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Flow Measurement
Measurements of surface water flow rates are important in determining
water balances (water in = water out + accumulation) and calculating
contaminant mass loading rates. Concentrations only describe the
mass of contaminant per volume. Concentrations may be high causing
reason for concern but if the flow rate is low, the total mass of
contaminant discharged could actually be fairly low. Conversely,
low concentrations might suggest low environmental impact, but if
flow rates are high, the total mass of contaminant discharged could
be fairly high. Therefore, measurements of flow rate (i.e. m3/sec)
combined with measurements of concentration (i.e. g/m3) can be used
to calculate mass loading rates (i.e. g/sec) which describe the
total mass of contaminant loading over a specified time period.
The most common method to monitor surface water flow is to use
a water depth-flow rate rating curve which correlates the depth
of flow to the flow rate based on measurements of both water depth
and flow over a range of flows at the location of interest. The
flow is estimated for the rating curve using a current velocity
meter and measurements of the cross-sectional area of the stream.
Appropriate locations for flow measurement stations need to be selected
to avoid backwater effects or other conditions that can induce errors.
Once a rating curve is developed, only water depth is required for
flow measurement. Water depths are commonly measured manually using
a staff gauge. Alternatively, float, pressure or electronic depth
measuring devices with an automatic recorder can be used for continuous
automatic measurement of depth. A V-notch weir may be used to translate
the depth of moving water to a flow rate without the need for developing
a rating curve (MEND,
2001).
Problem | Compliance
| Health & Safety | Sampling
| Analytical | Data
Quality
Site Assessment | Prediction
| Construction | GIS
| Monitoring & Assessment
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