Lasers shed light on the Shenandoah’s TMDL

Article by Ronda Nickey Lehman, President
Blue Ridge Watershed Coalition

Since the signing of the Chesapeake Bay Executive Order by President Obama, the calculation of “TMDL,” or “total maximum daily load” for the Bay’s tributaries has been a point of controversy for many people living in the Bay watersheds. Their concerns stem from the potentially flawed models the EPA used to calculate sediment loads for each watershed in the Chesapeake Bay. Since these calculations could result in mandates to reduce nutrient loads that could potentially affect tens of thousands of people, getting the numbers right is of the utmost importance.

In light of this controversy, the Blue Ridge Water Coalition set out to calculate how much of a nutrient load the Blue Ridge watersheds were actually contributing to the Bay via the Shenandoah River. We obtained funding through two grants to purchase special equipment to supplement our existing monitoring program with the Friends of the Shenandoah River.

The device we chose, made by Campbell Scientific, uses an underwater laser to take 21 readings in the space of 1/45 of a second. A connected computer uses these readings to calculate the average passing flow of sediment. The sensor is able to reliably distinguish biological material, such as algae or fish, from actual sediment to ensure more accurate readings, and it can be set to take readings as often as every 3 seconds. The data gained from this device will give us a better picture of how much of a nutrient load washes into the river during periods of heavy rain, allowing us a more accurate assessment of the actual TMDL at that point in the river.

The first of five of these devices to be purchased by the BRWC has been placed near a USGS gauge in Millville, WV. This sensor, in conjunction with the gauge, is a great step toward allowing the BRWC to reach a much more accurate estimation of our real TMDL for that area on the Shenandoah River. It is very difficult to calculate precise TMDL due to the fact that the sediment levels running along the bed of the river during heavy rains can be different from the amount of sediment floating along the top of the water. Obtaining a more thorough TMDL measurement would require stacking a bank of lasers from the bed of the river to the top of the water line. This sort of instrumentation would of course be quite costly, and obtaining funding for that amount would be very difficult.

The BRWC will nevertheless be gleaning very real and very helpful data on sediment levels, which will help us begin to locate problem areas. These lasers can be moved in an effort to try to trace the sources of the heaviest sedimentary inflows in Jefferson County. The BRWC will then be able to use this data to obtain grant funding to help target these problem areas, and to continue to monitor the success of any better management practices that are implemented.

The BRWC will also make this equipment available to other groups in our county as needed. Our focus is obviously the Blue Ridge, but we realize we are all part of a bigger picture, making it an easy decision to share our equipment with groups that are attempting to glean similar information in their areas of focus.

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