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Home My WebLink About2.0 Staff ReportREOUEST: • • BOCC 2/18/97 PROJECT INFORMATION AND STAFF COMMENTS Floodplain Special Use Permit and Amended Final Plat, Lots 1 & 2, Block 7, Rifle Village South Subdivision. APPLICANT: Laurence Bradley LOCATION: Lots 1 & 2, Block 7, Rifle Village South Subdivision; located approximately one (1) mile southwest of Rifle. SITE DATA: Approximately 13,000 square feet WATER: City of Rifle SEWER: City of Rifle ACCESS: Direct from Shotgun Street/Colt Drive EXISTING ZONING: R/L/UD ADJACENT ZONING: R/L/UD I. RELATIONSHIP TO THE COMPREHENSIVE PLAN The subject property is located in District A - Subdivisions, according to the Garfield County Comprehensive Plan's Management Districts Map (1981; 1984). II. DESCRIPTION OF THE PROPOSAL A. Site Description: The subject property is approximately 13,000 square feet in size, located in the Rifle Village South subdivision, one (1) mile southwest of the City of Rifle. The property is currently undeveloped and in native vegetation. There are a number of existing, single family dwellings throughout the subdivision. The tract exists on the alluvial fan created at the mouth of Helmer Gulch, which has been identified by the Federal Emergency Management Agency as a special type of flood hazard. See vicinity map, page • • • B. Adjacent Land Uses: Land uses adjacent to the subject property are limited to single family residential. C. Development Proposal. The applicants are proposing the construction of a new, single family dwelling on land that has been identified as existing within floodplain zone AO, a special flood hazard area that can be inundated by types of 100 -year shallow flooding where depths are one (1) foot or Tess with associated velocities of four (4) feet per second, therefore, requiring the issuance of a Floodplain Special Use Permit pursuant to Section 6:00 of the Garfield County Zoning Resolution. Additionally, the applicant is requesting an amendment to the Rifle Village South Subdivision Final Plat to merge Tots 1 & 2, Block 7. See site map, page 7• III. MAJOR ISSUES AND CONCERNS A. Floodplain: The subject tract of land is located within the "mapped" 100 -year floodplain of Helmer Gulch. In order to meet County zoning requirements, it is required that a Colorado registered professional engineer certify that the house will be built in such a manner that it would withstand expected hydrostatic, hydrodynamic and buoyant forces imposed upon the house in the event of a 100 -year flood. Section 6:00 of the Zoning Resolution requires an approved Special Use Permit for any development within a regulated (mapped) floodplain. The purpose of the Special Use Permit review is to assure compliance with various applicable state and federal regulations, as well as reviewing relevant technical data. B. Building Design: Pursuant to Section 6:09:02 of the Zoning Resolution, the construction of a new or substantially improved structure must meet certain performance standards to have a reasonable assurance that the proposed construction will be safe from flooding, to wit: The lowest floor, including basement, of any new or substantially improved building designed for residential occupancy shall not be Tess than one (1) foot above the maximum water elevation of the 100 -year flood; All new construction shall be reasonably safe from flooding; All new construction shall be designed and adequately anchored to prevent flotation, collapse or lateral movement, be constructed with materials and utility equipment resistant to flood damage, and be constructed by methods that minimize flood damage; Within any area subject to sheet flow (Zone AO on the Flood Insurance Rate Maps) all new construction and substantial improvements of residential structures shall have the lowest floor, including basement, elevated above the highest adjacent grade of the nearest street to or above the depth number specified on the Flood Maps. C. Engineering: The applicant has engaged Jerome Gamba & Associates to conduct an engineering review and certify that the construction will occur pursuant to the above requirements. Mike Gamba has evaluated the property and has submitted the following findings: 1. The 100 -year flood event from Helmer Gulch has been calculated to be on the magnitude of 760 cubic feet per second (cfs). 2. Based on the flood depth of flow of one (1) foot, the 760 cfs flood flow would have a total width of approximately 217 feet or 6% of the 3600 foot width of the alluvial fan floodplain. 3. Using these calculations, it is possible to estimate the average probability of any individual house being affected by this type of flooding, essentially resulting in an occurrence of once every 1667 years. 4. The hydraulic capacity of Colt Drive indicates a required flow depth of 1.54 feet in order to discharge a flow of 760 cfs. Based on these findings, Gamba can provide certification that the home is constructed to meet requirements and be "reasonably safe from flooding," given the following conditions of construction: I . That the minimum, finished floor elevation of the residence shall be no less than 5324.2 feet. 2. The foundation shall be cast -in-place reinforced concrete footers and stem walls and all foundation designs should be based on site specific geotechnical soil and foundation investigations. 3. That site grading around the perimeter of the residence be such that there is a minimum grade of 10% for a minimum distance of 10 feet and all patios, driveways and sidewalks slope away from the house at a minimum grade of 2%. Beyond these features, the 10%/10 foot grade shall be continued. 4. All foundation construction shalLcorn& with Uniform Building Code criteria. (See engineer's letter, pages r 17 Consistent with FEMA requirements, certified "as-builts" and an elevation certificate for the finished floor elevation must be conditions of approval. D. Soils: Hepworth-Pawlak Geotechnical has provided a subsoil study report detailing the type of soil encountered in a test bore hole. Based on this evaluation, the soils are considered low-density and hydrocompressive and footings placed on the undisturbed natural soils should be designed for a soil bearing pressure of 1000 psf. The report • • makes additional recommendations on foundation design and construction, which should b� followed, based on the exact type of foundation to be used. See report, pagesCw E. Amended Final Plat: The floodplain special use permit application is being combined with an application for amended final plat of Lots 1 & 2, Block 7, Rifle Village South Subdivision. Given the original platting of the Subdivision lots, owners have found it very difficult to construct residences and accessory buildings and still comply with mandatory setbacks. This amended final plat request would erase the line between the two (2) subject lots and create one (1) new lot that would comply with all applicable setbacks. As far as this portion of the application is conte ned, a conceptual approval from the Board is all that is required. See map, page : IV. SUGGESTED FINDINGS 1. That proper publication and public notice was provided as required by law for the hearing before the Board of County Commissioners. 2, That the hearing before the Board of County Commissioners was extensive and complete, that all pertinent facts, matters and issues were submitted and that all interested parties were heard at that hearing. 3, That the application is in compliance with the Garfield County Zoning Resolution of 1978, as amended. 4. For the above stated and other reasons, the proposed use is in the best interest of the health, safety, morals, convenience, order, prosperity and welfare of the citizens of Garfield County. V. RECOMMENDATION Staff recommends APPROVAL of the Floodplain Special Use Permit, pursuant to the following conditions: 1. That all representations by the applicant made before the Board of County Commissioners shall be conditions of approval, unless otherwise stated. 2. That the minimum, finished floor elevation of the residence shall be no less than 5324.2 feet. 3. The foundation shall be cast -in-place reinforced concrete footers and stem walls and all foundation designs should be based on the site specific geotechnical soil and foundation investigations. 4. That site grading around the perimeter of the residence be such that there is a minimum grade of 10% for a minimum distance of 10 feet and all patios, driveways and sidewalks slope away from the house at a minimum grade of 2%. Beyond these features, the 10%/10 foot grade shall be continued. 5. All foundation construction shall comply with the 1994 Uniform Building Code, as adopted by Garfield County. 6. Prior to the approval of a final/certificate of occupancy for the residence, the property owner shall submit an elevation certificate, signed and stamped by a licensed land surveyor, for the finished floor elevation of the single family dwelling unit. �- INiv bitSYivcr 5 94,461.13(c /qvEp 2/fi-o/ 1