Loading...
HomeMy WebLinkAboutDRB140312_Nugget Lot 2 Geol Consultants 2003_1408486140.pdf BmoeA.Collins,Ph.D.,dba COLLINS &ASSOCIATES GEOIAGICAL AND NATURAL RESOURCE]CONSULTANTS P.O.Box 23 . 1116 MINEDTA DRIVE SILT,COLORADO 81652 PHONEiFAx(970)876-5400 bacol@rof.net September 23, 2003 Michael Bloom 3615 Blake St. Denver, CO 80205 RE; 4288 NUGGET LANE—LOT 2, BIGHORN ESTATES SUBDIVISION,EAST VAIL Dear Mr. Bloom: At your request I have examined the subject property for the purpose of evaluating potential geologic hazards to which the lot made be exposed, and the effects of mitigating such hazards as may exist on adjacent properties,in accordance with Town of Vail Regulations Chapter 12-21- 15. The boundary of a high-severity rockfall zone related to the cliffs above the property cuts across the northeast corner of the lot, and the boundary of a high-hazard debris-flow zone associated with a small drainage as shown on the Town of Vail debris-flow/debris avalanche hazard maps passes through the center to northeast corner, both including most of the parcel. The property spans the transition zone between the glacial till filling the valley floor of Gore Creek and the steep slopes, ledges, and cliffs of limestone, sandstone, and shale that comprise the Minturn Formation of Pennsylvanian-Permian age to the southwest.Approximately the lower (northern)half of the property is mapped as Pinedale Till(Qtp,upper Pleistocene)by Kellogg,et at. (2003'), described generally as unsorted and unstratified bouldery till containing little clay and subangular to subrounded cobbles and boulders, mostly of Precambrian igneous and metamorphic rocks from the Gore Range to the east but occasionally of sedimentary rocks derived from the canyon walls.The remainder of the property is underlain by a narrow wedge of colluvium(Qo,Holocene and upper Pleistocene),essentially debris that has accumulated at the break in slope between the glacial till and the limestone ledges of the Robinson Limestone Member of the Minturn ([Pmr) above; it consists of unconsolidated to slightly indurated mostly non-stratified angular pebbles,cobbles, and boulders of limestone and sandstone in a matrix of sand, silt, and clay. The mapped rockfall hazard is in the form of ledges of limestone in the Robinson Limestone that crop out between 8,800 ft and 9,000 ft, or about 300 to 500 ft above the property, immediately below a substantial bench area developed between 9,200 ft and 9,400 ft below steep slopes capped by mostly sandstone ledges of the middle and upper members ((Ppm, (Pim) of the Minturn. The sequence is capped by the Jacque Mountain Limestone Member (jPmj) of the Kellogg,K.S.,Bryant,B.,and Redsteer,M.H.,2003,Geologic map of the Vail East quadrangle, Eagle County, Colorado:11.5,Geological Survey Miscellaneous Field Studies Map MP-2375. } Page 2 Minturn, although this unit as mapped by Kellogg, et al., does not appear to contribute to the hazard.Limestone outcrops above the property are limited and in fact so obscured by vegetation that none are visible from below.There is no evidence on aerial photographs of recent rockfall, and a report by Arthur I. Mears, P.E., Inc. (19902) regarding Lot 3 adjacent to the west suggests that there is no indication of rockfall within the last century. Boulders exposed on the surface below the abandoned irrigation ditch at approximately the tree line on the property are subrounded,suggesting that they are glacial in origin. More angular boulders located further up the slope may be of either glacial or very old rockfall origin. Rocks falling from the more extensively-exposed cliffs near the top of the ridge southwest will tend to roll to the northwest of the property,although the vast majority of such rocks will stop on the heavily-vegetated bench area, which in some areas is essentially flat. Therefore the actual rockfall hazard to which the property is exposed,while not nonexistent, is minimal. If the old irrigation ditch is left as-is, or is filled in but graded with a slight slope into the hillside (which will also mitigate the minor debris flow hazard as discussed below), it will act as a modest barrier, and assuming snow avalanche mitigating measures similar to those recommended for Lot 3,3 these measures — orienting the building to minimize walls and roofs facing directly up hill,minimizing openings on the uphill side of the structure, use of shatter-resistant glass and high-strength frames for windows and doors,and stiffening foundations,walls,and slope-facing roofs—will further reduce the rockfall hazard. The mapped debris flow hazard is apparently due to the presence of the southeast side of a small alluvial fan that may have had its source area cut off with the development of current topography and drainage on the bench above the property. Kellogg, et al., do not map this now heavily- vegetated feature as an alluvial fan (Qf). There