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Show Volume II, Issue XVI THE OGDEN VALLEY NEWS Page 11 15 August 2000 Getting the Pineview Dam Ready for “The Big One” Lorine Murtagh, Chair, Ogden Canyon Club One of the questions asked at the July 18 Ogden Canyon Club meeting between representatives from the Bureau of Reclamation (BOR) and Ogden Canyon, Ogden Valley, and local area folks was “what magnitude on the Richter scale can the Pineview Dam withstand at the present time?” Answer: “Not the big one.” “The Big One” or the largest earthquake that can be expected in Utah, according to the Utah Earthquake Preparedness Information Center, has a magnitude of 7.0-7.5. The Seismograph Stations at the University of Utah record about 700 earthquakes each year in Utah in the magnitude range of 4.0 or less. Recent geological studies indicate that large earthquakes (6.5-7.5 magnitude) occur on the central segments of the Wasatch fault about once every 350 years. A magnitude 7 earthquake is ten times larger than a magnitude 6 and one hundred times larger than a magnitude 5. (Each whole number step on the scale corresponds to a release of about 31 times more energy.) The Modified Mercalli Intensity (MMI) Scale measures the damage in relationship to the Richter Magnitude. For example, a magnitude 6 earthquake causes considerable damage to ordinary substantial buildings with partial collapse. Damage is great in poorly built structures. Chimneys, factory stacks, columns, monuments, walls, and heavy furniture is overturned. Flooding due to dam failure could cause the most property damage and loss of life. Major dams that may affect the Wasatch Front are Pineview and Causey. Fires and hazardous material spills have to be factored in–ten times this damage for a magnitude 7 earthquake. The chance of a large earthquake on the Wasatch fault during the next 100 years is about 1 in 4 (25%). The last such earthquake appears to have occurred between Mona and Nephi about 400 years ago. However, since 1850, at least 35 earthquakes of magnitude 5.0 or greater in and around Utah have occurred. The most active faults are the Wasatch and Bear River faults. Russ Findlay and Curt Pledger, Provo Area Office, Bureau of Reclamation presented a pictorial viewgraph briefing to the 40-plus area residents who attended the meeting. They briefed the proposed alternatives to correct the deficiencies that would bring the dam to current safety standards. The Bureau proposes a $26 million modification to correct known safety deficiencies of the existing dam. Pineview Dam was constructed by BOR as part of the Ogden River Project in 1937 and enlarged under the Weber Basin Project in 1957. The alternative chosen by the BOR requires the removal and reconstruction of a major portion of the dam to correct the spillway stability deficiency, the foundation liquefaction deficiency (water-saturated sandy soils react to vibrations and temporarily act like liquid), and the potential seepage deficiency. This alternative requires draw down of the reservoir below normal operating ranges, and closure of the portion of State Highway 158 that extends across the dam for an approximate period of two months. The BOR determined that the proposed construction on Pineview Dam would create no significant impact to the resources within the dam and reservoir project area. Power boating would be curtailed since the boat ramps do not accommodate use below the 4850-foot elevation. The water surface area would be reduced to 630 acres, 22% of the typical water surface area. Wetland and vegetation would suffer no permanent effects, wildlife resources should rebound quickly, fish habitat would be lost for a minimum of one year, and the fish population and predator prey balances would be temporarily affected. Modification of the dam is anticipated to start in July 2001. Ogden Valley residents voiced concern over the length of time State Highway 158 would be closed inconveniencing them by forcing use of alternate routes. Pledger stated the original estimated 18month period has been reduced to approximately two months, a significant reduction. Others thought the tradeoff of increasing their safety during an earthquake was a small price to pay. Other comments voiced were the possibility of a trail across the dam for pedestrian, bicycle, and horse traffic. Pledger responded saying UDOT has a right-of-way across the dam for the road, and it is not wide enough to safely add a trail. He concluded the presentation by saying that the BOR would work with UDOT within its authority to alleviate safety concerns. Senior Meals Meals provided at the Ogden Valley Branch Library. August 10: Beef Cube Stroganoff, Parslied Noodles, Buttered Peas, Yogurt Fruit Salad, Cookie and Buttermilk Roll. August 17: Chicken Tenders / Honey Mustard, Baked Potato with Sour Cream, Spinach, Pineapple Marshmallow Coleslaw, Chilled Fruit Cup and Blueberry Muffin. August 24: Ham and Cheese Sandwich, Pasta Salad, Three Bean, Salad, Lettuce, Onion, Tomato & Pickle and Watermelon. August 31: Roast Pork with Gravy, Sweet Potatoes, A copy of the Pineview Dam study “Safety of Dam Broccoli, Sunshine Jell-O Salad, Spice Cake with Icing Work Finding of No Significant Impact,” and the and Dinner Roll. pamphlet “Earthquakes—What You Should Know When Living In Utah” can be obtained by calling Lorine Murtagh, 392-9743. 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