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Western Dam Engineering Technical Note May 2017 4  Check for entrained sand or fines.  Observe and record increases or decreases in flow.  Observe and record secondary effects (slumping, sliding, erosion, deposition).  Check availability of off-site materials, equipment, labor, and professional resources – how much and how soon?  Assign roles to available support staff: monitoring the incident area, media/public management, coordination of off-site resources, coordination of emergency response staff, etc.  Begin brainstorming potential courses of action. Talk with the dam tender or other personnel familiar with the dam. See if they can be present as you do the assessment. Documentation does not need to be extensive and overly detailed; get the key facts and observations. Digital photos and videos are immensely helpful. Include recognizable features in the view and record from more than one vantage point to document proper perspective. Taking notes on a set of as-built drawings is efficient and effective. The results of the initial assessment will guide the appropriate next actions to take, as discussed in the following section. But be flexible and respond to changing conditions on the ground, including weather, availability of staff, equipment and materials, and especially the seepage behavior. Concentrated Flows vs. Disperse Seepage The characteristics of the exiting seepage related to the internal erosion incident will influence the appropriate emergency response, as well as influence interim and permanent solutions. For the purposes of this article, we will focus on two general types of seepage exit characteristics: concentrated versus disperse seepage. For those readers familiar, this is not to be confused with terminology conventions of internal erosion mechanisms (concentrated leak erosion, backward erosion piping, contact erosion, etc.). “Concentrated” versus “disperse” seepage in this context is simply a descriptor related to the relative discreteness of the identified seepage exit. Concentrated flows are those that have a distinct, identifiable exit over a relatively small area. Some signs of concentrated seepage exit flows are rupture or blow out at an isolated area near the toe of the dam, seepage around a contact or conduit penetration, a sand boil or set of adjacent sand boils, or identifiable points of pluming or sediment deposition. Conversely, disperse seepage is that which covers a broad area, as shown in Figure 1. Figure 1. Disperse Seepage at Dam Toe (Wyoming NRCS) Actions to consider and implement – and what not to do Potential intervention actions to control concentrated seepage until more permanent repairs can be made are described below. It is important to stay flexible and respond to conditions as they develop; don’t get overly attached to any given approach. Conventional Filter Blanket. Where seepage is dispersed and discharging at relatively low velocity a conventional filter blanket comprised of sand (e.g., ASTM C33 fine aggregate) can be placed directly over the area of seepage to trap fine eroded soil entrained in the seepage flow, and allow the clarified seepage water to pass freely downstream. Filter-compatible gravel would typically be placed over the sand blanket, acting as a drain and protective ballast (see Figures 2a and 2b). That ideal, however, is rarely achieved in the case of concentrated seepage flow. Filter sand itself is highly erodible under even moderate flows. If the flow from a concentrated seep is strong enough, it may not be possible to place a thick enough cover of filter sand (or even gravel) quickly enough to prevent it from washing away. And, if the filter sand cover is too thick, the risk of impeding sufficient drainage through the filter increases. To combat these conditions, the concept of an “inverted filter” should be considered as discussed next.