HomeMy WebLinkAboutColorado Geological Survey 12.14.16COLORADO GE,OLOGICAL SURVE.Y 1801 196Street Golden, Colorado 80401 December 14,2016 Kathy Eastley Garfield County Community Development Dept. 108 8o' Sffeet, Suite 401 Glenwood Springs, CO 81601 Location: SW% Section 10, T7S, R88W ofthe 6'h P.M. 39.4586, -107.2111 Karen Berry St¡te Geologist Subjech VelasquezMinorSubdivision- r¡te Wumter DIISA-10-1G8496: Gsrf¡eld Countv. CO¡ CGS Unique No GA-17-0005 Dear Kathy: Colorado Geological Survey has reviewed the Velasquez minor subdivision referal. I understand the applicant proposes to subclivide 10.9i acres located at3064 Cattle Creek Road, creating two lofs of approximately 5.5 and b.+ urr"r. proposed Lot2 (5.49 acres) contains an existing home, well (to be shared by both lots) and septic system. proposed Lot I contains a buried fiberglass watertank. With this referral, I received a copy of the ielasquez Minor Subdivision application (The Land Studio, Inc., Novembet 17,2016). CGS has several concerns about the properly's exposure to geologic hazards: Debris fan and debris flow hazard. Both the existing and proposed additional lot are located near the mouth of a drainage emanating from the north side of Cattle Creek Road, on a mapped, geologically young debris fan (Kirkhãm, R.M., an¿ V¿idr"nn, 8.L., 2008, Geologic Map of the Carbondale Quadrangle, Garfield Couity, Colorado: Colorado Geological Survey, Map Series MS-36, scale 1:24,000)' CGS recognizes that -uny -ho¡11"r, structures and other improvements have been constructed on mapped, geologically young debris fans in the Cattle Creek valley. However, the applicant should be made aware that large precipitation events could trigger debris/mud flows that may inundate both lots with dangerous amounts of water and sediment. Due to hìgh sediment content, ability to entrain and transport gravel-, cobble-, and boulder-size rocks and debris, and unpredictable flow characteristics, debris flows pose hazards that are very different from sheetflow or channelized water flow hazards' CGS recommends a debris flood hazard analysis by a qualifred professional and, if necessary, development and implementation of a hazard mitigation plan. The analysis should include a calculation of anticipated, bulked sedimenldebris volume, flooã height, and vetocity. Mitigation recommendations should include deflection/containment structure sizing location, construction details, and a maintenance plan. Mitigation cannot exacerbate hazards to adjacent properties. Debris flow hazard increases as a result ofevents that reduce hillside vegetation, such as avalanche, disease, wildfire, grading and other disturbances; debris flow mitigation structure(s) should be oversized to account for increased debris volumes a¡i a result of wildfire. Cattle Creek flood zone, erosion, and setback. Cattle Creek traverses the existing and proposed new lot' The Cattle Creek 1O0-year flood hazardzone as shown on FEMA Flood Insurance Rate Panel 08020514708 (January 3, 1986) appears to be corectly identified on the plat (High Country Engineering, November tS, à016). Â Uuitaing erwelope located entirely outside the mapped flood hazard zone should be identified on proposed Lot l. Homes and improvements should be set back as far as possible from the GA-17-0005_l Vclasqucz Minor Subdivision 4:40 PM,l2ll4f20$ Kathy Eastley December 14,2016 Page2 of2 river channel and flood zone to minimize risks associated not just with flooding but also river scour, erosion and undercutting. Additional concerns include unstable slopes in the southern portion of the site, and corrosive soits. G¿rfteld County geologic hazard mapping identifies the steep slopes in the southem portion of the property as potentially unst¿ble. The slopes should be specifically designated as non-buildable. The need for corrosion protection ofany buried steel (e.g. water supply pipelines) and use ofsulfate attack-resistant cement in project concrete should be evaluated as part ofa geotechnical investigation. Soil and bedrock engineering properties. A geotechnical investigation consisting ofdrilling, sampling, lab lesting and analysis will be needed, once a building location has been identified on proposed Lot l, to determine dbpths to groundwater and bedrock, characterize soil and bedrock engíneering properties such as density, strength, and swell/consolidation potential, and to design foundâtions, floor systems, retaining walls, and surface and subsurface drainage to minimize potential risks related to collapsible or hydrocompactive soils and bedrock. Thank you for the opportunity to review and comment on this project. If you have questions or need further review, please call me at (303) 384A643, or email carlson@mines.edu. C.E.G Engineering Geologist GA-17-0005_l Velasquez MirorSubdivision 4:40?M. t2¡t4f20t6