the range of field conditions in which they are to be used (Novak, 1957;
Hubbellet al., 1981).
8.4.6 Water qualityThe quality of water is judged in relation to its use (e.g. irrigation, drink-
ing, etc.) and its suitability to the aquatic life. Quality is defined by several
parameters such as its pH value, BOD, COD, TOC, etc., and for each cri-
terion there can be a variety of determinants.
These parameters are usually determined by laboratory tests on
samples collected from the river water, carried out either in situ(mobile lab-
oratory) or in central laboratory facilities. Continuous in situmonitoring is
preferable in order to avoid possible changes in characteristics due to trans-
portation and the time taken between sampling and analysis. The quality
standards are formulated on a statistical basis, and are more flexible for uses
of abstracted water since it is possible to improve it by treatment.
Dissolved oxygen (DO) sensors, pH meters and suspended solids (SS)
measuring devices, continuous monitors of temperature, ammonia, etc., are
commonly used to measure water quality. The reader is referred to James
(1977) for the quality requirements of water in relation to its uses and to
Kiely (1998) and Tebbutt (1998) for discussion of water quality parameters
and their control.
8.5 Flow-measuring structures
Flow-measuring (artificial control) structures are built across or in the
streams to be gauged. The flow is diverted through the structure, creating
critical flow conditions (flow upstream of the structure being subcritical),
thus producing a unique relationship between the discharge, Q, and stage,
h(upstream water level above the sill crest of the structure), in the form
QKhn (8.38)whereKandnare dependent on the type of structure which, in turn,
depends on the size of the stream and the range of flows to be gauged and
on the sediment load carried. Gauging structures can be divided into two
categories: (a) weirs – sharp-crested (thin plate) weirs or notches, spill-
ways, broad-crested weirs, or Crump weirs; (b) flumes – Venturi, Parshall,
or steep slope (Bos, Replogle and Clemmens, 1984).
A weir raises the upstream water level above its sill, water flowing
over it under critical conditions. They are not suitable in debris- or