Supervision of nearshore outfalls with ROVs on the island of Krk

Survey and documentation of 7km of nearshore outfalls and marine gravity drains within the public drainage system on the island of Krk The Ordinance on Technical Requirements for Wastewater Drainage Facilities and Deadlines for the Mandatory Control of Sewage and Wastewater Treatment Facilities (Croatian official gazette NN 3/11) defines nearshore outfalls as “public sewage structures” whose structural stability and function must be checked by a visual inspection during usage.

This visual inspection includes a review of all visible damage and problems in function. Legal regulations require such an inspection be undertaken once every three years.

During this project, we used a remotely operated underwater vehicle (ROV) equipped with high-resolution cameras and an underwater GPS system.

This was our first use of digital anchor technology (positioning the vessel above the outfall) and the SfM (Structure from Motion) method to better detect the state of nearshore outfalls.

The choice of a ROV that can turn on its x, y, and z axis proved to be critical; as nearshore outfalls are often used as anchor points for buoys, avoiding anchoring ropes during automated inspections became an imperative.

The project included:

  • Mapping the sea floor with lateral sonar mounted on a remotely operated underwater vehicle (ROV)
  • Programming inspection routes by ROV
  • Examination of nearshore outfalls using a ROV equipped with at least four high-resolution cameras and an underwater GPS device
  • Validating data using standard statistical methods
  • Drafting a report on the current state based on visual methods, including the creation of a 3D model of the sea floor and outfalls at each location, as well as defining specific damage using photogrammetric methods