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8 Letting it All Out: Hydraulic Design of Outlet Works Introduction The purpose of an outlet works is to regulate or release water impounded by a dam. This is done for a variety of reasons, including (1) passage of storm or run-of-river inflow; (2) releasing flow to meet demands downstream; or (3) draining the reservoir (in the case of a low-level outlet). In some cases, the outlet works might also be referred to as the principal or service spillway, if it is the primary outlet used to control the reservoir level. This article will discuss the major components of outlet works, the hydraulic analyses required to size and design each component, and general design considerations associated with outlet works of small dams. Structural analyses are an important component of outlet works design but they are not addressed in this article. Reservoir operation requirements for a dam vary based on federal, state, or local regulations, utility providers, and in some cases private individuals, group owners, or stakeholders. Because no two dams are the same and their operations, obligations, and impacts are specific to individual circumstances, this article will discuss outlet works design in generalities. For additional specific design information, the reader is encouraged to acquire the Embankment Dam Reference Toolbox (EDRT) described on Page 1 of this technical note and available from ASDSO. Outlet Works Components Intake Structures – Intake structures are those that draw water in from the reservoir for release. They can be located within the dam (with an inlet that extends to the reservoir), or immediately upstream of a dam as a free standing structure within the reservoir. It is often desirable to incorporate a combination of intakes within a single structure. For example, a gated conduit located near the bottom of a reservoir allows for draining the reservoir (see the article How Low Can You Go (Vol. 2 Issue 3) from our last issue), while a drop inlet located high in the reservoir allows for the uncontrolled spillway-type release to maintain normal pool levels. Intake structures can either be gated or un-gated (aka unregulated or uncontrolled). Most commonly, the intake for a principal or service spillway outlet is uncontrolled. Some examples include risers, drop inlets, and weir towers. Intakes set at elevations lower than the normal pool levels are generally gated. The elevation of the intake crest or sill is dependent on the desired control elevation of the reservoir. Gated intakes set at elevations below the maximum desired operating pool level may be required to control temperature of released water, to manage water quality considerations; and for reservoirs that have large fluctuations in pool levels, to allow release of water during low pool seasons. Photo 1: A glory hole is one type of drop inlet The choice of intake structure(s) depends on a number of factors including design capacity, available materials, cost, maintenance requirements, and degree of control required. See References [9], [10], and [13] for more information regarding the design of intake structures. Trash Racks – It is possible for debris in the flow to clog or damage the outlet works, especially during flood events. Trash racks are typically constructed around the intake structure to capture debris, which can later be removed. Trash racks reduce the hydraulic capacity of the intake structure, especially if they are designed with small openings or become clogged. Therefore, it is necessary to size the trash racks appropriately and ensure that they are regularly maintained. A good rule of thumb is: the rack spacing should be half the diameter of the pipe to pass small debris but also catch big debris that might clog the pipe.