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| © Eugene Brennan |
Some
more technical information here on the gauging station at the bridge over
the Mill Stream on the Mass path. The station measures water level and
flow rate. It was constructed in 1999. The weir on the downstream side
of the bridge is a flat-V type, not rectangular as I suggested in an
earlier post. These structures are used in hydrometrics for measuring
flow. They can be rectangular, V-notch, broad-crested etc. The idea of a
weir is to cause water to backup behind it and change in level. By
measuring the level of water above the base of the weir, flow rate can
be calculated. In theory, flow rate can be calculated knowing the cross
sectional area of flowing water and measuring its velocity. Simply
multiplying the two parameters together gives flow rate. So for instance
if a stream is 2 m wide and 1 m deep, and water flows at 1.5 m/s, flow
rate is 2 x 1 x 1.5 = 3 cubic metres per second. However, this would
still require water level to be measured because it varies depending on
whether there's high or low flow. More significantly however, velocity
varies within the flow. (Fast in the middle of a stream and slower
towards the edges and bottom because of drag). So making a velocity
measurement at one point would be inadequate. Using a weir simplifies
the calculations. Various types of weirs have been studied and
empirical equations worked out (i.e. by making measurement from models,
not worked out analytically) giving flow rate versus water level. Level
is usually measured using an ultrasonic transducer that sends out a
sound beam and detects an echo. Knowing the transit time of the beam and
the speed of sound, distance can be measured to the water surface,
which is then converted to a level measurement (similar to the way an
ultrasonic measuring tape works).