Smart Rocks Deployed!

Posted on: October 13th, 2017 by Leslie_Zucker

The Ashokan Watershed Stream Management Program and the NYC Department of Environmental Protection are working with the U.S. Geological Survey to study the rate at which larger rocks and cobble move down stream channels. Fine suspended sediments travel with stream flows and pose the greatest threat to water quality in the upper Esopus Creek. But larger material resting on the channel bed is also suspended during stream flows and then redeposited downstream. Streams in equilibrium with their surrounding landscape accomplish sediment transport in an orderly fashion. Rocks and cobble on the stream bed are lifted from their locations in riffles and redeposited downstream in another shallow area where riffles form. It’s when the channel is disturbed or thrown out of balance that the process can go awry. Large sediment accumulations in the center of the channel, like those deposited during Tropical Storm Irene for example, can split stream flows and push water against the banks. The banks may contain fine sediments that cloud drinking water or generate more coarse material that blocks bridges downstream. For this reason stream managers would like to know more about the coarse sediment – when and where it moves, to better maintain stream channel stability.

The large sediment, rocks and cobble, traveling down the channel bed is called “bed load.”
The AWSMP is partnering with the USGS to test different techniques for monitoring the amount and rate of bedload movement. Bed load monitoring is difficult, time intensive and expense to pull off. Several techniques being piloted are meant to save time and make the effort more manageable.

The USGS has installed hydrophones at two bridges in the watershed. The hydrophones are triggered when flows rise and record the sounds rocks make hitting against each other as turbulent water carries them downstream. USGS scientists will parse the data and check it against physical samples taken at the bridges at the same time to see if a sound signature can be used to quantify the sediment being transported.

RFID Tag on Smart Rock

An RFID-tag was drilled and glued into this rock pulled from the stream channel to allow for radio-tracking later.

A second approach is to embed RFID (radio-frequency identification device) tags into native rock material and then find these rocks later using a hand-held antenna during sweeps of the channel after bed-load moving storm events. Another “smart” way to track rocks is to put an accelerometer into pre-manufactured rocks along with the RFID tags to gather the rate at which rocks move.

Finally, USGS staff will deploy staff to capture sediment as it flows under the bridges. The bedload samplers are lowered from the bridge to the channel bed and fill with sediment that is dumped into buckets and later sorted and measured. Sampling continues as long as practical until stream flows recede!

Bed Load Sampler

USGS scientist Jason Siemion lowers the bed load sampler from a bridge.

The entire effort depends on having stream flows strong enough to move most of the sediment lying on the channel bed. Surprisingly, almost the entire stream bed moves during flow events that occur as frequently as every several years on average. And at least some of the bed load moves during smaller events. The project team is hoping for just enough rain to cause bed-load movement but not enough to cause any damage!

The study will run through 2019 with results to follow.

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