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20  What method should be used for collecting data?  What threshold level should be established for each instrument as a guideline for recognizing an usual reading? Answers to these questions will guide the decision to which instrument is best suited for the job. Selecting the right instrument includes having an idea on the known parameter. Do I need a 90-degree v-notch weir or a parshall flume? Should my piezometer be an open standpipe or a nest of fully grouted vibrating wires? The following table is one example that can be used in selecting the right instrument. This assessment is recommended to insure selection of the correct instrument to serve the required purpose. Note that the examples shown in the table are for illustration purposes only; actual information and recommendations would be very site specific depending on the potential deficiency and site specific conditions of the dam. Table 1: Instrument Planning Table Problem / Deficiency / PFM What information/parameter does the Instrument need to provide? At what location is the parameter best measured? (e.g. where is the source of water?) Construction Recommendation Potential Problems with Installation Reading Method and Frequency1 Example: Saturated foundation, dry embankment Are foundation uplift pressures present? Is a confining layer present? If so, how extensive? Fractured bedrock Water present at approximately 20 ft deep in boreholes 2 piezometers in same 6" borehole 1 in fractured bedrock 1 in presumed cap layer Isolating strata, can bedrock be augered through? Manually read with water probe, monthly Example: Seepage observed at downstream toe Flow rate of seepage Is sediment present within the flow? What is the relationship or time lag with pool level – does it stop or increase at different pool levels? Collect seepage at a location near downstream toe 90o v-notch weir Collect into an upstream basin to monitor for sediment Install water level gauge Collect and convey seepage to single point, avoid seepage bypassing weir. limit adjacent runon if possible Manually read water level and convert to flow (gpm), monthly 1. Record reservoir level and recent precipitation at every reading Implementation (Overview of Design Considerations) Key design considerations when implementing a monitoring program include:  Instrument Location  Instrument Design  Installation and Protection The following are some considerations for picking the best instrument location. Limiting impacts of the existing structure should be considered when selecting the preferred location. For example, in order to monitor the phreatic level within the foundation, is it necessary to drill the piezometer through the embankment or can it be installed at the toe? The location should consider both the existing conditions as well as the post-installation conditions. For example, toe drain outlets that look great during installation, may become inundated over time if installed in a location that does not allow positive drainage. The ability to measure the instrument should be considered when selecting the installation location. Can you get a bucket underneath the outlet pipe to measure outflow using the timed-bucket method? Staff gauges need to be easily visible from the shore, and within calm water, far enough away from any water intake gate, channel, weir or flume to avoid inaccurate readings due to drawdown effects. A staff gauge should be placed in a sheltered area that is not influenced by wave action or susceptible to ice damage.