and outfall channel are also proposed. Since this site is not located within any FEMA designated floodplains, a CLOMR or a LOMR is not needed. With the exception of the roadway and other drainage structures, there are no housing structures being proposed within the City's designated floodplain or floodway. All proposed structures on this site will be residential. There are no residential structures within the designated floodplains. Within the Single Family Area (eastern area), the groundwater elevation is not very deep therefore, full basements are not possible. The finish floor elevation of all residential structures located adjacent to the floodplains will be several feet higher than the base flood elevations. ' Crawl space foundations or garden level basements are being planned for the single family homes within this development. If garden level basements are used, their basement slab elevation must be higher than the 100 year flood plain. The duplex lots may have basements depending on the groundwater elevation. All garages will be elevated above the 100 year floodplain. 1 A floodplain use permit will be required prior to the construction of the roadway, utilities, bike paths and retention/detention ponds. A no -rise certification is required for all site elements in the floodway. All construction within the floodplains will comply with Chapter 10 of the City of Fort Collins Municipal Code. VIU. Erosion and Sediment Control ' This section describes methods recommended to control wind erosion, soil erosion and sediment from storm runoff during and after the construction of drainage structures and site grading. This development of erosion control tV. Request for Variance INo variance to the City's drainage requirements is requested. VI. Conclusions A. Compliance with Standards All storm drainage calculations followed the guidelines provided by the City of Fort Collins Storm Drainage Design Criteria. B. Drainage Concept The drainage system has been designed to convey the runoff to the proposed detention and retention ponds in a manner as safe as possible. For information on the sizing of the retention and detention ponds, please refer to the separated analysis prepared by URS. The following table is a list of the current and developed flows (cfs) onto the Calvary Baptist Temple and into the New Mercer Ditch. ICalvary Baptist Temple 2 yr 10 yr 100 yr Current Conditions 2.19 3.74 9.75 Developed Conditions 0.41 0.70 1.90 New Mercer Ditch 2 yr 10 yr 100 yr Current Conditions 8.69 14.83 38.62 Developed Conditions 1.71 2.91 7.42 VII. Floodplain Review information This section describes the existing and proposed floodplain per the City of Fort Collins Floodplain Review Checklist. Information for this section was obtain from URS, the City of Fort Collins web site and FEMA web site. There are two City of Fort Collins designated floodplains and floodways located on this site. The South Branch of the West Vine major drainage floodplain generally follows the historic pathway of Soldier Creek and is located in the southeast corner of this site. The other floodplain is the Central Branch of the West Vine major drainage floodplain and is located along the northerly boundary. There is no FEMA designated floodplains located on this site. There are no product corridor nor erosion buffer zones. 1 6 I 1 1 1 I I 1 1 Subbasin BB Subbasin BB consists of the landscape areas near the northern property boundaries. This subbasin will not be detained in the proposed detention/retention ponds. Subbasin CC Subbasin CC consists of the area that drains directly into the North -South Retention Pond. Subbasin DD Subbasin DD consists of the landscape areas that drain into the existing north - south portion of the outfall channel for the Hollywood -Irish Pond. This subbasin will not be detained in the proposed detention/retention ponds. Subbasin FF Subbasin FF consists of the landscape areas that drain into a private storm system located in the rear yards of the single family lots that are adjacent to the New Mercer Ditch. The private storm system will drain to the North - South Retention Pond. 1 Subbasin HH ' Subbasin HH consists of the landscape areas near the eastern property boundaries. This subbasin will not be detained in the proposed detention/retention ponds. 1 Subbasin KK Subbasin KK consists of "front" yards and landscape area. Runoff is directed undetained to the Calvary Baptist Temple site. The area of subbasin KK is significantly less than the current area that drains to the Temple site. Subbasin LL . Subbasin LL consists of the area that drains directly into the East Retention Pond. B. Specific Details The storm sewer system for the western portion will be designed to convey the 100-year runoff to the East-West Detention Pond. The storm sewer system for the eastern portion will be designed to convey the 100-year runoff to the North -South Retention Pond. 5 Subbasins G and GG Subbasins G and GG consist of pavement, a very small portion of rooftop and landscape area along Barton Drive. Runoff is directed to storm inlets and storm pipe that outfalls into