Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment Dams216• Length.—As with depth, the conduit length becomes a factor relating to theamount of time the diver has available at depth. If the conduit is extremelylong, it can take much more time to inspect than the diver has available. Theavailable dive time for a long conduit can be increased, but this can be costly.Safety also must be considered. Because the diver does not have a direct pathto the surface, the farther the diver must penetrate into the confined space, thefarther the diver is from a direct path to the surface.• Access.—Often the entrances to conduits are equipped with trashracks on theinlet side. The ability to remove enough of the trashrack bars to allow easyentry and egress is important. Since divers in such an overhead environmentwill be utilizing some type of surface-supplied breathing gas, it is important thatthe access point be such that the hoses will be able to be fed into the conduitwithout hanging up. A second diver is required to be stationed underwater atthe confined space entry point to tend the primary diver’s umbilical.• Leakage and currents.—The leakage of downstream gates or valves in a conduit isa safety factor that can affect whether a dive inspection can be safelyperformed. Currents can be unpredictable. Any inspection of this type shouldbe performed, such that the diver enters the conduit against any current andthen returns and exits with the current. In the case of an inverted siphon, thiscan be accomplished by entering from the downstream end, but in the case ofan outlet works, a submerged conduit will more than likely need to be enteredfrom the upstream end. Therefore, the condition of the gates or valves andhow much leakage is exhibited is a big factor with respect to the viability of adive inspection.• Conduit size.—A conduit should really be large enough that the diver can turnaround inside and exit head first. The size for this will obviously depend on thesize of the individual diver and also the exact type of equipment required.• Visibility.—The distance a diver can see is important to whether a diveinspection of a conduit is advisable. In poor visibility situations, the diver canuse their sense of touch for inspection. Sometimes a diver can use a hand toprobe areas that cannot be seen. In the event of zero visibility, there wouldlikely be little reason to pursue a dive inspection, as the shear magnitude of theentire surface of a conduit would be extremely difficult to inspect by touchalone. Also, in a circular conduit, a diver does not have a real edge or otherreference point to keep track of any findings. If a dive inspection (figure 123) isplanned for a conduit, consideration should be given to making a large releaseprior to the inspection as a means of flushing sediments from the conduit andthen allowing some amount of time for the water to settle out prior to diverentry. This time will depend on the type of sediments in the water, but couldvary from a day to a week. If visibility is good, the diver may want to use a high