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21 Directional drilling methods are sometimes proposed for installation of high-level outlet conduits. These are typically not allowed due to unavoidable disturbance of the soils surrounding the carrier pipe. This disturbed zone creates a seepage path that must be mitigated, which is often difficult and can add significant cost. The main advantage of this system is its ability to simulate the hydraulic characteristics and efficiencies of the outlet conduit systems located through the base of the dam. Unlike the siphon, this option can be designed to accommodate a wide variation in base flows into the reservoir without the corresponding large fluctuations in reservoir level. The advantages of such a system include the following: • Limited embankment excavation • Limited lake level fluctuation, as compared to a siphon spillway. • Can be installed without complete draining of the reservoir. • Cost-effective in that the components are typically small in size and length. • A stop-log structure, or gate, can be installed to maintain a desired normal high water level and to allow incremental releases for water rights administration. The system does have disadvantages, such as: • Cannot be utilized to drain the reservoir below the invert elevation of the intake. Complete draining of the reservoir is not feasible with this system. • Utilization of this system may require the use of bends or elbows along the conduit to allow for the discharge of water at or near the downstream toe. Conduit bends/elbows can be expensive. It may just require a downstream conveyance channel be excavated and adequately armored against erosion. • Foundation soils for the intake may be soft, which can cause settlement problems or raise costs due to over-excavation. • The height of fall in the conduit is limited. Pipe or culvert spillways should not be used for drops from riser invert to pipe outlet greater than about 25 feet, due to the danger of cavitation. Design and construction guidance on approach, entrance and terminal structures, and discharge channels, control features, and gate chambers supporting the high-level outlet conduit are outside the scope of this document. Additional guidance relating to various components of an outlet works is available in references, such as Reclamation’s Design of Small Dams (1987a), and USACE’s Structural Design and Evaluation of Outlet Works (2003b) and Hydraulic Design of Reservoir Outlet Works (1980). Implications Abandonment of low-level outlet conduits has significant implications on the operations of the dam, including the ability to drawdown the reservoir or make water releases. The reasons for water releases can vary widely based on the purpose of the dam. Dams are built for a variety of purposes including irrigation supply for agriculture, municipal water supply for communities, power generation, storage and attenuation of water during high precipitation or snow melt, to develop/restore various types of ecosystems, recreation, or combinations of all of these purposes. The purposes, schedules, rates, and magnitudes of regular (normal-operation) water releases are generally described in a dam’s Operation and Maintenance Manual (O&M). Water releases can occur for recreation, environmental considerations, and water rights and water supply administration. In addition to water release requirements under normal operations, reservoir drawdowns may also be periodically required for maintenance, emergency operations, or in advance of predicted floods. Drawdowns mandated by Dam Safety regulatory agencies can be driven by poor operating conditions or damage to the dam, stability concerns, design issues, maintenance, or repairs. All dams are, or should be, equipped with outlet structures or systems for releasing water. Dams can be outfitted with different combinations of discharge structures with varying degrees of redundancy. Low- level outlets provide a means of controlled reservoir release and drawdown below the invert of other discharge structures. A low-level outlet system is used