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Home My WebLink AboutDRB140327 Hazard ReportAugust 8, 2014 Eduardo and Pilar Vigil, Property Owners Attn: Bob Stephanoff, Col Construction PO Box 4573 Vail CO 81658 Re: Geological Hazards, Bighorn Addition Filing 1, Block 2, Lot 5A, 3946 Lupine Drive, Vail Dear Mr Stephanoff: At your request I have examined the subject property (Site) with respect to the geological hazards delineated by the Town of Vail as they may affect your proposed deck additions. Following are my observations, interpretations and conclusions. SITE TOPOGRAPHY The Site is located 250 feet to the south of Gore Creek, right at the break in slope where the mountain begins to rise more rapidly. The Site elevation is approximately 8400 to 8470 feet according to the 1993 site plan topography, versus approximately 8430 to 8500 feet on the latest USGS digital topography, and the contour orientations differ by about 20 ° -30 °, also. The current 2014 architectural site plan uses the 1993 detailed topography, but avoids some of the inconsistency by not assigning absolute elevations to the detailed topographic contours. The prominent rock cliff on the mountain slope above is at about 8720 to 8840 elevation, approximately 350 feet vertically above the proposed decks. The peak of the mountain above that cliff is about 11,000 feet elevation, or about 3500 feet above the Site. The bulk of the mountainside high above the Site drains into Waterfall Creek, which drains an area of perhaps 250 acres and debouches into the Gore Creek Valley about 250 feet west of the Site. The Site itself is subject to drainage from a much smaller area of the mountainside, on the order of perhaps 2 acres, and located on the lower slopes of the mountainside. SITE GEOLOGY Most of the Site lies on a debris fan deposit that has formed at the outlet of Waterfall Creek. The source of this material was the high upper slopes of the ridge that drain into Waterfall Creek. These upper slopes are underlain by the upper portion of the Minturn Formation and the basal Maroon Formation, comprising mudstone, sandstone, conglomerate and scattered limestone beds of Pennsylvanian age. The southern portion of the Site is underlain by colluvial material derived from the lower slopes that lie immediately above the Site itself. These lower slopewash sources are underlain by the Vigil /3946 Lupine Drive /Hazards Page 2 of 8 Robinson Limestone Member of the upper middle section of the Minturn Formation, primarily comprising sandstone and limestone beds of Pennsylvanian age. DELINEATED HAZARDS The Town of Vail has delineated two hazards that affect the built portion of this Site, and a third that only impacts the unbuilt portion of the Site. Snow Avalanche The relevant portion of the Vail snow avalanche hazard map may be found on page 3. An area of red avalanche hazard zone is at the southern corner of the lot, some 50 feet removed from the nearest corner of the proposed addition. The blue snow avalanche zone extends across the middle of the lot and encompasses the rear half of the existing structure, including both proposed decks. The direction of the avalanche threat is from the south, encountering the rear of the structure first. The source area appears to be around the cliff area, about 700 feet south and 350 feet vertically above the Site. Although apparently not extremely steep overall, the more open, unvegetated scar area from the large rockfall may favor greater snow accumulation and easier avalanche initiation. Probably the most recent visible event was in 1984, as reported by Art Mears, but two earlier events of larger size can be identified. The slope above the Site shows indications of at least 3 different avalanche events, one stopping about 240 feet short of the house, one stopping about 150 feet away, and the third of indeterminate runout length. It is likely that the house can be reached by the 100 year return period event. The blue zone is considered appropriate for the level of snow avalanche hazard at the Site. Rnckfall The relevant portion of the Vail rockfall hazard map may be found on page 4. The high rockfall hazard zone encompasses the entire lot, including all of the existing structure and both of the proposed decks. The direction of the rockfall hazard threat is from the south, similar to the avalanche threat. The rockfall hazard is created by the steep lower slopes above the Site, where erosion has preferentially removed the softer layers of the Minturn Formation, forming potentially unstable ledges of the harder limestones above. The undermined limestone cliffs are therefore at higher risk of sudden collapse and rockfall. Analysis of 2 foot diameter boulder falls by Art Mears in 1993 confirmed that the rocks could impact the Site with high kinetic energies. Vigil /3946 Lupine Drive /Hazards n <(5> V 3896 391 TOM OF VAT 50 m 200 ft 1060 18' 