Loading...
HomeMy WebLinkAboutbocc.packet.spec.03172008PITKIN COUNTY BOARD OF COUNTY COMMISSIONERS SPECIAL MEETING MARCH 17, 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. EXHIBITS SUBMITTED AT THIS MEETING ATTACHED . . . ... . . ... . . . . ... . . ... . . ... . . ... . . . ... . . ... . . . ~l~EIV O .. . . ... . . ... .. . . ... . . ... ~ GUL CH . . ... . . ... . . ... . . . . ... . . ... . . ... . . PROF~L E .~s~. ~,~. evm: ~ \s9 . •• EXISTING GRADE ;. ~ r ~~ : ~`~ eiaeia o s ~ ~ /. PROPQSfO. NEfy GRADE .OF. 4EBR/.S ~ ~•. . ~~sr ~ ~ ~ ~ • FLOW • CH.4N NEL• BOTTDAf y caw: • CHANNEL W/DENS AND FLATTENS OUT ~ aao aao ,.an ,uo rwo zroo TO CASTLE CREEK BUILD/NG AND LOWER BRIDGE 01/ERFLOW DIRECTION OF FLOW PARKING LOT /., Page 1 of 1 Susan Murphy ~~ ~' ~`~~`~~ °~"~ ",~ From: nanchromy@aol.com Sent: Thursday, March 13, 2008 5:09 PM To: Susan Murphy Subject: historic preservation BOCC- Re: Benedict Buildings I serve on the RHPC and have often felt the 'heat' so to speak from area citizens when it comes to preservation. but it doesnt ever really bother me personally, this heat....I adore our history in the Crystal Valley and Redstone. I am writing this brief email to urge you to listen strongly to your instincts on this current subject. When buildings are gone, they are gone. And, as we all know, buildings deteriorate over time, but to preserve at the very least, the Music Hall, would be honorable and appreciated by attendees and residents. Future uses and needs will be ever-changing. Mr Teague will certainly want to build a newer and better product today. But this should not sway your historic preservation decision. Aspen/Pitkin county has preserved much less interesting and historic buildings over the years. This should be one of the saved buildings. Something on the property should remain the same' for the sake of history. A Benedict building is a perfect choice. Thank you for your time. Nancy Chromy Redstone, CO Supercharge your AIM. Get the AIM toolbar for your browser. 3/18/2008 Memo To: Board of County Commissioners From: Alan Richman Date: 03/14/08 Re: Geologic Reports Regarding Castle Creek Campus Attached you will find two reports that we hope will provide useful background information for our continued public hearing on Monday, March 17. The reports are as follows: Letter from Mark Mahoney of Harry Teague Architects providing a summary of several of the technical reports prepared by our geologic experts that were included in our original application; and 2. Draft of the third party report prepared by Frank Harrison that he presented to you verbally last week. We hope these two reports will be helpful to you in preparing for Monday's meeting. See you then! • Page 1 HARRY TEAGUE ARCHITECTS i.z~ Per„ r~~a .=uire ~ Nasal*. ::n si>u e ~?a Gxl 'a2 r ~;o ~,z7 aN9z 1rfr..rvF.eaouearch.c;:r:! 8 March 2008 Jenny Elliot Director of Finance and Administrationo Aspen Music Festival and School Dear Jenny, This letter is in response to the Alan Richman's meeting summary entitled BOCC Issues List 2 (from the meeting on 2/25 There are three questions in the memo that address Keno Gulch and two questions that address avalanche events. The nature of the questions seem to express concern about whether or not the two types of events (debris flows and snowslides) have been adequately addressed, and whether the design criteria should be expanded from generally accepted practice. As part of my response, I would first like to distill the information contained in the Master Plan Application, in order to describe the design process for the mitigation of the Keno Gulch debris flow. Arthur Mears, of Natural Hazards Consultants, in his report dated July 3, 2001, describes the extent of debris flow, snow avalanche, and rockfall and associated potential hazards at the Castle creek Campus. He begins by establishing the limitations of his report, while maintaining that the current level of risk is acceptable according to those limitations: This report also has the following limitations, which must be understood by all those relying on the results and recommendations of this report: a. The debris flow, avalanche, and rock fall processes are reasonably foreseeable events determined from observations of current (June, 2001) topography, forest cover and traces of previous events; b. Any substantial changes in the current topography and forest cover as a result of nafural or man-induced activities could invalidate the results of this report.' He goes on to detail the sources and characteristics of Keno Gulch debris flows. 2 DEBRIS FLOWS 2.1 Source area and characteristics Debris flows initiate at the 9,600 to 9,800-foot level in upper Keno Gulch at an active landslide area (Figure 1). When the soil is saturated with spring snowmelt runoff, the landslide, which is approximately 150 feet wide and 330 feet long, slides down slope on water-saturated bedrock and soil info a flooding tributary of Keno Gulch. Field measurements made last month indicate that approximately 10,000 yd3 (cubic yards) of unconsolidated landslide material remains on and within the steep landslide scar. Additional potentially unstable material exists near the edges of the landslide and could double the volume, increasing it to 20,000 yd3. During heavy runoff periods, floodwaters erode the toe of the landslide and incorporate substantial amounts of mud and debris into the stream. Debris flows form, consisting of muddy water, soil, rocks, and vegetative debris. They continue down Keno Gulch and into the MAA campus. Individual flow surges maybe as much as 10-13 feet deep and 20 feet wide and can be moving down-channel at 10-20 mph on the steeper slopes above 8,20D feet elevation. Velocity will decrease to 5-10 mph on the flat area directly above the campus but will once again accelerate within the steep final channel to the campus." Mr. Mears later describes Additional Mitigation steps to improve the overall safety of the site. Debris Flows -The existing 