the East-West Detention Pond. Subbasins II, I, J and K Subbasins II, I, J and K consist of rooftop, pavement and landscape south of Barton Drive. Runoff is directed to storm inlets and storm pipe that outfalls into the East-West Detention Pond. Subbasins L and M Subbasins L and M consist of rooftop, pavement and landscape on both sides of Barton Drive. Runoff is directed to storm inlets and storm pipe that outfalls into the Southeast Retention Pond. Subbasin N Subbasin N consists of the Southeast Retention Pond. Subbasin O Subbasin O consists of the existing Right of Way of Briarwood Road. The runoff from this subbasin will continue to flow into the existing Southern Flow Path channel. Subbasins P Q R, S, T, U, V, W, and X Subbasins P, Q, R, S, T, U, V, W, and X consist of rooftop, pavement and landscape areas north of Barton Drive near the eastern property boundary. Runoff is directed to storm inlets and storm pipe that outfalls into the North - South Retention Pond. Subbasin Y Subbasin Y consists of the East-West Detention Pond Subbasin Z and JJ Subbasins Z and JJ consists of landscape area south of Barton Drive. These subbasin will not be detained in the proposed detention/retention ponds. Subbasin AA Subbasin AA consists of the landscape areas near the southern property boundaries. This subbasin will not be detained in the proposed detention/retention ponds. V.. D. Hydraulic Criteria The runoff produced by the site (with the exception of Subbasins Z, AA, BB, DD, HH, JJ and KK) will be conveyed to the proposed storm system and detention/retention ponds located within the development. All storm sewer will be sized for a minimum of the 2-year event, with most being sized for the 100-year storm event if there is no overflow conveyance provided. The equalization pipe located between the South -East Pond and the North - South Pond will allow for the transfer of storage volume between the two ponds. The North -South Pond volume is a approximately 4,700 cubic feet short of the required twice the 100 year runoff volume and the South -East Pond has approximately 8,600 cubic feet in excess volume. IV. Drainage Facility Design A. General Concept There will be four detention/retention ponds located within the development. The majority of the developed runoff will be conveyed to these detention/retention ponds. The East-West Detention pond is located within the outfall channel of the Hollywood -Irish Pond. The runoff to the East-West Detention Pond will be released at a predetermined rate to the east and into the existing channel. There are three additional retention ponds being proposed. Please refer to the additional drainage analysis prepared by URS. The subbasins will include the following: Subbasins A B C D E F, H, OS1 and OS2 Subbasins A, B, C, D E, F, H, OS1 and OS2 consist of rooftop, pavement and landscape north of Laporte Avenue. The runoff from these 9 subbasins is directed to the proposed storm system located in Barton Drive and will drain into the East-West Detention Pond. OS1 consist of the runoff from the Poudre R-1 School District land and building located at the northeast comer of Laporte Avenue and Sunset Street and will drain as overland sheet flow to Barton Drive and into the storm sewer. The storm sewer is sized to convey the 100 year flow to the East-West Retention Pond. Subbasin EE Subbasin EE consists of "front" yards and landscape area. Runoff is directed to a storm pipe which will drain to the East-West Detention Pond. I and the proposed Barton Drive. There is an equalization pipe connecting the Southeast Retention Pond with the North -South Retention pond. The portion of the site that is the farthest east (Subbasin LL) drains to a small retention pond located adjacent to the New Mercer Ditch. The offsite subbasins contributing runoff to the propose onsite storm system consists of ground cover, the northern half of Laporte Avenue and the rooftops of a few buildings. Not including the existing platted Right -of -Way for Briarwood Road, there are seven undetained onsite subbasins (Subbasin Z, AA, BB, DD, HH, JJ and KK for a total undetained area of 2.95 acres) located near the boundaries of this site. Of the undetained runoff only 0.06 acres is rooftop and there is no runoff from paving. All offsite subbasins which total 2.08 acres will be conveyed to the East-West Retention Pond. Of the offsite runoff, 0.11 acres is roof tops and 0.44 acres is in paving. The net un-retained area (onsite minus offsite) is 0.87 acres. The Southern Branch of the West Vine Major Drainage Floodplain crosses one of the access points to this development. The floodplain crosses Briarwood Road. During storm events, Briarwood road will be under water, however, there is a secondary access as well as an emergency access via a bike trail connection to the existing residential development to the north. It is our understanding that the depth of flow over Briarwood Road will be less that 18 inches during the 100 year event. III. Drainage Design Criteria A. Regulations The design criteria for this study are directly from the City of Fort Collins Storm