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5/11/2017 10:13:44 AM
WESTERN DAM ENGINEERING NEWSLETTER, VOLUME 5, ISSUE 1 MAY 2017
EMERGENCY RESPONSE, EROSION, CALIBRATION, HYDROLOGIC MODELING, EMBANKMENT DAMS, SEEPAGE, SINKHOLES
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Western Dam Engineering <br /> Technical Note <br /> <br /> <br /> May 2017 <br /> <br /> <br />4 <br /> Check for entrained sand or fines. <br /> Observe and record increases or decreases in flow. <br /> Observe and record secondary effects (slumping, <br />sliding, erosion, deposition). <br /> Check availability of off-site materials, equipment, <br />labor, and professional resources – how much and <br />how soon? <br /> Assign roles to available support staff: monitoring <br />the incident area, media/public management, <br />coordination of off-site resources, coordination of <br />emergency response staff, etc. <br /> Begin brainstorming potential courses of action. <br />Talk with the dam tender or other personnel familiar <br />with the dam. See if they can be present as you do the <br />assessment. <br />Documentation does not need to be extensive and <br />overly detailed; get the key facts and observations. <br />Digital photos and videos are immensely helpful. <br />Include recognizable features in the view and record <br />from more than one vantage point to document <br />proper perspective. Taking notes on a set of as-built <br />drawings is efficient and effective. <br />The results of the initial assessment will guide the <br />appropriate next actions to take, as discussed in the <br />following section. But be flexible and respond to <br />changing conditions on the ground, including weather, <br />availability of staff, equipment and materials, and <br />especially the seepage behavior. <br />Concentrated Flows vs. Disperse <br />Seepage <br />The characteristics of the exiting seepage related to <br />the internal erosion incident will influence the <br />appropriate emergency response, as well as influence <br />interim and permanent solutions. For the purposes of <br />this article, we will focus on two general types of <br />seepage exit characteristics: concentrated versus <br />disperse seepage. For those readers familiar, this is not <br />to be confused with terminology conventions of <br />internal erosion mechanisms (concentrated leak <br />erosion, backward erosion piping, contact erosion, <br />etc.). “Concentrated” versus “disperse” seepage in this <br />context is simply a descriptor related to the relative <br />discreteness of the identified seepage exit. <br />Concentrated flows are those that have a distinct, <br />identifiable exit over a relatively small area. Some signs <br />of concentrated seepage exit flows are rupture or blow <br />out at an isolated area near the toe of the dam, <br />seepage around a contact or conduit penetration, a <br />sand boil or set of adjacent sand boils, or identifiable <br />points of pluming or sediment deposition. Conversely, <br />disperse seepage is that which covers a broad area, as <br />shown in Figure 1. <br /> <br />Figure 1. Disperse Seepage at Dam Toe (Wyoming NRCS) <br />Actions to consider and implement – <br />and what not to do <br />Potential intervention actions to control concentrated <br />seepage until more permanent repairs can be made <br />are described below. It is important to stay flexible and <br />respond to conditions as they develop; don’t get overly <br />attached to any given approach. <br />Conventional Filter Blanket. Where seepage is <br />dispersed and discharging at relatively low velocity a <br />conventional filter blanket comprised of sand (e.g., <br />ASTM C33 fine aggregate) can be placed directly over <br />the area of seepage to trap fine eroded soil entrained <br />in the seepage flow, and allow the clarified seepage <br />water to pass freely downstream. Filter-compatible <br />gravel would typically be placed over the sand blanket, <br />acting as a drain and protective ballast (see Figures 2a <br />and 2b). That ideal, however, is rarely achieved in the <br />case of concentrated seepage flow. Filter sand itself is <br />highly erodible under even moderate flows. If the flow <br />from a concentrated seep is strong enough, it may not <br />be possible to place a thick enough cover of filter sand <br />(or even gravel) quickly enough to prevent it from <br />washing away. And, if the filter sand cover is too thick, <br />the risk of impeding sufficient drainage through the <br />filter increases. To combat these conditions, the <br />concept of an “inverted filter” should be considered as <br />discussed next.
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