« Back
  • Advancing dredging knowledge

    Dredging attrition and tailwater research
  • Dredging attrition and tailwater research
  • Dredging attrition and tailwater research
  • Dredging attrition and tailwater research
  • Dredging attrition and tailwater research

Dredging attrition and tailwater research

A variety of tests were evaluated for their ability to accurately simulate excavation of bed material via a Cutter Suction Dredger and transportation of dredged seabed material in pipes and pumps. Also several models were assessed in terms of their ability to represent processes affecting dredged sediments within reclamation areas.

Understanding how seabed materials will change during dredging is crucial for designing dredging projects.

Dredging is defined as intentional underwater excavation of bottom material, and the movement of this material to a different location. The entire process that undisturbed in situ soil/rock goes through has been considered in this work. During the removal, transport and deposition / re-use of the bottom material its physical characteristics can undergo significant changes. An understanding of these changes is essential in order to correctly design dredging projects.

Dredging activities can be divided into two main operations:

Looking at excavation of seabed materials

The first part of the research reviewed the processes involved in the operation of a cutter head (with a focus on developing a physical description of each) and how the actions involved affect different material types. It also evaluated existing laboratory tests simulating the processes at the cutter head and considered if or how some of these might be implemented at HR Wallingford. Although they are costly and require large sample sizes (which can be impractical) model cutter head tests appear to represent the best approach presently available.

Examination of different laboratory tests and their ability to simulate slurry transport

Next, the research examined the physical processes that affect a mixture of water and cut seabed material (slurry) when it travels hydraulically through transport pipelines. Laboratory techniques for simulating these processes were reviewed and assessed with a view to their potential use at HR Wallingford. The lab tests examined were: Los Angeles Abrasion Test; Micro-Deval Test; Slake-Durability Test; Beach Tumbler (HR Wallingford) Test; Closed Pipe Loop Test; and the Single-Pass Pump Test. Drum tests have the advantage of being economical but also have a number of shortcomings. Such tests are able to provide useful information at a qualitative level (e.g. whether a material has a high, moderate or low susceptibility to break up). Pipe loop tests have the significant disadvantage of being costly. They also tend to require significant volumes of intact sample material for testing, which can be impractical. However, these tests represent the best available in terms of their accuracy.

Evaluation of models for the estimation of tailwater concentrations at reclamation outfalls

A further part of the research looked at the estimation of tailwater concentrations at reclamation outfalls. The processes occurring in reclamations include sediment transport, settling, and consolidation; how these interact to influence suspended sediment concentrations in the tailwater is an important issue. 

Through understanding the processes that occur within reclamations, it was possible to evaluate available modelling tools (including the HR Wallingford Dredging Group Reclamation Spreadsheet; the TASS Hopper Model and the USACE ADDAMS modelling suite) against specific criteria.

This research indicated that an ideal method for estimating suspended sediment concentrations within tailwater does not currently exist, but several methods are available each having particular strengths and weaknesses. From the research a strategy for development and improvement of the existing approaches has been identified along with cost estimates for its implementation.

Authors

Mark Lee, Daniel Barber, David Middlemiss, Guilia Sforzi, Briain O’Dowd

Keywords

dredging, attrition, cutter head, sediment relocation, tailwater, disposal, reclamation

Completed

2012

Read more information at

Contact

Project information

Related projects