The Effect of In-Stream Construction Activities on Turbidity, Suspended Sediment, and Sediment Loads

About the report

Transportation construction activities involving in-stream work can mobilize sediment and elevate turbidity, which can affect sensitive aquatic species such as freshwater mussels. Although cofferdams are used to isolate construction areas and limit sediment mobilization, their installation and removal create some degree of sediment release as a result of unavoidable streambed disruptions. To support more informed impact assessments and survey requirements for aquatic species protection, this study determined the transport distances of sediment associated with the installation and removal of cofferdams. Related objectives were to compare the effect of cofferdam-related construction events on sediment loads and to document the factors that affect recovery times and sediment loads.

Streams associated with two bridge replacement projects in Virginia involving different but commonly used cofferdam types (sheet pile and sandbag or Jersey barrier) were instrumented with 19 turbidity sensors positioned upstream and up to 2,600 feet downstream of construction. In addition to continuous turbidity monitoring, data collection included water sampling and field measurements used to determine suspended sediment concentration and site-specific rating curve development. Suspended sediment loads were calculated using stream discharge, suspended sediment concentration, and duration of cofferdam-related construction events.

Across both sites, peak turbidity increases occurred immediately downstream of the cofferdam, with values as high as 238 Formazin Nephelometric Units at Site 1 and 3,324 Formazin Nephelometric Units at Site 2, which returned to background levels within 50 to 100 feet downstream. Sediment loads attributable to construction ranged from 3 to 636 pounds, and for most cofferdam-related construction events, more than 75% of the sediment load occurred in the first 50 feet downstream. The use of sandbag or Jersey barrier cofferdams generated higher turbidity and sediment loads than sheet piles, largely because installation and removal required more streambed disturbance. For all cofferdam-related construction events, turbidity levels returned to background levels between 5 and 90 minutes. Sediment load calculations from evaluated precipitation events were one to three orders of magnitude higher than loads from cofferdam-related construction events.

The findings can help inform assessments of potential mussel impacts from in-stream projects involving cofferdams conducted under comparable conditions. It is recommended that the Virginia Department of Transportation’s Environmental Division share this report with the Virginia Field Office of the U.S. Fish and Wildlife Service and aquatic program staff of the Virginia Department of Wildlife Resources to support agency evaluation of area-of-impact determinations and associated survey requirements for projects comparable with those this study examines.

_{Disclaimer Statement:The contents of this report reflect the views of the author(s), who is responsible for the facts and the accuracy of the data presented herein. The contents do not necessarily reflect the official views or policies of the Virginia Department of Transportation, the Commonwealth Transportation Board, or the Federal Highway Administration. This report does not constitute a standard, specification, or regulation. Any inclusion of manufacturer names, trade names, or trademarks is for identification purposes only and is not to be considered an endorsement.}

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