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14 reservoir or impoundment. Criteria vary by location and governing agency, but generally require the reservoir to be drawn down at a specified rate for inspection/maintenance purposes or in case of an emergency. The draw-down rate is a function of the combined stage-discharge relationship for the outlet works. A hydrologic modeling program such as HEC- HMS is commonly used for this analysis, although it can be done using a simple spreadsheet as well. A key consideration for water storage dams is to not draw down the reservoir too quickly. When the reservoir is full, the embankment becomes saturated and pore pressure within the embankment is high. During rapid drawdown, the pressure decreases on the upstream face of the embankment much more rapidly than the pore pressure within the embankment, causing instability that may result in failure of the dam. A discussion about reservoir drawdown was presented in How Low Can You Go (Vol. 2 Issue 3). Drawdown criteria vary by location. Summary Outlet works are a key component of most dams. Their purpose is to regulate flow through the dam, whether it is for flood control, water storage, or diversion. The major components of the outlet works include the intake structure, outlet conduit, energy dissipation device, filter diaphragm, and flow regulation and control mechanisms. Outlet works can operate as either pressurized or non- pressurized systems. It is important to avoid mixed flow conditions that may result in cavitation within the outlet conduit. Venting downstream of gates and valves within the outlet conduit is an important consideration. Hydraulic analysis of the outlet works involves evaluating the capacity of each control condition individually. Tailwater must be considered as well. Types of flow control include inlet weir control, inlet orifice control, and full pipe control with the outlet conduit. A stage-discharge relationship can be developed for each type of control and a combined stage-discharge relationship can also be developed based on the minimum flows over the range of reservoir elevations. This article presents general guidelines and considerations for the hydraulic design and analyses of outlets and provides the reader with references to more detailed approaches. Design criteria and regulations vary by location and readers should consult state and local regulations and guidelines when designing outlet works. Useful References [1] Brown, C. H., Tullis, B. P and Lindon, M. C. (2006). Air Venting Requirements for Low-Level Outlet Works, Does Size Really Matter? Association of State Dam Safety Officials. [2] Federal Emergency Management Agency (2007). Technical Manual: Plastic Pipe Used in Embankment Dams: Best Practices for Design, Construction, Problem Identification and Evaluation, Inspection, Maintenance, Renovation, and Repair. [3] Federal Emergency Management Agency (2005). Technical Manual: Conduits through Embankment Dams. [4] Natural Resources Conservation Service (2007). National Engineering Handbook Part 628 – Chapter 45: Filter Diaphragms. [5] Natural Resources Conservation Service (2005). Earth Dams and Reservoirs: TR-60. [6] State of Colorado: Department of Natural Resources. (2007). Rules and Regulations for Dam Safety and Dam Construction. [7] Tullis, B. P. and Larchar, J. (2009). Low-Level Outlet Works Air Vent Sizing Requirements for Small to Medium Size Dams. United States Geological Survey. [8] United States Department of the Army: US Army Corps of Engineers. (1975). ER-1110-2-50: Low-Level Discharge Facilities for Drawdown of Impoundments. [9] United States Department of the Army: US Army Corps of Engineers. (1980). EM 1110-2-1602: Hydraulic Design of Reservoir Outlet Works. [10] United States Department of the Interior: US Bureau of Reclamation. (1987). Design of Small Dams. [11] United States Department of the Interior: Bureau of Reclamation. (1990). Criteria and Guidelines for Evacuating Storage Reservoirs and Sizing Low-Level Outlet Works. [12] United States Department of the Interior: Bureau of Reclamation. (1984). Engineering Monograph No. 25: Hydraulic Design of Stilling Basins and Energy Dissipators. [13] United States Department of the Interior: Bureau of Reclamation. (2011). Appurtenant Structures for Dams (Spillway and Outlet Works) Design Standards. [14] United States Department of the Interior: Bureau of Reclamation. (1980). Engineering Monograph No. 41: Air-Water Flow in Hydraulic Structures. [15] United States Geological Survey, 1907. Weir experiments, coefficients, and formulas. Water Supply and Irrigation Paper No. 200. [16] Walther, Martin (2004). Guidance for Air Vents for Drop Inlet Spillways. Washington State Department of Ecology: Water Resources Program/Dam Safety Office.