Tech Briefs

Passive survey method provides less labor and resource intensive system for monitoring the effects of dredging on aquatic life in nearshore beneficial use sites.

The U.S. Army Corps of Engineers (USACE) Portland Districts (NWP) maintains safe and navigable waterways in Oregon through a variety of dredging projects, including channel deepening, and dredging sandbars. Placement of dredged sediment occurs at preselected, specific geographic areas categorized as nearshore beneficial use sites. Beneficial use sites are commonly monitored before, during, and after disposal, to evaluate dredge management practices and to avoid potentially adverse effects to the marine environment.

Two bottom contacting benthic sleds used by NOAA (left) and ODFW (right).

Ecological monitoring focuses on the survey of fish and epibenthic invertebrate species distribution and abundance. Surveys are conducted within, and outside the disposal area to assess changes in population or community structure, and to establish diversity indices for comparisons over time. A commonly used survey method to accomplish this goal is a bottom trawl survey, whereby a net is dragged along a preselected part of the ocean floor for a specific time and distance. The net is then retrieved onto the boat where the aquatic life can be more closely examined and the taxa are sorted. Traditional trawl surveys are labor and resource intensive, highlighting the opportunity to improve alternative “passive” survey methods.

A towed benthic sled is a passive method that has been used for several decades. A benthic sled is typically fabricated of aluminum or steel pipe with flanges to attach equipment like an imaging system and lighting. The sled is towed behind a boat across the sediment surface to collect video of organisms and physical features near the bottom. The benthic sled post-survey analysis usually relies on annotations made by human observers, which is often labor and software intensive. Therefore, long term video collection projects, occurring over many years, often require robust software tools to effectively analyze data sets.

For several years, the NWP has employed NOAA and ODFW custom built benthic sleds equipped with a video system to help monitor marine animals near the sediment surface at NWP’s nearshore beneficial use sites. The Video Annotation and Reference System (VARS), developed and used by the Monterey Bay Aquarium Research Institute (MBARI) for their deep-sea video annotations, was identified by the U. S. Army Engineer Research and Development Center (ERDC) to annotate the underwater video collected at nearshore beneficial use sites. Benthic sleds, or underwater sleds, have been used for several decades for video or image surveys. Sled designs are classified based on where the sled is assigned to operate in the water column as bottom contacting or bottom tending. A bottom contacting sled design is the one most commonly used. This design sinks to the bottom and maintains contact with the bottom while being towed on skids. A bottom tending sled is designed to be semi-buoyant, allowing the user to adjust the sled buoyance to achieve a specific depth. Bottom tending sleds are less conventional survey platforms, requiring some knowledge in control systems, fabrication, and electronics to successfully build, deploy, and operate.

Equipment such as image sensors or lasers for measurement scale can typically be mounted with limited modification to either sled design. Benthic sleds towed by NOAA and ODFW are bottom contacting designs and have been deployed in shallow and deep water (>70 m). NOAA sled accessories have included a high-resolution camera for the primary data acquisition (data camera, Canon Vixia HF R20 camcorder 1080p) enclosed in a waterproof housing, two light intensity-adjustable floodlights, a navigation camera (Multi SeaCam 2060) and two laser pointers (SeaLaser 100) to provide a measurement scale for items of interest in the video field of view. The data camera is internally powered, all other electrical and data transmission to the surface is accomplished via an umbilical coaxial cable. The data camera is usually set to a fixed focus and oriented to acquire video of an approximately 2-meter2 area in front of the sled, with the laser pointers positioned near the center of the camera’s field of view.

This work was done by Justin Wilkens, Jarod Norton, and G. Curtis Roegner for the Army Corps of Engineers Portland Districts. ERDC-0007

This Brief includes a Technical Support Package (TSP).

Monitoring a Nearshore Beneficial Use Site: Application of a Benthic Sled and Video Annotation (reference ERDC-0007) is currently available for download from the TSP library.

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