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<br />VII. 3 <br /> <br />001210 <br /> <br />The investigating hydrologist would be well advised to verify source <br />and antecedents of each capacity table. Many such tables have been based <br />on inadequate surveys of reservoir-site configuration, or derived from <br />operating records of inflow, outflow, and reservoir stage together with <br />records or estimates of evaporation and of minor factors in the water <br />budget. Capacity tables so derived are no better than the "weakest link" <br />in the chain of data put into (or omitted from) the derivation. They may <br />be adequate as a guide for reservoir operation, but misleading in a <br />hydrologic appraisal. <br /> <br />A once-accurate capacity table may become outdated owing to <br />acc~ulation of sediment in the reservoir. Such accumulation can be <br />determined periodically by repeat surveys along monumented ranges spaced <br />according to particular situations--closely spaced and localized if the <br />sediment is coarse and forms only deltaic deposits at the reservoir head; <br />widely spaced if the sediment is fine and is deposited extensively. <br /> <br />In such repeat surveys, an error in horizontal position may lead to <br />a fallacious value for thickness of accumulated sediment, especially over <br />a steeply sloping bottom. This type of error may be disclOsed by plotting <br />successive cross-sections one over the other,' or by drawing a profile of <br />highest apparent deposition; discordant depths of accumulation can then be <br />re-examined or even verified by "spud bar" samples. Adequately controlled, <br />sonic sounding is a convenient means for resurvey of reservoir capacity, <br />provided bottom slopes are flat or moderate. If slopes are steep, as <br />along either edge of a canyon reservoir, sonic measurements are likely to <br />be considerably in error. <br /> <br />Systematic errors in principal gage <br /> <br />Reported reservoir stage may be systematically in error by several <br />tenths of a foot at certain seasons, owing to the manner in 'Which recording <br />instruments are installed. For example, in a stilling well that is <br />insulated from the open reservoir by being placed deep within the mass of <br />a dam or one of its abutments, temperature and resultant density of the <br />water may at times differ substantially from temperature and density in the <br />open reservoir. Because the well and the reservoir act as the two arms of <br />a manometer with connection through an inlet pipe, any differenee in mean <br />density of the water in the two arms will result in a difference between <br />inside and outside water levels. Error in recording true level of the <br />reservoir follows. Magnitude of this error is proportional to length of <br />the two columns of water above the inlet pipe and to the difference in <br />mean temperature of those two water columns. This source of error can <br />be minilnized by providing several valved inlets at different levels and <br />opening only the highest inlet that will function, or by frequ.ently <br />resetting the recording gage in the stilling well to agree with a staff <br />gage that accurately measures the outside reservoir level. <br /> <br />