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XXVI
crew camp, daily rating performance may be expected, at least once whenever change of stage per day is
over 30 centimeters. Fixed boat by cable way across the river is one of typical means of flow measurement by
current meter for medium river, 100 m wide. Flow wider river, the boat is fixed in position by anchors.
The river cross section is requested to be made simultaneously during flow velocity measurement, so
that shifting of the section could be determined. This request is lawful because there had been in the older
day fashion of using cross section made during dry season for discharge computation.
The German A. Ott current meter has so far been adopted as standard equipment. The multiple -
point method has been used as velocity measurement practice. At present the two - point method is carefully
practiced.
Office data processing
The standard techniques used overseas are adopted in processing the data. In fact the standard
methods of U.S. Geological Survey and U.S. Department of Agriculture (for small streams) are adopted as
general basis.
Development of rating curve - Yearly mean rating curve is generally used as a tool in determining the
daily flow. Periodically developed rating is frequently used when dictated by the conditions, so that the
shifting control could be eliminated or reduced. Zero flow elevation of the rating section is determined by trial
and error method with logarithmic plotting of the rating curve. Extension of the logarithmic rating line is
frequently used to determine the channel flow at the over bank flow stage. The over bank flow is assessed by
either direct velocity measurement or slope - area method whenever applicable.
Determination of daily mean flow - Application of the adjusted daily mean gage height to rating table
gives the corresponding daily mean discharge from which the monthly and the yearly mean discharges are
computed. Shifting adjustment for observed gage height is made when necessary. Shifting design is made
basing on flood stages and times, if any. Other adjustments such as back water, rise and fall of flood, etc.,
which necessitate the use of other parametric factors, are also made, if any.
At some gauging stations where rises and falls of floods are immediate, hourly basis of gage height
application to the rating curve is used to determine daily mean discharge.
At stations where rises and falls of streams are gentle, gage height data are usually obtained by
manual reading.
It has been found that application of 1 to 5 times average reading (for daytime) gives an error of daily
mean flow not more than 15% for mountainous streams, less than 10% for larger valley streams and not more
than 5% for lower deltaic streams. Most of mountainous and valley streams are under recording operation,
only deltaic streams that daytime reading are still used.
The daily mean gage heights are obtained by arithmetic mean of the observed data available.
Adjusted gage heights due to shifting, which are applicable to rating curve for daily mean flow computation,
are practically made.