Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment Dams• Blockages at the conduit entrance (i.e., trash or debris) or at the exit (i.e.,vegetation, backed up water).In attempting to inspect the interior of any conduit, there may be difficulties toovercome, such as:• Unwatering.—A comprehensive inspection may be hindered, unless the conduitcan be unwatered. Proper precautions should be considered prior to anyunwatering situation. The possibility exists of external pressures being highenough to damage the unwatered conduit or vents being plugged, causingnegative internal pressures to develop and collapse the conduit. This is aconcern when pressurized conduits are unwatered. Unwatering a conduit maybe impractical or impossible for one or more of the following reasons:1. Lack of a bulkhead or closure device.2. The need to limit reservoir drawdown. Lowering of the water surface maybe restricted, which would prevent exposure of the conduit or entrancestructure.3. Structural adequacy of the conduit to withstand external hydrostaticpressures in a unwatered condition.• Poor air quality.—Poor air quality may exist within conduits. Poor air qualityconditions may include lack of oxygen and the existence of hydrogen sulfide.• Inaccessibility.—The conduit may be too small or too dangerous for man-entryinspection. The use of CCTV inspection equipment should be considered forinaccessible conduits. If this is not feasible, the inspection must then be basedsolely on the condition of the exposed and/or accessible portions of theconduit. Some details of the interior may be obtained by use of a bright lightand the zoom feature of a camera.For an examples of a man-entry inspections of a conduits, see the Dalewood Shoresand Salmon Lake Dam case histories in appendix B.9.5.3 Inspection of terminal structuresThe terminal structure may be dry or partially inundated, depending on the time ofyear and the schedule of releases through the conduit. If the terminal structure ispartially inundated, inspection of the structure above the water level will be fairlystraightforward. However, inspection of the portion of the structure below thewater level, such as the basin, chute blocks, baffle blocks, or end sill, will requirespecialized inspection services.212