Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment Damsand provide regulation of the outlet works flow. Regulating gates and valves aredesigned to provide a wide range of operation from closed to fully open. Theclosure times for gates and valves should be closely evaluated to keep water hammerpressures within reasonable limits. Guard gates are designed to provide closure onlywhen the regulating gates become inoperable or when unwatering of the conduit isrequired to inspect or repair the section of conduit downstream of the guard gates orto inspect or repair the regulating gates. In some applications, an emergency gatemay be used in conjunction with or in lieu of a guard gate. An emergency gate istypically provided only as a standby or reserve gate and is used when the normalmeans of water control is not available for use. Generally, slots are provided forstoplogs or bulkheads to be installed at the conduit entrance to allow for unwateringand inspection of the conduit. In some cases, if stoplogs or bulkheads can be quicklyinstalled during an emergency, guard gates may not be required. However, stoplogsand bulkheads are not intended for emergency closure under unbalanced conditionsor when the outlet works is operating. Specially designed stoplogs and bulkheadswould be required.The control features should allow for complete inspection by man-entry or CCTV.Certain types of gates or valves (e.g., butterfly valves) can act as an obstruction andmay preclude the use of robotic camera-crawler equipment, since it may not be ableto pass under or around the gate or valve. Alternate access using manholes may berequired to provide access around the obstruction.An important consideration in any closed conduit design for an outlet works is theproper use of air venting. Air vents can permit air to enter the conduit to preventcollapse or to prevent the formation of low pressures within flowing water, whichcould lead to cavitation and its possible attendant damage. Air vents can also beused to bleed air from a conduit prior to operation. Figure 32 shows an example ofan air vent leading from the conduit that daylights onto the surface of theembankment dam. For guidance on the location, airflow rates, and structuralconsiderations of air vents, see Reclamation’s Air-Water Flow in Hydraulic Structures(1980).The location for the control of the outlet works can be placed at the upstream end ofthe conduit, at the downstream end, or at some intermediate point. For illustrativepurposes, four arrangements for locating the control features within the outlet workshave been adapted from Reclamation’s Design of Small Dams (1987a, p. 446):• Arrangement 1—Intermediate control with downstream access (figure 33)• Arrangement 2—Intermediate control without downstream access (figure 34)• Arrangement 3—Upstream control (figure 35)60