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HomeMy WebLinkAboutTimber Ridge Rockfall Options Update 0201 1 MEMORANDUM TO: Vail Town Council FROM: Community Development Department DATE: February 1, 2011 SUBJECT: Timber Ridge Redevelopment Geologic Hazard Mitigation Update Vail Timber Ridge, L.L.C. and the Town staff have evaluated additional options for the mitigation of the geologic hazards directly affecting the Timber Ridge Redevelopment site. The purpose of this memorandum is to: 1. Identify the action the Vail Town Council needs to take to move the rockfall mitigation to the next step towards implementation. 2. Inform the Vail Town Council of the work that has been completed to date. 3. Outline three viable rockfall mitigation options for consideration. 4. Describe the next steps and timeline for the review process of the proposed Timber Ridge development review applications. Action Requested of Town Council The Town staff is requesting direction from the Vail Town Council on which rockfall mitigation option should be further evaluated for implementation. Pursuant to the pre- development agreement with Vail Timber Ridge LLC, the Town of Vail and Vail Timber Ridge LLC are obligated to cost share (50/50) the evaluation and subsequent design of the rockfall mitigation improvements. The Town of Vail, however, is obligated to pay 100% of the cost of constructing the mitigation improvements. The Town Council is being asked to select a final option and instruct Town staff to pursue that option for design and construction. Based upon the Town Council’s direction, staff will work cooperatively with Vail Timber Ridge L.L.C. to assemble final design drawings and construction cost estimates. A bid process for the rockfall mitigation work will be pursued concurrently. Background Pursuant to the Official Hazard Map of the Town of Vail adopted by the Vail Town Council, the Timber Ridge redevelopment site is located within a high severity rockfall hazard zone. A site specific geologic hazard study prepared by HP Geotech reaffirms this designation and has concluded: 1. that the site can be developed provided mitigation can be accomplished to reduce the danger to the public health and safety and to protect the inhabitants of the Town from the hazard. 2. that a future rockfall event is likely infrequent but has the potential to present a significant risk to the proposed redevelopment. 2 The Town of Vail may not exempt itself from the adopted hazard regulations without first adopting amendments to the Zoning Regulations. According to Section 12-21-13, Restrictions in Geologically Sensitive Areas, Zoning Regulations, Vail Town Code, in part, “If the findings of the engineer or geologist performing the geologic investigation is that the site is a geologically sensitive area, but that corrective engineering or engineered construction or other mitigation or alterations can be accomplished to reduce the danger to the public health and safety or to property to a reasonable level, and such mitigation does not increase the hazard to other property or structures, or to public buildings, roads, streets, rights of way, easements, utilities or facilities, approval of the development plan and/or the issuance of the building or grading permit shall be conditional and contingent upon approval of plans for the corrective engineering and engineered construction or other mitigation or alterations as set forth in this title.” Geologic Hazard Mitigation Options On August 3, 2010, the Vail Town Council instructed Town staff to further explore options for rockfall mitigation improvements on the hillside above Timber Ridge. As a result, Yenter Companies, a geologic hazard engineering consultant, located in Glenwood Springs, CO, was hired to conduct a site specific study of the rockfall hazard and provide recommendations for mitigation. According to the “Rockfall Mitigation above the Timber Ridge Apartments in Vail, Colorado” report, dated October 29, 2010, prepared by Yenter Companies, “Based on our initial site observations from the street level it did not appear that the entire outcrop was absolutely in need of steel meshing which was, at the time, the option the Town of Vail was most interested in.” Furthermore, based upon an up close visual investigation of the hillside, Yenter Companies has concluded, “It is very obvious that the entire slope (+/- 132,000 square feet) does not need steel mesh as much of this outcrop is a very solid, tight bedrock formation with little to no fractures present in most areas. Many of the individual rock features we witnessed from the air were not as unstable as they appeared after a ground level inspection was performed. There are some rocks that we do have a concern for that should be considered for stabilization. There are several common approaches that can be utilized for stabilization of individual rocks/boulders. These options range from drilling and bolting smaller boulders in place to applying a mesh drape and anchors at specific points around the perimeter of the larger features. In any event, the isolated treatment of specific features would be far less costly than globally meshing/bolting the entire outcrop. However, this approach of treatment of isolated features does not provide the global protection from weathering/fracturing of rock over time nor does it consider rocks smaller than 24” in diameter that can be dislodged during 3 freeze/thaw cycles, severe weather events, or disturbed by wild animals and/or rodent undermining.” Based on Yenter Companies’ rockfall mitigation and past construction experience, they have identified three viable options for rockfall mitigation on the hillside above Timber Ridge Village Apartments. The options are: 1. Mesh the entire rock outcrop (approximately 200,000 square feet of mesh) at an estimated construction cost of $1.4 - $2.0 million. 