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HomeMy WebLinkAboutbocc.packet.spec.03102008PITHIN COUNTY COMMISSIONERS AGENDA Plaza One Meeting Room MONDAY, MARCH 10, 2008 10:00 AM LAND USE PUBLIC HEARING Aspen Music Festival & School & Aspen Country Day School Rezoning, Public-Institutional Master Plan Review, GMQS Exemption for Essential Community Facility, Site Plan Review,(Continued from 2/27/08), PH (PN 12/23/08), S. Wolff 3:00 PM ADJOURN (Please note, there will be a lunch break from 12:00 to 1:00 pm) PITKIN COUNTY BOARD OF COUNTY COMMISSIONERS SPECIAL MEETING MARCH 10, 2008 ASPEN MUSIC FESTIVAL AND SCHOOL/ASPEN COUNTRY DAY SCHOOL REZONING, MASTER PLAN, GMQS EXEMPTION, SITE PLAN REVIEW PLEASE REFER TO THE MATERIAL PROVIDED IN THE JANUARY 23, 2008 REGULAR MEETING PACKET. NO ADDITIONAL INFORMATION IS BEING PROVIDED AT THIS TIME ADDITIONAL EXHIBITS SUBMITTED AT THE MARCH 10, 2008 PITKIN COUNTY BOARD OF COUNTY COMMISSIONERS SPECIAL MEETING EXHIBIT NO. AGENDA ITEM EXHIBIT DATE A Aspen Music Festival & Old Sepia photograph of NA School/Aspen Country Aspen Music School site Day School property from approximately 1910 showing the Administration Building and the Business Buildin B Aspen Music Festival & Progress Update on the NA School/Aspen Country Geologic Hazard Mitigation Day School Review by Independent Third Party, Frank Harrison to Jennifer B. Elliot, Director of Finance Administration for the Aspen Music Festival and School To: Ms. Jennifer B. Elliot Aspen Music Festival and School From: F.E. Harrison, P.E. Michael W. West & Associates RE: Geologic Hazard Mitigation Review Aspen Music Festival and School Date: March 10, 2008 cc: Mark Mahoney Harry Teague Architects EXHIBIT b D ,~' /D ` ~$ I am providing this memo as a progress update on my review of geologic hazard mitigation strategies for proposed improvements and new construction at the above referenced site. My review is being conducted in accordance with our proposal to the school dated February 13, 2008. I have reviewed a number of documents in conducting this review, including: • 100% SD plans from Harry Teague Architects' ftp site; • Reports by Yeh and Associates dated July 19, 2006 (revised April 10, 2007), April 11, 2007, and December 12, 2007; • Reports by Art Mears, P.E. dated November 10, 1992, October 18, 1993, July 3, 2001, and March 6, 2007 (2 reports); and • Report by Schmueser/cordon/Meyer dated December 14, 2007. I have also discussed issues related to hazard mitigation with Mark Mahoney of HTA, Matt Webster of SGM, and Richard Johnson of Yeh. My understanding of the geologic hazards at this site is based on Yeh's April 7, 2007, report, as well as previous reports by Mears. My current scope does not include a field review, and thus my opinions regarding mitigation strategies do not include a review of the geologic hazard characterizations presented by these consultants. The hazards identified as requiring mitigation, and addressed by the project team include avalanches or snow slides, rock fall, and debris flows. I briefly address my current understanding and recommendations for each below. In such as discussion it is important to understand several terms: • A Hazard is a naturally occurring condition. Natural Hazards are not created by a project; they naturally exist. • Risk indicates potential for injury, loss of life, or property damage due to a naturally occurring hazard. Risk is incurred when a project is subject to a hazard. Risk can be increased by poorly conceived projects, and reduced by well conceived projects. Typically, risk cannot be eliminated. • Mitigation is an approach to a project that reduces risk, associated with a hazard, to an acceptable level. Mitigation does not imply an elimination of risk. Avalanrhac The hazard is described by Mears in a March 6, 2007 report, and recommendations for snow and avalanche loading (or snow slide loading) are given in a separate report with the same date. As discussed above, I have not reviewed the characterization of the hazard. The loadings recommended by Mears seem reasonable and appropriate based on his characterization and my experience. I recommend that Mears confirm that he has calculations on file and available for review (if required) as a basis for the loadings presented on Table 1 of his loading report. Rockfall Yeh's characterization of rockfall hazard is presented in their April 11, 2007 report. They recommend (report issued July 19, 2006 and revised April 10, 2007) that roof structures of the Practice Rooms be designed for an impact force of 580 pounds due to the potential for rolling boulders from the slope above. I do not question this recommendation; however, I understand that calculations substantiating this recommendation are not currently available for review. I recommend that Yeh prepare calculations for this case and have them on file available for review if necessary. Keno Gulch Debris Flows Yeh's reports regarding debris flow characterization refer to and apparently rely upon Mears' work from 1992 through 2001. I have not reviewed Mears' field characterization of the debris flow event(s) described by his reports, nor will I review his characterization of landslide conditions at the debris flow source area, based on my current scope of work. However, I have reviewed the engineering approach to the mitigation of the hazard, based on the characterization presented by Mears, and relied upon by Yeh and SGM in developing design concepts. Yeh recommends a 10' by 10' box culvert for passing debris flows under a roadway across the Keno Gulch debris fan, and related improvements to the existing channel and deflection berm. They also present debris flow characteristics from Mear's work. They recommend a debris flow loading for buildings within the hazard area. The basis for this loading is not presented and is not available for review; however, I understand that no buildings will be in the hazard area, since the currently