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Our biggest threat to the safety of Broadway travel lanes, our water and sewer mains is the instability and weakened state of landslide area. The new valves and periodic monitoring and inspection are reasonable measures to manage the landslide risks in the short-term. Constructing permanent landslide mitigation soon is important to alleviating the risks to the public, utilities and private property. The Town through its consultants is developing designs for permanent mitigation and seeking the funds to accomplish it. The actions the Town is conducting demonstrates concern and a reasoned approach to resolving these risks.
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It should be noted that, if passed, this SPET would only fund restoration of public infrastructure (the western portion of the slide path). No public funding will be used to subsidize commercial property owners. These property owners will be responsible for mitigation of slide impacts on private property. The funds collected would be used to restore the Town’s road up to the neighborhood on the hillside, restoration of public water systems up to hillside residents, mitigate potential damage to the Town’s main water line and sewer line near West Broadway, and take measures to protect West Broadway.
The development on the north side of Broadway between Scott Lane and WY 22 sits on the remnants of a colluvium mass wasting and ancient landslides that occurred approximately 150,000 years ago. During this event the volcanic basaltic andesite, which overlaid the sedimentary layers, slid down to what would 150,000 years later become West Broadway.
Through a series of events over the past 60 years, this ancient landslide was reactivated. Between the dates of 1967 and 1968 WYDOT quarried approximately 52,000 cubic yards of materials to pave the roads which most residents and visitors now rely on throughout Jackson. In addition, during the years of 1963 until 1980 there were an additional 15,000 cubic yards of materials quarried through private operations. The Hillside building and Budge Drive Reconstruction removed 10,000 cubic yards of materials from the hillside during the years of 1998 and 1999. The Walgreens development removed approximately 6,500 cubic yards of materials from the site.
It should be noted that funds collected from the SPET initiative, if passed, would not be used to mitigate the eastern portion (Walgreens side) of the landslide. The Town of Jackson continues to work with Walgreens in an effort to develop a plan which will mitigate the eastern portion of the slide.
The landslide engineers, geotechnical engineers, and geologists have all said that there is a possibility that the West Broadway Landslide could move given the right conditions. The landslide is still moving as evident from the monitoring of inclinometers in the bore holes, extensometers at the scarp, and topographic surveys.
If there was a condition where there was rain on frozen ground that allowed significant water to flow into the scarp and the slide slip plane, as occurred in 2014, a very raining period and/or an earthquake that disrupted the friction that is resisting the slide movement there could be substantial movement of the slide. The United States Geological Survey (USGS) Letter (PDF) predicted that slide debris form a secondary slide would flow onto Broadway. The Landslide Technology Letter (PDF) stated that the possibility of debris flow from a secondary slide reaching Broadway was low probability, but high consequences.
Given the relative risks and potential consequences, the Town through its consultants are monitoring the slide with instruments and periodic site inspections to manage safety concerns. Over the past one and a half years, the slide has been creeping at rates between 0.1 and 0.2 inches per month with occasional increases to 0.5 inches during wet periods. These rates and magnitudes of movement are relatively small and indicate a very low risk to safety. The probability of the slide failing catastrophically is very low due to the granular nature of the slide materials and that the slide movement in April 2014 resulted in a more stable geometry.
The Town has taken temporary emergency actions to mitigate and/or reduce the possibility of slide movement by stock piling soil at the toe of the slide to add a resistive force, and they have constructed temporary drainage facilities above the slide to divert surface runoff away from the slide head scarp.
During the first two weeks of the landslide movement the Town installed 2 isolation valves on the 12-inch water main, one on the east end and one on the west end of the effective landslide area, which will help shut down 12-inch main water flow through the landslide area. We also installed 2 fire hydrants at the same locations which allows us to jumper/reconnect with a temporary above ground temporary water hose from the east fire hydrant to west fire hydrant through the affected landslide area. This only restores some domestic water pressure and flow but will not provide critical fire flow. The Town also continues to perform video surveillance of the sewer mains.
The Town recognizes if the slide is not mitigated in the next few years, that the accumulation of movement may over-stress buried utilities, including the water main. This is why the Town is actively pursuing mitigation of the slide and the funding in order to accomplish it. The Town mentions the risk of future water main problems because this is one of the risk factors for needing to construct permanent long-term slide mitigations. At the current rates of slide movement, it is our engineer’s assessment that it would take more than a few years to over-stress the buried water main. Therefore, in the short term, this risk is manageable with monitoring and pipeline inspections.
If there was substantial movement of the slide it could break the 12-inch diameter ductile iron water line on the north side of Broadway, similar to how the slide broke the 8 inch ductile iron pipes in Budge Drive. It could also break the 8 inch diameter PVC sewer main on the north side of Broadway. The broken 12-inch water line could discharge up to 6,400 gallons per minute of water, flooding Broadway and disrupting traffic. The Town maintenance staff may not be able to close the two isolation valves to prevent complete draining of the tank however there would be substantial damage to Broadway, loss of water supply to the area this 12-inch main supplies water to in Jackson. This could contaminate the water system from the sewage leaking from the broken sewer line.
There is an automatic valve that monitor’s pipeline velocity and automatically closes if there is a major break in the line, in fact there was an automatic valve installed at the base on the 2 million gallon tank in 1979 and 1980. The problem is that the valve cannot differentiate between a broken pipe and high fire flow demand. If there is a large fire the valve may close thus shutting off the water supply for firefighting purposes.
Our calculation indicate the maximum discharge from an open 12-inch pipe with the pressure created by a full tank is about 6,400 gallons per minute. In 30 minutes that would be 192,000 gallons. In reality the leak would start smaller and the Town would shut off the valves on both sides of the main before that much water was discharged.
The Town has taken several steps to help reduce the complete loss of the 2 million gallons of water as noted above, we have an automatic valve at the base of the 2 million gallon tank and we have installed 2 isolation valves and 2 fire hydrants on each side of the landslide area as means to help shut off the 12 inch waterline main. Please be aware if the 12-inch waterline main is damaged and with the water pressure between 80 and 90 pounds it will leak 6,400 gallons per minute and will damage a large area in Broadway travel lanes. There are a few variables that also could have impacts to this area: