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