Public Works
Public Works Blog
With river construction comes underwater sound monitoring
Posted On: August 12, 2020
(Photos were taken by The Greenbusch Group, Inc., of Seattle, the consultant doing the acoustical and audio work on the West Cashmere Bridge Replacement Project. The top photo shows the hydrophone in the Wenatchee River. Bottom photo shows the diesel hammer that was used to drive a piling into the river.)
When building a bridge, oftentimes extra care is taken to lessen the impacts of a large construction project on the businesses and neighbors in the project area. From watering the work site to control dust to implementing night-time road closures to avoid high traffic, steps are taken regularly with our neighbors in mind.
The same sort of consideration extends to the fish making their home in the river below the work area.
At the West Cashmere Bridge replacement project, you might get the chance to see an “acoustician” on site when the contractor is driving steel pilings into the riverbed using a large diesel hammer.
An acoustician monitors noise levels in the water so work can be stopped if the noise created is high enough to injure or kill fish in the river. When underwater sound pressure injures a fish, this is called “barotrauma.”
Sound monitoring is especially important in rivers with federally protected endangered species, such as the Wenatchee River. The river is home to bull trout, spring chinook salmon and summer steelhead.
Noise is monitored using a hydrophone, or an underwater microphone. In Cashmere, the hydrophone will be lowered into the water from a platform under the West Cashmere Bridge. Measuring is done about 30 feet from any impact hammer pile driving. If the noise level is too loud, work is stopped and adjustments are made, such as adding a wooden cushion block under the hammer.
A variety of factors can come into play when measuring noise underwater. Deeper water creates higher noise levels. (The deeper the water, the more contact with the pilings.) The rockier the riverbed, the more resistance – and noise – experienced when driving piles.
So far, Chelan County’s contractor on the Cashmere bridge project has been able to use vibratory pile driving when embedding the majority of steel pilings needed to build a temporary work trestle. Vibratory pile driving produces a continuous sound, with peak pressures lower than pulses generated by impact pile driving. The vibration method is easier on everyone – from the contractor to the fish.
A variety of state and federal agencies require noise monitoring if pile driving will be done when endangered species are present. But doing so is just good stewardship. In Chelan County, we place a high value on our rivers and streams and the wildlife that depend on them.
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