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15 370 feet. The method relates Froude number and air demand ratio and is generally applicable for slide and tractor gates operating in rectangular gate chambers. The envelop design curve may underestimate air demand in some cases, such as for Beltzville Dam, where actual air demand was 5 times higher than the air demand derived from the design envelop curve. This illustrates the necessity for the designer to check the limitations and applicability of a given method to ensure the specifics of their projects are consistent with the methods being employed. A spreadsheet that employs this design method is attached to this document. The 2011 paper titled, Determining Air Demand for Small- to Medium-Sized Embankment Dam Low-Level Outlet Works presents a design method for estimating air demand and sizing the air vent based on laboratory- scale low-level outlet tests with an inclined gated inlet on a 3H:1V slope. The design methodology presents a series of design curves that relate gate geometry (and corresponding discharge coefficient), driving head, gate opening (10, 30, 50, 60, 70, and 90 percent), and air demand ratio. The design method uses an envelope curve of all the observed model data; with the limitation that parameters such as conduit length and air vent geometry (and associated head losses) were not considered in the model, and the method may not be applicable for gates with inclinations different than 3H:1V. The 2008 thesis titled, Air Demand in Free Flowing Gated Conduits summarizes empirical design methodologies developed by previous researchers, and presents observations on significant parameters developed from a laboratory model study. The parameters studied included: Froude number, ratio of head to gate opening, surface water roughness, conduit length, and conduit slope. A possible limitation of this study is that the model air velocity measurements were not sufficiently detailed to draw conclusions. Air Vent Design Criteria and Guidelines The following criteria and guidelines are commonly employed in air vent design practice:  Limit maximum air flow velocity in the air vent to approximately 100 feet/second by increasing the vent size as necessary; above this velocity an objectionable, whistling noise occurs that can be damaging to hearing.  For safety reasons, keep children away from vent openings, and place personnel barriers around vents if the air velocity is expected to exceed approximately 50 feet/second.  A minimum air vent diameter of 4 inches should be used for all cases to facilitate vent cleaning and maintenance.  For valves, the air vent is typically located upstream from the point where the water jet impinges on the conduit walls.  If the air vent is of sufficient size to interrupt rebar in the conduit wall, use a series of smaller, side-by-side air vents.  Install an air vent through HDPE and CIPP pipe liners if there is susceptibility to internal vacuum pressures and liner collapse.  If steel vent pipes are used and will be in contact with corrosive soils, design appropriate cathodic protection, or use a protective coating or wrap.  A typical configuration for the end (open to atmosphere) of the air vent is to include a 90 degree elbow (see Figure 4) with an expanded or bell-mouth opening oriented away from the prevailing winds, with a stainless steel screen over the opening, which will help prevent debris from entering the vent, and help prevent water from entering the pipe, which could result in freezing blockage during the winter.  Avoid air vent design features that could result in large head losses such as a small-mesh steel screen, or an excessive number of vent pipe bends.  Take precautions against small objects (e.g., rodents, clipboards, etc.) getting sucked into the vent and creating a potential blockage; periodically inspect the air vent to ensure air is flowing freely through it and that there are no