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Western Dam Engineering Technical Note May 2017 7 to be balanced against the potential loss of more eroded fines than ideal. Another factor to consider is the timely shipping of a suitable geotextile to the site. In rural areas, typical concrete aggregates may be found more readily in the vicinity than geotextile. If more readily available, a woven geotextile (often used for silt fence) can be considered (good permeability, but less efficient filtering of fines). It may be appropriate to use the geotextile as a temporary emergency measure until a more robust engineered filter can be constructed as a permanent repair. Depending on the ready availability of either geotextile or aggregates, it may be prudent to pre-order and store on-site appropriate materials to provide more immediate response if a seepage event were to occur. This is especially true if there is a suspected seepage deficiency at the site. Other considerations in the construction of an aggregate or geotextile filter system include, but are not necessarily limited to the following:  Availability of equipment (haul trucks, tracked excavator, dozer and/or loader; portable light plant, etc.); consider equipment that may already be on-site or may be readily available from neighbors, highway department maintenance facilities, federal agencies (e.g., the U.S. Forest Service for dams on or near USFS lands), etc.  Inducing detrimental loadings (avoid equipment, to the extent feasible, with high concentrated tire or track loads to minimize bearing failures, and equipment producing strong ground vibrations to minimize potential liquefaction of susceptible saturated materials).  Site conditions (if seepage has resulted in extremely soft ground conditions, it may be necessary to mobilize low ground pressure [LGP] equipment and/or a long reach hydraulic tracked excavator [e.g., Cat 324DL, PC220-2, etc.]).  Under the most extreme site access conditions it may be necessary to employ a helicopter to transport mini-equipment and materials, and possibly to place the materials, if equipment access is not feasible. In some instances concentrated seepage occurring immediately at the downstream toe, or worse on the lower downstream slope, can result in severe erosion that may eventually undermine the slope and result in local or progressively larger slope failure. If such a condition is, already has, or may occur, then placing an earthen stability berm is warranted (see AV Watkins and Washakie Dams case histories below). If erosion and slumping or sliding is limited in the immediate vicinity of the concentrated seepage and does not appear to be progressing further upslope or laterally, then frequent and diligent monitoring can be implemented and used as the basis for determining if or when a berm is required. Often once the active seepage is controlled, the local erosion and instability around the seep ceases. Diverting Inflow to the Reservoir. If inflow to the reservoir includes flow conveyed by a canal or natural drainage with a controlled diversion or inlet structure or a waste-way that could be utilized to divert that flow from entering the reservoir, such a measure should be considered early during the incident. One key consideration in making the decision where this action is technically feasible is whether other potential damaging impacts may occur downstream in the drainage to which flows are diverted. Lowering the Reservoir Pool. The first action to be considered in the event of a serious or visibly worsening seepage discharge is lowering the reservoir pool. This should be implemented as soon as possible and at the maximum drawdown rate possible. The sustained drawdown rate implemented should consider what rate may result in slumping, damage, and instability of the upstream slope due to pore pressures in the embankment that are not able to drain (i.e., rapid drawdown failure), and temper that with consideration of the urgency of the seepage incident. Lowering can always be slowed or stopped if the concentrated seepage is determined not to be a threat to the dam or is stabilized by response actions. Most commonly, reservoir lowering will be by discharge through a low-level outlet conduit. However, if the dam has a gated spillway, opening the spillway gates should be considered. It is important to provide warning to downstream population and emergency managers prior to significantly increasing outlet and/or spillway releases. Consideration can also be given to supplementing the drawdown rate with siphons or pumps.