Test result triggers safety inspection of Warm Springs Dam

<p>A team of experts called in by the U.S. Army Corps of Engineers began inspecting Warm Springs Dam Tuesday after a monitoring well showed an unusual increase in the water level at its base.</p><p>The water level in one of a series of testing wells penetrating<NO1><NO> the earthen dam rose 14 feet in a week, the Corps said Tuesday. It is located near the base of the dam, about 700 feet from the top.</p><p>Corps officials stressed that no other monitoring device — there are 194 of them — showed anything unusual and a chemical analysis showed the water came from groundwater, not water from the lake.</p><p>The investigation is being done as a precaution, said Lt. Col. Laurence Farrell, an engineer and district commander.</p><p>“There is no concern, the dam is safe, there is no indication of seepage,” Farrell said.</p><p>Excessive water in earthen dams has the potential to weaken the structures, but engineers said there is no excessive water in Warm Springs Dam.</p><p>“We are looking at dozens of other instruments and they are reading as they have,” said Mike Dillabough, the Corps division chief of operations. “We are setting up sloped stakes and checking it to make sure there are no other changes to the dam face. There has been no change.”</p><p>The monitoring well that showed the increase on July 21 is a one-inch diameter pipe that goes 60 feet into the dam about 700 feet downhill from the top.</p><p>It was one of five monitoring wells and nine piezometers, which record the pressure of the dam’s core, that were installed between November and January.</p><p>Another monitoring well that is 100 feet away shows no unusual readings, officials said.</p><p>Among the possibilities, they said, is that the well has tapped into groundwater or a spring.</p><p>“Right now we are going through the process of elimination, our dam safety officers are read in on it, we have a dozen dam safety engineers working on it trying to figure out what it means. But there is no indication it is a danger,” Dillabough said. “We want to make sure it does not lead to that.”</p><p>Farrell said it might just be the discovery of a natural fluctuation in the groundwater at that well site or an underground stream or a spring.</p><p>A dozen engineers will be studying the dam, testing water quality, checking the dam for any signs of movement or seepage and looking at all of the monitoring gauges and running computer models of water flow.</p><p>“We’d like to know what it caused it,” Farrell said. “It is not a concern for safety, it’s an item of interest.”</p><p>The dam’s last annual inspection was in September, and no problems were reported except for minor rust on some control valves and minor rodent damage and erosion on the dam face, officials said at that time.</p><p>Warm Springs Dam was completed 25 years ago at a cost of $330 million, creating Lake Sonoma as a source of water and recreation and as a flood-control project, taking 5 to 8 feet off flood levels as measured on the Russian River at Guerneville.</p><p>The water from Lake Sonoma is released into Dry Creek, which flows into Russian River near Healdsburg.</p><p>The compacted earthen dam, made with 30 million yards of dirt and rock scraped from surrounding hillsides, is 3,000 feet wide and 319 feet tall.</p><p>The dam created a lake that when filled covers 3,600 acres and has a 73-mile shoreline. Its Dry Creek arm is nine miles long and its Warm Springs arm four miles.</p><p>It holds a water supply of 212,000 acre-feet and a flood pool of 130,000 acre-feet. </p><p>The Sonoma County Water Agency on Tuesday alerted the county’s Office of Emergency Services, other emergency service providers, property owners’ groups along Dry Creek, and local, state and federal official, said spokesman Brad Sherwood.</p><p>“At this point we just want more information and find out what the research will tell us,” Sherwood said. “Public safety is our priority and we want to make sure the dam is functioning and operating as it should.”</p><p>Sherwood said the agency will have daily updates on its Internet site.</p>