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HomeMy WebLinkAboutHIDDEN POND ESTATES - PDP - 11-07 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTINTERWEST C O N S U L T I N G G R O U P ` August 15, 2007 \ Basil Harridan City of Fort Collins Stormwater 700 Wood Street Fort Collins, CO 80522-0580 RE: Final Drainage and Erosion Control Report for Hidden Pond Estates Subdivision Dear Basil, I am pleased to submit for your review and approval, this Final Drainage and Erosion Control Report for Hidden Pond Estates Subdivision. I certify that this report for the drainage design was prepared in accordance with the criteria in the City of Fort Collins Storm Drainage Manual. I appreciate your time and consideration in reviewing this submittal. Please call if you have any questions. Sincerely, Michael Oberlander, P.E., L.S.I. F' U 4261 ii 1218 WEST ASH, SUITE C WINDSOR, COLORADO 80550 TEL 970,674.3300 • FAX. 970.674.3303 N FINAL DRAINAGE AND EROSION CONTROL REPORT I Hidden Pond Estates Subdivision Prepared for: Beld Developments, LLC 6215 Westchase Road Fort Collins, Colorado 80528 Prepared by: Interwest Consulting Group 1218 West Ash, Suite C Windsor, Colorado 80550 (970)674-3300' Revised August 15, 2007 May 7, 2007 Job Number 1055-039-00 0 LJ w w ja SITE E. HARMONY RD. HIDDEN POND ESTATES VICINITY MAP SCALE: 1 " = 1000' APPENDIX A VICINITY MAP A 6.2 Drainage Concept The proposed drainage concepts presented in this report and on the construction plans adequately provide for stormwater quantity and quality treatment for the proposed project. Conveyance elements have been designed to pass required flows and to minimize future maintenance. If, at the time of construction, groundwater is encountered, a Colorado Department of Health Construction Dewatering Permit will be required. 7. REFERENCES 1. City of Fort Collins, "Storm Drainage Criteria Manual", (SDCM), dated March, 1986. 2. Urban Drainage and Flood Control District, "Urban Storm Drainage Criteria Manual", Volumes 1 and 2, dated June 2001, and Volume 3 dated September 1999. 9 5. EROSION CONTROL 0 5.1 General Concept This site lies within the Moderate Rainfall and Wind Erodibility Zone per the City of Fort Collins Zone Maps. The potential exists for silt movement from the site and into the existing drainage areas on the west and north sides of the site. Potential also exists for tracking of mud onto existing streets which could then wash into existing storm systems. The required performance standard for the site 83%. During construction and after final paving and building construction, this figure is exceeded with the use of silt fence, wattle checks, inlet protection on the upstream ends of culverts, buried riprap, vegetation, and by using the new detention and water quality pond and a sediment trap during construction. The erosion control escrow amount is $10,950 5.2 Specific Details To limit the amount of silt leaving the site several erosion control measures shall be implemented during construction. Wattles will be used where appropriate (in ditches, swales and culvert entrances) and the boundaries of disturbance will have silt fence installed. A vehicle tracking pads will be installed at the proposed drive connection to Ziegler Road to control the mud being tracked onto the existing pavement. During overlot grading, disturbed areas are to be kept in a roughened condition and watered to reduce wind erosion. The proposed detention and water quality pond (Pond C) will be used as a sediment trap during construction. All disturbed areas not immediately built upon shall be seeded and mulched. This site is more than one acre and does require a State of Colorado Stormwater Discharge Permit Associated with Construction Activities prior to beginning work. 6. CONCLUSIONS 6.1 Compliance with Standards All computations that have been completed within this report are in compliance with the City of Fort Collins Erosion Control Reference Manual for Construction Sites and the Storm Drainage Design Criteria Manual. 8 El will release 0.8 cfs (0.18 cfs per acre) in the 100-year storm No new penetrations will be needed into the ditch, no work will be done in the FCRID right-of-way. Pond C only has 00.8' of freeboard between the 100-yr water surface elevation and the top of the proposed berm. The pond cannot effectively be made bigger because of efforts to limit disturbance in the Natural Areas Buffer. The spillway is 0.2' about the 100-yr water surface elevation, so there is about 15% excess capacity in the pond before the spillway would be used. U Basin 7 includes the area that will sheet flow to Pond C. This area just includes a couple of new lots in this basin; no common conveyance is required to the pond. Basin 8 includes the east portion of the site and the outside of the detention pond embanlanent. There is no way to detain this area, so it will free release (as it does today) into FCRID. This basin is only 1.71 acres, and is completely pervious, but still contributes 2.3 cfs to FCRID in the 100-year storm. This rate has been reduced from the 3.9 cfs allowed for the entire 17.0 acre site and is the reason for utilizing retention in Pond A and overdetaining in Pond B and C. Basin 9 is in the far northeast portion of the site. This area drains southeast into the existing irrigation crossing of FCRID and does not drain to the ditch. Because this area does not contribute to the release from the ditch, there is no detention proposed for the area — it will not be touched with the development of the subdivision. 