Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment DamsParticular attention should be given to seepage exiting ground conduits and thequantity of seepage flowing out of conduits. The quantity of seepage entering aseepage collection system is normally directly related to the level of the water in thereservoir. Any sudden change in the quantity of seepage collected without apparentcause, such as a corresponding change in the reservoir level or a heavy rainfall, couldindicate a seepage problem. Similarly, when the seepage becomes cloudy ordiscolored, contains increased quantities of sediment or changes radically in chemicalcontent, a serious internal erosion or backward erosion piping problem may bedeveloping. Moisture or seepage at new or unplanned locations on the downstreamslope or below the embankment dam also may indicate a seepage problem. Seepageshould be monitored regularly to determine if it is increasing, decreasing, orremaining constant as the reservoir level fluctuates. A flow rate not changing relativeto a reservoir water level can be an indication of a clogged drain, internal erosion orbackward erosion piping, or internal cracking of the embankment dam.Seepage may be measured with weirs of any shape, such as a V-notch, rectangular, ortrapezoidal; flumes, such as the Parshall flume; water exiting a pipe measured with astopwatch and bucket; and flowmeters. When a new seepage area that producesmeasurable flow is identified at an embankment dam, the seepage should bemonitored and, in some cases, measured. A qualified engineer should promptlyevaluate each new seepage area. In some situations, a change in the seepage regimeprecedes failures. The flow should first be confined and directed away from theembankment dam by excavating drainage channels or ditches. Then, the quantity ofseepage can be measured by creating a large enough drop in the drainage channel toinstall a pipe, weir or flume or to facilitate the measurement of the flow by means ofa stopwatch and bucket. The integrity of the seepage measurement devices shouldbe maintained so that seepage does not bypass the device and the device is kept clearof obstructions.Points where seepage measurement devices are added are often a good location tomeasure the amount of sediment that may be carried in the seepage. Sedimenttransport is often a sign of internal erosion or backward erosion piping failuremodes. Providing an area adjacent to a weir where water flow is stilled can allowsome of the sediment in the water to fall out and collect with time.Seepage into conduits should also be monitored where it is determined to beimportant. Note that if the seepage into a conduit is transporting material,operations of the conduit may be transporting material out of the conduit.Frequently, the highest seepage gradient at a site is associated with seepage into anonpressurized conduit. For this reason, inspection of the conduit is important.The internal erosion and backward erosion piping processes are frequentlyintermittent, and in many cases, the transport of materials in the seepage is sporadic.Inspection should look for signs of deposits, as well as clarity of the seepage.238