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22 to dewater a reservoir for inspection or construction activities or in the case of emergencies, and can also support regular water releases as summarized below. The decision to abandon the low-level outlet conduit without replacement results in the loss of all of the above functionality, which can prove to be detrimental in an emergency. Outlet abandonment may also result in more frequent use of the primary, auxiliary, or emergency spillways. More frequent operation of the spillway structure(s) may be undesirable as compared to passage through the outlet or lowering the reservoir level in advance of flooding by a controlled operation. Low-level outlet abandonment may alter the flood capacity of the reservoir and have impacts on the communities and environments both upstream and downstream from the dam. For the reasons listed above, abandonment of a low- level conduit without in-kind replacement is typically only applicable to smaller sized dams with no minimum flow or release requirements. The owner/designer should evaluate carefully the hydraulic impacts, effects of loss of use on operations, and needs for alternative water conveyance as a result of loss of use of the low- level conduit. Furthermore, the advantages of leaving the conduit in place must be weighed against the concerns of creating possible seepage paths, which could cause future problems, and continued conduit deterioration. For more information on the needs and considerations for low-level outlet conduits, see the previous Western Dam Engineering Technical Note article Volume 2 Issue 3: “How Low Can You Go? The Needs and Considerations for Outlets.” Means and Methods of Abandonment The most common method to abandon an existing conduit is by backfilling with grout or flowable concrete. This method is discussed in more detail in the following paragraphs of this article. Detailed information on conduit abandonment by grouting is provided in the FEMA Technical Manual, Conduit through Embankment Dams. Two methods are usually considered for backfilling with grout: • From upstream, downstream or center access—If conduit access is available from either upstream, downstream or center locations, these typically provide the simplest method for filling with grout or concrete. Removal of a portion of the entrance, terminal or center control structures may be required to attain sufficient access. • Through holes drilled from the surface of the embankment dam—When the upstream and downstream ends of the existing conduit are inaccessible and there is no center control structure, it may be possible to fill the conduit with grout or concrete through holes drilled from the surface of the embankment dam (Figure 5). To be successful, the precise location of the existing conduit must be determined, and the driller must be experienced and proceed with caution. The following are possible reasons or needs for water release that may be impacted by conduit abandonment: 1. Supply – Downstream releases to supply irrigation canals, pump stations, water treatment plants, and recreational waterways. 2. Water Rights Administration – Releases to satisfy downstream senior water right calls, out-of-priority storage and/or augmentation of evaporative losses. 3. Seasonal Operation –Provide storage space prior to seasons of high precipitation and/or snow melt runoff. 4. Flooding – Release of water stored during infrequent but significant precipitation events to reduce peak flood discharge downstream. After the event, the reservoir is lowered at a controlled rate to normal operating level. 5. Sediment Flushing – Scheduled releases to flush sediment to manage undesirable sediment accumulation. 6. Environmental/Biological – Releases to benefit downstream ecology and habitat. 7. Inspections, Repairs, & Modifications –Provide safe reservoir levels to inspect, repair, or construct modifications to dams. 8. Damage/Distress – Emergency evacuation of reservoirs as fast as safely possible to reduce risk of failure in case of damage to dams during extreme events or emergencies.