Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment Dams2083. The quality of any seepage, especially whether it is carrying soil particlesshould be analyzed. Water seeping into, out of, or along a conduit cancause problems by carrying particles with the flow. If the quality andquantity of the water flowing into the conduit is different from the waterflowing out of the conduit, then it is likely that open joints or cracks areallowing additional seepage flow to enter the conduit, or normal dischargeto leak out. The appearance of the flow at the area where water outletsfrom a seepage diaphragm is of particular concern. Any water flowing inthe vicinity of the conduit should be observed for evidence of fines beingtransported, such as cloudiness or discoloration. The internal erosion andbackward erosion piping processes can occur intermittently, with finesbeing transported sporadically. Evidence of fines being carried in seepageis cause for concern, further investigation, and prompt action.The monitoring of any condition involving seepage or discharge should alsoinclude the corresponding reservoir pool level. Any sudden change, or unusualtrend over time, which does not correspond to changes in the reservoir level,could indicate a seepage problem. For example, an increase in the seepage ratewhile the pool level is constant could be an indication of internal erosion. Poollevels may be measured by a staff gauge, by calibrations placed on a fixedstructure in the reservoir, or by water-level sensing devices.• Look for signs of internal erosion or backward erosion piping where theconduit exits the downstream slope of the embankment dam near the terminalstructure. Water flowing through cracks in the earthfill or along the conduitmay erode soils and cause a cloudy effluent with turbulent flow. Deposits ofsand may form at the exit point of seepage. Water escaping from intergranularseepage in granular soils may create sand boils, and the flow is less likely to beturbid. Other indicators of developing problems include deposits of sedimentnot associated with runoff, sinkholes, and signs of settlement, such asdepressions on the surface of the embankment dam or its foundation.• Any changes in the embankment dam or foundation in the vicinity of theconduit. Since the location of a conduit represents a unique condition in theembankment dam, and a potential seepage path through the dam, any changesin the vicinity of the conduit should be investigated. Such changes mightinclude slope movement, changes in vegetation, areas of new or unexpectedwetness or seepage, unusual piezometric readings, etc.• Check the exposed areas of the conduit for cracking, weathering, and/orchemical deterioration.• Look for any whirlpools in the reservoir in the vicinity of the conduit.