18" W 39'038' 22 " N tG, p'y Page 3 of 8 3907 Vigil /3946 Lupine Drive /Hazards n v C } TOWN OF 3896 50 m 200 ft 1060 18' 21" W 390 38' 23 N Page 4 of 8 M / l Vigil /3946 Lupine Drive /Hazards Page 5 of 8 A large section of limestone just above the Site appears to be the most recent large segment of cliff to have collapsed. This cliff area is a fresher lighter color, in contrast to the darker, greyer, more weathered color of the cliff areas to either side. Large (4 foot) limestone boulders at the Site are highly weathered on all visible sides, indicating that the collapse probably occurred before there were residents living in the area to observe it. The local collapse here may be interpreted as having already dispatched most of the largest, most dangerously unstable rocks, affording a somewhat greater measure of safety directly below due to there being fewer large boulders projecting from the face. But it probably also exposed a lot of fresher, though smaller, rocks to raveling as the slope continually retreats to stabilize its new configuration. The rockfall hazard is therefore relatively high in this area, as previously delineated. Debris Flow The relevant portion of the Vail debris flow hazard map may be found on page 6. The Waterfall Creek debris flow threat is apparently considered to be inactive in this specific area, and a more local debris flow hazard has been delineated only at the southern quadrant of the lot, not impacting the proposed deck addition. This hazard was therefore not further investigated. EXISTING BUILDING The existing building was built in 1994. It was designed to be protective from the two delineated hazards by virtue of a constructed catchment ditch in back, a low structural wall surrounding the existing rear patio, and by structural reinforcement of the rear wall of the house. PROPOSED DECK ADDITIONS The two proposed decks are located at the southern and western, rear (uphill) side of the building. They are at the second floor elevation, and exterior to the existing avalanche protection walls. Their locations may be found on the site plan on page 7. The proposed decks are not intended to be protected from the above hazards. They are only intended to provide more easily usable outdoor space in the unprotected area of the back yard. The northwestern deck will be essentially at existing patio and ground level. Any rock or snow that would cross the existing ground at that point would also cross the proposed deck, and its trajectory would be little affected. The deck and occupants would be expected to be damaged, but the threat to existing patio and house would be similar to their present threat situations. Similarly, it is not expected to cause any increase in threat to any neighboring property. The southern deck will be at the same second floor elevation, but the uphill terrain will be 2 to 5 feet below it. According to the structural engineer's representations to you, it will be Vigil /3946 Lupine Drive /Hazards Page 6 of 8 Vigil /3946 Lupine Drive /Hazards Page 7 of 8 9 /Vf -V I I I s :101 6719-486�5780 I C�39�h6 L pIN�17pIVENVAIp�AN Vigil /3946 Lupine Drive /Hazards Page 8 of 8 designed to collapse under the force or weight of an avalanche impact, rather than endangering the primary house structure by remaining intact. It is therefore not expected to cause increased risk to either the primary house structure or to any neighboring property. Occupants of the deck during an avalanche event would not be protected. SUMMARY The proposed deck additions will be subject to significant rockfall and snow avalanche hazards, and when they occur, those hazards will be extremely rapid in their descent upon the Site. These exterior decks would seem likely to be relatively low use areas compared to interior portions of the house, however the hazards associated with use of the decks should be made known to all potential occupants of the house. For increased safety, activities on the decks should ideally be ones that do not impair the mobility of occupants for rapidly retreating to the safety of the house interior. In some instances even a rapid retreat might not occur quickly enough for occupants to reach safety in time to avoid injury. Proper construction of the decks as breakaway structures will not increase the hazards that currently threaten either the primary structure on this Site or the neighboring properties. It should be realized by all concerned that the delineated hazards are actually expected to occur and impact the Site at occasional times in the near or distant future. This study is only intended to qualitatively characterize the nature of the expected hazards in relation to the proposed deck additions. Thank you for the opportunity to help with this project. Please call if you have any questions or concerns. Sincerely, POP REG /.s Willia Klaub rV .E.339Q Geologist and in;i a��n g � 719 - 486 -5780