2. Individual stabilization of specific large rocks/boulders (>24” in diameter) or small areas at an estimated construction cost of $70,000 - $100,000, plus annual maintenance. 3. Install a rockfall fence, 1,000 feet in length along the Lion’s Ridge Loop right of way at an estimated cost of $380,000 - $450,000. Option 1: Steel Mesh the Entire Rock Outcrop The anchored, steel mesh outcrop stabilization (hair net) will be achieved by pinning a flexible steel mesh on the outcrops with grouted rock bolts. A temporary access trail/road or helicopter would provide access to the barrier location for construction. The outcrop stabilization would be approximately 200,000 square feet or 4.6 acres in size. Estimated Cost – $1.4 - $2.0 million Pros • Protects the road and Timber Ridge residents • Mitigates the hazard at the source • Very low to no maintenance once constructed • Visually screened with native landscaping • One time expense • Complies with the hazard mitigation requirements of chapter 12-21 Cons • Mitigation to be constructed on Town of Vail Open Space • Requires consent and written agreement from property owners to construct • Severely limited construction access via helicopter or foot paths • Site disturbance caused by temporary access road/trail • Most expensive option Option 2: Individual Stabilization of Specific Large Rocks/Boulders The individual stabilization will be achieved by scaling the hillside and anchoring specific large rocks and boulders to the hillside. In some instances, small areas of hillside may be covered with steel mesh to provide the necessary protection. Loose rocks and boulders less than 24 inches in diameter would be removed from the hillside. Annual oversight and maintenance is required. Estimated Cost - $70,000 - $100,000. Pros • Protects the road and the Timber Ridge residents • Ease of access and maintenance • Results in less site disturbance • Most cost effective option 4 • Focuses the attention on the most susceptible rockfall areas • Less visually obtrusive • Visually screened with native vegetation • Complies with the hazard mitigation requirements of chapter 12-21 Cons • Requires permission from property owners to construct • Lower upfront capital cost/Higher annual maintenance cost • Requires mitigation to be constructed on Town of Vail Open Space • Focuses on current conditions only Option 3: Wire Rope Net Barrier along Lions Ridge Loop The wire rope net barrier would be constructed along the top of the road cut of Lions Ridge Loop. Construction would require crane access from Lions Ridge Loop. The barrier is approximately 1,000 feet in length and 11 feet in height. Estimated Cost – $380,000 - $450,000 Pros • Protects the road and the Timber Ridge residents • Ease of access and maintenance • Results in a lower net height • Cost effective option • One time expense • Maintains two way traffic on Lions Ridge Loop • 1,500 kilojoules of rock stopping strength • Complies with the hazard mitigation requirements of chapter 12-21 Cons • Requires permission from property owners to construct • More difficult to screen from view with landscaping • Greater negative visual impacts • Requires mitigation to be constructed on Town of Vail Open Space Next Steps The Developer, Vail Timber Ridge L.L.C. has not completed the development review process. Pursuant to the Zoning Regulations of the Town of Vail, a final review can not be heard until a final geologic hazard mitigation plan is submitted and approved in accordance with Chapter 21 of the Zoning Regulations. The Developer is scheduled to appear before the Town of Vail Planning & Environmental Commission on Monday, February 28th to present a revised landscape plan, geologic hazard mitigation plan, and site plan. Among other obligations, the revised site plan must include, or take into consideration, the geologic hazard mitigation measures proposed for the project. Staff Recommendation According to the consulting engineers with Yenter Companies, “Rockfall events are sporadic and unpredictable. Because of the multiplicity of factors affecting such an event, it is not and cannot be an exact science that guarantees the safety of individuals and property. 5 However, sound engineering principles to a predictable range of parameters, and by the use of properly designed protection measures in identified risk areas, the risk of injury and property loss can be substantially reduced.” The Town staff is recommending that the Vail Town Council instructs the staff and Vail Timber Ridge LLC to pursue Option #2 individual stabilization of specific large rocks/boulders. Staff’s recommendation is based upon our review and understanding of: 1) the applicable Zoning Regulations, 2) the engineering involved in constructing the mitigation improvements, 3) the pros and cons associated with each of the options, 4) the estimated cost of the improvements, and 5) practical realities associated with each of the options. 6