Drainage Design Criteria and Construction Standards Manual. B. Development Criteria Reference and Constraints The developed site will convey runoff to the four retention/detention ponds in a safe and effective manner. C. Hydrological Criteria In accordance with the Fort Collin policy, a minor and major storm for the Fort Collins area is identified as the 2-year and 100-year storms, respectively. A major storm for the Fort Collins area has a recurrence interval of 100 years and a total rainfall depth of 3.67 inches for a 2-hour design storm, with a peak intensity of 9.95 in/hr. These storms have been used as a basis for planning and system design. The peak flow rates for design points have been calculated based on the rational method as described in the Storm Drainage Design Criteria with storm duration set equal to the time of concentration for each subbasin. I. General Location and Description A. Location Sanctuary West site is located in the NE'/a, Section 9, Township 7 North, Range 69 West of the 6th P.M., the County of Larimer, State of Colorado. The site is bounded on the south by Laporte Avenue and on the east by the New Mercer Ditch, on the north by Green Acres Subdivision and on the west by Rostek Subdivision. B. Description of Property The site to be developed consists of approximately 26.6 acres and is currently being used as a hay field. The site has an irregular shape with two fingers extending to Laporte Avenue. The eastern portion of the site will be developed as single family development with the western portion being duplexes. The outfall channel for the Hollywood -Irish Detention Pond is located on this site. The southwestern portion of the site slopes southeasterly at about 2%. The northeastern portion of the site slopes easterly to the New Mercer Ditch with an average slope of approximately 0.6%. II. Drainage Basins and Subbasins A. Major Basins Description The site is currently located in the City of Fort Collins' West Vine Drainage Basin, floodway and floodplain. According to the West Vine Master Plan prepared by URS Corporation, dated November 2002, the Southern Flow Path will be diverted through this site to the Central Flow Path. The proposed channel will dissect this site. URS has prepared a specific report for Sanctuary West Subdivision addressing the sizing of the onsite retention ponds and the impact to the West Vine Basin. Please refer to the URS's report for additional information. B. Subbasin Description In the current condition, there are 7 onsite subbasins. All 7 existing subbasins eventually drains to the New Mercer Ditch. In the developed condition, there will be 36 onsite subbasins and three offsite subbasins that contribute runoff to the onsite storm system. The western portion of the site will drain to the proposed East-West Detention Pond located in the outfall channel for the Hollywood -Irish Detention Pond. The central part of the site will also drain to the East-West Detention Pond. The eastern portion of the site will either drain to the North -South Retention Pond located adjacent to the north -south portion of the Hollywood -Irish Outfall Channel or to the Southeast Retention Pond located between Laporte Avenue TABLE OF CONTENTS SECTION I - GENERAL PROJECT LOCATION AND DESCRIPTION Page No. Location I Description of Property 1 SECTION II - DRAINAGE BASINS & SUB -BASINS Major Basin Description 1 Sub -Basin Description I SECTION III - DRAINAGE DESIGN CRITERIA Regulations 2 Development Criteria Reference and Constraints 2 Hydrological Criteria 2 Hydraulic Criteria 3 SECTION IV - DRAINAGE FACILITY DESIGN General Concept 3 Specific Details 5 SECTION V — REQUEST FOR VARIANCE 6 SECTION VI — CONCLUSIONS Compliance with Standards 6 Drainage Concept 6 SECTION VII — FLOODPLAIN REVIEW INFORMATION 6 SECTION VIII — EROSION AND SEDIMENT CONTROL 7 SECTION IX — REFERENCES 9 EXHIBIT A - Drainage and Erosion Control Plan 0 p.. eaTl -h N �.: ? AL ENGINEER'S FICATION rosion Control Study for Sanctuary West direct supervision in accordance with the ►rainage Design Criteria for the owners ,LOO$44 a 04444C. 9", �. d fax 271815. oat a ed 8052? (170) 219' 4042 September 25, 2010 City of Fort Collins Stormwater Utility 700 Wood Street Fort Collins, CO 80526 ,.-]RE: Sanctuary West Subdivision Dear Sirs: Loonan and Associates, Inc. is pleased to submit this Final Drainage and Erosion Control Study for the proposed Sanctuary West site located north of Laporte Avenue, east of Taft Hill Road. Please review this Final Drainage and Erosion Control Study at your earliest convenience. We look forward to your comments and ultimate approval of the s"Final Drainage and Erosion Control Plan. Please feel free to contact me at (970) 218-4042 if you have additional questions. Sincerely, `,coowa« a«d r?aaoe[atea. 9�cc. James R. Loonan FINAL DRAINAGE AND EROSION CONTROL STUDY FOR SANCTUARY WEST SUBDIVISION PREPARED FOR: Solitaire Homes Mr. Bill Veio 6645 E. Heritage Place South Englewood, CO 80111 REV. July 2010 DATE: AUGUST 2008 PROJECT NO. 219 CONSULTING ENGINEER: LOONAN AND ASSOCIATES, INC. P.O. Box 217815 Fort Collins, CO 80527