existing Percussion Building is to be removed (see additional comments regarding the Castle Creek Building below). Yeh did not perform flow calculations for sizing of the box culvert; their recommendation is based on dimensions that facilitate cleaning. As I understand the design concept, the debris flow mitigation strategy is based on the assumption that the culvert is likely to clog, and thus debris flow events of the size characterized by Mears will spread onto the historic debris fan, and be directed to the north by the improved channel and berm, and related grading improvements (such as the culvert headwall design). I have discussed this mitigation strategy with Mr. Matt Webster of SGM. SGM reports (2007) that a 10' by 10' culvert will not fit the site geometry, and are currently designing for an 8' (high) by 10' (wide) box culvert. They are currently performing calculations regarding flows through this culvert that suggest that the culvert may (theoretically) have capacity to pass debris flows approaching the magnitude of event characterized by Mears. The reader should understand that substantial uncertainty exists with such calculations. However, the potential for clogging by trees, limbs, and other debris is high, and thus reliance upon this culvert to successfully pass the design event is not prudent, regardless of favorable calculations. I have discussed these calculations with Mr. Webster, and I understand they are currently in progress and/or being updated. Based on these discussions, I believe the mitigation concept developed by the project team for debris flows is reasonable and prudent. Most likely, the culvert will not pass events approaching that characterized by Mears; however, additional measures recommended and currently being designed are intended to direct the debris flow to the north side of the existing fan, including the proposed parking lot in this area. I believe this is a reasonable approach, given the constraints of the site. Users of the site should understand that debris flow risk is high in the culvert/parking lot area, and appropriate signage and education of staff/students would be prudent. This risk would include damage to cars parked here, as well as injury to individuals in the cars or on foot in the area. However, this risk exists now in this area (especially for the existing Percussion Building), based on my current understanding. The proposed project does not make it worse. Removal of the Percussion Building, and the proposed channel and grading improvements likely improves the risk. Mears' discussion (2001) indicates that both the Percussion Building and the north end of the Castle Creek Building are at risk. I understand the Percussion Building is to be removed, but potential risk to the existing Castle Creek Building may require further evaluation. I do not currently understand the existing building design or proposed grading sufficiently to evaluate this issue. It may be that the proposed grading/berm/channel improvements will be sufficient to protect the Castle Creek building, or that the existing building walls can withstand debris loading, but I recommend that the project team examine this detail as design progresses. As an additional check of the proposed mitigation, I have performed some rough estimates of available deposition volume compared to the debris flow volumes (10,000 to 20,000 cubic yards) mentioned by Mears in his 2001 report. Such estimates are difficult to make without using oversimplified assumptions. However, assuming an average deposition depth of 5 feet, over the entire debris fan area as constrained by berm and channel, a significant percentage of the volumes identified by Mears would not stop and would reach Castle Creek. However, if it does so, this appears to be a natural occurrence that is not substantially changed by the proposed construction. Some depositional area on the fan is lost due the berm, but improved channel construction likely will roughly offset this lost volume. Additionally, I have some question as whether the volumes described by Mears could be mobilized in a single event. It is possible that the 1996 event described by both Mears and Yeh involved less volume. In any event, the material reaching Castle Creek in a debris flow event is likely to be mostly water, and thus given typical creek flows occurring during runoff season, Keno Gulch flows due to such an event are unlikely to temporarily constrict the creek or adversely impact downstream locations beyond historic levels. The flow will likely mingle with the creek flow occurring at the time in a normal manner. Summary It does not appear to me that this project, as currently conceived, would increase risk due to the hazards discussed above, either to the Aspen Music Festival site or to downstream locations. This is a difficult site, and a significant debris flow hazard exists, whether this project is executed or not. However, the concepts currently advanced appear to reasonably mitigate existing hazards. The risk to occupied structures and thru-roads appears low, based on proposed concepts. Higher risk will be associated with the culvert area, parking lot, and adjacent riparian zones, but this risk currently exists. It can likely be mitigated through access limits, education, and signage. Thus I believe the project to be reasonable and prudent in regard to the hazards discussed above, based on my current understanding, and subject to the recommendations (provide above. My review is not complete at this time. New information may become available in the coming days or weeks that would influence my opinions. I expect to complete this review by March 17, 2008. This review is intended to be consistent with that level of care and effort exerted by other members of our profession, working in this location at this time, under similar constraints of access and scope. I am pleased to be of service in this matter. Please call if there are questions.