4.3 Drainage Summary All drainage facilities proposed with this project as well as the private drives will be maintained by the Homeowner's Association. There are no public facilities proposed with the development. The site drains to FCRID at a rate of 0.23 cfs per acre (3.9 cfs for 17.0 contributing acres). again will only have a single cross slope allowing for a single ditch rather than ditches on both sides. Basin 4 includes the new lots that will drain to the roadside ditch and ultimately to Pond B. The roadside ditch in this area drains to an area inlet and storm pipe (Storm Pipe 2), and then into Pond B. Storm Pipe 2 has been lengthened to prevent developed flows n from running down the steep embankment in this area. Basin 5 includes several new lots which drain to the north as well as Pond B has a water surface elevation of 4888.0 and is spring fed. It is proposed to install a water quality structure and 8" storm pipe with a headwall and orifice plate at the 4888.0 elevation. The plate will be designed to release 0.5 cfs (0.08 cfs per acre) and will drain on the south side of the pond where it will eventually reach the existing culvert into FCRID. With these structures in place the 100-year water surface will be at 4890.8. The current top of the pond is at 4891.5 to 4892.0, but there is not a standard spillway on the pond. It is proposed to build an appropriate spillway in the same vicinity as the outlet pipe at an elevation of 4891.0. This is not a tall enough structure to be designated as jurisdictional, so this work should require nothing more than City approval. This pond only has 0.7' of freeboard between the 100-yr water surface and the top of the berm. This is mostly because of the addition of water quality to the pond. The water quality depth provided is only 0.5', but the volume is more than double what is required. It is not possible to provide an outlet structure that will function with a shallower depth, so the 100-year WSEL was raised along with the water quality elevation. The spillway will be constructed in concrete (to double as a trail), so elevations should be exact with the construction, and this variance from freeboard standards should not be an issue. Basin 6 includes the south central portion of the site where now lots will be constructed as well as the proposed detention and water quality pond (Pond Q. These lots drain east via roadside ditches and overland flow to the proposed pond. The pond has been designed to be long and slim, basically just providing a berm to hold water against the existing slope from the east. The bottom of the pond varies from 1% to 20X0 and at this time, a trickle pan is not proposed in the bottom of the pond. The pond will have a standard water quality and detention structure and will drain to the existing ground on the east side of the pond. This area then drains to an existing culvert into FCRID. The pond 6 presently drains south into an existing channel and into the irrigation pipe which flows over FCRID. Since this land does not contribute to the ditch, it is not accounted for in any detention calculations. 4.2 Specific Flow Routing A summary of the drainage patterns within each basin is provided in the following paragraphs. V Basin 1 includes the north portion of the existing home site and the northwest corner of the subdivision. This area all.presently drains overland to the existing dry pond in this area (Pond A). This pond was a functioning irrigation pond, but the inflow was removed and it is now dry and only supports some wetlands in the bottom. It is approximately 5 feet deep and all water that drains to the pond is retained until it evaporates or percolates. It is proposed to leave Pond B as is with no improvements. Using the City's 3.67 inch 100-year rainfall and the rational method C value, there is approximately 0.35 acre feet of water draining to this pond in a developed 100-year storm. The pond has double the retention volume at a depth of 2.6 feet and at 5.0 feet deep has 1.36 acre feet of volume available. With four times the necessary storage (and the constraint of existing wetlands), it is proposed to not build any additional facilities in this area. Basin 2 includes the existing road right-of-way. Improvements in this area include . paving a 20' wide private drive and a 6' crusher fine path. A shallow roadside ditch will be constructed on the south side of the drive to prevent water from flowing to the property to the south. The north edge of the drive and trail will generally match the location of the north edge of the existing gravel drive. The trail and drive all will slope south (no crown). There will be no disturbance near the existing hedge between the road and the homes to the north. The drive and ditch are designed at approximately 0.5% because of the existing conditions and will act as a water quality feature by slowing flows from the area. This area flows east and into Basin 3 and is ultimately detained in the existing irrigation pond (Pond B). Basin 3 includes the southern portion of the existing home