4 foot diameter culvert could be replaced with a large box culvert 8 feet high and 12-14 feet wide while maintaining a steep stream gradient below the culvert. This would convey most of debris flows to Castle Creek where they terminated during pre-historic time. Any excess debris accumulating in the channel should be used to increase the height of the lateral berms. The current 4-foot culvert will be ineffective in conveying debris flows and the storage volume uphill of the culvert is insufficient to store debris from a large flow. The risk is to the Percussion Building, north end of the parking lot, and the north end of the Castle Creek Building. However, the Percussion Building is not a valuable facility and the Castle Creek Building presumably has been reinforced for debris-flow loading as specified by Lipkin-Warner Design Partnership in 1993. Furthermore, because heavy summer use of the area does not begin until mid-June which is approximately 2 weeks after the period of greatest debris flow risk; exposure of persons is not likely to be significant."' Roger Pihl of Yeh and Associates Inc., prepared a report dated April i 1, 2007 evaluating the geological hazards for redevelopment of the property. First he documents the database for the report: Our investigation consisted of a site visit and review of three map sets, the "Geologic Map of the Aspen quadrangle, Pitkin County, Colorado" and "Map showing areas of selected potential geologic hazards in the Aspen quadrangle, Pitkin County, Colorado" prepared by Bruce Bryant, U.S. Geological Survey, "Roaring Fork and Crystal Valleys-an Environmental and Engineering Geology Study" by the Colorado Geological Survey. We also reviewed a report titled "The Geologic Story of the Aspen Region" by Bruce Bryant for the U.S. Geological Survey, 1988. Numerous additional reports have been prepared for this site concerning geological hazards, foundation conditions, hydrology and environmental conditions. Of particular note are a total of six reports and letters by Arthur Mears, P.E. dating from 1983 through 2001 which identify potential areas of geological hazards and documentation of historic debris flows emanating from Keno Gulch. These previous, detailed studies of the conditions related to the recurring debris flows are the basis of this report. "' In accordance with Pitkin County Development Standards, he then makes recommendations for mitigating debris flows generated by Keno Gulch. Section 7-20-50 (d) This area contains the most recent primary flow channel for a historic debris flow for which the recurrence interval of the events is less than the service life of the proposed improvements. Debris flow mitigation should be considered during the site grading design and structure design. The 1996 debris flow event is likely fo be a typical design event. The severity of future events will be determined by the intensity of spring runoff, precipitation intensity and by the amount of material available for scour at the toe of the landslide on the north side of the basin. The likelihood of continued movement of the landslide is not within the scope of this report. The general approach for mitigation of this hazard should be to provide an enhanced lower channel, a CBC, and earthen berms or walls in the lower part of the channel. In addition, channel improvements and berming on the elevated bench area above the site should be considered. The site grading and CBC dimensions should be chosen to convey future flows under the parking lot access road then to the river along the existing flow path. Additional consideration should be given to controlling an overtopping event at the CBC to ensure that Building #13 and the lower bridge are not affected. If is likely that the impact from this hazard can be mitigated by reinforced earth berms, engineered ditches and selective site grading within the proposed building envelope. Sacrificial concrete walls or reinforced foundation walls which extend above grade are another option. High capacity debris and rock fencing are also available for sites with horizontal space constraints. The dimensions and capacities of mitigation systems should be determined during the geotechnica! investigation. The proposed improvements can be protected from debris flow hazard." Matt Webster, of Schmueser Gordon Meyer, prepared an engineering report which addressed the recommendations made by Yeh and Associates. The report describes improvements to the channel above the campus, as also the Concrete Box Culvert to be installed under the road leading to the summer-use parking lot. The report explains why the CBC was down-sized to 8' x 10', but is still capable of being cleaned by motorized equipment. Improvements to the Keno Gulch debris flow channel were recommended by Yeh and Associates, Inc. (Final Geotechnical Engineering Report, July 19, 2006). The channel on the upper bench above campus should be improved to maintain a channel invert of at least 15 feet deep. As the channel crests the hill and descends toward campus, the existing channel and berms will be improved to provide a minimum depth of 12 feet. The Master Plan calls for a road over the channel fo a parking lot in the area of the existing percussion building. To provide this crossing, a concrete box culvert (CBC) will be constructed to convey the debris flow under the road. Yeh and Associates recommended a 10' x 10"culvert, primarily to facilitate cleaning and maintenance following debris flow events. Due to difficulties encountered in reconciling grades between the fop of culvert and surrounding roads, the height of the CBC was reduced to 8 feet. This should still allow for motorized equipment to be used in cleaning and maintenance of the culvert. It is still possible that the 8' X 10' CBC could become clogged during a debris flow event. The uphill wingwalls of the CBC on the western side of the channel will be constructed to a height approximately 3' greater than those on the eastern