Summer Storm at
Tischer Creek
Have you ever noticed the flow of your local stream changing from a lazy clear trickle to a brown torrent after a rain storm? Ever wonder what kind of changes are taking place? Let's look at an example. Here’s what happened in Tischer Creek after a couple of inches of rain fell in the middle of the night of July 7, 2003:
The
black graph line shows how streamflow jumped an hour or two
after it started raining. The brown smudge shows how muddy
(turbid) the water became and how it tracked the changes
in flow.
See this data animated!
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Over 12,000 pounds of habitat smothering, gill fouling mud came past our sensor in just a few hours from this storm. High levels of turbidity and suspended sediments in streams can be a serious problem to stream health. It is a common issue in urban areas, where high percentages of impervious surfaces cause increased amounts of water runoff, leading to increased outputs of crud from streets and lots, and increased erosion.
What else happened?
Well, a lot of that rain
that
should have passed through storm sewers into the creeks and then
the harbor or the Big Lake got into the sanitary sewers that take
sewage to WLSSD, our wastewater
treatment plant. This Inflow
and Infiltration problem is
going to cost us a bunch of money to fix.
This event increased the flow into the treatment plant from 30 million gallons a day to 140 million gallons a day.
Although the plant can handle that much water, the collection system is not designed for that size load and bottlenecks are created. Because of the bottle necks, the wastewater backs up, popping manholes and escaping into the environment. Sewage spills such as these can cause environmental and health risks and may have contributed to beach advisories being posted. This particular event led to the discharges of several million gallons of partially treated wastewater. The I & I problem was compounded by a series of mechanical and electrical malfunctions that have since been corrected. WLSSD and the City of Duluth subsequently developed a Plan of Action (2.5 MB pdf) to prevent such overflows. Find out more about the plan at I&I or WLSSD. Pollutants from land runoff and the sewage spills are among a number of potential causes for the beach closing that have occurred in 2003 and 2004. The Minnesota Pollution Control Agency states:
Water at Minnesota's beaches can become contaminated by fecal material which carries harmful bacteria and viruses. This unsanitary condition can be due to several possible causes: animal feces (gulls, geese, pets), improperly disposed of diapers, children not properly cleaned after using the bathroom, a vomiting or fecal accident in the water, swimmers with diarrhea, malfunction in septic system in the vicinity, nearby boat moorings or marinas releasing sewage into the water, Stormwater runoff, and wastewater overflow/treatment plant malfunctions.
Beach closing information can be found at the MNBeaches.org website.
Flowing under the Lift Bridge
Just for fun, you can also see how this storm increased
the flow under the Aerial Lift Bridge from the harbor into
Lake Superior. (DataViewer)
Water sloshes back and forth under the bridge
because of the seiche* (“saysh”). The graph shows
much higher flow from the St. Louis River (the brown bands) into
the lake after the rain (the green bars). You can also see the “signature” of
the river water by its warmer temperature (black graph line)
and higher salt content (red line).
*
Seiches are lakewide
displacements of water that are wind-induced. Water pushed by the
wind can pile up on shore causing noticable increases in water
depth. When the wind is reduced the water mass continues to slosh
back and forth like water in a bathtub.