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Western Dam Engineering Technical Note May 2017 3 Salt Fork Dam Boil A 4-ft diameter boil appeared at the toe of the dam, following a record high pool. The boil was cloudy, with soil particles moving at the bottom. The boil was caused by surcharging of a clogged toe drain by flows from the right abutment. A sandbag ring was constructed around the boil and a V-notch weir installed to monitor flow (see Figure 7 for a similar installation). The amount of flow coming into the lake made it impossible to quickly lower the lake level. The use of large pumps to assist with the drawdown was not an option, due to high cost and availability. Excavating through an abutment to drain the lake was considered, but since both abutments are bedrock, this idea was abandoned. A weighted inverted filter berm over the boil consisting of pea gravel overlain by concrete sand was constructed (see Figure 3). Recognizing the First Signs of Trouble What should we be looking for? Examples of conditions that should be noted for vigilant monitoring include:  visible seepage exits  unexplained wet areas or lush vegetation  unexplained increases in downstream weir or channel flows Examples of conditions that should immediately raise concern are:  whirlpools in the reservoir  sinkholes anywhere on the dam, abutments, downstream toe or reservoir pool  new sand boils, blowout holes, pluming, or sediment deposited at a visible seepage exit.  concentrated seepage around conduits or elsewhere on the dam, at the downstream toe or in the abutments Dam inspections and monitoring are essential activities that can help avoid, or at least provide timely warning of, developing seepage conditions to allow for successful intervention. These topics are discussed in previous Western Dam Tech Note articles titled, “Dam Safety Inspections – A Closer Look” and “Does Your Dam Measure Up? – Developing an Effective Instrumentation Program for Small Earth Dams. Intervention Planning A key part of responding as effectively as possible to a serious seepage incident is preparation. One concept that is gaining attention is to develop an Emergency Intervention Plan (EIP). This can either be a stand- alone document or an appendix to the Emergency Action Plan (EAP). Guidance on developing an EIP is contained in the Dam Owner Emergency Intervention Toolbox [10]. Preparatory actions may include stockpiling of on-site materials, identifying owner’s on- site equipment, and making arrangements to be able to call on outside resources (experts, labor, equipment and materials), as addressed further below. Assess the Situation It is critical in responding to a serious seepage incident that conditions be assessed and documented as a basis for identifying and implementing the appropriate response actions. The assessment and early stages of the response should address all of the following topics, and others that may come to mind:  Perform an initial assessment – quick, but thorough; look for the big items and don’t get bogged down in details that can be dealt with later.  Determine the “emergency level” as defined in the EAP and make appropriate notifications and contacts.  Update initial assessment as often, as needed.  Confirm site access – are routes outlined in the EAP open and passable? Are alternate routes needed?  Note current and forecast weather conditions  Observe reservoir pool level – is it or has it been dropping at a rate greater than anticipated based on existing inflows and normal outlet releases?  Identify known or potential water entry areas (whirlpools, sinkholes in the reservoir pool area) and seepage exit areas (diffuse and/or concentrated seepage, sinkholes, around outlet or other conduits).  Estimate or measure seepage flow.