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Western Dam Engineering Technical Note May 2017 9 Place “Sandbag Ring” at Discrete Point(s) of Seepage Discharge. In cases where discrete concentrated seepage at a still manageable (i.e., relatively low) flow rate is exiting the downstream slope, toe area, or abutment(s) of the dam, “sandbag rings” have often proven effective as a response action (see Figure 7). The objective of a sandbag ring is to create a back- pressure head on the seepage discharge that is sufficient to slow the flow but not so much as to block or over-pressurize the seepage and cause it to divert to another flow path(s) and perhaps do more harm than good. It is important to provide an overflow to safely convey the reduced seepage flow from the ring and then downstream, and a means to measure the head within the ring and the discharge flow rate. Sufficiently frequent measurements of head and flow should be made until a reasonably steady state condition is achieved, or a condition of reducing head and flow that is commensurate with reservoir lowering, if implemented. Note that using sand bags is often the most efficient and rapid means to achieve the back- pressure pool desired. However, other materials can and should be used if more readily available and suitable to site conditions. This might include constructing a berm with: compacted clay, common soil, or aggregate lined with plastic sheeting or geomembrane; concrete blocks wrapped in tarp and stacked in a stable manner; or some other practical means. Figure 7. A Sandbag Ring with V-Notch Weir [4]. Internal Erosion along a Conduit. An especially challenging condition of concentrated seepage is uncontrolled flow from around a conduit through the dam or foundation. As seen in Figure 8, this can lead to severe consequences. In concept, the measures described above for an inverted filter using either aggregate materials or geotextile still apply to addressing this condition. The major challenges in this case are typically associated with seepage along an actively discharging outlet conduit. These challenges include access to the area of discharge and placing materials. As a result, it may prove more feasible to utilize geotextile as the filter material. Regardless of the material used, it is strongly recommended that the filter protection be placed around the full circumference of the conduit for at least several diameters beyond. This will provide protection in the event that additional erosion occurs around the pipe or the existing seepage moves in response to the placement of the filter system. Figure 8. Seepage Along a Conduit Led to Dam Breach (Courtesy of Wyoming State Engineer’s Office). Another intervention to consider is placing a dike with an armored overflow below the discharge end of the conduit to impound a pool that would submerge the area of concentrated seepage discharge while still allowing free discharge of the seepage and, in the case of the outlet works conduit, emergency releases from the reservoir. The intent of this approach is to mimic the effect of a sandbag ring as described previously. The dike would be most readily constructed with large rock (e.g., riprap); seepage through the dike would be minimized by placing progressively finer earth material, reinforced plastic tarp, or some other suitable seepage barrier on the upstream slope and basin floor. If appropriate to site conditions, dike constructability, and outlet operational needs or flexibility, the outlet conduit could be extended through the dike. In that configuration, the pool behind the dike would be filled by the concentrated seepage discharge, supplemented, if necessary, by pumping or siphoning from the channel below the dike