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19 Dam ownership, regulation, and operation vary from the federal government, state government, local municipalities, utility providers, and in some cases private individuals or group owners. Because no two dams are the same and their ownership, operations, obligations, and impacts are very specific to their individual circumstances; this article will discuss these topics in generalities. Alternatives for Water Conveyance For cases where rehabilitation or replacement in-kind is not an option, alternative means of water conveyance should be designed and installed prior to abandonment of the conduit. The selection of a means that is appropriate depends on the size of the reservoir, the physical features of the particular dam site, the availability of equipment and materials, the volume of water that may need to be released, and the required rate of release. Care should be employed in determining the means of reservoir evacuation during a specific emergency, to ensure that the reservoir releases do not cause loss of life or significant property damage downstream. The preferred and recommended method is to install a new low-level outlet. In situations where this is not feasible, installation of a siphon or high-level outlet conduit may be able to provide an alternate means of water conveyance that meet the operational needs and requirements of the dam. Requirements of the design shall include at a minimum: accommodating the desired range of releases, protecting against accidental overfilling, controlling normal reservoir level, and providing for emergency drawdown. Combining a high level outlet or siphon with an auxiliary spillway and/or overflow spillway may be used or needed to meet the requirements of the reservoir. Each method has its own advantages and disadvantages. The long term performance of the final selection and public safety considerations, rather than cost, should be the basis for the selected design. Installation of a Siphon Siphons can often provide alternative reservoir drawdown capability for low hazard dams. A siphon is generally installed over the dam or spillway, providing a safe and easily constructible, but usually temporary, outlet option for dam owners (See Figures 3 and 4). Figure 3. Siphon used to lower the reservoir water surface through the upper entrance of an outlet works intake structure. [1] Figure 4. A simple siphon constructed over the crest of an embankment dam. [1] Siphons used in reservoir drawdown operate by atmospheric pressure pushing water over an obstacle (i.e., reservoir water over an embankment dam) and discharging on the other side at a lower elevation than the reservoir. The maximum height, or lift, of a siphon is limited by the atmospheric pressure at the site. The height a siphon can lift water will, therefore, be lower for dams at higher elevations (for instance in the mountains of the western United States). There are several parameters that must be evaluated when establishing the feasibility and design of a siphon. Bernoulli's equation can be applied to estimate a siphon’s maximum lift, discharge capacity, diameter, and pressure. Articles in previous issues of Western Dam Engineering Technical Note have discussed methods for rehabilitation and replacement of outlet conduits and are referenced below: • Volume 1: Issue 1 – “Low-Level Conduits – Rehab or Replace?” • Volume 2: Issue 2 – “You Con-du-it; How to Fix a Leaky Pipe” • Volume 4: Issue 1 – “You Down with CIPP? – Yeah! You Know Me!” • Volume 5: Issue 2 – “Cellular Grout Use in Conduit Sliplining”