site. Flows in this area will be routed in a roadside ditch to a culvert crossing the private drive (Storm Pipe 1). The drive 1 5 4. DRAINAGE FACILITY DESIGN 4.1 General Concept Only 50% of the site will be developed with the project, but the City stated in the Conceptual Design Review Meeting that even the protected areas must meet the detention requirements for the site (0.23 cfs per acre, or 3.9 cfs for the entire 15.4 acre site and 1.6 acre right-of-way). This poses significant challenges with the existing Natural Areas Buffers on the site. Detention ponds are allowed within these buffers, but any significant grading to route flows to the ponds is not allowed. Because of the nature of the project (relatively large lots, and only 14 total), a private 20' asphalt drive is proposed throughout the site for access. The drive will have roadside ditches as required to achieve proper routing of the storm flows. The northeast portion of the site will continue to drain overland to the existing dry pond. It is proposed to use this pond as a retention pond and not disturb the pond or the buffer area in any way. Typically, this would not be acceptable, but in this case, the existing pond has available volume for about four times the runoff of a 100-year storm. The right-of-way and central portion of the site will be routed through grass lined roadside ditches and culverts to the existing irrigation pond on the north side of the site. It is proposed to utilize the upper portion of this pond to overdetain this contributing area to 0.07 cfs per acre. To achieve detention, a small water quality structure, orifice, plate and pipe is proposed above the operating level of the pond. If the pond reaches this level, it will release at a reduced rate to the east and into FCRID. It is proposed to add a spillway to this pond. The south central portion of the site drains to the east and into a new detention pond proposed adjacent to FCRID. This pond overdetains the contributing area to 0.18 cfs per acre. This pond will be designed with a standard water quality outlet structure. The proposed pond discharges to existing grade and then to FCRID through an existing culvert in the FCRID right-of-way. All of the land east of the proposed detention pond free releases to FCRID. This area contributes more than half of the allowed runoff for this site (which is the reason for the other ponds retaining or overdetaining). The land in the northeast corner of the site 4 0 3. DRAINAGE DESIGN CRITERIA 3.1 Regulations This report was prepared to meet or exceed the "City of Fort Collins Storm Drainage Design Criteria Manual" specifications. Where applicable, the criteria established in the "Urban Storm Drainage Criteria Manual" (UDFCD), developed by the Denver Regional Council of Governments, has been used. 3.2 Development Criteria Reference and Constraints The runoff from this site has been routed to conform to the requirements of the Fox Meadows Master Drainage Plan. The Plan specifies that this site is required to provide _ Stormwater detention to a rate of 0.23 cfs per acre. Water quality facilities are required for project. 3.3 Hydrologic Criteria Runoff computations were prepared for the 2-year and 10-year minor and 100-year major storm frequency utilizing the rational method. All hydrologic calculations associated with the basins are included in Appendix B of this report. Water quality volume was calculated using the method recommended in the "Urban Storm Drainage Criteria Manual". Detention volume was determined using the "Rational" or "FAA" method. All pond calculations are in Appendix D of this report. 3.4 Hydraulic Criteria All hydraulic calculations within this report have been prepared in accordance with the City of Fort Collins Drainage Criteria and are included in Appendix C of this report. ditch. The areas that drain to the existing dry pond in the northwest comer of the site and _ the areas that drain to the existing irrigation pond in the north -central portion of the site do not have outfalls to FCRID. The dry pond is quite large and was once used for irrigation storage, but the flow to the pond was cut off by the development to the west. dAny runoff to this pond simply is held until it can evaporate or percolate into the ground. The existing irrigation pond is spring fed and is in use. This pond has a head gate that releases water to an overhead pipe crossing of FCRID and to the land to the east. There is a small portion of land in the northeast corner of the site that also drains to the overhead crossing. There are no existing storm drainage facilities for these ponds and they have adequate volume to retain existing storms. There are no mapped floodplains on the property. It should be noted that it is the intent of the project to disturb as little of the landform as possible to keep the integrity of the project. The grading and drainage plans basically grade the roads, swales and the new detention pond, but not full overlot grading of all of the home sites. With the final design, there will be additional design so that the lot to lot drainage can be accomplished simply by the home builders, and minimum opening elevations will be specified. It is the desire of the Planning Department, Natural Resources Department, Historic Preservation and the Owner to try to limit the amount of disturbance with the project. 