side. This will allow for overflow to be directed toward the parking lot, minimizing the chances of a debris flow impacting the buildings, building #15 in particular. The parking lot also sits at the base of a steep slope with potential for rock fall. According to Yeh and Associates (Geological Hazard Evaluation, Letter Report to Aspen Music Festival & School, Revised, August 15, 2006), the parking lot can be protected from rock fall hazard using reinforced earth berms, walls, engineered ditches and selective site grading on the uphill side of the parking lot. ~' Please note that the report leaves open the possibility that the CBC will get plugged. Additional mitigation measures, such as creating a higher berm on the southwest side of the channel, will direct overflow toward the parking lot and away from the proposed buildings. Mr. Webster has said that even if an appropriate "break-away top" for the culvert was provided, the resulting U-shaped structure would still be subject to plugging or overtopping, and that the proposed additional mitigation measures would need to be implemented in any case. The question of whether we have designed fora "maximum" debris flow is a difficult one to answer. Debris flow events are not easily related to standard hydrologic modeling. The greatest risk for landslides may not correspond directly to periods of highest runoff potential. The planned mitigation places a priority on managing the direction of flow and directing overflow such that is poses less threat to structures and people. SGM used the NRCS (Natural Resources Conservation Service) TR-55 method to estimate the amount of runoff that will occur as a result of a particular precipitation event. Flows in Keno Gulch were determined from an analysis of the drainage basins of both Keno Gulch (229 acres) and Spar Gulch (246 acres). A 100-year, 24-hour rainfall intensity results in a runoff flowrate of 332 cfs (cubic feet per second). Based on this flowrate, the 8' x 10' culvert is adequately sized to pass rainfall runoff and some level of debris associated with the 100-year storm. It should be noted that the debris flow in Keno Gulch could carry particles up to 3' in diameter, as well as tree limbs and trunks and the behavior of such debris flows is difficult to predict and model. However, the debris flow mitigation plan does not depend solely on the capacity of the culvert. In the event that the culvert capacity is exceeded, or the culvert becomes clogged with debris, the channel and culvert wing walls will be designed to direct overflow through the parking lot -away from the Castle Creek building and lower bridge. We feel that, based on historic events and standards of professional care, we are providing an enhanced safety environment with these proposed Keno Gulch mitigation techniques for the occupants of the Music Associates and the Aspen Country Day School. Regarding the potential for the mine shafts above Keno Gulch to fill with water and burst, Yeh and Associates has asked for additional information, as well as time to research the number and types of open mine shafts in Aspen Mountain. There were two questions in the memo about avalanche mitigation design (Should we be using more than a 100 year avalanche design period? Should the practice rooms be "over- designed" (a wall form along the front of the roof) to ensure snow does not over-top the structured Both questions address design criteria. Mr. Mears has referred to discussions Pitkin County had a few years back where a 300 year standard was discussed, but a 100 year standard was eventually adopted. A 300 year design period could certainly be modelled and the practice rooms could be re-designed to conform to the additional snow loads. As it stands now, the design of the practice rooms does meet the current Pitkin County requirements, according to Mr. Mears. The practice rooms have been designed to withstand a 100 year avalanche event, and the roofs have been designed to stop the vast majority of snowslides from ever reaching the road. Mr. Mears confirmed as much in a recent a-mail Our design would contain this year's snow, but this year probably has a return period of no more than 30 years. I estimate the probability of exceeding our design is 1/100 or 1 %. It should be noted that even in the unlikely event of over-topping, the structures on that part of the site are not occupied during the winter months. In any case, the volume contained by the practice rooms would significantly reduce the amount of snow reaching the road, increasing the overall safety of the school, especially compared to the current campus conditions. The Project has commissioned, at the suggestion of the BOCC, a third party peer review of the design process. Mr. Frank Harrison, a geotechnical engineer from Michael W. West and Associates, Englewood, CO., will be at the BOCC meeting on Monday, March 10th, to present his report to the Commissioners. I hope this synopsislclarification has been helpful. Please contact me if you have any questions. Regards, Mark Mahoney Project Manager Castle Creek Campus Harry Teague Architects harryteaguearchitects. com 970.927.4862 ~ Master Plan Application Exhibit 8 p.i ~t Ibid, p. 2 iii Ibid, p.4 iv Master Plan Application Exhibit 6 p.1 v Ibid, p.3 vi Master Plan Application Exhibit 5, p. 9 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 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: 10090 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 • Reports by Schmueser/cordon/Meyer dated December 14, 2007 and _ 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. Avalanches 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. 1 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 characteriza#ion 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. ! 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. SummaN 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, 2048. 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.