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Tuesday, January 29, 2013

An urban stream and stormdrain

The two graphs below display the conductivity and temperature profiles for a few rain events in December. The event that began on 17-December fell as liquid precipitation (rain) the later event of 21-December switched between snow and ice then ended as rain. What's really interesting to see is how quickly the conductivity profile in the stormdrain increased as solids are washed into the drain from the street during a rain event. As more water flows into the stormdrain, the solids that were flushed from the street are diluted and the conductivity decreases. The stream also initially responded with an increase in the conductivity profile but the response time was delayed and not nearly as drastic. Notice the highest value displayed in the top graph is 2000uS/cm and the bottom graph is 12,000uS/cm.
So what does 12,000us/cm mean? For a point of reference, sea water has a specific conductance of ~50,000uS/cm and a sports drink is around 2,500uS/cm, regular tap water is 500-800uS/cm.
Also, notice that the warmer temp of the water in the stormdrain drastically decreases as colder surface water pours in from the street, it then rebounds to above 10°C. Air temperature is not shown on these graphs but the water temperature in the stream is much more stable and is closely tied to daily fluctuation of air temperature.

Urban Stormdrain- The sensors are secured to a concrete block and suspended from coated wire and attached to the drain cover, the entire block is lowered into the storm catchment. Additional water sensors are installed in the stream upstream and downstream of the storm water outflow.

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