Laserfiche WebLink
<br />00145S <br /> <br />Ice effects <br /> <br />Icing' in streams influences the accuracy of flow records in ways <br />too numerous and complex to review here. Published records commonly <br />identify the periods of ice effect, and usually fill in such periods <br />with estimated or average values of daily flOlf. Such estimates were <br />made with information at hand as to the surrounding circumstances and, <br />ordinarily, reliable adjustments are not practicable subsequently. <br />In general, it is advisable to include such possibly erroneous records <br />in hydrologic studies, but to give them less statistical weight. In <br />this connection it is wortQy of note that, during freeze up, a considerable <br />quantity of water may be :i1nmobilized as ice or detained in channel storage <br />behind ice;ttdams"; thus, streamflow cOlllll1only is less, in some instances <br />surprisingly so, than would be normal otherwise. Conversely, for a time <br />after the subsequent thaw, streamflow may be abnormally large. <br /> <br />Surface velocities <br /> <br />Some measurements of flood flows, and station ratings derived from <br />them, have: been based on velocities measured only at the water surface <br />or the 0.2\ depth, either by timing floats through a reach of kJiOWll length <br />or by current meter. Such measurements have been made not only when an . <br />approximate value of flow would suffice, but also when drift or extremely <br />large dep~s or velocities preclude use of the current meter in the uSUal <br />way. FlumeS in which the ratio of depth to width is large cOlllll1only have <br />been so me,sured. <br /> <br />Coefficients for reducing surface or 0.2-depth velocity to mean <br />velocity have been determined from curves that plot 0.2-depth velocities <br />against mean velocities for all high-water measurements that define the <br />station rating or for a follOlf-Up measurement made as Soon .after the <br />flood peakiasconditions permit. Of coefficients so determined for <br />stations o~ natural streams, the majority fall between 0.84 and 0.92; <br />however, sQme are as small as 0.80 or as large as l.00, depending on <br />depth and roughness of the channel. In measuring the discharge of <br />rectangular concrete channels below Hansen Dam, using small wood blocks <br />as floats, ,the Los Angeles District of the Corps of Engineers found the <br />coefficient to range between 0.95 and 0.99--that is, to be substantially <br />greater than is ordinary for natural channels. <br /> <br />In ge~eral" measurements based only on surface velocity should be <br />considered as approximations only. Whenever floats are used, they should <br />be described in the field notes, because results may vary somewhat accord~ <br />ing to shape and dimensions of the parts submerged in, and projecting <br />above the water, respectively. <br /> <br /> <br />1.5 <br />