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10 Spillways on Small Dams Introduction The purpose of a spillway is to safely convey reservoir inflows over and/or around a dam to the natural drainage channel, up to and including the Inflow Design Flood (IDF), thus protecting the dam embankment from failure due to overtopping. The IDF varies according to jurisdiction and dam hazard classification. In addition to providing sufficient flood capacity, the spillway must be hydraulically and structurally adequate and must be located so that spillway discharges do not erode or undermine the dam. Flow over spillways is a designed event and are usually not a cause for alarm on an appropriately designed and constructed dam and spillway. Different states in the US categorize spillways differently. For example in Colorado, the State Engineer’s Office (SEO) document titled Rules and Regulations for Dam Safety and Dam Construction defines an auxiliary spillway, as defined by US Bureau of Reclamation (Reclamation), as an emergency spillway. It is therefore recommended that the reader consult relevant state guidelines/legislation to understand the terminology used in each state. Design of Small Dams (Reclamation 1987) categorizes spillways as service, auxiliary, and emergency. These are described below. Service Spillway: A service spillway is the overflow structure designed to limit or control the operating level of a reservoir, and the first spillway to be activated. Service spillways are designed to pass part of the IDF unless the service spillway is unavailable at the time of flooding due to damage, blockage, or inoperability. Service spillways are designed to pass floods that occur frequently and damage to a spillway from the passage of these floods would not normally be tolerated. Service spillways rarely appear on small water supply dams, but frequently appear on flood control dams. Auxiliary Spillway: An auxiliary spillway is used in conjunction with a service spillway, if present, to pass the IDF. Auxiliary spillways are designed to operate for floods in excess of the flood flow used for the design of service spillways or, if a service spillway were not present, all inflows that are not stored or released through the outlet works. If used in combination with a service spillway, an auxiliary spillway may not need to be designed for the same degree of safety required for other structures and some flood damage may be considered tolerable for more infrequent flood events. In some cases, this may offer considerable construction cost savings. Spillway damage that would affect the ability of the reservoir to retain water or would threaten the integrity of the dam would not be tolerable. Flood damage to an auxiliary spillway and the associated repair and maintenance costs need to be considered during the design process. Auxiliary spillways are a common form of spillway on small dams and are the main focus of this article. Emergency Spillway: Emergency spillways are provided for additional safety should emergencies not contemplated by normal design assumptions arise. Such emergencies may include damage to, or issues with, the service and/or auxiliary spillway or damage/malfunction of spillway gates. Under normal reservoir operation, emergency spillways are never required to function and the crest level is set above the maximum reservoir water surface resulting from the IDF. Emergency spillways rarely appear on small dams, particularly small dams with a passive (ungated) auxiliary spillway. The design of a spillway that is both functional in terms of design, and economical from a construction cost standpoint, requires careful consideration of the associated topography, the geology, the spillway type, maintenance and operational requirements, the IDF, the dam type and the proximity of the dam to the spillway. To select an optimized and economical spillway and dam configuration, construction cost estimates are typically developed for various combinations of spillway capacities, spillway types, and dam heights. If possible, a spillway should be located independently of the dam itself, on the left or right dam abutment, or in a saddle or depression along the reservoir rim. The locations considered should lead to a natural waterway or a gently sloping abutment where an excavated channel can be constructed beyond the dam to avoid the possibility of damage to the dam or other structures.