is no indication of recent flow on this barely- perceptible fan, and the hazard is therefore minimal.Mears (1990a) states that debris flows will not reach the lot because there is insufficient accumulation area above the property,and that the form is actually that of a small debris avalanche path with potential debris avalanches consisting mostly of snow.The mitigating measures mentioned previously will further reduce what little risk of debris flow there is.In the case of debris flow,or simply heavy runoff drainage from the slopes south of the old ditch,leaving the ditch as is,so long as drainage from it can be properly directed and water is not allowed to accumulate in it, would provide modestly-greater protection than filling it in. There is no evidence that water remains in the ditch for any length of time now, but it must be remembered that unlined water-filled ditches can result in saturated soil downslope and subsequent soil and foundation problems if not properly designed for. In general, site drainage must be designed and constructed to provide positive drainage of surface water, including snowmelt, and groundwater away from proposed structures and properly integrated into the local drainage system. The property is included in the "Moderate" or "Blue" hazard zone of Town on Vail snow avalanche hazard maps.According to the Mears report(1990b)the hazard is modest regarding Lot 3 and there is nothing to suggest that different conditions affect Lot 2. However, snow and debris avalanche hazard recognition and mitigation are specialties in which I do not claim 2 Mears,A.I.,1990,consulting geohazard report,2 p. 3 Mears,A.I.,1990b,consulting snow and debris avalanche loading analysis report,6 p. Page 3 expertise,and if a more accurate assessment of the risk and recommendations for mitigation are desired you should seek the opinion of a qualified professional. The property is in an area of modest geological sensitivity with regard to rockfall and debris flow, but so long as overall site drainage is appropriately tied into adjacent street and other drainage systems and landscaping and grading is designed to conduct potential flow properly,construction on the lower portion of the lot, below the abandoned ditch,as described will not increase these hazards to other property or structures, or to public rights-of-way, buildings, roads, streets, easements, utilities, or facilities or other properties of any kind. This report is intended to comply with appropriate portions of Town of Vail Regulations Chapter 12-21-15, and nothing contained herein should be interpreted as suggesting that the subject property is not exposed to the mapped hazard, or that other geologic hazards do not exist. If you have any questions, or if I can be of further service, please do not hesitate to contact me.__�����N\y ie gt.rssisoN4/6.„‘ Sincer ° c<0� 46. %' i W1ry � 4 � �s ...,,,,, Bruce ... ,, i, e ,� r.�,[` �%'c"' Consulti :a�ete td '--- tea,— IMPORTANT Nonce:This report concerns natural processes that are unpredictable and in large measure poorly understood.it is intended to identify potential observable hazards within the scope of workto which the subject property is exposed and to suggest mitigating measures in compliance with applicable regulations.Nothing in this report should be construed or interpreted as suggesting the absence of the described hazards,or that the recommended mitigations will protect the subject propertyfrom the described hazards under all circumstances, foreseen or unforeseen.Nothing in this report should be construed or interpreted as suggesting that additional unidentified hazards are not present. It must also be understood that"mitigation"does not mean either the elimination of the hazard(s)or prevention of the consequences of a hazard event or events,only the reduction to the extent reasonably possible of the latter.Byaccepting this report all present and subsequent parties thereto agree to indemnify and hold harmless the preparer for any and all damages, direct, indirect or consequential, including personal injury or loss of life,above and beyond the original cost of this study,caused by or resulting from any occurrence of the described or other hazard(s),whether or not such damages may result from failure to identify said hazard(s) or front failure or inadequacy of properly engineered, constructed, and maintained recommended mitigations.The preparer of this report cannot and will not be responsible in anyway or manner whatsoever for the proper engineering,construction, and/or maintenance of recommended mitigations,or the inadequacy or failure of improperly engineered,constructed,and/or maintained recommended mitigations,or mitigations that have been altered in any way whatsoever from those recommended by the preparer.This report may be amended or withdrawn without notice at any time prior to receipt of payment.