2. DRAINAGE BASINS AND SUB -BASINS 2.1 Major Basin Description The proposed development lies within the Fox Meadows Master Drainage Basin. 2.2 Sub -basin Description The southern portion of the site drains to FCRID; the north portion of the site drains to the existing ponds and to the existing overhead crossing of FCRID. These drainage paths will be generally maintained with the project with some changes. No off site flows other than the existing right-of-way enter the site. 2 1. GENERAL LOCATION AND DESCRIPTION �1 1.1 Location Hidden Pond Estates Subdivision is located in southeast Fort Collins. The project is unplatted and located in the West Half of Section 33, Township 7 North, Range 68 West of the 61h P.M. in the City of Fort Collins, Larimer County, Colorado. See the location map in Appendix A. The project area is approximately 15.4 acres and is south and east of Woodland Park Estates PUD and north of the AVAGO campus (Hewlett Packard). The Fossil Creek Reservoir Inlet Ditch (FCRID) is adjacent to the east. The site access is from an existing private road right-of-way that connects to Ziegler Road on the west. 1.2 Description of Property The project contains approximately 15.4 acres along with an existing 60' wide right-of- way connection to Ziegler Road which includes about 1.6 acres. There is an existing house and garage on the property that will remain as well as an existing barn which will be moved to the west portion of the site — the house and barn are considered historic to the site. Approximately half of the site will undergo grading and other construction activity. The subdivision will consist of a single large lot (1.7 acres) around the existing house, relocated barn and garage as well as 13 additional residential lots which vary in size from 15,000 square feet to 22,000 square feet in size. The project is in the Urban Estate zoning and is being designed as a "cluster" development which requires that 50% of the overall project be reserved as open space. Tract A (mostly on the north and east sides of the site) meets this open space requirement. There are also several Natural Areas buffers on the site. These include buffers to the existing windrow on the west, the ponds and wetlands on the north, and the banks of FCRID on the east. The existing road right-of-way has a very shallow slope (0.5%) toward the site; the western portion of the site with the existing structures also has a very shallow slope. The central and eastern portions of the site are considerably steeper and range form 5% to 10% to the east. The existing drainage patterns on the site are relatively complex. The southern portion of the site and the existing right-of-way drain east and into FCRID via overland flow over the banks or through an existing pipe from the site into the side of the TABLE OF CONTENTS TABLEOF CONTENTS............................................................................................................... iii 1. GENERAL LOCATION AND DESCRIPTION 1.1 Location...................................................................................................................I 1.2 Description of Property............................................................................................1 2. DRAINAGE BASINS AND SUB -BASINS 2.1 Major Basin Description..........................................................................................2 2.2 Sub -Basin Description.............................................................................................2 3. DRAINAGE DESIGN CRITERIA — 3.1 Regulations..............................................................................................................3 3.2 Development Criteria Reference and Constraints....................................................3 3.3 Hydrologic Criteria..................................................................................................3 3.4 Hydraulic Criteria....................................................................................................3 4. DRAINAGE FACILITY DESIGN 4.1 General Concept.......................................................................................................4 4.2 Specific Flow Routing.............................................................................................5 4.3 Drainage Summary ...................................................................................................7 5. EROSION CONTROL 5.1 General Concept......................................................................................................8 5.2 Specific Details........................................................................................................8 6. CONCLUSIONS 6.1 Compliance with Standards....................................................................................8 6.2 Drainage Concept.....................................................................................................9 7. REFERENCES....................................................................................................................9 APPENDICES A Vicinity Map B Hydrologic Computations C Hydraulic Calculations D Detention and Water Quality Pond Sizing E Erosion Control Calculations F Figures and Tables iii