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18 measuring small, or large, flows than others. A weir can be installed to measure seepage flow and sediment transport out of a toe drain system or known surface seep that can be concentrated into a channelized path. A V-notch weir is shown on Figure 2 and Photograph 2. V-notch weirs are efficient at measuring low flows (less than about 450 gallons per minute (1 cubic foot per second)). The depth of water is typically measured with a staff gauge installed within the upstream pool, away from velocity drawdown effects as shown in Figure 2. Figure 2: V-Notch Weir and Staff Gauge Reference: FEMA, Dam Safety: An Owner’s Guidance Manual, 1987 Photo 2: V-Notch Weir Other seepage monitoring devices and methods include parshall flumes, pipes (toe drains), timed- bucket methods, and flow meters. Internal erosion can be detected by observing the water for increased turbidity and sediment. The Bureau of Reclamation provides a useful reference for details on methods to measure seepage in the Water Measurement Manual. Reclamation also provides download links for obtaining the USBRWeir.xls spreadsheet, used to determine rating tables and equations for sharp-crested weirs. Piezometers Piezometers installed in the abutments, foundations, and embankments of a dam are used to monitor phreatic surface levels, uplift pressures, and seepage gradients (through the use of piezometer groups). A line of piezometers installed at the dam crest, mid- slope and toe provides information to create a profile of the phreatic surface through the dam. A set of piezometers installed at different elevations in one location (nested piezometers) provides pore pressures in different soil strata and can be used to estimate vertical seepage gradients. Some common types of piezometers include open standpipe, fiber optic, and vibrating wire piezometers. The open standpipe, or observation well, installation offers the added benefit of being able to manually confirm the water level reading versus sole reliance on a digital instrument. A standpipe piezometer is shown in Photograph 3. Fiber optic piezometers are generally contained within a stainless steel tube and separated from the environment by a porous filter material. Some advantages include their immunity to vibration, lightning damage, and radio and electromagnetic noise interference. Vibrating wire piezometers contain electrical pressure transducers that read and record the pore-water pressure automatically, allowing for easier and more frequent data acquisition. Each of these types of piezometers has specific design specifications and a qualified engineer should be involved in specifying the appropriate piezometer type, location, and installation procedures. Piezometers, when strategically placed, can provide useful information on the overall seepage regime within and below the dam. However, since they are a single point measurement, they are not often effective at detecting or characterizing potential concentrated seepage paths unless they are installed to monitor an already known location of a defect.